9 replies to this topic

[m-kane]

Hi,

I am currently working on an upgrade project on an existing oilrig. Doing a verification on the flare system after gas export increases.

The problem:

The tail pipe from a PSV on a separator have 2 phase flow. The liquid fraction is very low but the presence of this small liquid fraction decreases the sonic velocity so much that the mach number in the tailpipe is higher than what the relevant standards allow (NORSOK, mach < 0,7 for tailpipe)

What I was wondering was if anyone has any experience on possiblilites for "ignoring" the liquids, because the tailpipe is within spec with regards to the gas alone. (When using just gas the mach number is lower than 0,7)

Calculations have been done in Flarenet, and flarenet help states that liquids in the flow can decrease the sonic velocity quite severely (up to 80%)

Any and all hints, tips accepted with open arms.

Edit: Fluidfraction is ~0,1% by volume, ~ 1% by mass. Sonic velocity reduction is >100m/s

[JoeWong]

What I was wondering was if anyone has any experience on possiblilites for "ignoring" the liquids, because the tailpipe is within spec with regards to the gas alone. (When using just gas the mach number is lower than 0,7)

Why not consider Homogeneous Equilibrium Flow as you have low liquid inventory and highly turbulence ?

Calculations have been done in Flarenet, and flarenet help states that liquids in the flow can decrease the sonic velocity quite severely (up to 80%)

What is the basis of this statement ?

[rxnarang]

Mach 1 ( adibatic or isothermal) is a term used for compressible fluids ( gases). Generally speaking gases will choke when velocity reaches mach 1 in piping or orifice.

For two phase flow the term " mach" is meaningless.

Choked flow and velocity is a better term to use for two phase flow.

It is well understood that two phase flow chokes much earlier than single phase compressible. This is the raison d'etre for the DIERS committee set up by AIChE.

Check API 520 Appendix D for more information on sizing for two phase orifice.

Or Check Guidelines for pressure relief and effluient handling systems , a CCPS publication.

My experience says that Flarenet will give you reasonable results, but a manual check is advisable.

Regards

[fallah]

It is well understood that two phase flow chokes much earlier than single phase compressible.

Why?Brief explanation would be highly appreciated.

[rxnarang]

Two Phase paper

Have a look at this. I think it shows the reduction of flow rate as the quality of fluid changes from 0 to 1, in a orifice and a pipe. It also shows other important data, which is not relevant to this question, but is very useful for a relief system design engineer.

Regards

[JoeWong]

m-kane,

Mach no may be used to evaluate how far your flow condition reaching the flow limit of a system. Once a flow condition reached Mach no of 1, it implies that there is no possibility of flow increase by introduce more differential pressure.

Flow condition with low liquid inventory where it is certain flow pattern such as mist flow, Mach no for two phase flow may be calculated by considering HEM (HNE for some system). However, once the liquid increases resulted other flow pattern such as stratified, slug, etc, it may deviate further away for prediction.

There is a paper (i am still looking for it) discussed Mach no for two phase flow. Check it out for better understanding.

One of the point that rxnarang pointed out that i do agree is the choke form "faster" for two phase flow. One of the reason is delay boiling due to slip flow.

[Art Montemayor]

I believe Joe Wong's description:
"there is no possibility of flow increase by introduce more differential pressure" stated in his first paragraph of his last post is not correct as written.

I think the description should state:
"there is no possibility of mass flow rate increasing due to a reduction in the downstream pressure."

Choked flow is a state of constant mass flow rate. This mass flow rate can increase if the the upstream presssure of the gas increases (increasing its density) and yielding a bigger differenctial pressure.

I agree with Joe's statement that 2-phase flow is basically an ill-defined state. One cannot generalize on the TYPE OF 2-PHASE FLOW. It makes all the difference in the world if one has either stratified flow, mist flow, horizontal flow, or vertical flow. The subject of 2-phase flow is dealt with as a common, ordinary calculation when in reality no one can tell us just what exactly is flowing inside the pipe and HOW IT IS DISTRIBUTED. Any calculations on pressure drop are simply either good or bad guesses. That is one of the main reasons why DIERS is still hard at work in trying to define what happens in 2-phase, high viscosity flow. The distribution of the liquid within the gas phase is critical to the prediction of the ultimate pressure drop and this problem will continue to haunt us. All we can do is speculate. A lot of hard, laborious work lies ahead in this area of research.

[JoeWong]

Mr. Montemayor,
Thanks for your correction. I agree with you.
"there is no possibility of mass flow rate increasing due to a reduction in the downstream pressure."

I would like to expand above statement to include "...with upstream condition is fixed."

I remember in one of the previous posts, there was a confusion if the mass flow will change with upstream condition (same pressure but different temperature).

I have the opinion where upstream condition change i.e. temperature change, the upstream fluid density will change and mass flow will change eventhough same upstream and downstream pressure at choke condition.

You input is very much appreciate.

[fallah]

I have the opinion where upstream condition change i.e. temperature change, the upstream fluid density will change and mass flow will change eventhough same upstream and downstream pressure at choke condition.

".....the upstream fluid density will change and.........eventhough same upstream......pressure at chocked condition."
I think there is contradiction between two parts of above statement.

Looking at another glance,it could be said :"same upstream pressure with different temperature (naturally accompanying with different density) leading to different mass flow with different downstream pressure at chocked condition (i.e. different critical pressure ratio)."

[vap]

Hi all

I was recently invloved in a plant flare network sizing (using Flarenet) where some of the tailpipe relief was 2-phase flow. In most of these cases, the Mach number criteria was exceeded. Rightly or wrongly, in order to confirm the tailpipe/subheader/header sizings for these lines, the velocity of the gas only (provided by Flarenet) was used to calculate the Mach number for the gas component only (mist flow regime assumed), i.e:

The Mach number for the gas-only phase calculated using:

Mgas = Vgas/Cgas , and Cgas=sqrt[ (Cp/Cv) x R x T / MW] , where;

Mgas = Mach number of gas-only phase
Vgas = velocity of gas-only phase, m/s
Cgas = speed of sound in gas-only phase, m/s
Cp/Cv = heat capacity ratio of gas component
MW = molecular weight, kg/kmol
R = universal gas constant = 8.314472 J/K/mol
T = temperature of gas component, K

Any thoughts on the validity of this procedure?