Ball valve partially open 2

[alimrheg]

i have reduced bore ball valve and i'm opening it partially,not fully open.
prssure differential acroos the valve is 3 bar
ball material is SS316
seat PTFE
any body know is that going to affect the valve,can i keep partially open?

 

[MikeHalloran]

You can't expect it to live forever, even passing an innocuous fluid. ... and by the way, you haven't stated what fluid is in your valve at what pressure and what flowrate.



Mike Halloran
Pembroke Pines, FL, USA

 

[alimrheg]

fluid is LPG
flow rate 15-20 M^3/h
valve upstream pressure is 8 bar and down stream pressure is 5 bar

 

[gerhardl]

... I agree with Mike, and could you please also state temperature,dimension and opening degree?

What you actually do seems to be to try to use a ball valve as a throtteling valve. Even at low delta p's and 'reasonable flow' this is not adviceable or generally recommended. A regulation will often require a very small opening to be used, hence very fast speed and maybee cavitation, abrasion of valve and sealing, or drawing out or misfiguration of the sealing.

Even if a calculation 'proves' that a 'large enough' opening is the one to be used, local situation or practical use or variation could limit the lifetime from very short and up.

Impurities in the flow will increase the risk.

 

[zdas04]

Valve manufacturers used to talk about "linearity" which was a measure of the change in flow rate assocaiated with a change in valve position. I always found this designation very useful, but it has gone out of vogue in favor of discharge coefficients.

The linearity of a standard reduced port ball valve is very pooor. A change from shut to 15% open would give you a change in flow rate from zero to 80% of full-open capacity. And within that first 15% the profile is very much non-linear.

Globe valve linearity was a function of the seat angle from flat (not very good linearity) to very steep (like a needle valve, very good linearity). Chokes and regulators are designed to be as close to a 1:1 as economically reasonable.

Now, if we assume that your M^3/hr is really m^3/hr then you are flowing a pretty small amount of LPG (assuming that it is still liquid at the point of the throttle valve), and taking that pressure drop with a ball valve would be among the dumber things you will do in your career.

David

 

[alimrheg]

the valve will be closed 50% and still on this position for ever
pressure drop is 3 bar
delta T =5 degrees
valve material
ball is SS
seat PFTE

what is the problem ?

 

[zdas04]

You're the engineer, you have to answer that for your system.

When I've seen fixed chokes installed in real flows that didn't work real well. Small changes in upstream pressure change mass flow rate through the choke which can actually make a very large impact in the downstream friction.

David

 

[gerhardl]

I presume that you wish your valve to regulate, and that you wish your valve to act as a throtteling device.

If this is the case I would strongly advice against it, the least you should do is to put in a v-port ball valve (inside 'ball' is part of a ball-shell with v-formed port for regulating).

By experimenting a bit with an online-calculator I suspect you might also be 'off the target' a bit with presuming that throtteling a reduced bore ball-valve would throttele sufficiently at 50% by the data given. You should perhaps double-check your calculation.

The link below illustrates the front edge of 'ball valves' for gas regulating purposes. Please note that modified seats are highlighted as necessary for precise regulation.

http://www.fmctechnologies.com/en/FluidControl/Technologies/CompactValves/ThrottleValves.aspx

 

[alimrheg]

thank you gents for this information
actually i'm process engineer,and if there are any checks you advise me to check before keeping this valve partially closed i welcome

 

[moltenmetal]

Actually, people do mount gear operators or actuators and rotary positioners on ordinary ball valves, plug valves and even butterfly valves, and use them for throttling and control applications. A slot or vee notch in the ball or plug (greatly) improves the Cv versus %travel characteristic but is not absolutely necessary.

If your service is free of solids, non-flashing, limited differential pressure, requires only coarse control and the downstream system can tolerate the full open Cv, the valve can last a very long time.

Your service is very likely flashing, and your differential pressure of 3 bar is not tiny but not gigantic either. You might be in trouble, but you probably won't be- unless the flashing generates autorefrigeration that the valve is not designed for.

I'm not primarily worried about the valve, though: I'm worried about what happens to the rest of your system if someone ever fully opens that valve. All it takes is less than 1/4 of a turn...a misplaced knee etc.

If the valve has a straight lever handle, get rid of it. Either remove the handle entirely or replace it with a gear operator, so that nobody can suddenly open it fully.

 

[MikeHalloran]

I have worked on systems where a ball valve was installed for throttling, and where leaving it full open would have no immediately noticeable effect, but would damage a lot of expensive machinery, where certifying organizations would not allow removal of valve handles, and where ranking idiots didn't like to see valve handles in odd positions, so they would just open it or close it fully during their 'inspection' tours,

After a few instances of being yelled at for the collateral damage, I took to placing hidden orifices in the system, e.g. at the root of a long branch of a fabricated tube tee, not accessible with a jobber drill. I sized the orifices to prevent collateral damage even if the throttling ball valve somehow went full open.

That tactic is not achievable or prudent in every system, but where random twiddlers can reasonably be expected, a little extra defense may be a good idea.



Mike Halloran
Pembroke Pines, FL, USA

 

[vpl]

My first impression on this question was "why not use an orifice" -- for much the reasons as expressed by Mike Halloran. However, after watching the thread evolve, I'm more inclined to zdas04's last comment ...

alimrheg asked a question and he's seen the various opinons. Now he has to make a decision that has to be based on his engineering judgement. Use of "someone on an Internet forum told me it was ok" isn't going to work well when something goes wrong. Especially in this case, when people appear to be mostly saying "not the best design."

Patricia Lougheed

******

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[casflo]

In the case of liquid fluid systems the throttled valves must run without cavitation and this is the main concern that must be attended. In the case of flashing liquids or gas systems take into account the valuable former comments.
For your information and help look for a paper published in the HYDROCARBON PROCESSING magazine, August 2008, about throttling valves.

Emilio Casado

 

[PeterIgg]

The issue that no-one seems to have addressed is that with a Ptfe seat and a ball in the half open position the seat will cold-flow around the bore of the ball so that when (if) the valve is ever closed the seat will have a dent in it and will not seal. This is one of the most common causes of rejected warranty claims on BV manufacturers. Standard ball valves (viz not V port), esp soft seated ones, should be fully open or closed as the ball protects the sealing surface of the seat in these positions; the seat is evenly loaded so will not develop dents. Which leads to the other issue, the exposed sealing surface of the seat will deteriorate, too. I pressume that the valve is small bore which is why a b/fly valve is not being considered? This being the case, why not a plug or needle valve, the latter would provide far better control.

 

[gerhardl]

Hello PeterIgg, checking the development after some time,I see that you rightfully have gotten a star for your direct comment! I could not agree more. I tried to indicate some of this in my first posting, but you manged to express it to the point!