Calculate API Plan 11 Flow rate w/throat bushing 1

[RS82]

I Have recently joined a seal company and have the following question.

If I am given an orifice size on the plan 11 line and throat bushing diametric clearance of 0.008"..what formulae do I use to calculate how much flow rate is being generated..and if I open up the throat bushing clearance to approx 0.020"..would that increase my flow rate as my orifice is still the same on the plan 11 line..would not my orifice dictate the flow rate or is it a combination of both? Thanks

 

[JJPellin]

There are simple programs available that do the sort of calculation you are asking for. I use one provided by our seal manufacturer. It will calculate flow for an orifice, a bushing, or an orifice and a bushing in series. If you increase the bushing clearance, the flow through the orifice will increase as the seal chamber pressure is reduced.

Johnny Pellin

 

[RS82]

Any chance of attaching the Excel File for these calcs? I am a complete novice to seal calcs at this point and would like to get more familiar with this. Thanks for your response.

 

[RS82]

To elaborate on my question, here is a scenario:

Discharge pressure: 100psig
Suction pressure: 10psig
vapour pressure: 15psia
pressure between throat bush & impeller: 15psig
orifice on plan 11 line: 0.125"

I would first determine the required seal chamber pressure: VP+50psi = approx. 50psig
For argument sake, with a press diff of 50psi and orifice size of 0.125", the flow rate is 3GPM. I would then insert the seal chamber pressure, 50psig, and flow rate, 3GPM, in the throat bushing leakage formula and calculate h, gap, required..

But if I change the gap to a larger number, then the flow rate will change and the pressure drop across the throat bush, 35psig, will change..how would I calculate both as they are related?

 

[flexibox]

If you work for a seal company why don't you ask your engineering dept to assist you in this? There are many methods and different approaches to this question.

 

[RS82]

We have a software to do the calc..but I am not comfortable using anything until I know where the values are coming from..Unfortunately the engineers here have not given any thought to where the values are coming from..scary..hence the questions..please dont state the obvious and give very generic answers. I have just joined the forum and I am pretty sure that is not what this is used for. So if you dont know, that is fine. Thanks anyways..anybody else?

 

[RS82]

Actually, judging for your comments on the other threads, you DO know what your talking about. I apologize for the previous comment and the comment on the other thread. Could you still please help me out?

I will quickly explain my situation, I recently got hired in a seal company and would like to progress in the company. I have asked 2 out of the 3 engineers who have an idea but are not sure how the calcs work. I refuse to accept the software as is and would like some more insight. If there are any sites you could refer me to or help me out on this forum, please do so.

Thanks.

 

[JJPellin]

If you really wanted to do this calculation long-hand, it would be an iterative process. The orifice calculation is a simple matter. Any text book on hydraulics would include the formulae. I use Cameron Hydraulic Data. You would have to assume a pressure in the seal chamber and calculate the flow through the orifice. Then, using that seal chamber pressure, calculate the flow through the bushing. If the two values were not the same, then you would adjust your assumed seal chamber pressure up or down and repeat the process. After a few iterations, you should be able to come up with the actual flow rate and seal chamber pressure. I don't know if the program I use uses this method. And, I don't have a good reference close at hand for the bushing calculation.

Johnny Pellin

 

[RS82]

Sorry, 1 more question. The attached document is what I used to calculate my orifice flow rate. I used the series circuit.
But nowhere in the formula does it take into consideration the pipe diameter. I remember there being a beta ratio = orifice dia/pipe dia..this shows the flow rate is not a function of the pipe dia which I remember it was.

Am I using the right formula here?