Pump System Improvement
by Ray Hardee
August 2, 2016
Figure 3. The pump curve for one of the operating cooling water pumps running after the third pump is operatedFigure 3. The pump curve for one of the operating cooling water pumps running after the third pump is operated

After inspecting the system and discovering that all the throttle valves were fully open, the plant engineer wanted to see if it would be possible to balance the system at the new system flow rate. Using the design data for the cooling water flow rates through the heat exchanger, the plant engineer determined that the design flow rate through the entire system was 2,650 gpm.

Looking at the pump curve, he could determine the flow rate through each pump would be 1,325 gpm, resulting in the pump operating close to its BEP. Next, the engineer had to determine if the pumps could deliver sufficient head for all the circuits in the system.

Setting the flow rate in the piping system model to the design flow rates and operating the model with two cooling water pumps revealed that the existing pump head was sufficient for the design flow rate through all the circuits. In addition, the model provided the calculated valve position of each throttle valve needed to balance the system.

The calculation performed using the piping simulation software was similar to the steps carried out by the balancing contractor. The software performed an initial guess of the differential pressure needed across the throttle valves to balance the flow rate in each circuit to the design valve. Once the differential pressures across the throttle valves were calculated, the software was able to determine the position of the control valve needed to balance the system.

Checking the Results

The team decided not to change the position of the throttle valves when the plant was operating to prevent downtime. During a planned weekend outage, all the throttle valves in the cooling water system were set to the calculated position. To make sure the system was balanced, an ultrasonic flow meter was employed to validate the flow rate though each circuit with only two cooling water pumps operating.

The test results of the physical piping system matched the design flow rates, indicating that the system was in balance. The test showed that the flow rate was balanced, but to ensure all heat exchangers met the thermal transfer requirement of each circuit, an operation test was planned when the plant was back in operation.

Once the plant was in operation with only two active pumps the engineers checked the outlet temperatures of all heat exchangers. With the exception of two heat exchangers, the outlet temperatures were within the allowable temperature. The two heat exchangers with high temperatures and the design flow rate indicated that the heat transfer within the heat exchanger was not as designed. To ensure the plant operated properly, they increased the flow rates through the two heat exchangers until the outlet temperature was within the allowable range.

By adjusting the throttle valve position on the two circuits that had problems with their heat exchangers, the plant engineer could determine how changing the flow rate through the two loads would affect the operation of the remaining circuits in the system. To accomplish this, he positioned all the throttle valves to the final position and performed a system simulation. The calculated results of the piping system model indicated that the calculated flow rates through each circuit were well within the range of system operation.

See more Pump System Improvement articles by Ray Hardee here.