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Control Valve

11/07/2013 11:07 PM

Dear all,

what is the difference between single seated and double seated control valve?

what is their sgnificance?

Thanx:)

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#1

Re: Control valve

11/07/2013 11:19 PM

One seat.

They control something!

Welcome:)

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#2

Re: Control valve

11/07/2013 11:19 PM

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

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#3

Re: Control valve

11/08/2013 12:28 AM

The two sets of disks and seats are arranged "flow up" and "flow down" so that the opening/closing forces are balanced to near zero.

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#4

Re: Control valve

11/08/2013 6:18 AM

single seated control valve controls the pressure by allowing the pressure to flow via single way but double seated control valve will allow the pressure via double ways. pressure affects the working of double seated but not single seated

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#6
In reply to #4

Re: Control valve

11/09/2013 3:41 AM

Tornado got it absolutely right and explained it correctly, read his post and learn!

Your explanation reads/appears to be as wrong as you can possibly get!!

Possibly bad English?

Just to sum up to make sure you understand fully:-

Pressure affects both. With the single seated valve the pressure "helps" movement of the valve in one direction and "hinders" in the other.

With the double seated valve, the pressure is balanced on either side and opening and closing of the valve requires far less effort in the normally "hindered" direction.

Naturally slightly more effort is needed in the "helped" direction, but in actual fact, it is as though there are as good as no pressure loads on the valve in either direction.....

In my very limited experience, I have only seen/noticed them fitted on the main steam control valves from the main ship's boilers to the propeller Turbines on RN Aircraft Carrier's main engines. Prop shaft revolutions were controlled with such valves on steam systems.

I remember as a teenager asking why and getting a really good explanation!!!

Its a simple but effective "very old" design.

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#5

Re: Control Valve

11/09/2013 1:43 AM

Dual seat valves enable much less opening and closing force for actuation, but due to differences in expansion rates, they are never fully closed. When one seat is tightly shut, the other will be slightly ajar.

For this reason, single seat valves are chosen when full closure is essential. If this is not a requirement then a dual seat valve would normally be preferred.

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#7
In reply to #5

Re: Control Valve

11/09/2013 3:45 AM

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....

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#9
In reply to #7

Re: Control Valve

11/09/2013 5:05 AM

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.

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#11
In reply to #9

Re: Control Valve

11/09/2013 6:09 AM

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.

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#12
In reply to #11

Re: Control Valve

11/09/2013 4:00 PM

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.

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#13
In reply to #12

Re: Control Valve

11/10/2013 2:46 PM

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.

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#8
In reply to #5

Re: Control Valve

11/09/2013 4:15 AM

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

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#10

Re: Control Valve

11/09/2013 5:42 AM

Single seated /ported Valves - has single seat. This is also known as single ported valves. These are used for tight shut-off. One of the disadvantages of single seated globe valve is that they require larger actuator due to unbalanced design compared to double seated valves.

Double seated /ported valves - here the two plugs ( top and bottom) are arranged in such a way that flow tends to open one plug while close the other plug. The opposite forces acting on the plug tends to cancel each other and hence requires smaller actuator compare to single seated valves. Generally it has higher flow capacity than single seated valves. The disadvantage is that they have higher leakage rates at shut-off. Also erosion may occur at higher pressure drop due to leakage characteristics.

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#14
In reply to #10

Re: Control Valve

09/10/2014 2:57 AM

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?

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