Hydraulic Institute Pump FAQs April 2009


Written by:
Hydraulic Institute


Q.

We have recently installed several double suction between bearing pumps and would like to verify that their vibration levels are within acceptable limits. How should this be done, and what is an acceptable upper limit for such vibration?

A.

The most appropriate method for vibration measurement is electronic velocity or acceleration measurements taken at various frequencies. Where the values are significant, these measurements are integrated in an appropriate electronic circuit to determine overall vibration in millimeters (inches) per second.

Direct measurement of velocity or acceleration filtered to unique frequencies is not intended by this guide. Such secondary measurements, including complete frequency analysis, are useful in diagnosing vibration problems.

It should be noted that relatively high velocity or acceleration readings at high frequencies result in small displacement values.

The vibration probes should be located as shown in Figure 9.6.4.12 for horizontal split case or double case pumps. Probes must not be located on flexible panel or cylinder walls, such as on motor end covers of vertical pumps. Such covers should be removed to allow measurements on a stiff part of the machine.

The vibration values shown in Figure 9.6.4.12 are for unfiltered RMS velocity readings. These values assume the following conditions:

  • Operation under steady state conditions at the rated speed +/- 10 percent
  • No entrained air or gas and adequate NPSH margin
  • Operation within the pump's Preferred Operating Region (POR) as recommended by the pump manufacturer (see ANSI/HI 9.6.3, Centrifugal and Vertical Pumps-Allowable Operation Region)
  • Pump must be installed so that shaft alignment and flange loads are in accordance with the manufacturers' recommendations
  • Vibration level recorded is the maximum of measurements taken in each of three planes-vertical, horizontal and axial-measured as shown in Figure 9.6.4.12
  • Pump intakes (wet wells) shall be properly designed in accordance with ANSI/HI 9.8, Pump Intake Design

The values in Figure 9.6.4.12 are not applicable to factory or laboratory acceptance tests. Experience has shown that vibration levels measured on temporary factory setups may be as much as two times higher than those obtained in the field.

These vibration values are to be used as a general acceptance guide with the understanding that vibration levels in excess of these values may be acceptable by mutual agreement if they show no continued increase with time and there is no indication of damage, such as an increase in bearing clearance or noise level.

Q.

How can we determine the proper size for the intake sump to accommodate several vertical turbine pumps taking input from a river?

A.

The answer to this question is complex, and sufficient space is not available to respond here. The basic design requirements for satisfactory hydraulic performance of rectangular intake structures include:

  • Adequate depth of flow to limit velocities in the pump bays and reduce the potential for formulation of surface vortices
  • Adequate pump bay width, in conjunction with the depth, to limit the maximum pump approach velocities to 0.5-m/s (1.5-ft/s), but narrow and long enough to channel flow uniformly toward the pumps

The minimum submergence, S, required to prevent strong air core vortices is based in part on a dimension­less flow parameter, the Froude number, defined as:

FD=V/(gD)0.5

Where:

FD  =  Froude number (dimensionless)

V  =  Velocity at suction inlet equals the flow per unit area, based on D

D  =  Outside diameter of bell or pipe inlet

g  =  Gravitational acceleration

Consistent units must be used for V, D and g so that FD is dimensionless. The minimum submergence, S, shall be calculated from (Hecker, G.E., 1987):

S=D(1+2.3FD)

where the units of S are consistent with the units used for D.

Q.

What is the most effective way to reduce energy consumption in existing centrifugal pump installations?

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