A control valve which handles steam .the up stream pressure is 4 kg/cm2(g) and down stream pressure is .9 kg/cm2(g).The line sizes are :Upstream 20 "
Down stream 24"Control valve size : 10".
Selecting bypass line size and bypass gate valve size.I typically prefer 75%-85% bypass valves; however, there are cases which require the 100% bypasses.  Either way, I wouldn't use a gate valve. size the bypass to provide more flow than the control valve, be sure to size the relief system appropriately. the bypass is line-size, it typically is capable of passing much higher flows than are desirable or safe. the bypass valve has a greater Cv value and is manually operated.  For many applications (and as is the case in the above example), the control valve is not line-size either.  If a smaller line is used for the bypass piping, it may be appropriate to use a line-size valve.  Also, for off-of service, a line-size bypass may be appropriate.
Even when the control valve and the bypass valve have an equal body size, the bypass will tend to have a higher Cv value since the control valve typically has some characterizing trim.

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