Blowdown (Reseat pressure) Vs Operating pressure. (ASME Sec VIII) 3

[Hari Mech]

Per ASME Section VIII, system pressure (Operating pressure) Should be always less than blowdown pressure range. This makes sure that the valve will reseat again after discharge before the operating pressure is reached.

I have a customer whose operating pressure is falling within my valves blowdown pressure range, where here the customer says that the constant pressure from his pump (Operating pressure) will wont allow the valve to reseat again since it is falling within the blowdown pressure.

Set pressure : 65 psig
Operating pressure : 52.6 psig
Blowdown%/Reseat pressure : 20% or 52 psig.

Our argument is that the operating pressure will not affect the reseat pressure. During a relief scenario, the pressure in the system will continuously be reduced by the opening of the relief valve to levels below the operating pressure.

Please advise how to tackle above situation.

Thanks in advance.

 

[gguliye]

Dear Sir,
U may proceed with the short blowdown valves such as pilot or 80 series from Emerson.
Regards,

 

[The Obturator]

(Just commented on your earlier question of AG Ser 400 Pilot SRV).

During a relief scenario as you have described, the system pressure will be constantly trying to get back to its operating pressure. So theoretically the SRV will never close. If it is a spring operated SRV, then this is how chatter then develops. Without knowing all of the service application detail, you simply need a SRV with a shorter blowdown. I guess your client has a spring SRV with a blowdown of 20% or so. There are other spring operated SRV's with short blowdown available as gguliye has suggested above.

Per ISO, only the term Safety Valve is used regardless of application or design.

 

[LittleInch]

I don't really understand the scenario here:

These valves are presumably Safety relief valves??

Hence if one of these goes off it means other forms of pressure trips have failed ( high high pressure trip).

Therefore you can't just have a relief valve blow off and merrily keep pumping away.

In any event the pressure from a centrifugal pump won't be constant but varies with flow. As flow increase through the valve then the pressure will fall.

If the relief valve is there to protect against shut in flow then the pump should trip or you need a higher margin between operating pressure and set pressure / design pressure of the system.

The issue with this valve is a smokescreen covering incorrect design, maybe.


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Hari Mech]

Hi Gguliye & avalveman,

Thanks for responding.

80 Series from Emerson has shorter blow down but the drawback here is that the customer requires metal to metal seat, which rules out all the 80 Series and pilot operated valves.

Requirement:
Size: 3/4 x 1 inch. (Other PRV models cannot be selected due to this size constrain)
Fluid state: Gas/Vapor.
Seat: Metal seat.

Type 81,83 & 81P & Pilot operated has only soft seats like Teflon & Buna-N...etc

 

[The Obturator]

Try an Emerson (Crosby) Series 800 spring operated SRV. It had adjustable blow down of 5 to 15 %.

Per ISO, only the term Safety Valve is used regardless of application or design.

 

[Hari Mech]

Hi avalveman,

Thanks for reply.

Sounds to be a good idea, but series 800 (Spring operated) is no longer in production. Its already obsolete.

 

[The Obturator]

Obsolete? It's only a modified OMNI Trim 900. Depends also which plant you deal with. USA, France, India or China and whomever there will support it.

Note the OMNI 900 returns a fixed 15% blow down on compressible fluids.

Per ISO, only the term Safety Valve is used regardless of application or design.

 

[Hari Mech]

Hi avalveman,

15% is for liquid application.For gas its 20%.

 

[The Obturator]

How can blowdown be less for liquid service? Catalog actually states max 20% for all fluids. My earlier 15 % comment on gas/vapour is for a correct "to the book" installation. Note your client application just borders on a 20% differential op/set.

Per ISO, only the term Safety Valve is used regardless of application or design.

 

[The Obturator]

You want to review the SRV products made by SEETHRU UK. They have designs like a metal ball for a seat claiming blowdown in the 10% area.

Per ISO, only the term Safety Valve is used regardless of application or design.

 

[Hari Mech]

How can blowdown be less for liquid service? Catalog actually states max 20% for all fluids

Answering above qs: The trim of the valve we selected was liquid,but the fluid passing through it is gas in nature (This specific requirement needs both liquid and gas i.e two cases, where the governing case is liquid). And thus it made us confuse. Thanks for clarification. Now we are pretty clear.

I will also go through products by SEETHRU UK. Thanks a lot for guiding my other thread regarding blowdown of 400 series.

Will get to you again in future, if we had any uncleared doubts in SRV.

Thankyou.

 

[The Obturator]

A pleasure to help and to challenge my SRV knowledge.

Per ISO, only the term Safety Valve is used regardless of application or design.

 

[Sabeau]

The question I've often had with some of these low blowdown valve designs is one of configuration differences between bench testing and in service performance.
Some manufacturers require specific ring positions/configurations for blowdown testing which are later returned to default for field installation.

Is this truly representative of the in service blowdown performance if a special ring configuration is required for testing?

 

[gguliye]

Spicyboy,
I don't think that the bliwdown test is possible in the factory since as the valve opens the pressure is relieved and the valve is closed immediately. Please correct me if I am wrong

 

[The Obturator]

"Real" Blowdown can only be realised on the protected vessel. That is why, in the main, ASME designed pressure relief valves, have an externally accessible adjustable blowdown ring. The volume of flow, rate of overpressure and decay, fluid type etc., etc., will influence the blowdown. ASME VIII requires manufactures to demonstrate their PRV designs can achieve blowdown (typically 7% for compressible fluids), during the approved laboratory flow testing (to demonstrate lift and determine floe coefficient of discharge). Blowdown tests are not done during production testing (too expensive and time consuming as would be required to be done in a lab each time). During production testing different ring settings, varied by manufacturer, are used to prevent damage to seating (from seats banging shut). After testing, the ring is set to "best setting" dependant on valve size. fluid, pressure etc.

Some testing organisations claim to be able to measure blowdown with their in-situ kit. While a result can be obtained (as it sure can even on a bench test), it is not the true blowdown. Blowdown can only be measured on the protected vessel without artificial lift, assistance or other.



Per ISO, only the term Safety Valve is used regardless of application or design.

 

[Sabeau]

avalveman, this is my point.

ASME Section VIII requires us to demonstrate that the blowdown figure can be achieved, however, I feel in some cases that demonstrations requiring special ring configurations under lab conditions are not representative of in-situ(real world) performance.

I have seen a blowdown test conducted on a full size vessel between an adjustable and non-adjustable blowdown design and the blowdown certainly was not what the manufacturer had stated.