Microsoft Word Course Control valves R. doc

STEP # 2:Calculate the valve coefficient (Cv)

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Control Valves Basics - Sizing & Selection
FMD3x00 98 DB Initialize (5), configuration-and-evaluation-software-pi-9045582-en-gb, configuration-and-evaluation-software-pi-9045582-en-gb, Document, colour present

STEP 3: Preliminary valve selection

STEP # 2:
Calculate the valve coefficient (Cv)
The next step is to figure out what size the valve will need to be. Key elements to consider in valve sizing are the pressure-flowrate relationship and the flow characteristics to ensure an appropriate size for the desired valve. For our system,
Cv = 150 (1/15)
½
= 38.7 = 39
STEP # 3:
Preliminary valve selection
The Cv value should be used as a guide in the valve selection, but before trying to match a valve with the calculated Cv value, some other considerations are
• Decide what type of valve will be used fora given process characteristic
• Never use a valve that is less than half the pipe size
• Avoid using the lower 10% and upper 20% of the valve stroke. The valve is much easier to control in the 10 to 80% stroke range. In our example we are using an equal percentage, globe valve. Obtain the valve chart for this type from the manufacturer typical shown below

For our case, it appears that the 2½ inch valve will work well for our Cv value at about 80 to
85% of the stroke range. Notice that we're not trying to squeeze our Cv into the 2” valve which would need to beat stroke to handle our maximum flow. If this valve was used, two consequences would be experienced the pressure drop would be a little higher than 15 psi at our design (max) flow, and the valve would be difficult to control at maximum flow. Also, there would be no room for error with this valve, but the valve we've chosen will allow for flow surges beyond the 150 gpm range with severe headaches So it looks like a 2½” valve will suffice, but before we make a decision, there are still some characteristics to consider as indicated instep.

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