Can someone suggest a good source to guide in the design of a gas control valve trim (plug and seat) set? I must comply with NACE MR0175.

Some of my questions are...

> What are my material options?

> What should I specify for hardness requirements (plug versus seat)?
Assuming you are a user of valves I recommend you talk to the valve supplier and get their recommendation. Some of the preliminary questions I have are 1) what is the gas?,2)what type of valve is this?, and 3) how big is the valve?

Typical valve trim materials are:
316 SS - ASTM A351,CF8M (cast)--- Notes 1,2
316 SS - ASTM A479,GR316,A (bar)--- Notes 1,2,3
416 SS - ASTM A487,CA6NM,CLB (cast)--- Notes 1,4
416 SS - ASTM A582,Type 416,A (bar)--- Notes 1,4
Note 1 - Any weld must be done to NACE welding procedures. Avoid this at all costs.
Note 2 - Material hardness HRC 22 Maximum, no cold working allowed.
Note 3 - Must be in the solution annealed condition per supplimentary requirement S5.
Note 4 - Material hardness HRC 23 Maximum, no cold working allowed.

The choice of valve trim material is largely based on service conditions (Temperature & Differential Pressure.) You can use 316 SS up to 300 psid and about 600F. Beyond these boundaries the hardened materials are generally used (17-4PH, 410SS, 416SS) but to reduce the corrosion potential NACE requires these materials to be in the "passivated" condition. For reference (Part 3):
17-4PH - double age hardened HRC33 max (table A.27)
410SS - austenitized/double tempered HRC22 max (table A.18)
416SS - austenitized/double tempered HRC23 max (table A.18)

Beyond this, would be the base material to which a coating such as hardfacing is added.

Generally valve plugs and seat rings are made of similar materials with similar hardnesses. For general service valves the HRC 33-38 is very poplular, therefore I would suggest using 17-4PH in a NACE valve.

The concept of "differential" hardness relates to valve trim components that slide on each other. Galling is an ever present danger especially in the 300SS and softer materials. A general rule of thumb is any sliding parts should have 6-10 points HRC differential.

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