9 replies to this topic

[pleckner]

A PSV is NOT a valve in the true sense of a valve but has a flow path that is more like a nozzle. Thus it is treated as an isentropic expansion, NOT an isenthalpic expansion. The equations used to size a PSV are derived from the formulas for isentropic expansion.

If you are asking HYSIS to model a PSV and it is calculating it as a valve then it is doing it wrong. If on the other hand you as the operator of HYSIS are choosing the wrong model, then it isn't HYSIS that is doing it wrong.

[latexman]

Jaimin,

The two models you mentioned can both be used, but for different reasons. As you said it’s up to the engineer and Hysys operator to know when to use the correct one.

Thermodynamic functions relate state properties at one point, where everything is known (inlet or Point 1), to state properties at another point, where there is an unknown (outlet or Point 2) that needs to be determined. Luckily, whatever happens between Point 1 and Point 2 (the path) does not affect the results that are desired at Point 2. As such, thermodynamic functions are called state functions as they are path independent.

To determine flow in a PRV the points of interest are in the pressure vessel (Point 1), where velocity ~ 0, and at the exit plane of the flow nozzle (Point 2), where velocity needs to be determined. If you look at the general energy balance equation for a steady-state process in your favorite thermo book and with the right assumptions, the energy balance equation for this specific process can be simplified to something like:

ΔH = - Δv²/2gc

This is called isentropic, because friction, an irreversible process, is ignored. Frictional effects are relatively small in a PRV, given the extremely short distances the fluid is at high velocity.

To determine unknown state properties in the discharge of a PRV using an isenthalpic process, the points of interest are in the PRV inlet (Point 1) and in the PRV discharge or tailpipe (Point 2). Since the inlet and discharge of a PRV are usually about the same diameter and about the same elevation, and with the right assumptions, the energy balance equation for this specific process can be simplified to something like:

ΔH = 0 or H1 = H2

This is isenthalpic.

If that is what you are doing with Hysys, determining discharge properties based on inlet properties, the correct terminology is being used. However, if you are selecting a PRV flow nozzle size, then the terminology is wrong and the method Hysys uses may be wrong. Hysys needs to use an isentropic, converging-flow nozzle model for sizing a PRV.

[JoeWong]

All,
Please allow me to drop my 2-cents worth thought.

Please correct me if i have mistake.

PSV is NOT a valve in the true sense of a valve but has a flow path that is more like a nozzle (~constant diameter)

Let see the sketch below.
 Nozzle.PNG   15.97KB   188 downloads

From PSV inlet (P1) to Vena contractar (Pvc), it will goes through REVERSIBLE process.
From Vena Contractar (Pvc) to PSV outlet (p2), it will goes through IRREVERSIBLE process.

PSV size will be determine by first process. Thus ISENTROPIC process shall be considered for PSV sizing.

However, for entire process perspective, the process is IRREVERSIBLE. Thus from PSV inlet to outlet, it shall be modelled as ISENTHALPIC process.

JoeWong

[latexman]

Well said Joe. I've never given much thought to a PSV exhibiting a vena contracta or not. It definitely could since the change in flow area is more abrupt than smooth. Part, maybe most, of the discharge coefficient, usually 0.8 to 0.9 for a gas or vapor, is probably correcting for a vena contracta too. Thanks.

[pleckner]

@joe:

Like Latexman, I've never considered the phenomena of a vena contractor in a PSV nor have I come across this in all of my readings on them. Can you state a source that is readily available to review that talks about vena contractors in PSVs?

Thanks.

[JoeWong]

@joe:

Like Latexman, I've never considered the phenomena of a vena contractor in a PSV nor have I come across this in all of my readings on them. Can you state a source that is readily available to review that talks about vena contractors in PSVs?

Thanks.

Phil & Latexman,
Good eye.

This merely a results of in-house study and interpretation of the vena contracta (VC) phenomenon. Apologize for not reveal it now.

Same as most the engineers, i am weak in thermodynamic. I just drop some points here. Please correct me if i am wrong.

Classical / conventional studies informed us that VC will only occur in sharp edge orifice. However, the flow restricting area in a relief valve is not an ‘‘orifice’’, but is actually a nozzle. Those there is NO VC downstream of the nozzle.

Phenomenon of flow through a PSV is actually a flow passing nozzle instead of orifice (as known by everybody) and the process is extremely fast. Choked flow is possible occur in some location "A" in the nozzle instead of outside the nozzle (probably closed to the exit end), from the inlet to "A" will be a REVERSIBLE process which generally accepted by most of us. It is ISENTROPIC process.

From location "A" to PSV outlet, the system is expanded and change in state. It will be slow down. There will be transformation energy loss however the enthalpy is maintain constant (ISENTHALPIC). This process is IRREVERSIBLE.

"A" is viewed as "vena contracta (VC)" in nozzle (many may disagree with me).

Those in sizing a PSV, we ignore the frictional loss in the nozzle and consider ISENTROPIC process and there is conservatism in it. However, from expansion process via safety valve ("A" to OUTLET), we view it ISENTHALPIC.

Sorry. That's all i can share and that's my limit for the time being.

JoeWong

[pleckner]

@Joe, thanks. That works for me.

[latexman]

A vena contracta forms in a sudden contraction. If the entrance edges of the PSV nozzle are not rounded enough (r >= 0.15D) one may develop.