Pressure Drop and Friction Factor in Flow of Gases

[shup0739]

Ok I am really stuck here and I need your help!

here is the information I have so far :

I am pulling facuum on a vessle using a jet, and I know the pressure at the jet and the pressure at the vessle. I have a valve mid-way and I need to know the pressure going into this valve and the pressure leaving the valve. I have measured and accounted for fittings from the vessel to the valve and after the valve up to the jet. I have calculated the pressure drops associated with the piping and fittings but I still do not know how to find the pressure drop across this valve.

inlet pressure : 49687 Pa
Size : 2 inch
Type : Ball Valve
Cv : 104
Type of FLow : Air ( this is a gas not water)

What is the correct formula to find the outlet pressure, this formula must take into account the Cv.

Also, what I want to do is see if this pressure drop will be different if I replace the valve with a 2 inch GLOBE valve with a Cv or about 50.

Please help me out on this one guyz...I would really appreciate it.


shup

 

[katmar]

Hi shup,

I suppose this sounds rather impolite, but you are asking a very basic question that could easily be solved by yourself spending half an hour reading almost any book on fluid mechanics, or with a quick internet search.

If you are too lazy to do that you should not expect the members here to jump to your assistance.

To get you on the right track here's a clue - you cannot calculate the pressure drop unless you know the flowrate. Now go find that book and read it.

 

[shup0739]

I have already tried to solve this but I am getting a pressure drop of zero and I know that does not make sense here.

I know how to do this in the case of a fluid but with gases its a little bit different.

I am familiar with the choked flow where the (Pin >=2*Pout) then Cv= Q *(sqrt(SG*T))/(816*Pin)

and the critical flow where (Pin < 2*Pout) then Cv= (Q/962)*(sqrt((SG*T)/(Pin^2-Pout))

I have tried both formulas and I've already calculated my flow but I am still gettin a pressure Pout that is about equal to my Pin. There has to be some pressure drop but I am using excel and it is ZERO.

I am not trying to be inpollite I joined the forum to learn more and if you had the time and energy to write that reply than I think you could have been more professional and perhaps give me a resource to go to.

If anyone has any other formula that they have used please let me know and If I am approaching this in the wrong way I appologize.

sam

 

[CRG]

I did a google search using:

"gas flow" Cv calculation



The first hit:

http://www.ces.clemson.edu/chemeng/undergraduate/uolab/valve6.htm

has a formula for sub critical flow.

 

[Montemayor]

Sam:

I have given you the tools to resolve your problem on the other forum where you double-posted the same query.

Katmar is correct in his professional response to your post. He is more than going the distance in trying to help you. He is trying to point you in the right direction because you are scattering your aim in a lot of useless and non-applicable areas. Katmar is doing the right thing by advising you to go back to basics and start again from there. The answer is very, very simple when you sit down and analyze the basic data you give. First, you have to assume you have sub-sonic flow in order to get the pressure drop. But you must know the flow rate as Katmar states.

We are all trying to help. But you have to realize that you are on the wrong track when you assume we are being negative in pointing out your errors. Professional engineers don't give importance to personal views or opinions. We are only interested in resolving the problem and perceptions and feelings don't contribute to that.

I hope you can get a copy of Crane Tech Paper #410 and see the clarity of resolving this problem. Good Luck.

Art Montemayor
Spring, TX

 

[katmar]

Hi again Sam,

I suppose I should not be surprised that you were a bit offended by my reply. If you spend a bit of time here you will see that I spend a lot (probably too much) of my time trying to help. I love solving technical problems.

So it is not that I don't want to help you - I could have given you an answer along the lines that Art gave in the other forum. Art has hit the nail on the head in terms of how we feel, so I won't say any more.

Except maybe one last bit of advice - if you are going to buy just one book on fluid calcs, follow Art's advice and get the Crane manual. It's a great start, and you will use it for the rest of your career.

 

[shup0739]

I appologize if I have been unprofessional, believe me that is not what I intended to do...I just got really frustrated after searching for a way to solve this problem and at times I found contradicting ways of doing this problem and I just wanted to get someones real time opinion and I guess when someone posts anything online the emotional aspect is often left and people may assume that the one posting is trying to get a quick answer, when I have spent a lot of time trying to solve it.

In anycase, I appreciate your help. I have already found a solution and I hope we can all just chill.


sam

 

[rmw]

Shup,

I also recommend Camerons Hydraulic data. I have two copies of Crane, but I will usually reach for the Camerons first, and then if it is not enough, out comes the Crane.

I know Art recommends it, and you can do a search on this site for it, and read the threads where he has extolled its virtues.

Air is a fluid, and you are correct, you cannot have zero pressure drop across the valve, although it may be so low that your calculator is reading zero, if you are not making a mistake in your calculatons or assumptions.

Greetings, Art, and Harvey (Katmar)

rmw

 

[katmar]

Hi shup,

I see that you were genuine in your query - you had done a lot of investigation already. We get a lot of chancers and lazy students asking questions here, and I took you to be one of those. Please accept my apologies.

If there is one thing I really try to teach young engineers it is that they should never be afraid of asking questions, and I would not like to discourage you from that by insulting your lack of knowledge.

I am pleased that you solved the problem, and that you have got a copy of Crane 410. Thanks for your mature response, I will take your advice and try to "chill".

best regards
katmar

 

[smckennz]

Hi Shup

what is the calculated velocity through the valve?
Or the volumetric flow rate?
If you have a small jet and a large valve, then the pressure drop through the valve may be very close to zero .
For the valve to have any useful effect, other than for isolation, then the effective valve window area must be close to the jet area.

Cheers

Steve