Process plant winterization strategies

07/05/2006


As winter approaches, processing plants and engineering firms begin to turn their thoughts toward preparing for the extreme weather conditions they will soon be facing. Typically, when individuals working in the processing industries think of “weatherizing” their plants, they conjure up images of “heat tracing.” However, proper winterizing efforts require a broader, more thoughtful approach. Of the many options available, the use of electric process heaters, although not exceptionally well known or understood, is often the most practical and effective approach to winterizing.

Although heat trace and process heating are both used to achieve the same goal (protect equipment and processes from extreme temperatures), they are not comparable approaches. Heat trace is used to maintain heat within a pipe, valve body, or other piece of equipment. Process heating is used to maintain a specific temperature, as well as drive heated processes, prevent condensation, provide boost heating in flow lines and a variety of other applications. At a large, intricate industrial complex or processing plant, winterization is often treated as a low priority. However, improper heating outputs can have significant negative impacts on the operation of a facility, even leading to a plant shutdown. Heaters (or the lack thereof) are comparable to the smallest ring gear in a transmission. Although they may be cheap and easy to ignore, when they fail, the entire transmission fails.

Most processing industries have multiple applications where winterization should be made a significant priority %%MDASSML%% to avoid maintenance headaches or simply to improve processes. Here are a few examples of applications where process heating is recommended and can have a substantial positive impact on production, maintenance and the bottom line.

Process heating

Large processing plants, such as refineries, electric power plants and pulp and paper mills, primarily use either steam or fired heat sources (gas, coal or oil) for large bulk heating and processing requirements. However, there are many instances where electric heating should be employed to overcome the limitations of these systems (especially steam) during periods of exceptionally cold weather.

Two good options are electric steam superheaters and trim heating systems. In both cases, these systems consist of in-line (circulation) heaters coupled with temperature control systems. Steam superheaters prevent steam from reaching its saturation point within long, remote pipe runs by boosting the steam temperature well into the superheated region. This same inline concept is used for electric “trim” heaters. These heaters are used in conjunction with steam systems to provide steam at the process point with the correct temperature and pressure. An electric heater and control system respond much faster to load variations and can be controlled much more accurately.

At a minimum, when steam heat loads reach plant capacity, an electric heater system should be considered as an alternative economical solution, instead of expanding the facility’s boiler capacity or adding an entire new boiler system.

For example, calendar rolls are located at the ends of paper machines and are used to increase paper smoothness, improve final surface finish and act as a final thickness gauge. Fluctuations in temperature naturally cause the roll to expand or contract, and therefore greatly impact the quality of the finished product. To avoid fluctuations (which can be significant in winter months), many paper mills use either electrically heated hot oil heat transfer systems or forced ducted air systems using tubular or finned heating elements. These types of electrically heated systems, mated with the proper control systems, can achieve temperature uniformity within 1 degree C or better.

One additional benefit of forced air systems is that they don’t suffer the long lag times associated with steam systems, or even hot oil heat transfer systems. Also, if inclement weather causes plant temperatures to drop, forced air systems usually have a 10% to 20% reserve built into the heaters that should be able to handle the lower ambient temperatures %%MDASSML%% something steam systems have difficulty with when they are already operating at close to capacity.

Operating temperature maintenance

As the thermometer plummets, oils become increasingly viscous, making pumping difficult %%MDASSML%% especially fuel oils. In addition, as temperatures fall below 0 C, oils such as diesel #2 have the added problem of paraffin wax precipitating out of the oil solution, causing wax build-up and deposits, which can potentially clog fuel nozzles on boilers, engines and other equipment.



Fig. 1. Over-the-side heaters can be used to heat underground storage tanks.

 

One solution to this problem is an “O”-shaped over-the-side heater (Fig. 1). These electric heater assemblies employ robust, rigid tubular heater elements in an “O” shape attached to an unheated riser mounted to a flange plate. Either a local or a remote control box can be used with the heaters. The system uses low-wattage surface loading to warm the oil, but not to the point of element coking and/or oil degradation. Additional precautions include a high-limit temperature sensor attached to the heater element bundle, and a high-limit control circuit set to trip well below the fuel’s flash point. Overall, the use of electric heaters within tanks is a simple solution to ensure that fuel oil is ready to flow for main system use, or that back up fuel tanks, used by standby generator systems, are ready at a moment’s notice.



Fig. 2. Tubular heater elements are used in stock and custom engineered duct, immersion and circulation heaters.

 

Another place where cold can wreak havoc is on tank farms. Tank farms for crude oil typically employ some type of temperature maintenance system %%MDASSML%% either steam loops or some form of internal electric heating. Yet, there are some lesser-known electric heating designs that often offer substantial energy savings over these methods. For example, one economical alternative to steam loops is the use of pipe insert (or bayonet type) electric heaters. These heaters can be removed and serviced without having to drain a tank’s contents. It is recommended that additional tubular elements be installed in the heater design to function as spares should any of the elements fail (Fig. 2). This significantly reduces downtime by allowing maintenance to simply rewire to the spare tubular heaters, without having to remove or replace the heater bundle.

Pipe insert heaters are essentially composed of pipe vessels mounted through the sides of storage tank walls. Screw plug or flanged immersion heaters are then inserted into these bayonet-like wells. The wells may remain dry or may be filled with a heat transfer fluid to improve heat transfer to a tank’s contents. The exact design selected depends on the type of oil, desired temperatures, tank capacity and other variables. These heaters normally need to be AMSE code stamped and/or obtain a CRN to ensure proper operation in hazardous locations.

Suction heaters, although not well known, are another good alternative for heating storage tanks. Similar in design to pipe insert heaters, suction heaters heat only the liquid pumped, not the entire contents of the tank. This not only significantly reduces energy consumption (heat is applied to only the oil that is drawn out), it also reduces pumping system requirements, which translates to savings on capital equipment investments.

The setup of a suction heater system usually involves bolting a flanged immersion heater directly to the storage tank through a pipe section. As the liquid closest to the pump is heated, it easily flows and is drawn through the outlet nozzle attached to the pump suction side. The power densities of these heating systems are kept low (0.6 W/cm2to 1.5 W/cm2tubular heater surface loading) to ensure minimal oil degradation and heater element coking. It is highly recommended to install a valve on the intake side of the pipe section to facilitate removal and service of the heater bundle, as needed.

Hydraulic and lube oils used in process equipment and mobile heavy equipment are usually lighter weight and have low viscosity. A good heater system is the critical component of winterizing efforts and will ensure that equipment operates properly and efficiently, regardless of the weather. Hydraulic oil heaters, like lube oil heaters, are usually screw plug or flanged style immersion heaters, and are inserted into oil tanks and reservoirs to ensure the maintenance of the proper oil temperature. If hydraulic oil is allowed to reach very low temperatures (%%MDASSML%%20 C or lower), the result is often excessive leakage, sluggish performance and lower efficiencies. At most facilities, hydraulic oil reservoirs are normally maintained between 30 and 60 degrees C to account for the loss of heat within oil lines as they get further away from the reservoir and are increasingly exposed to low ambient air temperatures. On occasion, in line circulation type heaters can be installed along long hydraulic lines to accurately keep the oil within a specified temperature range.

One final benefit of heaters in lube oil systems is that they play a key role in oil/water separation. When oil is heated to around 50 to 60 degrees C, emulsified and free water begin to vaporize more readily. Oil/water separation also ensures entrained gases and other undesirable volatiles are vaporized as much as possible. Ridding lube oil of entrained gases means there is less opportunity for cavitation within bearings and other key equipment components.

Heat tracing alternatives

Heat trace begins to lose effectiveness as equipment mass increases and temperatures climb into the 100 to 150 degree C range. Alternatives to heat trace are fundamentally better suited to overcome problems with temperature maintenance and system heat loss. But they are initially more expensive. For example, a company that manufactures carbon pitch was recently seeking a way to insulate the mating flanges on its pitch pipelines. The pipelines, which operate at around 370 C, were insulated, but all attempts to insulate the mating flanges using traditional heat tracing had failed.

To solve this problem the company installed a flat tubular heater element that was curved to fit the flange circumference. Appropriate terminal boxes were also installed on the heater ends, which were designed to bend up at a 90-degree angle from the flange surface to allow for easy installation and wiring. The solution proved to be simple, relatively inexpensive and much more reliable and controllable than heat trace.

Heat tracing also has limited value in high temperature and/or high mass valve applications. Traditionally, heat trace installers coil or wrap a large amount of heat trace cable onto valves to ensure they get enough heat energy. Even when this approach is effective, it increases the maintenance times required to service or remove a valve.

A more effective and more maintenance-friendly approach is to use small diameter cable heaters (non-self limiting) with controller packages and internal temperature sensors. The heater, control and installation expense is higher than a solution that uses heat tracing. However, the investment usually proves to be worthwhile, as the heater package provides a much higher power density, higher temperature capabilities (to 760 C) and the ability to shape the cable to the contours of the valve.

Freeze protection

Conveyor systems, including chutes and hoppers, can also have problems when temperatures drop far below freezing. Specifically, moist or wet particulate materials, even when covered, have a good chance of bunching up on, or freezing to, conveyor belts or a conveyor’s chains and rails. Freezing can also cause belts and chains to snap and chutes and hoppers to become blocked or clogged. Because of moisture within the conveyed materials, as well as from Mother Nature, a robust, water-resistant heating solution is required. For such applications, the flat tubular heater has ends that are sealed against moisture and contamination ingress. Clamping bars are used to ensure good physical contact between the heaters and conveyor rails. Heaters are also often placed at points along, and in direct contact with, conveyor belts.

Within industrial cooling towers, which are used to shed excess heat from industrial processes and HVAC systems, the primary winterization concern is preventing a tower’s catch basin (sump) from freezing. A frozen catch basin can result in pump failure, frozen lines and, potentially, complete system failure.

For towers used within air conditioning, refrigeration and light industrial applications, an immersion heater solution is best. In these applications, the heater is immersed in the cold water sump to keep the water above freezing. Factory installed heaters are normally through-the-side immersion heaters with mounting flanges and gaskets. Add-on installers may also use this type, or, if clearances permit, may choose to use an over-the-side type to avoid penetration of the basin. An adjustable thermostat, which includes bulb and capillary, also can be installed to energize the heater element contactor, maintaining a basin water temperature of at least 4.4 C.

A float switch wired into the control circuit ensures that the heater cannot energize unless completely submerged in water. A circuit breaker is necessary to disconnect the main power supply in case of excessive current draw in case of a short to ground somewhere in the heater, the heater controls or the wiring.

A bypass circulation system is an alternative to immersion heaters. A bypass circulation system uses two bypass lines to create a circulation loop independent of both the main pump and the cooling tower water distribution system. One line bypasses the cooling tower riser, diverting flow directly into the cold-water basin. The other line is installed in a warm area and connects the tower suction to the tower return, thus bypassing the main pump-process system. The line includes a small auxiliary pump with a magnetic starter, a small instantaneous water heater with a bypass (with globe valve), a gate valve preceding the auxiliary pump and a check valve following it.

The remaining components consist of an immersion thermostat %%MDASSML%% preferably located in the basin or suction line near the sump, a thermostat and heating cable system to protect the makeup line and a globe valve at the discharge point of the interior bypass.

The bypass circulation method, like the immersion heater system described earlier, effectively protects the tower’s basin from freezing. However, the bypass method has the additional benefit of protecting the tower’s exposed piping.

Conclusion

The use of electric heating technology offers process industries, such as pulp and paper, oil and gas, refining, electric power, chemical processing and waste water treatment, an effective and economical solution to:

  • Prevent freezing

  • Improve processes normally affected by cold temperatures

  • Overcome the limitations of steam systems used with process heating applications.

    • Although every application and industry is different, the use of electric heaters for winterizing is usually the best solution to recurring maintenance problems and other headaches caused by the effects winter has on process facilities.

      For more information, go to www.watlow.com





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