HI Pump FAQs: December 2012

Q. How should measurement uncertainty be accounted for when running a hydraulic acceptance performance test of a rotodynamic pump? A. Measurement uncertainty depends on the residual uncertainty of the measurement device and the measurement method. After all known errors have been removed by methods such as zero adjustment, calibration, careful measurement of dimensions and proper installation, an uncertainty will remain. This uncertainty cannot be reduced by repeating the measurements if the same instrument and method are used. After measurement device selection and setup, the best assurance of accuracy is ensuring that a zero adjustment is performed regularly and that device calibration is performed at proper intervals. Table 14.6.3.3.3 shows the maximum permissible measurement device uncertainty. Note that these maximum uncertainty values pertain to the measurements at the guarantee point. Many measurement devices have uncertainty based on their full-scale capability. When practically applied, the measurement uncertainty can be two to four times higher, meaning that the measurement device must often have a correspondingly higher accuracy (lower uncertainty). Q. What should be considered before installing a motor deepwell type-VS1 pump? A. When vertical pump types VS0, VS1, VS2 and VS3—either of the lineshaft or submersible type—are installed in wells, consideration should be given to the well before application and installation (see Figure A.2). Installing a unit in a crooked well may bind and distort the pump column or pump-motor assembly and could result in malfunction. Well straightness should be in accordance with the American Water Works Association (AWWA) standard AWWA-A100 Water Wells. If straightness is in doubt, the well should be gaged prior to installation by lowering a dummy assembly—slightly longer and larger in diameter than the actual pump or pump-motor assembly—on a cable. Gaging is also important when a stepped well casing is used, with the lower part of the well casing having a smaller inside diameter. Wells that have not been properly constructed or those that produce sand can be detrimental. If a well is suspected of producing excessive sand, a unit other than the production pump should be used to clear the well. Before installing pumps, these dimensional checks can preclude problems:

• Pump length versus pump depth
• Correct fit of anchor bolts to the soleplate and of the soleplate to the pump mounting base
• Satisfactory angular location of anchor bolts or correct lineup of discharge head to discharge piping
• Proper conduit location provided for the driver
• Sufficient head room for handling

Q. Where should an expansion joint be placed in the piping of a rotodynamic pump, and what are the limitations for using tie rods near the pump? A. If expansion joints are used, they should be placed between the pipe anchor and the check valve (see Figure A.3). If an expansion joint is installed in the piping between the pump and the nearest piping anchor, then a force equal to the area of the maximum inner diameter of the expansion joint, times the pressure in the pipe, will be transmitted to the pump. This force may be larger than can be safely absorbed by the pump or its system. If a pipe anchor between an expansion joint and a pump cannot be used, then acceptable installations can also be obtained using tie rods across the expansion joint or flexible pipe coupling, provided careful attention is given to the design of the tie rods. The total axial rigidity of the tie rods, including their supporting brackets, should equal that of the pipe. As an alternate, limit the axial deflection to 0.125 millimeters (0.005 inches) when subjected to the maximum working pressure and thermal loads of the system. Many standard tie rod designs are inadequate for use near pumps because they are based on maximum allowable stress only, and deflection is not considered. In fact, some standard tie rod designs result in high deflection values because of the use of high-strength steel in the tie rods that allow high stress values. Because deflection is proportional to stress, these high allowable stresses result in high deflections that transmit excessive loads to the pump.