6 replies to this topic

[cmp74]

Hello all,

We have a PSV (6") installed in a 14" line(Regeneration gas, not proccess gas).Now during the risk assessment a scenario is that the process gas which is at 15 times higher pressure might enter this line due to human error.I need to check how much relief capacity this exsting 6" PSV can provide.

I have calculated the capacity of the valve for the process gas.My problem is how to calculate the maximum flow possible in the line from 14" header to PSV.This is a 8" NPS line and has a reducer 8x6 at the end just u/s of PSV.

PSV is connected to DF header and for all practical purpose , d/s pressure can be taken as atmospheric.

Also the process gas is at 3700 KPag and PSV setpoint is 1165 KPag.Looking at these 2 pressures, it is quite obvious that there will be choking.

How do I clculate this choked flow??

regards and thanks .

CMP

[Jatin Shah]

you need to calculate the sonic velocity. This is the max velocity possible and then you can find the flowrate.

Jatin

[ARAZA]

Hi There,

You need to look up the existing PSV vendor data and find out the rated capacity of the valve 2) Evaluate the scenario again with the process gas entering the recycle gas line and evaluate how much will be the relieving capacity required 3) compare this flowrate with the rated capacity of the valve, if it doesn't match you will require another PSV.

Do not waste your time calculating choke flow in the PSV inlet, it doesn't serve any purpose since the maximum allowable pressure drop would be limited to 3 % of the set pressure if it is a conventional spring loaded PSV.

Hope this helps.

ARAZA

[Andrei]

cmp74,

I think Jatin gave you some indication about flow calculation method.

I am confused by your explanation: you've determined the PSV maximum capacity and you want to confirm it? How much flow can pass a inlet/outlet PSV line? It is not the rated PSV capacity? Why do you need the line flow when you know the maximum flow that can pass the PSV? It is not the same? What makes you believe that the line can pass a different flow than the PSV? Most probably a sketch will help a lot here.

Second, I am not used to "human error" case of a PSV. All I know is that humans make a lot of errors, and if we want to make a plant fool proof we will end up with a lot of PSV's installed, or with some humongous design parameters, doubled by very expensive materials. There are other methods of protection to avoid human error.

Third, I am looking at the pressure values you mention and I am seeing a ratio of 3, not 15, as you were saying. What are the design pressures of the two systems you are mentioning?

Based on the incomplete information you are posting I am questioning the requirement of a PSV for the situation you are describing.

[cmp74]

Thanks all for ur replies.Let me clear some confusions.

I have determined maxm capacity for the Process gas. The PSV is intended for regen gas. I used standard API formula and same set pressure to check how much relief will valve give in case Process gas entered the regen system.

What I want to know is whether this maximum flowrate will be passed by the 8" PSV inlet line. Because Process gas is at 3.5 times higher pressure than PSV set pressure and there will be choking based on inlet and outlet pressure (set pressure in this case) ratio.

I also know that sonic velocity is the maximum velocity when chking occurs and is indicative of maximum flow.

I also know the formula for to calculate choked flow, but this formula do not take into consideration the pipe length or/and outlet pressure.This formula is ,

m = C.A.SQRT (K.RHo.P.(2/K+1)^(K+1)/(K-1)).

[Andrei]

cmp74,

Araza already explained, you do not calculate the flow in the inlet line, the flow is determined by the PSV orifice size. A conventional 6" PSV orifice size can range from about 2.9" to about 3.8". If you were looking for these values you didn't even ask yourself the question if a 8" line can pass the flow passing through a lower size (half diameter) orifice.
What you should do is to calculate the actual velocity and pressure drop in the inlet line. If you want to use a conventional type PSV that pressure drop has to be maximum 3% of the relief pressure, this is standard requirement. Otherwise PSV chatter can occur during relief.

[JoeWong]

CMP,

When you stated 6" PSV, does it mean 6" PSV inlet pipe, 6" inlet flange, etc... Please be specific.


I understood you have a High pressure (HP) system with gas pressure at 3700 kPag connected to low pressure (LP) system protected by a PSV with set pressure of 1165 kPag. Two system is normally isolated by a device i.e. manual block valve, control valve, shutdown valve, etc. Your scenario is opening of the device due to scenario i.e. inadvertently opening, controller failure,etc lead to FULL opening of this device. In this event, HP gas flowing to LP system, overpressure the LP system and last lift the PSV.

Generally this device is the limiter of the flow and very likely the choking flow occur in this device. It is just like choking flow occur at restriction orifice, control valve, globe valve, etc. You may refer to Crane Fluid flow technical book. There are equation and examples illustrate choking flow (even non-choking flow).

If the flow turn out to be extreme large (it is common), you may need to add a flow limiter i.e. restriction orifice, globe valve, etc in the line to limit the flow to maximum flow of PSV.

HAH.