Why is there a pressure drop when the control valve was opened?

[Lonelywish]

I'm currently doing a design for a liquid gas separator. I would like to determine how to install my control valves, typically level controls. But I'm not too sure about the pressure drop when the valve is opened, therefore I do not know how to design my liquid depth and the maximum and minimum level control. Hope someone can give me a guide on how does the pressure drop works when the control valve is 100% opened.

 

[BigInch]

First your control valve should be designed to operate within a range of 30% to 70% open, as outside that range the flow vs valve %open position curve typically becomes a little flatter and some of the precision of the controlling action is lost.

When control valves are in the full open position, the area of the orifice through which the flow passes is still much much less than that of the adjacent pipe, so there will always be some pressure drop. Find the Cv vs Valve position curve for a control valve. That will tell you the flow that a control valve will pass at a given percent open for any differential pressure across the valve. You should also assume that when a control valve is closed, it will not be 100% closed and there will still be some possibility of passing a small flow as well.

I think that answers your basic questions, so now I would suggest that you have a look at the Fisher Control Valve Manual and study the example valve designs therein. Its here,

http://www.documentation.emersonprocess.com/groups/public/documents/book/cvh99.pdf

http://virtualpipeline.spaces.msn.com

 

[Lonelywish]

Actually I'm currently designing a separator which is used to remove excess air from saturated water after passing through a saturation tank but I'm not too sure about the level controller design.
1)How do I know at which point should I open my valve to release my air and to release my water flow into the next tank? Should I calculate the pipe flow rate out of the tank where the valve is controling to determine the opening % of my valve and the point to locate my level control?
2)Is there any other safety limitations that I need to consider?

 

[katmar]

Lonelywish, if you are a student then I must point out to you that student postings are not allowed on Eng-Tips. Please click on the Posting Policies link.

If you are not a student, then I can sympathise with you because there was a time when I was asking very similar questions to what you are asking now. In all honesty, you are not going to get the level of understanding you need by getting tips here. BigInch has, as he always does, given you some very good pointers. But you need to sit down with someone who has done this sort of work before and can talk it through with you while making sketches, graphs etc.

Go back to your supervisor and explain that you need help. There is no shame in not knowing everything. Hopefully someone in your organization can guide you through it. If there is nobody in your organization who can help you then you are putting yourself at an unfair risk by undertaking this work. Your supervisor needs to call in a qualified consultant who can help - and hopefully you can work with the consultant and squeeze as much knowledge out of him as possible.

Good luck

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[BigInch]

OK. First read Katmar's message 5 times.

Then assuming you're not a student and don't have anybody to talk to right now, I'd proceed with something like this, ... until you can find somebody to talk to...

Assuming you already have a fixed separator vessel size, and the air can be vented at intervals, (which may pull too much water into the separator too quickly, so be sure to check if you indeed can do that),

Calculate the air rate coming off the water in the separator. Initially the separator is full of water with say 10% air (by volume) at the top. The air fills the separator at your calculated rate. When you reach the point where the separator has filled with air to (I'd say) around 80% or so, you will probably want to vent, so put the air vent's OPEN level switch there at that elevation. The vessel will fill over the next interval to say about 85% water, which at that level you should put the air vent's CLOSE switch.

At 90% or so, you should have a HIGH LEVEL alarm.
At 95%, you should have a flow cut-off level switch connected to an automatic shutoff valve located at the water inlet to the separator, or somewhere else in the feed line.

Be sure to include a relief valve with the appropriate pressure setting on the vessel.

If you need a continuous vent, proportional air flow control to maintain a certain water level, you approach in a similar manner, but you would need an instrument, such as a continuous level monitor providing a proportional signal according to water level in the separator. The signal would then go to the air vent valve, such that the valve would be closed at lowest water level and open gradually until it would be 100% open when water reached the 80% level or so. HIGH level alarm and HI-HI shutoff w/valve and pressure relief valve.. still required.

Size the vent valve for the calculated air release rate from the water at the vessel's pressure when the water is low and the valve is say around 30% open, and then size using a full open Cv at the vessel's pressure at the high water level you calculated in the initial batched air release process.

http://virtualpipeline.spaces.msn.com

 

[Lonelywish]

Actually I've just finish my diploma and currently working as a chemical engineer trainee. My advisor is quite busy and I guess he has no time to instruct me on this matter. Thanks for the help

 

[BigInch]

Tell him its his job and responsibility.

http://virtualpipeline.spaces.msn.com

 

[Lonelywish]

haha.. actually he did guide me for the past month, but i guess he wanted me to move my brain more.
Currently my work is to design a range of separators which can be used for different kinds of flow rates and I guess the main problem I am facing now is the lack of any real life experience cause I'm doing only theoretical calculations. But anyhow, I will still try my best

 

[BigInch]

Well I can't think of a better place to start than on "theoretical separators".

http://virtualpipeline.spaces.msn.com

 

[dcasto]

Draw up you P&ID and label the air valve as a "VV-101". When he asks tell him its a irtual Valve matched to his theroretical separatotors (TS-101). Since he has made up the question ask him at what rate the air is coming into the vessel inorder for you to size your valve to match the theroretical air.

 

[katmar]

It is sad that all around the world the system of internal mentoring of young engineers seems to be falling away. I suppose it is admirable that Lonelywish makes the effort to come here and ask for advice, rather than just bumbling through it. Luckily we have people like BigInch who have the experience, knowledge and interest to help in a case like this. Lonelywish may well find that the reason his supervisor is reluctant to help is because he does not have the answers himself.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[Lonelywish]

I've discussed with my advisor today and actually what he wanted me to did was:
1st decide the amount of time before each purge of air (open of air valve).
2nd decide the volume accumulated before each purge of air.
3rd design the tank with 1 minute retention time.
He said I din't have to think too much because we don't have any stimulation or protocol system to test run on.
Anyways thanks everyone for the help

 

[Lonelywish]

We do run real test but for my current design I'm not going do any design/test run just yet. I'm just doing the basic calculation. So he just say put this as an early base assumption. When we do the real testing then only do the adjustment.

 

[Lonelywish]

But i guess i wont say that he is reluctant to teach me because he don't know. Recently he is just too busy and he does want me to think more
I admit I wouldn't be thinking much if he just tell me the 3 things that I was suppose to do ^^.
Anyways thanks everyone for the help.

 

[BigInch]

BTW I don't see any difference in being an engineering intern or being a practicing engineer. Both the same to me. We're all "students". Asking for the answers to some homework problem, ya... that's different.

http://virtualpipeline.spaces.msn.com

 

[Ashereng]

Lonelywish,

Another piece of advise. It is not just your immediate supervisor that you can go to. I go to whoever has time to answer my questions. In a plant, this includes:
- operators
- instrumentation tech/engineer
- controls tech/engineer
- mechanic
- the first guy/gal breathing that I see (my favorite)

Usually, if someone can't help me, they know someone else who can and usually send me there.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[ww2]

Hi,
why don't you check:
IEC534-2-1 incompressible fluids
IEC534-2-1 compressible fluids
IEC534-2-3 flow capacity test procedures
IEC534-2-4 flow capacities - inherent flow characteristics and range-ability
cheers

 

[rmw]

Is your work situation where you can go out into the plant and look at other examples of this type of equipment? If so, then go out and look at how exisiting systems are piped and plumbed. That may help give you a better "mind's eye view" of what is going on.

If not, then google the topic and see if you can find any photos or schematics.

http://www.sugartech.co.za

has some helpful information on condensate systems.

rmw