Hydraulic Sealing devices

Oil leakage in a hydraulic system reduces efficiency and leads to increased power losses. Internal leakage does not cause loss of fluid in the system as the fluid returns to the reservoir. External leakage represents a loss of fluid in the system. An improperly assembled pipe fitting is the most common cause of external leakage. Shaft seals on pumps and cylinders can get damaged due to misalignment, leading to leakages in the system.

Seals are used in hydraulic equipment to prevent excessive internal and external leakages and to keep out contamination. Seals can be of a positive or non-positive type and are generally designed for static or dynamic applications. Positive seals do not allow any leakage whatsoever. Non-positive seals permit a small amount of internal leakage.

Static seals are used between mating parts, which do not move relative to each other. Figure 11.1 shows some of the static seals including flanged gaskets and seals. The static seals are compressed between two rigidly connected parts. They represent a simple and non-wearing joint, which would be trouble free if properly assembled.

Dynamic seals are assembled between mating parts, which move relative to each other. Dynamic seals are subject to wear and tear as one of the mating parts rubs against the seal. The most widely used seals of this type are:
• O-rings
• Compression packings
• Piston cup packings
• Piston rings
• Wiper rings.

seal-flange-joints

V-ring packings
V-ring packings are compressible type seals, which are used in virtually all reciprocating motion applications. These include rod and piston cylinders in hydraulic cylinder applications, press rams and jacks. Here, proper adjustment is essential since excessive tightening will accelerate wear and tear.

Piston cup packings
Piston cup packings are designed specifically for pistons in reciprocating pumps and hydraulic cylinders. They offer the best service Ufe for this type of appUcation. Figure 11.2 shows a typical installation of piston cup rings for double acting and single acting operations.

piston-cup-packings

Sealing is accomplished when the pressure pushes the cup lip outwards against the cylinder barrel. The backing plate and the retainers clamp the cup packing tightly in its place, allowing it to handle very high pressures.

Non-metallic piston ring packings
These packings are made out of tetrafluoro ethylene (TFE), a chemically inert, tough waxy solid. Their extremely low coefficient of friction permits them to run completely dry and at the same time prevent scoring of the cylinder walls. This type of piston ring is very ideal for applications where the presence of lubrication can be detrimental or even dangerous.

The following are the most common types of materials used for seals:
• Leather. This is rugged and inexpensive. However it tends to ‘squeal’ when dry and cannot operate above 93 °C (200 °F). Leather also does not operate well at cold temperatures of around -50 °C (-58 °F).
• Buna-N: This material is rugged, inexpensive and wears well. It has a wide operating range between -45 °C (-50 °F) and 121 °C (250 °F) and also maintains good sealing characteristics in this range.
• Silicone: This elastomer has an extremely wide temperature range between -68 °C (-90 °F) and 232 °C (450 °F). Hence it is widely used in rotating shaft seals and static seals. Silicone is not used for reciprocating seal application as it has a low resistance to tear.
• Neoprene: This material has a temperature range of -54 °C (-65 °F) to 120 °C (250 “^F). It has a tendency to vulcanize beyond this temperature.
• Viton: This material contains about 65% fluorine. It has become a standard material for elastomer type seals for use at elevated temperatures up to 260 °C (500 °F). The minimum temperature at which these seals operate is about -29 °C (-20 °F).
• Tetrafluoroethylene: It is a form of plastic and is a very widely used seal material. It is quite tough and chemically inert in nature and has excellent resistance up to temperatures of 370 °C (around 700 °F). It additionally possesses an extremely low coefficient of friction. One major disadvantage with this material is its tendency to flow under pressure forming thin films. This can be neutralized to a large extent by using filler materials such as graphite, asbestos and glass fibers.

Hydraulic System Preventive Maintenance

Most of the production personnel carry the impression that a maintenance department exists primarily to repair the faults that occur. Unfortunately this is not the case. The most important part of the maintenance department’s responsibility is to perform routine planned maintenance otherwise known as preventive maintenance.

Preventive maintenance primarily deals with:
• Regular servicing of the equipment
• Checking for correct operation
• Identification of potential faults and their immediate rectification or correction.

As an often-overlooked side benefit, planned maintenance trains the maintenance technician in the proper operation and layout of the plant for which they are responsible. Most of the common problems listed in the introductory section of this chapter can be eliminated if a planned preventive maintenance program is undertaken.

More than 50% of the problems encountered in hydraulic systems have been observed to be related to hydraulic oil. This is why regular sampling and testing of the hydraulic fluid is a very important. A portable hydraulic fluid test kit is available nowadays. This helps in carrying out the basic tests at the site itself. Tests that can be performed include ones such as determination of viscosity, water content and particulate contamination.

It is vital that the maintenance personnel be trained to carry out maintenance activities effectively. A technician should also be able to recognize the early symptoms of potential hydraulic problems. For example, a noisy pump may be due to cavitation caused by a clogged inlet filter. It might also be due to a loose inlet fitting which permits air ingress
into the pump. If the cavitation is due to air leakage in the pump, the oil in the reservoir tends to get covered with foam. When air becomes entrained in the oil, it causes spongy operation of the hydraulic actuators. A sluggish actuator may also be due to the high viscosity of the fluid.

good reporting and recording system. These reports should include the following:
• The type of symptoms encountered and how they were detected along with the respective date
• A description of the maintenance repairs performed. This should include the replacement of parts, the amount of downtime and the date
• Records of dates when the oil was tested, added or changed
• Records of dates when the filters were cleaned or replaced.

Proper maintenance procedures with respect to external oil leakages are also essential. Safety hazards due to oil spillage on the floor should be prevented. The bolts and brackets of loose mountings should be tightened as soon as they are detected as they can cause misalignment of the pump and actuator shafts.

Hydraulic Cleanliness

Most hydraulic and pneumatic faults are caused by dirt. Very small particles can nick seals, abrade surfaces, block orifices and cause valve spools to jam. Ideally components should not be dismantled in the usual dirty conditions found on site. These should be taken to a clean workshop equipped with proper workbenches. Components and hoses come from manufacturers with their orifices sealed with plastic plugs, to prevent dirt ingress during transit. These should be left as they are during storage and removed only when the component is to be put to use.

Filters are meant to remove dirt particles, but only work until they are clogged. A dirty filter may cause the fluid to bypass and can make things far more worse by accumulating the particles and then releasing them all in one lump. Filters should be regularly checked and cleaned or changed when required.

The oil condition in a hydraulic system is also crucial for maintaining reliability. Oil, which is dirty, oxidized or contaminated, forms a sticky gummy sludge which blocks small orifices and causes the valve spools to jam. The oil condition should be regularly checked and the suspect oil changed before problems develop.

Hydraulic Safety

Electrical systems are generally recognized as being potentially lethal and all organizations by law must have procedures for isolation of the equipment and adopt safe working practices. Hydraulic and pneumatics are no less dangerous but tend to be approached in a far more laissez faire or casual manner.

A hydraulic system can present the following dangers to an operator:
• High-pressure air or oil released suddenly can attain explosive velocities and can easily cause an accident.
• The unexpected movement or drift of components such as cylinders can be harmful.
• Spilt hydraulic oil is very slippery and can cause accidents.

A few guidelines to ensure safety in hydraulic systems are listed here:
• The implications arising out of any action have to be considered before resorting to it.
• Anything that can move with change in pressure as a result of your actions should be mechanically secured or guarded.
• Particular care should be taken with regard to suspended loads. It must be remembered that fail-open valves will turn ON when the system is de-pressurized.
• Never disconnect pressurized lines or components. The whole system should be de-pressurized before disconnecting any of the lines.
• Put up safety notices to prohibit operation by other people.
• Ensure that the accumulators in the hydraulic system are fully blown down.
• Make proper arrangements to prevent spillage of oil on the floor.
• Where there is an electrical interface to a hydraulic system (e.g., solenoids, pressure switches, limit switches) the control circuit should be isolated, not only to reduce the risk of electric shock but also to reduce the possibility of fire.
• After the work is completed, keep the area tidy and clean. Check for any leakages and confirm correct operation of the system.
• Many components contain springs under pressure. If released in an uncontrolled manner, these can fly out at high speed and cause injury. Springs should be removed with utmost care.

In USA, the Occupational Safety and Health Administration (OSHA) of the Department of Labor describes and enforces safety standards at industry locations where the hydraulic equipment is operated. For detailed information of OSHA standards and requirements, the OSHA publication 2072 can be referred to. The general industry guide for applying safety and health standards, 29 CFR 1910 also provides us with a standard set of safety standards for operating hydraulic equipment.

These standards deal with the following categories:
• Workplace standards
• Machines and equipment standards
• Materials standards
• Employee standards
• Power source standards
• Process standards.

The basic rule to follow is that there should be no compromise when it comes to the health and safety of people at the place of their work.

Problems due to entrained gas in fluids

Entrained gas or gas bubbles in the hydraulic fluid is caused by the sweeping of air out of a free air pocket by the flowing fluid and also when pressure drops below the vapor pressure of the fluid. Vapor pressure is that pressure at which the fluid begins changing into vapor. This vapor pressure increases with increase in temperature. This results in the creation of fluid vapor within the fluid stream and can in turn lead to cavitation problems in pumps and valves. The presence of these entrained gases reduces the effective bulk modulus of the fluid causing unstable operation of the actuators.

The phenomenon of cavitation is in fact the formation and subsequent collapse of the vapor bubbles. This collapse of the vapor bubbles takes place when they are exposed to the high-pressure conditions at the pump outlet, creating very high local fluid velocities, which impact on the internal surfaces of the pump. These high-impact forces cause flaking or pitting on the surface of components such as gear teeth, vanes and pistons leading to premature pump failure. Additionally the tiny metal particles tend to enter and damage other components in the hydraulic system. Cavitation can also result in increased wear on account of the reduced lubrication capacity.

Cavitation is indicated by a loud pump noise and also by a decreased flow rate as a result of which the pressure becomes erratic. Air also tends to get trapped in the pump line due to a leak in the suction or on account of a damaged shaft seal. Additionally it has to be also ensured that air escapes through the breather while the fluid is in the reservoir or otherwise it tends to enter the pump suction line. To counter the phenomenon of cavitation in pumps, the following steps are recommended by manufacturers:

1. Suction velocities to be kept below 1.5 m/s (5 ft/s)
2. Pump inlet lines to be kept as short as possible
3. Pump to be mounted as close to the reservoir as possible
4. Low-pressure drop filters to be used in the suction line
5. Use of a properly designed reservoir that will help remove the trapped air in the fluid
6. Use of hydraulic fluid as recommended by the manufacturer
7. Maintaining the oil temperature within prescribed limits, i.e. around 65 °C or 150 °F.

Hydraulic components wear due to fluid contamination

Excessive contaminants in the working fluid prevent proper lubrication of components such as pumps, motors, valves and actuators. This can result in wear and scoring which affect the performance and life of these components and leads to their eventual failure. A typical example of this is the scored piston seal and cylinder bore of cylinders causing severe internal leakage and resulting in premature cylinder failure.

Common causes for hydraulic system breakdown

The most common causes of hydraulic system failures are:
• Clogged and dirty oil filters
• An inadequate supply of oil in the reservoir
• Leaking seals
• Loose inlet lines, which cause pump cavitations and eventual pump damage
• Incorrect type of oil
• Excessive oil temperature
• Excessive oil pressure.

A majority of these problems can be overcome through a planned preventive maintenance regime. The overall design of the system is another crucial aspect. Each component in the system must be properly sized, compatible with, and form an integral part of the system. It is also imperative that easy access be provided to components requiring periodic inspection and maintenance such as strainers, filters, sight gages, fill and drain plugs and the various temperature and pressure gages. All hydraulic lines must be free of restrictive bends, as this tends to result in pressure loss in the line itself.

The three maintenance procedures that have the greatest effect on system life, performance and efficiency are:
1. Maintaining an adequate quantity of clean and proper hydraulic fluid with the correct viscosity
2. Periodic cleaning and changing of all filters and strainers
3. Keeping air out of the system by ensuring tight connections.

A vast majority of the problems encountered in hydraulic systems have been traced to the hydraulic fluid, which makes frequent sampling and testing of the fluid, a vital necessity. Properties such as viscosity, specific gravity, acidity, water content, contaminant level and bulk modulus require to be tested periodically. Another area of vital importance is the training imparted to maintenance personnel to recognize early symptoms of failure. Records should also be maintained of past failures and the maintenance action initiated along with data containing details such as oil tests, oil changes, filter replacements, etc.

Oxidation and corrosion are phenomena which seriously hamper the functioning of the hydraulic fluid. Oxidation which is caused by a chemical reaction between the oxygen present in the air and the particles present in the fluid, can end up reducing the life of the fluid quite substantially. A majority of the products of oxidation are acidic in nature and also soluble in the fluid, thereby causing the various components to corrode.

Although rust and corrosion are two distinct phenomena, they both contribute a great deal to contamination and wear. Rust, which is a chemical reaction between iron and oxygen, occurs on account of the presence of moisture-carrying oxygen. Corrosion on the other hand is a chemical reaction between a metal and acid. Corrosion and rust have a tendency to eat away the hydraulic component material, causing malfunctioning and excessive leakage.