In the last three columns ("Troubleshooting Piping Systems" in the September, October and November 2015 issues of Pumps & Systems, read them here), we examined how a physical piping system can be mathematically simulated using basic engineering equations described in hydraulic textbooks, technical standards and manufacturer test data. Because all the calculations are based on well-understood equations, one can determine the pressure and flow rate in the system using the simulation. We also worked through an example system to calculate the head at various locations within the system and validate the simulation to the physical piping system using installed plant instrumentation. In the example, all calculated results closely matched the values on the installed plant instruments This series will evaluate piping systems for which the calculated results do not match the displayed plant instrumentation. The difference between the calculated values and the observed values will allow us to troubleshoot the example system. This month's column in particular will focus on the components of the pump and drive. The objective is not to show all the possible problems that can occur in a system; instead, it is to understand the connections of the various components of the system and how to use that information to identify system problems.
First of Two Parts
12/08/2015
Figure 1. Normal operating conditions for the example fluid piping system as calculated on the piping system model (Graphics courtesy of the author)
Figure 1 shows the system under normal operating conditions as described in last month's column.
Figure 2. Manufacturer's catalog pump curve for the installed centrifugal pump PU-101, which is a 6x4-17 end suction pump operating at 1,780 revolutions per minute (rpm). The installed pump was supplied with a 14.125-inch impeller by the manufacturer to meet the original design point.
Table 1. Comparison of observed plant instrumentation with the validated piping system model
Next, we will look at the pump suction pressure as indicated on PI-100. Because the observed pressure corresponds to the previously validated piping system model and the flow rate through the circuit of our piping system is controlled to 1,000 gpm, we can assume that no change in head loss in the pipeline connecting TK-101 to the pump suction has occurred. As a result, the suction pipeline can be ruled out as a possible cause for an increase in the pump discharge pressure.
The observed pump discharge pressure is 16 psi greater than the calculated value of the validated piping system model. The abnormal discharge pressure appears to be a result of the pump.
Part 2 of this series will investigate the possible causes of the increase in pump discharge pressure by 16 psi.
See other articles in this series here.