Re: Pressure Drop for Compressible Flow

Hi all,

Thank you for all the valuable input. The iteration is another set of problem by itself, at least for my limited expertise in using spreadsheet =p. But it has nothing to do with 'Piping Design' so I shall not ask about it in this forum.

Hi Amjad,

I'm sorry. I should not call it 'Darcy Equation for Compressible Flow'. It was just equation for compressible isothermal flow derived from Bernoulli's equation.

I use the following literature as my references: - Ludwig's Applied Process Design for Chemical Petrochemical Plants - Perry's Chemical Engineering Handbook
The equation for compressible isothermal flow can be found on both books.

Regards,
Indra

• In PipingDesign@yahoogroups.com, Christopher Wright <chrisw@...> wrote:
  >
  >
  > On Jan 27, 2008, at 11:56 PM, indra pratisto wrote:
  >
  > > I'm assuming isothermal compressible flow. Using pressure and
  > > temperature at the safety valve's discharge point I obtain the
  > > fluid properties (density, viscosity, Cp, Cv, etc). Afterwards,
  my
  > > step-by-step calculation is as follows:
  > > - Check for Mach number of inlet fluid: max possible velocity
  of
  > > a compressible fluid in a pipe is sonic.
  > > - Calculate friction loss using Reynolds number and relative
  > > roughness
  > > - Calculate loss coefficient due to fittings using Darcy's 3-K
  > > method
  > > - Calculate total loss (friction loss + loss due to fittings)
  > > - Calculate outlet pressure using Darcy's equation for
  > > compressible fluid
  > > - Get the pressue drop (Inlet pressure - Outlet pressure)
  > > - Check for Mach number of outlet fluid: max possible velocity
  of
  > > a compressible fluid in a pipe is sonic.
  > This is covered pretty well in my old undergraduate fluid
  mechanics
  > textbook by V. L. Streeter. Your best bet is to find a reasonably
  > good book on the subject to see the form of the relationships
  > involved and get some caveats on the limitations of the process.
  > Marks' Handbook is an excellent desk reference. The steps you've
  > outlined are correct, but the procedure is iterative--perfect for
  a
  > spreadsheet.
  >
  > If it were me, I'd start by assuming that the entire pressure drop
  > takes place across the valve and determining the first guess at a
  > flow rate from that. Then I'd figure what the flow rate would be
  with
  > zero pressure loss across the valve. The lower of the two figures
  is
  > an upper bound to the overall flow rate. Provided the pressure
  ratio
  > across the system is less than that for sonic flow in the valve,
  > which will be the absolute maximum flow rate. If that's the case
  your
  > problem is solved already. In that case the chances are that the
  > effect of the line will be small. In fact the line should be
  chosen
  > short enough and big enough so that the pressure drop will be an
  > absolute minimum--otherwise the pressure relief will allow the the
  > steam drum pressure to rise above the valve set point. You really
  > don't want that.
  >
  > Otherwise use the upper bound to estimate reynolds number and the
  > friction factor in the pipe and solve for the flow rate through
  the
  > system, then iterate until the Reynolds number and friction factor
  > are consistent with the flow rate.
  >
  > Christopher Wright P.E. |"They couldn't hit an elephant at
  > chrisw@... | this distance" (last words of Gen.
  > .......................................| John Sedgwick,
  Spotsylvania
  > 1864)
  > http://www.skypoint.com/~chrisw/
  >