Control Valve

My guess: one has 1 seat and the other... perhaps 2. Like for double sure, or for double control in stages?

I do understand what you are saying, but I also believe the design/manufacturing quality of the valve itself affects the efficiency of either closure.

I do not remember this being a problem once the valve was fully and correctly adjusted on RN steam systems despite really high pressures and extreme temperatures.....

I expect wear and tear required them to be replaced/refurbished every few years or so....

About all that I can offer apart from my limited experience with these is a link to people who make them that confirms my point.

Take note of where they state that very few of these valves will fully seal due to the reason that I gave.

I am sure that you can find more of the same with a google search of your own.

http://www.spiraxsarco.com/resources/steam-engineering-tutorials/control-hardware-el-pn-actuation/control-valves.asp

With regard to the spring loading suggestion, blowback pressure forcing back through the sprung seal would be a potential problem, so in effect the valve could not be claimed to be fully sealed.

What you are completely forgetting is that one valve will try to be "blown open" and the other try to be "blown closed".

If the spring(s) are on the "blown closed", you have a valve that seals at both positions.....simple.

The amount of movement allowed on the springs must/is very small as that valve side will tend to close first, but at the same time, providing a slightly more progressive "throttling" at the almost closed position....it must be a very small amount of movement.

The fixed seal will be unable to move but the spring loaded one, no matter how small the movement is restricted to will still leak.

I see that another poster has recently stated the same fact that I did, so I suggest you do a little more research on the subject if you really want to know the limitations of these valves.

I believe we are talking about different "styles" from different manufacturers....

We are on two different sides of the Atlantic I believe (you give me that impression, though you do not give a location - many US citizens like to keep it secret!), we both probably speak "English", but accent, spelling and meaning of some words have progressed in two different directions.....

Which is why I personally always speak about English and American......

Do you not believe its possible that such valves can be slightly different as well?

Also, you appear not to have read and understood fully what I wrote, so you will not understand either as well as you might!

Have a great day anyway.

If the seats are resilient or one of the disks is spring loaded, tight closing can be achieved.

Thanks for your valuable info. Can you temme is there any advantage of double seated valve over single seat valve other than Actuator size selection?