Blow Down Line Temperature Change 1

[ghamsa]

Hi everyone

Pls. help me with the following problem. There is 10 feet long and 4" in diameter blow down line coming out of LPG heat exchanger and I need to figure out the line temperature change along the line as result of opening block valve from this pressurized LPG heat exchanger to blow down system.

 

[Homayun]

You can do an adiabatic flash calculation from your LPG containing HX. FLash it from its operating pressure to the final let down pressure. You'll see a temperature drop. That is the temperature you need to know..

hope this helps

 

[JimCasey]

Agree with Homayun.
Adiabatic Flash across an orifice is isenthalpic. Pick up the enthalpy of the liquid before flash, then pick off the same enthalpy at the new pressure downstream. THere's your temperature. Also will reveal quality of the 2-phase flow.

 

[ghamsa]

Thanks for joining, I really need to find out what would be my adibatic flash temperature, say, 10 feet downstream where flash temperature starts off. For example, if the flash temperature goes down to 5F what would it be 10 feet down the road? 7F or 10F (the line is not insulated)

 

[Homayun]

It doesn't really matter what the exact temperauture is 10 Ft away from your equipment. the pipe attached to the equipment has to have a certain metalurgy and sepc to withstand the minimum temperature where the initial parts of the piping is gonna be exposed to anyway..so you will probably end up with a piece of pipe that can withstand 5F througout the whole length..

But if you really require to know the exact temperature 10 Ft away from the source, which is not far by the way, then you'll have to do a heat loss calculation to calculate how much heat will be transferred from the sorrounding, taking into account any insulation that is there,pipe thikness and diameter, wind velocity, minimum ambient temperature, etc, etc, into the pipe..

 

[JimCasey]

within 10 feet, the liquid phase will still be boiling off. Temp would still be the adiabatic flash temp.

 

[chemicalengineerthai]

If your initial pressure is within 300# before depressurization

and your piping downstream depressurization made of carbon steel it can tolerate up to -29 C

all calculation show not below -10 to -20 C minimum temp.

except your upstream system 600# including higher (900#,1500#) your let down can reach lower -29 C so, next step of material just change to SS which main handle -45 C..

this is overview for your view.

 

[786392]

There is definitely a need to carefully and accurately estimate final let-down temperature.
So as to explore certain Carbon steel's potential with -40 degrees Celsius withstanding capability.
As used for many LPG storage bullets manufacture.
Hope this helps.

Best Regards
Qalander(Chem)

 

[dcasto]

Even carbon steel can do -120 F during blowdown, there is no hoop stress on the blow down. We use cabon steel on all our ethylene blow downs, save a few where the pressure will be over 100 psig because of pressure drop and velocity.

The thing to pay attention to is piping stress due to line contraction.