Floating ball valves leaking at low pressure 1

[ocv]

I'm trouble shooting an instesting problum. Numerious 3" 300# ball valves (floating design) hold fine at high pressure (600 psi) but leak at low pressure (aprox 15psi head pressure) when pumps are off.Water service on pump discharge. Is this a common trait for floating ball valves?

 

[zdas04]

What do you mean by "leak". The two normal answers are "the valve leaks through" or "the valve-stem packing leaks to atmosphere".

I'm going to stick my neck out here and assume you mean the former. A floating ball valve seals by the ball shifting onto the downstream seat. The more dP, the harder the ball is pressed into the seat. When dP is very low, the ball is not held against the seat as hard (or at all) and the clearance between the ball and seat can allow leakage. If this is intolerable to your process I would replace the floating balls with a trunnion ball valve where the seats are held against the ball by springs.

David

 

[MikeHalloran]

Actually, low pressures are a problem for all pressure- energized seals, particularly if they are dynamic, more particularly if the surface speed is low ... i.e. in ball valves.



Mike Halloran
Pembroke Pines, FL, USA

 

[gerhardl]

.. and to add to above answer, seat leakage of floating ball valves may start at low pressure, but might increase over time and after a time also include leakage at increasingly higher pressure.

A thorough analysis will include a large number of parameters, in fact all normal when selecting a valvetype, plus some 'not normal'.

Among others:
Are the valves built for and with a quality suitable for the service?
Expected lifetime?
Operation mode, cycles?
Fluid within given parameters for the valve?
Clean fluid or particles or unexpected particles and additions?
Seal quality, materials?
Maintenance done according to manufacturer specification?
Abrasive or sticking fluid particles when drying?
etc, etc,....

Solution may be given by answers to the analysis, but might also be to change to trunnion mounted ball valves, or to another type of valve construction entirely.

 

[ocv]

Thank you for the replies. I will suggest replacing them with Gate or Plug valves for this on-off service.

 

[zdas04]

You are extrapolating to somewhere that I would never go. Gate valves are never bubble tight. Operators dearly hate them due to the difficulty of operation. The only place I preferentially select gate valves is in steam service.

There is no place that I prefer plug valves.

I think that the most cost effective answer to a low dP leakage problem is a trunnion ball valve, but if the MAWP of the system is low enough you might be able to solve the problem with plastic-body ball valves.

David

 

[NGiLuzzu]

ocv,
in addition to gerhardl questions, I would ask: have the valves to be "piggable" (i.e. completely free passage when they are opened) or you can tolerate the presence of a stem and disc within the pipe?
Moreover, which is the maximum leak rate, more or less, you can allow?

Perhaps high performance (double-offset), rubber lined or triple-offset, metal seated butterfly valves may be a way (for sure the second ones will give better performance than gate valves)... but, on such a little size, you should also consider possibly bigger prices respect to ball valves and, of course, decide if that's worthwhile ;-)

See, for example: thread408-237869, thread408-126483, thread408-64692 and thread408-190488 within this Forum.

(Maybe this is a little off-topic... but I think zdas04 already answered the original question).


Hope this helps, 'NGL

 

[ocv]

Again thanks, I agree Trunnion ball valves and triple off-set butterfly valves would solve this problum. They do not need to be piggable.The cost on both in 316 stainless are considerably higher that the original floating design. I must now convience the user that the floating "design" should not have been used in this application and they will have to use a valve design that will cost three times the price.

 

[hotbat]

Generally, PTFE seats for floating ball valve can withstand to high differential pressure up to 725 psig at 100'F
but, I guess that downstream seats damaged at your existing differential pressure(600 psi).
I strongly recommend to change a seat design in my experiences(reinforce a strength and dimensions)

 

[JimCasey]

There are a LOT of ball valve seat materials besides virgin TFE.

Most any floating ball valve comes out of the box with preload/interference fit on the seat so that it seals at low pressures. Virgin TFE is famous for having low "memory" so once it's exposed to 600 psi shutoff for a while the downstream seat is permanenetly distorted. So, when you go back to low-pressure, the preload is gone and it weeps.

There are veritable multitudes of formulated TFE-based blends with minerals, glass fiber, carbon in many forms, eyes of newt, (whatever). All make the seat less prone to cold-flow. So it will continue to seal after the pressure is reduced.

Delrin, PEEK, and other polymers are also available, with much better success at high pressure swings.

Gate valves leak.

Plug valves are hard to turn, but they can be adjusted to restore shutoff if they do begin to leak.

Double-offset butterfly valves also have resilient seats made out of essentially the same stuff as ball valve seats and they are also process-energized, so some of the same limitations apply.

Triple-offset butterfly valves have a lot of machinery that lives in the flow stream when they are "open" so capacity is affected. And they reqiure you to lean on them mightily to get the vaunted "zero leakage" . This is called "torque-seated" in the literature. You CAN get tight shutoff, but you'll need a strong helper and a bunch of bananas.