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 a petrochemical or even an aerospace application, must handle extreme swings in temperature.
Naturally, these applications require heating devices to maintain temperature, thus reducing the chance of failure. The heat produced from the heaters may be necessary to reduce the 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 necessary.
As varied as the industrial marketplace needing 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 pass-through devices regulating flow. A valve can be made of lightweight aluminum and incorporate a measurement device with an actuator for an oil pump line, or be as simple as a polymer ball valve for an irrigation system.
It is best to understand the application’s heating needs in the early stages of the system design. Too often the system heat is an afterthought and the design engineer or field technician must scramble for a solution. Fortunately, there are many heater types to choose from when it comes to valve heating.
For example, heaters can be applied by wrapping them around the valve. They also can 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 marketplace with respect to heater offerings. There are several heating options that warrant a more in-depth review before selection.
If placing heat close to the medium is important, a cast-in heater is a good option. Depending on the size of the valve, heating element designs such as cartridge, cable or tubular heaters can be utilized by placing them in direct contact with the valve body or used in open air near the valve. In smaller geometries where space is critical, cable or cartridge heaters are good options. If casting the heater as part of the valve is not possible, drilling holes and utilizing a cartridge insertion heater is another good option.
If the valve is in place, the heating solution may need to be retrofit. Creating a heated enclosure or “hotbox” around the valve and utilizing a tubular heating element, silicone rubber heater or small, finned strip heater are good supplemental heater options. The heater enclosure helps contain the heat while protecting the electrical connections from weather. These heaters also are good choices when heating manifold valve assemblies. In particular, some applications utilizing manifold valves might require blanket-type heaters in direct contact with the part. Blanket heaters can be designed with holes and notches to accommodate the obstructions. These heaters ease operator access to handles or instrumentation without requiring 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,” and therefore prevent the system from overheating.
If the heater is designed on a new OEM application or becomes an aftermarket requirement, pre-formed silicone rubber heaters with 0.25" insulation that act as portable ovens around valves can be used. At a recommended maximum watt density of 5 W/in2, these heaters can safely reach 300°F (149°C) and, in some cases, contain integral bimetal thermostats to maintain temperature. Some heaters include 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 heatup requirements, the cable heater may be suitable. Cable heaters can handle 30 W/in2 and easily reach 300 to 500°F (149 to 260°C) in minutes. They can be shipped straight and formed in the field or factory pre-formed according to application requirements.
The range of heating solutions possible means there are options for effectively heating the many varied valve designs. In a perfect world, the system designer would know early on whether the heat generated by the process is adequate to ensure proper valve operation. In those cases, the designer can then simply determine the best heater solution for the particular application. In those situations where the heating requirements are overlooked during the design process, however, all is not lost. Solutions such as blanket heaters can be retrofit to the valve to ensure optimum operation.
Case In Point: Cut-Off Valve
Obviously, safety is a top concern when heat is needed in any system. Consider this real-life example, where a refinery system failed because of the slow action of an emergency cut-off valve. The valve’s slow response compromised human safety and cost the company thousands of dollars.
At one refinery, an emergency shutdown procedure took place as a result of abnormally high system pressure. The operation of the pressure-relief valve was delayed due to a viscosity rise of the liquid, and this was a contributing cause of the failure. Due to the high pressure taking place, a leak occurred and ignited.
The slow action of an automatic switching valve was the result of low temperatures and high humidity. The instrumentation required heat in and around the valve to function properly, and the dehumidification typically prevents problems with the condensed water. A viable solution would be utilizing a molded silicone rubber heater with an integral sensor. This accident could have been prevented if heat was applied to the cut-off valve, which would have kept the moisture from freezing around the valve assembly.