Silt Lock in Hydraulic Systems Essay

Silt Lock in Hydraulic Systems
Many of today's hydraulic systems employ valves of the electro-hydraulic variety. These valves allow for the rapid conversion of electrical energy to mechanical energy. However, they are also of a far more intricate construction than their older control valve counterparts. As such, they tend to be more vulnerable to damage due to solids contamination.
Particle Size
The size of particles that enter a system specifically, the clearance between the bore and spool in the leakage path can contribute significantly to silt-induced failures in motion. For example, silt particles below 10 microns in size can easily enter these bore-spool clearances. Should a valve be actuated and the static friction of a silt-contaminated spool increase, the result can be a hard-over or slip-stick condition. But particles are not the sole cause of these types of failures.
Additional Risk Factors
Often it's a combination of issues which contribute to silt lock in hydraulic systems. Where small particles can pass through the clearance without interference, large particles that cannot enter are moved to the side. Although this may initially indicate less of an issue, the fact is that the smaller particles are what pose the largest risk. This is because they can exhibit a higher degree of static friction than particles of a larger size.
Valves at Rest
Whilst many of the valves in a hydraulic system move continuously, others don't move until and unless they are required to do so. These stationary valves can expose the fluid leakage path to increasing particle amounts. If a valve is at rest for long enough, particle build-up can be enough to impede the movement of the valve, if not completely prevent it.
Resting Position
Should silt lock be a problem in a valve, the cause can be the resting position of the spool in the bore. Spool weight can cause varied annual clearance. This variance can be as large as submicron to 50 microns. Indeed, in this situation, it can take just a single particle to sow the seeds of silt lock. This is because it only takes one particle to lessen the clearance amount. Once this occurs, secondary particles can invade the clearance space. It is these high-volume secondary particles which provide the silt needed for high-static friction, and ultimately stick-slip or seizure to occur.
Water
In water molecules, hydrogen bonding causes a strong enough attractive force as to mimic the clumping of wet sand. This kind of moisture can dramatically exacerbate silt lock conditions in flow paths having tight clearance.
High and Low Pressure Separation
As is commonly known, oil consistently attempts to move from high to low pressure. And it is the annular space between the bore and spool in an electro-hydraulic valve that separates high from low pressure. As well, as operating pressure increases, oil leaks through this area with higher velocity, which allows more exposure of particles to silt lands and a greater packing force.
Sludge
Sludge can collect on internal surfaces, usually more