Toluene line and heat tracing concern

[SrChemE]

Hello,
During cold weather conditions we are experiencing freezing on a system that is injected downstream of a toluene line installation. It has gotten too cold and the incoming line freezes at the injection point even so it is heat traced and insulated. (the toluene line is not heat traced and insulated, just the other line that is injected into a separate nozzle).

Due to this freezing problem, we are considering heat tracing the toluene line with a 5 W/ft BSX self-regulating heat trace, but there is the concern of toluene being highly flammable.

We would like to add a self-regulating heat trace on the toluene line prior to the injection orifice and after a control valve, usse a temperature regulator with two pipe temperature indications one for control and one for high temperature cut off. Control temperature will be set for 50 F. (To ensure the temperature of toluene does not reach an unsafe level, dual temperature indications will be installed. One will be the control to the self-regulating heat trace and the other is a high temperature shut off.)

Due to the flammability of Toluene, the Electrical Classification was reviewed, the non-welded piping connections are listed as Class I Division 2. All heat trace and components to be installed are Class 1 Div 2 rated as well. The selected heat trace and installation will meet or exceeds the Class I, Division 2 Group D rating.

Suggestions on how to design this safely and what to consider will be greatly appreciated.
Thank you,
Sarah

 

[don1980]

You don't have a flammability concern as long as you keep the fluid inside the pipe. That should be your focus. Evaluate the possible ways that the heat tracing can cause, or contribute to, a loss-of-containment failure. The one that immediately comes to mind is overpressure due to liquid thermal expansion. Another possibility is that of exceeding pipe stress limit due to excessive temperature caused by the tracing. There may be others - assess your specific case and find out. Once you've identified the possible hazards, determine the layers of protection needed to prevent those scenarios from happening. LOPA is a good way to do that evaluation.

 

[georgeverghese]

Heat trace is normally applied only when you need to keep pipe fluid contents warm in non flowing conditions. But here, the toluene will be flowing when injecting, so unless you have calculated the heat load to warm up the flowing TOL contents up to 50degF in such a short length of pipe, it is likely this scheme wont work. You need to install a heater of sorts to raise the temp of toluene at flowing conditions. The heat trace will still be required to keep the TOL line warm when it is not in use. Agreed, also add on a TRV for blocked in thermal overpressure.

With a little ingenuity, you may not need the 2nd TT for over temperature cut out. Check with Thermon if this BSX cable shows a decent current vs temp slope in its performance curve, and you can alternatively use the current ammeter for this HT circuit to perform the high temp cutout. This is what we did for a large number of HT circuits configured with Thermon HPT and HTEK cables.

Cannot comment on the electrical area classification requirements for toluene - a process safety or electrical engineer should assist. Agreed toluene is a dangerous chemical to deal with from a fire risk perspective.

 

[LittleInch]

I'm a little confused here.

Is the issue that the toluene line is cooling down? How long is the exposed line? What temperature does it come in at?

What insulation are you thinking of?
How big is the pipe?

I'm not sure you've grasped what self regulating tape is. It means the tape has inbuilt temperature control by means of how the tape works internally. It doesn't use an external temperature input.

An over temperature alarm / cut off is a good idea, but not one for control.

Other than that I can't see any big issues not already mentioned. Heat tracing is very common in Class 2 Div 2 zones so you should find suitable suppliers, but the lengths of each tape might be limited.

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

 

[Latexman]

Toluene freezes at -95[sup]o[/sup]C, so the problem must be whatever is injected. Sounds more like a solubility issue to me?

Good luck,
Latexman

Engineers helping Engineers

 

[SrChemE]

Thank you all of you that have relied, you gave me good ideas to consider during the PHA that I will be sitting in next week. These were good discussion points to consider.

Some clarification I wanted to make: the toluene line is all non insulated piping operating at ambient temperature.
The toluene is not heated. When it is injected in the winter month, it happens to cool down an adjacent liquid as both are introduced into a nozzle. The other liquid is heat traced and insulated, but due to the thermal load of the toluene flow, the temperature of the other liquid has very little impact compared to the toluene temperature.
The heat load was calculated and it indicates that we need toluene line to be maintained above 32F during the winter months.
The two temperature indications will be: 1. maintaining the temp at desired temperature >50F and 2. for high temperature shut off.

Thank you,

 

[itsmoked]

That's an easy temp to hit with tracing. As was mentioned you can easily find self regulating heat trace and then use a temp controller to run it below the intrinsic heat trace temp if you want to have complete control of your process.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[georgeverghese]

Have you actually checked if the heat transfer required to raise TOL temp to 32degF in flowing conditions ( MCpdT = UA. MTD) will actually be transferred to the TOL? What will the cable temp be at the required heat transfer rate, and is this temp within the BSX cable permissible operating limits ( or is it going to go red hot)? Looks like you're trying to get away with cheap heat tracing what should be an inline electrical heater (or similar).