3 replies to this topic

[sapna]

Hi All,

Thank for such a wonderful website. I really appreciate the efforts of each one of you.

I am new member and fairly new in process industry at Bangalore, India.
I have a problem -

While working on relief valve sizing and reading API code, I got a doubt

1. Design pressure is higher than operating pressure considering abnormal scenarios. 10% or 25 psig whichever is higher than normal operating pressure
of a vessel along with design temperature are used to determine the minimum permissible thickness or physical characteristic of each vessel component as determined by the pressure vessel design rules. The design pressure is selected by the user to provide a suitable margin above the most severe pressure expected during normal operation at a coincident temperature. It is the pressure specified on the purchase order. This pressure may be used in place of the maximum allowable working pressure (MAWP) in all cases where the MAWP has not been established. The design pressure is equal to or less than the MAWP.

2. Maximum Allowable Working Pressure (MAWP) = is the maximum gauge pressure permissible at the top of a completed vessel in its normal operating position at the designated coincident temperature specified for that pressure. The pressure is the least of the values for the internal or external pressure as determined by the vessel design rules for each element of the vessel using actual nominal thickness, exclusive of additional metal thickness allowed for corrosion and loadings other than pressure. The MAWP is the basis for the pressure setting of the pressure relief devices that protect the vessel. The MAWP is normally greater than the design pressure but must be equal to the design pressure when the design rules are used only to calculate the minimum thickness for each element and calculations are not made to determine the value of the MAWP.

3. Set Pressure = of a vessel along with design temperature are used to determine the minimum permissible thickness or physical characteristic of each vessel component as determined by the pressure vessel design rules. The design pressure is selected by the user to provide a suitable margin above the most severe pressure expected during normal operation at a coincident temperature. It is the pressure specified on the purchase order. This pressure may be used in place of the maximum allowable working pressure (MAWP) in all cases where the MAWP has not been established. The design pressure is equal to or less than the MAWP.

Design pressure is given by Process Engineer to fabricator.
Set Pressure at which prv is set/operate.

If Design pressure = MAWP, and if set pressure is less than or equal to MAWP.

then by ASME accumulation codes (10%, 16% and 21% for fire) over MAWP, are we violeting the rules?

Consider Example -

Operating pressure = 70 psig
Design pressure = 100psig
MAWP = Design pressure = 100psig
Set pressure with prv = 97psig

So, MAWP + Accumulation = 100 + 10% = 110psig will not violet the code?

According to this link
http://www.cheresour...asiseeit1.shtml


MISINTERPRETATION OF CODE
Capacity based on MAWP + Allowable Overpressure

CODE AS WRITTEN
Capacity based on Set Pressure + Allowable Overpressure

Please comment
Thanks and regards
Sapna

[pleckner]

Hi All,

Thank for such a wonderful website. I really appreciate the efforts of each one of you.

I am new member and fairly new in process industry at Bangalore, India.
I have a problem -

While working on relief valve sizing and reading API code, I got a doubt

1. Design pressure is higher than operating pressure considering abnormal scenarios. 10% or 25 psig whichever is higher than normal operating pressure
of a vessel along with design temperature are used to determine the minimum permissible thickness or physical characteristic of each vessel component as determined by the pressure vessel design rules. The design pressure is selected by the user to provide a suitable margin above the most severe pressure expected during normal operation at a coincident temperature. It is the pressure specified on the purchase order. This pressure may be used in place of the maximum allowable working pressure (MAWP) in all cases where the MAWP has not been established. The design pressure is equal to or less than the MAWP.

2. Maximum Allowable Working Pressure (MAWP) = is the maximum gauge pressure permissible at the top of a completed vessel in its normal operating position at the designated coincident temperature specified for that pressure. The pressure is the least of the values for the internal or external pressure as determined by the vessel design rules for each element of the vessel using actual nominal thickness, exclusive of additional metal thickness allowed for corrosion and loadings other than pressure. The MAWP is the basis for the pressure setting of the pressure relief devices that protect the vessel. The MAWP is normally greater than the design pressure but must be equal to the design pressure when the design rules are used only to calculate the minimum thickness for each element and calculations are not made to determine the value of the MAWP.

3. Set Pressure = of a vessel along with design temperature are used to determine the minimum permissible thickness or physical characteristic of each vessel component as determined by the pressure vessel design rules. The design pressure is selected by the user to provide a suitable margin above the most severe pressure expected during normal operation at a coincident temperature. It is the pressure specified on the purchase order. This pressure may be used in place of the maximum allowable working pressure (MAWP) in all cases where the MAWP has not been established. The design pressure is equal to or less than the MAWP.

Design pressure is given by Process Engineer to fabricator.
Set Pressure at which prv is set/operate.

If Design pressure = MAWP, and if set pressure is less than or equal to MAWP.

then by ASME accumulation codes (10%, 16% and 21% for fire) over MAWP, are we violeting the rules?

Consider Example -

Operating pressure = 70 psig
Design pressure = 100psig
MAWP = Design pressure = 100psig
Set pressure with prv = 97psig

So, MAWP + Accumulation = 100 + 10% = 110psig will not violet the code?

According to this link
http://www.cheresour...asiseeit1.shtml


MISINTERPRETATION OF CODE
Capacity based on MAWP + Allowable Overpressure

CODE AS WRITTEN
Capacity based on Set Pressure + Allowable Overpressure

Please comment
Thanks and regards
Sapna

It is often the case to set the PSV equal to the design pressure the Process Engineer specifies. We do this to minimize the size of the PSV and this is fully accepted practice by the Codes. The 10%, 16% and 21% allowable overpressures are relative to the true MAWP, not the design pressure or the PSV set pressure. If the design pressure, PSV set pressure and true MAWP are all the same, then the allowable overpressure are the typcial 10%, 16%, 21% of these values. If the design pressure and the PSV set pressure are less than the true MAWP, then the overpressure with respect to the design pressure and PSV set pressure may also be greater.

[sapna]

Thank you for reply.

But if MAWP = maximum gauge pressure permissible at the top of a completed vessel in its normal operating position at the designated coincident temperature specified for that pressure.maximum gauge pressure permissible at the top of a completed vessel in its normal operating position at the designated coincident temperature specified for that pressure.

Then above pressure above MAWP is unsafe and can have hazards, but by code which allows 10% accumulation over MAWP. Is it not unsafe for system?

Example - Pressure vessel system
Design pressure = MAWP = set pressure = 100 psig.
So, incase of nonfire single PRV it can test upto 110 psig & for ext. fire case it can test 121 psig.

Will the vessel safe? I fully understand code allows this. But we are operating PRV at unsafe conditions.

Please clarify, I will be thankful to all of you.

With regards,
Sapna

[Qalander (Chem)]

Thank you for reply.

But if MAWP = maximum gauge pressure permissible at the top of a completed vessel in its normal operating position at the designated coincident temperature specified for that pressure.maximum gauge pressure permissible at the top of a completed vessel in its normal operating position at the designated coincident temperature specified for that pressure.

Then above pressure above MAWP is unsafe and can have hazards, but by code which allows 10% accumulation over MAWP. Is it not unsafe for system?

Example - Pressure vessel system
Design pressure = MAWP = set pressure = 100 psig.
So, incase of nonfire single PRV it can test upto 110 psig & for ext. fire case it can test 121 psig.

Will the vessel safe? I fully understand code allows this. But we are operating PRV at unsafe conditions.

Please clarify, I will be thankful to all of you.

With regards,
Sapna

Dear Sapna Hello/Good Afternoon,

Are we not somehow confusing ourselves? by saying that Reliving Capacity is based on
MAWP (Maximum Allowable Working Pressure) plus permissible overpressure.

If we rephrase this by saying that
driving force to control (MAWP plus %age of MAWP permissible) is taken as basis for relieving capacity or flow rate design then the protected asset i.e

vessel, piping or other containment should not be exposed to higher pressure than their MAWP;except for a very very small initial time period.

Since the %age overpressure not only takes care of any Relief valve upstream piping's pressure drop, but it also ensures that protected asset's pressure at point temperature remains maintained as per design value in relation to specific relief valve's design/flow rate.

The above is my understanding of the issue.Might be helpful!

However there are many Forum Giants of this particular topic who will better contribute.
Best regards
Qalander