2 replies to this topic

[SA07]

I'm currently in the process of evaluating the inlet piping isometrics
for a pilot operated safety valve (internal pressure sensing
modulating type) in natural gas service.
The intention is to check whether the pressure drop for equivalent length as per the
isometrics match with the API 3% pressure drop criterion.

I've been reading the messages posted on this forum regarding the
safety valve inlet pressure drop restrictions and I believe I'm aware of the same now.

I have the following situation,
For a pilot operated safety valve (process contingency sizing case @
set pressure of 1000 psig), the calculated pressure drop in the inlet piping
system is 150 psi. This pressure drop is 15% of the valve set
pressure.

As people have discussed in this forum, I'm aware of the various
alternatives available to decrease the inlet pressure drop.
But before starting to evaluate the options, I have learnt from safety
valve vendors in the past that the pilot operated safety valves
(modulating type) can operate without any problem with inlet pipig
pressure drop up to 15% of the set pressure. What I could understand
from this statement was that even if the pressure at the inlet of the
pilot operated safety valve drops by 15% of the set pressure the valve
does not close or chatter to reduce the relieving rate. Thereby there
is no accumulation in the vessel to be protected.

Keeping this in mind I intend to approach the safety valve vendor and
ask for a confirmation on the allowable pressure drop for the valve to
be supplied.
If the valve vendor confirms that such high pressure drop is
acceptable then I need not modify the inlet piping design.
Is my undersatnding and approach correct?

[JoeWong]

SA07,

...
I have the following situation,
For a pilot operated safety valve (process contingency sizing case @
set pressure of 1000 psig), the calculated pressure drop in the inlet piping
system is 150 psi. This pressure drop is 15% of the valve set
pressure.

Safety perspective
Is your PSV protecting a vessel ? Is your vessel design to ASME sect VIII ?
If your PSV is protecting a vessel design to ASME sect VIII, with a pressure drop of 15% of set pressure, isn't the operating pressure at protected equipment exceed the 10% allowable by ASME for process contingency case ?

PSV chattering perspective
Pilot operated PSV normally modulating type is having higher blowdown (vendor can advise). Sometime pressure drop (>3%) at PSV inlet can be tolerated by resetting the PSV to get higher blowdown. This helps to reduce the cahttering frequency...personally i have experienced 5-6% pressure drop but not at 15% level.

Keeping this in mind I intend to approach the safety valve vendor and ask for a confirmation on the allowable pressure drop for the valve to be supplied. If the valve vendor confirms that such high pressure drop is acceptable then I need not modify the inlet piping design. Is my undersatnding and approach correct?

You may needs to clearly understand the meaning of "valve vendor confirms that such high pressure drop is acceptable...". This statement is a bit misleading to me. Back to the real situation, a process / process safety engineer who design the overprotection system should responsible for the design. One shall not rely on vendor "guarantee / warranty" for safety reason...as far as i know...""guarantee / warranty" sometime it does not work in real world...

JoeWong

[JoeWong]

SA07,

...
I have the following situation,
For a pilot operated safety valve (process contingency sizing case @
set pressure of 1000 psig), the calculated pressure drop in the inlet piping
system is 150 psi. This pressure drop is 15% of the valve set
pressure.

Safety perspective
Is your PSV protecting a vessel ? Is your vessel design to ASME sect VIII ?
If your PSV is protecting a vessel design to ASME sect VIII, with a pressure drop of 15% of set pressure, isn't the operating pressure at protected equipment exceed the 10% allowable by ASME for process contingency case ?

PSV chattering perspective
Pilot operated PSV normally modulating type is having higher blowdown (vendor can advise). Sometime pressure drop (>3%) at PSV inlet can be tolerated by resetting the PSV to get higher blowdown. This helps to reduce the cahttering frequency...personally i have experienced 5-6% pressure drop but not at 15% level.

Keeping this in mind I intend to approach the safety valve vendor and ask for a confirmation on the allowable pressure drop for the valve to be supplied. If the valve vendor confirms that such high pressure drop is acceptable then I need not modify the inlet piping design. Is my undersatnding and approach correct?

You may needs to clearly understand the meaning of "valve vendor confirms that such high pressure drop is acceptable...". This statement is a bit misleading to me. Back to the real situation, a process / process safety engineer who design the overprotection system should responsible for the design. One shall not rely on vendor "guarantee / warranty" for safety reason...as far as i know...""guarantee / warranty" sometime it does not work in real world...

JoeWong