7 replies to this topic

[CHEMSTRONG]

Hi All,

 

I do not have any standard simulation software like hysys for modelling restriction orifice, but want to model it using code .. So which all equations can be used for modelling it? I have blowdown line on compressor discharge cooler side for natural gas compression process..

 

Regards,

Chetan Chavan

[Zauberberg]

For orifice equations you can refer to Crane TP410. There are also two excellent FAQ's on Eng-Tips:

 

http://www.eng-tips....qs.cfm?fid=1201

http://www.eng-tips....qs.cfm?fid=1864

 

Basically - if you will be looking at the dynamics of depressurization process - there are three distinctive flow regimes during blowdown: compressible, transient, and incompressible flow. Each one of them is characterized with different behavior.

[breizh]

Hi Chetan ,

another document .

Good luck.

 

Breizh

[serra]

for compressible flow, assuming no heat transfer, at subsonic flow regime, velocity is proportional to pressure differential,
increasing dP you'll find a point (local sonic velocity) above which a further increase in pressure differential will not produce a mass flow rate increase.
For many fluids and correct RO installations chocked flow begins for values Pin / Pount about 2
Transonic flow (meaning pressure fluctuations due to mixed subsonic and sonic flow regime) is not frequent for gas flow and correct installations (with proper space after RO for normal expansion),

transonic flow may have a large impact for two phase flow.

Given a volume of gas at some initial T, P there are direct formulations which allow to calculate RO or time (there was a good page discussing the different correlations at air-dispersion.com),

as alternative to obtain values with accuracy equivalent to those calculated by simulators

you can simply integrate from P(initial) to P(final) in n time steps,
presuming that pressure downstream RO is low possibly you can model the RO for chocked flow condition,

for solving that (critical mass flow through RO) you need to calculate at each step fluid density and speed of sound,
a simple way is to adopt the ideal gas equations (PV=RT with related formulations for density and speed of sound),

to improve accuracy you may wish to use a EOS or similar methods (I do these calc's in Excel with Prode Properties but it shouldn't be too difficult to create a procedure for density or speed of sound for some simple EOS)

Edited by serra, 16 March 2015 - 03:22 AM.

[CHEMSTRONG]

Dear All,

 

Thanks for your reply.

 

There is a BDV in Compressor discharge cooler side with RO there for continous flow to HP flare during depressurization.

 

I am using API method 520 equations which relates flwo to some other parameters like orifice area, discharge coefficient, inlet pressure, outlet pressure.. I am assuming non-choked flow here.. IS this correct approach ?

 

W 0.03947OpAkdkbkcP1(Mk/TZ(2/k+1)^9k+1/k-1)) ^0.5

 

• The rupture disc coefficient will be set to 1

• The viscosity coefficient will be set to 1

• The discharge coefficient will be set to 0.953

• The back pressure coefficient will be set to 1

• The valve head differential coefficient will be set to 1

• The standard orifice should be set according to the datasheet

--------

 

Where:

W = mass flow rate (kg/hr)

 A = relief valve orifice area (mm2)

kc = rupture disk coefficient

 kd = effective coefficient of discharge

 kb = back pressure coefficient

 Op = fractional valve opening

M = molecular weight

Z = compressibility factor

T = inlet temperature (k)

P1 = upstream pressure (kPa)

P2 = downstream pressure (kPa)

 k = ratio of Specific Heats

Where:

W = mass flow rate (kg/hr)

 A = relief valve orifice area (mm2)

kc = rupture disk coefficient

 kd = effective coefficient of discharge

 kb = back pressure coefficient

 Op = fractional valve opening

M = molecular weight

Z = compressibility factor

T = inlet temperature (k)

P1 = upstream pressure (kPa)

P2 = downstream pressure (kPa)

 k = ratio of Specific Heats

 

Thanks.

 

Chetan

[Bobby Strain]

If you use the site search you should find a link provided by me to download a calculator for restriction orifice.

 

Bobby

[serra]

as noted before you may adopt different equations,

Crane gives details, another option could be Flow Measurement Engineering Handbook by Miller etc.

API is another option, API formulation is derived from ideal gas law (see my previous post)

and it is *normally* applied to critical flow (the usual design for PSV)

if you are not in that condition there are better alternatives (see Crane or Miller)

for critical flow, a more accurate alternative is to calculate density and speed of sound with some reliable EOS and apply

those values to a corrected area (with cd or equivalent factor) to estimate at each time step

the critical mass flux,

note that for compressible fluid density changes as pressure in your container decreases (from P1 to P2) during depressurization,

to avoid large errors you need to calculate different values as discussed in my previous post.

Edited by serra, 17 March 2015 - 03:11 AM.

[Padmakar Katre]

Hi All,

 

I do not have any standard simulation software like hysys for modelling restriction orifice, but want to model it using code .. So which all equations can be used for modelling it? I have blowdown line on compressor discharge cooler side for natural gas compression process..

 

Regards,

Chetan Chavan

 

Any fluid mechanics handbook should have the equation to estimate the flow or pressure drop thru restriction orifice. As suggested by other users you can check with Crane handbook or just google and you will get many useful links. Good luck