Use of Plug Valves 2

[RimShots31]

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.

 

[11echo]

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

 

[01101998]

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.

 

[quark]

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.

 

[joeswoes]

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.

 

[KenRad]

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.

 

[zdas04]

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.

 

[11echo]

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

 

[zdas04]

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

 

[KernOily]

...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

 

[zdas04]

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).

 

[dkf]

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