API 609 Butterfly valve trim definition 3

[svi]

The API 609 deifinition reads as,

"The term trim refers to shafts, seating surfaces on the body and disc, and internal keys, pins, screws and bushings in contact with the contained fluid are also considered trim".

Why is the disc not treated as a trim part? Why is it the seating surface alone of the disc is treated as the trim?

 

[gerhardl]

Remember the origin: American Petroleum....

I would imagine disc not described as trim derivates from the normal use of descriptions within the branch where body and disc are separately described (in detail) and with demands on material and other tests for the actual application, and that trim (as described above) might vary and also have a different set of demands.

Also the word in itself 'trim' describes in my opinion 'lesser details to finalize or make ready for use, and attached or in connection to an already existing or described larger item or construction' (My own interpretation)

Other opinions?

 

[bcd]

The intent of only stating "seating surfaces on the body and disc" is to avoid someone thinking that the entire part must be made from corrosion resistant material. Sometimes coating or facing the sealing surfaces with corrosion resistant or harder material is adequate for the service. And usually it costs less.

 

[svi]

This one is about a butterfly valve intended to be used in SOUR service. The specification calls for Inconel 625 trim. The body and disc are of ASTM A216 WCB casting. An acceptble material that may be used in accordance with NACE MR 0175.

What would the use of Inconel impart in this case. Will the disc have to be of Inconel, either solid or overlayed fully. If that is the case then even the body internal will need to be overlayed.

The additional material attributes required for the materials used for the trim components must be also from erosion and wear, in addition to corrosion. In this context will it be enough to have only the seating surfaces of Inconel as it says in the API 609.

 

[bcd]

A butterfly valve with an unprotected (bare) WCB disc will rust and leak at the sealing surface. You can sometimes get away with not coating the body as the sealing surface may be isolated from the media.

Inconel is applied by welding. It typically takes a few layers of weld before the top layer is mostly the corrosion resistant material. The first weld layer will be a mix of WCB and Inconel.

In this case, you need to challenge the speficiation and question what they are expecting. Otherwise, if you just want to coat the OD of the disc, make sure your quote clearly identifies what you are planning to do.

 

[gerhardl]

svi:

In myopinion bcd has pinpointed the correct answer in his last posting, ref. also the other good answers.

I will have to add following to my first answer:

The NACE specification has not a direct connection to the API trim definition.

In the 'day to day' language trim will sometimes be seen as to include the disc, other times not for a butterfly valve. The 'trim' definition is thus in day to day language not precise.

Conclusion: you will land as often necessary with unprecise technical descriptions, as bcd says, in asking the end user what is expected regarding disc material, eventual overlay or special surface and other details not directly specified in the constructional description or NACE requirement (centric, or double or triple eccentric), testing and strength details, any choices if several is given etc.

 

[hsbcn]

Hi svi:

Why is the disc not treated as a trim part? Why is it the seating surface alone of the disc is treated as the trim?

If a disc carries a coating as "surface", then that's the material which the medium will be in contact with. If the disc doesn't carry a trim the "surface" is the material of the disc itself.

The logic behind the wording? It's just API's shortcut way of saying: what matters is what the medium is in contact with, whether its a solid disc or the coating on the disc.

Trim is often referred to as "wetted parts", which is the more palatable intelligent lay-man's term!

Hope this helps!

Heather Smith
Alloy valves from The Alloy Valve Stockist

http://duplex-valves.com

 

[itascot]

Inconel, in butterfly (high performance/off-set) valves is normally associated to seal "ring", being in this material.
As bcd mentioned "Seating surfaces on the body and disc" the seal ring is assembled to disc and becomes the seating/sealing component, flexible...
Try checking out any high performance (try double off-sets)metal seated butterfly valve manufacturers, I am sure you will find mention of Inconel as seal ring material.
Ciao,

 

[itascot]

Just a note on what is commonly understood by trim in every day life...INTERNALS would be the best way to define what most people understand when they read "trim".
Ciao,

 

[JLSeagull]

Regardless of API I would list the disc when addressing trim in a butterfly valve. In control valves we use the phrase characterized trim. The disc characterizes the flow in a butterfly valve.
See ISA 75.05.01 for control valve terminology. 3.172 trim:
the internal components of a valve that modulate the flow of the controlled fluid.

 

[itascot]

Exactly what I tried to explain...would you say "generally" that a disc is part of a valve's internals?...

 

[hongdou]

The trim can be understood as parts inside the valve that will be in contact with medium, such as stem, seating surface on the body and disc, if there is no seating surface on the disc, then the disc itself is a part of the trim too.