2 replies to this topic

[sharadmagic]

I tried to use the Hysys Blowdown utility for the first time following the procedure in the tutorials. I was trying to do a simple exercise to see if the temperatures make sense.

For a flat end vertical vessel (10 M T/T , 5M OD), I input the following parameters

Gas composition- Pure Methane

Initial Pressure- 6000kpa

Final Pressure- 101.3 kpa

Initial Temperature-33⁰C

Vessel Thickness- 70 mm

Mode Adiabatic

Heat Loss Detailed

Valve equation supersonic

Cd 0.62

Allowing 15 minutes( Arbitary) depressurisation time, the depressurisation utility calculates,a 5 in² orifice size.

I have allowed 93% PV work contribution.

1% Vapour recycle efficiency and 100% Liquid

The only issue is that it calculates minimum temperature of -58⁰C at the valve outlet . Does this mean that the valve and the piping downstream the valve should be SS? If the same stream is let down, adiabatically through a valve, the coldest temperature achieved is 6⁰C.

In real life, if we check the P&ID's, the piping for this service is always CS.

 

 

[colt16]

The answer is not at all surprising.

Depressurization isn't something that fanciful. First of all, just think logically if you were to drop methane @ 6000kPa, 33degC to 101.3kPa with a control valve what will happen? Precisely that negative temperature scenario. A depressurization study is simply a calculation exercise to find the correct orifice size to control the "rate of depressurization". The final conditions can be predicted somewhat WITHOUT a depressurization tool. Now, where the depressurization tool comes in handy is to consider heat input along with the depressurization. So in your study you must consider everything that makes sense - wetted area, heat input, etc.

Note: I am answering depressurization in general, I have very little experience with Hysys' depressurization. I mostly use Pro/II.

[serra]

there are many threads discussing depressurization,

in most cases you can validate results with a step-by-step procedure (numerical integration),

at each step you solve mass & heat balance,

(there are descriptions for Prode Properties and Excel but you can adapt to any simulator),

low temperatures (gas discharge) are the result of high dp through valve (or orifice),

with mixtures I prefer the (above mentioned) numerical integration procedure to detect problems such as ice or hydrate formation...

(automatic procedures may not detect these problems)