piping theory

5/26/2018 Piping Theory

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The Designer, Stress Problems and StressTrainingBy: James O. Pennock

Stress related technical and execution problems in the design of process plant pipingare complex and must be addressed properly. There will be some Piping Designers,

Stress Engineers and others who read this and say that they agree. Others may saythat they do not agree. Others will just not know one way or the other. Thisdiscussion, while not covering solutions to every potential problem, is intended only

to highlight some of the most common stress related factors and designer trainingneeds.

There are five basic factors that influence piping and therefore piping stress in theprocess plant. There is temperature, pressure, weight, force and vibration. These

factors will come in many forms and at different times. Stress problems become allthe more complex because two or more of these will exist at the same time in thesame piping system. The main objective of the focus when dealing with problems

related to piping systems is not normally the pipe itself. In a very high percentageof the time it is not the pipe that is the weakest link. Note this: the pipe is normally

stronger and/or less vulnerable to damage than what the pipe is connected

to. Pumps are just one examples of equipment to which pipes are routinelyconnected. Misalignment problems caused by expansion (or contraction) in a poorlydesigned system can result in major equipment failure. Equipment failures can lead

to the potential for fire, plant shutdown and loss of revenue. At this point it shouldbe emphasized that the success (or failure) of the plant's operation, years down theroad can and will depend on what is done up front by all the members of the design

team during the design stage. An important point to remember, "While analysiscannot create a good design, it can confirm a good design" (Improved Pump LoadEvaluation," Hydrocarbon Processing, April 1998, By: David W. Diehl, COADEEngineering Software, Inc Houston, TX). On the other hand, proper analysis willidentify bad design and potential problems in a piping system design.

Stress Related Design Factors

Temperatures in piping systems may range from well over 1000o F (537.8C) on the high side to below -200o F (-128.8 C) on the low side. Eachextreme on the temperature scale and everything in between brings its own

problems. There will also be times when both high and low temperaturescan occur in the same piping system. An example of this would be in pipingthat is installed in an arctic environment. The piping is installed outdoorswhere it is subjected to -100o F (-73.3 C) over the arctic winter. Six to ninemonths later it is finally commissioned started up and may operate at fiveor six hundred degrees.

The problems that temperature causes is expansion (or contraction) in thepiping system. Expansion or contraction in a piping system is anabsolute. No matter what the designer or the stress engineer does theycannot prevent the action caused by heat or cold. Expansion or contractionin a piping system it self is not so much a problem. As we all know if a bare

pipe was just lying on the ground in the middle of a dry barren desert it willabsorb a lot of heat from just solar radiation. In the hot sun piece of pipe


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can reached 150o F (65.5 C). The pipe will expand and with both ends looseit would not be a problem. However, when you connect the pipe to

something, even if only one end is connected you may begin to haveexpansion related problems. When the pipe is anchored or connected tosomething at both ends you absolutely will have expansion induced

problems. Expansion induced problems in a piping system is stress. Thereare a number of ways to handle expansion in piping systems. Flexiblerouting is the first and by far the cheapest and safest method for handling

expansion in piping systems. The other way is the use of higher cost andless reliable flexible elements such as expansion joints.

Stress will exist in every piping system. If not identified and the properaction taken, stress will cause failure to equipment or elements in thepiping system itself. Stress results in forces at equipment nozzles and atanchor pipe supports. Two piping configurations with the same pipe size,shape, dimensions, temperature and material but with different wall

schedules (sch. 40 vs. sch. 160) will not generate the same stress. Force in

piping systems is not independent of the other factors. Primarily, force (asrelated to piping systems) is the result of expansion (temperature) and/or

pressure acting on a piping configuration that is too stiff. This may causethe failure of a pipe support system or it may cause the damage or failure ofa piece of equipment. Force, and the expansion that causes it, is best

handled by a more flexible routing of the piping. Some people suggest thatforce can be reduced by the use of expansion joints. However we mustremember that for an expansion joint to work there must be an oppositeand equal force at both ends to make the element work. This tends tocompound the problem rather than lessen it.

Pressure in piping systems also range from the very high to the verylow. Piping systems with pressure as high as 35,000 psi in some plants arenot unusual. On the other hand piping systems with pressures approachingfull vacuum are also not unusual. The pressure (or lack of) in a pipingsystem effects the wall thickness of the pipe. When you increase the wallthickness of the pipe you do two things. First, you increase the weight ofthe pipe. Second, you increase the stiffness of the pipe thus the stressintensification affecting forces. Increasing the wall thickness of the pipe isthe primary method of compensating for increases in pressure. Other ways,depending on many factors include changing to a different material. Withlow or vacuum systems there are also other ways to prevent the collapse ofthe pipe wall. Among these the primary method is the addition of stiffening

rings. Stiffing rings may be added internally or externally depending on thecommodity type and the conditions.

Weight in a piping system is expressed normally as dead load. The weightof a piping system at any given point is made up of many elements. Theseinclude the weight of the pipe, the fittings, the valves, any attachments,and the insulation. There is also the test media (e. g. hydrotest water) orthe process commodity whichever has the greater specific gravity. Pipingsystems are heavy, period. Everybody involved in the project needs tounderstand this and be aware that this weight exists and it needs to besupported. Ninety-nine times out of a hundred this weight will besupported from a structural pipe support (primary pipe support system) ofsome kind. However there are times when the piping (weight) is supportedfrom a vessel or other type of equipment.


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Vibrations will also occur in piping systems and come in two types. There isthe basic mechanical vibration caused by the machines that the piping is

connected to. Then, there is acoustic (or harmonic) vibration caused by thecharacteristics of the system itself. Typically the only place severevibrations will be found is in piping connected to equipment such as

positive displacement reciprocating pumps or high pressure multi-stagereciprocating compressors and where there is very high velocity gas flows.

All of the issues listed above that a piping system is exposed to need to becovered in a company specific or company sponsored piping designer,stress-related training program. This piping designer, stress-relatedtraining should be done at the department level, early in the designer'scareer and prior to the start of the project. Unfortunately however this isnot always the case.

By definition, the role of the piping designer is to design the plant pipingsystems. This means design all of the system. Design all of the system

means that the piping designer shall define the proper routing of each andevery pipeline required for the project. This includes each and every inlinecomponent (pipe, valves, fittings, flanges, instruments, etc.), every onlinecomponent (anchors, guides, hangers, etc.). It includes the definition ofany attached piece of equipment and the definition of every support point.To do this and do it properly the designer must know about piping stressissues and know what to do about them. The designer is responsible for alot and so they need to know a lot. Is there any risk involved to thecompany or the project in not doing this stress related designertraining? Yes! First, a designer who is nave about the cause and effect ofstress related problems would not be able to recognize the symptoms and

will burn a lot of budget hours and create bad designs. Second, bad designsare subject to the 'domino effect' when the need for corrective action isfinally identified and taken then other lines get "pushed" and thenmodifications to them are required. Third, when the bad design does get tothe stress engineer for analysis there is the potential for repeated recycleand a serious delay in the design issue schedule.

Designer Stress Training

What does the piping designer need to know? Piping design is more than justknowing how to turn on the computer, how to find the piping menus and the

difference between paper space and model space. So, appropriately, what else does

the designer need to know about piping design besides how to connect a piece ofpipe to a fitting?

Here is a list of some of the most basic of things that a good piping designer shouldknow. Thinking about every one of these items should be as natural as breathing fora good piping designer.

Allowable pipe spans All designer need to know and understand the spancapabilities of pipe in the different schedules for a wide variety of common pipingmaterials. When a new project introduces a new material with severely reducedspan capabilities; supplemental training may be required. Expansion of pipe All designers must understand that they need to treat apiping system as though it is alive. It has a temperature and that temperature


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causes it to grow and move.That growth and movement must be allowed for andincorporated in the overall design. Not just of that specific line but for all other lines

close by.The process of expansion in a pipe or group of pipes will also exert frictionalforces or anchor forces on the pipe supports they come in contact with.

Routing for flexibility The piping designer must understand how to routepipe for flexibility.Routing for flexibility can normally be achieved in the most naturalrouting of the pipeline from its origin to its terminus.Routing for flexibility means (a)do not run a pipe in a straight line from origin to terminus and (b) building flexibilityinto the pipe routing is far cheaper and more reliable than expansion joints.

Weight and loads (live loads and dead loads) The piping designer needs tounderstand the effects of weight and loading. They need to know and understandthat everything has a weight. They need to be able recognize when there is going tobe a concentrated load. They need to have access to basic weight tables for all thestandard pipe schedules, pipe fittings, flanges, valves for steel pipe. They also needto have the weight tables for other materials or a table of correction factors for

these other materials vs. carbon steel. They need to be able to recognize whendownward expansion in a piping system is present and is adding live loads to asupport or equipment nozzle.

Equipment piping The piping designer needs to know the right and thewrong way to pipe up (connect pipe to) different kinds of equipment. This includespumps, compressors, exchangers, filters or any special equipment to be used on aspecific project.

Vessel piping The piping designer also needs to understand about theconnecting, supporting and guiding of piping attached to vessels (horizontal or

vertical) and tanks. They need to know that nozzle loading is important and doeshave limitations.

Rack piping The designer needs to understand that there is a logicalapproach to the placement of piping in (or on) a pipe rack. It does not matter howwide or how high the rack or what kind of plant, the logic still applies. Starting fromone or both outside edges the largest and hottest lines are sequenced in such amanner that allows for the nesting of any required expansion loops. The spacing ofthe lines must also allow for the bowing effect at the loops caused by the expansion.

Expansion loops The designer needs to understand and be able to usesimple rules and methods for sizing loops in rack piping. This should include the

most common sizes, schedules and materials.

Cold spring/Pre-spring Designers should understand the basics rules of coldspring and pre-spring. They need to understand what each one is along with whento and when not to use each.

Piping Designer or Piping Drafter

Any piping designer that has this type of training, this type of knowledge and thenconsistently applies is indeed a piping designer. He or she will also be a morevaluable asset to their company and to themselves in the market place. On the other

hand anyone who does not know or does not apply the knowledge about these


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issues while doing piping work is nothing more than a piping drafter or a CADoperator.

James O. Pennockis a former Piper with more than 45 years experience coveringprocess plant engineering, design, training, pipe fabrication and construction. He is

now retired and lives in Florida, USA.

Section - IV

B: The Problem with Piping "Lift-off"By: CAEPIPE (visithttp://sstusa.com)

Contemporary commercial piping analysis programs deal differently withthe problem of apparent lift-off of an operating pipe at a rod hanger or a

one-way vertical support, such as a pipe on a support rack. A few programsprovide error messages; others show a vertical movement with a possible

increase in sustained (weight) stress (see NOTE below for CAEPIPE). Aproper understanding of the standard piping design practice is the key to

correct interpretation of these results from different programs. Suchstandard piping design practice was generally understood when the

sustained and flexibility analysis rules were introduced in the 1955 Editionof the ASME B31 Code for Pressure Piping.

The problem with lift-off is compounded by the intention of the pipinganalysis being performed - whether the intent is to design new or revamp

existing piping or the intent is to analyze as-built. The intention of thevarious sections of ASME B31 Code (B31.1, B31.3, etc.) is to provide

guidance for new construction. Note, since the publication of the 1935Edition of ASME B31.1 (which included the predecessor of B31.3 as achapter, Paras. 101.6 and 121.4 and their predecessor paras.) state:

Piping shall be carried on adjustable hangers or properly leveled rigid hangers or

supports, and suitable springs...

Hangers used for the support of piping, NPS 2 (NPS 2 in 1935 edn.) and larger,

shall be designed to permit adjustment after erection while supporting the load.

While not quite as explicit, the current ASME B31.3 Para. 321.1.1 states:

The layout and design of piping and its supporting elements shall be directed toward

preventing... piping stresses in excess of those permitted by in this Code;...

unintentional disengagement of piping from its supports;... excessive piping sag in

piping requiring drainage slope;...

These paragraph excerpts define standard practice in piping design. That is, during

operation, it is neither the intention of the code nor standard practice to allow piping

to lift-off. Piping is normally designed to be supported in the operating condition.

The means to achieve this is through proper adjustment of the supports during

operation. This is important in piping because unadjusted supports will permit thepipe to sag and create locations in steam or condensable gas piping where
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condensates can collect or concentrate. And it is especially important for piping

operating above 800 degF, where unadjusted supports will allow the pipe to

permanently deform (creep) over time.

Small gaps are inevitable in actual construction because of fabrication and

installation tolerances and would normally be closed by support adjustments. But, so

long as the pipe is prevented from significant lateral movement, small gaps below

pipe during operation ( inch and less in moderate size piping) may be tolerable

because the weight analysis is a very simplified and conservative method that the

ASME B31 codes use to guard against collapse. Stresses caused by takeup of a small

gap below the pipe could even be considered expansion or building settlement type

stresses and thus would not need to be considered in the weight analysis. Weight

analysis with the intent of designing pipe normally considers all the weight supports

perform their intended function. Any significant gaps determined by analysis could

either indicate that a support is not required, or that adjacent supports need to be

modified, or that an alternate means of support is needed, e.g., a variable or

constant spring should be used.

However, if the purpose of an analysis is not to design a new or revamp an old

piping system, but to evaluate an as-built and maintained piping system, small gaps

may have more significance in as much as they would indicate that the pipe support

system may not be acting as designed and maintained. A lack of or improper

adjustment of the supports in the operating condition may cause lift-off at rigid

supports. Improperly designed or adjusted or maintained or degraded variable or

constant spring supports may cause lift-off, too.

The interpretation of the results of the analysis of as-built piping systems need not

necessarily conform to the rules of the ASME B31 codes. Remember, the rules in the

B31 codes are required for new construction, not the evaluation of existing piping. It

is understood that a greater factor of safety is required for the design process

because the pipe and its components are not yet available to be measured and

materials confirmed, as well as the knowledge of how the piping is to be actually

used. The interpretation of the analysis results of as-built piping may be able to take

advantage of what the actual piping dimensions and materials are and how the

piping has been operated. Competent engineering judgement based on knowledge of

the intent of the respective ASME B31 codes must then become part of theevaluation process.

For the reasons noted, it is important to distinguish between the design and analysis

of piping. If designing, certain assumptions are normally made with regard to

whether the piping is supported in the operating condition. Such assumptions might

include tolerating a small gap at a given support but realizing that the installation of

the given support will require adjustment. Alternately, a larger gap at the given

support may require support relocation to be effective or the selection of a different

type of support, most typically a constant or variable spring. If merely analyzing

existing piping, no assumptions need be made regarding supports acting andanalysis gaps may become important considerations. That said, however, the


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analyst must realize that the piping analysis model is a very idealized estimation of

the as-built piping and for the analysis results to be meaningful, the analyst needs

to consider how well the results correlate with the actual performance of the in-situ

piping.

NOTE: In case of lift-off, CAEPIPE will show a gap and possibly increased sustained

stresses. The user must interpret the gaps according to whether the user is

designing new or revamping existing piping or is analyzing an existing condition.

Author: Mr. Ron Haupt, P. E., of Pressure Piping Engineering(www.ppea.net)is a

member of several piping code committees (B31, B31.1, B31.3, BPTCS, and others).

He consults with us in the capacity of Nuclear QA Manager.

Section - III

A: Pipe Supports, Part - 1By: James O. Pennock

The subject, "Pipe Supports" is a much more complex subject than the termsuggests. There are so many situations that a pipe can find itself in and in everycase it will need to be supported. Pipe supports is a general term that actually issplit into two families. There is what I call the primary pipe support systems, andthen there are the secondary pipe support systems.The primary pipe supports systems are those supports that are a part of theinfrastructure and fall under the prime responsibility of the structural department.The secondary pipe support systems are more a part of the piping systems and as

such fall under the prime responsibility of the piping department. You notice I usedthe words 'prime responsibility' with each of these there is still a cross overresponsibility to provide proper, accurate and timely information and then action.

Primary Pipe Support Systems

As noted above the primary pipe supports are a part of the infrastructure. This istrue of most all projects. For simplicity the emphasis here will focus on "Grass Root"

or new construction plants. These primary pipe supports systems may also bereferred to as piperacks, pipeways, pipe alleys. These support systems may bemajor or minor and they may be overhead or sleeper pipe racks. It is important to

understand that even though they are called pipe racks they support and carry morethan just piping. These other items may include the cables for electrical andinstrumentation services.

For clarification, overhead pipe racks are elevated to the point where you can walk

and/or drive under the supported piping. Sleepers or sleeper ways are low to theground so there is no passage under the supported piping.

Pipe racks (overhead or sleeper) are normally established and sized early in thepreliminary engineering phase of a project. This time of the project is normallycalled the plant development phase or the plot plan development phase. Once they
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are established and sized they are one of the first things the structural departmentcan work on. The terms 'establish' and 'size' requires a lot of wisdom and work.

The wisdom and work means thinking one, two or three years into the future anddeciding where (location) the primary pipe support systems will run. Other critical

elements include the configuration, height, width, spacing and the materials ofconstruction/fabrication method. Let's take these elements one at a time.

Location - In order to set the location of the primary pipe support systems thetotal plant layout must be established. This means that all the various disciplinesmust have a very good idea what equipment is required and it's size. The "Plot Plan"must be reviewed by all the key people on the project and then approved by theclient.

Configuration - This is the selection of "fit-for-purpose." Each main run, minor runand branch run must be looked at to determine its configuration. Will it be an

overhead rack or a sleeper way? Will each be single deck (layer) or multiple deck?Will the support be a single column ("T") support or multi-column support? Howmany columns? A second part of the configuration issue effects pipe racks in theprocess units themselves. This is the question of whether or not the pipe rack willsupport equipment such as Air Coolers (Fin Fans). Another part of configuration isthe issue of intersections. Poor planning on this issue can cause problems later withthe piping.

Height - How high should each run of rack be? Should they be elevated or lowsleepers. The sleepers are concrete with an imbedded steel plate on the top. Forsleepers, they need to be off the ground to allow for maintenance and drainage also

to prevent corrosion. For elevated multi-level racks what should the separation be?For elevated racks you must plan the height and the separation of the whole systemtogether. A key element in the determination of separation is the line sizes to becarried on the racks.

Width - This requires a detailed study of the total piping systems for the wholeplant based on pipe rack routing. In the past, a study (called a "Transposition") wasdone to, as best you could, account for each line on each pipe rack. From this study,a berth sequence was established and the line spacing set. A percentage was addedas an error factor and then the clients "future" reserve was added. This thenconstituted the minimum rack width. The final width would be set after all rackswere "sized" and then some might be rounded up in width for consistence based on

the materials of construction/fabrication method.

Spacing - This issue can be addressed after the transposition has been completed.The transposition identifies all the rack piping from the largest to the smallest Fromthis the average line size for each leg of the rack system can be established. Withthe pipe size information (largest, smallest and average pipe size) the number andspacing of the pipe support bents can be set. A cost tradeoff is evaluated and madebetween more pipe supports spaced closer together or fewer pipe supports andsome sort of intermediate support system.

Materials of construction/fabrication method - What materials are the pipe racks

to be made of and what will be the fabrication method? Pipe racks can be bare steel,steel w/a concrete encasement (fireproofing), reinforced concrete or a combination.The steel can be steel structural shapes or pipe shape. The concrete fireproofing can


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be cast in place onto (or around) the steel columns and beams or it can be pre-castonto the columns and beams prior to installation. The reinforced concrete pipe

supports can also be cast in place or pre-cast then field erected. The spacerequirement dimensions for a reinforced concrete column or beam is about twicethat of bare steel.

The piping design group on the project (at the company where I came from) was thelead group in all of the above issues except the last one, materials ofconstruction/fabrication method. This issue was properly the responsibility of thestructural department, construction and the client. There is no doubt thateconomics, the jobsite location, labor and material availability played a part. Piping,however must know what the materials of construction/fabrication method will bebecause it can affect one or more of the other issues.

Secondary Pipe Support Systems

The secondary pipe support systems are a broad family of devices with two branchesand actually include more than just supports. The two branches are defined as (a)"engineered" devices and (b) "miscellaneous" pipe support devices.

The term "engineered" pipe supports relates to devices that are non-static, one-of-

a-kind, location and condition specific. They are identified at the time the need isrecognized and then designed and engineered for that specific need. Constantsupport spring hangers and snubbers are just two of the devices in this category.

The piping stress engineer is the party/person who is responsible for the engineeringof these. However, the piping designer working in the specific area has a sharedresponsibility.

The term "miscellaneous" pipe support refers to a broad array of devices thatincludes items such as Anchors, Base Supports, Cradles, Dummy Support Legs,Guides, Hanger Rods, Pick-ups, Shoes, Trunnions, etc. All companies have their ownoperating methods and may not use a different approach to miscellaneous pipesupport devices. Some may allow each piping designer to pick and choose piecesand parts from various catalogs to design their own pipe supports. Others may use amore organizational approach and "pre-engineer" these supports.

The term "pre-engineer" means that the various devices are an existing companystandard that may be used on the project. Secondary support devices typically havemultiple or repetitive point of use subject to similar conditions. Having these devices

"pre-engineered" and available to the piping designer on the project saves money,provides consistency of design, and results in a safer design. The configurations,hardware and materials have already been established, the load calculations havebeen performed (and are on file). There is also an "If-then" selection key and criteriaestablished (If you have "X" support problem, then you can/must use "Y" supportdevice). The extensive use of computers and plant design software makes thisapproach more viable. Having these support devices "pre-engineered" anddocumented allows for the inserting of the item's specific electronic symbol requiredfor model generation and document (plans, elevations and isometrics) extraction.

Secondary pipe support devices

(Item name, purpose and frequency of use)


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Name Purpose Frequency

AnchorsPrevent the movement of the pipe line

normally in a pipe rackHigh

Base AnchorsPrevent any movement of a piping assembly

normally at grade

Low

Base GuidesAllows only vertical movement (up or down)

of piping assemblies at gradeLow

BaseSupports

Provides support under piping assembliesnormally at grade

High

CradlesProvides protection for cold insulation when

crossings a pipe support in pipe racksHigh for cold

service

DirectionalAnchor

Restricts the movement of a pipe line to aspecific direction pipe racks

High

Dummy

Support Legs

Provides added length to a pipeline for the

purpose of support. Not restricted to only piperack usage High

FieldSupports

A catchall term sometimes used by a pipingdesigner that includes any type of non-

infrastructure support. These items are notlocation specific.

High

GuidesProvides restraint to keep a pipe line in placein horizontal pipe racks or vertical pipe racks

in buildings or up tall equipmentHigh

GussetsProvides added reinforcement for small

(fragile) branch connections on a largerheader or pipe

See note #1

Hanger RodsA wide verity of top-down pipe supports

situations, not location specific. High

Hold DownsPrevents or controls mechanical vibration in

piping systems. See note #2

Load

DistributionPads

Provides additional mass for thin wall pipe at a

point of concentrated stress loading.This item is not location specific.

Low

Pick-upsProvides support of pipes from other pipes or

overhead beams and is not location specific.Moderate

ShoesProvides "mini-supports for lines with hotinsulation normally only used only at pipe

support pointsHigh

Trunnions

Provides load-carrying points for verticalpipelines most often used to support pipes

attached to tall vertical vessels or hung fromtall structures.

Low

Note #1 - This item is normally used only for (a) services subject to heavy vibrationsuch as at reciprocating compressors or (b) services that contain highly hazardous

or toxic material.


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Note #2 - This item is normally only used for the suction and discharge piping atreciprocating compressors.

Now, lets look at and discuss each of these "miscellaneous" or "pre-engineered"devices. The description for these items is based on my own experience. Others will

no doubt have other and even better ways. Everyone is encouraged to create "abetter mouse trap."

Anchors

The anchoring of a pipe in place can be achieved in a number of ways. An anchor

will normally require some additional material regardless of the line size. You cannotjust weld a pipe to a pipe support. For some small lines in the right situations youcan use "U" bolts over the pipe (tack-welded to the pipe) and through-bolted to a

bare steel pipe support. Another way for small line sizes (2" and 3") uses 1-1/2"angle iron 6" long. Weld one leg of the angle iron (horizontal) flat to the top of thepipe support with the other (vertical) leg against the pipe. Stitch weld (1" fillet weld

on 5" centers) to the vertical leg to the pipe. For larger lines use a pipe guide torestrain the side-to-side movement and add a piece of steel ("T" or channel) to thebottom of the pipe (or shoe) at the pipe support to restrict longitudinal. Anchors willbe required for both bare (uninsulated) pipe and insulated pipe. The requirementsfor anchors for cold insulated and hot insulated pipe is different.

Base Anchors

This will occur most often at control valve manifolds (or stations) situated close tograde or a platform. Base anchors are simply a stub of pipe (dummy leg) attached

to the lower portion of an elbow and extended to grade (or platform). A square steelplate is welded flat to the pipe. The plate may have holes in it and be cinch-anchored to the paving or welded to platform steel. The sizing of the "pipe leg" canbe the same as for Dummy Legs.

Base Guides

This item is constructed of the material and methods as the base anchor except that

the bottom plate is not bolted or welded down. For this item angle iron strips areinstalled on two opposite sides (depending on desired movement) to control thedirection.

Base Supports

This is another name for one of the items that sometimes falls under the name FieldSupport. This item also has a dummy leg type pipe extension (or stub) welded downfrom an elbow. However, the bottom end if the stub is threaded using a straight(conduit) thread machine. A straight thread, conduit coupling in then used to makeheight adjustments to the support. When this is required for high cost pipingmaterials that require post weld heat treating the stub is shortened and added in theshop. The balance of the stub is added in the field from carbon steel. Anothervariation of this is restricted to small diameter piping. For this a 3'-0" (1 meter)length of 3"x3" steel angle is welded to a 6"x6" plate. Holes are drilled in the angle

at the proper elevation and a "U" bolt secures the pipe to the angle.


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Cradles

This device is normally fabricated from carbon steel that is shaped to fit the outsidediameter of cold insulation. The potential number of sizes for this item can be vast.The sizing requirements are based on (a) the pipe size, (b) the insulation thickness,

(c) the load bearing capability of the insulation, (d) the length of the required cradleand (e) the thickness of the cradle material. The pipe size, the insulation thicknessand the load bearing capability should be easy to understand. The length if thecradle is influenced by questions such as: Does this line require an anchor at thiscradle? What kind of pipe supports do we have at the point of this cradle? How muchthermal movement will this line "see" at the point of this cradle? All of these itemseffect the cradle length. If there is to be an anchor at this cradle and the forces aresubstantial then the cradle thickness may need to be increased.

Directional Anchor

This item could also be called a Directional Guide and is most often associated with

hot piping. This item is designed to allow for thermal movement in a specific axis.The design may require longitudinal movement or it may require side-to-sidemovement of a line. This item has two versions, one for longitudinal movement anda second for the side-to-side movement. Remember this most often occurs in hotpiping. Hot piping also requires shoes to elevate the line and the insulation abovethe pipe support. So we have a pipe, a hot pipe, already on a shoe. Now, to allow forlongitudinal movement we simply add (weld) Guides to the top (steel) surface of thepipe support. To allow for side-to-side movement in the pipe we DO NOT ADD

GUIDES. We add two pieces piece of steel ("T" or channel) to the bottom of the pipeshoe, one on each side of the pipe support with a small (1/4") gap to avoid binding.

Dummy Support Legs - (or Dummy Legs)

This is simply a piece of pipe extended from an elbow to provide support when apipe line enters or leaves a pipe rack short of a support and is left improperlysupport. A stub or length of pipe sized to carry the load is welded to the elbow andextended beyond the support. The length and the wall schedule of the pipeextension are a rather complex formula based on the parent line size and the totalload. The total load is based on the distance (indirection of flow) from the lastsupport to the drop, the distance of the drop, the distance from the drop to the nextsupport, the weight of any insulation plus the weight of the hydrotest water orcommodity which ever is greater.

Field Supports

This "catch-all" term is used to describe a simple piece of steel angle or channelwelded to a column or beam intended to provide a support point for a pipe. Asmentioned above (Base Support), this term is also used for the support undercontrol valve stations and pump suction or discharge piping.(The term "Field Support" (or F.S.) is seen on old drawings from existing plants ofyears ago. It was used on drawings with only a simple symbol indicating a location.This may have occurred when the piper got lazy or did not know enough about pipesupports. The intention was for the installation contractor "Field" to do what everthey chose to do with whatever material that was available.)

Guides


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Guides are predominantly in elevated pipe racks or sleepers to keep the pipes intheir assigned berth. Guides are most often short lengths of properly sized steel

angle welded to the pipe support on each side of each pipe. For small lines usingsmall angle the angle is installed with the point up, like a pyramid. For largeruninsulated lines with larger angle one leg of the angle is flat on the support and the

other is vertical. For the installations of guides care must be taken by thew installersto leave a small gap between the pipe and the angle to avoid binding. Because ofthe close spacing of the pipes in a rack guides are attached to alternate pipe bents

in staggered fashion.

Gussets

This is a simple piece of angle steel welded or clamped to a header pipe and to a(small) branch to prevent breakage due to vibration or other action. There are somelocations and services where the use of gussets is highly recommended.These are:1. Suction and discharge piping of reciprocating compressors and pumps

2. Lines in mixed phase flow subject to slug flow or surge3. Lines in hydrogen service4. Lines in toxic service (category "X" or "M")5. Branches in piping low to grade (or platforms) that may be used as a step byoperators

Hanger Rods

These devices are one of the most dangerous items used in the piping field. In manyif not most cases they are not properly "designed". Hanger Rods, Rod Hangers andPipe Hangers all terms for the same device. There are three basic types of Hanger

support devices: (type 1) beam-to-pipe, (type 2) pipe-to-pipe and (type 3) beam-to-beam (or trapeze). In general they all have three components, a top connectioncomponent, a connector component and a bottom component. For the type 1Hanger the top component normally connects to a structural beam. The connectorcomponent is normally steel rod. The bottom component is normally a pipe clamp.For the type 2 Hanger the top component is also a pipe clamp. Other componentsare the same as type 1. For the type 3 Hanger there are two top connectorcomponents and two connector rods. The bottom component is a piece of steel angleor channel sized to span the distance and carry the intended load.The danger with the design of these items is in the lack of knowledge of the peopledoing the design. They do not know how to calculate all the actual dead and liveloading that the Hanger will support. Then they choose the wrong type or strength

of component for the intended load.

Hold-Downs

These items are a combination of clevises, steel shapes, bolts and compressionwashers. The are used to hold down the piping on the suction and discharge ofreciprocating compressors and pumps. Normally this type of piping is low to theground and supported on sleepers. The hold-down is a bridge assembly over thepipe and welded to the sleeper steel plate. The combination of clevises, steel shapesbolts and compression washers exert tension on the pipe to suppress vibration.

Load Distribution Pads


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This is simply a 120 degree section of pipe about 18" long. The Pad is cut from thesame material as the subject line. The Pad is opened up a little to fit the pipe O. D.

and then welded to the pipe at the required location.

Pick-ups

This is a set of devices used to provide intermediate support for small diameterpiping that will not span the existing distance. Its use is normally restricted tolocations where the small size pipelines run parallel to one or more large diameterpipelines. This is also used to save the cost in time and material from adding aformal (primary) structural pipe support. This is simply a length of properly sized,steel angle and one or more "U" bolts. The angle is cut long enough to span underboth the supported and the supporting lines. The "U" bolts are sized based on thelarge pipes that will be doing the supporting.

Shoes

This device is required to raise a hot insulated off the structural support surface. Thereason for this is to prevent damage to the insulation as the pipe expands as it heats

up and shrinks as it cools down. For pipe sizes 3" thru 10" a simple inverted "T"shoe with a flat bottom plate and one (single) vertical plate should be used. For pipesizes 12" thru 18" a shoe with a flat bottom plate and two (double) vertical plates

should be used. For pipe sizes 20" and larger consideration should be given to theaddition of a Load Distribution Plate (see above) where thin wall pipe may exist. Thematerial for pipe shoes will normally be carbon steel. However, where the pipeline is

an exotic material this would cause a weld of dissimilar metals to exist where theshoe is attached to the pipe. For shoes used on exotic materials only the bottomplate is carbon steel. The (single or double) vertical plates are made of the same

material as the pipe. For piping that requires post weld heat treating (PWHT) afterfabrication the shoes must be added by the shop. Some company's (engineering andclient) will also require the use of shoes (with the Load Distribution Pad) for alluninsulated 24" and larger piping where the pipe wall is below a certain limit.

Trunnions

For this device a vertical pipeline will have two (2) stub pipes attached horizontallyto opposite sides of the pipe. One end of these stub pipes is shaped to fit the O.D. ofthe vertical pipe the other end is normally square cut. The shaped end of the stubsare welded to the vertical pipe with a full penetration (*) fillet weld. When used on apipe attached to and supported from a vertical vessel the vessel department

supplies the primary support. Coordination of size, type, elevation, orientation, etc.between the piping designer and the vessel group is required. When used on a pipeattached to and supported from a vertical structure the structural departmentsupplies the primary support. Coordination of size, type, elevation, location, etc.between the piping designer and the structural group is required.(*) This full penetration refers to the wall thickness of only the stub pipes not thevertical pipe.

The recommended practice for all of these secondary pipe support devices is todetermine what is needed. Start out with items that are found to have consistent

and repetitive use within the company's past projects. Document each devicecomplete with parts list and installation instructions. (Documenting also includes theupdates required for any electronic design system database, AutoCAD, PDS, PDMS


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or other) Qualify each device by the specific use criteria based on pipe size, loadlimitations and application. Define the selection criteria for each based on the

qualification criteria. Then train all the piping designers, stress engineers, materialgroup and construction contractors on the responsibility, purpose, use, applicationand limitations.

What about responsibility? Who is responsible for pipe supports or the supporting ofthe piping? Some may say, "That it is the structural groups responsibility." That isonly partly true. They are only responsible for providing a support of the size; shape

and strength based on information given to them. If nobody tellsthem to put a pipesupport (of a specific size, shape and loading) in a specific location they are notgoing to do it. So, who is responsible for doing the telling? The piping designer is

responsible for the piping, which means allthe piping and allaspects of allthepiping. The piping designer is responsible for telling the structural group what isrequired for all primary pipe support systems. And, the piping designer is also

responsible for telling the structural group when a secondary pipe support device willbe attached to and impose a load on a structural member.

There are of course other opinions on this subject and there are no doubt questionsand more that can be discussed. The other opinions I will warmly accept. And, as for

the questions, please ask. If you don't ask you will never give others a chance tooffer answers.

Pipe Supports, Part - B, Will discuss data requirements and the process for theselection and qualification of typical pipe supports.

James O. Pennock is a former Piper with more than 45 years experiencecovering process plant engineering, design, training, pipe fabrication and

construction. He is now retired and lives in Florida, USA.

Section - IIIA: Training - Pipe Supports, Part - 2

By: James O. Pennock

Pipe supports as we stated in Part 1 (of Pipe Supports) is a much more complexsubject than the term would first suggest. We also want to make it clear that thereare many ways that errors can be made when designing or selecting pipe supportsthis includes the various secondary pipe supports.

In Part - 1, we saw a chart that described some of the many different types ofsecondary pipe support devices. In this, Part - 2 of Pipe Supports we are going to

focus on specific data required to properly size, qualify and select a support.To dothis we will look at one specific device. The specific device we will focus on is theHanger Rod.

You will remember that in Part - 1 we said there are three basic types of Hanger Rodsupport devices: (type 1) beam-to-pipe, (type 2) pipe-to-pipe and (type 3) beam-to-beam (or trapeze). They all have three major components, a top connectioncomponent, middle or connector component and a bottom component. For the type

1 Hanger the top component normally connects to a structural beam. The connectorcomponent is normally steel rod. The bottom component is normally a pipe clamp.


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We also said that the danger with the design of these items is in the lack ofknowledge of some of the people doing the design. They do not know how to

calculate all the actual dead and live loading that the Hanger will support. Then theychoose the wrong type or strength of component for the intended load.

In order to bring attention to some of the potential problems lets take a hypotheticalpiping configuration and plant situation for study. We will look at two cases. We willuse the same configuration with different conditions for each case.

Case #1

Let's take the following as an example scenario for the basis for our discussion.

>> The project is a process plant in a multi-story structure

>> The line is 12", standard weight carbon steel pipe located in a lower level of thestructure

>> The line will carry a process liquid with a specific gravity of .85

>> The line is subject to hydrotest

>> The line is not insulated

>> The piping travels horizontal north in a well supported manner, then aftercrossing the last normal pipe support (support 'a') it travels 40 feet, then dropsdown (3'-0") and turns east (right) with two elbows (fitting-to-fitting) and travelsanother 40 feet to the next normal support (support 'b').

>> There are no additional horizontal support beams available at or near the turnpoint and at the exact piping elevation.

>> The closest steel available as a possible support point is 24" deep majorequipment support beam located 6'-0" (top-of-pipe to bottom-of-beam) above thepipe and 4'-0" from the pipe drop.

It is logical and factual that structural support 'a' will carry one half of the pipe loadof the north-south run. And the structural support 'b' will carry half pipe load of theeast-west run. However, the L-shaped "dog-leg" in this scenario is obviously

excessively overspanned and the pipe will be over stressed. The piping designermust provide some type of additional support at or near the corner. Because of theavailability of the overhead beam a hanger rod is chosen as the best possible andmost economical method of support for the pipe.

We must now look at the factors so we can choose the correct Hanger Rodassembly. The factors include all the weight to be supported.

The component weights are as follows:

>> 20'-0" of pipe in the north-south run (1/2 the 40' run)

>> 20'-0" of pipe in the east-west run (1/2 the 40' run)


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>> Two 90 degree elbows

>> 43 lineal feet of hydrotest water in the 12" Standard Weight pipe

With this information the next step is a simple look-up of the correct data.

Case #1-12" Standard Weight, Carbon Steel Pipe

Pipe Weight Fitting Weight Insulation Weight Water Weight Total Weight

1984 lbs. 246 lbs. 0 2107 lbs. 4337 lbs.

We now have what we need to select a hanger rod assembly to support our pipe.There are two ways that this can be done. First, the designer can use the "pick-and-choose" or "do-it-your-self ' method. This is the process of picking up a hanger partscatalog and then selects each individual piece and part. The hope is that thedesigner knows what they are actually doing.

The second method is that we select from a pre-packaged Hanger Rod assemblythat fits our need. One that comes complete with all the proper and matched piecesand parts. The term "pre-packaged hanger assembly" also means that the assemblyhas been "tag named," has been pre-designed, pre-engineered, pre-qualified andfully documented including the related needs for the applicable computer aideddesign system, material procurement and installation.

The assembly we need for our "Case #1 includes the following:(All components and load data are taken from "PTP" Piping Technology and Productsonline catalog, see pipingtech.com)

Load Capacity*

>> Figure 110, Eye Rod (Welded), Size 1" 4960 lbs.>> Figure 20, Welded Beam Attachment, Size #8 (for 1" Rod) 4900 lbs.>> Figure 40, Weldless Eye Nut, Size #2 for 1" threaded Rod 4960 lbs.>> Figure 80, Heavy Three-Bolt Pipe Clamp, for 12" pipe 7000 lbs.>> Beam attachment welds " fillet, 2 sides 12000lbs.

* It is normal practice for components of this type to be designed with a plus 50%safety factor. The safety factor is not to be considered as available when making a

selection.**The Beam Attachment is 3" on each side, " attachment fillet weld 1" long israted @ 2000 lbs. Per inch.

We now compare our pipe weights against the Hanger Rod load capacity data andsee that (not using any of the safety factor) the Hanger' weakest link is the Welded

Beam Attachment (4900 lbs.) but it is more than enough for our piping needs (4337lbs.).

If we were using the "pick-and-choose" method then the designer must indicate thehanger in the design then identify each and every piece and part. The detailed partidentification is required for proper procurement and installation.


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If we use the "pre-package" method the designer is only required to indicate thehanger and the item name or tag number (example: HR-1-12".) All the procurement

and installation details are included in the hanger documentation.

Now Case #2

Later someone else has a similar problem. They had seen what was done by anotherdesigner with the Case #1 problem and decided they would just copy it and calloutfor the same Hanger Rod Assembly. Why not? They too had a 12" line. They had thesame configuration. And, they also had the same span distances. No problem, right?However, all things were in fact not the same.So what was different?

Case #2

>> The project is also a process plant in a multi-story structure

>> The line is 12", Schedule 160 carbon steel pipe located in a lower level of thestructure

>> The line will carry a process liquid with a specific gravity of .85

>> The line is subject to hydrotest

>> The line is insulated with 3" of Calcium Silicate

>> The piping travels horizontal north in a well supported manner, then aftercrossing the last normal pipe support (support 'a') it travels 40 feet, then drops

down (3'-0") and turns east (right) with two elbows (fitting-to-fitting) and travelsanother 40 feet to the next normal support (support 'b').

>> There are no additional horizontal support beams available at or near the turnpoint and at the exact piping elevation.

>> The closest steel available as a possible support point is 24" deep major

equipment support beam located 6'-0" (top-of-pipe to bottom-of-beam) above thepipe and 4'-0" from the pipe drop.

With this information we look-up of the correct data.

Case #2, 12" Schedule 160, Carbon Steel Pipe

Pipe Weight Fitting Weight Insulation Weight Water Weight Total Weight

6412 lbs. 794 lbs. 528 lbs 1462 lbs. 9196 lbs.

We see here that the total load to be actually carried by the Case #2 hanger is morethan twice the safe capacity any of the components included in the original HangerRod. This will not work! This is an example of the type of errors that result when

there is a lack of thinking or laziness on the part of the piping designer.


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All of the items identified, as Secondary Pipe Support Systems are subject to thissame kind of miss-design and miss-use. It is incumbent on the piping designer to

become trained and knowledgeable about these issues.

Having identified the need for the hanger in the case study above and selected the

correct hanger is not the end of the piping designers responsibility. That hanger iscarrying a load and the top of that hanger is attached to a steel beam. The load isbeing transferred to that beam. That hanger and the pipe it is carrying is anabnormal load added to that beam. It is a load that the structural engineer wouldnot normally be aware of. It is the piping designer's responsibility to document thatloading and advise the proper member of the structural engineering group. Thatbeam may be a very large beam and is at or very near it's safe design limit. Youmight think "Oh it is okay, it can carry my pipe" However, you are not a structuralengineer and this is not your decision to make. Whenever an abnormal piping load isadded to a structural beam (steel or concrete) the structural group must be advised.

James O. Pennock is a former Piper with more than 45 years experience

covering process plant engineering, design, training, pipe fabrication andconstruction. He is now retired and lives in Florida, USA.

Section - III, Pipe Supports

C: SNUBBERS: A GENERAL OVERVIEW

By: Hyder Husain

Article courtesy of Piping Technology & Products, www.pipingtech.com

Introduction:PT&P produces various kinds of snubbers. Why snubbers are usedand how they function are briefly discussed here.

What are they?:Snubbers are restraining devices used to control the movement ofpipe and equipment during abnormal dynamic conditions such as earthquakes,traveling shock waves caused by turbine trips, safety/relief valve discharge, rapid

valve closure or accidental rupture of piping.

Where are they used?:Snubbers are extensively used in various applicationsincluding chemical plants, power plants (both conventional & nuclear), refineries,and structures such as suspension bridges and tall rise buildings in earthquake

prone areas.

How do they function?:The design of a snubber allows free thermal movement ofcomponents during normal operating conditions. Abnormal conditions activate thesnubber to become momentarily rigid (locked condition). While locked, the snubbertransmits the transient force to the ground or to a permanent structure withoutcausing any damage to the downstream components. As soon as the transient force

ceases, the snubber resumes its normal operation.
http://pipingtech.com/


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Types of Snubbers:There are two types of snubbers: (i) hydraulic and (ii)mechanical snubbers with various types of designs. However, the function of any

design is the sameto protect the downstream structure from abnormal shocks.Snubbers are designed for various load ratings depending upon the magnitude ofseismic activities and the criticality of fluid induced shocks.

Hydraulic Snubbers:

This type consists of either two concentriccylinders or two parallel cylinders and their

respective moving pistons. Both the maincylinder and the compensating cylinders arefilled with fluid. The main and the

compensating cylinders are connected tovelocity limiting valves and a main piston

which works in either a push or pull mode.Under normal operating conditions, the valvesremain open and allow the piston to movefreely under thermal expansion/contraction ofthe supported component. When thethreshold velocity (typically 8 in. per minute)

is reached, the valve activates by closing theflow through the valve (also known as valvelocking) and the flow through the system

stops momentarily. At this point, the mainpiston that takes the shock load stops movingand the load is transmitted to the ground or

to a permanent structure, thus avoiding any

damage to the structure downstream of thesnubber. As soon as the shock wave passes,

the snubber resumes normal operation.

Hydraulic Snubbers

Mechanical Snubber:

Similar to hydraulic snubbers, this type of

snubber is comprised of a moving cylinder/rodarrangement. Unlike hydraulic snubbershowever, mechanical snubbers use mechanicalmeans to provide the restraint force.

Mechanical Snubbers

MSA Mechanical Snubber:

With this type of snubber, the linear movement of the rod connected to the pipingcomponent is converted to rotary motion. When the centrifugal acceleration exceedsa certain threshold acceleration (typically 0.02g), a centrifugal type clutch flares outand locks at the peripheral slot of the cylinder and restricts linear motion.

Anchor-Darling Mechanical Snubber:

With this type of snubber, the linear motion of the central rod that is connected tothe structural component is converted to oscillatory motion via a verge mechanism.


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This oscillatory motion is in turn converted to rotary motion via a set of gears. Asthe linear velocity increases, the inertia force generated in the oscillating verge and

the train of rotating gears increases. The extent of this increase depends upon theamount of inertial mass and gear trains angular velocities thereby limiting thevelocity of the piping components within the safe limit.

Section - III, Pipe Supports

D: VARIABLE SPRING SUPPORTS VS. CONSTANT SPRING SUPPORTS

By: Hyder Husain

Article courtesy of Piping Technology & Products, www.pipingtech.com

What is the difference between a variable & a constant spring support?

In a variable support, the force acting on the spring and hence the reactive forcevaries during the pipe travel, while the moment about the line of action is zero. Incontrast, in a constant support, the fixed applied load remains uniform throughoutits travel but the moment around a pivot point varies.

What is a variable support?

A variable support is essentially a spring, or series of springs, in a container.When the installed load w is applied, the spring is compressed through the

distance W/k (where k is the spring rate) such that the reactive force exertedby the spring is also w under the equilibrium condition. As the pipe movesdue to thermal expansion, it produces a deflection (L), causing a differential

load (W=k L), to act on the spring(s). Depending upon the direction ofthe movement, the change in load (W) will either add to or subtract from ourinstalled loadw to reach our final operating load (w1). In order to minimize the

stress variations, the differential load (W) for a given variable spring support islimited to a maximum of 25% percent of the operating load (w1).

What is a constant support?

A constant support is a device comprised of a spring or series of springs and anintegral cam mechanism. The external load of a constant support is fixed while itsmoment about the fixed pivot point varies during its travel (because the moment

arm length changes). In order to maintain an equilibrium condition,the external force moment is balanced by the internal momentproduced due to the springs compression or decompression aboutthe pivot during the displacement of the pipe.

With an appropriate choice of moment arms, as developed by thecam geometry, and spring properties (i.e. spring rate), a resisting force can beprovided that is nearly independent of position during its travel.
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At each travel location of the applied load, the moment caused by the external loadis balanced by the counter moment produced by the (compressed/decompressed)

spring force with the appropriate moment arm. Typically, the variation of the activeand reactive forces is very small (with a maximum deviation of 6%) and can betaken as a constant force while moving either upward or downward.

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