isolation valve
isolation valve
(OP)
would anyone suggest using an isolation valve at a pump discharge as a means of controlling operating pressure in a closed system
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RE: isolation valve
RE: isolation valve
RE: isolation valve
There are other strategies available.
1) Use a VSD.
2) Use "by-pass approach". The pump runs continuously at the maximum process requirement and the excess fluid flow rate is continuously diverted and sent back to the supply.
3) Trim the impeller (when the pump has been sized wrongly, that is oversized, machining the impeller is an option to reduce energy added to the fluid).
RE: isolation valve
RE: isolation valve
You risk wearing the valve, and they you've got no isolation valve to use when you want to isolate.
RE: isolation valve
Other than that, you will likely cause damage to the valve thus negating it's effectiveness for isolation as mentioned by TenPenny.
Generally, just not a good idea but it would really help to know specifically what type of valve you are talking about (gate, ball, butterfly, etc.).
RE: isolation valve
**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch
http://virtualpipeline.spaces.live.com/
RE: isolation valve
RE: isolation valve
The most energy efficient way to do pressure control is to never have increased the pressure beyond what you need. That means that a VFD or a constant pressure valve (i.e., a valve that has a pressure regulator that dumps excess flow) are both far better than throttling the full stream. Anything is better than throttling the full stream with a butterfly, gate, or ball valve.
David
RE: isolation valve
**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch
http://virtualpipeline.spaces.live.com/
RE: isolation valve
It's easy enough to program the response into a function block to linearise the response, and the electronics are plenty fast enough. Getting a realistically sized actuator with the speed and power to rapidly swing the valve through more than 3/4 of its physical travel for (e.g.) a few percent change in the upstream pressure, and also finding a valve which will put up with that kind of abuse, takes a bit more doing.
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If we learn from our mistakes I'm getting a great education!
RE: isolation valve
**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch
http://virtualpipeline.spaces.live.com/
RE: isolation valve
Show me a valve which can pulse at 100kHz. I have a lot of money to make...
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If we learn from our mistakes I'm getting a great education!
RE: isolation valve
**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch
http://virtualpipeline.spaces.live.com/
RE: isolation valve
I stand by my last paragraph. It is true that throttling a non-linear valve is less energy efficient than either a VFD or a constant pressure valve. It is not always true that the most important characteristic of a system is efficiency. There are times that the most effective control scheme is to throttle the full stream. Just because something has a better process result does not mean that it has the best energy-consumption result.
ScottyUK,
Like I said, I've seen people try to control systems with a non-linear valve and the results have been poor every time. I think that the reason is that the amount of flow that you can get through a throttled block valve is a function of some really subtle things (like what direction the valve moved last--if you are changing direction then you have to account for gear lash which changes with time) and early in the travel, very small changes in position are large changes in flow.
David
RE: isolation valve
Apologies.
**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch
http://virtualpipeline.spaces.live.com/
RE: isolation valve
RE: isolation valve
**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch
http://virtualpipeline.spaces.live.com/
RE: isolation valve
On the other hand there are people who do use globe valves for isolation. In that case, the linearity is pretty good and control schemes work well, but the act of throttling is hard on disks and seats and they often wear badly (as you said above) and don't isolate the flow when fully shut.
Bottom line is that a valve designated for "isolation" should not be used for control and a valve designated for "control" should not be relied upon for isolation.
David
RE: isolation valve
RE: isolation valve
**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch
http://virtualpipeline.spaces.live.com/
RE: isolation valve
----------------------------------
If we learn from our mistakes I'm getting a great education!
RE: isolation valve
Throttle valve that actually shut off flow are common. Throttle valves that can be counted on to shut off flow are nonexistent. I've seen hundreds of globe valves that actually give you a bubble-tight seal and dozens of globe valves that leak like a barn door, but I still stand by the statement that it is poor design to rely on them for isolation.
David
RE: isolation valve
OK. I'll give you that one.
Now I see that your globe is malfunctioning.
**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch
http://virtualpipeline.spaces.live.com/