Before Winter Strikes, Check the Heat Trace
With the cold months on the precipice, November is a great time to review the actions that plant managers and operators should take to better ensure a winter free from incident. Whether the industry in question is power generation, petrochemical, or chemical processing, winterizing a plant before temperatures freeze is a critical practice. The pitfalls that one needs to be on the look- out for in the scorching heat of July is not the same as those which present themselves in the middle of January.
If left unaddressed, certain elements caused by the cold have the possibility of resulting in catastrophic failures inside the system leading to unexpected shutdowns, significant downtime, and loss throughput. One element of a plant that cannot go overlooked heading into winter is its heat trace. Between the condensate return, exhaust line, chemical feed lines, instrumentation, and freeze protection for cold water pipes, heat trace is a critical element for plants in cold weather environments. Ensuring that the technology is operational during this time of the year is essential.
Checking the Alarms
Heat trace is not traditionally used during the summer months. This means that heading into winter, there is significant risk that something may not be working properly when the heat trace needs to kick back on. Adding to the surprise and misery, there may not be any signs of a problem until the heat trace is re-energized.
Most heat trace systems, if properly configured, should have some type of alarm mode, alerting operators if there is a problem. These alarms typically recognize common problems such as circuit interruptions or whether the temperature of a process is slipping outside of a predetermined range. Although many heat trace control panels are programmed with an alarm setting if something is amiss, often operators and managers will not realize that the alarm has been triggered. Commonly, the alarms on the panel are not integrated within the entire system and the panel itself is hidden away in a less frequented area of the plant. This makes it even more likely that alarms may be “sounding” without anyone’s knowledge. Ideally, these alarms should be integrated with the system, but the reality is that many of these systems in the plants are old and do not have SCADA integration.
The Amp Draw Test
It is not enough to simply energize the heat trace and see that there is no alarm. No alarming on the panel is certainly desired and ultimately a very good thing, but it does not necessarily mean the heat trace is working. The only way to properly check is to energize the heat trace and monitor a controlled amp draw from the fully energized system. Then, the results can be compared to what the reading should be to see if the system is operational. Remember, the alarms alone are not going to indicate whether the heat trace is operational, so an amp draw is required.
The equation for determining amps is watts/volts, and heat trace cables are classified by watts per foot. Thus, if an operator knows how many feet of linear heat trace cable is in play and how many volts are being applied, he or she can determine the number of amps that should be expected. For example, if a plant is utilizing 100 linear feet of 10-watt heat trace cable, this is equivalent to 1,000 watts (10 watts x 100 feet). If the operator applies 120 volts, a draw test should yield 8.3 amps. The equation also dictates that whoever is conducting the test must know how long the runs are. If, after running the test, the amp draw shows a number that is lower, for example three or four amps, there’s a problem. Although the circuit is complete, and the heat trace is “working,” it will not achieve the desired thermal output, thus the pipe could still freeze. This means that the plant is not ready for the coming winter conditions.
There is an additional consideration to the above equation. Self-regulating heat trace provides a given output at a given temperature. Say the trace is rated at 10 watts/foot at 50 F. Therefore, prior to running the calculation above, you must consult the product data sheet and see what the expected wattage output is per linear foot. For example, 10-watt cable at 100 F will only output eight watts, thus to continue with the above example, the proper amp draw would be 100*8/120 = 6.6 amps.
Heat trace has a significant inrush. A general rule of thumb is 2X, so on a 6.6-amp system, the operator can expect just over 13 amps of initial current, which will then drop down to the expected value of 6.6 after a few minutes. It is also important to be aware of the lighted end seal. Too often, plant managers are satisfied with placing a lighted end seal at the end of the run to test the circuit. If they see the light is on, they will assume the heat trace must be working due to the presence of voltage at the end of the run. This may be true, but if the proper amp draw is not being achieved, the temperature of the heat trace will not reach the desired level and will completely lose its effectiveness.
A properly functioning heat trace application is one of the most critical elements to consistent, high volume output of a plant during the cold winter months. Checking and maintaining a plant’s heat trace before winter begins through an amp draw test is the best way to reduce the risk of failure during these inopportune times.
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