Valve sizing for flashing liquids
Valve sizing for flashing liquids
(OP)
Hello,
What would be the proper methodology for sizing a control valve with liquids flashing downstream?
Fisher recommends allowable pressure drop method that results in a larger valve than using actual pressure drop. The question is, how can I get the required pressure drop if the much lower allowable pressure drop is used for sizing?
Any idea or commends would be appreciated.
What would be the proper methodology for sizing a control valve with liquids flashing downstream?
Fisher recommends allowable pressure drop method that results in a larger valve than using actual pressure drop. The question is, how can I get the required pressure drop if the much lower allowable pressure drop is used for sizing?
Any idea or commends would be appreciated.





RE: Valve sizing for flashing liquids
Sorry, while I refer to them often, I downloaded them long ago (onto my work computer), so I don't have to go to where I am sending you so I don't know the path right off hand (and I will leave looking it up to you).
rmw
RE: Valve sizing for flashing liquids
http:/
Hope this helps!
My focus: Alloy Valves Duplex Valves Super Duplex Valves Monel Valves Inconel Valves Titanium Valves www.6-moly-valves.com
RE: Valve sizing for flashing liquids
When flashing occurs in a valve, the flow is choked. "Choked" is an unfortunate term but when choking happens the actual downstream pressure is independent of the flowrate. The effective DP is P1-the pressure at which the flow becomes choked.
Download the sizing program from a major control valve manufacturer. Flowserve (Valtek) , Emerson (Fisher), Dresser (Masoneilan) all offer free software.
Water expands around 1000 times its liquid volum when it flashes completely, although typical flashing usually vaporizes around 15% of the mass of the liquid. That's enough to make the downstream flow very fast. Rotary valves serve well for flashing because the outlet is straight out and the high-speed droplets don't scour the valve internals. It's a good idea to calculate the velocity downstream of the valve and to transition to a larger pipe diameter to keep the velocity within reasonable limits so the downstream system erosion is managed. Also, Chromium-bearing alloys resist flashing erosion a lot better than carbon steel, so select piping accordingly.
Calculate the percent flash by setting up an isenthalpic balance:
Hf1=XHg2+(1-x)HF2 Where H is Enthalpy, F is Liquid, G is vapor, 1 is upstream and 2 is downstream. X is the percent of vapor phase.
Once you have the percent flash you can calculate the specific volume, then the velocity.
RE: Valve sizing for flashing liquids
When you say effective dp, is that allowable dp used for sizing?
If P1=150 psig, p2 needs to be 10 psig (process requirements). However, the allowable dp is only 20 psi. Then 20 psi is used to size the valve, say, passing the total flow at 70% opening. So, the valve will be 70% open with 20 psi dp. How can I get P2 to 10 Psig?
RE: Valve sizing for flashing liquids
In a situation like this, you would have to have multiple let down valves, or choose a valve that has a trim capable of taking a large pressure drop without becoming choked. Those valves usually work on the basis of multiple let down stages within the trim itself so that no stage becomes choked.
Fisher has valves that will do it, but in my experience, there are others that specialize in this type of service and do a better job of it. (Not to knock Fisher, I bought 3 of their valves today and there were 4 bidders.)
rmw
RE: Valve sizing for flashing liquids
RE: Valve sizing for flashing liquids
Imagine you were discharging to atmosphere. Downstream pressure is 0 and will always be 0. Like JimCasey mentioned; the effective pressure drop (choked) is used for calculation of flowrate, not to calculate downstream pressure.
rmw, I'm afraid your post is incorrect. Multiple letdown valves or multistep trim is only required for high-pressure (>500psip)cavitation elimination or cavitation/flashing with multi-component streams. You can only eliminate flashing by reducing the vapour pressure (cooling) or increasing the downstream pressure to above vapour pressure.
For small Cv's select a globe valve with reduced stellited trim (to allow expansion within the valve) or for larger Cv's Camflex style rotary valves installed flow-to-close are good and swage up your downstream line to minimize erosion.
PaoloPemi; I'm afraid your answer scares me because you are relying on someone who may not be competent. In the good old days Fisher and Masoneilan had good reps who provided great support but they have cut resources just like everyone else. I recommend you study and try hard to understand exactly what is being proposed by vendors.
RE: Valve sizing for flashing liquids
RE: Valve sizing for flashing liquids
rmw
RE: Valve sizing for flashing liquids
rmw, don't take it personal but if someone suggested using multiple valves for 150psi pressure drop they would not be taken seriously. In addition, your solution of multiple valves will not prevent flashing.
RE: Valve sizing for flashing liquids
I still stand by my post and will continue to do so, although I will not continue this diatribe.
rmw
RE: Valve sizing for flashing liquids
True enough, but Valve application engineers are, in my experience, expert at what they do and a valuable resource to process engineers.
I wish I knew how many times process engineers had sent me data sheets where they neglected to specify or omitted the downstream pressure (What size valve do I need for 100 psi steam?) , or specified the same upstream pressure and downstream pressure across a wide flow range.
RE: Valve sizing for flashing liquids
JimCasey: I used to have your faith in valve application engineers (I was a valve application engineer for Masoneilan and Fisher many years ago) and I'm sure if you work in Houston, London or other large engineering centers they still have good application engineers but it's sadly not the case everywhere. Good luck with dealing with your process engineers.
RE: Valve sizing for flashing liquids
What exactly is the definition of flashing here? Think about the following two cases:
1) Dehy contactor level control valve: some dissolved gas flashes out from the glycol downstream of the valve.
2) Refrigeration chiller level control valve: propane liquid is let down to almost atmosphere presure and C3 vapor flashes out from the C3 liquid.
The second case is definately a flashing case for valve sizing. How about the first one?
RE: Valve sizing for flashing liquids