Isolation Ball Valves vs Gate Valves 3

[jacsc]

Both Gate Valve and Ball Valves can be purchased with Class VI shut-off. Are ball valves best to use in a steam process line in lieu of gate valves? Wondering which is best to use in a hospital facility especially since ball valves will open easily and faster than gate valves.

 

[psafety]

Steam valves in my mind should be slow opening to prevent thermal shock to the system.
If piping is large use a warm-up bypass to allow the traps to remove excessive condensate formation at start-up.

 

[TBP]

I've used lots of ball valves in steam systems with no problems at all. I've found that much more important that the type of valve, is the design, installation & maintenance of the system, along with trained operators. A proper system will allow condensate to drain so there are no "surprise" pockets of condensate to trigger waterhammer events. Check out Wayne Kirsner's articles at kirsner.org.

Spirax Sarco's publication "Hook-Ups" is an excellent resource for practical design information on steam systems, and I highly recommend it. I've used it for years, and it has never steered me wrong. It's worth noting that they also have their own line of steam-rated ball valves.

 

[psafety]

I've read more than once of people killed by a couple of "good design practices" failing, traps go bad, warmup procedures are overlooked. Ball valves may be ok for instrument connections, and drip legs, trap lines, but use in transmission lines is looking for trouble unless small in size.

 

[YoBob]

I once specified/installed 8" ball valves for 80 deg F, 500 psig condensate and it was a disaster because the valves were infrequently opened/closed,i.e, once per year or less. With non-use, the balls rust and take a 'set' in the seat. When one tries to open/close them, they will break the valve operator trying overcome the seat friction.

 

[TBP]

psafety -

Good design practices are just that. Then there's proper installation. If the facilities involved in the incidents you mention failed to provide adequate trap maintenance or operator training, then that's hardly the fault of the design, or valve selection. Are you seriously suggesting that Spirax Sarco, Apollo and all the other ball valve manufacturers are selling valves that are unsuitable for the intended service? C'mon...

Look at Kirsner's site, and I believe you'll find that pretty much all of the incidents he's investigated involved gate valves. But the type of valve itself was not the source of the problem. It is simply not possible open any valve - including a gate valve - slowly enough to prevent condensation induced water hammer, if conditions exist where it may occur. Kirsner has investigated a number of fatal accidents where operators assumed that the operating technique of slowly opening or "cracking" the valve would keep them safe. Those men are dead because of that erroneous assumption.

Anyone relying on the operational differences of gate, ball or butterfly valves to prevent accidents in steam systems is very much inviting disaster.

 

[TBP]

YoBob - The ball valves I'm familiar with for steam & condensate service have stainless steel balls. I would suspect that rather than the ball itself rusting, it was an accumulation of rust carried from the condensate piping itself that built-up in the valve over time. You can often suffer similar trouble with gate valves - accumulated crud won't allow the valve to seat when it comes time to isolate something. All things considered, I've suffered FAR more operating problems with gate valves than ball valves. Periodically stroking the valves as part of a regular maintenance/operating procedure often helps.