Safety Valve Conversions 4

[panther8288]

If I have a sect VIII safety valve rated at 100 Psi air.
Why do you have to change Nozzle,and nozzle ring and at times other parts,when converting to liguid.
And why does the spring change in this type conversion.
I have noticed many valves which are N.B. coded that have
set pressure Eg. 100Psi Air with capacity in both air and
liquid on the name plate. Thanks for any reply

 

[CHD01]

You don't necessarily have to change anything for the relief valve. However, you need to use the correct formula and coefficients to calculate the required orifice and capacity. Refer to vendor catalogs for the formula required for the use of liquid-trim and non-liquid trim valves in liquid service. Liquid trims to guarantee a 10% overpressure are now offered routinely by vendors; but it did not use to be that way. The more you learn, the less you are certain of.

 

[xcrosby]

Changing from a Section VIII gas valve to a Section VIII liquid valve usually requires changing some parts. Most API type flanged valves will require some of the parts to change. It will depend on the manufacturer. It is not unusual to have to change the spring or one of the other internal components. Some valves have dual certifications. It is not unusual for a liquid service valve to have dual certifications. It is uncommon for a gas service valve to have both. Typically a gas service valve has a blowdown of about 7%. The same valve on liquid service will have a much shorter blowdown and will not achieve the required lift until greater than 10% overpressure. That is why the parts usually have to change. A liquid service valve used on gas service will usually have a blowdown much longer than 7%. This is a longer blowdown than is typically acceptable but may be acceptable for some applications.

 

[CHD01]

The key to whether internal relief valve modifications are required is whether the service requires ASME certification because it is protecting an ASME vessel. If cerification is NOT required, then either a liquid-trim or non-liquid trim valve may be used without modification so long as the correct equation and coefficients are used as explained in my note (an over-pressure coefficient is required - equal to 0.62 for a 10% overpressure; and a different equation is used). My point was that we normally do not see non-liquid trim valves used in liquid services anymore; since liquid-trim valves guaranteeing a 10% overpressure for liquid service are now offered by all major vendors; but there are still many such valves installed in the world. The more you learn, the less you are certain of.

 

[JAlton]

COnversions from compressible to incompressible fluid depend on the age of the PRV. Liquid certification of PRVs came along in 1984. Many PRV manufacturers came out with Liquid Trim to meet the new code standards. Some used weaker springs to try and achieve overpressure by 110% of set. Hence the need to change springs. Other Part changes, disc holder, adjusting ring, etc. are mfg. specific. A few design need no change. However, the method of certification of capacity is different, and requires consideration. To detrermine if a PRV is certified for one or more fluids, go to

http://www.nationalboard.org

and click on NB-18, Pressure Relief Device Certifications. It is a great reference.