Leakage Control in Hydraulics

Cost Of Leakage
Concern for safety at work and the rapidly increasing cost of oil makes industry sensitive to leakage. Leakage creates safety hazards, wastes costly oil, increases machine down-time, decreases production rates, generates product spoilage and increases replacement parts inventory. The cost of effective leakage control is minor when compared to the long term costs of leakage.

Leak-Free Design
Hydraulic systems do not need to leak. Today’s designer must create a more leak resistant system, where static seal leakage should not occur and dynamic seal leakage will be controlled. Before presenting some design practices proved effective in stopping leaks, we should consider the sources of most leaks.

Cause Of Leaks
Almost all hydraulic system leaks occurring after extended service result from three conditions:
– Loosening of fittings and connectors by shock and vibration
– Wear of dynamic seals and mating parts especially in hydraulic cylinders
– Deterioration of the elastomer because of elevated fluid temperatures or an incompatibility with the hydraulic fluid

Combating Shock and Vibration
Many things can be done to minimize leakage from loose fittings and connectors subject to shock and vibration:
1. Support all pipe lines with damped mountings to absorb both shock and vibration.
2. Reduce shock with low-shock valves or accumulators.
3. Use pressure controls with low override and strategically placed to protect all parts of the system.
4. Use a minimum number of fittings and connectors. Use welded joints wherever practical.
5. Use parallel thread connectors, tees and elbows in place of tapered pipe threads.
6. Use manifolds instead of individual lines wherever possible.
7. Specify proper bolt and plug torques for expected peak pressures to prevent surface separation and static seal nibbling.
8. Stress good workmanship to avoid poorly assembled fittings and connectors.

Reducing Dynamic Seal Wear
Most dynamic seals are well designed and will provide long, relatively leak-free service if given reasonable chance. Four things a designer can do to extend the life of dynamic seals are:
1. Eliminate side loads on cylinder rod and drive shaft seals.
2. Protect cylinder rods from abrasive dirt with scrapers, shields or rubber gaiters.
3. Provide the requisite filtration and easily cleaned reservoirs to prevent dirt build-up in the oil.
4. Keep cylinder rod and shaft speeds as low as possible.

Requirements For Static Seals
A static seal retains fluid between rigid, stationary surfaces. The seal must be compressed as with a gasket or deformed as with an “O” ring, to flow into the microcrevices in the mating surface and also raise the seal’s internal stress level higher than the pressure to be sealed. When parts are not rigid enough or bolt preload is not high enough, the mating surfaces will separate under the action of fluid pressure, creating clearances of enlarging those that might exist because the sealing surfaces were not initially flat enough. With movement of mating surfaces, the static seal becomes a dynamic seal. Rough surfaces will wear the seal and changing clearances nibble seal edges.

Mounting Plates
When valve packages or sub-plates are bolted to mounting plates, the condition of the plate is important to obtain a satisfactory initial seal and prevent extrusion and wear. Requirements are:
– Flat mounting surfaces
– Good sealing surface finish -64 micro-inches with no radial scratches
– High enough bolt pre-load to prevent surface separation.

Preventing Seal Deterioration
Premature deterioration of the seal can result from other factors. A primary factor is excessive fluid temperature. A good guide is that seal life is halved by every 20o F. rise. The cure: Incorporate sufficient heat exchangers to keep fluid temperatures below 150o F.

Another factor may be compatibility of the fluid with the seal material where special fluids are used. If a doubt arises, contact your Vickers representative. The following brief review of seal materials may be helpful.

Nitrile (Buna N) is the most widely used and best all-around elastomer for petroleum (mineral) oils, fuel and fire-resistant fluids – with the exception of phosphate esters.

Fluoroelastomer (Viton or Fluorel) costs more than Nitrile, can be used instead of Nitrile but has the added advantage of longer life when fluid temperatures consistently run above 150o F. It can be used with phosphate ester fluids (except Skydrol).

Polyurethane shows extrusion and abrasion resistance superior to Nitrile in petroleum oils, fuel and silicate esters, but deteriorates if contaminated with hot water. Refer to “Stop Leaks” bulletin 394 for more comprehensive coverage of leakage control.