Did I size this orifice plate correctly?

[fdchamb]

I referenced the Orifice plate FAQ: Link

The following calculator was recommended: Link

I have a pump that we want to operate at a higher pressure. Currently it is operating at 300 psi, and we would like it to operate at 350 psi. To do this right now, operations is running with the shutoff valve on the discharge partially-closed, but they want an orifice plate as a long-term solution.

I know that the atmospheric pressure at my location is 14.484 psi. To use the equation on the bottom of that page I did the following:

Ff = critical pressure ratio factor = 0.96 - 0.28*sqrt(Pv/Pc)

Pv = the vapor pressure at my operating temp which is 180F -> Pv = 7.51 psia
Pc = the critical pressure of water = 3208.2 psia

Ff = 0.96 - 0.28*sqrt(7.51/3208.2) = 0.9465

Cd = discharge coefficient = 0.61 for a sharp-edged orifice.

300 gpm = 18,000 gal/hr (units used in the given equation)

P1 = primary pressure

I know the inlet pressure is the pressure prior to the future orifice plate. We have a gauge there, and we are currently seeing 300 psi when the valve is fully open. However, we eventually want this to be whatever corresponds to 300 gpm on the pump curve. The pump curve shows this as being 810 feet of head, which means 351.1573 psi. That would be the gage pressure, so adding 14.484 psi for our atmospheric pressure means P1 = 365.6413 psia.

SG_water = SG_water @ 180F / SG_water @ 4C = 60.569 lb/ft^2 / 62.43 lb/ft^2 = 0.9702

The final equation is: Q (gal/hr) = 60*Cd*(d_0/0.183)^2 * FL * sqrt((P1 - Ff*Pv)/SG)

When I plug this in I get d_0 = 0.976 inches.

My main question is, did I use the proper P1? Is it bad to pull both the P1 and Q from my pump curve, or is that not proper practice?

Thank you!

 

[Latexman]

we would like it to operate at 350 psi

Why is P1 [≠] 350 + 14.484?

Good luck,
Latexman

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

 

[chief]

Flow thru' orifice, Q gpm = 25 x A x k x sq.root h

A = Cross sect area in2
k = 0.6 for water
h = differential head, Feet
Convert to Metric if required.

Use the gauge pressure instead of the pump curve, too many variables not allowed in curve, pipe losses ,filter restriction, worn pump etc.

Offshore Engineering&Design

 

[georgeverghese]

To size this orifice, you also need P2, the pressure downstream of the orifice - the expression you've posted doesnt have this P2 input..
To get P2, you have to work backward from the minimum permissible pressure required at the downstream user, say P3, then run a frictional dp calc at 350gpm and the corresponding line length. This will give you P2.

 

[ieseberri]

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