Testing the Model
After completing the cooling water system hydraulic model, the plant engineer ran the calculations to show how the system was designed to operate with only two cooling water pumps.
The physical piping system required three cooling water pumps to avoid overloading the pump motors, so the physical system was not operating according to the computer simulation.
Now the engineer needed the differences between the physical piping system and the simulation. A quick look at the throttle values in the physical piping system pointed out the problem. Reviewing the manual throttle valves’ position in the physical system revealed that the majority were fully open instead of set to an intermediate position to limit the flow rate through each circuit.
The plant engineer discovered a year earlier that many new loads were added to the cooling water system. After those system changes were made, however, the system was not rebalanced as had been done when the system was first placed in operation.
The cooling water system was placed in operation, and the plant operators manually adjusted the valve positions to each throttle to maintain the required outlet temperature. As each operator independently adjusted a throttle valve, the flow rate through each circuit was affected. Before long, the flow rate through each load in the cooling water system was much higher than required by the design.
As the summer progressed and the outlet temperature increased, the operators continued to open the throttle valve, further increasing the cooling water flow rate. The flow rate through the system was greater than the two pumps could handle, causing one to trip out on high motor overload.
The plant engineer now had a good understanding of how the throttle became open, but he still did not know if his model accurately represented the physical system. The plant engineer changed the piping system model to reflect how the physical system was working. After these calculations were performed, the pump suction and discharge pressure values accurately matched the pressure readings found in the plant. Comparing the motor current on the three running pumps, the plant engineer determined that the flow rate through each pump accurately matched the model’s calculated values. The engineer now had an accurate model of the total system.
Using the Accurate Model
After validating the piping system model, the engineer set the design flow rate through each circuit in the model to the design value. Using the valve manufacturer’s supplied flow coefficient (Cv) data versus the valve position for each throttle valve, the simulation calculated the valve position needed to meet the design flow rate through each circuit.
The result was a calculated valve position for each throttle valve to balance the entire system. The physical plant had three cooling water pumps in operation and all throttle valves open, so the plant engineer wanted to balance the system without affecting the plant’s operation.
The engineer presented the results of the actual system and the model to plant management along with a process to return the system to its normal operating conditions without adversely affecting plant operations. The installation of the fourth cooling water pump was scheduled to take about four months, so management decided to attempt to balance the system.
The plan involved setting the position of the throttle valves to the value calculated by the plant simulation with two pumps operating in a planned shutdown.
Balancing the System
During the plant shutdown, the operators set the physical throttle valves in each circuit to the calculated position for the two-pump operation. Then the cooling water system was started with two pumps in operation. The operators recorded the suction and discharge pressures for the two pumps.
The observed differential pressure across the two operating pumps matched the calculated results of the system model, confirming the flow rate. Another test confirmed the flow rate through each pump by comparing the motor’s required electrical power to the manufacturer’s pump curve. To confirm flow rates through each circuit, an ultrasonic flow meter was used to show whether the flow rates matched the flow rate entered in the piping system model.
As a result of these tests, the plant engineer determined that the system was balanced for the two-pump operation and was ready to return the cooling water system to its normal two-pump operation.
When the plant was brought back to service, personnel performed further checks to ensure that the cooling water system was operating properly by checking the outlet temperature of each circuit load. The plant engineer determined that all but two of the heat exchangers were operating properly.
The cooling water flow rates through these two circuits on the physical system were increased so the heat exchanger outlet temperatures were within normal operation. Once the outlet temperatures were returned to normal, the positions of the two control valves were measured.
The new positions of the throttle valves were entered into the piping system model. The calculated results indicated that the increased flow rates through each load in the system were still within the acceptable range of operation.
This example shows how adding cooling loads to an existing cooling water system results in unintended consequences for a critical system in an operation plant.