Integrity of atmospheric storage tanksSPC/Tech/Gen/35

Author unit / section:  HID CI1G, HID CI5B

Target audience:   HID Discipline Specialists (Mechanical Engineering, HID Regulatory Specialists, FOD Specialist Group (Mechanical Engineering)

Purpose Background Introduction Aim Objectives Scope Good practice benchmarks Storage tank background Procedures and documentation Damage mechanisms Original manufacture, modification, and repair defects Tank inspection; inspection bodies and competencies Inspection practices Inspection periodicity Inspection reports GRP/HDPE tanks Horizontal tanks Piping, bellows & flexible joints Venting and relief Second hand tanks Coatings and linings Hydrotesting Codes and guidance Supporting guidance (inspection and continued fitness for service) Supporting guidance (general)

Purpose

The purpose of this circular is to raise awareness of the factors to consider when operating storage tanks containing hazardous substances.  Information was drawn from a series of site inspections ranging from refinery tank farms, large speciality chemical storage facilities as well as a number of smaller sites where bulk storage tanks were used to store raw, intermediate and final product materials.

Background

While some of the information is general and can be applied to a wide range of mechanical plant, other parts of the report are equipment-specific.  It is intended as a reference aid to assist regulators and those concerned with the operation, and in-service integrity of storage tanks, by identifying good practices.  These are set out in Section 5.

Other sections of the report cover issues such as general integrity procedures, damage mechanisms, manufacturing and repair defects, inspection practices, non-metallic tanks, venting and relief, coatings and linings, and hydrotesting.  It concludes with a summary of the most relevant codes and standards.

Introduction

1  During the 2005/2006 work year Inspectors in HID CID carried out a number of interventions examining the way in which the integrity of atmospheric storage tanks was managed. 

Aim

2  The aim of this SPC is to provide Mechanical Engineering Inspectors with information for use when inspecting atmospheric storage tanks at COMAH sites and other hazardous installations. 

3  Regulatory Specialists are not expected to attempt a detailed technical inspection using this guidance. It may be useful, however, in supporting interventions associated with the HID Chemicals Industry Strategy particularly Key Goal 1 ‘Prevention of Major Accidents’.

4  The SPC also gives a summary of the principal management arrangements to reduce the risk of loss of containment from storage tanks that were identified as representing good practice for this sector. HSE expects duty holders to adopt relevant good practice as a minimum. Therefore these measures, or others that have been demonstrated to be at least as effective, would normally be considered enforceable as part of the package of measures that reduce risks ALARP.

Objectives

5  The objectives of the project were to:

identify key issues affecting integrity management raise the profile of atmospheric storage tank integrity assurance benchmark and promote good practice across sites.

Scope

6  The scope of the interventions was limited to in-service issues relating to tanks containing hazardous substances, their appurtenances, and piping up to the first isolations.  The principal focus was on those measures to prevent loss of primary containment.

7  Most designs of tank were included in the exercise e.g. flat-bottomed vertical cylindrical, skirt and leg supported domed end, and horizontal. Tanks constructed from non-metallic materials such as GRP and HDPE were also studied in the project and appear to be gaining popularity for certain applications, notably the storage of mineral acids.

8  No tanks constructed from aluminium were inspected nor were any with double skins or double bottoms.  A number of stainless steel tanks were seen during the course of the project and many claimed to have been designed and manufactured generally in accordance with BS 2654 ‘Specification for Manufacture of vertical steel welded non-refrigerated storage tanks with butt-welded shells for the petroleum industry’[1]

9  Key issues examined included:

integrity management policy and procedures codes and guidance deterioration mechanisms inspection practices  (including RBI techniques; hydrotesting etc.) competencies

Note:  because of the wide range of topics areas some were intentionally left out of scope, for example:

refrigerated storage buried or above-ground mounded tanks gas holders bolted tanks silos level controls, alarms, trips and instrumentation bunds and tertiary containment flow control valves, block valves and rosov’s

Good practice benchmarks

10  This SPC identifies reasons why the integrity management arrangements of atmospheric storage tanks are usually not as comprehensive and robust as for other process plant and equipment.  The following list summarises good practices identified during the interventions together with existing guidance and standards:

Tanks containing hazardous substances should be identified and entered in the plant register 

Operators should maintain tank data files  Compatibility assessments should be undertaken (e.g. tanks on multi–product

service)  Tanks should be subject to formal periodic maintenance and inspection Inspection and maintenance of tanks should only be carried out by experienced

and qualified competent persons   Schemes of inspection should be established and agreed between Operator’s and

Competent Persons  Operators and Competent Persons should have knowledge of and adopt the

recommendations given in the relevant guides, codes and standards   Tank examination schemes should include both internal and external

inspections   Appropriate inspection techniques should be utilised, depending on deterioration

mechanisms 

Inspection reports and checklists should be of high quality Where necessary recommendations from inspection reports should be actioned

promptly  Assessment for fitness for service should be carried out following tank

inspection and significant changes to process or operating conditions

Storage tank background

11  Storage tank integrity needs to be well managed since tanks can contain large inventories of hazardous liquids and their failure has the potential to result in serious and dramatic events.

12  Many of the tanks around the UK are still in operation after well over 50 years of service.  Use of riveted tanks installed in the first half of the 20th century is still widespread.  The oldest tank seen still in service was of riveted construction, manufactured in 1893, and used to store diesel oil. 

13  Failures, though not as common as those in piping systems, occur regularly. Permeable bunds are still found at many sites and it is by no means unknown for a tank to be leaking through the base without any indication or the Operator’s knowledge.

14  Tanks are often perceived as simple structures that require little attention. Nevertheless damage mechanisms associated with them can be complex and varied.  The measures Operator’s have in place to maintain tanks in a safe operating condition were found to be variable and in some cases fundamentally inadequate.

15  Atmospheric tanks are similar to much process piping in that they are non-codal type equipment (i.e. not subject to PSSR 2000 regulations). Integrity management regimes are not nearly as mature and well developed as for pressurised plant.  In the UK there is no specific duty or regulation that sets out prescriptive rules for storage tank inspections, though clearly duties under HSW, COMAH and PUWER regulations can apply.  The generally high reliability of tanks has meant that in many instances maintenance approaches have tended to be reactive.  Usefully, there is extensive guidance available on periodic/proactive tank maintenance and inspection practice .  References to ‘Supporting Guidance’ are given in Section 21.

16  It is often the case that thorough inspections of tanks are only carried out if the tank can be taken out of service, which, in many instances, can only be achieved by agreement between the Operator and their customers who are renting the space under 3rd party tank lease agreements.  Clearly this can be a major problem where shortage of tanks or lack of flexibility is an issue.

17  At many sites tanks are not dedicated to a single substance and may have been used to contain a range of products.  This can have a significant effect on their integrity, not only from the threat of direct chemical attack and corrosivity (e.g. incompatibility between the substance stored and the tank material of construction) but from other factors such as product density, chemical stability, volatility, concentration, temperature and erosion.

18  At the best-managed sites this difficulty is clearly recognised and strict procedures are in place to assess the properties of different products.  These controls were not in place on many of the less well-managed sites.

19  Despite their importance storage tanks are frequently perceived as ‘infrastructure’ and not fundamental to process.  This can lead to issues such as poor record keeping, insufficient design, construction and condition information, unsatisfactory design drawings, process diagrams and P&ID’s.  Often repairs, modifications and other changes, some which have been substantial, have not been documented and captured on records.

Procedures and documentation

20  Well-produced policy and procedural documentation describing the systems in place to provide integrity assurance of storage tanks were seen at the best managed sites.  These specified the principal requirements of an integrity management regime and were used to underpin all of the good practices seen at these sites. The documentation often included references to relevant guidance and standards. 

21  Key information in good procedures included administrative and organisational information, statements on general integrity strategy, roles and responsibilities, personnel qualifications, tank databases and operational duties.

22  High-quality procedures often incorporated annexes giving details of inspection scope and techniques associated with the main elements of tanks e.g. foundations, floors, roofs, and shell.  Where relevant, additional detail would set out particular requirements (e.g. for inspection and maintenance of floating as opposed to fixed roofs, or requirements for insulated tanks).  Individual sections included different requirements for external and internal examination.  Guidance was also given on the format, production and retention of records along with corrective actions and review procedures.

23  Other general information incorporated elements such as schemes of examination specifying the frequency and scope of inspections (in and out-of-service), recommended NDT techniques, cleaning and surface preparation standards, retirement limits, level and settlement surveys, repairs and hydrotests.

24  In the best cases procedures also defined the requirements for maintenance, inspection and testing of protective devices such as pressure/vacuum valves and emergency relief.

25  Procedures often specified inspection strategies drawn from the three leading guidance documents: API Std 653 ‘Tank inspection, repair, alteration and reconstruction’, API RP 575 ‘Guidelines and Methods for Inspection of Existing Atmospheric and Low-Pressure Storage Tanks’ and EEMUA 159  ‘Maintenance and inspection of above ground vertical steel cylindrical storage tanks’. 

26  Other specific guidance material, such as HSE PM 75 ‘Glass reinforced plastic vessels and tanks: advice to users’, National Sulphuric Acid Association (NSAA) ‘Recommended guidelines for the bulk storage of sulphuric acid, oleum and liquid

sulphur trioxide’ and HSG 235 ‘Guidance on the storage of Hydrochloric and Nitric Acid in tanks’ were referred to where relevant.

27  Use was also made of HSG 176 ‘The Storage of Flammable Liquids in Tanks’ which makes an important recommendation that there should be a ‘written scheme of examination’ (WSE) agreed between the user and competent person that should  include the scope and frequency of the thorough examination.  It is worth noting that although the WSE and the scope of inspection may well be similar to that employed on pressure plant, this WSE is not, from a regulatory perspective, the same as that required under Pressure Systems Safety Regulations 2000.

28  The best Operator’s maintained tank records that contained information such as original design codes and construction data, date of installation, operational history, piping systems or process schematics, drains, foundation data, repairs and modifications, maintenance and inspection histories.  In the best cases the key document was the scheme of examination, each adapted to the individual tank.  These schemes identified the forseeable deterioration mechanisms and then specified the maintenance and inspection practices to prevent or control them.

29  Some of the less well-managed sites had little if any formal in-service inspection and maintenance procedures for their tanks.  Indeed in many cases tanks were not even individually identified in plant registers or included in any proactive inspection and maintenance regime, even where these contained hazardous substances. 

30  While it is essential to examine tanks on an individual basis some useful information can be obtained from sample inspections.  Guidance on the circumstances where the results from sample examinations can be helpful in modifying recommended inspection frequencies or the scope of an inspection are given in API Std 653 and EEMUA 159.

Damage mechanisms

31  A wide range of damage mechanisms can cause a storage tank to deteriorate and fail.  Many can operate simultaneously. This section cannot identify all of these but does aim to address those that pose a threat to the integrity of the structure.  The following section on inspection methods will describe how the failure mechanisms can be detected.

Corrosion

32  The vast majority of storage tanks are constructed from carbon steel and corrosion is a prime cause of deterioration of them and their accessories. It can be associated almost equally from external attack (atmospheric side) or from an internal (product side) mechanism. By way of example tanks in crude oil service can be particularly susceptible to sulphate reducing bacteria (SRB) attack. 

33  Corrosion is rarely uniform, though this is not unknown.  However, random, localised, pitting corrosion attack, particularly of flat-bottomed tank floors appears to be the most common failure.  This can be topside down (especially where there is an aqueous phase) or underside up.  Product temperature appears to be an important element. The condition and materials of construction of tank base along with the

effectiveness and durability of the floor to base seal, and the slope angle of the berm or tank pad away from the base are crucial factors in prevention of bottom up corrosion.[2]

34  Corrosion attack of the lower shell strakes is also common as is attack of annular plates where these are fitted.  This is often, though by no means universally, caused by poor bund drainage.  Edge lamination of annular rings is also commonly seen in these circumstances.  BS 2654 recommends tanks > 12.5m diameter should be constructed with annular ring plates.  API 650 takes a different approach and uses allowable stress to determine when annular plates are necessary.

35  For the reasons outlined above many Operators arrange to have the bottoms and typically the first metre of their tanks painted or coated to provide increased corrosion protection.  Linings and coatings for tanks is a subject in itself and some further detail on coating protection is given later in the report.

36   Deterioration and failure of tank drains is common.  Drains are often the single most vulnerable point on a tank, especially those that run through channels under the floor.  They are regularly overlooked, difficult to inspect, the culverts are often submerged in rainwater and full of debris.  At one site the channel under the tanks and the drains had simply been concreted in.

37  Water draw-off sumps are also vulnerable parts of the tank, particularly those which are centrally located on cone-down type floors.  While wall thicknesses are generally greater than that of the floor both the inner and outer walls are at potentially greater threat from corrosion.  Guidance on inspection techniques for sumps is poor and it is arguable that full ultrasonic thickness scans should be carried out here.  API 650 gives information on centre draw-off sumps and drains.

38  Examples were seen where preferential attack had occurred to lower shell strake welds and in another case the floor directly under the dipping point on one tank containing diesel oil had been pierced.  This may be attributed to fretting-related corrosion (a mechanism seen with floating roof support leg striking plates) where repeated contact removes any protective layer of rust scale that may have formed.

39  External corrosion is often found also where fittings are welded to the tank shell or at water traps for example around and below wind girders, stairwells and vertical/spiral stairway connections.

40  Other areas of increased corrosion attack are typically at the liquid/vapour interfaces and areas around vents and breathers where oxygen and humidity levels can be higher.

41  Care must be taken walking on tank roofs where there are doubts about its condition.  It is by no means unusual that the first sign of deterioration found is when daylight is seen through holes in the roof at the time of internal entry.  Internal damage can occur where the product is more corrosive in its vapour phase and due to general condensation in the roof space.  It can be more prevalent around vents and breathers.  Some Operators highlighted roof walkways by painting them in a different colour or used a non-slip coating and carried out additional inspections in these areas.

42  There are a number of different designs of floating roofs and inspection of their many features such as rolling ladders, deck plates, seals, drains, pontoons and guides is carried out from the top.  Like that for fixed roofs care should always be taken when walking on them and, if the tank is in service, access is normally restricted if the roof is below a certain level (normally top dip or no more than 2m from the top).  Access to pontoons or the inter-space of double deck roofs should also be restricted if the tank is in-service.

43  The other most common external corrosion attack is CUI (corrosion under insulation).  Here the operating temperatures are critical and many tanks contain liquids that are stored in the ‘at risk’ range of 50 to 100°C.  Also where insulating material is high in chlorides.  Again the damage tends to manifest itself as pitting attack predominantly around the bottom edge particularly if the insulation is not cut away causing a ‘wicking’ effect.

44  Effective preventative measures here incorporate good insulation and cladding standards, effective sealing at vulnerable areas for example the shell to roof joint, insulation supports, small-bore penetrations and include an insulation holiday for about the first 8 to 10” (200 to 250mm) from the tank base.  Recommendations for the design and application of insulation can be found in Appendix B of BS 2654.

45  Chloride stress corrosion cracking is the main cause of degradation with stainless steel tanks.  It is temperature dependent and the main problem areas are the nozzles and attachment points around steam heating coils.

Erosion

46  Is generally found at points of increased flow or change in flow direction particularly adjacent to fill and discharge nozzles.  It is especially common in tanks containing sulphuric acid.  In many cases increased thickness of materials or backing plates are used where erosion is identified as a threat.  One instance was found where damage to a tank sump, which eventually leaked, was caused by poorly installed vibrating piping which eroded the side wall of the sump.

47  Erosion of a floating roof tank inner shell wall was identified at one site where the tank was sited downwind in a particularly dusty location.  It has also been observed directly under a mixer on a tank in crude oil service.

Creep

48  Cold creep (or stretching over a period of time) is a well-understood deterioration mechanism but is limited to non-metallic thermoplastic tanks.  These tanks can be temperature sensitive as well with HDPE in particular losing strength as temperature increases.

Fatigue

49  Does not appear as significant a potential failure mechanism as for pressure vessels since the membrane stresses are generally much lower.  Nevertheless for tanks in high cyclic fill/empty service areas of high stress such as the floor to shell/annular to shell

welds could be threatened as well as some areas around nozzles.  It is common to feel tank floors flex when walking across them.  Many operators utilise dye penetrant and MPI NDT techniques to inspect for fatigue induced defects.

50  Fatigue is recognised in GRP tanks and a service life is defined.

Chemical attack

51  The threat to tank integrity through chemical attack and incompatibility between the products stored and the material of construction is obvious.  The most common construction materials are carbon and stainless steels, thermoplastics, polypropylene, PVC, PVDF, HDPE, and laminates such as GRP.  Aluminium tanks are used more commonly for hoppers and silos and in low temperature applications but appear less general for normal liquid storage.  Tanks utilising a variety of lining and coating materials are used although metallic clad tanks are quite rare.

52  Carbon steel is suitable for the storage of a large number of chemicals, however, some, such as dilute acids will react with carbon steel.

53  Stainless steels are often used where purity and quality of the stored chemical is paramount.  However welding of stainless steel tanks and system components is critical as these can be susceptible to stress corrosion cracking.

54  Additional information of HDPE and GRP tanks is given later in the report, however poor high temperature properties, UV light degradation and environmental cracking are all well understood failure modes for non-metallic tanks.

55  It is essential that the materials used for the construction of the storage tank are compatible with the chemicals to be stored.  Additional consideration may be required where chemicals are stored at elevated temperatures. The best Operators had robust systems which identified the issues as well as applying formal change procedures to assess the effects of any anticipated product/ tank changes.

Mechanical damage

56  Mechanical damage to storage tanks can be caused by impact, differential or non-uniform settlement, over-pressurisation, vacuum, excess dead loads such as snow or ice, and wind inflicted damage.  Buckling of shell plates and other externally inflicted damage should be identified from good inspection. Shell buckling can have severe effects where tanks are fitted with floating roofs as can differential settlement and tilt.

57  Snow and wind loads appear to be the most common threats to a tank’s structure and buckling of the shell due to wind gusts is relatively common.  Buckling of shells and catastrophic failure have been caused by internal vacuum where vents have become blocked.  Serious damage has also occurred on floating roof tanks where rainwater drains have blocked or failed.

58  Subsidence can occur particularly on weak and compressible ground. Many tank farms are sited on recovered land, often alongside rivers and other environmentally

sensitive areas.  All of the tank codes stress the importance of Operator’s/Designer’s understanding the subsurface conditions and the soil properties.

59  Frost heave and frequent freeze-thaw of the ground can affect tank foundations as can exceptionally high tides in tidal areas.  This can lead to uniform or differential settlement, planar tilt and edge settlement. In addition to roof binding on floating roof tanks and cracking of welds it can also affect connected piping systems many of which are fitted with bellows or flexible joints of varying design. 

60  The interventions revealed that many Operators did not consider it necessary to carry out periodic verticality and settlement surveys.

61  It is worth noting that anchoring flat-bottomed tanks using bolts is not a mandatory design requirement.  Many factors are taken into consideration such as tank foundation type, deadweight, product weight, pressure uplift and roof frangibility.  These must be balanced with other risks e.g. tanks floating if bunds fill with rainwater or leaking product or wind loads.  Where fitted the design codes do specify a minimum diameter and spacing.  Clearly condition evaluation of anchor bolts is fundamental if they are fitted. This can be carried out in conjunction with the foundation inspection.

62  The poor impact resistance of non-metallic tanks is well recognised.  Mishandling during transport and poor installation have resulted in their premature failure

Brittle fracture

63  Brittle failures of tanks have occurred shortly after construction especially during hydrotesting or on the first filling in cold weather.  Experience has shown that once a tank has demonstrated satisfactory service the risk due to brittle failure in normal operation is minimal.  More information on tank hydrotesting is given later in the report.

64  Section 5 of API Std 653 is devoted to an assessment procedure for tanks in brittle fracture service.

Original manufacture, modification, and repair defects

65  This section of the report gives examples seen where original manufacturing defects or questionable repairs and alterations have been carried out.

66  Examples of loss of containment have arisen from weld defects particularly from poor standards of single pass butt weld in floors, shell to floor welds, and from fatigue cracking of welds in the floor and around nozzles.

67  Patch over plating of metallic tank floors is common as is whole floor replacement.  It is not unusual for floors to be replaced with thicker plates e.g. 6mm being replaced with 8mm plate.  Floor replacement also allows the opportunity for foundation material checks and any remediation to be carried out. This is often carried out by jacking the whole tank up, or by cutting out letterbox type sections in turn.  Jacking can also be used to correct uneven settlement.  Good advice on tank jacking is given in EEMUA 159.

68  Annular plates typically suffer from corrosion attack around the outer exposed plate edge, which, in extreme cases, can lead to a ‘laminated’ appearance.  In addition both top-side and bottom-side corrosion are relatively common.  At one site through-thickness cracking occurred from a region of undercut along the inner shell to annular plate fillet weld.  Most of these problems are caused, or at least exacerbated, by inadequate foundation support under the annular plate and poor drainage.  It is also possible that cyclic loading could also play a part.  Modifications to improve the drainage in this region based on the recommendations in CIRIA 598 ‘Chemical storage tank systems – good practice: Guidance on design, manufacture, installation, operation, inspection and maintenance’ are often made at the time when new annular plates are installed.

69  In one other case it was found that a tank floor and annular plates had been replaced and when the tank was subsequently hydrostatically tested it was reported that a lower shell plate moved inwards towards the tank centre and became distorted, along with two other plates higher up the tank shell.  This was attributed to an incorrect alignment between the lower shell course and the new annular plates prior to welding.

70  General shell wall thinning and subsequent failures are much less frequent than floors but can occur on external floating roof tanks.  Articulated roof drain failures are also common on these types of tank.

71  Repairs to lower shell strakes generally involve cutting out and re-welding areas of replacement shell plate (including one or more entire shell plates or full height segments of shell plate) or welding into place lapped patch plates.  It should be noted that lapped patch shell repairs are considered to be an acceptable form of repair for butt-welded, lap-welded and riveted tank shells in API Std 653 but only under specified circumstances.  If more extensive areas of shell plate require repair then the use of butt-welded shell replacement plate is preferred.  The repair of a buckled tank shell should always be done using insert plates.

72  At one site repairs had been made to corrosion holes in two tank shells using epoxy or polymer based type materials. No formal assessment of integrity threat had been carried out or any indication given of when these would be replaced by a proper repair.

73  The installation of a second drain line into a tank required a repair and modification.  Unfortunately, to accommodate both drain pipes in the sump one of the vertical sections of the new steel drain pipe had to be located in close proximity to the sump wall.  This eventually resulted in perforation of the sump and a considerable quantity of product was lost.  Details of this failure are given in the ‘News and Alerts’ section of the Refineries Issues Group[1] page.

74  Roof repair procedures are described in API Std 653.  If it is necessary to replace roof plates consideration should be given to strengthening areas used as designated walkways (subject to proper design assessment).

75  Repairs to wind girders are also covered briefly in API Std 653 plus repair to other features such as penetrations. This document makes frequent reference to the original design code (i.e. API Std 650).

76  During the course of the project numerous examples were seen where the competence of those organisations engaged to carry out repairs of storage tanks varied markedly.

Tank inspection; inspection bodies and competencies

77  A number of Operator’s employed Type B or 2nd party inspection bodies to carry out the inspection activities while others employed Type A or 3rd party inspection bodies.  Some were independent sole traders (not UKAS accredited) while in other cases the inspectors were the engineer/surveyors from the major engineering insurance companies accredited as Full or Associate members of the Safety Assessment Federation (SAFed).

78  HSE has growing concerns over the differences in performance of the inspection bodies, particularly in respect of COMAH major accident sites. Contacts with the representative trade associations have become more regular recently in order to address matters of common interest.

79  It was not uncommon to find sites that did not employ dedicated tank inspectors or utilised competent contractor support.  Inspector qualification and competencies are covered later in this section.

80  The competencies of those involved with tank inspection varied markedly.  In the organisations with the better integrity management programmes examinations were conducted by inspectors qualified to the standards required by API Std 653 or EEMUA 159 using a range of techniques. There appeared to be widespread knowledge of the EEMUA 159 guidance document even if not all operators followed its recommendations rigorously.

81  HSE recommends that only qualified and competent persons are engaged for the inspection of atmospheric storage tanks.  Provider information is available on the internet giving details on formal courses which train inspectors to either API Std 653 or EEMUA 159 standards[1][2].

82  Examples were seen which demonstrated that the inspections performed by single operator inspectors or others employed by more reputable bodies were not of a high standard and many had never received any formal training.  The following issues were identified:

1. In a number of cases companies were not clear what service the tank inspector was providing, who had responsibility to interpret the inspection results and then decide if a tank was fit for continued service until the next inspection.  Some companies were under the mistaken impression that an inspection report simply listing the inspection findings without any analysis and setting no date for the next examination was sufficient to justify continued operation.  This is not good practice.

2. Cases were seen where the original tank design, manufacturing and operating details were unknown and in addition, in some instances there was little, if any, previous tank inspection history available.  Despite these complications some companies had adopted inspection frequencies without first:

1. Comparing the design and manufacturing details of the tank against appropriate design codes; and,

2. Establishing the ‘datum’ condition of the tank.

Again, this is not good practice.

3. After many years of inspecting tanks at fixed inspection intervals one site decided to implement a risk based inspection (RBI) approach.  The new inspection interval determined from the RBI assessment was applied from the date of the review and not from the date of the last inspection under the ‘old’ regime.  As a consequence the period between inspections on some tanks was significantly outside the maximum inspection period recommended from the RBI review.

4. Many examples were seen where recommendations from inspection reports had been ignored.  This points to a failing in the integrity management arrangements within the company and is poor practice.

5. Other cases were seen where it appeared that the person carrying out the inspection was not aware of the contents of the scheme of examination.  This is a potentially difficult area for the inspector if a subsequent failure points to an omission in the scope of the last inspection.

6. It was not uncommon to find that large numbers of tank (and other equipment) inspections had been completed at a single visit.  This implies a cursory and superficial approach to the examinations.

7. At one site, the inspection resource had been concentrated on plant falling within the scope of the Pressure Systems Safety Regulations 2000 (as amended) and, possibly as a consequence, one storage tank inspection was found to be 15 years overdue.  At the better companies no distinction was made between plant falling within the scope of PSSR and plant outside the scope of PSSR.

8. The quality of the inspections carried out on HDPE tanks was particularly variable and a number of tanks were found to be operating beyond their original design life without any supporting technical justification.

9. A failure investigated during the course of the project involved the piercing of the tank floor due to repeated contact with the nose of a weight attached to the dip tape.  The failure mechanism does not appear to be given due prominence in either EEMUA 159 or API Std 653.  This is an issue that may benefit from wider publicity as the location of the potential failure is easily checked during internal tank inspection and the solution (e.g. an additional striker plate) relatively easy to install.

Inspection practices

83  This section gives an account of the main inspection practices seen during the project.  There is an abundance of guidance and other publications on this topic and only a brief description is given here.

84  The interventions revealed that thorough tank inspections were often very limited in scope before the 1990s.  In many instances they were visual only.  This situation is improving and numerous techniques were encountered with most Operators carrying out external visual checks as part of the routine maintenance function.  Nevertheless

examples were found where no proactive inspections of the internal condition of the tanks were being undertaken.

85  It is worth stating that thorough internal inspections of storage tanks can involve a very high investment of time and resource.  Opening up a tank for safe entry and to allow meaningful inspection may require months of preparatory work.  The costs can be very substantial and a tank may be out service for extended periods of time.

Visual

86  Visual inspection remains the principal technique and used both during external intermediate examinations and for thorough internal examination.  Checklists set out in the EEMUA and API codes provide comprehensive guidance on those areas where visual examinations should be made.

87  As well as the provision of good access equipment such as ladders, scaffolding, machines etc. it is to be emphasised that detailed visual internal inspections of tanks can only be carried out under adequate lighting conditions.  BS EN Standard 970:1997 ‘Visual Examination of Fusion Welds’  recommends lighting intensity level for inspection of no less than 350 lx while 500 lx is preferred.

88  Tank cleanliness is also essential, and, depending on the product stored, may involve many weeks of intensive cleaning effort. Other preparatory work can include supporting floating roofs and removing equipment such as heating coils and drains. 

89  Frequencies of external inspections varied widely.  In some cases a number of external visual inspections would be carried out before one internal thorough inspection was completed.  Generally, however, inspections alternated between an intermediate external examination and a thorough internal (and external) examination.

90  Visual inspections of shells on insulated tanks can be problematic.  Unless there was evidence of cladding or insulation damage, discolouration etc, little was removed. In some cases Operators required areas to be stripped though the procedures in general permitted a considerable degree of latitude. 

91  The most common approach was to remove sections of cladding and insulation adjacent to spiral staircases.  It is essential however that careful examination is made at other vulnerable areas such as below wind girders, other penetrations like pipe stabbings, handrails etc. and at the roof to shell joint.  One example was seen where the Operator had incorporated purpose-designed sealed vertical strips into the insulation at the cardinal points around each insulated tank.  Each of these contained a number of removable plugs that permitted direct access to the shell.  This allowed easy access for repeat thickness checks and also some limited opportunity to check for CUI.

92  The principal codes do not give detail on the amount of insulation removal, however excellent advice is given in Section 7 of API RP 575 ‘Inspection of Atmospheric and Low Pressure Storage Tanks’.

Radiography

93  This technique was not encountered during the interventions and appears to be restricted to inspections carried out during construction or following repairs and modifications.

Ultrasonics

94  Use of spot pulse echo ultrasonics to carry out thickness checks of shells, floors and roofs became more widespread from 1990 onwards. It is arguably the most practical way of measuring the thickness of the shell, roof, floor and nozzles of a tank. Generally tank floor inspections consisted of a pattern of 5 inspection points per floor plate.

95  Other methods such as long-range ultrasonics have proved popular for checking corrosion deterioration on annular rings.  At present this technique cannot be universally applied since there are limitations in regard to the minimum length of projection of the annular and its topside surface condition.

96  Crawler type scanning techniques can be used on shells but are not the norm.

Magnetic flux leakage (MFL) and saturated low frequency eddy current (SLOFEC)

97  The use of floor scanning techniques using magnetic flux leakage and saturated low frequency eddy currents appear to be gaining acceptance as the preferred method for detecting and recording tank floor corrosion, even to the extent that, where it is practical Operators now remove hot water, steam and electrically heated pipes temporarily in order to permit the technique to be used in heated tanks.  There are a number of specialist providers and confidence in the techniques is now high. 

98  There can be limitations in areas of coverage e.g. adjacent to the shell.  However it is a relatively simple task to cover small localised gaps by hand scanning.

99  HSE has just supported a research project on magnetic floor flux scanning, ‘Recommended practice for magnetic flux leakage inspection of atmospheric storage tanks floors’, Research Report RR 481, which has concluded that the technique has many advantages over spot ultrasonics.

100  An important consideration when carrying out MFL and SLOFEC inspection techniques on tank floors is the level of cleanliness of the floor.  This should be clear of magnetisable debris and fit for a detailed visual inspection as a minimum. 

101  Where grit blasting is required it is generally accepted that a standard equivalent to BS EN ISO 8501-1 Sa 2 or Sa 2.5 (or NACE and SA standards) is necessary to carry out meaningful inspection.  High pressure water jetting can be a practical alternative to grit blasting though generally it is not thought to be as effective in some circumstances. Depending on thickness of application the equipment can scan directly through some types of lined floors.

MPI and dye penetrant NDT

102  In the past these techniques have been employed selectively following repair and alterations rather than as a primary in-service inspection practice. They are useful however where floor plates may be subject to fatigue loading, especially where empty/full cycles are high and for crack detection of floating roof plate welds.

103  When used they tend to be applied more extensively around nozzles and at the floor to annular joint of flat-bottomed tanks.  However, the general application of MPI NDT and its capability of revealing weld defects during tank thorough examinations, is an issue which HSE would support.

104  Dye penetrant NDT is used where stress corrosion cracking of stainless steel tanks is a threat.  A particularly vulnerable location can be the area around steam heating coil nozzles.

Acoustic emission

105  This technique is used, but not extensively, and appears more commonly employed as a secondary, supporting technique and a screening tool. AE used at very high sensitivity can evaluate the floor condition by listening for active corrosion and leakage without taking tanks out of service. AE has also been used on cryogenic and refrigerated storage where non-invasive inspection is the norm.  In certain circumstances the equipment can be installed on line to provide continuous data.

Vacuum box

106  The technique, which is very effective, is used more often following a floor repair or where there is a suspected leak rather than as a normal inspection tool.

Other techniques

107  Under floor pressurisation using low pressure air and soap solution or helium and gas sniffers have been used to detect small corrosion holes and weld defects where tanks have leaked.

108  Less common methods such as Pulsed Eddy Current (PEC) to determine wall thickness without the removal of insulation and where direct access to tank floors and shells is difficult, have been employed with some success.

Inspection periodicity

109  The API and EEMUA codes offer procedures for calculating next inspection intervals and remaining life from trendable degradation rates.  Minimum thicknesses for floors, shell plates and roofs are given in both.  The interventions revealed however that there is only limited use of these procedures.  This is perhaps not too surprising given that the period between inspections is so wide and the inherent difficulty in obtaining reliable and accurate data (particularly for flat-bottomed tank floors) that can be compared with previous results to derive a trend.  An additional challenge arises from the unpredictability of uniform corrosion loss.

110  Whilst the periodicity of tank inspection was drawn from the guidance given in the codes it was often the case that the Operator’s set maximum endorsement intervals for examinations less than those given in the codes.  In other instances more frequent access into tanks for cleaning and product quality purposes (e.g. aerofuels) gave opportunities for additional thorough examinations to be carried out.

111  One Operator, whose inspection strategy involved external examinations every two years with a thorough internal at the 12th year, introduced a procedure making use of enhanced visual inspection supported by additional NDT at the mid-point between their intermediate and thorough inspections.

112  Section 6 of API Std 653 suggests frequencies based on service history and from corrosion rates determined by previous inspections.  Where not known it recommends external inspections every 5 years and internals at 10 years. Inspection intervals should never exceed 20 years.

113  Table B.3-1 in Section B3 of EEMUA 159 sets out frequencies of tank inspection which include external routine visuals, detailed external inspection and internal inspection.  The frequencies can vary dependant on service conditions and products stored as well as climate conditions. By way of example the maximum interval for internal inspections in temperate climates is given as 16 years.

114  Both the codes now recognise the value in adopting RBI principles to improve the availability of equipment while ensuring its integrity is not compromised, though  they also highlight some limitations with respect to non-uniform degradation mechanisms and sparse data.

Inspection reports

115  The quality of tank inspection reports varied widely.  In a small number of cases these were very comprehensive and included photographs, sketches or more detailed drawings along with checklists.  The best incorporated floor, shell and roof sketches for vertical tanks and elevation schematics for horizontals.  Many included positions of man ways, main nozzles and small-bore branches, heating coils, drains, sumps etc. Sketches were often used to identify thickness measurement locations and the results.

116  At other sites the quality of the inspection reports and the standard of information recorded was poor. This is unsatisfactory. Thorough internal examinations of tanks are carried out infrequently, and it is their quality which offers the principal means by which the integrity of the tanks can be determined, often many years on.

117  Many examples were found in reports where unhelpful comments such as ‘satisfactory where seen’; ‘shell not visible – insulated tank’; ‘parts inaccessible’ etc. had been made by inspectors without challenge by the Operators.  Inspection procedures found at some sites acknowledged difficulties for examinations, for example with insulated tanks or around masked or problem access areas etc. but offered no suggestions of alternatives or of how these could be satisfactorily resolved.

118  Use of the inspection reference survey point or grid system set out in EEMUA 159 was not common though it was employed at a small number of sites.  The principle was

used more widely for tank floors.  There appeared great reluctance to adopt the guidance given in the EEMUA or API codes to domed bottom or horizontal tanks even when much of the detail in these was of obvious value.  The use of the checklists set out in both of the codes was variable though there is some evidence that recognition of their value is increasing.

119  Frequent examples were encountered where deterioration and defects that should have been found by inspectors had been missed. This raised serious questions about the thoroughness of the process and the competence of those carrying out the examinations.

120  In other instances defects had been correctly identified and recorded during inspections but no action taken by the Operators.  In one case this led to the Operator modifying their procedures by introducing corrective actions punchlists which then had to be signed off by a senior engineer.

121  The interventions revealed a general requirement for improved inspection reporting particularly as the period between thorough tank examinations is wide.

122  Increasing use is being made of floor scanning techniques.  The quality of the reports from these surveys is generally high, providing good detail. 

GRP/HDPE tanks

123  It is a commonly held misconception that non-metallic storage tanks are suitable for indefinite service and that they require little or no maintenance and inspection.

124  Four visits were included in the project to follow-up serious leaks, failures or issues associated with GRP and thermoplastic storage tanks.  In three of the incidents the tanks had exceeded their original design life (calculated on the basis of creep deformation) and had split in or near to the weld seams. Some evidence of fatigue damage was also found in the shell of one of the tanks that leaked. HSE was also informed of a serious fire in a Polypropylene tank caused when an electric immersion heater failed to trip on reaching a high temperature limit.

125  Of particular concern are aspects such as material compatibility and continued fitness-for-service for tanks approaching, or even exceeding, their original design life. 

126  There are a number of codes, standards and guidance documents in widespread use for the design and manufacture of non-metallic storage tanks.  These are referenced at the end of the report.  Two of these relating to thermoplastic tanks are worthy of mention here:  DVS 2205 Parts 1-5, ‘Design calculations for containers and apparatus made from thermoplastics’, from Germany and BS EN 12573 Parts 1-4 ‘Welded static non-pressurised thermoplastic storage tanks’.

127  In a report produced for HSE - HSL Report HSL/2006/21, ‘Specification and Inspection of Thermoplastic Storage Tanks’ [PDF 1.87MB] [2][3], by Mr J Stonehill of Stonehill Materials Services it was suggested that DVS 2205 would be withdrawn in favour of the new European Standard BS EN 12573 once it was published.  However, DVS 2205 has not been withdrawn (as it is not considered to the German National Standard and so does not need to be withdrawn under CEN Rules) and, in fact, has been

continually updated since the publication of BS EN 12573.  As a consequence manufacturers of thermoplastic tanks are currently able to choose which standard to follow, though they should be clear of their legal duties to reduce risks as low as is reasonably practicable whichever document is used.

128  One site had experienced a catastrophic failure of a GRP tank.  The probable cause was identified as environmental stress cracking.  As a result of this incident (and another similar failure recently) and a number of failures of thermoplastic storage tanks work is in hand to revise HSE Guidance Note PM 75, ‘Glass reinforced plastic vessels and tanks: advice to users.’

129 As a result of concerns about the design, material selection, manufacture, installation, use and inspection of HDPE tanks, HSE has formed a working group to try and resolve a number of issues, update supporting technical references and, where necessary, publish new guidance.

Horizontal tanks

130  The arrangements for managing the integrity of horizontal tanks on most sites visited during the course of the project were poor.  Sites, which had comprehensive tank integrity management programmes in place for their vertical storage tanks, had nothing so thorough for horizontal tanks.  This was not uncommon.

131  Many riveted construction horizontal tanks of age were still seen in use. Other examples were observed where tanks clearly designed for vertical use had been modified and installed horizontally. Little or no information was available concerning the thickness of the shell wall directly under the saddle supports.  On many bona-fide horizontal tanks these areas are reinforced with doubler plates.

132  A maintenance and inspection regime at one Operator’s site treated horizontal storage tanks in a similar manner to that in place for their pressure vessels even though these were not in pressurised service.

133  BS 2594: 1975, ‘Carbon steel welded horizontal cylindrical storage tanks’ is a recognised manufacturing standard for atmospheric horizontal storage tanks[3], however it does not deal with issues of in-service inspection and maintenance matters and it has been noted that there is no other specific guidance material for this type of tank.

134  Horizontal tanks can be vulnerable to corrosion above the saddles (which may or may not be integral to the tank). In certain applications, where thermal expansion is expected, the tank may be supported on steel saddles which are not physically attached to the shell.  In other instances the saddles may be fully seam welded to the shell while in other cases the welding is not continuous.  Concrete type saddles often have bitumen or other packing material between them and the tank shell.  These can often be a trapping point for water. The interventions revealed many instances of serious corrosion around the saddle areas.[4]

135  Localised corrosion was commonly found around small-bore piping stubs and at gantry, handrail attachment points plus other vulnerable areas such as bottom drains.

136 At one site a tank, originally designed as a vertical tank, was being used in the horizontal position to store waste hazardous liquids without any technical justification.

137  The quality of inspection reports for horizontal tanks was, if anything, worse than that found for verticals. In only a small number of cases were drawings and sketches employed to detail inspection locations and record results.

Piping, bellows and flexible joints

138  Questions involving piping, bellows and other flexible joints were not central to this study.  Nevertheless because of their close association with tanks some of the issues are worthy of comment.

Piping

139  It was not uncommon to find poor piping standards in tank farm areas.  There seems little doubt that piping in these areas is not given the same level of attention as process piping.

140  Examples of badly aligned, supported and corroded piping were widespread.  Other features such as buried piping and ‘masked’ areas through bund penetrations were also of concern due to the difficulty of carrying out meaningful inspections at these locations.  Failures are not uncommon due to wall thinning corrosion of piping caused by trapped water in the bund wall.

141  It was common to find piping lying unsupported just above and on the ground.  Even where there was poor bund drainage piping was seen partially or completely submerged under water.

Bellows and flexible joints

142  At one site piping angular displacement and expansion directly away from the tanks was managed by employing flexible type couplings.  Following a failure it was established that the coupling pressure rating was less than the maximum delivery head of the associated pumping system.

143  Bellows and other purpose designed flexible joints are often an integral part of tank installations.  They are installed where settlement or other movement between the tank and the downstream piping is greater than can be accommodated in the piping design or where expansion and contraction of the shell is expected.  They are often fitted at the tank side of the first isolation.

144  Inspections of these joints may be difficult owing to their construction.  Often bellows convolutions are protected by steel covers.  Operators should ensure that the integrity of these vulnerable components is not missed during the tank examinations.

145  Ideally they should be subject to close visual inspection and measurement (for corrosion, alignment, squirm, seizure of guides, hinges, links etc.) at intermediate inspections, and, where possible, removal with thorough NDT plus pressure testing

during tank outages.  Detailed inspection should also be carried out following any system excursion or abnormal external event.

146  In some instances Operator’s had established that tanks were no longer subject to settlement problems and examples were found where technical reviews were being carried out to determine whether flexible connections could be removed by utilising piping expansion loops and spring type supports. 

Venting and relief

147  Proper venting and relief is critical to protect against possible catastrophic failure of atmospheric storage tanks.  The interventions revealed however that this is not a well-understood concept, and numerous examples were found of poor maintenance and inspection standards of venting equipment.

148  It is important to understand the differences between normal pressure/vacuum relief and emergency pressure relief.  Normal venting rates, which cater for liquid movement pumping-in and pumping-out, plus thermal outbreathing and inbreathing, can be calculated using appropriate formula.  Emergency venting in the case of external fire may require that the tank has a frangible type roof or additional equipment such as hinged lids fitted (sometimes referred to as Whessoe Lids).  Advice on emergency venting and frangible roofs is given in Appendix F of BS 2654; API 650 and EEMUA 180:96 ‘Guide for Designers and Users on Frangible Roof Joints for Fixed Roof Storage Tanks’.

149  For normal venting tanks can be fitted with a variety of pressure and vacuum relief devices.  These may range from simple open vents (swan neck or Chinese hats) to more complex combined pressure-vacuum valves.

150  Roof vents and pressure–vacuum valves should be inspected at regular intervals to ensure they function properly.  It is not unknown for vents to be blocked by bird’s nests, vermin, debris and ice, or even purposely during maintenance.  Open vents should be installed with wire netting with openings of at least 6mm to prevent birds nesting.  Note:  It is not normal to fit flame arrestors to tanks equipped with PV-valves.

151  Pressure–vacuum valves are vulnerable to blockage or mal-operation caused by contact with the vapour of waxing, sublimating, crystallising or polymerising materials.  Phenol, aniline and styrene are typical examples. In some of these cases PV-valves can be trace heated and insulated and sometimes low temperature alarms are fitted.

152  Good schemes of examination adopted frequencies of examination based on the likelihood of vent or PV valve failure, and the scope included removal of the valves for checks, cleaning and resetting of pallets.  Other considerations for inspection may be required if tanks are equipped with nitrogen blanketing systems.

Second hand tanks

153  The inspections revealed that the use of second-hand tanks is common.  In many cases the design and codes used for the tank construction or the history of use were not known.  This was not limited to smaller sites and in many cases re-evaluations had been

undertaken on tanks where there was no history or where the history was lost when the site ownership changed.

154  In some cases Operators have been requested to carry out design gap analyses where original design information was not available, and particularly where there was little or no baseline data and tank history.

155  At one site, most of the tanks had been purchased from suppliers of second hand tanks and pressure vessels without any supporting documentation.  They had then been put into service without any detailed technical fitness-for-service assessments.  This is not good practice.

Coatings and linings

156  Comprehensive guidance on tank coatings and protection is given in API RP 652, ‘Lining of Aboveground Petroleum Storage Tank Bottoms’, and EEMUA Publication No. 183: 1999, ‘Guide for the prevention of bottom leakage from vertical, cylindrical, steel storage tanks’. 

157  Epoxy coatings are frequently used to protect tank floors and the lower shell strakes but care is needed to ensure that the tank surfaces are properly prepared before the coating is applied and that the correct thickness and curing procedures are followed as breaks in the coating can lead to accelerated corrosion.  Other coating products are available that offer improved resistance to chemical attack and the manufacturers advice should be sought for particular applications.

158  Cathodic protection systems for tanks were not in widespread use on the sites visited during the course of the project.  It should be noted that cathodic protection systems should not be used where the tank is founded on bitumen/sand or similar, as a hydrocarbon layer will provide a high resistance barrier to prevent the current passing through.

159  The subject of testing and examination requirements for coating repairs does not appear to be addressed by API Std 653 or EEMUA 159 but some information is contained in API RP 575 and also API RP 652 ‘Lining of Aboveground Petroleum Storage Tank Bottoms’

Hydrotesting

160  There are a number of reasons why British and API Standards require that storage tanks are filled to the top of the shells with water for testing [5]. 

For example:

to demonstrate that the tank is strong enough to withstand the liquid pressure during service

to demonstrate that there are no leaks to demonstrate that the tank foundation can carry the load of the tank and its

contents.  Note: settlement checks should be undertaken during the test.

to allow plastic yielding in welds where there are localised high stress peaks (shakedown)

161  Almost all hydrotesting is therefore carried immediately after construction of the tank.  The codes do however specify that hydrotesting is also carried out after significant repairs or modifications have been made.

162  Re-hydrotesting is normally required after:

partial or complete replacement of bottom plates partial or complete replacement of annular plates repairs to the shell to bottom fillet welds installation of new shell plates or insert plates installation of nozzles or manways

163  The principal codes also highlight important factors that should be considered before carrying out hydrotests such as water quality and temperature, filling rates, hold times, filling and emptying procedures.

164   API Std 653 sets out a number of conditions which allow a tank back into service after modifications or alterations without hydrotesting.  Section 16 of EEMUA 159 also gives advice on this matter.

165  It should be noted that the requirements for hydrostatic testing following repairs are more stringent in EEMUA 159 (as amended in February 2004) than API Std 653, particularly following shell jacking operations.  It is also worth noting that API Std 653 (clause 12.3.2.7) permits fitness-for-service assessments to be adopted to exempt a repair from hydrostatic testing but is silent on the procedures and acceptance criteria that should be followed.  This is an area that may merit further discussion.

Codes and guidance

Design codes

Steel vertical tanks

BS 2654: 1989

Specification for Manufacture of vertical steel welded non-refrigerated storage tanks with butt-welded shells for the petroleum industry, (now superseded by BS EN 14015)

API  Std  620

Design and construction of atmospheric and low pressure storage tanks

API  Std  650

Welded steel storage tanks for oil storage

BS EN 14015: 2004

Specification for the design and manufacture of site built, vertical, cylindrical, flat-bottomed, above ground, welded, steel tanks for the storage of liquids at ambient temperature and above.

Steel horizontal tanks

BS 2594: 1975

Carbon steel welded horizontal cylindrical storage tanks, (now superseded by BS EN 12285 Parts 1 and 2)

BS 799-5: 1987

Oil burning equipment –specification of oil storage tanks

UL 142 Steel Aboveground Tanks for Flammable and Combustible Liquids

BS EN 12285-1:2003

Workshop fabricated steel tanks. Horizontal cylindrical single skin and double skin tanks for the underground storage of flammable and non-flammable water polluting liquids

BS EN 12285-2:2005

Workshop fabricated steel tanks. Horizontal cylindrical single skin and double skin tanks for the aboveground storage of flammable and non-flammable water polluting liquids SS  

Vertical GRP tanks

BS 4994: 1987

Specification for design and construction of vessels and tanks in reinforced plastics

API RP 12R1 Fibre reinforced plastic tanks

Vertical thermoplastic tanks

DVS 2205 Parts 1-5 Design calculations for containers and apparatus made from thermoplastics

ASTM D1998-97 Standard specification for polyethylene upright storage tanks

BS EN 12573 Parts 1-4

Welded static non-pressurized thermoplastic tanks

Supporting guidance (Inspection and continued fitness for service)

EEMUA Publication Number 159: 2003 (amended 2004 and 2005), ‘Users’ Guide to the Inspection, Maintenance and Repair of Aboveground Vertical Cylindrical Steel Storage Tanks’

API Std 653:  Tank Inspection, Repair, Alteration, and Reconstruction

API RP 575:  Inspection of Atmospheric and Low Pressure Storage Tanks

PM 75:  Glass reinforced plastic vessels and tanks: advice to users

Supporting guidance (General)

 A number of sources of guidance and further information were consulted during the course of the project and the following were found to be particularly helpful:

Chemical storage tank systems – good practice: ‘Guidance on design, manufacture, installation, operation, inspection and maintenance’ (C598), published by CIRIA in 2003 (ISBN 0 86017 598 7).

EEMUA Publication Number 183: 1999, ‘Guide for the prevention of bottom leakage from vertical, cylindrical, steel storage tanks

HSE Research Report RR 481, ‘Recommended practice for magnetic flux leakage inspection of atmospheric storage tanks floors’

EEMUA 180:96 ‘Guide for Designers and Users on Frangible Roof Joints for Fixed Roof Storage Tanks’

‘Guide to Storage Tanks and Equipment’, Bob Long and Bob Garner, Professional Engineering Publishing, 2004, ISBN 1860584314 [6]

API RP 651:  Cathodic Protection of Aboveground Storage Tanks

API RP 652:  Lining of Aboveground Petroleum Storage Tank Bottoms

HSL Report HSL/2006/21:  Specification and Inspection of Thermoplastic Storage Tanks

HSG 41:  Petrol Filling Stations: Construction and Operation

HSG 176:  Storage of flammable liquids in tanks

HSG 235:  Guidance on the storage of Hydrochloric and Nitric Acid in tanks

National Sulphuric Acid Association (NSAA) ‘Recommended guidelines for the bulk storage of sulphuric acid, oleum and liquid sulphur trioxide’

Footnotes - Link URLs in this page

1. API Std 653 or EEMUA 159 standardshttp://www.eemua.org/t_stanks_l1.htm

2. HSL Report HSL/2006/21, ‘Specification and Inspection of Thermoplastic Storage Tanks’ [PDF 1.87MB]http://www.hse.gov.uk/research/hsl_pdf/2006/hsl0621.pdf

3. Footnotes - Link URLs in this pageAPI Std 653 or EEMUA 159 standardshttp://www.eemua.org/t_stanks_l1.htmHSL Report HSL/2006/21, ‘Specification and Inspection of Thermoplastic Storage Tanks’ [PDF 1.87MB]http://www.hse.gov.uk/research/hsl_pdf/2006/hsl0621.pdf

[3]Footnotes1. Stainless steel tanks fall outside the scope of this standard.  Appendix S of API

620 or Underwriters Laboratory Inc. Standard for safety, UL 142, ‘Steel aboveground tanks for Flammable and Combustible Liquids’ may have been more appropriate. [back to 1]

2. Although it is common to refer to some tanks as flat-bottomed, the floor may actually be designed cone-up or cone-down. Cone-up floors are the most common and allow settled water or bottoms product to gravitate to sumps around the periphery of the tanks.  Cone-down floors normally have a sump at the centre of the tank.

For tanks in crude oil or other liquid hydrocarbon service any entrained water in the product or entering through seals or natural breathing, will naturally collect as a layer in the bottom. This is often referred to as a ‘water bottom’.  Along with any sediment it can often contain aggressive compounds and in some instances monitoring the pH of drained water may be required as a corrosion control.  It is important that Operators adopt good drainage procedures where water can accumulate in the bottom of tanks. [back to 2]

3. Now superseded by BS EN 12285 Parts 1 and 2. [back to 3 ] 4. At the present time it is understood that NDT techniques are not available which

can detect localised shell corrosion above and around the saddle areas from outside of the tank.  In most cases assessment of deterioration in these areas will require internal access. [back to 4 ]

5. Note that unlike BS 2654 tanks designed and manufactured to API Std 650 are designed for a stated product specific gravity.  If the tank operator wishes to store products of higher specific gravity in the tank at some later stage then a lower maximum filling height should be determined to prevent the tank being overstressed (see page 51 in ’Guide to Storage Tanks and Equipment’, by Bob Long and Bob Garner). [back to 5 ]

6. A comprehensive description of the development of internationally recognised design and construction codes for both steel and concrete storage tanks is given in Section 2.9 of this reference book. [back to 6 ]