Electrical heat tracing for process piping

[Sawsan311]

Dear All,

I need your views on the use of electrical heat tracing for flare vent subheader having a minimum temperature 0f -50 C during depressurization and it has to connect to an existing line of -29 C MDMT. Is the use of electrical heat tracing practically possible and are there any limitations?

regards

 

[Compositepro]

Heat tracing is used to maintain temperature of insulated pipe. You cannot use it to heat a high flow of cold gas unless it is a small amount and you are relying on the heat capacity of the warm pipe to warm the gas.

 

[LittleInch]

"Is the use of electrical heat tracing practically possible and are there any limitations?"
1) NO
2) The heat capacity of the trace heating system

Low temps ni a flare header are not unsual but it loooks like you're below the design min.

Now in practice is that an issue?
Don't know - Is there other higher temp gas in the header?
How much material is there at -50?
What happens at -50 for your -29 material?
Is -50 only there for a very short time when it increases as you blow the unit down?

To many options / variables to be dealt with here.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

Try using a more sophisticated simulation model to see if this value of -50degC is realistic (other than Hysis or Pro II). Suggest using BLOWDOWN from Shell, Dynsim from Simsci or LNGDYN from Technip.

 

[Sawsan311]

Thank you for your replies,

Actually the temperature downstream the depressurizing globe valve on the manual globe valve was modeled using the HYSYS isentropic depressuring utility and further validated with the J-T throttling valve modeling, in both cases, the temperature is in the range of-50 C in the vent piping and where no credit can be given to the thermal mass inertia of the pipe due to the very high convection heat transfer coefficient i.e. high velocity unlike the case of the vessels systems where credit can be taken to the vessel's thermal mass pertained to its thickness.

Considering heat tracing is impractical, there are two options:
- Either attempt to perform exemptions in MDMT as per ASME B31.3... anyone performed such MDMT reduction which takes into account the credit for the low exposed pressure/stress in the flare line to allow alower considered MDMT for carbon steel i.e. if stress ratio is less than 0.3 then MDMT for CS can be nominated to -155 F / - 104 C.
- estimate a minimum length for enabling heat transfer from the ambient by free conviction prior to tie-in with the lower exotic metallurgy , still SS piping will be selected for the new piece of piping.

Thanks

 

[georgeverghese]

Yes, standards allow for this lower temp provided stress on pipes is low; piping engineers are usually wary of quoting stress values on flare piping. Safer option is to use LTCS or SS316L or better till the flare stream gets into a bigger dia pipe where velocity will be lower and inside htc will be correspondingly low also.