From: <indra>

Date: Mon Jan 28 2008 - 20:47:00 EST

Date: Mon Jan 28 2008 - 20:47:00 EST

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/*

*>*

*
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