Help - Material balance at relievibg conditions 1

[sheiko]

I have a very basic question to ask you.

Context:
Relief valve scenarios - Material balance at relieving conditions

Nomenclature:
T= temperature
P = pressure
Q = volumetric flowrate
W = mass flowrate
D = density
1: state 1
2: state 2

Problem:
When dealing with scenarios involving the material balance for a project, I have used the material balance as it is and not at relieving conditions because to me, the mass flowrate does not change with T and P for gases and liquids. As a result, D and Q are functions of T and P for gases and only T for liquids (incompressible).

To sum up, if W is independent of T and P:

1/ Liquids:
D(T) => Q(T)

2/ Gases
D(T,P) => Q(T,P) and Q2/Q1 = (T2/T1)*(P1/P2)

May I have your opinion on it? Is it correct to consider that the mass flowrates are independant of T and P for gases and liquids?

Thanking you in advance


"We don't believe things because they are true, things are true because we believe them."
"Small people talk about others, average people talk about things, smart people talk about ideas and legends never talk."

 

[JoeWong88]

Sheiko,
There are many scenarios i.e fire, gas blowby, blocked discharge, etc lead to PSV lifting... You must define the relieving scenario.

Each scenario would drive to different relieving conditions.
For example, when a vessel containing vapor & liquid under fire, external heat may lead to two phases relieving if poor vapor-liquid disengagement or only vapor relieve due to perfect vapor-liquid disengagement or all liquid has converted to vapor when shifting from Gas-liquid phase to pure vapor phase...

Fluid characteristic may lead to different possible relieving scenario... may read more in How Fluid Characteristic affect 2 phase Relief via PSV on Liquid filled Vessel Exposing to External Fire".

It is difficult to provide good response if you don't define your problem statement in specific way...



JoeWong
Chemical & Process Technology

 

[CMA010]

You can't base your relief rates on your mass/material balance because the mass flows that occur for a number of relief cases (e.g blocked outlet) are dependent on the differential pressure. For the fire case you could use the composition from the mass/material balance.

 

[sheiko]

So if i well understand, the mass flow is independant on T and P, but dependant on the pressure differential DP?

Please explain

"We don't believe things because they are true, things are true because we believe them."
"Small people talk about others, average people talk about things, smart people talk about ideas and legends never talk."

 

[CMA010]

Driving force.

During a relief scenario the pressures in your system change. And therefore the differential pressure, or driving force change. You could end up with larger or smaller mass flow rates than stated in the mass balance.

 

[MortenA]

Sheiko

If it were simple then you wouldnt need engineers to do it.

First you must identify your cases as Joe writes
Then you must examine each of them and find the largest relief area.

For instance - if your case is a pump that dosnt trip - then your pump will have less capacity at relieving pressure than normal operating (if its a centrifugal pump).

If your flow is from a well - then it would be fair to assume that the flow WILL drop at increased pressure - this may however be difficult to quantify - or if you choke your well back - then no decrease at all. So in most cases i would assume unchanged mass/molar flow. I would also assume that the temperature is unchanged - but the vapour fraction changes - and i would use HYSYS or similar to calculate this.

Best regards

Morten

 

[sheiko]

I agree with you MortenA

Many thanks to all contributors

"We don't believe things because they are true, things are true because we believe them."
"Small people talk about others, average people talk about things, smart people talk about ideas and legends never talk."