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Maximum Flashing Rate from a Control Valve

Maximum Flashing Rate from a Control Valve

Maximum Flashing Rate from a Control Valve

I'm working on a system that uses a control valve discharging into a flash tank to concentrate a stream. Currently the system flashes about 10% of the flow; the client has suggest looking into flashing 52% of the flow.

I've been trying to find a guideline on the maximum amount of flashing allowable through a control valve, with no luck. Has anyone worked on a similar process and can tell me if there is a guideline?

RE: Maximum Flashing Rate from a Control Valve

You need to define what flashing 10% of the flow. Does this mean you are taking 10% of the total steam and dropping the pressure on it through a valve to a lower pressure to get a saturated steam at a lower pressure?

Or are you saying that the whole stream is sent through a valve where 10% of that stream flashes as a vapor stream of steam?

If the first is true, then just get a bigger valve, and possibly a larger flash drum.

If the second is true, then the only way to get more of the stream to vapor is to either heat the stream going into the valve of lower the outlet pressure of the valve to get more vapor.

RE: Maximum Flashing Rate from a Control Valve

It is the second. We knew we would have the heat it before sending it through the control valve to flash it, however we weren't sure if there were any guidelines on the percentage of the stream we could flash at a time. It sounds like there isn't, as long as we can design the system to allow it.

Thank you for your help.

RE: Maximum Flashing Rate from a Control Valve

The current system may, or may not, reach a physical limit where the flow is choked in the control valve or in the pipe and fittings between the control valve and flash tank as more and more liquid is flashed. Not enough information was given to determine. It's probably unlikely that the current system will develop choked flow, but if it does, do as dcasto said, "get a bigger valve, and possibly a larger flash drum".

Good luck,

Technically, the glass is always full - 1/2 air and 1/2 water.

RE: Maximum Flashing Rate from a Control Valve

The limit and the maximum amount is set by your valve. Increasing the volumetric vapour flow by five times coming out of the valve sounds rather a big ask to me, but you need to ask the control valve vendor. Even if the valve can physically do it you could end up changing the trim rather more often than before...

Is this steam or some other liquid?

My motto: Learn something new every day

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

RE: Maximum Flashing Rate from a Control Valve

It is a toluene based solution with a solute in it that could precipitate out if the solution goes below a certain concentration, which is another concern we had. It looks like we are going to stick with the initial design. Thank you all for the help.

RE: Maximum Flashing Rate from a Control Valve

Regarding your question about "guidelines on the percentage [that can be flashed]"...
It is possible that a [throttling] valve not designed for flashing could be damaged by cavitation. You should consult with a valve vendor to know what valve trim is best suited to this application, and delta-P limits, if any.

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