gate valve to reduce downstream pressure

[gelsi]

hi
i would use a gate valve to reduce downstream pressure i a system, you must consider upstream pressure is constant.
I can't use an orifice because i do not know the exact upstream pressure but only its range.
I have seen that the valve could be set at position 15-50% open and i am not sure that it can be set without difficults.
Who can suggest where finding intrinsic characteristic of a gate or can give me a comment?
tx

 

[Latexman]

Gate valves are NOT very good as a control valve. The classical approach is to use a globe valve. What is the line size?

Good luck,
Latexman

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[gelsi]

line size is 8"
valve size is 6"

it's a fire-fighing application so fluid velocity is high

 

[racookpe1978]

Do you want to "set" (control) the downstream pressure?
Or do you want to simply reduce (by an arbitrary amount) the downstream pressure because your current upstream pressure is too high?
A fire-fighting system usually does NOT want any flow restriction.

Why not use a simple pressure-reg valve, if you cannot use a variable speed pump controller.

 

[gelsi]

i want to reduce downstream pressure to a known value, without auto-regulating valve (that is my first solution there is no time to buy).

i have not upstream pressure and can't calculate it by pumps and net caracteristics, but i know that it is in a range of pressure that is too high for my system.

 

[bimr]

You need a control valve, not a gate valve. The gate valve will only reduce pressure when the line if flowing and will have to be constantly adjusted as the flow changes.

 

[LittleInch]

If your flow is zero then a gate valve just will not work to reduce pressure. For higher flows, gate valves are very bad at controlling pressure, but if you have to do it this way, look up gate valve Cv versus percent open. Most throttling will occur in the last 20% closure and be very sensitive to small movements of the valve.

You can get wholly fluid powered control valves off the shelf, especially for water so I don't understand why you can't get one. They need no power or instruments, but work very well for what sounds like your duty.

If your system has items not rated for the pressure, especially a fire water system, then you have more trouble than just a gate valve.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way