Restriction Orifice Design

[sothpau]

When going through orifice design calculations I find that one must assume a pressure differential to calculate a velocity based on a specific Beta and other design information such as upstream pipe internal diameter. My question pertains to determine if there is a standard guideline for pressure differential when designing an orifice? I have found anywhere from not exceeding 100" h2o to 100 psi being acceptable. Keeping velocity in check and ensuring Beta is within reason are considerations but I can't get my head around that I really could guess any pressure differential for my design but I really don't know what my downstream pressure will be (assume none is recoverable by nature).

Example:
I want to design a 1" recirculation line that will allow 40 GPM (min req. pump flow from specifications).
Square cut 1/8" orifice with a K of 0.62.
Water Service at 225 degree F with a discharge pressure of 250 psig.
I can assume an orifice diameter of 0.5" with a dp of 50 psig and obtain approximately 35 GPM @ 14.15 ft/sec.

Where I lose myself is that I can change the estimated dp (since this is a design, I cannot read pressure gauges and know what my dp is) and affect flow and velocity but how am I sure what the downstream pressure truly would be with this particular orifice installed. How do I know the dp won't be 40 psig or 100 psig?

I have Crane's book and have reviewed other sources but none of them deal with initial design and more so with using given information to solve for one unknown (like flow or velocity).

I did perform a search prior to starting this thread and I am new to the forum so please go easy on me if I am just a bone head here...

 

[georgeverghese]

You've got to work out the pressure downstream of the RO by working backwards from the final point of discharge where this recirc stream ends (a tank or a vessel ??), at which point the pressure is known. Work backwards and add up all the frictional and static head differentials till you get to the point downstream of the RO, using the min flow of 35-40gpm you've selected.

 

[hacksaw]

Your pump curves fix the discharge pressure and the minimum flow condition, and discharge pressure.
Given the fluid properties and the orifice coefficient you have all the data needed....

For recirc applications the restriction orifice needs to be thicker than that used for measurment.

As the previous response shows, the rest of your piping system sets the pressure drops.

 

[Latexman]

The difficulty is because it's not just a restriction orifice design, it's a recirculation system design. It can vary from a system with only pipe or tubing of the correct diameter (no RO) to give the desired flow to a system with pipe large enough such that 99.9% of the pressure drop is taken by the RO.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[georgeverghese]

If the final discharge point pressure is variable ( say tank level is not constant or vessel pressure varies for some reason or other), then the pressure at the discharge of the RO will also vary, and hence for a fixed RO, the flow through the RO will also fluctuate. In such cases, it would be better to set up a min flow control loop with FCV rather than this fixed bore RO.

 

[zdas04]

One thing that you haven't talked about is the thickness of the plate. With that beta ratio, you are in the range of flow measurement (which is where the 100 inH2O number comes from, the equations don't give you accurate flow measurement above 100 inH2O because of some simplifying assumptions they made in derivation), and a thin plate (basically thinner than the hole) will only have a permanent pressure drop of about 40% of the dP (the rest is recovered through velocity effects). To actually get a permanent pressure drop that is nearly all of the dP I use a billet at least 5 times the orifice diameter, but never less than 4 inches (I hit on that number through trial and error) and drill the hole. Your dP will be due to friction instead of velocity effects and that pressure is not going to pop back up 10 pipe diameters downstream.

Have a machine shop thread and drill a peice of bar stock that is the same as the OD of your pipe and use hammer unions.

[bold]David Simpson, PE[/bold]
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[LittleInch]

Or use an automatic recirculation valve (ARV). Much more efficient.

The problem I have with orifice based recirc lines is that, in essence, you have to oversize your pump as you are constantly leaking flow back to your start point.

If your min recirc flow is 40GP, my guess is you have a pump of 200GPM?, but you can only use 160. 20% of your energy is just thrown away.

A complete waste of energy for the sake of a little more in the design.

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