Valve Seat Design

[shaff]

Hi,

I have 2 doubts with regards to valve seats

1. What is the meaning of this statement "The ball and metal seat ring assembly shall be part matched by lapping". It would be great if you could guide me to some technical article or paper.

2. What is Primary Metal Secondary Soft (PMSS) means? I tried to figure out referring many catalogs but the concept is not clear. Where exactly the metal seat is coming and where the soft? I am of the understanding that soft seat will be in contact with ball and metal seat will come in contact with ball only if the soft seat is damaged. I am not sure it can be the other way round...

I am not deeply into valves in my job but curious...

 

[valvit]

Shaff,

1. Often valve manufacturers will match-lap a seat to a ball to ensure high performance in the closed position. This means that they will take the seats, and label one Seat-A, and the other Seat-B. The ball then, will have a corresponding (closed position) Side-A and Side-B. Seat-A is lapped to match Side-A, and same for Seat-B/Side-B. By lapping as a matched set, the surface of valve ball is fine-tuned to match the exact shape and profile of the seat that it will be closing against. Doing so requires attention during assembly, as matching Seat-A with Side-B will generally result in a high leak rate through the valve. Spare parts are often provided as a match-lapped set.

2. PMSS seat design was invented by Marvin Grove (Grove Valve and Regulator) in the early 1960's. The seat design is a two-piece configuration that provides both metal and soft sealing, in one seat. The "primary metal" makes contact with the spherical surface of the ball, and alone provides fairly good sealing. The "secondary soft" is an elastomer o-ring that fits into a groove created by the assembly of the inner and outer seat ring. The o-ring will provide zero leakage on both gas and liquid test (ISO 5208 Rate A). Both the metal portion of the seat, as well as the o-ring, are in contact with the ball at all times. The PMSS is considered a soft seal by the industry. Valve manufacturers (at least everyone who has been copying Grove designs forever) will use this seat design up to and including Class 600 valves, in sizes up to NPS 60.

valvit

 

[mbelly84]

PMSS is often cause of troubles / misunderstandings between EPC/end users and valve manufacturers.
It is clear that PMSS by definition, as correctly described by Valvit, cannot be used for class 900 and above due to the risk of extrusion / explosive decompression of the elastomer.
Anyway there are some EPC/end users that insists asking for PMSS in class 2500.
So what happens is that a lot of valve manufacturers sells them some nylon seats, claiming they are PMSS.
Other "honest" ones try to explain customer this is not possible, and they risk to be excluded technically from the bid.