Low Temperature Trunnion Ball Valve

[aggieengineerrmb]

I have a very low temperature application, -70F, which may not be truly cryogenic, but appears below the level that most valve companies are experienced. It will almost never see those temperatures, the vast majority of the time it will be above freezing. We have had some companies come back with what appear to be almost standard trunnion valves, some utilize spring entergized lip type seals for the stem and valve seat ring, and others have very exotic extended bonnet designs. Of course, the price-delivery increase in direct proportion to how non-standard the design is that they offer.

It's very difficult to sort through all the claims from the different MFG's to determine what is really required. Does anyone know of a good reference on the design of cryogenic valves, or as a minimum some basic information you have gained from experience?

 

[joeswoes]

-70 does not even touch "cryogenic", which to me is ARO -250 F. Avoid ductile iron and stay with stainless steels for low temp. ductility. You might want to consider some version of live-loading of stem seals.

 

[PeterIgg]

Basically I agree with joeswoes. The issue is: Shall the valve be required to operate (viz move from closed to open or vice versa) whilst the valve is below 32°F (0°C)? The reason being that ice can form around the stem (and actuator). The ice can cause the stem seals to be damaged, hence the suggestion to use a bonnet to keep the stem seals at or above zero Celsius and anyone who expects a valve to operate reliably whilst covered in a ball of ice is hopeful, to say the least! At -70°F there are few seal materials that will not embrittle, Ptfe lip seals being one of the exceptions.

I suggest that you need to be 'upfront' with your suppliers and state just exactly what you need out of the valve to avoid either over or under specifying the valve so that you end up with a product fit for the purpose at the most economic cost to yourself.

 

[mqas]

A few thoughts from a manufacturer

Manufacturers who have quoted a standard design have assumed that the valve will not be operated at the design temperature. e.g. operated from close to open and the pressure allowed to drop with resultant drop in temperature. The valve will then not be operated until the prssure is virtually nil and the temperature of the valve has started to increase.

The manufacturers who have added PTFE spring energised lipseals to the stem and seat components have considered that the valve is expected to operate at the design temperature. This is simple modification of the standard design and they have assumed that the main body seal which is static and may be an oring or block PTFE seal is suitable without modification.

The manufacturers who have added a stem extension is considering all possibilities i.e. a valve that will be fully operational at all pressure and temperature design limitations.

PeterIgg is correct in that only you know the true design aspects of your system and what you need the valve to be able to do. Discussing this with each of the valve manufacturers may help you obtain the optimum solution.

 

[aggieengineerrmb]

Thanks for the valuable information and insight provided so far. It does help a great deal. One question however, how does the formation of ice cause potential problems with the stem seal as PeterIgg mentioned. I'm sure you are correct, as I have seen pictures of valves covered in ice, but I can't see how ice on the exterior of the valve would affect the seal on the internals.

 

[scalleke]

Ice outside the valve will not affect the sealing of the valve But it will affect the sealing of the packing on the stem. Ice can get attached to the stem and go through the packing when the valve is cycles and the stem moves. this is more an issue with gate valves (that have graphite packings) than it would be with a ball valve.

At lower teperatures the soft seats will harden and will deform less in the ball to seat contact area. This is why it is adviseable to have narrower seats at lower temperatures. The narrower seat to ball contact will increase the contact stress and the valve will seal better at low temperatures.

Best regards.

Scalleke