Sizing an orifice / nozzle for pressure drop

[Rajder]

OK guys and gals, I'm new here but I'm hoping that you guys may be able to help me out. I'm tasked with creating a fixed flow restriction to test our pumps against. I know the pressure required, the flow rate, the inlet pipe size, etc. Basically I know everything I just need to calculate what size orifice or nozzle that I need to put inline to maintain a constant pressure at a fixed flow rate. I have two different flow conditions. Both of which we are simply pumping water for a fixed amount of time to commission our pumps.

Condition #1 - 727.2 GPM at 3000 PSI
Condition #2 - 727.2 GPM at 4000 PSI

Basically I'm looking for a way to calculate the orifice size required to achieve the 3000 psi and 4000 psi pressure against our pumps while flowing the required 727.2 GPM. The liquid is water and we will be pumping through a 2" XXH wall pipe (1.5 inch I.D.).

I've been using the calculator and formulas from this website:

http://www.efunda.com/formulae/fluids/calc_orifice_flowmeter.cfm#calc

And specifically I've been starting with the equation P1-P2 = 1/2*rho*V2^2 - 1/2*rho*V1^2

I combine those results with a coefficient of discharge Cd based upon the type of orifice or nozzle that I use and theoretically that should give me the pressure difference across the orifice correct? Or am I off base here? Thanks for the help!

 

[Compositepro]

Use a back pressure regulator (relief valve) or a needle valve that you can adjust to get exactly the pressure drop you want.

 

[cvg]

use the orifice equation
assume a c value and you can calculate the theoretical value very easily. you don't really need an online calculator to do this, it is a simple formula
Q = CdA(2gh)^2

problem is that you will never assume the correct orifice coefficient nor will you be able to verify exact pressure so you will never get an accurate flow rate, at least not to the accuracy implied in your post.

you would be better off installing a pressure gauge, flow meter and a throttling valve

 

[Rajder]

Yeah, I realize that I will never exactly be able to design an orifice that gives me those exact parameters. I should have specified that the important thing that we have to hold is the pressure on the pumps. We can vary the flow rate (pump speed) a little to achieve the pressure that we need. I would just like to stay close to the original flow rate that we defined. We actually currently use a throttling valve to commission the pumps. The problem is that the valve is very expensive and needs to be rebuilt regularly. Our thinking was a fixed orifice or nozzle would be a big cost savings over the valve assembly. Thanks!

 

[bimr]

You are correct in your thinking.

The orifice plates are drilled out. You can adjust the size of the plate by removing more material. Alternatively, you can have several orifice plates fabricated and use the size that fits your needs.

 

[LittleInch]

The problem for a single orifice at those sort of pressures is that whatever hole size you start with, fairly soon after won't be the hole size you actually have. Work out the velocity in your single orifice and you'll soon figure that out.

I would say that a series of orifices ( 4,5 or 6) to take out 75% of your pressure drop then regulate with a control valve to fine tune your rather precise flow rate.



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[Rajder]

Yeah, I'm definitely worried about the orifice washing out fairly quickly. We are moving a lot of volume at high pressures. That's why I was originally thinking about a nozzle instead of an orifice thinking that a nozzle may last a little longer. The choke valves that we use right now are so expensive we could replace orifices or nozzles fairly often and still come out ahead.

Do you have any feeling on if a nozzle will last longer than an orifice in the application?

 

[LittleInch]

Don't understand what you mean by Nozzle versus orifice.

At the velocities you must be having, an orifice would wear out faster than a valve.

I really don't understand why a correctly sized and built choke valve is wearing out as fast as you imply. Can you supply details?

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[Rajder]

When I talk of orifice I mean a simple sharp edge hole in a flat plate. Nozzle is probably the wrong term but I'm meaning a cone that transitions down to the final hole diameter. The thinking is that a sharp edge orifice will wear out pretty fast but maybe if we add smooth transition from the larger diameter down to the orifice diameter it will promote longer life.

It's not that the choke valve isn't working or wearing out too quickly. It's just that currently we rent the choke valve setup and it has a very expensive monthly cost associated with it. And for something that really isn't used very often (or for very long periods of time) we feel like we are spending too much money on renting the choke valve. We're just testing pumps a couple times per month. We were just thinking that a simple fixed flow restriction would allow us to accomplish the same thing for a lot less money.

Thanks!

 

[LittleInch]

Ok I thought the expense was was replacement.

I still think you want a stage spool, maybe a many as 10.

Your small orifice might easily cavitate or vapourise the water, see,

https://neutrium.net/fluid_flow/cavitation-in-restriction-orifices/

The noise may also be very loud

I got about 3mm orifice hole diameter as a single plate which is just crazy.


These guys make some and others will as well. They advise multi stages above 300psi drop.

http://www.tmtecnomatic.com/multi-stage-restriction-orifice.php

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Also: If you get a response it's polite to respond to it.

 

[Rajder]

Thank you for bringing up the cavitation point. I wasn't considering cavitation and it is something that I definitely need to consider.

I am curious as to how you got the 3mm orifice diameter. When I use the online calculators or the orifice equation listed about they all show orifice diameters of somewhere between .60 - .75 inches depending on what we assume the coefficient of discharge to be. I may be doing the calculations wrong though.

Considering the Orifice Equation Q=Cd*A*SqRoot(2*G*h) Where h=pressure head, G=Gravity, Q= Flow Rate and A=Orifice Area.

I'm assuming that I'm trying to achieve 3000psi of pressure on the pumps and that would equal 6930 feet of head.
A flow rate of 727.2 GPM equals 1.6202 ft^3/sec
Gravity at 32.2 ft/sec^2
If we just assume Cd is 0.8

That leaves us with the Area of the Orifice A = Q/Cd*SqRoot(2*G*h)

That gives us an orifice area of 0.00303 ft^2 or 0.43655 in^2, solving for the diameter of the orifice that gives us around a 0.745 inch orifice diameter. Am I doing something wrong in my calcs?

 

[LittleInch]

I did the same calc using metric units and came out at about 3mm again. A couple of the online calculators did the same thing.

However I think noise and cavitation make a single plate hole unsuitable so I think it's not the best way to go forward.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.