The make-up water was frozen because the electric heat tracing failed or was poorly installed. There was no time to troubleshoot — I had about an hour to come up with a circulation-conservation plan before we'd lose every fiberglass vessel in the plant on that cold January night in Portage, Ind. Tracing, often ignored, becomes critical at times like these. Electric tracing is often favored over steam.
The advantages of electric versus steam tracing are: 1) less operational and less maintenance problems — no steam traps cracked or broken by frozen condensate, no corroded copper tubing ruining soft fiberglass insulation, no crystallized product that got too hot, no slip hazards from ice rinks created by working traps, unreturned condensate and troubles with condensate systems and, what's more important, no long steam lines to maintain; 2) much easier to control — you can put the heat where you want it; and 3) electricity is easier to move to a remote location than steam.
The disadvantages are: 1) not as high a temperature is possible — tracing with steam can provide the highest temperature, followed by electric and then heating oil; 2) skilled installation is required because of lower heat flux — tracing must be carefully banded to the surface being heated; 3) 120/240/480-V AC power isn't intrinsically safe — electric tracing is suitable for Class 1, Division 2 areas but not Division 1; 4) steam has residual heat, electric tape doesn't if power is cut off; and 5) tracing imposes an additional load on an electrical system.
If electric tracing seems your best option, let's consider how to make it work for you. I'll divide this into engineering, construction and maintenance.
First, let's identify the classes of heat tape: self-regulated (SR) low temperature (<~185°F); SR medium temperature (<250°F); SR high temperature (<482°F); and constant watt (power-limited) (<900°F). Resistance temperature detectors (RTDs) normally monitor the temperature, although thermocouples have been used. A well-engineered system has: 1) no flammable or corrodible materials of construction; 2) allowances for extra loads, such as loss of heat from exposed flanges, valve bonnets, etc.; 3) complete assembly drawings showing termination and anchor points to pipe and insulation — don't count on straps alone to keep tracing in place, adding a hook to the pipe will secure the tape; 4) short runs or multiple breakers to limit current — starting current can be three times the operating current and last 2–5 minutes or longer; 5) socket heaters replacing, or supplementing, tracing when three or more traces are required; 6) verification via alarms and trips in the distributed control system — where tracing must work to ensure safety or reliability; and 7) RTDs located properly, i.e., near the area most likely to be cold — avoid dead legs like isolated pump suctions and protect them with socket heaters to supplement tracing.
Now, let's consider construction. Tie the tape, without undo pulling or kinking, to the pipe at eight o'clock. If using two tapes, position the second tape at four o'clock. For three tapes, place the first at seven o'clock, the second at five o'clock, and the third at eight o'clock. For more than three tapes, consider supplemental socket heaters. Put the RTD at three o'clock regardless of the number of tapes. Secure RTDs to the pipe wall 45° from any tape. Remove burrs that could tear the tracing. Inspect all components — the wrong tape could cause a fire. Don't bend a tape more than six diameters, e.g., a 1-in. tape can bend 6 inches.
Carefully stage your tests. Conduct pressure tests, and equipment and instrument inspections before installing the tracing. Test tracing in sections, as you build, to more easily identify problems. Bench-test beforehand components like RTDs. The first commissioning test is to verify the connections; disconnect the tracing and ensure the RTD is controlling the correct tape. Next, disconnect the power and the thermostat. Disconnect the tails. Using a voltmeter, connect the + to one tail and the - to the other. Read the resistance; a low value shows a short. Then, do a resistance test with a mega-ohmmeter: connect to the + lead of the cable sheath and the - end of the heating cables at 1,000 VDC. Apply voltage for one minute. The meter should jump then stop moving; if not, then the cable is grounded to the sheath. Forget splicing — replace the cable.
On to maintenance: every 1–3 months, confirm the currents measured by the trace controllers match the baseline measurements, which ideally should be taken in winter. If a heater is on all the time, the RTD could be disconnected from the pipe and measuring air, or the insulation is breached. A cold RTD means a broken or burned out trace. If the tracing won't heat, suspect the RTD first.
DIRK WILLARD is a Chemical Processing contributing editor. He recently won recognition for his Field Notes column from the ASBPE. Chemical Processing is proud to have him on board. You can e-mail him at firstname.lastname@example.org