Spring-Operated Pressure Reilef Valves 4

[IdanPV]

Dear Experts,

I heard and read that for spring-operated pressure relief valves, small amount of leakage start to occur at 92-95% of the set pressure.
I have read about it, and found this statement in several articles but I would like to know if there is any formal reference (e.g. Code or Standard)

Thank you very much.

 

[The Obturator]

Depending on the manufacturer/design of PRV, spring operated pressure relief valves will have a small amount of leakage prior to set pressure. This is referred to a "simmer". A good reference to get you on board with such definitions etc is API-RP-520 Part I.

Per ISO-4126, only the term Safety Valve is used regardless of application or design.

 

[IdanPV]

Hi The Obturator,

Thank you very much for your answer.

 

[LittleInch]

Vendors will supply you with their set point accuracy.

If you need closer than 8 to 10% then you need to look into pilot operated relief valves which are good for 2-3% below set point.

But I don't think you'll find it written anywhere specifically because it depends on the vendor and type of relief valve.

Any pulsations in the fluid will also affect the simmering action.

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

 

[IdanPV]

@ LittleInch,

Thank you for your comment.

 

[gguliye]

Hello,
normally spring operated valves with metal seats are recommended till 90 % of the SP
regards,

 

[IdanPV]

Does the seat material has any influence?
Does safety valve with PTFE seat might be better?

 

[The Obturator]

REVISED for correction and clarity

Your initial question was of spring operated PRVS just before opening. The phenomenon advised is simmer.

Seat tightness is normally the measure at typically a pressure of 90% of set pressure, simulating the normal operating condition. Ie., a 10 % operating gap.

The standard API-527 specifies seat tightness levels.

For all metal to metal seated PRV''s there is an allowable leakage rate at 90% variant by size, pressure..

For soft seats there is no leakage allowance at 90%.

Some companies offer enhanced/better than API-527.

You can expect leakage for spring operated PRV's at 95% metal and O ring seta, regardless what the claims are.

Using soft seats - O Rings, the leakage is minimised. It is more difficult with PTFE. PTFE Seats(which are not O Rings) require some degree of heat to plasticise the seat. Testing at room temperature requires the PRV to be warmed up.

If you are looking for zero leakage at a higher factor then a pilot operated PRV is recommended. The higher the operating pressure the tighter the seats until September point. Pilot PRV''s use O Rings.

What is your specific application and leakage criteria?

Another way is to install a bursting disc upstream of the PRV.

Per ISO-4126, only the term Safety Valve is used regardless of application or design.

 

[gguliye]

Hello IdanPV
so the normally CROSBY says that soft seated valves are tight till 95-98 percent of SP no matter material is. i havent seen the material problem.
HOWEVER,
if u are still in doubt, please proceed AG 81 83 series for gas and thermal relief applications and 81P for the liquid applications. 81P is hard to calibrate on the field though. they are super tight OR POPRV
regards,

 

[IdanPV]

The Obturator,
Thank you for your detailed response.
The application is power boiler which is designed and constructed in accordance with ASME Section I.
MAWP=5BAR @ 160°C - Steam.

I don't have any leakage criteria, I just want to minimize it.

gguliue,
Thank you very much sir.

 

[The Obturator]

Then we need to talk about about a completely different thing...

More information required to make a better judgement/recommendation;-

1. What Safety Valve are you using (incl. size and type)?
2. What is your normal operating and set pressure?
3. How has existing valve been tested?
4. What is current SV experiencing (leakage, blowdown etc.)?
5. Installation of SV, if other than vertical on top of vessel with inlet pressure loss no more than 3%

Per ISO-4126, only the term Safety Valve is used regardless of application or design.

 

[gguliye]

Sir,
if u are talking about ASME Section i that's completely another story
AG 80 series is not used for this purpose. as Obturator said, we need to know the several details
regards,

 

[IdanPV]

1. Ø3/4" , Spring Operated Safety Valve, metal seat.
2. Normal operating pressure=4.5bar, set pressure (of the safety valve)=5bar.
3. I don't know.
4. The current SV experiencing "simmering".
5. The SV is installed in acc. with the requirements of ASME I

 

[The Obturator]

It is simply simmering because it is most probably not a boiler designed safety valve to ASME I. You appear to have a close operating to set pressure differential for this size of valve.

What is the make and model number?

Better still, do you have a data sheet you can share of the valves application?

Per ISO-4126, only the term Safety Valve is used regardless of application or design.

 

[IdanPV]

The Safety Valve is approve for power boilers and it is marked with "V" designator.
The SV discharge (Kg./Hr @ 3% overpressure) is 397 Kg./Hr. (90% of actual) as per the SV data sheet.

 

[The Obturator]

It;s difficult to help you more if you can't provide the name/model and/or SV Data Sheet.

Per ISO-4126, only the term Safety Valve is used regardless of application or design.

 

[IdanPV]

Hello all.
The SV is Apollo 19 Model:
19K-DC-L(or A)-40
Model 19 - Brass Trim/Teflon Seat - 1/2M x 3/4F - For VIII-1 Steam or Sec. I - 40psi

 

[LittleInch]

Well according to the manuals etc, the -40 in your model is 40 psi ( 2.6 bar). Now sure you can reset the valve, but how accurately was this done?

did you actually buy a valve pre set to 5 bar?? or did someone "adjust " it?

But operating these simple valves at 90% of set pressure is going to risk simmering, especially when the margins are so tight.
How accurately is your operating pressure being measured? Is it really 4.6 or 4.7?

Was the valve bench tested to 5.01 bar?





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

 

[The Obturator]

Another thing. Acc to the valve style, the seats are Teflon. Testing at ambient conditions will result in leakage. The valve is designed to work on steam and ideally should be tested on steam. Really need to know your comments on Little Inch's pressure comments above and also whether it is ASME I or VIII.

Per ISO-4126, only the term Safety Valve is used regardless of application or design.

 

[IdanPV]

Correction:
I made a mistake.
the valve is
Apollo: 19K-DC-A-72.5
It's a brass trim/teflon seat, Section I Steam Service at 5BARG (72.5 psi)

Sorry for that.