Jack Claxton, P.E., has been involved with pump engineering for Patterson Pump Company since his graduation from Georgia Tech. in 1975. He is currently vice president, engineering.
These forces can have destructive effects if not carefully considered in piping design.
For example, studies by the author indicate the equivalent axial stiffness of the M11 thrust rod designs to be 1/4 to 1/3 of the equivalent axial stiffness of the recommended piping designs on which the thrust rods are used. The use of more rods or larger rods on fittings near pumps than recommended by the M11 manual is likely needed, and this is a suggested solution that can be examined by designers. More information on thrust rod designs is also contained in the Hydraulic Institute publication ANSI/HI 9.6.6, Pump Piping.
Table 1. Sources of pump load (Graphics courtesy of Patterson Pump Company)
Typical Vulnerabilities of Piping Systems
Tables 1 through 3 provide general guidance as to vulnerabilities and potential problem areas for some typical piping systems. This information is not provided as a substitution for considering all potential load sources for a particular installation.
Table 2. Vulnerabilities of piping systems
First, consider the summary of how the three categories of pump loads discussed are generated, as shown in Table 1. Both approaches have advantages and disadvantages that must be carefully evaluated and managed to produce a satisfactory installation, without excessive piping loads being imposed on the pump.
Table 3. Summary of possible piping problems
Typical trends to watch for and recommended precautions are shown in Table 3. Combining the tables above produces the generalized overall summary in Table 3.
Troubleshooting to Determine Objectionable Piping Load Effects
Once a system is designed and in place, tests can be conducted to verify that the pump is not subjected to excessive loads from the three load sources discussed here. These procedures require the use of six dial indicators or, alternatively, the use of computer measurement machine (CMM) technology.
Note that the placement of the dial indicators should be done so as to assess movement relative to the foundation.
Determining Mechanical Pipe Strain
With the pump not operating, place dial indicators on the pump flanges (X, Y and Z directions). Isolate the pump by closing isolation valves in suction and discharge lines, and drain the water. Remove the flange bolts. If the movement exceeds 0.005 inches, there may be a problem that warrants further investigation. To determine the effects of the fluid mass, the flanged joint alignment can be corrected as applicable, re-bolted, and the system filled with fluid with the dial indicators in place. If the movement exceeds 0.005 inches, there may be a problem that warrants further investigation.
Determining Thermal Pipe Strain
With the system (and pump) not operating and at ambient temperature, place dial indicators on the pump flanges (X, Y, and Z directions). Start the system and operate until the temperature reaches a stable normal operating temperature. If the movement exceeds 0.005 inches, there may be a problem that warrants further investigation.
Determining Hydraulic Pressure Pipe Strain
With the pump not operating, place dial indicators on the pump flanges (X, Y and Z directions). Start the pump. If the movement exceeds 0.005 inches, there may be a problem that warrants further investigation.