Depressurization in HYSYS - Simulation Problem 3

[EmmanuelTop]

Dear All,
My question is regarding depressurizing in HYSYS.

I want to depressurize 24” (ID 547.72 mm, thk 30.94 mm) 52 m length pipe. Pipe volume is 12.3 m3.

Geometry:
2” line present on 24” line. On 2” line there is one RO (Restriction Orifice), upstream of RO there is globe valve and after 600 mm (minimum) there is ball valve after ball valve a spool piece and 24” line. Downstream of RO is routed to vent line open to atmosphere, having approximately 0.2 bar drop in line.

From ball valve to vent line material is LTCS having design temperature -50°C.
After running this model in HYSYS, I am getting final vapour temp as 0.18°C and minimum temp as -2.8°C. Upstream of ball valve line design temperature is 5°C. I want to restrict vapour minimum temp to 5.5°C

Input:
Operating Condition: 86 bara & 22°C.
Ambient Temp = 5°C
Natural Gas: 84.46mol % Methane

Regards,

http://antwrp.gsfc.nasa.gov/apod/astropix.html

 

[MortenA]

You dont tell if you have the SS or the dynamic license - but in any case: I think you must solve this by manually BD time so that the minimum temperature is met.

Best regards

Morten

 

[balaplus]

Dynamic license is not required for depressurisation run in HYSYS.

 

[MortenA]

I know - but hes askine for optimizing a system. Not that this could be done having a dynamic license anyway.

So maybe my question was a little useless - but i consider my answer correct

Since he havnt been back for 16 days then i assume hes found a solution

Best regards

Morten

 

[dcasto]

You don't need HYSYS depressure to run this, its a heat exchange problem. As you drop the pressure in the 24", the line will drop temperature at a constant enthalpy Delta H=0. At some FIXED pressure you will reach your 5.5C temperatyre, EXCEPT for heat input from the surroundings. so you need to know the heat input rate into the 24" to set the flowrate out.

Now having said all this, whats the problem with just letting the system depressure to your calculated temperature of -2.8C. It is below the 5.5C minimum, but due to reduced stress from lower pressure, your material should be able to drop below your 5.5C minimum temperature.

If you cannot let the temperature drop below 5.5C because of something other than pipe materials, then get out your heat transfer books (simulate a heat exchanger in HYSYS).

 

[MortenA]

dcasto

should have seen that myself. You are right.

Best regards

Morten

 

[sailoday28]

dcasto (Chemical)
What is your basis for constant enthalpy Delta H=0?
I believe the process p, T, will lie between that of an adiabatic or isothermal process.
Regards

 

[MortenA]

Not the upstream temp (when disregarding heat transfer to surrondings=worst case) - downstream end temp is temp same as upstream since dP is very small.

Best regadrs

Morten

 

[sailoday28]

MortenA (Petroleum)Do you mean no heat transfer, adiabatic? If, so, and dp is small, then the process over time is almost isentropic?

Regards

 

[MortenA]

No the depressurization of the upstream volume is adiabatic (when disregarding heat exchange with the surroundings). The process over the valve / RO is not - but at the end then the pressure difference across the valve is very little and the gas up/down stream the valve /RO will have allmost the same temp=adibatic depres. temp.

If you really need to coorect for heat flux e.g. a fire case then you must do it step by step and is back at the HYSYS solution (you will of course use HYSYS to determine the adiabatic depres temp - but not the depres module).

Best regards

Morten

 

[sailoday28]

MortenA (Petroleum)For the adiabatic case with negligible dp in upstream piping, I am trying to say that for a perfect gas constant specific heats, upstream conditions will follow
pv^gamma= constant or p/mass^gamma=constant
Regards

 

[dcasto]

Thanks MortenA for finishing up.

 

[sailoday28]

dcasto (Chemical) 8 Oct 07 22:29
Thanks MortenA for finishing up.
Basis for delta H=0?????

 

[dcasto]

No work is performed (added or subtracted) when gases pass through a valve or orifice therefore delta H = 0 adiabatic throttling.

What you are looking for is the JT coefficient or the change in temperature per change in pressure.