Appeared in Valve World Magazine on November, 2016.
Valve selection is never a “one-size-fits-all” process. Any engineer who has had the responsibility of designing a hydraulic or pneumatic circuit knows that selecting the right components during the initial design process will alleviate a lot of headaches down the road. System safety, useful life and overall efficiency can be greatly improved by understanding the full range of the system’s operating parameters. This is certainly the case when selecting valves for high-pressure water systems.
There are essentially two types of hydraulic systems where high-pressure water is used—open-ended systems and closed-loop systems. Both systems typically operate at pressures between 1,000 and 5,000 psi. In an open-ended system, water leaves the circuit at high pressure to perform a process, such as in a waterjet, pressure washer or steel descaling operation. Conversely, in a closed-loop system, water or a similar low-viscosity fluid stays in the system so that hydraulic pressure is converted into mechanical power to move a cylinder, or to turn a turbine or gear motor, for instance.
As is the case with the selection of a valve for any flow media, moving water at high pressure through valves and other system components presents its own unique set of challenges. This article discusses several methods for establishing the operating parameters for high-pressure valves. It also explains how to use these parameters to select the best valve for a high-pressure water application.
Establishing System Design Parameters
Being given the task to design a safe, efficient and economical high-pressure water hydraulic system, the engineer needs to clearly establish what job the system is being asked to accomplish. If designing a closed-loop water hydraulic system, such as one used in a forge press, the required tonnage of the press will need to be established. From that, the parameters of system pressure, cycle frequency and operating temperature can be derived, which are of paramount importance when selecting the valves for the system.
Similarly, for open-ended systems, such as steel descaling using high-pressure water media, the same system parameters of pressure, cycle frequency and temperature need to be defined as well as the required flow rate, velocity and consistency of the flow media. Again, these factors are all of critical importance when selecting the appropriate valves for the system.
Another factor in valve selection is pressure drop, which is defined as the difference in pressure measured before and after the valve. Different types of valves will produce different pressure drops, as pressure drop is affected by the size of the orifice and the flow path through the valve.
As flow rate increases, so does the pressure drop associated with the valve. Conversely, a decrease in flow rate will result in a lower pressure drop. Valve manufacturers typically publish the pressure drops for their valves, which can be especially helpful to hydraulic system designers.
Unique Applications Drive Specific Considerations
Since this discussion focuses on water as the flow media for closed-loop and open-ended systems, the cleanliness of the water must also be considered as a relevant factor in proper valve selection. While this may not be an issue for closed-loop systems that are closed to the atmosphere, contaminants such as sand or scale particles from a descaling operation may enter an open-ended system. Small particles travelling at high speeds through a valve will eventually erode and prematurely wear valve components that come in contact with that water. While this may be unavoidable, valve component materials can be selected with a focus on protecting against premature wear.
Another potential problem when using water as a flow media is water hammer. This potentially destructive pressure wave is created by an instantaneous change in fluid velocity when a valve is suddenly opened or closed. If not accounted for, water hammer can destroy hydraulic system components due to pipes bursting, pipe supports being pulled from their anchor points or valves exploding. To mitigate the occurrence of water hammer, flow velocity within the piping should be maintained at 25–30 feet per second and the valve shift speed for low-viscosity media, such as water, should be controlled. There are also certain types of valves that are designed to limit water hammer. It is vital to consider the potential for water hammer while choosing the valves during the design process rather than waiting until after installation.
Proven Valve Designs
Once the operating parameters of the system are defined, the engineer can begin specifying valves that will perform different functions in the hydraulic circuit. Two of the most commonly used valves in high-pressure water hydraulics are poppet valves (Figure 1) and spool valves (Figure 2), and both types which can be used to perform a variety of tasks.
In a poppet valve, a cone- or ball-shaped plug is held in place on the valve seat by a bias spring or hydraulic piston when operating as normally closed. When the plug is permitted to move away from the valve seat, fluid flows through the valve. Poppet valves can controlled by manual operation, a solenoid, pneumatic/ hydraulic pressure or a combination of methods. See Figure 1 for a diagram of a poppet valve.
Spool valves contain a radial-ported spool to control hydraulic or pneumatic flow. Flow through the valve is determined by the position of the radial ports in relation to the spool seals. This can be accomplished by either shifting the position of the spool or shifting the position of the housing that retains the spool seals as shown in Figure 3.
The hollow bore spool valve can operate over extended periods of time with water or air that contains some degree of contamination. With this type of valve, flow is transmitted through the spool, which is a hollow tube that contains a series of radial ports spaced evenly around the circumference. To permit fluid or air flow from the inlet to outlet ports, the seal housing is shifted to a position where the radial ports for both the inlet and outlet flow chambers are centered between the spool seals, as shown in Figure 3. To block flow from the inlet to outlet ports, the seal housing is shifted to a position where only the inlet chamber flow slots are located between the two spool seals, permitting flow from the outlet to the exhaust, as shown in Figure 4. With the hollow bore spool configuration, the spool seals are only briefly subjected to direct contact with the water or air flow as the seals transition across the radial flow ports when switching between valve flow configurations, which is advantageous when working with flow media that may contain contaminants.
Common Valve Configurations
Both poppet and spool valves can be configured for a variety of high-pressure applications:
Isolation: When configured as a two-way isolation valve, the valve will have two ports, an inlet and an outlet. The purpose of the isolation valve is to either allow or shut off flow.
Directional Control: These valves are used to extend and retract hydraulic cylinders such as those typically used in a forging press.
Pump Bypass: In most high-pressure water hydraulic systems with centrifugal pumps, the pump needs to maintain a minimum flow for cooling. Pump bypass valves are ported to direct water back to the flume or reservoir to recirculate through the pump when there is no system demand, providing minimum flow so that that the pump does not overheat.
Check: Check valves prevent flow back into the circuit that is caused by multiple pump sources of pressure. Flow can only occur in one direction through a check valve.
Prefill: Used to reduce water hammer and damage to the upper header nozzles, prefill valves allow the filling of a system header with low pressure water from an external low-pressure source or with system pressure when used in conjunction with a pressure breakdown orifice.
Safety: Safety valves are typically poppet style and are installed in circuits to prevent system overpressure. When the valve detects a pressure greater than the set limit, flow is directed to a relief port.
Making an Informed Choice
The valve selection process must be approached both objectively and subjectively. Some system parameters are absolute, such as flow media, pressure ranges, flow rates and cycle frequency, and will have a significant impact on the type of valve that will be used. Decisions regarding valve type, valve material and maintenance requirements also need to be made. A high-priced valve may last twice as long as a less expensive model, but the associated downtime with replacing the less expensive version may be unacceptable.
Designing high-pressure water hydraulic systems presents unique challenges for engineers. Clearly defining the operating parameters of the system is imperative before selecting system components such as valves. With the numerous configurations that are available in both spool and poppet valves, the engineer has many options to choose from when designing a high-pressure water hydraulic circuit. Considering the required operating parameters of the system as well as the materials of construction, ease of maintenance and cost will help determine the best valve for the application.
About Hunt Valve
Hunt Valve brings nearly 100 years of fluid power engineering innovations and solutions to a wide range of industrial and military customers. It specializes in severe-duty valves and complementary engineered components and system solutions for applications that include primary metals (steel, aluminum), energy (nuclear, hydro, downstream oil & gas), process (chemical) and U.S. Navy nuclear-powered vessels, including all submarines and carriers in operation as well as the Virginia Class, Ford Class and soon-to-be-in-production Ohio Replacement.
About the Author
Mickey Heestand, Vice President and Senior Mechanical Engineer
Heestand’s leadership role at Hunt Valve includes oversight of the company’s welding and nondestructive test procedures, Welder Workmanship Training and Examination and contract engineering reports. He is also responsible for a portion of NDT Level II inspector training and testing. On June 23, 1998, Heestand was granted a U.S. inventor patent (US 5769123 A) for developing the cylinder actuated descale valve. This patent is owned by Hunt Valve. Prior to joining the company in 1986, Heestand worked at GM as a welder maintenance repair supervisor, gaining experience in hydraulics and control. He is an ASNT NDT Level III who is certified in several NDT methods (VT, PT and MT). He is also tested to meet HVC-specific requirements and certified by Hunt Valve as a NAVSEA Examiner. Heestand has a bachelor’s degree in mechanical engineering from Youngstown State University.