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Improving Reliability and Boosting Energy Efficiency of an Ammonia

Plant by Revamping Natural Gas Pipe Line and Associated Compressor

Train

Petrokemija's ammonia plant has been in operation since 1983 and designed to use low pressure natural gas of 7 barg (101.5 psig) at the inlet of natural gas compressor train. The new possibility to

extract higher pressure natural gas of 30 barg (435.1 psig) from natural gas pipeline network triggered revamping scenario to boost the energy efficiency and improve reliability. In total, the

overall natural gas consumption of ammonia plant was reduced for 1200 Sm3/h which gives 2.1 % better energy efficiency at nameplate production rate. The revamping process from concept to

commissioning is presented, including planning, procurement, installation, commissioning, safety procedures and practical difficulties which encountered during startup. Although safety comes first

as a part of each and every revamping process phase including developing a risk analysis matrix and HAZOP study, our experience shows that occurred leakage of natural gas during startup could

jeopardize the complete plant safety, and even further steps should be taken in order to eliminate all possible risks.

Nenad Zecevic and Dejan Mudric Petrokemija Plc. Kutina, Croatia

Introduction etrokemija Plc. operates Kellogg design ammonia plant on site in Kutina, Croatia. The ammonia plant was originally de-

signed with a nameplate capacity of 1360 MTPD, and commissioned in 1983/84. The pro-jected energy consumption was 34.3 GJ/t (8.2 Gcal/t, 32.5 MMBTU) with possibility of using

the natural gas, oil or combination of both as fuel for steam reforming section. Subsequently, Petrokemija Plc. made several modifications to the plant in order to reduce specific energy de-mand without increasing capacity. After several improvements and modifications in the plant, the facility never achieved the projected energy consumption of 34.3 GJ/t (8.2 Gcal/t, 32.5 MMBTU). Before described revamp scenario, the plant was successfully running at 100 % ca-

P

2372017 AMMONIA TECHNICAL MANUAL

pacity (yearly average about 1360 MTPD) in self-sustaining mode with the average energy consumption of 36.5 GJ/t (8.72 Gcal/t, 34.6 MMBTU), using natural gas as feedstock. A project of installing the new high pressure natu-ral gas pipeline was the crucial project in terms of plant energy savings and to achieve the planned target value of 32 GJ/t (7,64 Gcal/t, 30.3 MMBTU) through the step phase manner procedure. The revamping process from concept to commissioning is presented, including plan-ning, procurement, installation, commissioning, safety procedures as well as practical difficulties experienced during plant startup. Particular em-phasis is directed to safety part of the revamping project which includes development of a risk analysis matrix and HAZOP study. An isolated case during startup of the installed equipment shows that leakage of natural gas could jeopard-ize the overall plant safety. The newly installed high pressure natural gas pipeline represents the base for further improvement of plant energy ef-ficiency by the time when the pressure in the natural gas network will be on a value up to 75 barg (1087 psig). Revamping Process Description According to original design natural gas feed flows through a gas pipeline in length of ap-proximately 3 km with inner diameter of 18 inch and pressure of 14 barg (203 psig). At the inlet of the battery limit the natural gas feed is re-duced at the letdown station to a pressure of 7 barg (101.5 psig) and enters to knock out drum. The outlet of knock out drum feeds natural gas two case (LP/HP) compressor driven by a con-densing type steam turbine. Downstream of the natural gas compressor, flow is directed through convection and radiant coils of the feed gas heater fired by natural gas. Due to the low suc-tion pressure of natural gas compressor, the overall energy consumption needed for intro-ducing the natural gas at the designed value of 42 bara (609 psig) in primary reforming section is approx. 3300 kW or 16 t/hr of medium pres-sure steam (40 bara (565 psig) and 395 oC).

With development of the public distribution natural gas pipeline network imposed by the EU natural gas regulatory rules the new possibility was created for using higher pressure of natural gas at the battery limit of Petrokemija’s produc-tion site. The aforementioned condition generat-ed energy approval for construction of the new high pressure natural gas pipeline from the in-terconnection measuring station situated at the public network up to the inside of the battery limits of production sites. The issued energy ap-proval guarantees the minimal pressure of the natural gas at the value of 30 barg (435 psig) and volume flow of 100,000 Sm3/h. The natural gas minimal pressure of 30 barg (435 psig) ena-bles the savings of steam consumption at natural gas compressor steam turbine and in subsequent step possibility for electrical energy generation due to future higher pressure of the natural gas at 75 barg (1087 psig). The executed project is one of the steps in the overall project for energy enhancement at the existing ammonia plant. Thus far, the conducted project can be consid-ered as a standard, state of the art project exe-cuted solely from Petrokemija’s engineering, maintenance and process staff. The aim of the agreed revamping scheme was to meet the following objectives:

1. modification of the interconnection measuring and letdown station at the public distribution natural gas network,

2. construction of the new high pressure natural gas pipeline outside and inside of the battery limits and new letdown sta-tion,

3. modification of the natural gas compres-sor and steam turbine,

4. modification of the feed gas heater, 5. pre-commissioning and commissioning

procedure, 6. startup procedure, 7. HSE (Health, Safety, Environment) and

process safety management.

238 2017AMMONIA TECHNICAL MANUAL

The main objectives of the technological re-vamping project are shown through block dia-gram in Figure 1 with subsequent short descrip-tion section by section. Complete process description, electrical instrumentation, safety, relief, layout, fire mitigation system, fire net-work, DSC/PLC/EWS integration, hydraulic studies, piping integration, civil works etc., scope of the project was included in detailed en-gineering scope, in addition to new equipment’s detailed specifications, preliminary drawings is-suance, P&IDs and isometrics update.

Figure 1. Block diagram of the revamping process. Modification of the interconnection measuring and letdown station at the public distribution natural gas pipeline network According to the issued energy approval from the national operator the interconnection meas-uring and letdown station was completely re-constructed to ensure the possibility of using the nominal pressure of 50 barg (725 psig) and 100,000 Sm3/h volume quantities from the natu-ral gas pipeline network. The construction of the new knock out drums for natural gas with asso-ciated natural gas pipeline and appropriate re-ducing pressure valves in ANSI300 standard was carried out at all three measuring lines. Consequently, the existing measuring devices were replaced with a new ultrasonic devices cal-ibrated for the nominal pressure. The new SCADA system for monitoring of the volume flow of the natural gas enables the interconnec-tion with the Petrokemija DCS system at pro-duction facilities for tracking the natural gas consumption in real time.

Construction of the new high pressure natural gas pipeline outside and inside of the battery limits and letdown station for end consumers inside the battery limits The new high pressure natural gas pipeline is designed for 75 barg (1087 psig) nominal in or-der to meet future demands by the time when the mentioned pressure will be available from the public natural gas pipeline network. The overall length of the natural gas pipeline is 3770 m from which 2750 m is placed underground, while the remaining part of 1020 m is construct-ed over ground. The inner diameter of the natu-ral gas pipeline is 300 mm in API 5L X52 quali-ty (12 inch). For long lasting purposes the natural gas pipeline with associated equipment was properly cathode protected. The new let-down station inside of the battery limits was constructed with the following features:

1. regulation of the outlet natural gas tem-perature by the steam heat exchangers,

2. mechanical particles, moisture and liquid removal from the natural gas by the knock out drums,

3. reducing natural gas valves for the pow-er generation plant and other end users within fertilizer complex,

4. reducing natural gas valves for the fuel at the ammonia plant,

5. reducing natural gas valves for the pro-cess feed at the ammonia plant,

6. installation of ultrasonic measuring de-vices for all volume flows of natural gas, interconnected optically with outside measuring station at the public network,

7. tie-in points of the pipeline for the in-stallment of the natural gas expander and associated power generation device, by the time when the nominal pressure of the natural gas will be at 75 barg (1087 psig).

Regulatory requirements imposed to Petrokemi-ja’s technical personnel have been directed for


ensuring the special environmental study. The environmental study mandatory includes all necessary elements for protection of the living world surrounding high pressure natural gas pipeline. The location of the constructed high

pressure natural gas pipe line in relation to pub-lic measuring interconnection station and Petrokemija’s production facilities is shown in Figure 2.

Figure 2. The location of the constructed high pressure natural gas pipe line in relation to public measuring interconnection

station and Petrokemija’s production facilities. Modification of the natural gas compres-sor and the steam turbine The compressors as well as steam turbines were originally designed and manufactured by Ther-modyn, France which is now part of GE Group. The original design of the natural gas compres-sor train and the steam turbine is shown in Fig-ure 3. In cooperation with GE Oil & Gas the re-

vamping of natural gas compressor and steam turbine was performed. The new operating con-ditions correspond to a suction pressure higher than the existing one, 30 barg (435 psig) com-pared to 7 barg (101.5 psig).


TURBINE102-JT

COMPRESSORLP CASE

6stg.

MP Steam

To condenser

COMPRESSORHP CASE

6stg.

Discharge relief valve

To 103-B

CW

169-C

Suction relief valve

120-FNATURAL GAS

FEED

CW

144-C FV-102

Figure 3. The original design of the natural gas compressor train and the steam turbine. The new compression line has one compressor casing instead of previously installed two com-pressor casings. The existing LP (low pressure) compressor case was removed and the existing HP (high pressure) compressor casing, after modification was installed in lieu of the existing LP compressor casing. The existing HP com-pressor bundle is revamped with 3 impellers in-stead of previously installed 6 impellers. The existing shaft ends (bearings, oil seal) were re-used. The power requirement is significantly

lower and amounts 800 kW with subsequent consumption of the MP steam of approx. 2.5 to 4.0 t/hr, comparing to 16.0 t/hr in original de-sign. The existing nozzle block of the steam tur-bine was replaced by a new one in order to meet the steam turbine new operating conditions and to improve efficiency. The new set up of the natural gas compressor train and the steam tur-bine is shown in Figure 4.

TURBINE102-JT

COMPRESSORHP CASE

3stg.

Suction relief valve

MP Steam

To condenser

Discharge relief valve

To 103-B

CW

144-C

120-F

FV-102

NATURAL GASFEED

Figure 4. The new set up of the natural gas compressor train and the steam turbine. The existing anti-surge valve was suitable for the new process conditions, while the existing anti-surge control system, which was pneumat-ic, was replaced by anti-surge system monitored by PLC. The PLC delivers 4 to 20 mA signal to

the anti-surge valve. This PLC is sized in order to manage the future anti-surge systems of other ammonia plant centrifugal compressors as well. The existing hydraulic type speed regulation system is replaced by a speed regulation system


monitored by Woodward 505 control system. The existing servo-motor is driven by an elec-tro-hydraulic converter VOITH. The control system is installed in a stand-alone dedicated cabinet suitable to ZONE 1 explosion protection standard. The executed electronic speed regulation scheme is shown in Figure 5.

Figure 5. New electronic speed regulation system. The existing seal oil skid (HP oil supply) was changed because the design pressure of the ex-isting seal oil system was too low. Nevertheless, some equipment, such as the steam turbine of main HP oil pump and the electric motor of aux-iliary HP oil pump were reused. The performance of modified steam turbine 102-JT is shown in Table 1.

Table 1. 102-JT Steam turbine performance chart.

A steam consumption at maximal power B steam consumption at minimal power

Reconstruction of the feed gas heater The original design of the compressor train en-sured the outlet temperature of the natural gas in the temperature range from 120 oC to 140 oC. The mentioned temperature was the inlet tem-perature in the natural gas fired feed heater 103-B which increases the outlet temperature up to 400 oC for the purposes of hydrogenation and desulphurization section. The main energy input is secured through radiant section of the feed heater. The new design of the natural gas com-pressor brings outlet feed temperature at the lev-el of approximately 45 oC to 60 oC due to higher pressure of the natural gas at the inlet of the compressor. The open space in the upper part of the convection section of the feed heater 103-B, was used for the installation of the new finned tube heating coils. With additional finned tubes compensation has been achieved in terms of the natural gas consumption due to the lower inlet temperature of the natural gas. Pre-commissioning and commissioning procedure All pre-commissioning activities were carried out by developing detailed system packages, and pre-defined sequence of activities. Mechan-ical completion was achieved as per system packages and in order of priority. Petrokemija's engineers developed systems and procedures to ensure verification and logging of all pre-commissioning steps including piping cleanli-ness through air blowing, flushing, steam blow-ing and chemical cleaning wherever applicable. Activities include:

• checking for conformity with P&IDs, drawings and project specification (Me-chanical Completion (MC) checklists),

• cleaning and/or flushing of piping and equipment,

• pressure testing (including removal/re-instatement of instruments and control valves as applicable),

• drying of piping lines/systems,


• cleaning of static vessels, • cleaning, final alignment and preparation

for running in of rotating equipment, • preparation of lube oil and seal oil sys-

tems (lubricant filling), • powering up motor control centers and

electrical motor solo run, • instrument calibration and PSV recali-

bration, • DCS power-on and functional control

loop check.

The technical team reported the positive out-comes of these checks and tests on the appropri-ate test forms for the applicable Inspection Test Plan (ITP). As part of commissioning activities, the follow-ing actions were performed:

• functional tests on process and auxiliary units (utilities, oil units, packages, etc.),

• checks and tests on operational and emergency sequences (interlocks),

• permanent power utilization of electrical systems,

• drying and inertisation of piping lines/systems,

• final leak test, • introduction of process fluids into sys-

tems. Startup procedure Plant startup activities were initiated as planned on 01st of August 2016 after conducted general overhaul of the whole ammonia plant. However, several issues arose during plant start-up:

1. heat exchanger 144-C used for kick-back in case of open anti-surge valve of the natural gas compressor – serious leak-ages of the natural gas,

2. natural gas compressor casing leakage, 3. instability in operation of the Woodward

505 control system and associated elec-tro-hydraulic converter VOITH

In spite of the aforementioned issues the plant was subsequently normalized and ramped up gradually in a satisfactory time (72 hours) to achieve target production after revamp. HSE (Health, Safety, Environment) and process safety management During revamp project main objective and strat-egies were set in early stages to ensure safety, health, and protection of environment, with spe-cial focus on safety, product quality, reliability and flexibility. Several hazard reviews and pro-cess hazards analysis (PHA) were carried out during revamp project to ensure inherently safer design selection and assure safe operation of the facilities. Furthermore, all other elements of process safety management were employed throughout the revamp project which not only assured safety of employees but safeguarded company assets as well. All changes were stew-arded and approved through facility and tech-nology changes mechanism for revamp project. Elements of process safety management that provided higher importance and helped to achieve highest standards in quality and safety were:

1. PHA's and facility siting reviews, 2. development of new sets of standard

procedures for operation and mainte-nance teams,

3. training and validation of operators and technicians,

4. quality assurance programs for procure-ment and installation of equipment,

5. mechanical integrity inspection and test program,

6. emergency planning and response pro-gram as necessary,

7. contractor safety management program for construction safety,

8. pre-startup safety reviews, 9. process safety information system up-

date.


At later stages of project multiple HAZOP ses-sion were conducted until finalization of order-ing, commissioning and facility siting as well as supplier involvement which shows as an essen-tial part of the process hazard analysis element. Process hazard analysis was conducted with supplier and licensors in order to take into ac-count prior experience and concerns for devel-oping the final PHA actions. Pre-startup safety review also remained the mandatory element prior to startup of every piece of new or modified equipment and system. Lessons Learned 1. Natural gas leakage of kick-back heat

exchanger 144-C Kick-back heat exchanger was engineered and specified by Petrokemija’s technical personnel. The purpose of the same is to cool down the natural gas feed in case of anti-surging action of the valve that protects natural gas compressor from surge. It is a common design shell and tube heat exchanger with the natural gas on the shell and cooling water on the tube side. Manufactur-ing was performed by the domestic company specialized for construction of heat exchangers. Petrokemija’s technical personnel were present several times at supplier’s work shop in order to check all critical points during the manufactur-ing phase. The heat exchanger was delivered with all technical documentation according to national legislation standards (pressure test, welding test, measuring protocols, etc....). With the satisfactory pressure test protocol delivered and knowing the fact that Petrokemija’s tech-nical personnel witnessed the procedure at the supplier’s workshop, it was decided that the subsequent pressure test procedure with the nat-ural gas pipeline after installation on site was not necessary. This failure and deviation in the safety procedure was shown as critical one due to the near missed accident. Namely, after ap-plying the pressure of the natural gas close to working through the system, serious leakage of

the natural gas occurred at the shell side of the kick back heat exchanger, located inside of the compressor building. The plugs mounted on threaded positions used for venting, draining and carrying out pressure test procedure frac-tured and jumped out, leaving the serious leak-age of the natural gas with the pressure of 30 bar gauge. With extremely fast reaction of the pro-cess personnel the associated valves were closed and inlet of the natural gas was immediately blocked. Couple of minutes was needed for de-pressurization of the entire system. The building and surrounding area was evacuated without any consequences. Performed analysis showed that the installed plugs were plastic ones. Plastic plugs shown in figure 6 are used during the an-ticorrosive protection of the heat exchanger shell side– application procedure for the anticor-rosive dye.

Figure 6. Plastic plugs at heat exchanger shell side.

According to prescribed procedure, after corro-sive protection the plastic plugs must be re-placed with appropriate metal plugs and tight to satisfactory force. In spite of all conducted pre-cautions and safety prescriptions the supplier made serious and almost fatal mistake. Petrokemija’s technical and process personnel visually checked the heat exchanger but due to the same shape and color of the plastic and met-al plugs, there were no signs for the remediation action.


2. Natural gas compressor casing leak-age

The second leakage occurred at the modified natural gas compressor HP case barrel. The aforementioned leakage were minor one, which was evident at the bolts of the outside blind flange of the compressor casing – opposite side of the steam turbine coupling – figure 7. The leakage was determined with portable flamma-ble gases detection device and checked with foam test.

Figure 7. Compressor HP case barrel leakage.

Safety standards imposed mandatory trip of the machine to correct the mentioned leakage. After performed disassembly procedure it was noted that the casing gasket was improperly installed and caused the leakage. After replacement of the rubber gasket the tight seal condition was achieved. 3. Instability in operation of the Wood-

ward 505 control system and associ-ated VOITH I/H converter

The original hydraulic controller and all its ac-cessories were replaced by an electronic Wood-ward 505 control system with following inputs:

• speed sensors installed in the front bear-ing housing facing one tachometric wheel,

• the external 4 to 20 mA signal coming from existing DCS.

The new system is previously shown in Figure 5. The existing servo-motor is driven by VOITH I/H converter which is suitable to Zone 1. The existing DCS system sends the electronic signal to the Woodward 505 control system which ensures the 4 to 20 mA signal for VOITH electro-hydraulic converter. From the mentioned converter, hydraulic force executed by the con-trol oil system governs the regulation of the MP steam control valve and subsequently deter-mines the speed of the compressor train. During the startup of the compressor train, the oil linkage between electro-hydraulic converter VOITH and associated transducer created a massive disruption and vibration of the whole control system. The level of vibration was ex-tremely intensive which affected the stability of the speed controlling action and did not allowed the adequate ramp up of the machine. After several modifications at the connected oil pipelines and sizing change inside the transduc-er body the satisfactory results were achieved, but with further difficulties in controlling the speed of the machine. The general scheme of the problematic control detail of regulation system and included devices is shown in Figure 8.


Figure 8. The general scheme of steam turbine regulation system.

In spite of the conducted improvements inside of the transducer, the compressor train is cur-rently working with limited capability and will need certain corrections during the next over-haul period. Conclusion One of the main key features of the revamp scheme of ammonia plant is complete modifica-tion of the natural gas compressor from two stages to a single stage due to a higher natural gas pressure supply at battery limit and revamp of the steam turbine to improve energy efficien-cy. New conditions in the public distribution natural gas network enabled construction of the new high pressure natural gas pipeline as a first prerequisite in the overall ammonia plant mod-ernization project. The executed project consists of documentation preparation necessary for EPCC contract (feasibility study, basic engi-neering, detail engineering, etc.) which was per-formed as the state of the art project from the Petrokemija's technical personnel. During startup several issues were successfully man-aged taking into consideration all safety stand-ards. The project not only increased sustainabil-ity of ammonia plant, but also resulted in lower energy consumption and production flexibility

which provided better operational capabilities and opening wider horizon for execution of all other steps in further improving of energy effi-ciency at the ammonia plant. References 1. HAZOP Training Guide, Manufacturing

Training Committee, 2. Faisal I. Khan, S. A. Abbasi, Techniques and

methodologies for risk analysis in chemical process industries, Journal of Loss Preven-tion in the Process Industries 11 (1998) 261–277.


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