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Realign hot-running recips with adjustable chocks.

For almost 40 years, epoxy chocks have been used to maintain precise, permanent alignment of reciprocating and centrifugal compressors. Because these chocks are poured in place, there are no high spots to fret and wear. Intimate contact with compressor bedplates and a high coefficient of friction between the epoxy chocks and machinery bedplates help to prevent anchor bolts from working loose, thus assuring excellent maintenance of alignment.

By allowing a free flow of air under hot-running compressors, epoxy chocks "thermal humping" and any other misalignment caused by thermally induced distortions. Other advantages of epoxy chocks include excellent vibration damping characteristics, permanent dimensional stability, a 24-hour cure time, and no corrosion or degradation by fuels or lubricants.

Fast-cure epoxy chocks and epoxy grouts help to speed the installation of all types of machinery. But permanent alignment on monolithic structural epoxy foundation systems isn't the perfect solution for every installation problem.

Recips Preferred For

Variable Pipeline


When gas pipeline pressures change, centrifugal compressors cannot be operated under optimal design conditions. The heat rate of a centrifugal win increase more rapidly than a recip. Hence, if pipeline pressure vary widely, recips are preferred because they will provide better fuel efficiency.

Centrifugals are usually preferred if pipeline conditions are steady or when ultra-low nitrogen oxide (NOx) emissions are required. The gas turbines that power centrifugals are steady-flow devices. They can be operated with lean fuel/air ratios to achieve complete combustion with inherently low exhaust emissions. (Conversely, to bring many older recips into compliance with more stringent NOx emission regulations, it has been necessary to retrofit those older recips with new turbochargers and pre-combustion chambers.)

Reduced manpower has forced many pipeline managers and compressor superintendents to take another look at adjustable chocking systems. Developed to compensate for the most severe thermal humping of hot-running machinery, adjustable chocks allow convenient shimming, up or down, to realign even the most massive reciprocating compressors within hours.

Recips Require Frequent


Recips for gas transmission service almost always incorporate integral engines with connecting rods for compressor cylinders connected to the same or adjacent crank throws of the power pistons. During operation, engine loads and compressor through-puts are controlled by opening or closing compressor pockets (clearance chambers) to vary volume of the compressor cylinders.

Reciprocating engines utilize very heavy modular construction that provides service life in excess of 40 years. Massive foundations are required to mount these units. Traditionally, major overhauls and routine maintenance for recipes -- including weekly balancing of power cylinders -- are performed by station personnel.

The turbines for centrifugals require less routine maintenance, and are generally operated with a smaller staff. Major overhauls at 20,000-hour intervals are usually performed off-site by the turbine manufacturer or by a rebuild contractor.

Reduce Wear

And Harmful Emissions

In addition to reducing downtime for realignments, adjustable chocks with steel soleplates embedded in a thick-pour epoxy grout damp (absorb) vibrations and shock loads. Effective damping of both static and dynamic loads plus precise alignment help to reduce wear on compressor components, while minimizing potential liabilities -- or mandated shutdowns -- associated with NOx emissions, leaks or spills from misaligned compressors.

Despite staff downsizings, many compressor superintendents rely on Vibratherm adjustable chocks made by ITW Philadelphia Resins, Montgomeryville, PA, for re-alignment of hot-running recips. These two-piece adjustable chocks are two inches thick to provide a free flow of air. The air flow reduces the effects of thermal humping on crankshafts; reduces the temperature of steel soleplates by 35-50 degrees F; and allows relatively fast alignment corrections, if necessary.

Dual-purpose "level-lift" bolts, incorporated in the soleplate design, relieve some or all of the machinery weight on the patented two-piece chocks to expedite the insertion or removal of 316 stainless steel shims. The adjustable chocks and their soleplates are machined to 0.001-inch tolerances and slotted for easy installation under large recips.

A special anchor bolt, trademarked MagBolt, with a built-in tension monitoring device, permits "wrenchless checking" of hold-down loads. Repeatable accuracy, within plus or minus 5 percent of desired tension, makes torque wrenches, with their plus or minus 30 percent accuracy, obsolete. It also saves time and money from the day of installation.

Better Control Of Tension

The-MagBolt tension monitor measures the actual stretch in an anchor bolt, thus eliminating the friction errors of a torque wrench. (Frictional forces between nuts, threads and washers affect the accuracy of all torque wrenches.) Any under- or over-tensioning of each patented anchor bolt is indicated immediately by the position of an indicator pin to visual reference marks. In addition to convenience, this built-in system allows much more accurate bolt tensioning to maintain alignment of hot-running recips.

Vibrations that occur when recip engines start up or shut down, as well as vibrations caused by variations from no-load to full-load on the compressors, are transmitted through the bedplates down through the adjustable chocks to the steel soleplates, embedded in epoxy grout. The epoxy adheres to the underlying concrete foundation with tensile and shear strengths superior to concrete, effectively transforming the compressor baseplate into a monolithic structure for subsoil absorption of destructive vibrations.

Don't Over-Engineer


Many compressors, crackers, gas turbines, turbochargers, pumps and other critically aligned equipment do not experience severe thermal distortions. The foundation system of choice should then be cast-in-place Chockfast Black or Chockfast Orange chocks, if an air flow is needed under the machinery.

For pumps and other equipment installations with little or no heat-dissipation problems, full-bed Chockfast Red epoxy grout is recommended.
COPYRIGHT 1996 Oildom Publishing Company of Texas, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1996 Gale, Cengage Learning. All rights reserved.

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Title Annotation:reciprocating compressors
Publication:Pipeline & Gas Journal
Date:Oct 1, 1996
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