6 replies to this topic

[pdender]

for a blowdown line I am evaluating mach number for two design cases,
gas and vapor + liquid,
mach number is calculated as

mach number = average speed of fluid / calculated speed of sound

where the speed of sound is calculated with method

= StrMSS(stream)

in PRODE PROPERTIES which returns the speed of sound for gas/vapor,liquid
and gas + liquid (with HEM methodology),

while for gas I adopted a limiting value of 0.6
for vapor+liquid I would consider a lower value
to take in account for the higher liquid density
and additional induced stress,
does that decision seem reasonable ?
(I mean to adopt a much lower mach number for vapor+liquid compared to gas only)

Edited by pdender, 10 March 2014 - 06:52 AM.

[marchem]

I would consider separately critical flow and stress (ρv2 criteria) design limits,

if you use the HEM model in PRODE PROPERTIES (which is a conservative approach)

a mach number of 0.5-0.6 could be a reasonable guess,

while for stress, ρV2 < 200000 kg/ms2 where ρ is average density for two phase conditions in kg/m3
and V is the velocity in m/s

but in some cases different values are considered

[pdender]

thanks marchem,

actually, for design purposes, with vapor+liquid, I utilize a mach number of 0.4,

does that value seem too low ?

My concern is about vapor fraction,

which may be higher than expected, that would change density and speed.

[PaoloPemi]

you are correct,

density of vapor fraction may have a large impact

while densities of liquids show limited variations.

You may consider a safety factor.

[twell]

marchem,

do you have a specific reference for the suggested limit ?

 

"ρV2 < 200000 kg/ms2"

[marchem]

it's reported in several textbooks,

perhaps from NORSOK P-001,
I think it's good only for a preliminary screening,
if there are process specifications for vibration and support checks
these can require a flow induced vibration analysis and specify acceptable limits

[frpe]

twell,

my copy of NORSOK STANDARD, PROCESS DESIGN P-001 under paragraph  LINE SIZING CRITERIA says

"In general, all flare lines shall be designed to keep the

rho*V*V < 200 000 kg/ms2 criteria

(rho is the fluid density or mixed density for two phase conditions in kg/m3).
Where the rho*V*V criteria will not be met, additional calculations will be required to document that the selected pipe size is still acceptable.
This involves evaluating piping stress levels, supporting, noise etc."

Edited by frpe, 03 June 2014 - 03:06 AM.