Manage leakage in hydraulic-valve design
Appeared in Plant Engineering on 6/11/2016
Damage to the valve body resulting from leakage past the worn seal. Courtesy: Hunt Valve, Inc.Plant managers who oversee a manufacturing facility where hydraulic systems are used have most likely been told that leaks in those systems are inevitable. However, it is more accurate to say that design and maintenance are the main factors that drive valve leakage. Leakage robs hydraulic systems of efficiency, increases costs and exposes workers and the environment to potentially harmful conditions.
Explaining hydraulic-valve leaks
There are two types of leakage in hydraulic systems: internal and external. The drips and puddles on plant floors or equipment are evidence of external leaks. These can be hazardous to workers, the general public and the environment, depending on the flow media. Even if the flow media does not contain substances that present an exposure risk, there is an added slip-and-fall risk, which, according to OSHA, is among the top five causes of lost-time injuries in the workplace. Additionally, the loss of flow media in a closed-loop system has a negative impact on the system's efficiency. For instance, if a hydraulic press has a cycle time of one minute, but leakage is causing that cycle time to drop to two minutes, then the efficiency of that press has been cut in half. There can be other possible detrimental effects associated with operating all of the system components at suboptimal operating conditions, as well.
External leakage can result in lower operating pressure, which can lead to vibration and cause excessive stress on system components.
Internal leakage, on the other hand, is sometimes designed into a valve to provide lubrication to various valve components, such as seals, spools or pistons. Without that lubrication, friction would age those components prematurely. In addition, this form of intended leakage maintains the pressure within a system at a safe level.
This is the case with some types of pressure-reducing valves, which may have an orifice designed to vent pressure to the tank as part of its ability to control pressure on the reduced circuit.
Unintentional internal leakage, though, can be just as problematic as external leakage. Internal leakage through a valve can result in a system that operates inefficiently and impact the pressure, temperature, flow, velocity and other parameters. This type of leakage will most likely result in equipment inefficiencies, leading to inconsistencies in the processes that rely on the hydraulic circuit. The unintended variation in the process leads to poor quality, which means that scrap is being produced and dollars are being lost.
There is a common assumption that if a machine is running, it is running at optimal performance. Internal leakage is a hidden culprit that discredits that mindset.
External leakage due to erosion in the piping system. The inset shows the extent of the leak. Courtesy: Hunt Valve Inc.
How much is too much leakage?
How much leakage should be accepted as inevitable? That question will be answered differently by any entity that uses high-pressure hydraulics systems. An acceptable amount of external leakage in a forging operation will most likely be unacceptable in a food-processing plant. Internal leakage in valves in a hydraulic system used to create high-cost parts will be tolerated less than internal leakage in a system where a commodity item is being manufactured. Industry standards have been developed to help quantify the acceptable amount of leakage for different applications. ANSI, Fluid Controls Institute (FCI), American Petroleum Institute (API) and Manufacturers Standard Society (MSS) have all created leakage standards for different types of valves:
API Standard 527: Developed specifically for pressure-relief valves, API 527 specifies the methods for gauging seat tightness and defines acceptable leakage rates.
API Standard 598: Acceptable leakage in butterfly, ball, gate, globe and plug valves is established by API 598.
MSS Standard SP-61: A standard intended for establishing allowable leakage in valves that operate as isolation or check valves.
ANSI Standard 70-2: Specifically established for control valves, ANSI 70-2 sets different leakage classifications ranging from Class I, which allows an unspecified amount of leakage, to Class V, which essentially allows zero leakage.
These standards were developed with the intention of establishing design valve guidelines for valve manufacturers. The standards use a set of specific operating parameters, such as viscosity, pressure and temperature, when measuring leakage. These guidelines can be helpful in the valve-selection process, but do not offer any guarantees on how a particular valve will operate in service conditions—where the actual operating parameters can exceed those that are in the leakage-testing protocol of a particular standard. The valve will need to be tested under actual service conditions to verify that it is providing the desired leakage control.
Proper hydraulic-valve selection
It is vital to take the time to size and select the proper hydraulic valve for the process, to decrease the chance of a valve leaking. An oversized valve is one of the most common causes of valve leakage.
Flow velocities that are incompatible with an oversized valve will result in eventual leakage. It is important to recognize potential incompatibilities between the flow media and the materials used to construct the valve, especially the wet areas. Additionally, additives in the flow media can wreak havoc on valve components. For instance, glycol added to a water-based hydraulic system for fire resistance will degrade a polyurethane or polyester seal in short order, which will result in a leakage through the seal. Valve, seal and gasket manufacturers typically publish incompatibility charts for materials, which are essential tools for engineers selecting hydraulic valves.
Damage to the poppet from internal valve seat leakage. Courtesy: Hunt Valve Inc.
Proper hydraulic-valve positioning
Once the correct hydraulic valve is selected and installed into an operational system, leakage can be minimized by operating the valve within its design parameters. This is likely easier said than done, but it stands to reason that a spool valve that is designed to operate within a certain range of temperatures, pressures and flow velocities will not operate efficiently if it operates outside of that range. When inspecting a valve leak, it is important to remember that the valve may not be operating as intended.
The problem also may be that the system itself may be presenting conditions that are causing the valve to leak.
One of the conditions that will lead to leakage is contamination in the flow media. Dirt or unintended particles in the media can cause a valve to leak for a couple of reasons. First, those particles will cause premature wear as they travel at high speed through the circuit. Second, these particles can prohibit the mating surfaces of the valve shut-off point, allowing gaps where leakage can occur. Contamination is a condition that feeds on itself; if flow media is capable of escaping the system in the form of a leak, then contaminants can enter the system through that same point. Cylinder rods are prime offenders in carrying dirt into the system, as they extend and retract into unclean environments. Wiper seals on cylinder rods can help prevent this from happening. Contamination can be controlled by filtering the flow media and by consistently monitoring it for cleanliness.
Another cause of leakage is misalignment between the valve stem and the actuator stem. If the two stems are misaligned, there can be an uneven force applied to the valve shut-off surfaces, which can allow leakage to occur at that point. Any uneven force on the valve stem will also create issues with the packing surrounding the stem. The valve actuator also needs to be inspected regularly and tested to ensure that it is operating properly. If it is not capable of mating the shut-off surfaces with the required seating load, then leaks will occur.
Adhering to a regular preventive maintenance program can minimize the majority of leaks. But these programs are typically dictated by the organization's tolerance of leakage or the eventual failure of a valve. How organizations handle maintenance varies; however, more companies are implementing a total predictive maintenance program in comparison to reactive maintenance. Industry studies have shown that predictive maintenance can be more cost-effective than reactive maintenance.
Internal and external leakage in valves can be minimized if the organization makes an effort to not fall into the archaic mindset that leaks are inevitable. It all starts with sizing and selecting the right valve for the process.
Once the valve has been installed into the system, it needs to perform under a consistent set of operating parameters and serviced regularly to prevent leaks. Maintaining a disciplined approach in all of these areas will lead to valves that function in a safe and efficient manner, with minimal leaks.
- Mickey Heestand is vice president and senior mechanical engineer with Hunt Valve Inc. His 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.