Unbalanced force in Doubly/Triply offset butterfly valves 1

[rgprasannakumar]

thread408-126483

When the valves are doubly/triply offset, I understand that the pivot and stem is offset from the centre of pipeline. In such cases, the highly pressurized fluid at one side causing unbalanced force. Wont this make the valve to open? If not, Why?

 

[hsbcn]

Hi!

The following thread, and the threads referred to in it, will be of help to you:

http://www.eng-tips.com/viewthread.cfm?qid=126483

Good luck!

My focus: Alloy Valves Duplex Valves Monel Valves Inconel Valves [link titanium-valves.com]Titanium Valves[/url] Hastelloy Valves

 

[rgprasannakumar]

Hi HSBCN!

After reading those entire thread articles only I got this doubt.Pl clarify

 

[NGiLuzzu]

It must be taken in account that triple offset valves are torque seated, so when pressure is applied on one side, they need less torque to seat and reach the required tighness, and more torque to unseat (against pressure); the opposite when pressure is applied on the other side.


Hope this helps,
'NGL

 

[rgprasannakumar]

Gotcha!!! Now I understand. Earlier I thought the longer offset section is open outward to make the fluid flow upstream. But now we need to set the valve in such a way that we need to open against the pressure.. Right?

 

[bcd]

Not in all cases. SOme valve designs do work better when pressure is on the shaft side and is trying to push the valve closed. Some double offset designs work better when pressure is trying to open the valve. It depends upon if the valve has a pressure energized seal when the shaft is on the downstream side.

The amount of shaft offset which theoretically causes an unbalanced pressure force on the disc is very small. Just enough so the disc does not touch the seat when the valve is open. When you multiply the total pressure force on the disc by this very small offset, you get a very small torque. Typically, the packing friction can be enough to resist the torque from pressure. Bearings and seat friction also help hold the disc in place. Consider that the offset is only a few millimeters. Maybe only 1 mm on small valves up to 5 mm for very large butterflies.

 

[rgprasannakumar]

@BCD: Assuming an 150 NB pipe with 1 mm offset, operating at 8.8 bar pressure, approximately it is exerting 13.2 Kg (130 Newton) of force. Will it sustain?

 

[bcd]

It may, or it may not, depending upon how much friction there is. And friction is completely unreliable. If you have graphite packing, and the packing is tight, it will hold. But if the packing is not tight, it may not because then all you have is bearing friction. Does the valve have metal or PTFE bearings? This would also affect how much friction there is.


One thing I have learned in over 30 years of valve engineering is that you can never rely upon friction. When you need it, it is not there. When you don't want it, it appears. Vibration, inadvertant lubrication, temperature chagnes, etc. can happen at anytime and drastically change the coefficient of friction in an undesireable way. So if there is any risk of harm or damage that can occur if the valve opens under pressure, then ensure that it cannot.

 

[Allent2002]

Lets not forget - In general there are operators like Latching Handles or actuation packages that will also lend a specific force to maintain tight closure.

 

[gerhardl]

Back to original question:

1. Valve closed will remain closed.

Reason as given by others above: friction, torque, actuators holding force and locking. In addition: geometry will not give anything but increased pressure against the whole seat by increased pressure on the disc, in preferred flow direction.

True: for most valves of this type, the construction will normally set a limitation of the overpressure in the 'not prefered' flow direction. The construction is however always such to include (and specificfy) the maximum allpowed pressure this way. Normally this pressure will be at least up to and including the pressure classes working test pressure with closed valve (both ways), but practical pressure might be allowed higher (by the manufacturer) the prefered way.

2. By opening valve, forces will be balanced and caretaken by construction.

Any valve, offset or not, will be exposed to different forces by the fluid flow and pressure when opened. Any good constructed valve is arranged and tested with maximum conditions given. Quality valves can be recognized by solid stems, stem bearings, sealing constructions, solidity and methode of stem fastening to disc, disc construction and actuation construction.

 

[rgprasannakumar]

Ya. Got the point.
Anyway I'm getting information for every post.