17 replies to this topic

[namvuvu29]

Hi Sirs/ Madam

 

Currently, i'm designing for a only gas regulator system including:

- Two Control Valves in series for regulating pressure from 100-137 barg (ASME 900#) down 85.7 barg (600#). First valve is set Fail Close and second one is Fail Open

- I install the PSV at downstream of CV to protect system and set pressure @ 93 barg

My question is: When Valve is Failure, How  to determine Peak rate of Valve and which value in this case?

 

i study myself and find out the table leakage valve (link below), but i'm really upset to select what proper is here

http://www.flowcontr...-leakage-rates/

 

I'm looking forward from your advice!

 

 

 

 

[Bobby Strain]

A sketch or drawing is necessary for us to provide help. And is this in design phase?

 

Bobby

[Pilesar]

I would calculate the peak rate with both valves 100% open. You must protect the system for this scenario. Sometimes valves are opened all the way on purpose and not just because of valve failure. Your equipment must be protected from overpressure for all reasonable scenarios. If these are control valves, the control system might open both valves because of misleading instruments.

[namvuvu29]

Hi Bobby & Pilesar

Thank you very much for your help

i'm based on your comments to refer some standards and select Class IV tightness leakage to calculate PSV at downstream system

So sorry because i dont know how to load photo to clarity my issue.

 

Warm regards!

[Bobby Strain]

I think you missed Pilesar's point.

[namvuvu29]

Hi Bobby,

As my understanding. We are have a Pressure Regulator System includes two series Control Valves installation for regulating gas from:

110-137 barg -> to 86 barg

Q range: 1.01 - 18.0 MMSCMD

My mention above is set for first Control vale FC/ Second one is FO. and they are sized of 10 inch

If two CVs are FO meaning full gas flow of 18 MMSCMD goes to down stream & PSV is very bigger than PSV sets for leakage case

I think Vendor to confirm about this guarantee. Right?

 

By the way, if i have a Fuel Gas system including two CV1 (set @ 8.5barg); CV2 (Set @8barg) route gas to Scrubber.

If Cv Failure Open. PSV will open to protect Scrubber.  I would like to know how much gas flow for relief case. 

is gas flow assumed from one CV or two CVs. Pls help me clarify this issue

 

Thanks & Regards

[Alexsandres]

Hi Bobby,

As my understanding. We are have a Pressure Regulator System includes two series Control Valves installation for regulating gas from:

110-137 barg -> to 86 barg

Q range: 1.01 - 18.0 MMSCMD

My mention above is set for first Control vale FC/ Second one is FO. and they are sized of 10 inch

If two CVs are FO meaning full gas flow of 18 MMSCMD goes to down stream & PSV is very bigger than PSV sets for leakage case

I think Vendor to confirm about this guarantee. Right?

 

By the way, if i have a Fuel Gas system including two CV1 (set @ 8.5barg); CV2 (Set @8barg) route gas to Scrubber.

If Cv Failure Open. PSV will open to protect Scrubber.  I would like to know how much gas flow for relief case. 

is gas flow assumed from one CV or two CVs. Pls help me clarify this issue

 

Thanks & Regards

 

Namvuvu,

 

You need to understand your control how is it working. Are these control valve could opened simultaneously in case or controller failure, or only one valve could open. This is also pointed out by Pilesar

"If these are control valves, the control system might open both valves because of misleading instruments."

 

Two valve could be opened together, 100% open, and lead to over pressure on scrubber. Your flow rate could go much higher than control valve capacity. Flow rate could go higher than 130-160% of control valve capacity (depend on rated CV value of control valve). In this case, if two control valve fail together, potentially relieving rate could be 260-320% of one control valve capacity. And this is not good. 

 

Pilesar, once again pointed out this concern. And this is true. It can happen if it is not designed properly.

 

 

Your equipment must be protected from overpressure for all reasonable scenarios.

 

I or Pilesar could not say whether PSV should be sized for two control valve or one control valve because we do not know your control system work. Sketches would be better to describe how your control work, or PID drawing would be better.

 

Alex

[namvuvu29]

Hi Alexsandres; Pilesar

The attached files are shown control valves configuration. 

Hopefully your supports to clarify my issues

 

Regards,

Namvuvu

Attached Files

•  30-1.pdf   90.38KB   31 downloads
•  30-2.pdf   97.27KB   21 downloads

[fallah]

I would calculate the peak rate with both valves 100% open. You must protect the system for this scenario. Sometimes valves are opened all the way on purpose and not just because of valve failure. Your equipment must be protected from overpressure for all reasonable scenarios. If these are control valves, the control system might open both valves because of misleading instruments.

 

If the PV's have dedicated control loop and one of them is FO while another one is FC; then to take two PV's simultaneously in full open position as worst case is going to double jeopardy conditions which cannot be acceptable, because in instrument air failure case only one PV would be in full open position and simultaneous loops malfunctions leading to two PV's in full open position is rarely occurred.

 

Hence, appears taking each PV in full open position and another one in normal position and select the case with higher passing flow among these two cases as relevant PSV sizing basis could be more reasonable.

[Bobby Strain]

It appears to me that you should enlist a professional to help if you follow Naser's advice. The upstream valve will be fully open in normal operation. I always premise that all valves remain in their normal operating position when another valve fails. Be careful. And investigate the use of HIPPS to protect the system against overpressure. If I designed your system, I would specify that both valves fail closed, too.

 

Bobby

Edited by Bobby Strain, 25 August 2017 - 09:37 AM.

[namvuvu29]

 

I would calculate the peak rate with both valves 100% open. You must protect the system for this scenario. Sometimes valves are opened all the way on purpose and not just because of valve failure. Your equipment must be protected from overpressure for all reasonable scenarios. If these are control valves, the control system might open both valves because of misleading instruments.

 

If the PV's have dedicated control loop and one of them is FO while another one is FC; then to take two PV's simultaneously in full open position as worst case is going to double jeopardy conditions which cannot be acceptable, because in instrument air failure case only one PV would be in full open position and simultaneous loops malfunctions leading to two PV's in full open position is rarely occurred.

 

Hence, appears taking each PV in full open position and another one in normal position and select the case with higher passing flow among these two cases as relevant PSV sizing basis could be more reasonable.

 

Mr. Fallah & Mr. Bobby,

Thanks for your helps,

i have conducted double check with Instrument team and specified the notes for control loop function in specification & DTSheet to make sure these valve operating abide by requirement. Besides, the later issue will discuss in working with Valve Vendor next stage.

 

Regards,

Namvuvu

Edited by namvuvu29, 27 August 2017 - 08:28 PM.

[Bobby Strain]

So, what did you conclude?

 

Bobby

[Hiren Parmar]

 

I would calculate the peak rate with both valves 100% open. You must protect the system for this scenario. Sometimes valves are opened all the way on purpose and not just because of valve failure. Your equipment must be protected from overpressure for all reasonable scenarios. If these are control valves, the control system might open both valves because of misleading instruments.

 

If the PV's have dedicated control loop and one of them is FO while another one is FC; then to take two PV's simultaneously in full open position as worst case is going to double jeopardy conditions which cannot be acceptable, because in instrument air failure case only one PV would be in full open position and simultaneous loops malfunctions leading to two PV's in full open position is rarely occurred.

 

Hence, appears taking each PV in full open position and another one in normal position and select the case with higher passing flow among these two cases as relevant PSV sizing basis could be more reasonable.

 

There may be case when FO valve is 100% and FC is struck open/malfunction open . Why we are not considering malfuction of control valve irrespective of the fail safe condition. When Control valve achieves fail safe condition in case of air failure that means that control valve is working perfectly well but there may be chances when it will malfunction/struck Close/Open irrespective of FO/FC.

[fallah]

 

There may be case when FO valve is 100% and FC is struck open/malfunction open . Why we are not considering malfuction of control valve irrespective of the fail safe condition. When Control valve achieves fail safe condition in case of air failure that means that control valve is working perfectly well but there may be chances when it will malfunction/struck Close/Open irrespective of FO/FC.

 

 

 

For one valve is ok; but two or more valves are rarely considered to be stuck full open simultaneously...
 

[namvuvu29]

So, what did you conclude?

 

Bobby

Mr Bobby,

We decided to keep configuration one PCV (FO), another one (FC) and just sizing PSV for valve leakage only. It has to require in DTSheet for instrument.

[Pilesar]

When this forum is consulted by others as a reference in the future, I hope they realize that just because someone asks for advice and then decides on a course of action does not mean that the correct conclusion was reached. Determining equipment overpressure protection is a grave responsibility and this example should not be used as a go-by.

[Bobby Strain]

And the conclusion is in error. So you take full responsibility. We don't know what you mean by leakage rate as you never told us. But you won't hear more from me on this subject.

 

Bobby

[namvuvu29]

And the conclusion is in error. So you take full responsibility. We don't know what you mean by leakage rate as you never told us. But you won't hear more from me on this subject.

 

Bobby

Bobby & Pilesar,

i dont know why you say me like that.

As far i mentioned in above content:

i have two issues when i need to help:

1. Determine which case for PSV sizing (i mean PCV failure open or PCV failure leakage) when installation a series two PCV (one set FO, another one set FC). According to discuss i appreciate all your help especially Fallah's experience. I double check with Instrument team and they take notes in attachment file. Meanwhile PCV leakage for Class IV in case of failure. That's why i will use it for sizing PSV with flowrate Cv_select = 0.01*Cv rate of PCV

2. For two PCV install parallel (i.e IG/FG system). I asked about flowrate using for PSV sizing mount on Scrubber. and advice receives is Q using for PSV takes in account for one PCV FO + one PCV normal operating

 

i dont know how to conclusion is in error. Help me clarify your opinion?

Attached Files

•  30-3..pdf   484.96KB   26 downloads

Edited by namvuvu29, 29 August 2017 - 05:00 AM.