installation of pipeline pumpsproceedings.asmedigitalcollection.asme.org/data/... · upon...

International Pipeline Conference — Volume IIASME 1998

INSTALLATION OF PIPELINE PUMPS

Jim W. Horner, P. Eng.Trans Mountain Pipe Line Co. Ltd.

Kevin W. Savage, P. Eng. Trans Mountain Pipe Line Co. Ltd.

ABSTRACTThis paper reviews Trans Mountain's recent experience with the

installation of main line pumping units ranging in size from 670 to 1900 kw (500 to 2500 Hp). This includes a review of vibration problems encountered due to a structural resonance, how this problem was resolved and its impact on subsequent baseplate designs and their associated installation techniques.

NOMENCLATURE Hz Herte (cycles per second)IPS Inches Per SecondRPM Revolutions Per MinutefN Natural Frequencyk System StiffnessW Weightg Acceleration due to Gravity

INTRODUCTIONIn 1989 Trans Mountain started construction on its Stage I

expansion of the pipeline's capacity. The expansion plans included the construction of three new pump stations, and replacement of all the main line pumps. The new pumps could not be adapted to the existing baseplates.

Trans Mountain's existing pump stations had seen a number of modifications over the years as the pipeline's capacity changed. The early pump stations were initially composed of engine driven multistage pumps operating in parallel. As the line's capacity increased, newer station designs included electric motor driven single stage pumps connected in series. Trans Mountain then replaced the engine drivers at all but one of its Pump Stations in the 1980's.

The scope of the expansion included replacement of all the existing pumps with a new diffuser design (Horner, 1995). The construction program included installation of these pumps at both existing stations and new facilities. To expedite the installation process it was decided to build a common baseplate for all locations.

The baseplate design, developed by the pump vendor, supported both the pump and the motor. This design was developed for the 1500 kw (2000 Hp) motor in common use at that time. The design could be adapted for use with a 1100 kw (1500 Hp) motor with the addition of some extensions that bolted in between the motor and the baseplate.

This concept simplified installation, but it was less than satisfactory from an operational perspective. The installations were plagued with vibration problems and in place modifications were required for most of the baseplates. Subsequently, Trans Mountain has used independent methods of support for the pump and motor.

BASEPLATE DESIGNThe function of the baseplate is to secure the rotating equipment

to the concrete foundation. The function of the foundation is to provide sufficient mass to prevent the tendency to respond to exciting forces and transfer vibrations. Dodd (1995) suggests the following rules of thumb for good foundation design:1) The mass of the concrete foundation should be five times the

mass of the supported equipment2) Imaginary lines extending downward at 30° from either side of

a vertical line through the machinery shaft should pass through the bottom of the foundation and not the sides.

3) The foundation should be 150 mm (6") wider than the baseplate for machinery over 670 kw (500 Hp).

IPC1998-2128

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Pump BaseplateUpon installation of the new pumps, high vibration levels were

experienced at some locations. These vibration levels ranged form0.6 to 1.0 ¡PS,.* for about 30 seconds, or until the pump discharge valve was substantially open (fixed speed pumps are started against closed discharge valves). These vibrations did not occur at any discrete frequency, but were characterized by pulsations within a band from 20 and 50 Hz.

This vibration phenomena was associated with flow conditions well below the recommended minimum of the pump. This problem was alleviated by attenuating the vibration monitors for a fixed time during start-up and shutdown. Of greater concern was the tendency for the pumps to trip off on high vibration during upset conditions (eg. loss of an upstream station), and then have this problem cascade down the pipeline.

System Resonances. A vibration consultant was retained to evaluate the problem. The analysis found that a “system" resonance was being excited (system being a collective term for the Interaction of the pump, baseplate, foundation and piping). This resonance took the form of a torsional mode of vibration of the pump about its vertical axis, at 44 Hz (2640 cpm) as shown in Fig 1.

System resonances are not an uncommon problem. As part of an investigation into the dynamic behaviour of multistage pumps, Bodeter et al (1984) encountered natural frequencies within a multistage pump’s operating range. Their analysis covered six possible modes of structural vibration: rotation about the vertical [1] (similar to our case) horizontal [2] and axial [3] axis and translation in the vertical [4], horizontal [5] and axial [6] planes. Due to the differences in pump geometry the horizontal translational mode had the lowest natural frequency. Their analysis lead them to optimize the pedestal design also.

Baseplate Modifications. The pump baseplate was modelled using finite element techniques and a workable solution was developed through an iterative design process. The design had to minimize interference with the pump and its ancillaries, such as the seal flush and drain ines, while avoiding an unfavourable adjustment in another vibrational mode (while correcting one problem, we could induce another by bringing the frequency o f one o f the other modes of vibration into the critical 20 to 50 Hz range).

The final design required additional anchor bolts, welding and in place machining of the pump support pedestals. Additional reinforcement was welded around and in between the individual pump pedestals to limit relative movement Anchor bolts and back up bars were added around the perimeter of the pedestals to minimize any flexing or pivoting at their bases. The mounting surfaces on top of the pedestals then had to be machined because of distortion caused by the welding. The cost of these modifications averaged $15,000 per pump.

This work was completed in operating pump rooms, so unique isolation procedures were developed to permit the ’ hot work* required. Each pump was isolated with a ventilated, tarp covered enclosure during the modifications and continuous gas testing was conducted during any hot work. Due to operating requirements we were only able to modify one pump baseplate at a time. It took almost a year to complete all the necessary changes, but the modifications were successful in reducing vibrations to acceptable levels.

Baseplate Stiffness. The pump pedestals on the original baseplate had a lateral stiffness of one million Ib/in. The in situ modifications increased the lateral stiffness of the pump pedestals to 2.5 million Ib/in. Based on this experience, Trans Mountain has specified that subsequent pump baseplate designs have a structural stiffness exceeding 5 million Ib/in. in the lateral and axial directions and 2.5 million Ib/in. in the vertical. As observed by Bolleter et al (1984), a major difficulty in predicting the stiffness of the pedestal is the modelling of the baseplate/foundation interface. To simplify this issue, Trans Mountain specifies that the design stiffness shad be obtained without the benefit of grout

Additional Recommendations. Hrivnak (1996) makes the following additional recommendations for baseplate design:1) The baseplate should be fitted with one 100 mm (4") fill hole for

each square meter (fO square ft.) of baseplate and/or subdivided section or raised cavity.

2) 12.7 mm ('A") vent holes should be provided for each bulkhead compartment at all corners, high points and perimeter edges (perimeter holes should be should be on 18” centers maximum).

3) 12.7 mm ('A’) vertical levelling screws should provided around the baseplate perimeter to facilitate alignment of the baseplate.

4) Machined mounting surfaces for the equipment and driver should have 9.5 mm (% ') horizontal jacking screws.

5) All welding on the baseplate should be completed prior to machining equipment mounting services.

6) Machined mounting surfaces should be coplanar to 50 microns (0.002”).

7) Machined mounting surfaces should extend 50 mm (2") beyond equipment feet on all sides with a 125 micro inch Ra finish.

8) A 3.2 mm (0.125") shim allowance should be provided under the driver feet for alignment

9) Anchor bolt holes should be 6.35 mm ('/*') larger in diameter than the anchor bolts.

10) AD comers of baseplate flanges should be radiused a minimum of 25 mm (1"). All surfaces which will contact the grout should be rounded to eliminate stress risers.

11) Angle, channel or stud anchors should be welded to the bottom surface to act as a shear key in the grout


Motor SupportLarge electric motor frames are inherently more flexible than the

pumps they are coupled to. The rotating mass of the motor rotor is also usually much larger than that of the pump. This mass is supported on large "end bells’ or covers. These endbells are required on each end of the frame to allow the rotor's removal. The end bels are normaly installed in large diameter, tight clearance fits in the frame. Final alignment of the end bell is usually secured with modest sized tapered dowels. The bearing housings are usuaHy attached to the end beds in a similar fashion.

Due to the electric motor's flexibility, its installation can have a significant impact on its reliability. Trans Mountain has had experience with motors with poor assembly tolerances where installation errors have compounded the vibration problem. These vibration problems have contributed to enlargement of journal bearing clearances and cracked rotor bars.

Investigation of units with poor motor vibration histories revealed baseplates where motor support pedestals were as much as 2.54 mm (0.100") out of plane. Alignment histories had shown that the units were characteristically difficult to bring into alignment Although the motor can be shimmed for alignment, the support surfaces are no longer ’ in plane’ . This causes a distortion of the motor frame.

The original baseplate design had the motor mounted on channel supports with insufficient reinforcement It was dear that these baseplates did not have adequate rigidity as they were not even able to withstand the rigors of transport and installation. The baseplates and motors are normally transported to site separately. In spite of this precaution, one baseplate was so badly distorted when it was improperly removed from the transport that it had to be returned to the vendor to have the support pedestals machined.

Sole Plates. Trans Mountain now specifies fabrication of a compact baseplate for the pump only. The motor is supported on sole plates or‘chocks' grouted directly into the concrete foundation. The unit foundation is ‘stepped’ or has two different levels for the pump baseplate and the motor sole plates as shown in Fig. 2.

The basic sole plate design is shown in Fig. 3. Motor anchor bolts are cast directly into the concrete foundation or holes are cored and the bolts are epoxy grouted. The bolt is sleeved to allow some lateral movement and to distribute bolt-up strains over a longer length. A heavy hex coupfing is tac-welded on the end of the anchor bolt below the sole plate. This coupling can accept a removable stud (in case o f thread damage) or bolt through an oversized hole in the sole plate. The motor mounting stud is smaller in diameter than the anchor bolt so it will fail instead of the anchor bolt

The sole plate designs used have been continuous longitudinal rails, with each rail supporting a front and rear motor foot This concept was used to allow the mounting of different motor sizes by drfflng and tapping new holes in the rails. Four independent pads, one for each motor foot, could also be used.

Alternative Design. Some motor vendors prefer a different approach. They recommend anchoring the motor to tapped holes in the sole plate. This allows the use of the motor to locate and position the sole plates. The motor is bolted to the sole plates and the anchor bolts are used to fasten the sole plates to the foundation. The motor and sole plate assembly is then levelled with jacking nuts on the anchor bolts, under the sole plates. Final positioning is determined by alignment with the pump.

This alternate method is not favoured by Trans Mountain because motors are often moved to other locations. This alternate method can ‘ imprint* a given location with a particular motors idiosyncrasies. The prior method has the added advantage of anchoring the motor directly to the foundation, placing the sole plate and grout in compression.

Adjustment Luos. As shown in Fig. 4, the sole plate is typically a 75 mm (3“) thick plate to allow for some engagement with the grout and still provide enough clearance for removable motor adjustment lugs. All edges of the sole plate that will contact the grout are given a minimum radius of 6.35 mm (0.25") to minimize stress concentrations in the grout All surfaces that are in contact with the grout are abrasive blasted to obtain a profile of 50 to 75 microns (2 to 3 mils).

The function of the adjustment lugs is to provide jacking screws that are used to make small lateral and axial adjustments of the motor position. These adjustments devices are an aid in positioning the motor for alignment These lugs are bolted to the sole plates so they can be removed to facilitate insertion and removal of shims (Murray, 1995). These lugs could be welded in place, but this introduces a risk of marring the mounting surfaces with weld spatter.

GroutingThe sole plates incorporate threaded holes for levelling screws

to support and position it during grouting. The sole plate is temporarily held down by the anchor bolts against the levelling screws. The levelling screws are tapped or greased and removed after the grout has cured so that the sole plate is supported by a uniform layer of grout with no discontinuities or variations in support stiffness.

The epoxy grout is selected on the basis of its compressive strength and thermal coefficient of expansion. The grout must have a compressive strength greater than or equal to that of the concrete foundation. Trans Mountain’s pump stations are unheated, so the equipment will experience an annual ambient temperature variation of 55°C, therefore the thermal coefficient of the grout should be as dose to that of the baseplate and foundation as possible (6.1 x 1(f in/in/’F for steel and 5.9 x 1 (f infm/°F for concrete). Some epoxy grouts have expansion coefficients that are more than twice that of steel or concrete.


Grout Troughs. In the earty installations the sole plates were installed on a continuos layer of epoxy grout which capped the top of the motor section of the foundation. The objective of the grout is to support the sole plate, so this practice is wasteful and because of the large surface area involved, it can also contribute to delamination of the grout

In subsequent installations troughs have been cast in the concrete foundations at the sole plate location. These troughs form a reservoir for a pond of grout around the sole plate. This method minimizes the grout required and virtually eliminates delamination problems.

Foundation Requirements. The concrete foundation should be at least 150 mm (6") wider than the outermost edge of the grout trough, to ensure the epoxy is well contained. The grout layer under the sole plate and pump baseplate should be at least 25 mm (1”) thick. For each additional meter of grout flow length, this clearance should be increased by 40 mm (14* per ft).

INSTALLATIONCare taken in the selection of the equipment installation

contractor will save time and minimize any problems that may be encountered. Equipment alignment and grouting is specialized millwright activity, and not normally an area of strength for the average general contractor. These requirements should be addressed in the construction specifications, but there is no assurance that the contractor will have read or understood them. Therefore, Trans Mountain reserves the right to approve the equipment installer or contracts this work separately to insure a qualified contractor is used.

InspectionAs has been mentioned, the installation of the Stage I

baseplates was less than ideal. Inspection of the installations was imited to witnessing of final pump to motor alignments. Subsequent installations have been subject to continuous third party inspection with a number of inspection hold points in the installation process to ensure the work is completed satisfactorily.

Surface PreparationProper preparation of the foundation is essential to realize the

full potential of the grout After the concrete foundation has cured for 28 days the surfaces to be grouted are roughened (this period could be shortened depending on the mix). Normally the top 25-50 mm (1-2”) of the concrete foundation contains less aggregate and more sand. This portion of the concrete, often called laitance, is weaker and needs to be removed. This is accomplished with a small chipping hammer. The objective is to roughen the surface and expose the aggregate. The surface must be free of any dust, oils or water prior to grouting.

Due to the baseplate's stiffness requirements, there are significantly more anchor bolts than are required by the standard baseplates in Appendix M of API 610. Fig. 5 shows a 670 kw (500 Hp) pump baseplate design that includes 26 bolt holes (re. six distributed around the open faces of each pedestal). In spite of the large number of anchor bolts, installation is not problematic due to the use of oversized anchor bolt holes in the baseplate.

GroutingPrior to grouting the following baseplate checks should be

completed:1) That the baseplate has the adequate number and size of grout

fill holes.2) That the baseplate has adequate vent holes.3) That sharp edges that may be in contact with the grout are

removed.4) That the underside of the baseplate has been cleaned and

abrasive blasted to achieve the proper anchor pattern.5) That 50 mm diameter by 9.5 mm (2” x %’) stainless steel discs

are grouted under the levelling bolt locations.6) That the anchor bolts and levelling bolts have been sleeved or

coated to prevent them adhering to the grout7) That forms are property coated to prevent adhesion to the grout

and any seams are filled with silicone sealer.8) The equipment mounting surfaces are level and coplanar with

in specified tolerances

Pump Baseplate. Grouting the baseplate takes at least two pours. One to seal between the baseplate and the foundation and one to fill the baseplate. On the second pour, the grout is poured into the fill holes with a ‘head box’ or ‘head pipe'. The head box provides some head pressure to help force the grout in and push the air out (an inverted traffic cone makes a good funnel/head box). The epoxy grout can be coaxed into position by tapping the baseplate, but vibrators will cause the aggregate to separate from the resin.

Motor Sole Plates. After the pump has been set and grouted, the motor sole plates can be positioned. The motor anchor bolts have already been cast into the concrete foundation at this point The pump is installed without shims and the final elevations of the motor sole plates are determined from the pump shaft The final sole plate elevation is lowered a further 3.2 mm (0.125") as a shim allowance.

Steps 3, 4, 5 and 6 from above are repeated with the sole plates. The plates are aligned with the levelling screws and then secured for grouting with the motor anchor bolts. The sole plates are epoxy grouted in place and the levelling screws are removed when the grout is cured. The grout is introduced at one side of the sole plate to force air out the opposite side.


RESULTSStiffening baseplates and improving motor installations has

eliminated vibration trips due to low flow problems and has also reduced operating vibration levels. It is difficult to determine the precise contribution the equipment mounting changes have made to these vibration reductions because this work was often completed in concert with motor overhauls or replacements.

Where the same motors have been reused at new locations with updated installation techniques we have observed a 50% reduction in operating vibration levels. Where it was not uncommon to have overall operating vibration levels of 0.25 to0.30 ips, the vibrations dropped to 0.15 ips or less in the new installations. On installations with new motors, these levels are usually less than 0.10 ips.

Generally, pump baseplates are inherently stiff in the vertical due to lack of offsets that can create bending moments. Baseplates tend to be weaker in the lateral and axial directions due to the offset between the pump's shaft and the base of the pedestals. As a rough role of thumb, the required lateral and axial stiffness can be determined by considering the pump installation as a simple mass-spring system and transposing the natural frequency equation to yield:

k = — (2n fNŸ (1)g

W is the weight of the pump, f̂ is the natural frequency and k is the required baseplate stiffness. The natural frequency used should be at least 30 Hz above the operating range of the pump to ensure adequate stiffness.

CONCLUSIONSThere are a number of remedies for the control of rotating

equipment resonances. As discussed in this paper, the natural frequency can be raised by stiffening the baseplate. Another method is to lower the natural frequency by weakening the pedestals or inserting elastic elements between the baseplate and the foundation. A third solution could include the application of a dynamic damper.

The best solution, obviously, is the optimization of the support pedestals during the design stage. Ideally this optimization would include a dynamic analysis, which would take into account the interaction of the pump, baseplate, foundation and piping. This may be difficult to coordinate, with tight project schedules.

It is desirable to have the pump vendor be responsible for the supply of the pump baseplate. It is doubtful that the vendor can take any responsibility for the impact of a third party’s foundation, piping or installation in their design. A minimum requirement could include the specification of the baseplate stiffness given by equation (1) to avoid a natural frequency within the pump's operating range.

The installation of large motors requires considerable attention to detail. Due to the relative flexibility of large motors separate support systems for the motor and the pump should be considered to ensure that the motor obtains all the benefit of the foundation’s mass. Supporting the motor on sole plates instead of a common pump baseplate results in a more complex installation, but the superior reliability obtained outweighs any minor capital cost saving.

Installation of rotating equipment is a specialized activity and it should only be completed by a competent and experienced contractor. If the installation work is part of a larger general construction contract, the installation sub-contractor should be reviewed and approved separately or the work should be contracted separately.

ACKNOWLEDGEMENTS

P.T. Huddleston, Trans Mountain Pipe Line Co. Ltd.

REFERENCESJim W. Homer, Trans Mountain Pipe Line Co. Ltd., February,

1995, “Development of a Diffuser Pump for Pipeline Service", Pipelines, Terminals and Storage conference, Houston, Texas

Ulrich Bdleter, Amo Fret and Dusan Florjanic, Sulzer Brothers Ltd., 1984, “Predicting and Improving the Dynamic Behavior of Multistage High Performance Pumps“, International Pump Symposium, Texas A&M University, College Station, Tx., pp. 1-8

V. Ray Dodd, Chevron USA, October, 1995, “Foundation Tips”, Pumps and Systems, pp. 44

Steven J. Hrivnak, Eastman Chemical Company, 1996, “Computer based Reliability", Proceedings of the 13th International Pump Users Symposium, Texas A&M University, College Station, Tx., pp. 115-124

"Centrifugal Pumps for General Refinery Service", API Standard 610, Eight Edition, August 1995

Malcolm Murray, April, 1995, “Alignment Methods”, Pumps and Systems, pp. 42 - 47


Figure 2: Stepped Foundation


Figure 3: Sole Plate Installation

Figure 4: Motor Positioning or Adjustment Lugs


Figure 5: Typical Pump Baseplate Design


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