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5

# Too Much Allowable Overpressure!

27 replies to this topic
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### #1 shahidulislam48

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Posted 28 December 2016 - 10:32 AM

Hello,

I have a pipe line of liquid where a relief valve is installed with a set pressure of 3.5 barg. The design pressure of the pipe is 20 barg. If I take overpresssure factor of 10% then the allowable overpressure (%) becomes 528.57%. Is it acceptable??

Calculation: 20 barg x 1.1 =22 barg

allowable overpressure = 22 - 3.5 =18.5

in % = (18.5/3.5)x100%=528.57%

Would be very grateful for getting any suggestion.

Many Thanks

### #2 breizh

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Posted 28 December 2016 - 11:09 AM

hi ,

Consider to review the scenario associated to the design of the relief valve !

Probably good to consider this resource too .

http://www.wermac.or...ure_relief.html

Good luck .

Breizh

Edited by breizh, 28 December 2016 - 11:13 AM.

### #3 fallah

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Posted 28 December 2016 - 02:01 PM

Hi,,

With such high difference between operating and design pressures having such high allowable over pressure isn't so strange.
Let's know your exact query which should be clarified.

### #4 christopherchoa

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Posted 28 December 2016 - 10:05 PM

http://process-eng.b...erpressure.html

### #5 shahidulislam48

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Posted 28 December 2016 - 10:06 PM

Hi,,

With such high difference between operating and design pressures having such high allowable over pressure isn't so strange.
Let's know your exact query which should be clarified.

I am thinking about having so high allowable overpressure due to high difference between set and design pressure, how will vendor think about it?

Will it be still acceptable despite having this high allowable overpressure?

Thanks.

### #6 Bobby Strain

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Posted 28 December 2016 - 11:17 PM

This is a strange situation indeed. Maybe you should look at connected equipment upstream and downstream of the relief valve to ascertain that the relief valve is not to protect something other than the piping from overpressure. Maybe you did not ask the right question. Or convey information on the whole system, which you should always do. It seems to me that if you really need the set pressure of 3.5 bar(g), you also need to specify the required relieving rate. Liquid relief valves are typically sized for 25% overpressure, as I recall. Unless you want a more expensive valve.

Bobby

### #7 fallah

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Posted 29 December 2016 - 02:07 AM

Hi,

Vendor nothing to do with such high difference and just may recommend for set pressure increment which will lead to smaller valve.
Anyway, the situation you described although isn't a normal one could be acceptable.

### #8 mirandomka

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Posted 29 December 2016 - 10:20 AM

Your vendor may come back with a full open overpressure lower than allowable overpressure but as long as it's below your allowable overpressure it's acceptable.

What you need to check is the PSV capacity (with vendor Kd) at relieving condition (MAWP+allowed accumulation by code), is greater than your required relief load.

Edited by mirandomka, 29 December 2016 - 10:20 AM.

### #9 Pilesar

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Posted 29 December 2016 - 11:03 AM

The term 'allowable overpressure' properly refers to the relief valve set point. An allowable overpressure of 10% means that the pressure upstream of the PSV is allowed to exceed the set point by 10% during the relieving scenario and still be properly configured. The design pressure of your pipe is not considered when calculating allowable overpressure. The design pressure of your pipe would only be considered when determining the PSV set point.

Calculation: 3.5 barg x 0.1 =0.35 barg

in % = (0.35/3.5)x100%=10%

allowable overpressure = 10% of set pressure

Maximum pressure allowed upstream of the PSV would be 3.85 barg

Edited by Pilesar, 29 December 2016 - 11:11 AM.

### #10 colt16

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Posted 29 December 2016 - 11:46 AM

If the design pressure is 20barg, why is the relief valve set @ 3.5barg? This is the first item to check.

If you have a relief valve set @ 3.5 barg, then a question to ask is why didn't we simply use 3.5barg as the design pressure instead of 20barg? The cost of the material will be so much cheaper.

Usually however the set pressure and design pressure is the same. And when this is the case, the 10% of allowable overpressure for the short period of relieving is considered short term and hence the word "allowable".

It is very puzzling why the relief valve set pressure =/= design pressure.

### #11 mirandomka

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Posted 29 December 2016 - 12:24 PM

There can be many possible reasons that set pressure is lower than design.

The most likely reason is the disposal system has excessive back pressure that cannot be mitigated due to piping configuration or other reasons ie static.

API520 Figure 15 shows the relationship between allowed accumulated pressure, operating pressure and PSV relieving pressure and set pressure. Set pressure can be between max expected operating to MAWP.

### #12 mirandomka

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Posted 29 December 2016 - 12:26 PM

The problem with lower design pressure is you may need a bigger PSV to cover blocked outlet case or control valve failure case.

### #13 fallah

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Posted 29 December 2016 - 01:47 PM

The term 'allowable overpressure' properly refers to the relief valve set point. An allowable overpressure of 10% means that the pressure upstream of the PSV is allowed to exceed the set point by 10% during the relieving scenario and still be properly configured. The design pressure of your pipe is not considered when calculating allowable overpressure. The design pressure of your pipe would only be considered when determining the PSV set point.

Calculation: 3.5 barg x 0.1 =0.35 barg

in % = (0.35/3.5)x100%=10%

allowable overpressure = 10% of set pressure

Maximum pressure allowed upstream of the PSV would be 3.85 barg

### #14 fallah

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Posted 29 December 2016 - 02:03 PM

term 'allowable overpressure' properly refers to the relief valve set point. An allowable overpressure of 10% means that the pressure upstream of the PSV is allowed to exceed the set point by 10% during the relieving scenario and still be properly configured. The design pressure of your pipe is not considered when calculating allowable overpressure. The design pressure of your pipe would only be considered when determining the PSV set point.

Calculation: 3.5 barg x 0.1 =0.35 barg

in % = (0.35/3.5)x100%=10%

allowable overpressure = 10% of set pressure

Maximum pressure allowed upstream of the PSV would be 3.85 barg

Allowable over pressure is any value equal to or lower than following value:

1.1*MAWP (or 1.16*MAWP, 1.21*MAWP)-Set Pressure=Maximum Allowable Over Pressure

### #15 fallah

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Posted 29 December 2016 - 02:03 PM

term 'allowable overpressure' properly refers to the relief valve set point. An allowable overpressure of 10% means that the pressure upstream of the PSV is allowed to exceed the set point by 10% during the relieving scenario and still be properly configured. The design pressure of your pipe is not considered when calculating allowable overpressure. The design pressure of your pipe would only be considered when determining the PSV set point.

Calculation: 3.5 barg x 0.1 =0.35 barg

in % = (0.35/3.5)x100%=10%

allowable overpressure = 10% of set pressure

Maximum pressure allowed upstream of the PSV would be 3.85 barg

Allowable over pressure is any value equal to or lower than following value:

1.1*MAWP (or 1.16*MAWP, 1.21*MAWP)-Set Pressure=Maximum Allowable Over Pressure

### #16 fallah

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Posted 29 December 2016 - 02:04 PM

term 'allowable overpressure' properly refers to the relief valve set point. An allowable overpressure of 10% means that the pressure upstream of the PSV is allowed to exceed the set point by 10% during the relieving scenario and still be properly configured. The design pressure of your pipe is not considered when calculating allowable overpressure. The design pressure of your pipe would only be considered when determining the PSV set point.

Calculation: 3.5 barg x 0.1 =0.35 barg

in % = (0.35/3.5)x100%=10%

allowable overpressure = 10% of set pressure

Maximum pressure allowed upstream of the PSV would be 3.85 barg

Allowable over pressure is any value equal to or lower than following value:

1.1*MAWP (or 1.16*MAWP, 1.21*MAWP)-Set Pressure=Maximum Allowable Over Pressure

### #17 shahidulislam48

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Posted 29 December 2016 - 02:51 PM

Thanks everyone for sharing your valuable knowledge.

That helped me a lot.

Many Thanks.

### #18 Bobby Strain

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Posted 29 December 2016 - 07:42 PM

So, what did you conclude from all these guesses based on almost no information?

Bobby

### #19 shahidulislam48

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Posted 30 December 2016 - 03:14 AM

Dear Bobby,

Your first reply was unclear to me. I just have mentioned my system as it appears. And the data I have calculated, according to API 521, posed some confusion to me as I am new in this field. That is why I needed someone's help. I just have wanted to is there any wrong approach in my calculation or is it okay.

On the other hand, "Liquid relief valves are typically sized for 25% overpressure" is only used for PRVs Not Requiring Capacity Certification.

Regards,

shahidulislam48

### #20 Pan Nainggolan

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Posted 05 January 2017 - 03:14 AM

Basically, you have to understand the function of psv itself. Simply, psv has function to protect your boundary system from any potential or possible overpressure scenario.

if your protected boundary system has higher design pressure than your potential overpressure system (maximum pressure that could be happened, in this case describe by your psv set pressure) then your psv become useless, that psv protect nothing in my perspective.

Rgds,

-PN-

### #21 shahidulislam48

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Posted 05 January 2017 - 05:49 AM

Hi PN,

Respective PSV datasheet provides that sort of data in this case.

And I have no choice other than this to look for a solution.

Regards

Shahidul Islam

### #22 mirandomka

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Posted 05 January 2017 - 02:18 PM

PSV is not useless. its still protecting the system from pressure.

It's only useless when your sp is above MAWP.

Basically, you have to understand the function of psv itself. Simply, psv has function to protect your boundary system from any potential or possible overpressure scenario.

if your protected boundary system has higher design pressure than your potential overpressure system (maximum pressure that could be happened, in this case describe by your psv set pressure) then your psv become useless, that psv protect nothing in my perspective.

Rgds,

-PN-

### #23 Pan Nainggolan

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Posted 08 January 2017 - 09:17 AM

PSV is not useless. its still protecting the system from pressure.

It's only useless when your sp is above MAWP.

Basically, you have to understand the function of psv itself. Simply, psv has function to protect your boundary system from any potential or possible overpressure scenario.

if your protected boundary system has higher design pressure than your potential overpressure system (maximum pressure that could be happened, in this case describe by your psv set pressure) then your psv become useless, that psv protect nothing in my perspective.

Rgds,

-PN-

Well, interesting. I feel you've lost my point.

ok, let's say overpressure happened. As per thread, pressure of system raise up to psv's set pressure 3.5 barg and assume in the same time psv fail to open because of mechanical failure. is it your protected system still safe?. if it is yes, so what is protected or guarded by your psv?

Rgds,

-PN-

### #24 mirandomka

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Posted 08 January 2017 - 05:27 PM

PSV is designed for protecting overpressure, of course if the PSV is failed, the system won't be protected......

But that doesn't mean your PSV is not needed. In deed it's code mandated for pressure equipment per ASME VIII

I don't understand your idea at all. If your adjacent systems have higher pressure than your PSV protected system, your PSV has to be sized adequately to relief the pressure and keep the system under MAWP+allowed accumulation, if the outlet is blocked,control valve failed or check valve failed, that's the whole point of having a PSV...

### #25 mirandomka

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Posted 08 January 2017 - 05:35 PM

And in this case, his PSV protected system's MAWP is much higher than the PSV set point, so the PSV opens early. The PSV will work fine for overpressure protection purpose.