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# PSV sizing

## PSV sizing

(OP)
I am trying to properly size a relief valve for a batch reactor and was wondering if I could get some help determining the relieving capacity. I haven't done a calculation like this before and want sure how to calculate the flow rate. The scenario would be a runaway reaction. The reaction is the partial oxidation of methane to methanol. Any help would be greatly appreciated.

### RE: PSV sizing

Sounds like homework. Most industrial routes use syn. gas. Plus, on the relief sizing difficulty scale of 1 to 10, you've asked a 30! Since you offer no details, I'll answer similarly. Develop worst case scenarios, collect lab data to evaluate, educate yourself on runaway reactions (DIERS), determine "THE worst case scenario", collect more data on "Twcs", size the relief. It shouldn't take over 6 months working full time on it

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

### RE: PSV sizing

(OP)
It does sound like homework, something I may have done as an undergrad but it isn't. There are papers out there on this research. That's correct syngas is more common which is why this is being looked at

### RE: PSV sizing

A typical route from methane involves natural gas reforming, a shift gas reactor, syngas compression and the methanol convertor, so how do you plan to do all this in a single batch reactor ? Is this some new process ?

In another process, the steam reformer is replaced with a partial oxidation reactor where methane is converted to CO with pure O2.

Where exactly is this RV located ?

### RE: PSV sizing

So, it's a research project? What size is the batch RX?

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

### RE: PSV sizing

(OP)
Yup it's for research. It's a 300 cc reactor

### RE: PSV sizing

Solution
This sample problem for relief valve sizing calculation can be solved in following steps, which are based on pressure relief valve sizing procedures described in API RP 520 Part I. The first step is to determine whether the gas flow type is critical or sub-critical.
Step1
The first step is to determine if the flow conditions are critical. For this purpose the critical flow pressure (Pcf) downstream to the relief valve has to be calculated using the following equation from API RP 520 Part I,

P1 : Relieving pressure upstream to relief valve in bara
P1 = (including 10% accumulation)
k : Ratio of specific heats of the gas .. (CP/CV)
k =
Pcf : Critical flow pressure for the given flow conditions in bara
Pcf =
If the pressure downstream or back pressure of the relief valve is lower than the critical flow pressure, then the flow is said to be of critical type. In this case, the actual pressure downstream to the relief valve cannot fall below the critical flow pressure even if much lower pressure exists further downstream to the valve. Mass flow relieved cannot increase any further, for the given set of flow conditions upstream to the valve.
Step2
Next step is to use the relief valve sizing procedure given in the API 520 part I for critical flow type gas relief valve. The following equation is used,
... (1)
A : orifice are requirement in mm2
W : flowrate in kg/hr
W = ………… kg/hr
T : Temperature in Kelvin
T = ………………….. K
Z : compressibility factor of gas
Z = …………
C : coefficient for fire case as a function of k (CP/CV) as per following equation,

Here, C = 351.6
P1 : Relieving pressure upstream to relief valve in bara
P1 = ………… (including 10% accumulation)
Kd : Coefficient of discharge
Kd = …….. for gas service
Kb : capacity correction factor due to back pressure
Kb = 1 (for conventional relief valves with near atmospheric backpressure)
KC : combination correction factor for use of rupture discs
KC = 1 (for absence of rupture disc upstream to the valve)
Plugging this data into equation (1), the minimum required orifice area for the relief valve is calculated to be,
A = ……………… cm2
Step3
The next higher available orifice area should be selected for the pressure relief valve to be actually installed.

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