Early relieving of Pilot Operated Safety Relief Valve

[Mohammad Saad]

Hey you all, hope you are having a good day.
There has been an issue of early relieving with 3800 series POSRV with backflow preventer, installed on LPG service. The normal operating pressure is around 10 kg/cm[sup]2[/sup] while the set pressure is around 12 kg/cm[sup]2[/sup]. A few days ago, the POSRV popped at 9.6 kg/cm[sup]2[/sup]. The backup POSRV was taken into service and yesterday it also popped around 9.5 kg/cm[sup]2[/sup]. But when the POSRV was commissioned on-site, it popped at its set pressure.

It has caused a confusion, and I was wondering if the ambient temperature could have caused overpressure in the "sensing line" of POSRV because on both days (when each POSRV popped), the outer temperature was quite high i.e. 48[sup]o[/sup]C and they popped around the same time of the day. If any of you can educate me on this matter, if the above-mentioned reason is even possible or not and what could be the other reasons, I would be thankful to you.

 

[The Obturator]

Need some information from you;-
What is the inlet piping configuration from the protected vessel to the inlet of the pilot operated pressure relief valve like? drawing. photo?
How was the valve set pressure tested in the workshop and with what fluid?
Has the valve set pressure ever been changed from the original supply?
Do you have a data/specification sheet of the valve? or more details size, full model etc.



*** Per ISO-4126, the generic term
'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

 

[LittleInch]

Liquid LPG or gas?

If liquid you were probably boiling the liquid which will create some pulses in the sensing line or in the main pipe itself.

LPG is a vague terms. What actual liquid or mix of liquids? What's the vapour pressure curve?

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

 

[Latexman]

Any source of pressure dynamics/surges nearby, like a pump or valve?

Good Luck,
Latexman

 

[Snickster]

I suspect it has something to do with vaporization of the LPG inside the pilot valve. The vapor pressure/boiling point of 70% propane / 30% Butane LPG mixture is about 12 kg/cm[sup]2[/sup] at 48 deg. C which is sounds like is very close to what you may have. So even with the main piping at 10 kg/cm[sup]2[/sup], if somehow the LPG is vaporizing in a location where it can be trapped inside the pilot valve and not be able to be relieved to the lower pressure of the piping then it may be popping the valve. For instance if it is trapped in small spaces underneath the pilot piston where it can't escape fast enough. If it were not trapped then the pilot line would just equalize the pressure even with vaporization to the main line and everything would remain at the main line pressure. This would be also true if the LPG was not vaporizing and the liquid LPG would thermally expand the pressure could not build up because the pilot line will always keep the pressure equalized with the main process line. Therefore I believe somehow the LPG when it vaporizes at 48 deg. C is being trapped at vapor pressure in locations where it exerts a opening force on the pilot piston.

What is the vapor pressure of your LPG at 48 deg C?

Another thing is that the backflow device in the pilot valve vent line could also be trapping vapor at 48 deg. C vapor pressure and not allowing it to bleed down to the downstream pressure piping pressure that the vent line is connected to, which may be also causing pressure to build up under your pilot piston and activating the pilot.

 

[Snickster]

I would discuss this with the manufacturer. This may be a common issue when the fluid being relieved is close to the vapor pressure and they may have already developed a fix for it.

 

[georgeverghese]

Where is the sensing line for the pilot connected to ? It should be connected to the equipment being protected, and not the piping just upstream of the safety valve.

 

[georgeverghese]

And you should be reading the pressure at the equipment being protected at the time of relief(assuming that is where the pilot sensing line is connected to), and not from the piping just upstream of the safety valve.

 

[Latexman]

The pressure transmitter reading the upstream pressure could be wrong, and both POSRV’s popped correctly?

Good Luck,
Latexman

 

[gguliye]

theoretically, there can be something wrong with the backflow preventer also, that would allow the backpressure to help to lift the valve piston