Pump Baseplate Design and Installation

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Pump baseplate design and installation Consider these baseplate design and installation options to solve alignment and piping stress problems in centrifugal pumps

motor, and the combined pump-motor weight can be supported by practically any structure including the floor, concrete pad, steel ''I" beam or the like. No mechanical attachment such as foundation bolts are necessary for most typical piping systems.

J. H. Doolin, Dresser Industries, Inc., Liberty Corner, N.]. BASEPLATES for most centrifugal pumps are somewhat taken for granted, and have evolved into the familiar steel or cast iron structure which supports a pump and driver. In addition, precast concrete and molded epoxy are sometimes used. However, it is useful to look at the functions of the baseplate so that alternatives to the more conventional approach might be considered. The primary functions of a pump baseplate include:

1. Support the weight of the pump and driver 2. Maintain shaft alignment between the pump and driver to allow for successful performance of the flexible shaft coupling 3. Resist the forces of gravity and from piping connected to the pump which might cause unacceptable shaft misalignment 4. Resist the movement due to thermal expansion of the pump or driver which might also cause unacceptable shaft misalignment 5. Collect leakage or spillage from the pump which may occur during operation or maintenance so that it can be piped to a suitable disposal system 6. Partially protect the pump and driver from damage caused by fork lift trucks and other such vehicles 7. Absorb noise and vibration originating in the pump and driver. Let's look at these functions in more detail and explore possible alternatives for doing the same thing. Support weight of pump and driver. For most pumps up to about 250 horsepower, weight support is not a serious concern and there are alternatives:

• Close coupled pumps (Fig. 1) require minimal structural support. The pump end is supported by its mounting on the

Fig. 1-Close coupled pump.

• Vertical in-line pumps, either close coupled or separately coupled (Fig. 2) present the opportunity to support the weight of the pump-motor by the piping. The vertical orientation keeps the center of gravity of the assembly over the center of the piping thereby eliminating any torsion loads on the pipe. If the pipe supports are not adequate to also carry the pump-motor weight, the pump casing can rest on the floor, a concrete pad or other structural device. Maintain shaft coupling alignment. Without the disruptive forces from connected piping, maintaining shaft coupling alignment is relatively easy. The mechanical connection between the pump and driver need only support them and resist any torsion forces resulting from the power transmitted from the driver to the pump. The most common solutions include:

• Simple structural baseplate. An inverted channel steel beam is the most common (Fig. 3) Continued Hydrocarbon Processing, July 1990

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Fig. 4--, Hydraulic forces on pump nozzles.

• Face mounted motors which are common on vertical in-line designs (Fig. 2) • Close coupled pumps (Fig. 1). Withstand forces and moments from system piping. The most serious problem associated with the maintenance of shaft coupling alignment is the disruptive effect of forces from the connecting piping. ?\ot only can these forces be substan· tial, but they may also vary with time so that they cannot be compensated for by realigning the pump and drive after the piping forces have been applied. To put this problem in perspective, the American Petroleum Institute in their standard API-610 for centrifugal pumps includes the following values for forces and moments on the pump nozzles: Nozzle size (ln.)

2 Fig. 2-Vertical in-line pump.

Fig. 3-Channel steel baseplate. 60

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Force (lb) 290 430 570 1,010 1,560 2,200 2,600 2,900 3,300

Moment (ft-lb) 460 950 1,330 2,310 3,500 5,000 6,100 6,300 7,200

Note that these are the forces and moments on each nozzle so that the total forces and moments will be approximately double, or more precisely the sum of the forces and moments on both the suction and discharge nozzles. Resisting these forces is difficult and requires reasonable rigidity built into both the pump and its supporting baseplate. In an effort to relieve these loads, expansion joints are sometimes used. However, this is a case where the cure is worse than the disease. \Vhen unrestrained expansion joints are used, a hydraulic force equal to the dischar:ge pressure times the pipe area must be resisted by the pump. Fig. 4 shows the magnitude of these hydraulic forces compared to those specified by API-610. As shown, even pressures as low as 100

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psi can cause forces many times higher than those from pipe strains. These forces on the pump can be eliminated by restraining tie rods on the expansion joint (Fig. 5). However, even with tie rods, expansion of connecting pipe will tend to relieve the stress in the tie rods resulting in hydraulic thrust on the pump nozzle. One may ask, "If tie rods are required what good is the expansion joint?" One answer is that they may still minimize bending moments. Assuming that pipe forces will be present, several alternatives can be considered:

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• Typical structural baseplate with sufficient structural rigidity, bolted to a foundation and filled with grout material. This is the most common solution) • One alternative is a structural baseplate of sufficient rigidity to maintain alignment without the additional support from a foundation or grout. Sometimes such baseplates are supported on stilts or springs. This approach also has the ;cclvantage of minimizing pipe loads. If the pump and baseplate are free to move horizomalh·. then horizontal expansion in pipes will simply mm·e the baseplate instead of applying a force. Vertical expansion forces can be eliminated by the spring supports. \\'ith this approach, however, care must be taken to ensure that the baseplate is rigid enough to withstand reasonable forces and still maintain coupling alignment. ~ Venical in-line pumps also a\·oid the problem of coupling misalignment. If separately coupled (Fig. 2) any pipe forces or movement of the pump casing \\·ill not affect the coupling alignment. The face mounted motor and relatively rigid motor support will maintain good alignment. Furthermore, no forces or moments are transmitted through the motor support to cause misalignment. • Another approach is to pro\·ide greater flexibility between the pump and driver with universal joint couplings. This is most common in vertical water and sewage pumps where the driver is mounted on the floor above the pump to minimize the danger from flooding. The connecting drive shaft is usually arranged with universal joints (Fig. 6). Thermal expansion of ~umps. When operating temperatures exceed 800°F, as in the petroleum processing industry, thermal expansion of the pump casing is substantial and could easily result in movement of the pump shaft and coupling misalignment. The proven solution to this problem is to support the pump at the shaft centerline of the casing (Fig.7). This allows the casing to expand and contract vertically without having an effect on the shaft position. However, horizontal expansion of the casing must be allowed for by some flexibility in the pedestal supports under the casing. For example, a typical end suction pump may be 17.25 in. between pedestal supports. TP.e thermal expansion of this casing can be calculated as follO\\S:

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Fig. 6- Universal joint couplings.

thermal coefficient (length) (b. temp.) 6. 33 x 10·6 ( 17 . .:25 in.) (870- 70)°F 0.087 in.

Deflection of one pedestal will be half of this, or 0.0435 in. If we assume the pump is mounted to each pedestal with two 3/4-in. bolts, a stress in these bolts of 20,000 psi would allow a force of: force stress (area) 20,000 psi (2) (0.551 in.2) force 22,000 pounds A plot of this force versus deflection (Fig. 8) gives some measure of the relative stiffness of a properly designed pedestal. Even when designed with this degree of stiffness, such pedestals will probably allow slippage between the :-~~~~~

Fig. 5- Restrained expansion joint.

Fig. 7-Centerline support pump.

corn)-;iq: chemicals from thr. stuffing box or seal and pipe them w ~l dispt,sal system before t'-ley can reach the pump frame or baseplate and cause c:;rrosion. • If leakage is not corrosive, a larger pan extending under the entire pump, including the suction and discharge flanges will sen·e the purpose ~md take care of bearing and repair leakage as well (Fig. 7). • Vertical in-line pumps rend to have a natural collector formed by the casing cm·er (fig. 2). HoweYer. it is necessary to prm·ide a tapped opening to allow for piping leakage away. This an·angement, howe\·er, does not contain leakage from most pump repair actinty. • Sealless pump~ ciii:::nate the problem of stuffing box or seal leakage. Hc1\\·enT. pump repair will still contribute to occasional problems.

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Fig. 8- Baseplate pedestal deflection.

Mechanical protection. \\·here horizontal pumps-drivers are supported on a common structural basepbte. it should be designed to extend beyond all extremities of the pump and driver. On the pump end it should extend beyond the pump flanges as \vell as the end of the outboard bearing housing (Fig. 7). Similarly, the baseplate should extend beyond the back end of the motor, and if practical C\·en the electrical conduit box. Turbine dri\·es and gears also should be similarly protected. Vertical in-line pumps also should have protecti\"C ~truc­ tures surrounding them. or pos,iblv a concrete pad under the pump which extends bC\·ond the pump flanges.

Fig. 9-lnertia block baseplate support.

Noise and vibration. A properk designed centrifugal pump \\·ill naturally emit a reasonable amount of noise and ,-ibration. The role of the baseplate is to pt-e\-cnt transmission of such noise and Yibration to the surrounding air and structure. Airborne noise is sometimes emitted from resonam 1·ibration of drain pans that are built into baseplates to collect leakage. Grout beneath such pans will help eliminate noi,;c, and for pans not supported by grout, stiffeners will raise natural frequencies to aYoid resonance. Structure-borne noise and \·ibration can be minimized by a\·oiding direct structural support of pumps b1· steel melllbers in the building structure. If a pump baseplate is bolted to a floor steel beam, the chance of noise transmission is incn·asccl. To minimize this ellect, inenia blocks and isolation pach (Fig. 9) are very effective.

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support flange and the pedestal. If do\1·el pins are used for location of the casing, they will probably be distorted due to the high expansion force. On the other hand, pedestals designed for much less rigidit\· will not withstand the piping forces and maintain good coupling alignment. If we assume an 8 in. discharge pump with 12 in. suction is used, the sum of the side to side forces on the pump, Fz = 850 + 1,500 = 2,350 pounds. Assuming one half of the force is resisted by each pedestal, this will result in a deflection of 0.0025 in. This is acceptable to API, but demonstrates that a delicate balance in pedestal stiffness is required. If the pedestal is too stiff, the casing is not free to expand. If the pedestal is too flexible it will not withstand the pipe forces. Several alternatives to the conventional baseplate design exist and they are the same as in the previous section on piping forces: Rigid baseplates without foundation bolting or grout, vertical in-line designs and universal joint couplings.

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Seal and casing leakage. Leakage from most pumps comes primarily from the stuffing box packing or mechanical seal. In addition, oil leakage from bearings is common, and pump repair also contributes occasionally when pumps are dismantled or removed from the piping system. When pumping liquids other than water, such leakage will usually not evaporate readily and must be collected and possibly piped away to a disposal system. Drain collectors are common, and following are some of the design~ that are used.

• Stuffing box drip collectors are designed to collect

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Hydrocarbon Processing, July 1990

Conclusion. The variety of pump designs and applications prohibits general conclusions about baseplate designs. Ho1,·e1·er, the following comments may be helpful:

• \\'hen practical, a rigid baseplate without foundation bolting or grout should seriously be considered • Consider close coupled or vertical in-line designs • Avoid the use of unrestrained expansion joints. LITERATCRE CITED 1

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The author John H. Doolin, director of technology for Dresser Pump Division, Dresser Industries Inc., Liberty Corner, Ne-w Jersey, has 40 years of pump design experience. He holds several patents for pump design and has written 16 technical papers or articles. He has been active in pump standards activity including ANSI-B-73, API-610 and the Hydraulic Institute. Mr. Doolin received BS and MS degrees in Mechanical Engineering from Newark College of Engineering, now NJIT.