Sizing of orifice? 2

[ddkm]

Hello all. I'm a newbie here, searching for opinions on orifice sizing.

For a vapor through a hole (in this case, saturated steam), I was recommended the following equation (from Crowl/Louvar):


Qm (choked)
= Co A Po SQRT{(k gc M / Rg To)*(2/k+1)^[(k+1)/(k-1)]}

(Equation is provided in the old units, not SI)

Then you vary the orifice diameter - which changes the cross sectional area A - and thus, changes the maximum flow Qm (choked). Keep varying the diameter til you get the max flow that is required.

Is this correct? I'm asking because I'm getting a very low calculated result compared to the expected result. Example I'm getting only 11kg/hr instead of, say, 1100kg/hr.

 

[sailoday28]

ddkm
Flow thru the orifice thru its exit prior to expansion to the larger down stream piping is two phase. The ratio of specific heats has no meaning unless the flow is in a metastable condition. That condition is possible however, in a metastable condition, a shock/discontinuity to two phase normally occurs.

 

[katmar]

ddkm,

You cannot really define a maximum flow for non-critical conditions. For a fixed upstream pressure the flow will increase as the downstream pressure decreases, until the choked condition is reached.

There must be hundreds of references on the internet, and in any standard fluids books you can lay your hands on (or even Perry), that will give you the necessary formula. But be aware that there will be two types of formulas. One is for using an orifice for metering flows, and the other is for so-called restriction orifices. In a metering application the pressure drop is basically the drop from the upstream pressure to the vena contracta, whereas in a restriction orifice you are looking at the overall pressure drop to a few pipe diameters downstream from the orifice.

There are a few computer programs that will give you the k value for steam at various temperatures and pressures. Googling for "Water and Steam Properties" will take you there. I can't comment on what Sailoday28 has said - when it comes to discontinuous metastable shocks I am lost.

 

[sailoday28]

In the metastable state the fluid stays as a single phase.
For example, in the depressurization of sat steam thru a ventui or nozzle, it has been observed that the steam will not go two phase as the pressure drops. Then at a certain point along the axis of the nozzle two phase suddenly apprears. This is a discontinuity in the flow. What has been approximately an isentropic proscess yields a shock and entropy increase. That process has been observed/measured in nozzles of steam turbines.

A similar result is in steam traps. Sat water flows thru the trap. In the process where flashing should start, the water stays single phase.-Then suddely flashes. The result is higher than normally calculated flows for the trap.

 

[mbeychok]

Wow! This thread has taken on a life of its own and seems to go on forever.

Back in the 1950's, when we didn't even have electronic calculators (much less computers), we developed process flow sheets, heat and material balances, and equipment specs for complete refineries in 3-5 months using slide rules. If we developed a Ph.D. thesis every time we sized a restriction orifice, it would have taken us 3-5 years or even more.

And some of those refineries are still operating and operating well.

Milton Beychok
(Contact me at www.air-dispersion.com)
.

 

[sailoday28]

I hope you are not comparing the responses to Thesis material. This topic thread should be on the thermodynamics forum.