Mininimum Allowable Differential Between Operating Pressure and PSV Set Pressure.

[Pavan Kumar]

Hi All,

I am trying to fix the maximum operating pressure of Steam piping downstream of a Control Valve. The Upstream pressure is 12 bar(g) with saturated steam. The downstream pressure needs to be fixed so that the users in the network get at least 5 bar(g) pressure. I performed a flow model and got that with a operating pressure (pressure downstream of the control valve) of 6.3 bar(g) the users can get at least 5.3 bar(g). There is a PSV downstream of the control valve which is set at 7 bar(g). The design pressure of the downstream piping is 10 bar(g). For some reason they donot want to set the PSV at 10 bar(g). I saw some posts which says the maximum differential between PSV Set pressure and maximum operating pressure is the larger of 10% of PSV set pressure or 25 psi. The PSV will be conventional type.



In my case, 10% of PSV Set Pressure = 0.10*7*14.5=10.15 psi, so I should use 25 psi differential which means my maximum operating pressure would be 5.27 bar(g) = [(7*14.5-25)/14.5]. This is too low as the farthest user will now only get 4.3 bar(g). I really cannot go 6 bar(g) operating pressure. I want some expert guidance and reference to code if possible in this problem.

https://www.cheresources.com/invision/topic/2581-psvoperating-pressuremawp/


Thanks and Regards,
Pavan Kumar

 

[MJCronin]

Review ASME B31.1 on what is required (both setting AND FLOWRATE) for these types of relief stations

What is the piping design "Code of Record", Pav ?

 

[LittleInch]

You need to question this 10 bar rating. Makes no sense to set the PSV below that.

The 10% "rule" is a decent one based on vendor data over decades.

The 25 psi isn't. Don't believe everything you read.

 

[pierreick]

To me, the Psv set point makes no sense. Set point should be in accordance with pipe spec and or equipment downstream. you should work on the control valve to get the right pressure downstream. more users will lead to pressure drop. Check the control valve parameters or ask the E&I engineer to tune them. Psv is not there for process control.
Pierre

 

[Snickster]

The minimum set pressure of a relief valve is related to process conditions. In typical process systems the pressure can vary based on the process like in chemical reaction vessels or other cases so you want good differential to keep the PSV from popping all the time during excursion in the process pressure.

If you have a system that the pressure downstream is determined by a very good reliable regulator at 6.3 bar then this will be the pressure downstream period. Then having a relief valve set at 7 bar will give you 0.7 bar differential to set pressure which seems sufficient to me.

 

[goutam_freelance]

The philosophy of safety valve setting should be to protect the piping and the downstream equipment. Irrespective of the achieved pressure at the downstream end, if the piping and equipment downstream can sustain a design pressure of say, 7 or 8 bar(g), then the safety valve set pressure can be 7 or 8 bar(g) as the case may be.

If you have a lot of margins available in the design pressure then it may be beneficial to take advantage of this, which may reduce the number of safety valve discharges, thereby reducing the wastage of steam.

For some reason they donot want to set the PSV at 10 bar(g)

The reason is not understood.

 

[Pavan Kumar]

Review ASME B31.1 on what is required (both setting AND FLOWRATE) for these types of relief stations

What is the piping design "Code of Record", Pav ?

Hi MJCronin,

The piping code used was EN 13480. I am not sure how this corresponds with ASME 31.1 or 31.3.

Pavan

 

[Pavan Kumar]

You need to question this 10 bar rating. Makes no sense to set the PSV below that.

The 10% "rule" is a decent one based on vendor data over decades.

The 25 psi isn't. Don't believe everything you read.

Thank you LI. They might want to protect the downstream users from overpressure and that's why they want to set the PSV lower than the piping design pressure of 10 bar(g). The 25 psi was from a well-respected person Phil Pleckner from CHEE resources. I saw him posting on Eng-Tips as well. Obturator from Eng-Tips mentioned that for PSV set Pressure between 71 - 1000 psig the differential is 10%.

PSV Set Pressure vs Operating Pressure - Safety Relief Valve engineering (PSV)

Is there a limit between the PSV set pressure and the operating pressure? For example, if a system was designed with a MAWP of 1000 psig and the safety relief valve has a set pressure of 1000 psig, would this be acceptable if the system operating pressure was reduced to 500 psig? The system is...

www.eng-tips.com

Also he is referring to ASME Sec VIII (2021) Appendix M-10 in which I could not locate this information.



Thanks and Regards,
Pavan Kumar

 

[Pavan Kumar]

To me, the Psv set point makes no sense. Set point should be in accordance with pipe spec and or equipment downstream. you should work on the control valve to get the right pressure downstream. more users will lead to pressure drop. Check the control valve parameters or ask the E&I engineer to tune them. Psv is not there for process control.
Pierre

Hi Mr. Pierre,

It appears to me that downstream users might have lower Design pressure and that's why the are setting the PSV lower than the piping DP. I am the one sizing this control valve using the normal operating conditions of 12 bar(g) upstream, 6.2 bar(g) downstream for a normal flow rate of 29,410 kg/hr. The required CV is 264 and I selected a 6" Globe Style Control valve.

Thanks and Regards,
Pavan Kumar

 

[Pavan Kumar]

The minimum set pressure of a relief valve is related to process conditions. In typical process systems the pressure can vary based on the process like in chemical reaction vessels or other cases so you want good differential to keep the PSV from popping all the time during excursion in the process pressure.

If you have a system that the pressure downstream is determined by a very good reliable regulator at 6.3 bar then this will be the pressure downstream period. Then having a relief valve set at 7 bar will give you 0.7 bar differential to set pressure which seems sufficient to me.

Ok thank you. Is there a Code reference to use 10% differential. Obturator from Eng-Tips post said ASME Sec VIII(2021) Appendix M-10. I tried looking for it and couldn't find it. See post # 6 in the below thread.

PSV Set Pressure vs Operating Pressure - Safety Relief Valve engineering (PSV)

Is there a limit between the PSV set pressure and the operating pressure? For example, if a system was designed with a MAWP of 1000 psig and the safety relief valve has a set pressure of 1000 psig, would this be acceptable if the system operating pressure was reduced to 500 psig? The system is...

www.eng-tips.com

Thanks and Regards,
Pavan Kumar

 

[Pavan Kumar]

The philosophy of safety valve setting should be to protect the piping and the downstream equipment. Irrespective of the achieved pressure at the downstream end, if the piping and equipment downstream can sustain a design pressure of say, 7 or 8 bar(g), then the safety valve set pressure can be 7 or 8 bar(g) as the case may be.

If you have a lot of margins available in the design pressure then it may be beneficial to take advantage of this, which may reduce the number of safety valve discharges, thereby reducing the wastage of steam.

The reason is not understood.

I agree with you Mr. Goutam.

Pavan

 

[Snickster]

I know of no code requirement for the 10% value but most large company process clients such as oil and gas, chemical plants, etc use this rule of thumb.

 

[pierreick]

hi,
I won't comment on your calculation, you seem not consider the Pid's parameters of your controller, the reason why I asked you to talk to E&I engineer.
Pierre

 

[georgeverghese]

How you've drawn the process controls on this wont work - you dont have any feedback to the PCV to throttle this valve when downstream pressure exceeds an upper limit ( controls action prior to PSV activation).
From the looks of it, there isnt enough headroom on pressure to allow for some pressure fluctuations in this supply line when consumers swing up and down on flow.

 

[The Obturator]

For the record. Recommended differentials between operating and set pressure are to be found in current ASME VIII-I (2023) M-10 (c) as copied below (same as earlier edition ASME VIII (2021) M-10 referenced earlier in this post). It does exist. strange that you say you cannot find it. Look it up in the index at the front.

 

[Pavan Kumar]

For the record. Recommended differentials between operating and set pressure are to be found in current ASME VIII-I (2023) M-10 (c) as copied below (same as earlier edition ASME VIII (2021) M-10 referenced earlier in this post). It does exist. strange that you say you cannot find it. Look it up in the index at the front.

View attachment 5826

Thank you The Obturator, I was able to locate this section in Non-Mandatory Appendix M-10(c) in ASME Sec VIII (2021). I was searching in the Mandatory Appendix 10 which is why I could not find it. Apart from 10% differential I must also consider 7% blow down for the PSV set pressure correct?. So with my Set pressure of 7 bar(g), with 7% blowdown the the re-seating pressure would be 6.51 bar(g) and with 10% differential the maximum operating pressure would be 6.3 bar(g). So having an max operating pressure of 6.3 bar(g) with PSV set at 7 bar(g) is adequate.

Thanks again.

Thanks and Regards,
Pavan Kumar

 

[Pavan Kumar]

How you've drawn the process controls on this wont work - you dont have any feedback to the PCV to throttle this valve when downstream pressure exceeds an upper limit ( controls action prior to PSV activation).
From the looks of it, there isnt enough headroom on pressure to allow for some pressure fluctuations in this supply line when consumers swing up and down on flow.

Yes you are correct. There is indeed a feed back from the downstream PT that modulates the Control valve depending on the pressure. Yes there is very little pressure margin to allow for fluctuations in steam consumption. My preference is to set the PSV set Pressure at 8 bar(g) while the maximum operating is still at 6.3 bar(g).

Thanks and Regards,
Pavan Kumar

 

[georgeverghese]

8barg would be better. Try to find out what are current constraints that limit you to a design pressure of 7barg at each of these users.

 

[_1angelia23]

Thank you The Obturator, I was able to locate this section in Non-Mandatory Appendix M-10(c) in ASME Sec VIII (2021). I was searching in the Mandatory Appendix 10 which is why I could not find it. Apart from 10% differential I must also consider 7% blow down for the PSV set pressure correct?. So with my Set pressure of 7 bar(g), with 7% blowdown the the re-seating pressure would be 6.51 bar(g) and with 10% differential the maximum operating pressure would be 6.3 bar(g). So having an max operating pressure of 6.3 bar(g) with PSV set at 7 bar(g) is adequate.

Thanks again.

Thanks and Regards,
Pavan Kumar

As mentioned in ASME BPVC 2023 Section XIII 2023 part 9.2.1 blowdown "
(a) Pressure relief valves for compressible fluids having an adjustable blowdown construction shall be adjusted prior to testing so that the blowdown does not exceed 5% of the set pressure or 20 kPa (3 psi), which-ever is greater.
(b) Pressure relief valves for incompressible fluids and pressure relief valves for compressible fluids having nonadjustable blowdown shall have the blowdown noted and recorded.
(c) Pressure relief valves for Section I power boilers for steam service to be marked with the Certification Mark and V Designator shall be adjusted so that the blowdown does not exceed 4% of the set pressure. For pressure relief valves set at or below 700 kPa (100 psi), the blowdown shall be adjusted so as not to exceed 30 kPa (4 psi). Pres-sure relief valves used on forced-flow steam generators with no fixed steam and waterline, and pressure relief valves used on high-temperature water boilers shall be adjusted so that the blowdown does not exceed 10% of the set pressure. The reseating pressure shall be noted and recorded.
(d) Blowdown adjustment is not a requirement for Section IV (HV Designator) pressure relief"

Take into consideration inlet pressure loss.
"inlet piping to pressure relief valves should be designed for the lowest practical inlet pressure drop (including exit loss and piping and isolation valve pressure drop). API RP 520 guidelines as well as manufacturers’ recommendations dictate an inlet pressure drop of no more than 3% of set pressure at maximum valve relieving capacity.
This 3% limit is particularly important for pressure relief valves in liquid service.
In the case of pressure relief valves in low pressure vapor services where the set pressure is below 50 psig (3.4 barg, 3.5 kg/cm2 G), or for certain valve sizes (particularly larger valves), this may be difficult to achieve and the manufacturer should be contacted for guidance. Loss up to 5% may be acceptable for certain valves."


I hope that helps you

 

[The Obturator]

As mentioned in ASME BPVC 2023 Section XIII 2023 part 9.2.1 blowdown "
(a) Pressure relief valves for compressible fluids having an adjustable blowdown construction shall be adjusted prior to testing so that the blowdown does not exceed 5% of the set pressure or 20 kPa (3 psi), which-ever is greater.
(b) Pressure relief valves for incompressible fluids and pressure relief valves for compressible fluids having nonadjustable blowdown shall have the blowdown noted and recorded.
(c) Pressure relief valves for Section I power boilers for steam service to be marked with the Certification Mark and V Designator shall be adjusted so that the blowdown does not exceed 4% of the set pressure. For pressure relief valves set at or below 700 kPa (100 psi), the blowdown shall be adjusted so as not to exceed 30 kPa (4 psi). Pres-sure relief valves used on forced-flow steam generators with no fixed steam and waterline, and pressure relief valves used on high-temperature water boilers shall be adjusted so that the blowdown does not exceed 10% of the set pressure. The reseating pressure shall be noted and recorded.
(d) Blowdown adjustment is not a requirement for Section IV (HV Designator) pressure relief"

Take into consideration inlet pressure loss.
"inlet piping to pressure relief valves should be designed for the lowest practical inlet pressure drop (including exit loss and piping and isolation valve pressure drop). API RP 520 guidelines as well as manufacturers’ recommendations dictate an inlet pressure drop of no more than 3% of set pressure at maximum valve relieving capacity.
This 3% limit is particularly important for pressure relief valves in liquid service.
In the case of pressure relief valves in low pressure vapor services where the set pressure is below 50 psig (3.4 barg, 3.5 kg/cm2 G), or for certain valve sizes (particularly larger valves), this may be difficult to achieve and the manufacturer should be contacted for guidance. Loss up to 5% may be acceptable for certain valves."


I hope that helps you

​

@_1angelia23 The referenced ASME BPVC 2023 Section XIII 2023 part 9.2.1 (ie., the whole of Part 9 of ASME XIII) is purely a requirement for Pressure-relief Valves undergoing Capacity and Flow Certification. These are the type tests observed and certified by the National Board of Boiler & Pressure Vessel Inspectors (NB), and then published in NB-18. The blowdown test is intended for the valve design to demonstrate it can meet the blowdown criteria. In the case of adjustable blowdown on compressible fluids, the blowdown should not exceed 5% or 3 PSIG whichever is the greater (as you have copied), as is probably in the OP's case above.

These type tests are done under laboratory conditions and are not production tests required of the manufacturer. The key word here is adjustable blowdown. Actual blowdown that will be realised would be dependant on many things including inlet piping as you have elaborated on. The user of course can adjust blowdown in the field, but in reality 7% blowdown would be a more true figure for adjustable valves on compressible fluid (ASME VIII, prior to the requirement transitioning to XIII, used to specify 7% blowdown some years back but tightened the requirement).