3 replies to this topic

[yizsteph]

Hello all,

I have a question on what is the basis to choose pilot type PSV?
I noticed some vendor use conventional type and other choose pilot for a LNG tank.

I look forward to your reply.

Many thanks

Yizsteph

[ankur2061]

Yizsteph,

 

Some selction criteria are provided in the link below for selection of Pilot Operated Safety valve abbreviated as "POSV".

 

http://www.cheresour...f-pig-launcher/

 

Regards,

Ankur.

[fallah]

Yizstesh,

 

In brief:

 

1- When operating pressure is so close to set pressure

2- Back pressure is higher than 50% of the set pressure

3- Inlet line is so long the friction loss may pass 3% of the set pressure

...........

The POSV is selected

Edited by fallah, 04 August 2014 - 06:12 AM.

[Art Montemayor]

I am familiar with pilot-operated pressure relief valves as applied on LNG storage tanks.  As a DNV GL employee I have participated in evaluating LNG tanks and written assessment reports on this type of application on an LNG terminal and can relate to this query.  The LNG tank relief valves I am familiar with are Anderson Greenwood, model 931216FA - which I believe are standard in the industry.  The body and bonnet material are 316SS.  The set pressure and MAWP are usually around 2 psig.  At this low pressure, it is very important to attain full valve capacity as soon as possible when the set point is reached.  A pilot-operated type safely does this - as opposed to a pallet, gravity valve.

 

Pilot operated pressure relief valves consist of a main valve with piston or diaphragm operated disc and a pilot.  Under normal operating conditions the pilot allows system pressure into the piston chamber.  Since the piston area is greater than the disc seat area, the disc is held closed.  When the set pressure is reached, the pilot actuates to shut off system fluid to the piston chamber and simultaneously vents the piston chamber.  This causes the disc to open.

 

The pilot operated pressure relief valve has several advantages.

 

As the system pressure increases, the force holding the disc in the closed position increases.  This allows the system operating pressure to be increased to values within 5% of set pressure without danger of increased seat leakage in the main valve.

 

Pilots are generally designed with a separate control for set pressure and blowdown.  Valves can be set to open fully at the set pressure and close with a very short blowdown.

 

Another advantage of pilot operated pressure relief valves is the reduced cost of larger valve sizes (which is the case in LNG tanks).  The large spring and associated envelope is replaced by a small pilot, thus reducing the mass and cost of the valve.

 

 

Pilot operated pressure relief valves are supplied with filters to protect against foreign matter and are always recommended for relatively clean fluid service (which is typical of LNG).