How does hydraulic oil contamination damage valves?

Damage to valves from hydraulic oil mostly through wear and abrasion on seats and spools. It also causes silting, which leads to the valve becoming sluggish or sticking, as well as chemical degradation, which expands or strengthens seals. Solid particles smudge metal surfaces, and they increase the clearances within, while water and air encourage erosion and corrosion as well as the resultant leakage pathways, and unstable motion can cause insufficient control, excessive heat, and ultimately, valve failure.

Hydraulic valves are precision parts. Spool-to-bore clearances on the directional control valve may be as small as several microns. Similarly, proportional valve or servo valve meters are machined to even more precise tolerances. When contaminants enter the area, even tiny amounts of them can cause significant harm. Knowing exactly how contaminants affect the valve—physically or chemically—is a great way to justify filtering and fluid-care procedures, which ensure that the valve is not damaged prematurely.

Why are valves so sensitive to contamination?

The function of a valve is to regulate the flow direction, flow rate, direction, and pressure in hydraulic fluid through the movement of an exact-fitting element such as a spool, a poppet, or ball within the mating bore or seat. This fit requires very tight clearances that limit leakage inside while providing smooth, low-friction movements.

Because these clearances are tiny, small particles that would be innocuous within a cylinder or pump can get caught between a spool's bore or jam the seat of a poppet or block a tiny pilot orifice. Valves, especially proportional and servo types, are often the most contamination-sensitive components in an entire hydraulic circuit.

Wear of abrasives from solid particles

The most obvious cause of damage is the three-body abrasion. The hard particles—usually silica dust, metal particles, or even debris from wear elsewhere in the system—get trapped between the spool and the bore wall. When the spool moves back and forth, the particles function as grinding compound that smudges both surfaces.

The wear can have the effect of compounding. As clearances increase as more fluid passes through the metering edges, causing the leakage inside. This leakage manifests as a slow actuator response and a decrease in the force of holding and heat generation when pressure energy is absorbed across the wider gap. As time passes it is possible for spool sticking to occur when wear debris deposits in a different way, or when the enlarged clearances allow the spool be cocked slightly off-axis in the bore.

The size of the particle is as important as the particle count. The contamination that is roughly the same as the gap between them tends to result in the greatest wear because particles that are greater than this gap are stuck and drag repeatedly across the surface, rather than just flowing through the fluid. This is the reason measurement of particle count in the size range, not only the total level of contamination is a crucial aspect of the standards for cleanliness of hydraulic systems like ISO 4406. ISO 4406.

Silent and slow valve response

Some particle damage is abrasive-scoring. Fine particles, typically within the range of 2-5 microns are able to accumulate in the narrow space between the bore and spool, in a process referred to as silting. Instead of cutting the metal, the particles encapsulate into the space between bore and spool which binds the spool together using a combination of mechanical welding as well as hydraulic locking.

Silting is one of the main causes of valves that operate slow, is stuck intermittently or does not change at all despite the correct pilot or solenoid signal. It is especially problematic with proportional and servo valves where fine metering control relies on a spool that is completely free of movement. A valve with a silty seal might pass a functional test but still fail under the precise control conditions that the application requires.

Wear corrosive at the areas of metering and orifices

When the velocity of fluid increases -- for instance, at meters, spoollands as well as small pilot orifices contamination can cause an entirely different type of damage, known as erosion wear. In this case, particles suspended in fluids moving fast collide with the surfaces with an angle slowly cutting sharp metering edges and expanding orifice diameters.

Since the performance of a valve is dependent on the exact shape of the edges that determine the flow, erosion can alter the valve's flow rate and response curve before leakage is severe. A proportional valve that has been erosion-prone metering edges could not provide the same flow when triggered by an incoming command signal which can cause unstable control that may be difficult to identify because the valve is still moving without restriction and does not show any obvious external signs.

Corrosion and water contamination

Water is among the most destructive non-solid pollutants that a hydraulic system could be carrying. It is introduced through damaged seals on cylinder rods breather caps, cylinder rod seals leaks in heat exchangers or even condensation in reservoirs which are subject to temperature cycles. Once it is in the reservoir, water can attack valves in a variety of ways.

The presence of water in the environment can lead to corrosion on ferrous valve components. The the products of corrosion then create contaminants that are abrasive and circulate throughout the system. Water can also alter the film that lubricates the hydraulic fluid which increases the contact between metal and the spool-bore junction in operation. When the temperature is cold or there is a high water content fluids, ice crystals or droplets of water emulsified can cause problems with clearances for fines similar fashion to silt.

Furthermore, water speeds up the decomposition of base oil, creating acidic byproducts which further corrode the valve's internals and can cause varnish formationwhich is a distinct but related pollution mechanism.

Sludge and varnish deposits

Degradation of oil, caused through oxidation, heat, and catalytic reactions with metallic surfaces, results in soft organic byproducts, referred to as varnish. Contrary to hard particulate contaminants, varnish is a squishy, insoluble layer that is formed on internal surfaces, such as bores and valve spools.

The buildup of varnish reduces clearances gradually instead of suddenly, creating symptoms that resemble silting. It can cause slow reaction, inconsistent sticking and a decrease in reliability. Because varnish develops within the fluid instead of having to be introduced via external means, it may occur even in systems with high filtration, if the fluid has gone beyond its lifespan of service or the system is running hotter than what the fluid is intended for.

The chemical attack can damage seals, as well as elastomers.

Valve seals and elastomeric parts are susceptible to chemical contamination within the fluid, such as water mixed with incompatible fluids, and the degraded byproducts of oil. The seal material and the contaminant can result in expanding, hardening, or embrittlement.

A swollen seal could be a binding device for a spool or a poppet, whereas a hardened or embrittled seal ceases to fit to the sealing surface and can cause leak pathways. Both failure modes add to the issues of mechanical contamination already addressed, as a leaky or stuck seal alters the flow behavior and pressure the valve was intended to regulate.

The effect of compounding on the system's performance

In their own way, these mechanisms are harmful. Together, they affect each other. Wearing abrasives creates metal particles which can cause particulate pollution. Water-driven corrosion creates more particles. Varnish reduces clearances, which in turn hold larger particles. A valve degraded by any one mechanism typically accelerates the others, which is why contamination-related valve failures often appear to worsen suddenly after a long period of seemingly normal operation -- the system has crossed a threshold where multiple damage mechanisms are reinforcing each other.

Preventing contamination-related valve damage

Since valves are on the most sensitive side of the spectrum of contamination, their protection begins at the beginning by ensuring that the filtration is sized correctly to the most sensitive part of the circuit, routine fluid testing to detect the ingress of water and oxidation prior to they occur, proper sealing and breather maintenance to prevent contaminants from entering from the start and adhere to the fluid change intervals based upon conditions rather than on calendar time as the sole factor. Regularly weighing particles in relation to an ISO cleanliness goal is the most reliable method to verify that filtration is keeping up with the source of contamination within the system.

What is the most common contamination-related cause of valve failure?

Wear and tear from solid particles that are trapped between the bore and the spool is usually the most popular method, as it directly expands the gap between the spool and bore, as well as causes internal leakage to increase over time.

Does a valve break down because of contamination but there is no obvious metal wear?

Yes. The buildup of varnish and silting could result in sticking, slow response or an erratic motion without causing any noticeable scoring as these mechanisms function by reducing clearances instead of cutting through metal.

What is the effect of water pollution? specifically impact valves for hydraulics?

Water can cause corrosion to internal valves, degrades fluid's lubricating layer and speeds up oxidation which results in acidic byproducts and varnish, which further impact seals and clearances of valves.

Why proportional and servo valves more susceptible to contamination than conventional valves with directional design?

Their more precise clearances and designed metering edges result in less contamination causes an equal impact on flow performance and response precision, as well as reproducibility.

Does contamination-related valve damage happen gradually or suddenly?

Both. Both wear and accumulation of varnish are generally gradual, however because the mechanisms are able to reinforce one another and are able to work together, systems can end abruptly after the clearance or deposit levels reach a threshold.