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Use of Plug Valves
2

Use of Plug Valves

Use of Plug Valves

(OP)
We have a design on an influent pump station at the start of a wastewater treatment plant that uses Check and Plug Valves on the discharge side of the pumps. I need to know why plug valves are better for this operation versus using gate valves? Thanks.

RE: Use of Plug Valves

Well I can think of one right off the top of my head. A plug valve is a 1/4 turn operation, as a posed to a gate valve, where you'll have to turn the hand wheel a number of times to get it closed (something operators like!). You could replace the plug with a ball valve ...they might be a bit less expensive than a plug. Comparing cost of gates and balls are pretty close up to about 4". Good Luck!  ...Mark

RE: Use of Plug Valves

may be there is possibility of sediments and impurities filling the gate valve seat making it unoperable.If line is big and cost is the consideration butterfly will be a good choice.

RE: Use of Plug Valves

Rimshots!

That may be a wrong (unintentional) option. It is always better to use control valves on the pump discharge rather than on/off valves. A gate valve (if disc seat contact is good) is a better option.

Regards,

Truth: Even the hardest of the problems will have atleast one simple solution. Mine may not be one.

RE: Use of Plug Valves

The only calim to fame a plug valve has, is that it is the only valve design that can be in-line adjusted to stop through leakage. Barring this requirement, plug valves are very hard to turn ball valves in disguise.

RE: Use of Plug Valves

joeswoes,

You stole my thunder...I have yet to find an application where a plug valve is the best choice.  In fact, I have outlawed plug valves from my factory.  After a few years, they usually turn with great difficulty or not at all.

One caution on the quarter-turn valves;  operators need to understand what water hammer is, and how to prevent it by operating these valves slowly.  Other than that, I would go with a good quality ball valve any day over a gate.  They're compact, easier to operate, and usually hold better and have less stem leakage.

RE: Use of Plug Valves

Ball Valves Good.  Plug Valves Bad.  

I'm with Ken, I've never found a case where a plug valve was among the top 5 choices.  Even if you follow the mfgr recommendation and grease them frequently, they are still hard to operate and don't seal all that well.

As to using gate valves for a throttling application, they have very similar (bad) throttling characteristics as a ball or plug valve.  None of the three should ever be installed in an application where they would normally be partially closed.  

RE: Use of Plug Valves

zdas04  ...you ever heard of a ZEE Ball? It's a ball valve that you can throttle with, pretty trick!.                  Good Luck! ...Mark

RE: Use of Plug Valves

Yea, I use Fisher's version (they call it a V-Ball) all the time, it has throttling characteristics very similar to a globe valve.  The problem is when you take them apart the "ball" looks real strange.  Depending on manufacturer, the ball sometimes has a large groove cut out of it so that in a throttled position it has a larger flow area, other times the "ball" is an oddly shaped plate.  I guess they call it a ball valve because it can be 1/4 turn (but many of them are 1/2 turn).

My comments above apply to traditional ball valves with a simple hole bored through a ball.

David

RE: Use of Plug Valves

...although you can certainly get away with using a traditional Jamesbury or whatever ball valve for throttling service IF it's properly selected, sized, and installed.  I see it done successfully all the time, in installations that have run for 10-15 years or more.  Just like anything else you have to weigh the trade-offs: cost vs. serviceability vs. process criticality.  Obviously in a critical throttling service you would never use an el cheapo ball valve.

I run into lots of plug valves on older installations all the time.  You see them a lot in old gas plants, oilfields, and old LPG/NGL terminals.  I think it's because 50 years ago there were no PTFE seals, so if a 1/4 turn valve was wanted, you used a plug valve.  I'm with all you guys - they are usually stuck open or closed and they frequently leak by when closed.  A plug valve is not acceptable by most plant owners for use in double block and bleed service, for example.  We replace them routinely nowadays with some other valve.  I don't know of anyone who specifies them for new installations.

Thanks!
Pete

RE: Use of Plug Valves

Rules of thumb make the world go 'round, and one of my "never violate (except sometimes)" rules is I never let folks throttle a valve that I might need later for a bubble-tight seal.  Sure people throttle ball valves, done it myself, but the throttling characteristic are so poor that it is always a last resort.

We have a well known gathering, processing, transportation company here in the four corners that still specifies that ONLY plug valves will be installed.  When I asked one of their engineers why she said that no one had revisited their bid specifications since plug valves where the only choice for 1/4 operations (which was really less than 20 years ago).

RE: Use of Plug Valves

Out of sheer curiosity.
The plug valves mentioned in this thread are not the "TWIN SEAL " kind ? Or could they also be included?
TIA
DKF

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