6 replies to this topic

[Preeti Shetty]

Sir,

I need to do the Adequacy check for PSV installed in line,downstream of PIG-Launcher and it is set at 93 barg pressure and they have mentioned it is pilot operated PSV. The outlet of PSV is to an open end drain ,so there may not be any back pressure in that line.Then why pilot operated PSV is being used ?

[ankur2061]

Pilot-Operated valves are recommended for applications where the back-pressure on the valves are very high (typically 50% or higher). The other criteria for selecting pilot-operated valves are as follows:

a. Low accumulation rates are required.
b. The set pressure is required to be close to the operating pressure.
c. Calibration is required without removing the valve.
d. Handling of large relief flows are required.
e. The service is non-fouling.

These are some general guidelines. However, you have not furnished the complete details of your application such as what is the material to be relieved from the POSV, the difference between the set pressure and the operating pressure, the relief scenario considered for providing the POSV etc. Without this information it would be very difficult to arrive at any conclusion why a POSV has been provided in your case.

Regards,
Ankur.

[Preeti Shetty]

Thank you very much Sir ,

The Reliving Liquid is crude Oil and it is operating at 69 barg pressure and the PSV set pressure is 1350 psig,the relief scenario is not mentioned but PSV size is given as 1" X 2"

[ankur2061]

Preeti,

Installing a POSV on crude oil pipeline does not seem the right choice since the orifice of the pilot valve (orifice dimensions are small) can get plugged due to high viscosity and fouling characteristics of crude oil. The set pressure of any PSV installed on a pipleine would normally correspond to or be lower than the design pressure @design temperature of the pipeline. As an example a 600# Carbon Steel Pipe (Material Class 1.1) according to ASME B16.5 would have a design pressure of 93 barg @95 deg C. In your case the operating pressure (69 barg) and set pressure (93 barg) are not even close and there is no backpressure on the relief valve since you have mentioned it open to atmosphere.

Relief valves on pipelines are generally installed for thermal expansion between blocked-in sections of pipe where stagnant liquid can expand due to rise in temperature of the stagnant liquid in the blocked-in section most likely due to solar radiation. Thermal expansion relief valves (TERVs) are generally of the size 3/4"-1" or 1"-2" and calculations for relief rate may not be done since the relief quantity from TERVs is very small.

Looks like installing a POSV is not the right choice.

Regards,
Ankur

[Preeti Shetty]

Thank you sir for this information,

Sir there are two PSV's at the outlet of Pig launcher and in between them an ESDV valve is installed,and these two PSV's are connected to each other through an equalizing line too maintain the pressure drop across ESDV when it gets closed.The POSV is placed downstream of this ESDV Valve and that line is 44km long.The line is 600# carbon steel pipe.
May be this line is long and exposed to atms ,thats why they have taken such PSV ?

[Bobby Strain]

Sounds like you need to do a lot more work to determine the appropriate maximum relief case. Then you can establish the rate according to applicable procedures.

Bobby
.

[fallah]

Preeti,

Would you please upload a simple sketch of the system. It can clarify the system more than too many explanations. Anyway at a glance because the equipment is liquid filled seems POSV (probably modulating type; please confirm/comment) has been applied to prevent chattering during relief. Accurate judgment needs to more detailed information along with a sketch.

Fallah