By Gary Baumgardner, Chief Engineer, Pneumatic Div. North America, Parker Hannifin Corp.
Components operate at wide temperature ranges for trains, construction vehicles, agricultural equipment and implements, and semi-trucks.
Pneumatic valves work in a variety of applications to control the operation of actuators, including cylinders, clutches, rotary actuators, and air bags. However, using valves for applications in low-temperature environments—such as those found in transportation applications—can pose special challenges from both a manufacturer and OEM perspective. To understand why, we’ll look into what goes into a valve manufacturer’s thought process in designing a standard valve. This will help further the understanding of valve design for applications where a standard product isn’t “good enough.”
Standard pneumatic directional control valves
A basic valve is typically manufactured with a metal or plastic body containing a sliding spool or disk. Actuated by a solenoid, air pilot or manual operator, the spool shifts to provide air flow from one or several communication ports to others. Standard valves feature general design characteristics to meet the considerable variation in standard performance requirements that is common across factory automation and heavy industrial applications.
In great part, pneumatic valve manufacturers build valves with common features that meet or exceed customer expectations for performance and quality. In terms of performance, manufacturers consider size and flow expectations for the valve package size. They also take into account response time, cycle life, and leakage. Response time is literally how long the valve takes to shift position. Response times typically vary between 5 and 40 msec. Cycle life equates to how many cycles the valve can operate over its lifetime, with the number of cycles typically ranging from 20 to 40 million. This assumes the use of proper filtration, such as 5 to 40 µm, so the valve and actuator receive clean air. Previously, designers basically relied on manufacturers’ spool technologies as a general rule to determine performance and cycle life. Today, valves from the major suppliers operate for tens of millions of cycles in non-lube systems; however, a majority of applications typically don’t require a cycle life of 20 million cycles. In fact, from the OEM’s point of view, the life of the valve usually exceeds the manufacturer’s warranty.
For standard valves, basic temperature specifications are often considered to be in the range of –14° to 122° F, with pressures ranging from vacuum to 145 psi. Valve manufacturers choose from a combination of internal designs containing seals, spools, lubrication, solenoid components, and connections based on these temperature ranges. Once the basics are designed and tested, the manufacturer fine tunes design characteristics to limit the overall leakage factor of the valve. Unless the valve is specifically designed to be bubble tight—that is, with no measurable leakage of pressurized air—the valve will typically demonstrate some amount of leakage.
Valves have different spool designs, including spools that use O-rings for seal, spools over-molded with a rubber seal, poppers with over molded or crimped seals, and metal to metal spool and sleeve construction. Sealing surfaces and general construction can often be altered to ameliorate the leakage factor. Standard valve leakage can vary by manufacturer, but typically is under 10 cc per minute. Valves are tested for leakage during factory assembly at ambient temperatures. Some standard valves can be used in “critical” applications, such as suspension control on semi-trucks. These valves can also be equipped with a unique solenoid for operation in intrinsically “safe” applications.
Valves for transportation applications
The design of pneumatic values used in “extreme” transportation applications requires a different mindset for both the manufacturer and the design engineer. These valves might be engineered differently or they might need a different spool design, low temperature sealing materials, or a different type of lubrication. The two major considerations for these sorts of valves are pressure and temperature.
Temperature is important in applications on railcars because they experience ambient temperatures as low as –40° F. Standard valves contain lubricant that become less viscous as the temperature decreases. Therefore, they can become sluggish or can fail to properly shift position. As such, these valves require the use of a special lubricant.
Additionally, at lower temperatures, sealing materials become increasingly hard and brittle. Normal manufacturing imperfections affecting the surface finish on the sealing surfaces and O-rings have a greater effect and can cause valve leakage or premature wear. With a standard valve operating under typical pressures and temperatures, the hardness factor remains fairly constant. At lower temperatures, sealing materials get harder and harder—which means the valve can ultimately leak. Different seals have their own coefficients of expansion and physical properties that either let the seal operate or allow it to remain static and to maintain an acceptable level of sealing. Many valves use special compounds, such as, among others, low temperature nitriles or ethylene propylene rubber.
To understand why pressure is important, consider the case of a customer that tests a valve at –45° F and notes that it properly shifts at this temperature. It’s important for the customer to know the valve’s leakage at that temperature and its inlet pressure when it is shifted. Valves typically have a minimum operating pressure of about 40 psi to ensure proper operation in extreme environments. Most vehicles will have adequate compressed air reserves to overcome any leakage and friction in such a situation. But a vehicle might require an elevated inlet pressure, in some cases 80 or 90 psi, so leakage through the valve does not affect the valve reliability.
Another factor in extreme applications is a circumstance wherein the valve is used at different times for multiple functions—this can make leakage a main consideration. For example, a street sweeper with a large rotary brush that rotates as the vehicle travels must adjust the rotary brush up or down according to the terrain. When the vehicle is sweeping, the valve works continually. However, when the operator parks the sweeper for days, the air supply reservoir remains pressurized and the control valves remain static for long periods of time. It’s important that the valve doesn’t leak, because any leakage can potentially drain the system. Typically, pressure is requireed at startup for immediate operation.
Other examples include heavy-duty mass transit buses, which have kneeling modules that include pneumatic poppet valves to inflate and exhaust air bags attached to the vehicle axles. This causes the bus to “kneel” or lower so passengers can more easily get on or off. These pneumatic components encounter drastic temperature swings in northern and southern regions during seasonal changes in temperature. The pneumatic systems must be reliable, have repeatable response times, and have the least amount of leakage possible, so that, upon the vehicle’s initial start-up, the driver does not have to wait several minutes for the system to achieve operational pressure.
Designing an extreme valve
A valve manufacturer can take two basic approaches to design valves that can withstand extreme temperatures and pressures. When cost is a major consideration, the company must further develop something it has already created, perhaps even modifying a standard valve to withstand harsher environments. Extreme applications usually do not entail production volumes comparable to those associated with more common applications. Consequently, an engineer might need a valve that will operate on a one-off prototype on a farm implement or on construction or forestry equipment.
A better approach can often be achieved by starting from scratch. This eliminates the need for a compromise in terms of cost, design, or anything else. Of course, this approach might necessitate the use of special tooling and parts.
Drawing a line between a standard and a custom or extreme valve depends on customer specifications. Should an engineer need a valve that will work at –60° F, it might be possible to use a standard valve and simply modify it for use at that temperature. But what if he/she has space restrictions that preclude the use of a standard valve body? Each of the performance criteria will lead the manufacturer down a narrower path of useful technologies it can adapt, or whether the project must start from a clean sheet of paper.
Specifying a pneumatic valve for low temperatures is also not a simple or obvious process. Russian rail companies use a guideline that defines climate regions around the world and gives a statistical measure of how cold a region gets within so many years. For instance the guide might indicate that in far northern climates, temperatures can reach –70°F for one day out of every 15 years. The railcar OEM might then perform a risk analysis based on this information to determine the design specification, answering questions such as, “Does the valve have to work successfully at that temperature?” Alternatively, the question posed might be, “Does the valve just have to withstand that temperature because when it warms up, the valve will then work again?”
Overall, few OEMs—even rail companies—can correctly specify how a pneumatic valve should work at low temperatures. Instead, the designer generally relies on the valve manufacturer to know what valve will work in settings of –40°F or below. Additionally, consider that when a valve leaks at –40°, is it really a critical loss of air pressure? Or, is the main concern whether or not the valve continues to shift? Therefore, in general, a train manufacturer would likely specify an extreme pneumatic device to ensure an acceptable level of performance in all conditions. Additionally, OEMs should not jump to conclusions by simply using catalog specifications for general operating parameters and assume that the same specification holds true in extreme environments such as those found at –40° F. Jumping to such conclusions is a common mistake.
Parker Hannifin Corp.