Hydraulic&Pneumatic Seals--Importance of -Low Pressure- Sealing
This is the most important aspect of reliable hydraulic / pneumatic actuators and ultimately overall system reliability. Low pressure is more difficult to seal than high pressure. First we must define "Low Pressure" and "High Pressure" as it relates to the sealing of hydraulic/pneumatic actuators.
High Pressure is any pressure created in the system that causes a differential in pressure across the sealing device. Take, for example, the pressure developed on one side of a piston head to move a piston in one direction or the other. There is a seal present to prevent flow past the piston so that resistance can create pressure. The piston moves from high pressure toward low pressure. That difference in pressure is called a pressure differential. Pressure acts at 90 degree angles to the elastomeric seal surface, forcing it against the containment surface which affects a seal. The pressure actually helps the seal work.
High pressure leaks are not acceptable in systems as they allow high pressure leakage of the medium which is dangerous and costly. High pressure leaks are considered "Sporadic Failures" and are repaired as soon as possible.
Much more common, however, is low pressure leakage. A low pressure differential across the seal is when the pressure on both sides of the sealing device is almost equal. This would include the time when the actuator is at rest, between cycles or on the low pressure side of the piston. Always remember, the pressure created in a system is only as high as is necessary to overcome the resistance of the load. When there is little resistance, the pressure is relatively low. When actuators are cycling, much of the cycle may have very little resistance until it meets the load. And so, seals could be in low pressure conditions for hours or days or as quick as a few seconds. When this condition is present in the actuator, the sealing device is allowed to relax in the groove or containment space. The seal relaxes and returns to the shape it was, prior to high pressure deformation. This is when reliability issues occur. In the relaxed state, the seal, which has seen system heat and frictional heat, will begin to compression set. Compression Setting is when an elastomeric sealing device, which was molded or machined in a shape larger than the containment space it was designed to be installed in, begins to "re-mold" and take the shape of the containment space. The seal is losing its interference and ability to seal low pressure. This is a of the seal material's physical characteristics, temperature and time. Once the seal has started to compression set and when high pressure conditions are not present, a potential leak path occurs between the sealing device and the containment surfaces. The severity of the leakage is determined by the surface finish and condition, the degree to which the sealing device material has compression set, and the time between low pressure and high pressure conditions. It is evident by medium weeping, hissing, leaking or dripping out around the sealed areas.
As the low pressure leakage occurs; dirt, contaminants and other foreign particles are introduced between the sealing device and the smooth machined (RMS) finish of the dynamic surface. When high pressure conditions occur, the pressure acts against the sealing device surface, forcing the sealing device's material and the foreign particles against the smooth dynamic surface. This causes scoring damage to the dynamic surface which will cause the sealing device to leak and fail prematurely. It is also very expensive to refinish the dynamic surface at the same time the seals are replaced. Low pressure leaks are the cause of the following avoidable expenses and issues:
. System Reliability Issues
. Maintenance Costs: Labor and parts
. Downtime / Production losses
. Lost Medium or Fluid costs
. Safety Issues
. Environmental Issues
Low pressure leaks have been (and still are) traditionally accepted in hydraulic / pneumatic systems as users have come to expect them. Low pressure leaks are considered "Chronic Failures" and present the area of greatest return on investment and system reliability improvements.