thread391-246002: steam flow calculation through a fixed orifice
Just joined because I too am trying to double check flow calc of steam thru an orifice. Mr Rosse gave the flow in lb/hr answers for various drops but I've been struggling with all the forumulas to come up with proper units.
Mr Rosse, what formula (or calculator) did you use to get the flow answers?

I think you can use Q=AV that is the same Q=mv
Q is flow
m mass flow (lb/hr)
v specific volume
you need to know the properties of the steam it depends on the pressure temperatura etc.

It is the typical equation to calculate the flow through an orifice. Flow(pounds/h) = 1891(d**2)YC(PD/v)**0.5
d ... orifice diameter, in
Y ... expansion factor for steam
C ... flow coefficient for squared edge orifices
v ... specific volume of steam (ft**3/pound)
You must see for example the Crane Technical Paper 410.
NOTE: The steam flow through squared edge orifices increase with pressure drop without limits by critical conditions.

Careful with any equation for steam flow, they presume 100% dry quality steam.  Any condensate in the steam will flash through the orifice choking the flow (like if you are trying to calculate the steam loss through a steam trap).

carbluff, the missing critical piece of information is where you are measuring the pressure differential.

Orifices are used for 2 entirely different purposes.  The one is as a flow element to be able to measure the flow rate, and the second is to add resistance into the system to restrict the flow.  It seems from your numbers that your orifice fits into the first category - i.e. you are trying to measure the flow rate. In this case the pressure differential is measured very close to the orifice plate. Examples of this are corner taps, flange taps or D and D/2 taps.

However, when you are trying to restrict the flow you are interested in the pressure differential relatively far away from the plate - maybe 2 diameters upstream and 10 diameters downstream.  This is called the overall pressure drop. The examples in Crane are for this type of orifice. Crane addresses the overall pressure drop of a system so that it can be matched to a pump or other pressure source. The Crane manual makes no pretense of being an instrumentation manual and does not address the use of an orifice for measurement purposes.

The pressure drop measured across the close tapping points is always higher than the widely spaced points because you get pressure recovery downstream of the orifice.  You must be aware which of these two pressure differentials your formula is trying to calculate.

The value of 200 inch of water seems reasonable to me for close taps - i.e. those designed for measuring a flow rate.  I do not have my software with me at the moment so I cannot say it is exactly right, but it seems OK.  Certainly not way out.

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