Control valve by pass size
Control valve by pass size
(OP)
Dear all,
i'm having 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".
Can any one help me in selecting bypass line size and bypass valve size (gate)?
i'm having 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".
Can any one help me in selecting bypass line size and bypass valve size (gate)?





RE: Control valve by pass size
Hit the books from your vendor and start picking.
RE: Control valve by pass size
I2I
RE: Control valve by pass size
RE: Control valve by pass size
I2I
RE: Control valve by pass size
Normally both the bypass and control valve is 1 to 2 standard sizes smaller than the pipe size. So in this case a bypass valve of 8" or 10" would be reasonable.
Most companies prefer globe valves for bypass valves smaller than roughly 8". For 10" and above, you may have to use a gate valve (have a look at the company std specs).
RE: Control valve by pass size
RE: Control valve by pass size
RE: Control valve by pass size
"We don't believe things because they are true, things are true because we believe them."
"Small people talk about others, average people talk about things, smart people talk about ideas and legends never talk."
RE: Control valve by pass size
I2I
RE: Control valve by pass size
RE: Control valve by pass size
Section 10.2.4
(They are called "bypass manifolds" in this document)
http://www.pip.org/downloads/Sample-PCECV001.pdf
-MJC
RE: Control valve by pass size
You said: "if the bypass is line-size, it typically is capable of passing much higher flows than are desirable or safe. "
Is it because the by-pass line in longer than the portion with the control valve?
"We don't believe things because they are true, things are true because we believe them."
"Small people talk about others, average people talk about things, smart people talk about ideas and legends never talk."
RE: Control valve by pass size
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.
I2I
RE: Control valve by pass size
About the valves:
1/ Both control valve and By-pass valve to be 1 or 2 standard sizes than the main line size.
2/ Cv of by-pass valve to be equal or less than Cv of control valve (both 100% open).
3/ At equal size, the by-pass valve (which is manually operated) has a greater Cv than the control valve
About the lines:
1/ The by-pass line is usually smaller than the main line. If the case, the by-pass valve can be line-size.
Am i right? Other guidelines?
"We don't believe things because they are true, things are true because we believe them."
"Small people talk about others, average people talk about things, smart people talk about ideas and legends never talk."
RE: Control valve by pass size
1. Validate the risk assesment does not rely on the automatic valve for any safety system.
If it does make sure you document any change in a formal safety review.
I've seen operations folks demand bypasses but when you review the safety cases the demand often goes away.
RE: Control valve by pass size
for what its worth
control by-pass,
maintence by-pass,
start-up by-pass
and various combinations of these...
RE: Control valve by pass size
I think you got it right with your 12June08 summary. Quite simply, there is no reason to have a bypass valve (manifold) that can pass even more flow than the normal circuit (which is thru the control valve in this case). Furthermore, an oversized bypass valve can create a hazardous situation that is easily overlooked. Therefore, to keep it simple, do not allow the bypass valve to be larger than the control valve. I actually prefer that the bypass valve be (typically) one size smaller than the control valve. That is because the control valve often has a small Cv for its size relative to most valve types. Not an ironclad rule, but a starting point.
Doug
RE: Control valve by pass size
Paul
www.ostand.com
RE: Control valve by pass size
That said, if I have 4" PRV and a 3" PRV in parallel, what size should the by pass be?
Thank you.