Editors Note — This article has beencreated to provide one perspective onthe need for adequate analysis and de-sign for proper foundations for recipro-cating and rotating equipment. In com-pleting the article, COMPRESSORTechTwo

interviewed three Calgary-based com-panies involved in foundation design,construction and inspection services.

Beta Machinery Analysis (BrianHowes, chief engineer), PrimusEngineering (K. Pak, president) andAccurata Inc. (Frank Zahner, com-pression consultant) were interviewed,with each providing insights and ex-periences in compressor foundations.Although the companies and individ-uals perform many more services thanmentioned in this article, their com-ments have been limited to certain ar-eas for brevity.

There are a number of factors thatcause gas compression equipment tovibrate, and eventually break down —and poor foundation design is one ofthem. Recent discussions with indus-try consultants revealed that manychallenges in designing and imple-menting effective foundations exist.One challenge includes the lack of in-dustry specifications to guide equip-

ment users in the design of founda-tions where dynamic loads will beused. As a result, many equipmentpurchasers lack a consistent approachto foundation design, thereby decreas-ing the reliability of their facilities. Insome cases, the lack of proper foun-dations can create disastrous results.

Why worry about vibration? Vib-ration is one of the leading causes ofhigh-frequency maintenance and fail-ure of compression equipment. The

damage it causes may be obvious andeasy to detect, such as the loosening ofoil lines and other attached equipment.But it can also be insidious, includingfatigue of equipment components andpiping that may go unnoticed until fail-ures occur. In both instances, excessvibration leads to higher operatingcosts, and failures in pressure contain-ment systems can result in gas leaksand serious safety concerns.

Several important elements are in-

FOUNDATION ANALYSIS AND DESIGN FOR RECIPROCATING AND ROTATING EQUIPMENT

The Lack of Proper Foundations Can Create Disastrous ResultsBy Neil Purslow

APRIL-MAY 2006 COMPRESSORTechTwo

n Dynamic foundation analyses are prepared to provide recommendations for pile locations and foundation stiffness. Through the completion ofthese analyses the likelihood of resonance by the equipment is reduced, which results in reduced vibration within the package.

n A variety of dynamic forcesfrom a reciprocating com-pressor package will affectthe analysis and design ofthe package’s foundation. It isthe action of the pistons thatcauses the biggest vibrationconcern when designingfoundations because the pis-tons create unbalancedforces and couples in thecompressor.

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cluded in a foundation analysis and de-sign process to reduce the likelihood ofvibration in reciprocating and rotatingcompressors. First, a site and equip-ment analysis must be completed.Second, a dynamic foundation analysisshould be performed to identify wherefoundation stiffness is required to avoidresonance of machine components.And finally, field implementation mustbe carefully managed to ensure foun-dations are properly installed and con-nected to the package’s skid members.

When Primus performs a founda-tion analysis, a series of factors aretaken into consideration. First, testingis performed to determine the proper-ties of the soil at the proposed site.Next, data on the type of equipmentto be used at the site are gathered. Allof this information is required to de-termine the dynamic response of thefoundation system.

In a package using a reciprocatingcompressor, for example, the model ofcompressor, the number of throws,cylinder sizes and the speed of theequipment determines the dynamicforces and couples produced duringoperation. In the majority of larger com-pressors, opposing pistons are mountedhorizontally and offset (i.e., the centerline of the pistons do not line up).Since opposing pistons are connectedto the same crankshaft and travel in andout together, the forces they create al-most cancel each other. But eventhough the forces are minimal, the pis-tons create torque as they travel out-ward, and create torque in the oppositedirection when they travel inward.

It is this action by the pistons thatcreates unbalanced forces and couplesin the compressor, and it is these forcesthat cause the biggest vibration concernwhen designing foundations. Besidesmechanical causes, unbalance forcescan also be induced by pulsating gas inthe system, counterweights on the

crankshaft and crosshead forces in thecompressor, said Pak and Howes. Othertypes of vibration may also arise, suchas vibrating cylinders, scrubbers, bottlesand piping. These are often the result ofpoor support or the equipment beingoperated at or near their resonance.

These forces and their location onthe skid are then incorporated into adynamic analysis, which can be per-formed by Beta or Primus. The resultis a recommended design that resistsforces and adds stiffness in areas sup-porting key machine components.The required stiffness is critical toavoiding resonances in certain areas,such as scrubber bases and cylinderassembly supports.

The foundation design may entailthe construction of an engineeredgravel pad. “Pads are normally cheapto install, but do not always providegood solid contact between the pack-age skid and the gravel pad that is nec-

essary to transmit vibration into theground,” said Pak. “It becomes apainstaking process to ensure propercontact between the two surfaces.There is no easy way to fix a gap, sinceshims and grouting are not suitable op-tions. The only way to fix a gap is toremove the package and fix the pad.”

Using pile foundations is another op-tion. Piles are effective at suppressingvertical movement, but are weak atdampening horizontal movement, saidPak. Many compressor packages inWestern Canada are installed on metalpiles, which generally consist of 8, 10or 12 in. (203, 254 and 305 mm) pipeor I-beam, added Howes. Steel pipe isthe most common because it is fast toinstall and can be used immediately,whereas concrete piles require time tocure. Metal piles are often used in re-mote locations because of the difficultyof obtaining concrete.

Primus recommends that if piles areused, a pre-cast concrete slab be in-stalled on top of the piles. This optionprovides a more forgiving design whileoffering a large contact surface to sup-port the skid package. Steel plates canbe embedded onto the top of the con-crete and used for welding the skid tothe slab. As well, plates can be in-stalled for welding the skid underscrubbers, pipe clamps, bottles andbuilding columns to the concrete base.

After the piles have been driven totheir designed depth, they should becut to the proper elevation andcapped. Pile stick-up above the groundshould be minimized wherever possi-ble, since the amount of stick-up ad-versely affects the lateral stiffness ofthe piles. When compressor skids arewelded directly onto metal piles, the

APRIL-MAY 2006 COMPRESSORTechTwo

n Gravel pads are gen-erally cheap to install.However, they oftendon’t provide good,solid contact betweenthe skid package andthe gravel pad fortransmitting vibrationinto the ground. Gapsbetween the two sur-faces, as shown in thispicture, can only befixed by removing theskid and fixing thegravel pad.

n In this field project, the foundation was modified. A beam was added under the skid and weldedto the existing piles. By adding an extra beam, stiffness was provided midspan to support theequipment above it. If a foundation analysis and design had been completed before construction,a pile would have been installed at that location.

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amount of stick-up must provide suffi-cient crawlspace to accommodateproper welding from below. Some pre-cast concrete slabs come with weldingsleeves embedded into the concrete,which allows for the slab’s installationclose to the ground and accommo-dates welding from atop the slab.

The installation of enviropans oncompressor packages has causedother challenges, said Howes. The in-clusion of these pans requires pack-ages be installed higher to providewelders access to the pilings from be-low. This additional height increasesthe likelihood that resonance will oc-cur in the foundation and add to fu-ture foundation concerns.

“Good foundation designs are im-portant for minimizing vibration,” saidZahner, “but so is sound construction.It is very important to ensure thatequipment and foundations are in-stalled correctly.” Zahner cited a num-ber of examples where improper con-struction resulted in vibration inoperating equipment.

In one situation, a skid package hadbeen built on a wide flange for instal-lation at a particular site. When thepackage was installed, it was notshimmed before being welded to thepiles. This resulted in the welds onlyoccurring along the tips of the wideflange, thereby creating a diving boardeffect and vibration in the package.

In another case, a compressorpackage had been fabricated with asolid main skid in the center contain-ing all the major equipment compo-nents. Along the two lengthwise sideswere removable wings that containeda small amount of process piping. The

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wings had been properly fastened tothe center skid, but the foundationsunder the wings had been reduced. Itwas believed that because of thewing’s lighter weight, the foundationunder the wings could be diminished.The result was a package that resem-bled a bird in flight, with the move-ment in the wings causing vibrationthroughout the package.

“It is important that all constructionactivities be adequately supervised,”said Zahner, “and that the proposedfoundation be delivered precisely asdesigned. All alterations should be ade-quately analyzed and their effect on theoperation of the package determined,before any alterations are undertaken.Construction deficiencies not only costmoney and time to fix, but generallyresult in equipment downtime and lostproduction, which can add up quickly.”

“We’re often asked to determine whya customer’s equipment shakes and vi-brates,” said Howes. “If a customer isfortunate, the exciting element can be

calmed down by merely adjusting ormaking small repairs to the equipment,or by making a minor modification tothe package. If not, the problem mayinvolve more extensive repairs to thefoundation or stiffening of the skid.”

One solution used to decrease vi-bration in a package is to add a beamalong the bottom of a skid using theexisting piles. By installing an addi-tional beam, stiffness can be providedmidspan to support the equipmentabove it, which reduces the resonanceand resulting vibration. The beam isslid under the package and lifted. It isdeformed by jacking up the middle,then welded into place. All repairs oc-cur while working in the crawlspacebetween the package and the ground.This solution is generally used whenthere are not enough piles or the ex-isting piles are incorrectly located,plus, there must be enough space un-der the package to enable this type ofrepair. These repairs are generally ex-pensive to complete.

Vibration tends to be less of a prob-lem with screw and small reciprocat-ing compressors because of smallerforces, but nonetheless, problems stillexist. Beta recently completed ananalysis of two electric-driven screwcompressors that were vibrating andcausing a number of operational is-sues. Even though the units had beenmounted on a concrete pad and piles,analysis showed that the piles wereinstalled in the wrong places andthere were not enough of them, re-sulting in a skid that was too flexible.

These examples illustrate the chal-lenges that exist in analyzing and de-signing proper foundations for com-pression equipment. To assistequipment users, the following recom-mendations were identified by theconsultants.

All equipment users need to beaware of the importance of founda-tion design, including dynamic analy-ses and field installation practices.

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nVertical pipe sleevescan also be embeddedinto the pre-cast con-crete slab. Each sleeveis placed in the slab tofit over an installedpile. After the slab is inplace, the sleeves arewelded to the pilesfrom atop the slab.

n Pre-cast concrete slabs can be constructed with steel plates embedded onto the top of theconcrete. These plates are used for welding the skid onto the slab. Plates are also installed forwelding the skid under scrubbers, pipe clamps, bottles and building columns. Before the con-crete is poured, the plates are positioned to match the corresponding skid members where tie-downs are required.

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should be clearly outlined in the con-tracts between the parties involved inthe project. When not contractuallystated, all consultants agreed that theproducer/owner is responsible. Since itis the owner who is responsible for allfacets of a project, then, by default, theyare responsible for foundation design.

In the absence of dynamic founda-tion specifications for equipment,users should use the services of com-panies who specialize in foundationdesigns. Many facilities are installed

using similar foundation designs, re-gardless of soil and the forces andstiffness associated with the installedequipment. Companies with years ofexperience and developed expertisein geotechnical engineering should beconsulted to reduce the likelihood ofequipment vibration.

One common theme for all was thatonce a producer experiences a vibra-tion problem with a compressor, theyquickly realize the need for proper en-gineering for each project, including

the foundation. It is important to getthe foundation right, they said, sincethe last thing an owner wants to do isfix a problem after the package is in-stalled. Producers can either pay up-front to complete a proper foundationanalysis, design and implementation, orpay later to fix a vibration problem.The latter is generally more costly. Theother alternative is to do nothing andlive with the vibration, safety issues,higher maintenance cost and shortenedequipment life. n

REPRINTED FROM APRIL-MAY 2006 COMPRESSORTechTwoCopyright Diesel & Gas Turbine Publications

Printed in U.S.A.

n After the piles have been driven totheir designed depth, they are cut to theproper elevation and capped. The needfor a crawlspace beneath the slab for un-derside welding can be eliminated whenslabs are welded from the top using ver-tical pipe sleeves. When stick-up is keptto a minimum, the lateral stiffness of thepiles is increased.

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