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Which Valve First
- shrivallabha
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13 years 5 months ago - 13 years 5 months ago #5571 by shrivallabha
Replied by shrivallabha on topic Re: Which Valve First
I am just trying to guess the possibility here...
Leakage is a possibility and if it is the case then:
1. Valve leaking the least should come first so that there is very little leakage to the next one.
2. Or someone may say the exact opposite is better i.e. the valve with very little leakage should be second so as not to let anything pass by.
I do not know which condition is better: 1 or 2.
If we just look at the purpose of bypass then it is to control or regulate the flow when the CV is under maintenance. Also we know there is a definite pressure drop across each valve. So when a single globe valve is not capable of achieving the stipulated pressure drop then a gate valve is introduced. In such case, gate valve will be first to reduce the pressure at inlet of globe. As I was typing this I remembered:
1. Sometimes you will see Restriction Orifices as well on the bypass. They probably serve the same purpose.
2. To regulate the flow, generally the bypass line is a pipe size smaller than the main.
So someone may also would say why not even smaller size which sounds bright but process engineers keep verifying the fluid velocities so reducing the size of bypass may increase the velocity to such an extent that erosion may become a possibility. So the size can not be reduced drastically.
Hth,
Leakage is a possibility and if it is the case then:
1. Valve leaking the least should come first so that there is very little leakage to the next one.
2. Or someone may say the exact opposite is better i.e. the valve with very little leakage should be second so as not to let anything pass by.
I do not know which condition is better: 1 or 2.
If we just look at the purpose of bypass then it is to control or regulate the flow when the CV is under maintenance. Also we know there is a definite pressure drop across each valve. So when a single globe valve is not capable of achieving the stipulated pressure drop then a gate valve is introduced. In such case, gate valve will be first to reduce the pressure at inlet of globe. As I was typing this I remembered:
1. Sometimes you will see Restriction Orifices as well on the bypass. They probably serve the same purpose.
2. To regulate the flow, generally the bypass line is a pipe size smaller than the main.
So someone may also would say why not even smaller size which sounds bright but process engineers keep verifying the fluid velocities so reducing the size of bypass may increase the velocity to such an extent that erosion may become a possibility. So the size can not be reduced drastically.
Hth,
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- Jop
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13 years 5 months ago - 13 years 5 months ago #5572 by Jop
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Replied by Jop on topic Re: Which Valve First
Please Stop all this guessing and justifying something you do not know the background for.
It is NOT the role of a Piping Designer to do this. It is your role to find out WHY there are two valves in a CV bypass when only one is normal.
When you see something different on a P&ID you need to pin down the Process Engineer and get a real valid reason for doing "it" what ever "it" is.
If the Process Engineer does not have a valid reason for doing something then every one with authority (the Piping Department Manager and the Project Manager) should get into the picture and demand answers.
This Process Engineer may have seen a two valve bypass somewhere that was a mistake and he or she thought "Wow! That looks like a good idea".
Someone needs to do the math on this and be sure the Client is willing to pay for it. There are already three valves associated with a CV station (not counting the control valve it self). These valves all have a cost. Class 150 Gate and Globe Valves are not all that expensive. But, if the systems with an extra valve are high temperature, high pressure with a toxic commodity requiring Bellows Seal 9 Chrome valves then you are talking big bucks ($). The increase of that one valve (from three to four) is an increase in cost of 33% per each CV station.
Stop sitting there speculating about which valve goes where and go down the hall to the Process engineer and demand answers. All the answers.
It is NOT the role of a Piping Designer to do this. It is your role to find out WHY there are two valves in a CV bypass when only one is normal.
When you see something different on a P&ID you need to pin down the Process Engineer and get a real valid reason for doing "it" what ever "it" is.
If the Process Engineer does not have a valid reason for doing something then every one with authority (the Piping Department Manager and the Project Manager) should get into the picture and demand answers.
This Process Engineer may have seen a two valve bypass somewhere that was a mistake and he or she thought "Wow! That looks like a good idea".
Someone needs to do the math on this and be sure the Client is willing to pay for it. There are already three valves associated with a CV station (not counting the control valve it self). These valves all have a cost. Class 150 Gate and Globe Valves are not all that expensive. But, if the systems with an extra valve are high temperature, high pressure with a toxic commodity requiring Bellows Seal 9 Chrome valves then you are talking big bucks ($). The increase of that one valve (from three to four) is an increase in cost of 33% per each CV station.
Stop sitting there speculating about which valve goes where and go down the hall to the Process engineer and demand answers. All the answers.
Do it once and Do it Right
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- kumaraba
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13 years 5 months ago - 13 years 5 months ago #5636 by kumaraba
Replied by kumaraba on topic Re: Which Valve First
I agree with JOP. We need to check up with the process Engineer to get a valid reason. However I want to pin down one thing here. There were assumptions that because of the valve leakage another valve (gate or ball) has been called off. To reply this I've an other question "Won't the globe valve one flange will not have leakage even though the ball valve or globe valve is shut off because of the other potion which is in contact with the medium?".
So leakage doesn't matter at all. There is something else which should be questioned?
So leakage doesn't matter at all. There is something else which should be questioned?
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- shrivallabha
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13 years 5 months ago - 13 years 5 months ago #5724 by shrivallabha
Replied by shrivallabha on topic Re: Which Valve First
ashokaa,
You say that the P&ID shows two valves in the CV bypass.
Ok, which valve is shown first (in flow direction)?
Now since the Process Engineer is responsible for developing the P&ID, please go ask him (or her) the following]
This was the very reason I tried to guess...to think of a probable answer (from the academic perspective). What you have said is absolutely perfect as well as pragmatic and shall be followed when working on a project. However, can we not discuss this from academic angle and think of some reasons?
And in case where the process engineer insists on two valves, as a piper we are bound to follow the P & ID.
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- Peter Pyper
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13 years 5 months ago - 13 years 5 months ago #5633 by Peter Pyper
Replied by Peter Pyper on topic Re: Which Valve First
We have this situation in the project I am currently working on. We have a 2" bypass around two 36" HP steam valves. These are warm ups though and the first valve is a gate valve and the second a Y pattern Globe. There is also a spec blind between the 2" valves and they are set up as double block and bleed with the bleed between the gate and the spec blind. These are welded valves and the spec blind c/w RTJ 900# flanges just upstream of the globe valve. In this case the block valve is upstream of the throttling valve. The thinking is that the stream is more accurate from the globe and helps control flow shock in controlling water hammer from the amount of condensate created.
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- Jop
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13 years 4 months ago - 13 years 4 months ago #5991 by Jop
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Replied by Jop on topic Re: Which Valve First
ASHOKAA,
Did you go and ask the process Engineer why there are two valves in the Control Valve Manifold Bypass?
If you did not ask, please tell us why you did not ask.
If you did ask, please tell us what he or she said.
We are all waiting.
Did you go and ask the process Engineer why there are two valves in the Control Valve Manifold Bypass?
If you did not ask, please tell us why you did not ask.
If you did ask, please tell us what he or she said.
We are all waiting.
Do it once and Do it Right
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