Electric Heat Best Practices

Submitted by Patrick Bartell || Valin Corporation
on Mon, 04/02/2018
Heat Trace and Electric Process Heaters are used in many critical processes in all plants.  It is extremely important that you and your team know how to properly test, install and maintain your electric heaters. Lack of attention in regard to these fundamentals will lead to costly heater shutdown and will cause an interruption in your process.

Let’s start with the basics. The startup instructions provided by the electric heating manufacturer are normally very limited.  No real explanation is provided, and specifics are normally lacking.

Example:
Instruction Example

Instructions like, “Before energizing the heater, be sure to check that the termination is tight”.  What does that mean?  How tight is too tight or not tight enough?  The number one maintenance issue we see at Valin comes from lose terminals. So how tight should they be?  No manufacturer will tell you in the instructions for some reason, but we will. 

Torque Driver and Megger
The required torque is 20-inch pounds, and a device like the torque driver on the right is used to properly tighten.

What about item number 4, “Megohm is within acceptable limits”?  What’s acceptable?  How can you test the limits?  In order to measure Megohm’s you need a Megger.  What is a megohm, and why would you need to check it? A “meg” test is the resistance of the inner resistance element to grounding on the inside of the heater sheath.  Heater elements are designed to have a live resistor inside of a metal sheath. That metal sheath touching our grounding out to the inner element tube will shut you down quick.  We are checking for resistance to ground. A meg test will predict or prove that failure.

There are several forms of Meggers. The image to the right shows a special type of Ohmeter designed to check the integrity of the insulation of the resistance wire in the tubular element. This completes the required test referenced above.

Ohmeters will come in values of 500, 1000, 2500 and 5000 meg ohms.  Nearly every heater and heat trace manufacturer will test their heaters up to 5000 VDC before leaving the factory. A low reading shows a high potential for failure. 

Normally we like to see 20 meg ohms or higher to fire a heater.  A perfectly insulated unit will show infinite resistance to ground.  Over time, resistive heaters will get progressively lower meg readings indicating a risk of potential for failure of all or part of the heater. This causes down time and process upsets that you don’t want to occur.

Using a Megger is the best and only way to ensure heater health and life.

Heat Trace Meg

Many companies make the fatal mistake of not running a megger on heat trace at any point during its use. Heat trace consists of two parallel bus bars with plastic molded around them. It will degrade over time.  Normal resistance tests will not catch what a megger would.

But many feet of heat trace are bad before they even get commissioned. You need to first check it with a megger on the roll before using it, after it has been cut and installed on the pipe (before it gets insulated over), after the insulators are done and at commissioning.  Any viable PM plan must include megging heat trace circuits annually at a minimum.

Meg Heat Trace
So how do you meg heat trace correctly? First disconnect all power to the heat trace if it is installed, also disconnect any thermostats, temperature sensors, anything attached to the heat trace. Connect the negative lead of the megger to the heat trace overbraid after making a “tail” with it. Connect the + lead of the megger to both of the heat trace bus wires. Begin at 500 vdc, then go to 1000 vdc and if your megger has it, use 2500 vdc.  Record the values and report.


When should you meg electric heat trace?

  • Meg in the box
  • Meg after cut and installed (pre-insulation)
  • Meg after insulation
  • Meg at Commissioning
  • As a PM punch list item annually

Heat trace actually begins to degrade very soon after it is first energized.  In hi temp applications where it is used year-round, it degrades much quicker.

Your maintenance team should test each heater circuit for insulation resistance to ground annually. 


Heat Trace Low Meg

What should you do?
  • Roll out cable and inspect for damage
  • Cut damaged cable off and re meg
  • Order a replacement roll and send it back

When your heat trace is not megging out, there are a few things you can try:

Look at the cable roll and see if there is physical damage to the cable.  If you spot any, roll it out and cut the cable off inside of the damaged area.  Take a note of how many feet were removed and proceed accordingly. If nothing works to improve the meg reading, FIRST order a new spool. Work with your Valin Representative to return the non-compliant spool.


Megging an Electric Process Heater

Megging Electric Process Heater
Megging an electric process heater is not quite as easy as heat trace or an electric motor.

A typical large electric process heater may have dozens or even HUNDREDS of leads.  It’s important to know where to use the megger, and how to use it.

Again, as with any megger reading, anything over 10 is “OK”.  We like to see over 20.  From the factory it is Infinite.  Hitting it with 500 vdc, you see 500 as the value.

That demonstrates perfect resistance to ground.  High level of integrity on the insulation value.

Most heating elements are tagged 1L1, 1L2 and 1L3, 2L1, 2L2…etc.

Schematic
A typical schematic is shown here. On a 3-phase heater, each circuit has three termination points on each circuit.  A circuit could have many elements on it.  In the example heater above, there are actually 36 elements on just one circuit!  For the whole heater, which has 12 circuits, the entire heater has 432 elements in it.

Your first meg each circuit to ground.  If the readings are “Good”, over 20 meg ohms, you can go on to the next set of elements. 

Keep in mind, 20 is very low.  It is possible after taking all of your readings that out of 12 circuits, one of them megs low.  Make a note of it and move through the rest of the circuits.

If all meg out perfectly, but one megs at 20, it may be wise to service that circuit.  That’s where Valin can help.

Meg the heater in the box before ever installing it.  Meg it after it is mechanically installed, prior to wiring it up.  Meg at commissioning and then meg it as often as your PM will allow, at least yearly.

How:
  • Disconnect all wires/t stats etc.
  • Connect (-neg) lead to ground
  • Connect (+pos) lead to circuit group
  • Set meter to 500 Vdc.  Test for 1 min
  • Record values
  • Repeat at 1000 and 2500 Vdc and record if available

Getting a Low Meg Reading on your Heater

Heaters that meg low have a high potential for failure.  If meg readings are continuously falling, you can plan on putting the unit back in service but make plans to have a replacement on its way.

What should you do? Contact Valin.  We have the ability to provide a bakeout in our oven at our Houston location.  We can also look at other solutions that may be causing the problem.

Having spares is one of the most commonly neglected practices for most facilities running electric process heaters. If spares are on hand, it’s not very common to see them stored correctly.

Valin can also provide this service for you.


No matter what the brand…

Brands do not matter.  All heaters use ANSI flanges and fit in the same mating flange.  If it’s custom, we can handle that also.

Many heaters can be repaired.  Our Team at Valin can evaluate your heater on site or in our Tomball Texas facility. We can also come to your site and check it out.

During the evaluation process, we can let you know the best route to take for repair or replacement.

Process Heater Spares Program

Local storage at our facility or on-site health checks available for any process heater, including heat trace.  
  • Store your spare units at one of our local facilities in a clean, dry, climate-controlled environment
  • Monthly health check reports on all spares
  • Deliver your spare to you within hours of your request
  • Help you set up a storage process at your facility, with our periodic on-site health check
  • Help you design a PM system to monitor your own heaters

Our facility has the correct equipment to service your electric process heaters.  We can accommodate heaters that need to be baked out at high temps, and we can maintain your heater’s health in our low temp ovens.

Monthly fees cover the storage of your heater, monthly health checks and reports, and a 24-hour response time to your dock when the heater is needed.

We can also perform this same service at your facility if you have suitable accommodations for storing heaters.  We can come out on a monthly basis and check them out for you.

We can also check your in-service heaters for you and provide data to help make decisions on when to replace installed units.

Taking a look at failed or underperforming heat trace circuits is also a specialty at Valin. If that line is not staying at temp, or fails frequently, let us take a look.

Contact us today at (866) 351-4328 if you would like to speak to one of our heating specialists.