Markets and applications such as petrochemical, freeze protection, metal casting, pulp and paper, and transportation are just a sampling of the many areas that have valve heating needs. The markets requiring valves seem to be without boundaries. For example, an arctic mining operation might utilize flow valves hundreds of feet below the surface that must withstand up to 5,000 pounds of pressure per square inch while operating at a constant temperature. Or perhaps an instrumentation valve, which is a key component in an aerospace application, must handle extreme swings in temperature.
Naturally, these applications require quality heating devices to maintain temperature, thus reducing the chance of failure. The heat produced from the heaters may be necessary to reduce viscosity of the medium as it flows through the valve. If the heater fails it can cause serious damage, halt the process, or compromise safety. Additionally, agency requirements such as UL®, CSA, CE, or RoHS might be involved.
As varied as the industrial marketplace for valves is, so too are the many valve geometries being utilized. While valve shapes and sizes ─ along with application requirements and pricing ─ are unique, the basic functions of valves are quite similar. In general valves are a pass-through device regulating flow. A valve can be made of lightweight aluminum and incorporate a measurement device with an actuator for an oil pump line, or can be as simple as a polymer ball valve for an irrigation system. (Watlow’s wide variety of silicone rubber and flexible heaters with insulation and jackets use an easily removable design for hard-to-reach valves; foam-in-place technology enables heating of complex geometric shapes.)
Ideally, the application’s heating needs should be understood in the early stages of system design. Too often, system heat is an afterthought and the design engineer or field technician has to scramble for a solution. Fortunately, there are many heater types to choose from when it comes to valve heating.
Heaters can be wrapped around the valve. They can also be integrated as part of the assembly at the time of the initial design. Choosing the appropriate heater for the valve is as important as choosing the appropriate valve for the application. Such a task means understanding the heater offerings in the marketplace. Several heating options warrant a more in-depth review before selection.
A Closer Look at Heating Options
If placing heat close to the medium is important, a cast-in heater is an excellent option. Depending on the size of the valve, heaters such as FIREROD® cartridge, cable or WATROD tubular heating elements can be used by placing them in direct contact with the valve body or used in open air near the valve. In smaller geometries, when space is critical, cable or cartridge heaters are excellent options. If casting the heater as part of the valve is not an option, drilling holes and utilizing a cartridge insertion heater is another good option.
Opportunities exist in the aftermarket for heating valves. Creating a heated enclosure or “hotbox” around the valve and making use of a tubular heating element, silicone rubber heater, or small, finned strip heater are excellent supplemental heater options. The heater enclosure helps contain the heat while protecting the electrical connections from the elements of weather. These heaters are also good choices when heating manifold valve assemblies. In particular, some applications using manifold valves might require easy-to-install, blanket-type heaters in direct contact with the part. Blanket heaters can be designed with holes and notches to accommodate the obstructions. These heaters offer the operator easy access to handles or instrumentation without significant disassembly.
Some valve heating applications require good controllability as a result of temperature limitations of the medium. In addition, temperature-sensitive parts such as O-rings must not exceed melting temperatures. Incorporating a sensor (such as a thermocouple or RTD) as part of the heater solution can save headaches down the road. These sensors work in concert with the control system and keep the heater from over-temping, thereby preventing the system from overheating.
If the heater is designed on a new OEM application or becomes an aftermarket requirement, preformed silicone rubber heaters with ¼-in. insulation that act as portable ovens around valves can be used. This type of heater works well on snow-making equipment. The machine produces snow continuously as long as the water lines and valves are protected from freezing. At a recommended maximum watt density of 5 W per square inch, these heaters can safely reach 300°F (149°C). In some cases they can contain integral bimetal thermostats to maintain temperature. These heaters also have an optional removable blanket with snaps for quick assembly. The blanket holds in heat while holding the heater in place.
When higher watt densities are needed for higher temperatures or faster heat-up requirements, the cable heater is an excellent choice. Watlow’s cable heaters, for example, can handle 30 W per square inch and easily reach 300-500°F (149-260°C) in just minutes. They can be shipped straight and formed in the field or factory preformed according to application requirements.
Safety -- Always a Concern
Obviously safety is a top concern in any system when heat is needed. For example, a refinery system failed because of the slow action of an emergency cutoff valve, compromising human safety and costing the company thousands of dollars. An emergency shutdown procedure occurred because of abnormally high system pressure; delayed operation of the pressure relief valve due to a viscosity rise of the liquid was a contributing cause of the failure. Due to the high pressure, a leak occurred and ignited. The slow action of an automatic switching valve resulted from low temperatures and high humidity. The instrumentation required heat in and around the valve to function properly; dehumidification typically prevents problems with the condensed water.
A viable Watlow solution would have utilized a molded silicone rubber heater with an integral sensor. This accident could have been prevented if heat had been applied to the cutoff valve, keeping the moisture from freezing around the valve assembly.
Multiple valve designs and heating solutions lead to versatility in the marketplace. The heat required in an application can get overlooked at times. It is up to the system designer to identify early on when the heat generated by the process is simply not enough and then determine the best heater solution for the particular application.
