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bmoorthy (Mechanical) (OP)
12 Sep 05 11:08
Hello All

Our client has specified Trunion mounted ball valve.

As per my understanding in trunion mounting, at the bottom of the ball, there will be a central rod (Which is called trunion) which is similar to the dimension of the stem, and trunion is supported to the bottom boss.

I now have a couple of valve manufacturer's brouchers and Cross section drawings which does not have the bottom central trunion, instead there is a step in the side (Just after the seat). The manufacturer still terms this as turnion mounted,  they claim that it is not seat supported and the ball is supported by the body (The step i mentioned)and hence trunion mounted.

1)Can some body tell me the correct definition of trunion mounted ball valve.

2) Are the claims made by the valve manufacturer correct

3) If it is correct, does it meet the intent of trunion mounted valve

4) In which standard can i find the details of trunion mounting design


Thanks

B.Moorthy
Sircrashalot (Mechanical)
12 Sep 05 13:25
A trunnion mounted valve means that the ball is constrained by bearings and is only allowed to rotate.  The bearings act on the trunions which may integral to the ball, or may be separate depending on the valve design.  The key feature is that the ball does not shift as it does in a floating valve to press the ball into the downstream seat.  Instead, the line pressure forces the upstream seat onto the ball to cause it to seal. As the area on which the pressure acts is much lower, the amount of force exerted on the ball is much less, leading to lower friction values and smaller actuators or gear boxes.

Andy Harbin, P.Eng
zdas04 (Mechanical)
12 Sep 05 15:52
Sircrashalot,
That isn't quite right.  No manufacturer's trunion ball valve that I've looked at relies on upstream pressure to shift the seat into the ball.  Every valve I've looked at uses a series of springs to push both seats into the ball.  This configuration allows a body bleed to be used to evacuate the body cavity (for a double-block-and-bleed that meets many company's hot-work requirements).  Without the springs, the downstream side would keep the body pressurized.

bmoorthy,
Look at the manufacturer's drawings to see if there is a body bleed and at the seats to see if they are spring loaded.  If the valve has both of these then the step could be about as good as a trunion and trunion bearing, but I have a hard time seeing how the step you've described could be effective to prevent ball movement from either direction, maybe I'm just missing something.  If the ball can move, or the body-bleed is missing, or if the seats are not spring loaded, then avoid the valve at all costs.  

I've never seen any of the standards define the difference between floating ball valves and trunion ball valves.  To me the difference is that a floating-ball valve relies on the ball shifting into the downstream seat to seal and a trunion has components to prevent lateral ball movement from either direction and has spring-loaded seats.

There are several good manufacturers of Trunion Ball Valves.  I've had really good luck with PBV (http://www.zy-tech.com/) but there are many others that are as good.

David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
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bmoorthy (Mechanical) (OP)
13 Sep 05 0:36
Thank you both for the input.

Could you please clarify the following (Some basics)

A) Does all ball valve with trunion mounting need body bleed.

B) If the bleed is provided then does it mean that all the trunion mounted ball valves are doulbe block and bleed type.
Zine2 (Mechanical)
13 Sep 05 7:06
bmoorthy,
First, all valves need body bleed; the trunion mounted ball valves aren't doulbe block because rotating the ball (close-open or open-close) the pressure is retained only by the body.

Trunion ball valve designer.

Regards

zdas04 (Mechanical)
13 Sep 05 7:07
Every trunion ball valve I've ever seen has a body bleed.

Satisfying double-block-and-bleed is an individual company decision.  Since the seats are spring loaded, each seat in a trunion ball valve is a more positive seal than the downstream seat in two floating ball valves, so company's should accept it, but the size of the vent is so small that some don't (and some require blowing the body down and then removing the plug to give a larger vent).  The regulations describe the concept and then require you to meet it without specifying technologies.  I prefer to get double-block-and-bleed with a trunion to using a pair of valves separated by (sometimes) miles of pipe, but not everyone agrees.

David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
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bmoorthy (Mechanical) (OP)
13 Sep 05 7:49
Thankyou all for the valuable input.
One more query.
The manufacturer offers Thrust washer instead of bearing. Does this mean the Trunion mounted design requirement is met?
Is thrust washer (Which is essentially a metallic ring (Disc plate) with PTFE coated) a bearing?

zdas04 (Mechanical)
13 Sep 05 8:59
Nickollas,
I don't think I understand your comment that "every valve needs a body bleed".  A body bleed on a floating ball valve would never blow down since the ball has "floated" to the downstream seat and is slightly off the upstream seat (keeping pressure on the body cavity so it wouldn't blow down if it had a body bleed).  I've never seen a floating ball valve with a body bleed.

If I shut a trunion ball valve and open (or remove) the body bleed then I have isolated an evacuated space between two seating surfaces--that is the definition of double-block-and-bleed as I understand it.

Bmoorthy,
I'd have to see the design of the thrust washer to see if I would use it in a place where I would specify a trunion ball valve.

David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

JimCasey (Mechanical)
13 Sep 05 11:17
David, the seats in trunnion valves ARE commonly pressure energized AND they use a spring load to bring them into contact with the ball.  Look at the seat.  The area exposed to the upstream pressure is greater than the area exposed to the downstream pressure, results in a net pressure loading of the upstream seat. As soon as the ball is rotated closed and the seats can seal against the ball, the differential is generated to "suck" the seat against the ball.  

(2) Body Bleed
It is quite common in a floating-ball valve to drill the ball so that there is a hole between the bore and the upstream port when the valve is closed.  Any trapped liquid that might vaporize and expand can then do so at no greater than upstream line pressure.  In a trunnion valve if the cavity pressure gets much above the line pressure, the spring-loaded seats >should< relieve the pressure before the valve is damaged.  However this does not guarantee that line-pressure fluid is not trapped in the cavity.  Venting the ball DOWNSTREAM in a trunnion valve, notching or removing the downstream seat, or providing a cavity vent, are all ways to ensure that the cavity cannot overpressure due to expansion and is depressurized for disassembly.    
zdas04 (Mechanical)
13 Sep 05 11:48
JimCasey,
I think we're saying the same thing.  I see the springs as sealing the seat against the ball and the differential area as sealing the seat against the valve body.  

The drilled balls in the floating ball valves are very common for tank load-valves that you don't want to risk trapped liquid flashing or freezing.  The issue is a rate of change of state.  For freezing (for example), the frozen liquid will plug the (very small) path around the ball to the upstream side and you can develop very high pressures within the cavity.

I've never used (or recommended) a trunion ball valve in liquid service, I'm not sure I'd trust that the springs will reliably release in a significant temperature-change situation.  Before I'd notch or remove a seat in a trunion ball valve I'd just install a less expensive floating ball valve.  In gas service it is unlikely that the condensible liquids in the gas stream will be of adequate volume to result in the body cavity becoming liquid-full.

For an absolute spec break I've put a PSV on the body bleed of a trunion ball valve used as an ESD.  This works well.

David
Sircrashalot (Mechanical)
15 Sep 05 12:33
David,

Yes, the seat is energized initially using the springs, but the main sealing force comes from the line pressure differential between the seat and the cavity.  The springs get a seal started so that the pressure can build up and to seal the valve at low pressures.

For cavity relief, all the cavity pressure has to overcome is the load generated by the springs - once the cavity pressure and line pressure are the same, the springs are the only thing generating the seal.  It does not take much cavity overpressure to have this relieve and the valve will always relieve to the low pressure side.

I have used trunnion valves in numerous liquid services on offshore platforms (heating medium, water injection, crude offloading, etc.).

You can also get into double piston effect seats that DO NOT self relieve (seat seals by pressure in BOTH directions - if upstream seat fails, downstream seat maintains seal).  Valves with DPE on both seats do need PSV's - API6D even talks about these.

Trunnion valves tend to be more expensive than their floating cousins and build by more specialized manufacturers - there tends to be a lot more parts in a trunnion valve.  But, depending on the application, they can often be a better choice, expecially for higher pressures.  Most of my clients specify all ball valves 6" and larger or class 600# and higher and ALL actuated valves are to be trunnion mounted.

Andy

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