Documents Available from GMRC

The following documents are available from GMRC, and most, if not all, can be downloaded from the GMRC website. They were written strictly with integral engine/compressors in mind, but the material provides worthwhile background to the subject of mounting high-speed separable compressors.

1.7.9.1 GMRC TR97-2 – “Foundation Guidelines” by A. J. Smalley and P. J.

Pantermuehl, January 1997

This document would be helpful to those contemplating a block mounted separable compressor.

1.7.9.2 GMRC 97-3 – ”Friction Tests: Typical Chock Materials and Cast Iron,” by P. J. Pantermuehl and A. J. Smalley, December 1997

The data in this report would help with the engineering of tie-downs with interfaces of particular material combinations.

1.7.9.3 GMRC TR97-5 – “Epoxy Chock Material Creep Tests” by A. J. Smalley, December 1997

The data in this report would help with the engineering of tie-downs with using epoxy materials.

1.7.9.4 GMRC TR97-6 – “ Compressor Anchor Bolt Design”

This document guides the sizing and engineering of anchor bolts with guidelines for minimizing the effects of creep and for achieving a particular holding capacity through anchor bolt tension.

1.7.9.5 GMRC TA94-1 – “Parameter Studies for Enhanced Integrity of Reciprocating Compressor Foundation Blocks,” by J. S. Mandke and A. J.

Smalley

This is an early research report on a series of investigations supported by GMRC into foundation block mounting.

1.7.9.6 “Recommended Practice for Control of Torsional Vibrations for High Speed Separable Reciprocating Compressors,” by R. E. Harris and A. J. Smalley, February 2002 (GMRC)

This document provides detailed recommendations for torsional design and analysis for medium and high-speed separable compressors.

1.7.9.7 GMRC TR92-2 “Dynamics of Compressor Skids,” by J. S. Mandke and P. J.

Troxler, March 1992 [26]

This document was an early GMRC investigation of packaged compressor dynamic issues. At the time, the experience base was limited almost entirely to upstream applications.

The report includes a literature search, methods for predicting local and global shaking forces and couples, and engineering guidelines. Key overall points recognized even then were the need for the end user to ensure that critical issues are addressed and that the various contractors involved communicate and coordinate. The importance of keeping the CG low, of gusseting beams, of alignment, of the need for similar analyses recommended in these guidelines were recognized by GMRC then and is even more important today with larger horsepower installations. The present guidelines are, therefore, not inconsistent with the findings of TR92-2, but they, of course, bring to bear a wealth of experience and knowledge accumulated in fifteen years.

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This section of the guidelines more specifically defines important factors in procurement, engineering, and installation of medium and high-speed compressors in pipeline service, and the challenges faced by the end user’s project engineering team. It comprises five sub-sections:

The Compression System (2.1) Physical Interfaces (2.2)

Forces to Recognize and Manage in Designing the Mounting System (2.3) Geometries to be Managed (2.4)

Experience Base (2.5)

Technical Background and Topics to Support Specific Guidelines (2.6)

The last sub-section includes the identification of state-of-the-art limitations, pointing to areas where research is needed to comprehensively support the reliable engineering of separable reciprocating compressors in the future. The incomplete knowledge base most commonly involves the interaction of forces defined in Section 2.3 with the mounting system, and how to engineer this interaction. Research is also needed to quantify alignment criteria in terms of damage risk when the criteria are exceeded.

2.1 THE COMPRESSION SYSTEM

A skid-mounted pipeline compression system built around a medium or high-speed reciprocating compressor comprises (without limitation):

A driver (natural gas engine or electric motor)

The compressor frame (with crankshaft, bearings, connecting rod, and crosshead) A coupling which connects driver output shaft to compressor input shaft

The compressor cylinders (attached to the frame by crosshead guides/distance pieces)

Suction and discharge nozzles (sometimes two of each per cylinder) Often nozzle orifices at the cylinder to nozzle flanges

Suction/discharge primary bottles (normally with internals to control pulsation) Sometimes suction or discharge secondary bottles (with chokes)

Attached suction and discharge piping Often a discharge gas cooler system Often a suction separator

The skid on which many of these components are mounted

A concrete foundation on which the skid and some components are mounted Sometimes pilings under the concrete foundation

Mounting system between compressor frame and skid or concrete foundation Mounting system between engine frame and skid or concrete foundation Mounting system for compressor cylinders and crosshead guides

Fuel, control, and ignition systems for engines Electric power for a motor drive

Lubrication systems for driver and compressor Water cooling systems for driver and compressor Sometimes a speed control for a motor

An unloader system for the compressor

Sometimes added steel structures for support of bottles Instruments for protection, control, and condition monitoring

If compressor and/or engine are mounted directly on a block, part or all of the skid will be removed from the above list, but essentially all the other items remain.

The list is provided for the purpose of identifying items, which must be considered. It may not be exhaustive, and site-specific additions may be needed and should be added and considered. Almost all these items have some direct or indirect relationship to mounting.

2.2 PHYSICAL INTERFACES

The above list of components gives rise to a large number of interfaces, most of them very important to installation integrity, and many of which carry with them the potential for divided or neglected responsibility. The list follows (without exclusion):

Compressor frame to skid

(Cylinder to head end support)

(Head end support to foundation block) Compressor nozzle to suction bottle Compressor nozzle to discharge bottle Discharge bottle to foundation block Suction bottle to its structural support

Suction bottle structural support to the foundation

Left side suction bottle to right side suction bottle (Primary suction bottle to secondary suction bottle) (Primary discharge bottle to secondary discharge bottle) Left side piping or chokes to right side piping or chokes Discharge gas to coolers or to station headers

Station piping to suction separator or compressor suction lateral Interfaces for fuel gas, oil, air, water, electric power

Items in parentheses may not be required in all installations. Of course, the full installation will involve further interfaces not covered in relation to mounting.

2.3 FORCES TO RECOGNIZE AND MANAGE IN DESIGNING THE MOUNTING SYSTEM

The forces to be managed or carried by the mounting system can be distinguished broadly as weight forces, inertia forces, gas forces, drive forces, installation forces, and thermal forces.

More specifically, they can be categorized as follows:

Weight of skid

Weight of compressor Weight of driver

Weight of cylinders and crosshead guides Weight of discharge bottle(s)

Weight of suction bottle(s) Weight of other vessels

Local rotating unbalanced forces

Local reciprocating unbalanced forces at rotating frequency Local reciprocating shaking forces at 2X rotating frequency Local reciprocating engine unbalanced forces

Local horizontal gas forces in compressor cylinder Potential unbalance in horizontal gas forces

Differential cylinder stretch forces

Axial gas shaking forces in discharge bottles Lateral gas shaking forces in discharge bottles Axial gas shaking forces in suction bottles Lateral gas shaking forces in suction bottles Vertical gas forces

Torques which produce differential vertical forces Installation and fit-up forces

Piping thermal forces Anchor bolt forces

Grout to concrete thermal growth forces

Compressor and engine frame thermal growth forces These are discussed briefly below.

2.3.1 S^KID W^EIGHT

A skid with concrete can weigh well over 200,000 lbs. Figure 2-1 shows a large skid whose beams alone weighed over 85,000 lbs and which weighed over 220,000 lbs with concrete added. Skid weight and, of course, all components mounted on the skid must be carried and located by the concrete foundation below the skid. Thus, whether or not a unit is directly mounted on the block, its weight, together with any skid weight, must be satisfactorily supported by the block. In turn, the soil bears this weight over the area of the foundation block, so the soil must be able to carry the implied pressure (maximum weight per unit area).

Figure 2-1. Large Skid – for 5,000 HP Motor Driven JGU/Z, with Beams Alone Weighing Over 85,000 lbs and Completed Skid Weighing Over 220,000 lbs After Addition of Concrete

Prior to grouting the skid, its weight with added concrete, and mounted equipment must be carried on a series of vertical jacking screws. It is not unknown for these jacking screws to be undersized (or too few) and to deform under the required load—particularly since, during alignment, load sharing between jacking bolts can be far from even; thus, care must be taken to ensure and document adequate jacking screw strength.

2.3.2 WEIGHT OF COMPRESSOR AND DRIVER

The weight of the compressor and its driver must be accommodated on the skid (and/or block) with some attention paid to the location of the center of gravity (CG) of the components and to the distribution of the weight about the CG. If either compressor or driver is mounted on the skid before lifting into place, the skid must be able to carry its own weight and the additional weight of equipment during a crane lift (see Figure 2-2 for a skid being lifted into place with compressor alone in position on the skid).

Figure 2-2. Crane Lift of Skid with 6-Cylinder Ariel JGD Compressor Already Mounted

2.3.3 WEIGHT OF CYLINDERS AND CROSSHEAD GUIDES

The cylinders and crosshead guides are cantilevered from the compressor frame; they must be provided additional direct line structural skid support at points of significant weight load, and they must be carefully aligned with the frame and crosshead bearings. As a minimum, a solid A-frame support should be used under each crosshead guide near their joint with the cylinder, and these supports must themselves be appropriately mounted with direct line structural skid support to the block (or, in the case of a block-mounted compressor, directly to the block), with engineered anchor bolts, and well-aligned mounting plates or chocks, with all metal-to-metal interfaces machined and free of rust, coating, dirt, oil, or paint. This area of the support structure needs to be carefully engineered and carefully installed to manage the weight and associated dynamics.

The cylinder weight is also supported by the nozzle attached to the discharge bottles, whose weight support depends, in turn, on the thickness of the bottle wall and the method used to support the discharge bottle and its adjustment. Stress at joints under weight load should be considered in the pipe static stress analysis required and performed for pipeline compressors (to satisfy ASME B31.8). The wedges under the bottles must be robust, stiff, and appropriately adjusted.

The compressor OEM may further recommend a head end support for heavy cylinders typical of pipeline applications (or first stage of storage applications), provide bolt holes for the attachment of a head end support to the cylinder, and procedures for installation and adjustment.

The compressor OEM will normally provide detailed directions for appropriately shimming at the crosshead guide support to offset any droop of the cylinder; such shimming should be undertaken with the recognition that it affects the clearances and alignment of the crosshead slider bearing, and these clearances should be checked after shimming to be sure they remain within OEM tolerances, with further adjustment to satisfy these tolerances as needed.

In addition to supporting cylinder weight and managing deflections under that weight, the potential must also be considered for the mass associated with the weight to cause low natural frequencies which can be excited by high energy at lower orders of compressor operating speed.

Such natural frequencies can involve both vertical and horizontal (parallel to the crankshaft) motion of the cylinder. Options for their control include: confirming that in-line and transverse skid beams exist directly beneath A-Frame attachment points; locating anchor bolts at skid perimeter directly beneath crossheads; installing gussets on beam webs at bolted joints associated with the crosshead supports; increasing the stiffness of individual cylinder A-frame support;

combining the support of crosshead guides for two or three cylinders into a single, stiff, and reinforced beam; increasing discharge bottle wedge support stiffness; increasing discharge bottle wall thickness; adding a cylinder head end support. As will be further discussed, the head end support option needs careful consideration and should not be applied without OEM recommendation, or analytical evidence that it is needed. This option should also not be applied without attention being paid to the mounting and flexibility (under cylinder stretch) of the support, its possible geometrical interference with the discharge bottle, and its potential to add new modes of vibration within the support itself.

2.3.4 P^RIMARY D^ISCHARGE B^OTTLE W^EIGHT

The geometry of larger discharge bottles typical of gas transmission leads to them being most commonly (though not universally) supported below skid level on the foundation block and normally with wedges and/or clamps. Use of an extra chamber on the discharge bottles is quite common as a pulsation control approach and recommended (and elsewhere discussed in these guidelines) whenever possible (i.e., four chambers for three cylinders on one side). The extra length and weight must be accommodated in the discharge bottle supports and clamping. For pipeline applications, it is strongly recommended that the discharge bottle always be supported directly from the concrete foundation by solid wedges under each clamp and each discharge nozzle, with a pair of robust, side-by-side, tie-rods (which must be adjusted to accommodate vertical thermal growth of the cylinder discharge nozzle once operating temperature is reached after start-up). Discharge bottles are designed on the basis of pulsation analysis, and their diameter may well impact the width of the skid in the compressor area.

2.3.5 P^RIMARY S^UCTION B^OTTLE W^EIGHT

The primary suction bottles are sometimes supported and restrained only by the suction nozzles; in principle, their weight can be supported by the suction nozzles, but the concern to be addressed is that with large bottles typical of transmission, the suction bottle lateral resonance may be excited if they are not more completely constrained. Some suction bottles employ an extra chamber (four chambers for three cylinders on one side) on the basis of pulsation analysis,

and this is recommended whenever possible in these guidelines (see Figure 2-3, for example). In this case, the added mass and rotary inertia makes it desirable to support the suction bottles more fully for dynamic considerations as well as for weight support. A solid beam structure specifically designed for the suction bottles is recommended, together with an effective method to attach the suction bottle to this structure. Figure 2-3 shows an example of a retrofitted 4-chamber bottle on a compressor with three cylinders per side with stiff support structure and robust clamping between bottle and support.

Figure 2-3. Rugged Support Structure for 4-Chamber (3 + Common Chamber) Suction Filter Bottle on JGU6, Replacing 3-Chamber

Note clamp and wedge support for bottle with 2-tie bolts on wedges, tying bottle to stiff I-beam structure.

An option sometimes considered for suction bottle support is close coupling the bottle, structurally, to an adjacent vertical vessel. This option should be carefully analyzed, if considered.

Cross-bracing similar bottles on opposite sides of a compressor is widely applied practice and is recommended as general practice in these guidelines (see below). It is a conservative design approach to include the brace attachment points in the vessel construction, even if the analysis indicates that they are not needed.

2.3.6 VERTICAL VESSEL WEIGHT AND INERTIA FORCES

Vertical vessels, whether used to support suction bottles or not, require weight support and solid mounting to control their “reed mode” vibrations. A solid mounting plate is essential with a 2-inch plate and multiple anchor bolts a normal design. If the vessel is on the skid, it must be attached to main skid members (never to the floor plate) or to heavy, structural plate welded firmly to skid members. Heavy vertical vessels may need bracing, in addition to a heavy, structural base support, to manage dynamic loads and natural frequencies, and this potential need must be carefully engineered, as part of the piping and/or structural analysis.

2.3.7 OTHER HORIZONTAL VESSEL WEIGHT AND INERTIA FORCES

Horizontal vessels, including secondary suction or discharge bottles, must be solidly mounted, and this mounting subject to careful engineering and structural analysis for weight support and control of natural frequencies. Figure 2-4 and Figure 2-5 illustrate solid mounting structures for secondary bottles, including side-to-side bracing in Figure 2-5.

Figure 2-4. Cat G3616/Ariel JGD6 Installation (5 Units)

Rugged support structure for secondary suction and discharge pulsation filter bottles for four of the units.

Figure 2-5. Ariel JGD4 Installation Showing Modifications to Control Observed Vibrations:

Cylinder Supports Grouted with Anchor Bolts at Their Base; An Increased Number of More Rugged Clamps; An Increased Number of More Rugged Wedges; Cross-Bracing on Suction

Chokes; Added Cross Beams to Skid Structure

2.3.8 ROTATING UNBALANCED FORCES

The compressor and engine’s crankshafts have the potential to generate rotating unbalanced shaking forces at 1X rotational speed. Most commonly, the obvious rotating unbalanced forces attributable to individual throws are effectively and directly balanced by the crankshaft geometrical design. There remains small residual rotating unbalance due to manufacturing imperfections, which result in small mass eccentricities of the crankshaft, and the manufacturer can be expected to provide the maximum residual unbalance values, which should be considered in any structural analysis.

2.3.9 RECIPROCATING UNBALANCED FORCES FROM COMPRESSOR

2.3.9.1 Forces and Couples from Individual Throws

The shaking forces from reciprocating motion of the connecting rod, crosshead, piston rod, and piston are not so readily balanced by geometrical design. The effective frequency of these forces is predominantly at 1X and 2X rotating speed. The approach used in most medium and high-speed separable compressor designs is to use the opposed motion of closely adjacent throws to achieve as much balance as possible between the reciprocating unbalanced forces of these throws.

Viewed as a system, an adjacent pair of throws, oriented at 180 degrees to each other on the crankshaft, with cylinders on opposite sides of the crankshaft, combine to produce a rotating force and rotating couple at 1X and at 2X rotating speed. These act on the bearings supporting these throws. Equalizing all the weights on this pair of throws can make the 1X and 2X rotating forces for the pair of throws close to zero, but with any axial offset between the throws, the rotating couples cannot be reduced to zero (the Ariel website explains this with animations).

2.3.9.2 Counterbalancing

On a 2- or 4-throw compressor, it is normal practice for compressor OEMs to provide some level of rotating counterbalance on the crankshaft to offset the primary couple induced by the reciprocating motion on a pair of throws. Typically, this counterbalancing is a compromise because the reciprocation is horizontal while the rotating counterweights produce both a vertical and horizontal component. In addition, the components at 2X produced by the slider crank mechanism of each throw cannot be directly balanced by counterweights rotating at 1X (although the peak-to-peak variation in the complex wave of unbalanced force may be influenced by counterweight magnitude).

For 6-throw compressors, it is normal to phase the throw pairs at 120 degrees to each other, and for a single-stage compressor as a whole the sum of the primary and secondary couples is balanced in this way.

2.3.9.3 Effects of Frame Flexibility

What compressor manufacturers will not always emphasize is that the effectiveness with which an entire compressor is balanced for forces and moments depends on the rigidity of the frame. In fact, the frame is not rigid. This subject has been intensively explored and analyzed

What compressor manufacturers will not always emphasize is that the effectiveness with which an entire compressor is balanced for forces and moments depends on the rigidity of the frame. In fact, the frame is not rigid. This subject has been intensively explored and analyzed