Pressure Safety Valve (PSV) Material Selection

Specify the Pressure Safety Valve (PSV) material could be simple and might be difficult for some situation. It becomes simple because the API 526 only state the requirement of the body, bonnet, and spring material. Nevertheless, the API 526 standard leave the internal part material to the manufacturer’s standards. If the project requirement or Company standard require the material of the PSV shall follow the API 526, then the material selection of the PSV is simple. Just specify the body/bonnet material of the PSV in accordance with the related PIPING globe valves and leave the other parts to manufacturer’s standard. It means, we need to know the piping class of our PSV, see the globe valve code number for the specified piping class, and then open the manual valve datasheet of the globe valve. We will then just simply state the disc, guide, nozzle, etc, to become manufacturer’s standards.

If the project requirement or the Company standard require that the PSV material shall be in accordance with the piping class material, then it would be a little bit complex. The way we select the PSV material is a little bit different with control valve material selection where we just simply copy and paste the selected material from the piping globe valve datasheet of the related piping class. However we could also do this kind of selecting the material of PSV as we do for the control valve by a little bit ‘synchronization’ of PSV and control valve part as below.

 

1.           We will find the following part of the globe valve in the manual valve datasheet specified along with its material.

Body

Bonnet

Disc

Seat

Stem

Gasket:Body/Bonnet

Bolt

Nut

 

2.           We will find the following part of the PSV in our PSV datasheet that need to be specified along with its material.

Body

Bonnet

Spring & Spring Washer

Disc

Nozzle

Disc Holder

Spindle

Guide

Adjusting Ring

Bellows

Internal Gasket Material

 

3.           Specify the material of the PSV as follows (Note 1):

PSV Part	PSV Material Equal to ….of Globe Valve
Body	Body
Bonnet	Bonnet
Spring & Spring Washer	Body/Bonnet (Note 2)
Disc	Disc
Nozzle	Seat
Disc Holder	Stem
Spindle	Stem
Guide	Stem
Adjusting Ring	Stem
Bellows	Stem
Internal Gasket Material	Gasket:Body/Bonnet (Note 3)

 

Note:

1.           Just use this as a guidance, if any doubt, material specialist engineer shall be consulted for further review.

2.           For spring and Spring washer, just specify the material series of the body or bonnet material, for example Carbon Steel for A216 WCB, Stainless Steel for 316 SS. This simplification is necessary because the material code of the spring is different with the body/bonnet material standard. The spring material in typical is having more strength rather than body/bonnet material. The corrosion issue in the carbon steel spring could be negligible due to the infrequently operation of PSV and also a very minimum of process fluid entering the bonnet chamber.

3.           The internal gasket of PSV could be the same with the internal gasket of globe valve. But typically the vendor would offer 316 SS with graphite filler as standard gasket even though for carbon steel body/bonnet. This is acceptable and become a common practice. The worry about galvanic corrosion could be negligible because the gasket will be fully enclosed inside the body/bonnet connection and will only in contact with the process fluid. The galvanic corrosion would only occur if the two dissimilar metals are in contact with a medium which acts as an electrolyte and allows migration of ions from the anodic metal to the cathodic metal through the electrolytic medium. As long as the gasket could make sure the integrity of the body/bonnet connection, compatible with the process fluid, and safe to use (no asbestos contain), then it should be acceptable.