Relief Valve - Understanding Allowable Inlet & Discharge Line Pressure Drop 2

[chemks2012]

Hello engineers!

As I understand, to avoid chattering, there is a limit of allowable pressure drop for relief valve inlet and discharge lines. My queries & understanding is summarised as below.

1) I believe, inlet pressure drop is always 3% of set pressure which is independent of overpressure. Please correct me if I am wrong.

2) However, discharge line pressure drop depends on overpressure i.e. generally we allow 21% overpressure fire case and other process upset conditions we allow 10% overpressure.

• I believe, if I design a valve for fire case allowing 21% overpressure, am I correct that the allowable overpressure at the valve discharge line is 21%?
• And therefore, if the overpressure is 10%, am I correct that the valve discharge line allowable pressure drop is 10%?

3) Also, I have one query for one specific example

If the design pressure of the vessel is 10barg and the relief valve is installed independently at set pressure of 5barg [please note that the set pressure of valve is quite less compared to the design pressure]. I am designing it for the external fire and process upset conditions. Please advise what overpressure should I allow for external fire case and process upset case?

Thank you very much.
KS

 

[bljnv]

I think you should get clarity on the definitions.
However I give below my comments point wise for your immediate consumption.
(1) Correct. It is recommended to keep the inlet pipe size equal to or greater than inlet size of the valve.
(2) I think here you are confusing between over pressure and back pressure (discharge line pressure)
Overpressure : Pressure increase over set pressure allowed by codes so that the valve attains its full lift and delivers rated capacity. Overpressure can be maximum equal to the accumulation pressure of vessel being protected. One recommendation is discharge line pipe size should be equal or greater than valve outlet size.
(3) Set pressure should be 10 % above the normal operating/working pressure in the vessel being protected.
Regards

 

[don1980]

Here are my responses to your questions:
1. Correct. Inlet losses are limited to 3% of the set pressure, regardless of the overpressure or accumulation.
2. Correct. Built-up backpressure is limited to 10% of set pressure or the allowable overpressure, whichever is greater.In other words, for a fire case, the built-up backpressure can be 21%. Alternatively, one can reduce the set pressure below MAWP to allow more overpressure. In such cases the % built-up BP can be as high as the % overpressure, but not higher than that. (Ref: API 520 Pt 1, 8th ed. 5.3.3.1.3).
3. There are limits for accumulation but not overpressure. Overpressure isn't directly limited by any code. Instead, it's effectively limited the rules which apply to accumulation.In the case you describe, the peak relieving pressure is 11 barg for process scenarios and 12.1 barg for fire scenarios.You're free to reduce the set pressure (below MAWP) to any value you wish, resulting in high overpressure values, so long as the accumulation doesn't exceed the code limits mentioned above.

 

[chemks2012]

Thanks bljnv

Thanks don1980

1. I assume that 3% rule does not apply to a bursting disc inlet if it is installed independently. Please comment.
2. In the case, if I have constant or variable superimposed back pressure, I assume superimposed back pressure is not included in that 10% or 21% allowable pressure. I meant 10% or 21% should be compared to only built up back pressure only. Is that correct?
3. So, in this case [i.e. vessel design pressure of 10barg and set pressure of 5barg], can I keep 10% overpressure [i.e. relieving pressure of 5.5barg] for process upset and 21% [relieving pressure of 6.05barg] for the fire case? I believe, if I am keeping 10% or 21%, I am oversizing the valve or bursting disc?

Thanking you in anticipation.
KS

 

[don1980]

Responses:
1) Correct. The 3% rule doesn't apply to rupture disks. It's just for pop-acting valves.
2) Correct. Superimposed back pressure is fundamentally different, and not part of this discussion. The outlet loss limits (10% and 21%) are just for built-up backpressure.
3) Yes, you're free to design the installation for 10% and 21% overpressure, even though this particular case allows higher values. And, yes this does result in the relief device and tailpipe being larger than it has to be. To me, that's a waste. I wouldn't limit the overpressure to values that are less than the allowable limits, unless there's a specific reason for doing so (e.g. limiting the relief temperature to prevent a undesired chemical reaction, etc.).

 

[chemks2012]

don1980

Thank you very much.

You are a star.

 

[SmilerFDL]

I would like to pick up don's answer regarding overpressure limitation (or lack there of). As don already pointed out, the MAWP in the above example would allow relieve pressure of 11 barg (process scenario). With a set pressure of 5 barg it would mean allowable overpressure of 120%. Subsequently the allowable built-up backpressure maybe also 120% (answer 2 of post #3). Don't you think that's something wrong, if the built-up backpressure exceeds the spring force of the valve?

CU, Smiler

 

[Latexman]

When the PSV is open, the increased area of the huddling chamber makes the force balance work such that "% built-up BP can be as high as the % overpressure, but not higher than that" is fine. Remember the set pressure (opening force) is about 7% higher than the reseating pressure (closing force) in gas/vapor service.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[SmilerFDL]

Hello Latexman,
thanks for your comment, I agree on the basic steady-state force balance, but I think we have to consider the system dynamics. Assume the valve is really designed to open at 5 barg vs. atmospheric and reach the desired relief rate at the code allowed limit of 11 barg inlet (120% overpressure) and 6 barg built-up BP (120% built-up).
In the moment the PSV fully opens on (latest) 5.5 barg inlet pressure vs. the atmospheric BP, the developing built-up BP is way higher then the actual 10% overpressure and will immediately close the valve again. Built-up BP vanishes and the valve opens again, in short: the valve starts to chatter. From my understanding the positive opening force is only kept, if the pressure rise in the protected vessel is faster then the development of built-up backpressure (which is sonic).

CU, Smiler

 

[chemks2012]

SmilerFDL

I seem to agree with you and I have the same queries.
Not sure which one is correct out of two cases below

case1) If we design a valve with 10% overpressure[i.e. 5.5 relieving pressure]then built up back pressure allwable could be up to 10% [conventional relief valve , process upset]
case2) If we design a valve with 10% overpressure[i.e. 5.5 relieving pressure]then built up back pressure allwable could be up to 120% [conventional relief valve , process upset]

Hope we both are playing the same tune?

 

[Latexman]

Hello SmilerFDL,

I agree, the dynamics should be considered. The size of discharge piping must be checked to ensure that piping discharge pressure drop under any expected condition will not adversely affect set pressure or capacity (due to backpressure, chattering, fluttering, etc.).

Furthermore, this also means that discharge piping pressure drop should be checked for each fluid type possible. This could mean up to three independent pressure drop checks are needed. The controlling gas/vapor case, the controlling two phase case, and the controlling liquid case, if they are credible.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.