Ball Valve Open/Flow Relationship

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barts
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Re: Ball Valve Open/Flow Relationship

Post by barts » Thu Mar 31, 2016 2:45 am

Lopez Mike wrote:I get by with a ball valve for my throttle but it is a bit sudden when opening. I haven't researched it but is there any off the shelf 1/4 to 1/2 turn alternative? And I don't mean some wonderful machining project with castings.
I found a 'needle' 1/2" valve that has a metal seat and a rounded knob that seals reasonably well; I use an angle valve w a soft seat to shut off flow when I'm done w/ the engine.

My guess is you could easily use a larger globe valve that would give you full throttle w/ only a bit of a turn; you could also silver solder in a faster thread.

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fredrosse
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Re: Ball Valve Open/Flow Relationship

Post by fredrosse » Thu Mar 31, 2016 4:38 pm

I have moderate steam conditions (100 psig, slight superheat), and the ball valves have all worked well. The "quick opening" characteristic that is intrinsic to ball valves can be remedied with the use of a "V-Ball" valve. Several control valve manufacturers make these, but cost is far far beyond what we might agree to pay.

Contrary to conventional wisdom, I have used ordinary gate valves in throttling service for many years, always without the often stated difficulties that are mentioned as reasons to NOT use gate valves in throttling service. I have heavily throttled gate valves up to 24 inch size, and have used a 3/4 inch gate valve in 150 psi steam service, throttled to give only about 30 Pounds Per Hour (PPH) flow, just enough to keep a typical launch engine ticking over, with no load. That bronze gate valve was used in steam service for about 16,000 hours, and was just fine after that length of service, no wiredrawing of the seats whatsoever.

I presently use a 3/4 inch gate valve to throttle the flow for the very large whistle on my launch. This was chosen because it is much shorter than a ball or globe valve, and it is self draining of the condensate which keeps the whistle valve and whistle hot. I would guess that the opening on this valve is about 5% to 10% of the full open area. When the whistle valve is opened there is no shower of hot water preceding the sound. Without this gate valve, when opening the whistle valve the boiler has great carryover, spewing great amounts of hot flashing steam/water mix all over the boat.
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Lopez Mike
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Re: Ball Valve Open/Flow Relationship

Post by Lopez Mike » Thu Mar 31, 2016 5:12 pm

When I return to my boat (I Winter in Baja) I'm going to take apart one of the 1/2" ball valves and see if there is enough material to allow me to notch either the ball or the seat for a more gradual initial opening.

I often find myself in situations where I am reversing quickly so I feel I need a quarter turn valve. Just would like that first bit of steam to come on a bit less suddenly.
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