Heat Tracing Thermostats 1

[NickParker]

I read a Heat Tracing specification that reads, "The use of thermostats for controlling oversized self regulating circuits shall be avoided".

Why does it says so? any specific reason as it is quite common to provide thermostat for heat tracing applications. In this particular application, it is going to be in Hazardous area.

 

[waross]

I suspect that the point is;
Use the correct sized heat trace. Don't use an oversized trace with a thermostat as an alternative to calculating the correct size trace.

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Ohm's law
Not just a good idea;
It's the LAW!

 

[NickParker]

Use the correct sized heat trace. Don't use an oversized trace with a thermostat as an alternative to calculating the correct size trace.

Don't the correct sized trace require thermostat?

 

[davidbeach]

“Self regulating” may be the operative term. If properly sized it should be able to be energized continuously.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[waross]

There are many uses for heat tracing. Use may range from freeze protection to process heat input.
I have seen hundreds of heat trace circuits on PID controllers.
Thermostatic control always involves undershoot and overshoot. Process thermal lag may result in temperature swings greater than the differential of the thermostat. Oversized traces may result in even greater temperature swings.
Some products may be damaged by over-temperature.

"The use of thermostats for controlling oversized self regulating circuits shall be avoided"

The specification writer may be aware of real world issues with his plant or similar plants due to oversized trace.
On the other hand, undersized heat trace is common.
Example:
Heating requirements are 15 Watts per foot.
Available trace is 10 Watts per foot and 20 Watts per foot.
Supply 10 Watt per foot trace.
Installation instructions:
Trace shall be installed so as to provide 15 Watts per foot of run.
The trace is installed zig-zag fashion so as to provide 15 Watts per running foot of pipeline.
Extra cost; 150% greater length of trace to be purchased. Extra labour to install zig-zag fashion.
A straight run of thermostatically controlled 20 Watt per foot may be cheaper to buy and install, but the designer does not want that.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[mc5w]

When I built somebody a pipeline antifreeze system using self regulating heat tapes and thermal insulation I installed line voltage thermostats that would turn off the power when room temperature went above about 40 Fahrenheit. There is nothing illegal about that. At 50 Fahrenheit self regulating tapes still draw a considerable amount of power even if room temperature is 45 or 50 F. This guy needed for these to be drawing power when his unheated warehouse was well above freezing like he needed a giant hole in his wallet. I also used GFCI protection because that was cheaper than the equipment ground fault protection that National Electrical Code requires. The lower trip level of GFCI worked just dandy.

You can use 20 watt per foot self regulating tape in a 15 watt per foot application it will self adjust as it warms up. You can also use 10 watt per foot tape in a 15 watt per foot application provided that the tape is 1.5 times the length of the pipe and wrapped around the tape to use the length and it will use larger diameter pipe insulation than when running straight.

The specification writer may have some false beliefs as to what is legal or possible and is writing the specs on those false beliefs. His prohibition may be based on the idea that thermostatic control can cause a heat tape to catch fire. That is only possible if the thermostat is the only controller for tape that is NOT self regulated and is not in intimate contact with the pipe being heated. Self regulating tape will never get hotter than about 70 Fahrenheit. He may also think that a thermostat when used with self regulating tape is illegal ( false ) or will fail to turn on the power. One time a ratlord tried to tell me that National Electrical Code requires a receptacle once every 6 feet* in a dwelling. Also false. Some people think that streetlights are controlled by timers. Not correct they are controlled by photocells and they are very reliable. When somebody wants a timer for their outdoor lights I tell them I would still need to install a photocell so that the timer settings do not need to be changed each month. We do sometimes use a 7 day timer plus a photocell so that the timer will take care of when they are open and the photocell takes care of sunrise and sunset. I have seen parking lot lights that were on during the daytime and off at night because they used a timer and the timer was on the wrong noontime or had lost time during a power failure. Usually, the only maintenance for a photocell is to clean its lens once every few years. Sometimes photocells do fail on ( seen that ) and failed off ( seen that too ) but they are reliable.

I have an Ohio electrical contractor license number EL45,008 and a Bachelor of Science degree in Electrical Engineering and Physics from John Carroll University. To look up my name just go to the Ohio Construction Industry Licensing Board website and then go to their license lookup. When you pass a 100 question NEC test and a 40 question business law and accounting practices test for a master electrician license like I have then you can tell me what is legal and not legal and the talk to an electrical inspector to see who is right.

*The rule for receptacle spacing in NEC uses the simplest possible prose but the prose is very confusing to people who did not pay attention in geometry class and mechanical drawing class.

 

[waross]

There are more things in heaven and Earth, Horatio, / Than are dreamt of in your philosophy

The protection of water lines is only one of many uses of heat trace.
What is the maximum temperature of Self Regulating trace?
I have never seen SR trace used on an asphaltene line for instance. Depending on the source the melting point of asphaltene ranges from about 215 degrees C to about 300 degrees C. The trace may be capable of 350 degrees C.
A petro-chemical plant will have hundreds of heat trace circuits that are mineral insulated trace rather than self regulating.
This is nothing to do with the NEC. It has to do with process control and if a spec writer states "The use of thermostats for controlling oversized self regulating circuits shall be avoided", then that is the condition for that project. By the way, those hundreds of circuits were PID controlled, not thermostatically controlled.
You do know the difference between PID control and thermostatic control?
When designing heat trace for non-water applications, a designer may want to avoid overshoots, undershoots and hot spots.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[FacEngrPE]

The companies that sell self regulating heat tape all sell (usually it is no charge, but only provides a BOM containing products from the sponsor) software which allows determination of how changing watts/ft, and insulation thickness/pipe size and design conditions change the pipe temperature. The software also will calculate the temperature attained with a thermostat welded closed.

Thermostats on freeze protection heat trace usually measure air temperature, and just enable / disable freeze protection. It is unusual to attempt temperature control of the pipe with the thermostat.

 

[waross]

Plants processing heavier petroleum fractions are an exception. Many Mineral Insulated traces run well above boiling. In the heavy oil plants even steam may not be hot enough. Often the traces are de-energized, or only working at a very low output. There may be millions of dollars worth of heat trace sitting mostly unused. But if the plant is shut down for any reason, for long enough for the product to get cold, the heat trace is needed to heat it up so the plant may be restarted.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!