Lubricated plug valve vs. ball valve for natural gas service 2

[koneill44]

Why would one use a lubricated plug valve over a ball valve for shutoff in natural gas service?

Is it a temperature issue if the valve is located outdoors?

Is there a fire safety issue?

Appreciate your comments.

Thanks.

 

[zdas04]

A lot of people have opinions on this, I'll give you mine: I cannot find a single place where I would prefer a plug valve to a ball valve.

The problem I have with "lubricated" plug valves is that they don't get lubricated in the field (plants may be different, but I don't work in plants) and after a few months or years they can become impossible to operate. I've seen people try to operate them with a length of pipe tied to a rubber-tired hoe and not been able to budge them.

With recommended maintenance, maybe they would have a proper place in the world, but field maintenance on small valves is spotty to non-existent.

If you ignore a ball valve for a decade it will still most likely operate and there is a reasonable chance that it will hold pressure. Not so with a plug valve.


David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

"Life is nature's way of preserving meat" The Master on Dr. Who

 

[gerhardl]

Principally I agree with zdas04, but the real valve selection should be based on the full technical appplication set of data for any valve and service, for instance minimum pressure for and after, temperature (medium and surrounding), flow, purity and full descroption of fluid, on/off or regulation, manual or mechanical operated (with details), any design or safety requirements, any material or sertificate requirements, any material preferences, operation time and details, expected or required lifetime and operation dependabillity, price/lifetime cost, references for similar use, etc, etc, etc....

And then you might well end up with a standard ball valve suitable for the service, but mind you, there are alternative constructions. But greased plugvalve? No!

 

[rmw]

Back in the day, the only way it was done was plug valves, and lubricated plug valves were popular. That was before ball valves were able to be manufactured using some of the metallurgies and had some of the seat materials that are available today. It was an attempt to try to guarantee a tight seal against leakage into a furnace, or at least that is where I ran into them, and about the only place I encountered them.

Given today's technology and what is available, I am not sure what I would want to be out there keeping the gas out of any furnace I was in, double block and bleed or not. I have been in plenty where it was a plug valve greased who knows when. David's comment about stuff not getting greased in the real world is real truth.

rmw

 

[JimCasey]

OK, I will be the Devil's advocate.
Lubricated plug valves are inherently firesafe.
They are also quite inexpensive. After all it's a simple shape to cast and not really a lot of machining.
As for lubrication, there's the weak point. Widely misunderstood that the stuff pumped in is "sealant". Its job is to seal and to lubricate and to hydraulically lift the plug out of contact with the body. The sealant should be selected for compatibility with the process material so that it neither is soluble in it nor contaminating or reactive. Companies such as Nordstrom have approximately one bazillion different sealant formulations. The down side of that is that you might not get the right formulation, so the maintenance guys will either skip maintaining the valve or pump it full of the wrong goo. Some valves have a sealant injector built in the top of the plug shaft. Just give THAT a crank, THEN start horsing on the lever.

 

[MortenA]

well for one thing you can get a DBB plug valve (real DBB) with the same measures as a single ball valve (API 6FA) - so that you can re-fit for DBB by replacing the valve.

http://www.bch.dk/index.php?page=double-plug-valve

But maybe these are not lubricated then?

Best regards

MOrten

 

[JimCasey]

>>But maybe these are not lubricated then?<<

Yep, those are lubed plug valves....well THAT one isn't 'cause the cutaway body lets the lube leak out, but you know what I mean.

NONlubricated plug valves have a polymer (usually PTFE) sleeve, that seals and lubricates.

 

[moltenmetal]

Jim gets the star, since not every ball valve is firesafe- even those stamped "water, oil or gas" (WOG) with pretty yellow plastic handle covers...

A valve's not much good if it can't be closed when you need to close it, but a valve which leaks fuel into a fire isn't much good either!

zdas04: plug valves are also king in certain mining/mineral processing applications (and other services where the lack of a cavity is a benefit), and when you need a 100% teflon-lined valve. Just about everywhere else, I'm with you, with the proviso that the ball valve be 607 fire safe when needed.

 

[MortenA]

The valves that i have referenced i know is used in gas storage facilities where additional trains were installed and they wanted DBB isolation of existing line.

Best regards

Morten

 

[MTPipeliner]

Unlike most of the previous posts, my company has lots of lubricated plug valves in service, and we still spec and use them quite frequently.

They are used in natural gas pipeline service. Typically in a blow off or side valve situation. A few old ones are still in a mainline block setting.

The advantages we've found with them is that they will almost always shut off completely when properly greased. We've had trouble getting ball valves to completely shut off. They also seem to perform better than ball or gate valves in a throttling or partially open situation such as pigging or purging. They tend to be cheaper than the same size ball valve.

The downsides are that they won't pass a pig (at least not in one piece), and they can be awfully stiff to operate if they are neglected year after year.

 

[zdas04]

PatrickR,
I promise I'm not going to start a Ford/Chevy argument here. I'm just fascinated by the statement

They also seem to perform better than ball or gate valves in a throttling or partially open situation such as pigging or purging

I've written both pigging and purging procedures for several companies, dozens of pigging facilities, and hundreds of purges and there is not a single place in any of them where throttling is either required or even allowed.

I don't mean to hyjack this thread, but where are you throttling in either pigging or purging?

David

 

[MTPipeliner]

We use valves in a throttling (partially open) service when we purge lines back into service after maintence work.

High pressure gas is taken from the in-service side of a block valve and transfered through a bypass with a partially open valve to the section being purged.

Now you've got me curious. How do you control the amount of gas (or other product) in the purge procedures you've written. I'm always interested to hear other ways of doing something.

 

[zdas04]

Actually, the problem I always have is people throttling the purge outlet valve and the procedures I write strongly warn against that. Of course you have to throttle the supply valve, I don't know why that left my tired brain.

I just reviewed the last pigging procedure I wrote and I have a purge-the-barrel step, so obviously I require throttling there too. After the barrel is purged, I want each valve that has to be operated to be fully open or fully shut (although on the launcher, the last valve to be operated prior to the pig launching is the side valve and I say "slowly shut the side valve, if the pig passes the pig signal prior to the valve being fully shut, it can be reopened without fully shutting it" so maybe that is a throttle as well.

I don't see any difference in throttling a plug valve and a ball valve. Both have very non-linear throttling characteristics and the ball valve tends to move easier. If I was going to specify a preference, I call out a globe valve for throttling.

David