Appeared in Flow Control on October 10, 2016.
Experienced plant and maintenance engineers know that designing a high-pressure water hydraulic system is no easy task. During the design and planning phase, it is vital to take the time to specify the system parameters and analyze the additives flowing through the hydraulic circuit. Then, the engineer can begin the high-pressure water valve selection process, considering common valve configurations and popular materials of construction. To optimize a military or industrial application, steps should also be taken to minimize leakage and implement a preventive valve maintenance program.
Planning ahead for valve selection, leakage and maintenance will help ensure the long-term safety and reliable performance of the high-pressure hydraulic system.
Part 1 of this two-part article series will focus on high-pressure water valve selection and the characteristics of industrial valve materials and components.
Select the ideal high-pressure water valve
Moving water at high pressure through valves and other system components presents several unique challenges. Understanding the full range of the system’s operating parameters will help the engineer select the best high-pressure water valve for the hydraulic system and overcome these challenges.
Establish system parameters
Here’s the best process for defining design parameters:
- Define system function — Clearly establish what job the system will need to accomplish and define associated parameters.
- Closed-loop water hydraulic system — The required force, flow rate and total volume required will need to be established. Then, define system pressure, cycle frequency and operating temperature.
- Open-ended water hydraulic systems — Define pressure, cycle frequency and temperature. Also determine the required flow rate, velocity and consistency of the flow media.
- Consider pressure drop — Pressure drop is the difference in pressure measured before and after the valve. Different types of valves will produce different pressure drops since 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.
Pick the appropriate valve design & configuration
Once the operating parameters are defined, the engineer should specify the high-pressure water valves needed to meet the operation and actuation requirements of the application. The following are the most common high-pressure water valve designs for hydraulic applications:
- 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. This industrial valve offers a higher flow rate than spool-style valves. Its internal seals experience less wear, extending its operational life.
- Spool valve — This industrial valve contains a piston or spool that extends and retracts in the valve housing. As the spool travels, it opens or closes ports in the valve body. Flow through the valve is determined by the position of the spool. Spool valves are not affected by pressure, requiring less force for actuation. They are balanced and can be used for vacuum operation.
- Hollow bore spool valve — With this type of industrial valve, the spool is hollow and contains a series of slots spaced evenly around the circumference of the spool. This valve can operate over extended periods of time with water that contains some degree of contamination from sand or scale.
Poppet and spool valves can be configured for many 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 allow or shut off flow.
- Directional control valves — 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, the pump needs to run continually. 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 so the pump does not overheat.
- Check — Check valves prevent flow back into the circuit 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.
Remember, the industrial valve selection process should begin during the system design phase. In addition to choosing the best valve design and configuration, the engineer also needs to consider the available valve materials.
Choose the proper materials of construction
Materials of construction have long-term impacts on the performance of the industrial valve and the overall hydraulic system. When choosing high-pressure water valve materials, the following factors should be considered:
- Material incompatibilities — Understanding material incompatibilities is especially important for those parts of the industrial valve that will come into direct contact with the flow media (i.e., the “wetted” areas). Additives may cause a negative interaction with the valve materials. For example, a water-glycol mix will degrade a polyurethane or polyester seal quickly. To identify potential material incompatibilities, consult the incompatibility charts published by industrial valve, seal and gasket manufacturers.
- Flow media cleanliness — For open-ended systems typically used in descaling operations in steel mills, the process water recycled through the system contains particles that can have harmful effects on the wetted areas of the valves. These particles will impinge upon those surfaces and cause
- Temperature — The temperature of the flow media as well as the temperature of the operating environment must be considered. Corrosion will occur faster at higher temperatures. Additionally, extremes in temperature can cause different materials to expand or contract at different rates. These changes may cause leakage issues, undue binding stress on valve components or seizure.
Common materials for valve bodies, seats & spools
When selecting industrial valve materials, consider the corrosion level, temperature and pressure of the application. Also, certain materials are less susceptible to wear. Some characteristics of popular materials for valve bodies, seats and spools are:
- Ductile iron — Low in cost, readily available, easily absorbs shock, poor corrosion resistance
- Brass and bronze — Low in cost, readily available, improved corrosion resistance over ductile iron
- 316 stainless steel — Excellent corrosion resistance, high tensile strength at elevated temperatures, costlier than cast iron, brass and bronze valves
- Monel (nickel-copper alloy) — Superior corrosion resistance, ideal for cladding of valve trim parts
- Inconel (alloy of nickel, chromium and iron) — Ideal for handling corrosive media, maintains strength at high temperatures
Popular materials for valve seals, gaskets & packing
Consider the following characteristics of popular materials of construction for various valve components:
- Polyurethane — Highly durable, designed for temperatures up to 200°F, ideal for pressures up to 6,000 psi, excellent abrasion resistance
- Viton — Excellent chemical resistance, works well at high temperatures
- Polytetrafluoroethylene (PTFE) — Extremely low friction, designed for a wide range of pressures and temperatures
- Glass-filled PTFE — Improved creep resistance, improved wear resistance and heat transfer
- Polyetheretherketone — Ideal for steam applications, high temperature rating, good corrosion resistance
After selecting the optimal valve and materials of construction, steps should be taken to minimize leakage and ensure the reliable performance of the high-pressure hydraulic system. Stay tuned for Part 2 in this article series, which will discuss best practices for managing industrial valve leakage and preventive maintenance.
Mickey Heestand oversees Hunt Valve’s welding and nondestructive test procedures, Welder Workmanship Training and Examination and contract engineering reports. He is an ASNT NDT Level III certified in several NDT methods (VT, PT and MT). Hunt Valve brings nearly 100 years of fluid power engineering innovations and solutions to a wide range of industrial and military customers. Learn more at huntvalve.com.