Case Study Balancing a System

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Liliana Gardner
5 years ago
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1 Case Study Balancing a System Piping systems providing heat transfer fluid to a variety of loads can present their own special problems. Since there are multiple paths for the fluid to travel, the system naturally balances so the pressure drop around each loop is equal. It is highly unlikely that when all of the valves are fully open, the flow rate will equal the design flow rate needed to provide the necessary heating or cooling to the various loads. As a result the system must be balanced using flow control valves or orifices. If the heating or cooling system is not balanced, some loads will get excessive flow and other loads will be starved for flow. This results in the system not operating as designed. Anytime a looped heat transfer system is built or expanded, the system should be balanced to insure each load gets its required flow rate. This tedious step can take a crew of operators many days to complete. In this case study, we will see how PIPE-FLO can be used to determine the position of a valve to balance the system, cutting days off the balancing process. In addition, we will see how balancing the system can provide extra system capacity along with minimizing the system's pumping cost. The Piping System In looking at the FLO-Sheet you can see the system consists of an open loop cooling system, which takes suction from the cooling tower sump, through one of two circulating water pumps, then to three buildings. Each building has four cooling loads

2 requiring varying capacity. After exiting the heat exchangers, the fluid goes through a throttle valve, then a common return line to the spray header of the cooling tower. Here the heat is released to the atmosphere. A system expansion was recently completed at which time the loads in Building 123 were added to the system. The circulating water pumps CCW-15 and CCW-16 were sized so one pump could supply the entire cooling loads including the Building 123 expansion. After 6 months of operation with Building 123 online, the circulating water pump CCW-15 tripped due to excessive electrical load. To correct this problem, operations started running both cooling water pumps continually. Since then, there have been no problems with either pump tripping on overload. The plant's engineer at this site was instructed to add a third circulating water pump as a standby to improve the system reliability. Before adding the third pump, the plant engineer wanted to get a better understanding why the existing pump arrangement would not meet the loads as designed. How the System Operates The first step is to determine how the system is currently operating and why the system will no longer work with a single pump. It can be seen that the flow rate through the pump CCW-15 exceeds the pump's run out of 2,978 gpm, which is why the motor for the single pump tripped on overload.

3 Next, see how the system operates with both pumps running: In reviewing the results, with two pumps running neither pump is in a run out condition. That explains why the pumps no longer trip on overload with both pumps running. Notice, however that many of the components are red, indicating the flow rate through those components exceeds the manufacturer's limits. This could cause a problem over time due to high fluid velocities in the heat exchanger tubes. After a trip to the field, the plant engineer noticed all the throttle valves were wide open. After further investigation, it was discovered that the system was never rebalanced after Building 123 was added to the system. When the new building went online, the flow rates to the loads in the two existing buildings dropped. The operators opened the valves in these two buildings to increase the flow rate. As each throttle valve was opened, the flow to the adjacent valves decreased. Before long, the operators opened all the throttle valves in the system to make sure all loads received their required capacity. Before sizing the third pump, the plant engineer decided to use the computer model to see if a balanced system was possible using a single pump, as originally designed. Calculating the Balanced System To adjust the flow rates to the design values, balancing valves in each loop are throttled to increase the differential pressure across the loop, thereby lowering the flow rate in that loop. This also causes the flow rate in the adjoining loops to

4 increase, resulting in increased differential pressure in these. To balance a system, all the valves must be throttled and the system results checked multiple times until the flow rates through each loop are at the design value. This is a tedious and timeconsuming process when done by trial and error in the field. To accomplish this in PIPE-FLO, FCV's are inserted into each loop and set to the design flow rate. The program then calculates the differential pressure across each throttle valve needed to balance the flow for each loop. The table below shows the design flow rates required for each load in the system Load Design Flow Rate (gpm) Load Design Flow Rate (gpm) HX HX HX HX HX HX HX HX HX HX HX HX Regulating the flow rate through the pipelines Now we will set each of the FCVs to the specified flow rate.

5 PIPE-FLO calculates the differential pressure needed to limit the flow rate through the FCVs to the set values. When the system is balanced, the total flow rate through the system is 2,655 US gpm, which is less than the run out flow rate for the pump. As you can see, the system can run with only one CCW pump in operation. This eliminates the need to purchase and install a third standby pump. By having only one pump operating, we also save on annual pumping costs. Finally the system only requires 2655 gpm, which is well within a single pump's operating curve, giving the plant future system capacity as well. Balancing the System The final step in balancing the system is to insert restrictions in each pipeline. These restrictions provide the differential pressure needed in each loop to balance the system as calculated by PIPE-FLO. The FCV restriction calculated by PIPE-FLO can be supplied by: A modulating control valve controlled by a flow control circuit A balancing orifice The method chosen is based on the degree of control desired. Regardless of the method, PIPE-FLO can meet your needs.

6 For a modulating valve, you can have PIPE-FLO select a control valve to be placed into the system. You can then insert the control valve and PIPE-FLO will modulate the valve and display the valve position based on the set point. For a balancing orifice, you can insert an orifice instead of a FCV.You can then use the flow meter sizing feature of PIPE-FLO. This calculates the size of a balancing orifice needed to provide the required pressure drop at a certain flow.

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