hydraulic fluid

How Long Should a Hydraulic Pump Last?

How long should a pump last?

Unfortunately, it is not as easy to answer because the same pump can be applied in millions of different systems under widely varying conditions of pressure, temperature, speed, fluid and will be used and maintained in many different ways.

Considering the purpose of the manufacturer of the pump, the pump was built to last infinitely. The manufacturer guarantees that the fluid will be sent to lubricate abundantly every moving part inside the pump and therefore will not generate wear. So how can one ensure that the hydraulic pump will last effectively infinite number of years? What factors affect the life of the pump?

Next, we list the main factors that decrease the life of the pump:

RPM and maximum flow

Constant work near maximum RPM (revolutions per minute) and peak flow can shorten the life of the pump due to the fact that the pump has the suction and discharge connections with sizes that cannot be changed. When working near these limits the fluid enters the pump at very high speeds that can generate cavitation and therefore internal damage due to lack of lubrication.

Also keep in mind that factors such as temperature, viscosity of the fluid used, and entry of air into the intake can lower the speed limit (rpm) and peak flow of the pump.

Change in the properties of the fluids used

A new hydraulic fluid will provide all necessary lubrication for internal pump parts and prevent wear of the same, but depending on the type of fluids (oil from petroleum, synthetic, biodegradable or water – oil mixtures) are degraded or additives lost over time, some more than others and of course, factors such as temperature, humidity and pollution can accelerate degradation.

The end result is that the oil does not lubricate as before and cannot avoid in the same way that metal parts rub against each other and wear. The periodic change of the fluid is required to maximize the life of the hydraulic pump.

Control of contamination

As we know, any element other than hydraulic fluid is treated as contamination, e.g. air, water, and solid particles of all kinds. This is perhaps one of the most important facts and one of the least known.

The installation of filters suitable for the application and its placement within the hydraulic system are vital for controlling contamination; tank vent, suction filters, return and pressure filters are essential, as well as good selection of micronage. But the filters get clogged doing their work after a while, so it is imperative to change the filters elements regularly to extend the life of the pump.

Bearings

All pumps have some bearings to support the drive shaft and some other internal parts. There are thrust bearings, radial ball bearings, cylindrical roller bearings, tapered roller and so on. These elements are responsible for supporting the loads generated against the housing and covers of the pump by the movement transferred from the drive shaft to the rotating group which in turn generates the pumping of the fluid.

The bearings are subjected to cyclic loads that are repeated with each shaft revolution, or at a rate of thousands of cycles per minute (RPM) which when multiplied by the hours, days and years of work generate millions of cycles in their life. The bearing load in each cycle varies according to the working conditions of the hydraulic system such as pressure, displacement and temperature among others, creating a phenomenon called ‘material fatigue’ which reduces its mechanical strength with the number of work cycles or over time. The load, this factor and the friction between the rolling elements (balls, rollers and tracks) are what determine the bearing life.

Obviously, the bearings used in pumps vary tremendously depending on the type of pump. For example, the bearing loads in an axial piston pump or bent axle piston pump are gigantic; however the case is very different in a balanced vane pump where the loads are minimal.

Material fatigue

This is a condition that affects all the parts of the pump that are subject to fluctuating or cyclic loads as explained in the bearings. Vanes that pass from the zone of high pressure to the suction zone with each turn of the shaft; or the piston in a piston pump that pumps fluid under pressure in a half of a turn of the shaft and sucks fluid in the other half; or the teeth of the gear pump as they move from the suction to the pressure side.

These cyclic loads generate cracks starting at a microscopic level and will get larger over time and the number of cycles until the part gets broken. A determining factor in the onset of fatigue is how close is the value of the load on the piece to its maximum resistance, the closer to this limit; the fewer cycles are needed to break it. Finally, the original ultimate strength of the part decreases with the number of cycles as well.

With that said, the pump working under extreme pressure conditions that are very close to the maximum recommended by the manufacturer makes the real life of the pump to be reduced dramatically.

Taking into account all the factors listed, the life of the pump is determined from the start with a good selection of it at the time of design, good working conditions without reaching extremes, using the proper fluid at the proper temperature and a good maintenance policy (changing fluid and filters regularly).

Craig Cook

Dangers while Working with Hydraulic Fluids

Health Problems While Using Hydraulic Fluids

People can become exposed to the chemicals in hydraulic fluids. The exposure to chemicals may be due to inhalation, ingestion, or touch. There are instances of people suffering from skin irritation or weakness in hands while handling hydraulic fluids. There are also cases of intestinal bleeding, pneumonia, or death through hydraulic fluid ingestion though no serious hazards are reported with hydraulic fluid inhalation. These are only some of the dangers of hydraulic fluid.

Similar to ingestion, fluids can be accidentally injected into the skin as well. This takes place when the high-pressure hydraulic system hose is disconnected and toxic fluids are leaked and injected into the skin. If there is a small leak in the hydraulic pipe and someone runs there hand along it, at 2000 psi, they can easily incur an injection of hydraulic fluid and may not even be aware that it happened until gangrene begins to set in.

Fire Dangers Associated with Hydraulic fluids

When working with hydraulic fluid, there is every chance that the hydraulic fluid gets heated to high temperatures. And it is evident that most petroleum-based hydraulic fluids will burn and thereby create explosions and burns.

Environmental Problems Related to Hydraulic Fluids

Another hazard of hydraulic fluid is that when the hydraulic hose or pipe leaks, the chemicals of the fluids can either stay on top of the soil or sink into the ground. If the chemicals get mixed in a water body, they will sink to the bottom. In fact in such cases the chemicals can stay there for more than a year. Aquatic life can absorb the toxic hydraulic fluid, leading to illness or death to the animal or anything higher on the food chain. For example, a hawk that eats a fish that has been contaminated by hydraulic fluid that was mixed in water could become ill as well.

Fluid Texture Problems

Although the slimy texture of hydraulic fluids may not seem like a danger or a problem, a spill can cause a person to slip and fall. Also when there is fluid on the hands of a person, it can cause him to slip while climbing on a machine. It can also cause the operator to lose steering control.

Safety Precautions

There are numerous dangers of hydraulic fluid involved, like skin irritation, fires, explosions, environmental damage, and a slippery workplace. But hydraulic fluids are required for many machines to function. Therefore it is necessary to follow certain precautions while using these fluids. With proper knowledge of these hazards, working with hydraulic fluid can be safe.

In order to avoid skin irritations, it is necessary to wash contaminated skin immediately. It is also necessary to keep your clothing clean.
Wearing masks and gloves while using hydraulic fluids is also helpful.
To avoid environmental dangers, there is a biodegradable hydraulic fluid option, though it is more expensive.
To avoid fires, materials and fluids soaked in hydraulic fluid should be stored in sealed metal containers and disposed of at proper places.
To check for leaks, use cardboard.

Craig Cook

Fluid Power Safety Products

Fluid Power Safety 101

Safety is a critical aspect to any fluid power system, not just from the basic level of keeping components plumbed properly, but also in overall levels of machine safeguarding. It is critical to evaluate the entire system, including the electrical portion, to minimize exposure to unnecessary risk. Systems are rated based on the weakest link in the control chain. a Several standards (including ISO 13849-1:2006, ANSI/ASSE Z244.1-2003 (R2008) and ANSI/PMMI B155.1-2011) define the control system as including input, sensing, and interlock devices as well as output devices such as pneumatic and hydraulic valves. a The function of a fluid control valve mimics that of an electrical-control relay and, therefore, is subject to the same rules for classifying safety integrity. Thus, properly specified machine safeguarding systems include provisions for pneumatic valves, including:

Must be functionally redundant.
Must be monitored for faults (including diminished performance faults, which may create the loss of redundancy), without depending on external machine controls or safety circuitry.
Must return to a safe position in the event of a loss of pressure or other such event.
Must be able to inhibit further operation upon detection of a fault condition until such condition is corrected.
Should have a dedicated, specific function-reset input and should prohibit the ability to perform a reset by simply removing or re-applying pneumatic or hydraulic power.
Must not automatically reset.

Providing control reliability with fluid power is not quite the same as with electrical controls, however. For instance, plain redundancy in a safety circuit requires the equivalent function of four valve elements, not just two. Two of the four valve elements handle the inlet function while the other two elements handle the stop function (energy release). Many self-designed systems risk having hidden, potential flaws, which can lead to unsafe conditions because they are unseen, unexpected and, therefore, excluded from design and safety reviews. A good example is the spool cross-over conditions or ghost positions of a valve, which are usually not shown on schematics. a

Two general abnormal conditions can affect valve safety. The first is similar to an electrical-control fault, such as when a relay might be stuck in the open or closed position. The second is when a valve develops diminished performance, as when a valve becomes sticky or sluggish. In such cases, the valve reaches the proper position, but slower shifting affects safe stopping distances or precise timing. The ANSI B11.19-2010 Standard mandates a monitoring system that detects these conditions for critical applications and the ANSI/PMMI B155.1-2011Standard requires diminished performance monitoring if stopping time can be affected. An easy solution is to use a self-monitoring, Category-3 or -4 valve, designed to detect both conditions. a

The use of double valves remained relatively unheard of for many years except in a few select industries, such as stamping presses, which first initiated control reliability requirements. Double valves provide dual internal functions (redundancy) so that an abnormal function of one side of the valve does not interfere with the overall normal operation. At the same time, the double valves sense abnormal operation on either side of the valve and then inhibit further operation until the problem has been corrected and the valve deliberately reset. This sensing and inhibiting function is commonly referred to as monitoring. a

Two standard air valves, whether in parallel or in series, cannot perform the same safeguarding function as does a double valve critical function. By simply incorporating two standard air valves into the circuit, no provision is made to sense the abnormal operation of one side of the valve or, even more preferable, diminished performance such as slow shifting. In addition, there is no provision for inhibiting further operation of the circuit until the valve is repaired. If one valve actuates abnormally, the second one continues to function and redundancy is lost. The circuit doesn’t recognize lost redundancy nor would it halt operations as a warning that redundancy has been compromised. Then, if the second valve also actuates abnormally, there is no back up and control integrity no longer exists. a

Double valves are appropriate for pneumatic and hydraulic equipment anytime reliability is an issue. Typical applications include E-stop, two-hand-control, light curtains, safety gates, pneumatic locking devices for safety gates, hydraulic brakes, air brakes, amusement rides, hoists, elevators, pinch-point applications, or any other application where control system integrity depends on valve operation. a

Craig Cook

How Air Effects Your Hydraulic System

Air can be present in your hydraulic system in four forms:

Free air – a pocket of air trapped in part of a system.
Dissolved air – hydraulic fluid contains between 6 & 12 percent by volume of dissolved air.
Entrained air – air bubbles typically less than 1 mm in diameter dispersed in the fluid.
Foam – air bubbles typically greater than 1 mm in diameter which congregate on the surface of the fluid.

Of these four forms, entrained air is the most problematic.

Pre-filling components and proper bleeding of the hydraulic system during start-up will largely eliminate free air.

Small amounts of foam are cosmetic and do not pose a problem. However, if large volumes of foam are present, sufficient to cause the reservoir to overflow for example, this can be a symptom of a more serious air contamination and/or fluid degradation problem.

Negative effects of entrained air include:

– Reduced bulk modulus, resulting in spongy operation and poor control system response
– Increased heat-load
– Reduced thermal conductivity
– Fluid deterioration
– Reduced fluid viscosity, which leaves critical surfaces vulnerable to wear
– Cavitation erosion
– Increased noise levels
– Decreased efficiency

As pointed out above, hydraulic fluid can contain up to 12 percent dissolved air by volume. Certain conditions can cause this dissolved air to come out of solution, resulting in entrained air.

When fluid temperature increases or static pressure decreases, air solubility is reduced and bubbles can form within the fluid. This release of dissolved air is known as gaseous cavitation.

Decrease in static pressure and subsequent release of dissolved air can occur at the pump inlet, as a result of:

-Clogged inlet filters or suction strainers
-Turbulence caused by intake-line isolation valves
-Poorly designed inlet
-Collapsed or otherwise restricted intake line
-Excessive lift
-Clogged or undersized reservoir breather

Air entrainment can also occur through external ingestion. Like gaseous cavitation, this commonly occurs at the pump as a result of:

-Loose intake-line clamps or fittings
-Porous intake lines
-Low reservoir fluid level
-Faulty pump shaft seal

Like other hydraulic problems, proper equipment maintenance will prevent the occurrence of most air contamination problems.

Craig Cook

How Much Water is in Your Hydraulic Oil?

Water in hydraulic fluid is a contaminant that can be just as damaging as hard particles and in some cases, more so.

Water in hydraulic fluid:

Depletes some additives and reacts with others to form corrosive by-products which attack some metals.
Reduces lubricant film-strength, which leaves critical surfaces vulnerable to wear and corrosion.
Reduces filter ability and clogs filters.
Reduces the oils ability to release air.
Increases the likelihood of cavitation occurring.

How much water is too much?

A number of factors need to be considered when selecting water contamination targets, including the type of hydraulic system and your reliability objectives for the equipment.

It’s always wise to control water contamination at the lowest levels that can reasonably be achieved, but certainly below the oil’s saturation point at operating temperature.

Water removal methods

Methods for removing free (unstable suspension) and emulsified (stable suspension) water include:

polymeric filters

vacuum distillation

headspace dehumidification

Polymeric filters – These look like conventional particulate filters, however the media is impregnated with a super-absorbent polymer.

Water causes the polymer to swell, which traps the water within the media. Polymeric filters are best suited for removing small volumes of water and/or maintaining water contamination within pre-determined limits.

Vacuum distillation – This technique employs a combination of heat and vacuum. At 25 inches of mercury, water boils at 133F (56C). This enables water to be removed at atemperature that does not damage the oil or its additives.

Headspace dehumidification – This method involves circulating and drying the air from the reservoir headspace. Water in the oil migrates to the dry air in the headspace and is eventually removed by the dehumidifier.

Vacuum distillation and headspace dehumidification also remove dissolved water.

Prevention is better than cure

Like all other forms of contamination, preventing water ingress is ten times cheaper than removing it from the oil.

Craig Cook

Dealing with Water in Hydraulic Fluid

If you have worked with hydraulic equipment for any length of time, it’s likely that you’ve come across a hydraulic system with cloudy oil. Oil becomes cloudy when it is contaminated with water above its saturation level. The saturation level is the amount of water that can dissolve in the oil’s molecular chemistry and is typically 200 – 300 ppm at 68°F (20°C) for mineral hydraulic oil. Note that if hydraulic oil is cloudy it indicates that a minimum of 200 – 300 ppm of water is present. I recently audited a hydraulic system with cloudy oil that was found to contain greater than 1% (10,000 ppm) water.

Why is water in hydraulic fluid bad?

Water in hydraulic fluid:

Depletes some additives and reacts with others to form corrosive by-products which attack some metals.
Reduces lubricant film-strength, which leaves critical surfaces vulnerable to wear and corrosion.
Reduces filterability and clogs filters.
Increases air entrainment ability.
Increases the likelihood of cavitation occurring.

How much water is too much?

A number of factors need to be considered when selecting water contamination targets, including the type of hydraulic system and reliability objectives for the equipment. It’s always wise to control water contamination at the lowest levels that can reasonably be achieved, ideally below the oil’s saturation point at operating temperature.

Water removal methods

Methods for removing free (unstable suspension) and emulsified (stable suspension) water include:

polymeric filters;
vacuum distillation; and
headspace dehumidification.

Vacuum distillation and headspace dehumidification also remove dissolved water.

Polymeric filters

These look like conventional particulate filters, however the media is impregnated with a super-absorbent polymer. Water causes the polymer to swell, which traps the water within the media. Polymeric filters are best suited for removing small volumes of water and/or maintaining water contamination within pre-determined limits.

Vacuum distillation

This technique employs a combination of heat and vacuum. At 25 inches Hg, water boils at 133°F (56°C). This enables water to be removed at a temperature that does not damage the oil or its additives.

Headspace dehumidification

This method involves circulating and dehumidifying air from the reservoir headspace. Water in the oil migrates to the dry air in the headspace and is eventually removed by the dehumidifier.

In the case of small systems with high levels of water contamination, changing the oil may be more cost-effective than using any of the above methods of water removal.

Prevention is better than cure

Like all other forms of contamination, preventing water ingress is cheaper than removing it from the oil. A major point of water ingression is through the reservoir headspace. Many hydraulic system reservoirs are fitted with breather caps that allow moisture (and particles) to enter the reservoir as the fluid volume changes through either thermal expansion and contraction, or the actuation of cylinders.

Replacing the standard breather cap with a hygroscopic breather will eliminate the ingression of moisture and particles through the reservoir’s vent. These breathers combine a woven-polyester media that filters particles as small as 3 microns, with silica gel desiccant to remove water vapor from incoming air. The result is relative humidity levels within the reservoir headspace that make condensation unlikely, therefore reducing water contamination of the oil.

Craig Cook

What to do When Your Hydraulic System is Contaminated

There are generally only two conditions that necessitate changing the oil in a hydraulic system:

1. Base oil degradation

2. Additive depletion

Contaminants of the hydraulic fluid such as hard and soft particles and water can be removed from the oil, and therefore, don’t mandate an oil change.

Techniques for flushing hydraulic systems vary in cost and complexity. Before I discuss some of these methods, let’s first distinguish between flushing the fluid and flushing the system.

The objective of flushing the OIL is to eliminate contaminants such as particles and water.

This is usually accomplished using a filter cart or by diverting system flow through an external fluid-conditioning rig.

The objective of flushing the SYSTEM is to eliminate sludge, varnish, debris and contaminated or degraded fluid from conductor walls and other internal surfaces, and system dead spots.

Reasons for performing a SYSTEM flush include:

Fluid degradation – resulting in sludge, varnish or microbial deposits.
Major failure – combined with filter overload disperses debris throughout the system.
New or overhauled equipment – to purge ‘built-in’ debris.

Common methods for flushing hydraulic systems include:

* Double oil and filter change.

* Mechanical cleaning.

* Power flushing.

The technique or combination of techniques employed will depend on the type of system and its size, your reliability objectives for the equipment and the reason for the flush.

Double oil and filter change

This technique involves an initial oil drain and filter change, which expels a large percentage of contaminants and degraded fluid. The system is then filled to the minimum level required and the fluid circulated until operating temperature is reached and the fluid has been turned over at least five times.

The oil is drained and the filters changed a second time. An appropriate oil analysis test should be performed to determine the success of the flush.

To maximize the effectiveness of this technique, the system should be drained as thoroughly as possible and the reservoir mechanically cleaned.

Mechanical cleaning

Although not technically a flushing technique, the selective use of mechanical cleaning may be incorporated in the flushing strategy.

This can involve the use of a pneumatic projectile gun to clean pipes, tubes and hoses, and disassembly of the reservoir and other components for cleaning using brushes and solvents.

Mechanical cleaning is labor intensive and, therefore, costly. It carries with it reliability risks associated with opening the hydraulic system and intervention by ‘human agents’.

Power flushing

Power flushing involves the use of a purpose-built rig to circulate a low viscosity fluid at high velocities to create turbulent flow conditions (Reynolds number > 2000).

The flushing rig is typically equipped with a pump that has a flow rate several times that of system’s normal flow, directional valves, accumulators, fluid heater and chiller and of course, a bank of filters.

The directional valves enable the flushing direction to be changed, the accumulators enable pulsating flow conditions and the heater and chiller enable the fluid temperature to be increased or decreased, all of which can assist in the dislodgment of contaminants.

Analysis of the flushing fluid is performed regularly during the flushing operation to determine the point at which the system has been satisfactorily cleaned.

What about components?

The question of how to deal with system components arises when contemplating a hydraulic system flush. Plumbing should be flushed first in isolation from pumps, valves and actuators. Once the conductors have been flushed clean, valves and actuators can be gradually included in the flushing circuit.

The decision to disassemble and mechanically clean components will depend on the type of equipment, your reliability objectives and the reason for the flush.

Craig Cook

How Hot is Too Hot?

When it comes to the oil’s operating temperature – how hot is too hot?

Heating of hydraulic fluid in operations is caused by inefficiencies. Inefficiencies result in losses of input power, which are converted to heat.

A hydraulic system’s heat load is equal to the total power lost (PL) through inefficiencies
and can be expressed as:

PLtotal = PLpump + PLvalves + PLplumbing + PLactuators

If the total input power lost to heat is greater than the heat dissipated, the hydraulic system will eventually overheat.

Hydraulic fluid temperatures above 180F (82C) damage most seal compounds and accelerate degradation of the oil.

So while the operation of any hydraulic system at temperatures above 180F (82C) should be avoided, fluid temperature is too high when viscosity falls below the optimum value for the hydraulic system’s components.

This can occur well below 180F (82C), depending on the fluid’s viscosity grade (weight).

To achieve stable fluid temperature, a hydraulic system’s capacity to dissipate heat must exceed its inherent heat load.

For example, a system with continuous input power of 100 kW and an efficiency of 80% needs to be capable of dissipating a heat load of at least 20 kW.

It’s important to note that an increase in heat load or a reduction in a hydraulic system’s capacity to dissipate heat will alter the balance between heat load and dissipation.

As you’ve probably gathered, there are only two ways to solve overheating problems in hydraulic systems:

1. Decrease heat load; or
2. Increase heat dissipation.

Decreasing heat load is always the preferred option because doing so increases the efficiency of the hydraulic system.

Craig Cook

Tips for Choosing the Right Hydraulic Hose

When choosing a hydraulic hose you should pay close attention to the following characteristics:

Working Pressure – Choose a hose that is suitable for the working pressure of the machine
Wire or Sheathing – Install wire or sheathing when fabricated if the hose will be used in an area exposed to damage from pinching or crushing
Fluid – Make sure you use a hose compatible with the fluids that are used in the machine
Size/Inner Diameter – Choosing the right size hose is important in order to avoid unwanted friction. When fluid rubs against the inner surface of the hose, friction is created, which creates heat, back pressure increases, and the rate of flow is reduced
Match the Fluid Viscosity to the Operating Temperature – In order to achieve maximum component life, the fluid’s viscosity grade should be correctly matched to the operating temperature range of the hydraulic system

Craig Cook

Water in hydraulic fluid is a contaminant that can be just as damaging as hard particles and in some cases, more so.

Water in hydraulic fluid:

Depletes some additives and reacts with others to form corrosive by-products which attack some metals.
Reduces lubricant film-strength, which leaves critical surfaces vulnerable to wear and corrosion.
Reduces filter ability and clogs filters.
Reduces the oils ability to release air.
Increases the likelihood of cavitation occurring.

How much water is too much?

A number of factors need to be considered when selecting water contamination targets, including the type of hydraulic system and your reliability objectives for the equipment.

It’s always wise to control water contamination at the lowest levels that can reasonably be achieved, but certainly below the oil’s saturation point at operating temperature.

Water removal methods

Methods for removing free (unstable suspension) and emulsified (stable suspension) water include:

polymeric filters;

vacuum distillation; and

headspace dehumidification.

Polymeric filters – These look like conventional particulate filters, however the media is impregnated with a super-absorbent polymer.

Vacuum distillation and headspace dehumidification also remove dissolved water.

Prevention is better than cure

Like all other forms of contamination, preventing water ingress is ten times cheaper than removing it from the oil.

Craig Cook