Steam density with velocity

[dbday]

Hi,

If I have a pipe with steam flowing in it at what I think is around 200 metres/second and I want to calculate the density of the steam do I simply use the static pressure and temperature shown on my gauges (85 Psia and 330 Deg F) and pick the appropriate specific volume from steam stables and convert it to density, or, in some way, should I account for the velocity of the flow and include something for dynamic pressure ?

I know this seems a basic question, but I don't want to miss something obvious.
Thanks in advance for any help.

 

[vt2012]

Density is a function of temperature and pressure. You are correct that you can use your steam table to get the density based on process conditions.

Velocity is only a function of flow rate and pipe size. Also - something seems off with your estimate of 200 m/s velocity. That would be extremely high, even for superheated steam (and you only have about 15 degF superheat).

 

[dbday]

The duty is on the outlet of a pressure reduction valve and the numbers are what we have been given.

I am only questioning my own sanity because it has been stated categorically by a respected engineer in a large organisation that you pick the steam specific volume from steam tables based upon Total pressure, ie static pressure plus dynamic pressure.

From my point of view I can't quite see how you do that as it is surely a circular function.

I get that there might be an argument about compressibility affecting the result, but 85 Psia is not a high pressure and in perfect gas terms compressibility would certainly be ignored.

 

[georgeverghese]

The kinetic energy component of a stream flow doesnt play a part in the stream density.
Agreed total energy = stream enthalpy + kinetic energy + potential energy (due to static height differential from datum).

 

[Latexman]

It depends on how much error you can tolerate. 200 m/s is about Mach = 0.5 in this case. Static pressure measurement is close to the stagnation pressure, not the dynamic pressure. I recommend you refresh your recollection of Compressible Flow. At least, read the first paragraph. I'm pretty sure at M = 0.5 the density change exceeds 10% error. At least, that's what the k = 1.4 tables in my copy of Compressible Fluid Flow by Shapiro is telling me. It only has k = 1.4 tables.

Good luck,
Latexman

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