2 replies to this topic

[arraziy]

Dear All,

My name is Razi. I'm a junior engineer process, doing calculation for relief rate of PSV with scenario fire case in the Fuel Gas Mixing Drum from H2 Rich Gas Unit.

The equipment protected is Fuel Gas Mixing Drum with op.press : 3 barg and 65 C, Design Pressure: 5 barg and 120 oC. The document from the contractor shows that the Reliefing Properties below:

Relieving Temp. 277 C,
MW: 32.7,
Z; 0.9933
Cp/Cv: 1.081 (I know that the ratio specific heat is ideal gas at relieving temp, as per API RP 521)

I'm little bit confuse since I got the properties bit different from the contractor causing the PSV size one step bigger than contractor's. Please help me to correct mey work.

The compositions of the fluid are below:

First step I did was:
1. Determine wether the fire case happens in wetted area or unwetted area.
(Since I think this is Fuel Gas Drum with datasheet liquid level exist, this is considered as wetted area)
I was a bit confuse when I used simulator, with the composition above at the relieving pressure and also operating pressure, the simulator shows this is all vapor. (Which one should I choose? Wetted?Unwetted?

2. Continue (Wetted), I used the simulator to predict the latent heat.
Actually, I can get the latent heat by the (enthalpy vapor - enthalpy liquid) of the fluid.
Get the Relieving temperature and Latent heat, by Shell DEP methode, But I was stuck when I saw this fluid is all vapor. So I took the latent heat by setting the pressure at relieving at bubling point and dew point to get hv and hl.

But the problem is....
How do I get the MW and Z factor? I got the MW is around 9.7 (vapor), but the contractor is 32.7
as if the fluid become heavier when at relieve caused by fire. Please help me.

Thank You

Regards,

Razi

[arraziy]

Sorry: The composition are below:

H2: 0.7001
Nitrogen: 0.0239
Methane: 0.1534
CO: 0.0061
CO2: 0.0045
Ethane :0.06
Ethylene: 0.0053
Propane:0.0086
Propene: 0.0061
i-butane:0.0113
n-butane:0.0111
1-butene: 0.0008
i-pentane:0.0019
n-pentane:0.0040
n-hexane:0.0029

Thank You

[Dacs]

Since Fuel Gases are not expected to have liquid under normal conditions, you have to think of a scenario that will introduce liquid in the Fuel Gas. One that I can think of is an upset in your upstream process that will introduce liquid carryover that will ultimately be sent to your Fuel Gas KO Drum. You have to check the upstream process (Fuel Gas Unit?) to estimate the liquid carryover properties (maybe mainly C5-C6?)

Try to carry out two scenarios: one for wetted (with the info I mentioned above) and unwetted (only Fuel Gas), then pick one that will produce the larger relief load and have it as a basis for your fire case, but if I were in your shoes, I'd carried out a wetted fire case since Fuel Gas KO Drums drains out at ON/OFF basis at best and manually at worst, so you can expect liquid inside the drum at worst case.

That said, if you continue to work on the wetted scenario, you'd have your latent heat from your liquid carryover. I dunno how the contractor got the MW of 32.7 (which is lighter than C4) though.

Hope this helps