So I have a 4-inch pipe with a pressure of 1.5 bar drop to 1.01325 bar, with a temperature of 120 degrees Celsius. How can I calculate or estimate the steam flow rate for this system?

  • $\begingroup$ I do not know much about the subject, but I think that formulas exist for calculating what you are asking ... i would also think that the length of the pipe may be a factor, as well as possibly the pipe cross section shape $\endgroup$
    – jsotola
    Jul 1, 2022 at 16:09
  • $\begingroup$ I have no formulas for it. But check if this has something usable for you: forbesmarshall.com/Knowledge/SteamPedia At least it may reveal something useful that you haven't thought. $\endgroup$
    – user33233
    Jul 1, 2022 at 20:58

1 Answer 1


Is this a very long pipe? Generally this type of pressure loss (aka head loss), assuming it is due to the pipe and not any features such as bends, valves, contractions, or other obstructions, is calculated as:

$$ \Delta P = \left(\frac{fL}{D} \right)\frac{1}{2} \rho \left( \frac{Q}{A} \right) ^2 $$

Where $L$, $D$, and $A$ are the length, inner diameter, and cross sectional area of your pipe, $Q$ is the volume flow rate you are after (e.g. in $m^3/s$),$\rho$ is the density that you would look up in a steam table for your temp and pressure conditions, $\Delta P$ is your overall pressure drop which you know, and $f$ is a friction factor that you can estimate using a Moody chart for the type of pipe you have.

This analysis is based on incompressible flow (bernoulli's equation), which will generally be satisfied for the situation you describe, but you should check that the steam is not too near its boiling curve or its critical point.


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