Temperature Effects on Actuated Valves

The performance and service life of actuated valves are affected by temperature in various ways. There are several factors to consider, and this article will discuss these factors and common failure modes for both valves and actuators. First, however, a primer on heat transfer principles and terminology.

Convection vs Conduction vs Radiation

Heat transfer occurs via three basic phenomena. Thermal convection is heat transfer by the movement of a fluid (ie a gas or liquid). Factors that affect convection are temperature differential, surface area and flow velocity. Heat transfer is proportional to temperature differential, so the greater the differential the greater the heat transfer. Similarly, heat transfer is also proportional to contact surface area. The third factor, flow velocity, brings us to two types of convection: natural and forced. Natural convection is heat transfer that is caused by buoyancy forces. Mist rising from a lake into cold, still air is an example of natural convection. Forced convection occurs when the fluid is acted upon by an external force like a fan or pump. Hot fluid flowing through and heating up a pipe is an example of forced convection. The heat transfer rate for forced convection is 2- 5x greater than natural convection.

Thermal conduction refers to the transfer of thermal energy (ie heat) between two solids in contact with each other. A pot on a hot stove is a typical example of conduction. Conduction depends on the temperature differential between the bodies, the area in contact and the thermal conductivity of the material. Some materials such as aluminum are excellent conductors while others like ceramic are poor conductors and function instead as insulators. This difference is accounted for by each material's thermal conductivity constant. The thermal conductivity constant differs markedly. For example, brass is about 7x more conductive than stainless steel, and stainless steel in turn is ~70x more conductive than PVC.

Radiation is heat transfer via electromagnetic radiation, usually in the form of infrared waves. Heat transfer of this type rarely occurs in valve applications and the subsequent discussion will be limited to convection and conduction.

Heat Transfer vs Heat Energy

Finally, we need to make a distinction between heat transfer and heat energy. Heat transfer occurs per the phenomena above. Hot media that starts flowing through a cold valve will have a higher rate of heat transfer than that same media flowing through a warm valve. It will take time for the cold valve to heat up, so the valve will not reach its temperature limit until after a longer period of time. The closer the valve temperature is to the media temperature, the slower the heat transfer, but the heat energy in the valve is greater than when it was cold. Therefore it is possible to flow media at a higher temperature than the valve's upper temperature limit, but only if the valve does not absorb enough heat energy over time that it raises its temperature beyond the upper limit.

So time at temperature is a factor. Another factor affecting valve temperature is the valve size and thickness. A smaller valve will heat up quicker than a larger valve because the larger valve has more material to absorb heat energy. The amount of material available to absorb heat is referred to as an object's thermal mass.

Similarly, the length of the thermal path also affects temperature. Consider the simple case of two pipes of different lengths with a blow torch heating them at one end. The opposite end of the shorter pipe will heat up faster than the longer pipe because it takes more time for the heat energy to conduct through the longer pipe.

Temperature Effects on Valves

Valve temperature is largely a function of convection. Consider the case of hot media flowing through a valve in a cold room. Heat transfer occurs not only from the media to the valve, but also from the valve to ambient surroundings. Media flowing through a valve is a type of forced convection, and as we learned earlier forced convection increases heat transfer. Therefore, higher flow rates will heat the valve quicker. Heat transfer to the surroundings occurs via natural convection and depends on ambient conditions- ie a valve in a warm room will reach its temperature limit sooner than a valve in a cold room. (Of course, ambient heat transfer can be forced convection as well by a fan blowing over the valve.)

Valves are composed of a shell, a metering device such as a ball or disc, and a stem connected to the metering device to rotate it. Sealing between these elements is accomplished by elastomeric seats and gaskets. For metal valves such as brass or stainless, the seals are generally the temperature-limiting component. Common seal material like PTFE (Teflon) generally begin to soften and lose their sealing strength around 100°F. Complete failure occurs around 400°F. In between these temperatures the seals can hold pressure, but below the shell rating. The de-rated pressure in this region is often plotted vs temperature and is referred to as a pressure-temperature, or P-T curve.

At the other extreme, cold temperatures cause the seals to become stiff and brittle, thus compromising their sealing ability. This lower limit is usually around 0°F for typical seal materials, but specially designed seals can withstand cryogenic temperatures (<-320°F).

Temperature Effects on Actuators

Actuator temperature is a function of both convection and conduction. For purposes of discussion, let's consider a case where the valve has reached its upper temperature limit and the temperature is stable. How is the actuator affected?

Actuated valves are valves fastened to a motor (usually pneumatic or electric) that rotates the ball or disc. The motor is referred to as an actuator.

Heat transfer occurs by thermal energy flowing through the valve stem and mounting pad into the actuator. Since the faying surfaces of the valve and actuator are in contact with each other, heat transfer occurs via conduction. As we learned previously, factors affecting conductive heat transfer are temperature differential, surface area and material type.

An actuator connected to a brass valve will heat up much faster than if it were connected to a stainless valve because brass is a better conductor of heat. (PVC, despite its lower conductivity, is usually not considered for elevated temperature applications because of its overall temperature limitations.)

Similarly, an aluminum actuator will heat up faster than a stainless steel or polyamide actuator.

Heat transfer into an actuator can be reduced by lengthening the thermal path and reducing the contact area. These are the principles behind high temperature brackets, which are installed as a spacer between the valve and actuator. In addition, installing thermal insulators between the actuator and valve such as insulating washers further reduce heat transfer.

Like valves, convection also plays a role in actuator temperature in that heat transfer takes place between the actuator and ambient surroundings. Considering our case of a valve at its upper temperature limit, actuator temperature is a function of heat flowing in through the valve via conduction minus heat removed via convection to ambient surroundings. It is possible that the media temperature could greatly exceed the upper temperature limit of the actuator, but, with a high temperature bracket limiting heat input and cold surroundings serving as a heat sink, the actuator remains at an acceptable temperature.

The failure modes due to temperature depend on actuator type. High temperatures can damage electrical components, especially the performance of resistors, the dielectrics in capacitors, and the integrity of solder joints. Temperature extremes also affect sealing material in the same way as valves, ie at higher temperatures sealing strength is compromised and at low temperatures seals become brittle and can crack. Failed seals could compromise weatherproofing, allowing moisture into the actuator and, for electric actuators, corroding electrical components. For pneumatic actuators, damaged seals could reduce the ability to hold air pressure, thus reducing torque or even rendering the actuator inoperable.

Saturated Steam

Since steam is a common (and often misunderstood) media, it warrants additional discussion. Saturated steam is the temperature and pressure at which water can exist either as a liquid or vapor. At one atmosphere (14.7psi), the temperature of saturated steam is the familiar 212°F (100°C). Steam that is heated above the saturation point is referred to as superheated steam.

Saturated steam has approximately six times the energy of boiling water. (The difference is the energy it takes to change phase from liquid to vapor and is called the latent heat of vaporization.) As such saturated steam is an excellent carrier of energy and has a high heat transfer coefficient which is why saturated steam is often used in heat exchangers. In addition, the temperature of saturated steam can be controlled via pressure:

Saturated steam at 10psig has a temperature of 239°F; at 25psig the sat steam temperature increases to 267°F, and so on. The temperature-pressure relationship for saturated steam is well-characterized and can be found in steam tables. Superheated steam is not as commonly used. It has a low heat transfer coefficient, and the temperature can vary even at constant pressure. Superheated steam is used in steam turbines, where the steam undergoes both a pressure and temperature drop across the turbine. However, since it is still superheated it does not condense which greatly reduces the chance of corrosion.

We make mention of the pressure-temperature relationship of saturated steam because the valve's temperature limit is often exceeded before the pressure limit, and saturated steam pressure increases nonlinearly with pressure. For instance, a stainless steel valve may be rated for 1000psi and have a temperature limit of 400°F. Saturated steam reaches 400°F at around 230psig, well below the valve's room temperature pressure limit.

The actuator temperature limit is typically lower than the valve's, especially for electric actuators. Most electric actuators have an upper temperature limit of ~130°F (vs ~400°F for the valve). This would preclude using electric actuators for steam applications entirely, given the saturated steam temperature starts at 212°F at atmospheric pressure.

For this reason, high temperature brackets must be used for steam applications. The question becomes what is the allowable maximum saturated steam temperature so as not to exceed the actuator temperature limit. As discussed earlier, actuator temperature is a complex heat transfer phenomenon and cannot be derived theoretically via a closed-form solution; the maximum allowable steam temperature is a function of the specific valve and actuator combination.

Therefore, manufacturers typically either measure actuator temperature empirically or estimate the steam temperature/actuator temperature relationship for each actuated assembly combination.

Since the saturated steam P-T relationship is well characterized, knowing the maximum steam temperature allows us to specify a maximum steam pressure. Valve manufacturers refer to this as the maximum saturated steam pressure, and list this separately from the valve pressure limit.

Conclusions

Valves and actuators have different failure modes at different temperatures. Being aware of these limitations, failure modes and effects- as well as avoiding them- will help to ensure proper operation and maximize service life.