Knife gate valve 4" to MSS-SP-81 -- leakage issue 2

[hsbcn]

Hi there, team:

We're having some issues with a metal seated knife gate valve 4" 150# made to MSS-SP-81 standards. Our customer indicates that the valve is leaking; they're operating the valve with steam.

Please see the attached video. The valve has been tested successfully at 3.75bar (body) and 2,75 (seat) at an external test-bench facility.

These are early days, and I'm still not clear on the details, but I have the impression that they're expecting a "bubble tight" seal; in this case, absolutely no leakage from a metal-seated knife gate valve whilst running steam through the system.

First question: assuming 2 bar and 60ºC, would it be correct to assume that the valve would cut off all the steam?

Any input on questions we need to ask and actions we could take to get this clarified would be appreciated.

Thanks!

I work with valves in exotic alloys at The Alloy Valve Stockist. This is our blog, and this is me on Twitter and LinkedIn. Let's connect!

 

[gerhardl]

I can think of several issues here. First question: What are the purchasers specification ordering the valve? Any specification of use for the given fluid (gas/steam) or temperature given?

As the standard referred to, MSS-SP-81, only mentions hydro-static tests, one sided thightness, fluids between 0-66 deg. C, and test fluids limited to 50 deg. C, the MSS description in itself excludes any requirement for tightness at the given operational conditions.

On the other side: I have seen large-size ( I think I remember up to 500 mm) steinless steel knifegate valves subjected to combined media (woodcut filings for production of wood brickets for burning warmed by direct injection of high-temperature steam) at 35 bar.

There is this way no doubt that the market could supply knife-gate valves for the purpose given by you, but theese would have to be produced to further specifications than the standard MSS mentioned.

Additionally: any hair-crack leakage of steam, even at this low pressure, could easily give cavitation and eat the steal away to give larger leakages.

 

[bcd]

A metal seated knife gate is not the correct valve choice for this application if no leakage is to be expected. With a lot of effort, you may get a metal seated knife gate to have zero leak in a factory seat tightness test. But it will not stay zero leak for long in service.

Knife gates are excellent for shutting off against solid, abrasive materials like wood chips, ash, etc. That is what they are intended for. Trying to use one for tight shut-off of a gas (steam) is not what they are good at so it will lead to problems like this.

I would recommend a gate, ball, or butterfly valve for this service, which can give the tight shut-off with no leaks. Also a soft-seated knife gate would work better.