Fluid Mechanics Solutions

SEA  WATER COOLING SYSTEM. 

 

Balancing the 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 or throttling valves.

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 study, PIPE-FLO will be used to determine the position of a valve to balance the system, cutting days off the balancing process..

 

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 seawater intake pumping station, through one of two circulating water pumps, then to the pumping room of BF3. The pumping room  has 9 ( 4 stand-by) cooling loads requiring varying capacity. After exiting the heat exchangers, the fluid goes through a common return line to the Darse . Here the heat is released to the sea.

The circulating water pump G13 was sized to supply the entire cooling. The water pump G14 is a standby to improve the system reliability.

 

Stand-by Pump G14

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Pump G13

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

How the System Operates.

 

The first step is to determine how the system is operating without balancing.

In reviewing the results(results 1), with one pumps running (G13)and the by-pass full open the pump is in a run out condition. To control the flow at the BEP condition the by-pass has to be partially closed. (results 2)

All the throttle valves at the PHE are wide open. After further investigation, it can be seen  that the system is unbalanced. The flow rates will not equal the design flow rates needed to provide the necessary cooling to the various loads.

 

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 by-pass to increase, resulting in increased differential pressure in this valve. 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 time-consuming 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.(Results 3)

The table below shows the design flow rates required for each load in the system

Flow Rates Required for BALANCED System:

 

 

Load   Design Flow Rate (m3/h)

 

E301    832

E307    145

E302    832    

E308    145      

E303    832

E309    75         

E304    552

E310     75

E305     552

 

 

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 inserted into the pipeline

        • A manually throttled valve

 

The method chosen is based on the degree of control desired. In this case the second solution is chosen.

For a balancing orifice, the flow meter sizing feature of PIPE-FLO is used. The module calculates the size of a balancing orifice needed to provide the required pressure drop. Then the orifice is inserted into the pipeline as a fixed K value. Once this is done the FCV control is removed and PIPE-FLO calculates the flow rate through the pipeline with the additional losses caused by the balancing orifice.