3 replies to this topic

[cmp74]

Hello folks,

I know this point has been discussed in some blogs previously but I would like to once again bring it to forefront.

API 520 Appendix - D mentions using 0.85 as Kd value for preliminary sizing and advises to use vendor's data for final sizing.But most vendors I talked to do not have Kd values for 2-phase.They test valves for liquid and gas only and provide Kd values respectively.

Recently I have done some literature survey(see below) and have found some recent papers indicating thatin a 2 -phase application Kd may well be higher than 0.85 , especially if the flow is choked in the valve (which is the case most times).

1) A recent paper by J C Leung(who proposed omega method) suggest that Kd for 2-phase flow is a function of "omega" value at PSV inlet and certified Kd value for gas or liquid depending on the valve trim.Basically, once have certified Kd value,one uses a chart (Kd vs. w) to get Kd value for 2 phase.


A theory on the discharge coefficient for safety relief valve

Joseph C. Leung

Comparing the flow through an SRV to that through an orifice, a simple theory is proposed to relate the discharge coefficient in two-phase compressible flow to the value in liquid incompressible flow. This approach made use of the author’s omega method applicable for both flashing discharge and non-flashing discharge. The two-phase discharge coefficient is shown to be a smooth function of the omega parameter. For non-flashing two-phase discharge, the discharge coefficient is shown to lie between the liquid coefficient and the gas coefficient. For flashing two-phase discharge, the discharge coefficient is higher than the gas coefficient. Comparison with recently published SRV data seems to support the present recommendation.

2)An article by Dilip K Das in Chemical Engineering -October 2008 also mentions above paper and suggest that Kd may as well be same as for the gas flow in choked flow conditions.


My question is ,

1) Does anyone has used this method to get Kd value for 2 phase ?
2) If you are buying a new valve for 2 phase, should it be gas trim or liquid trim ? One vendor suggested that liquid trim valves are allowed by ASME to be used for Gas service as well and hence for 2 -phase .But Gas trim valves are not allowed to be used in liquid service and hence not for 2-phase service.

Give your inputs.
thx.

[latexman]

1) I have not.

2) The practice I have been taught and used is use gas trim and gas coefficient for two phase flow.

[pleckner]

Dilip subscribes to Ron Darby's theory that if the PSV is choked in the throat then the valve body effects are essentially nill and thus the capacity correction will be close to that of a gas. If the PSV is not choked in the throat then the valve body will have a significant effect on valve capacity and the Kd for a liquid should be used. Fauske years ago provided an equation that is a type of weighted average between the gas and liquid Kds; this sounds like Leung's equation and I think this is where the API figure comes from but I won't bet my salary on that.

Darby's assessment seems more reasonable to me so that is what I follow in my calculations unless otherwise instructed by my clients.

Phil Leckner

[cmp74]

Thanks Latexman and Pleckner.

I also agree that if flow is choked, valve body should have no affect on flow and Kd for gas should be used.

But our company's standard says to follow API.

The valve in question,is undersized if I use 0.85 (As per API) , but adequate if I use gas Kd (0.953).

Im thinking of upsizing valve. There is no need to change the piping as the pressure drop is below 3% even for larger valve.