Control valve by pass size 1

[Process879]

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)?

 

[dcasto]

put is the same size by pass as the control valve. By size I mean Cv value. A 10" valve may have a 8.5" seat size may have a Cv of 800, and a 8" manual globe valve may have a 750 Cv (both at 100%) opem. I'd put in the 8" globe valve by-pass.

Hit the books from your vendor and start picking.

 

[insult2injury]

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.

I2I

 

[SeanB]

Our practice is that the bypass valve should have a Cv equal to 100-200% of the Cv of the control valve.

 

[insult2injury]

If you size the bypass to provide more flow than the control valve, be sure to size the relief system appropriately.

I2I

 

[CJKruger]

Process879, your application is unusual because the pipe is oversized (control valve much smaller than pipe 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).

 

[djack77494]

It's so important that I'm going to restate I2I's comment. Please size your bypass valve to have a Cv less than (no bigger than) the control valve Cv. Very often, relief valves get sized for fail open control valves. There is an implicit assumption that the Control Valve's Cv sets the maximum flowrate that can occur in such a fail mode. That assumption is defeated if the bypass valve can pass more flow than the control valve.

 

[Process879]

my bypass valve size is 10". then what could be my pypass line size...

 

[sheiko]

Why not the same size than the pipe 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."

 

[insult2injury]

if the bypass is line-size, it typically is capable of passing much higher flows than are desirable or safe.

I2I

 

[hacksaw]

what is your designated purpose for the bypass valve?

 

[MJCronin]

To all of those above who have never laid out piping systems.....

Section 10.2.4

(They are called "bypass manifolds" in this document)

http://www.pip.org/downloads/Sample-PCECV001.pdf

-MJC

 

[sheiko]

insult2injury,

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."

 

[insult2injury]

No, it's because 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.

I2I

 

[sheiko]

To sum up:

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."

 

[jamesbanda]

On bypass valves.. Ive seen many company us these succcessfully. But.. please ensure you over your self in any changes by

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.

 

[hacksaw]

rules of thumb are only for specific processes, e.g. refinery or pipeline, etc., you need to define the purpose for your valve, then size it.

for what its worth

control by-pass,
maintence by-pass,
start-up by-pass

and various combinations of these...

 

[djack77494]

sheiko,
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

 

[proinwv2]

Reading this thread brought the following question to my mind. What do you do then to provide some assurance that both valves are not flowing simultaneously (with the associated problems of that scenario)?

Paul

http://www.ostand.com

 

[Prajna21]

Why is a by-pass needed around a steam PRV? If the PRV is off-line, and the by pass is used, doesn't this let HP steam through where only MP or LP steam was meant to go?

That said, if I have 4" PRV and a 3" PRV in parallel, what size should the by pass be?
Thank you.