Hydraulic reliability is much more than checking oil levels and regular filter changes. In our experience, these are the ten most commonly missed hydraulic reliability checks. Just a few minutes each month can help you spot potential hydraulic problems so they can be corrected before they become an outage.

1. Case Drain Flow

All variable displacement hydraulic pumps (and certain fixed displacement pumps) have a case drain. The purpose of the case drain is to provide a flow path to tank for oil that bypasses across the tight clearances between internal components. Without this flow path, case pressure would build and cause a failure of the shaft seal. The internal tolerances are exceptionally tight, usually two or three ten thousandths of an inch. Naturally, as the pump wears, the clearance between the internal components will increase, resulting in an increase in case flow. If the case flow is measured on a regular basis, the condition of the pump can easily be tracked.

By far the best way to measure case flow is to permanently install a flow meter in the case drain line. The meter can then be monitored easily as the load on the pump varies and record the highest reading.

If there is no flow meter installed, an approximation of case flow can be determined by measuring the temperatures of the suction line and the case drain with a laser temperature gun or thermal imaging camera (a thermal imaging camera is more accurate and less susceptible to erroneous readings because of technique). The above formula can then be used to estimate case flow where GPM is the case flow in gallons per minute and PSI is the system pressure in pounds per square inch.

Since temperature changes are relatively gradual, this formula will represent an average amount of case flow rather than the instantaneous measure that would be shown with a flow meter. It  should therefore be used only for recording reliability trends and not for troubleshooting.

2. Sound Checks – Cavitation & Aeration

Cavitation

Two common audible problems with hydraulic pumps are cavitation and aeration. These two conditions sound similar and are often confused, but while they do sound somewhat alike, cavitation is characterized by a steady, high-pitched whining sound while aeration is a much more erratic whine and is often accompanied by a sound like that of gravel or marbles rattling around inside of the pump. Many people believe that these very prominent sounds indicate that the pump needs to be replaced. While prolonged cavitation or aeration can indeed destroy a pump, if they are corrected early the pump will likely retain a substantial amount of useful life.

Cavitation occurs whenever the pump is unable to get as much fluid into its suction line as it is attempting to output. Suction pressure drops very low, pulling air molecules from the oil forming a cavity. The cavity, once delivered to the pressure side of the pump, implodes, causing the distinctive whine. The most common cause of cavitation is a plugged suction filter or strainer. Suction strainers are inside the reservoir below the oil level, out of sight and out of mind. Often, they go for long periods without being checked or cleaned. There should be easy access to the suction strainer so that it will be checked periodically. We recommend checking it quarterly.

Another common cause of cavitation is high fluid viscosity, usually a result of starting the system with the oil too cold. The system should never be started unless the oil is at least 40º F (4º C) and should never be placed under load until the oil is at least 70º F (21º C).

A somewhat less likely cause is high drive motor RPM. On occasion, we have found that a drive motor has been replaced with one of an RPM that exceeds the rating of the pump. Since hydraulic pumps are positive displacement devices, meaning that a very specific amount of oil is delivered with each rotation, if the drive motor turns faster than oil can be delivered through the suction port, the pump attempts to deliver more oil than it can get into its suction and will cavitate.

Aeration

Aeration occurs whenever outside air enters the suction line of the pump. Outside air will not enter at a constant rate, causing the more erratic sound than that of cavitation. One common cause is an air leak in the suction line. Since the pressure in the suction line is below that of atmosphere, oil doesn’t leak out – air leaks in. To test for an air leak, oil can be squirted along the suction line. If the aeration stops momentarily, the leak has been found.

A worn shaft seal on a fixed displacement pump can cause it to aerate. A good way to check the shaft seal is to apply shaving cream around the seal. If air is getting in, little holes will appear in the foam.

The most common cause of aeration is a bad installation – couplings not properly aligned or torqued, or the wrong shaft rotation.

3. Minimum & Maximum System Pressure

In many facilities, system pressure is checked only when the machine appears not to be running properly. Minimum and maximum system pressures should be checked and recorded monthly. In most systems, the pressure should be monitored through a complete machine cycle. On many systems, the pressure should remain stable while on others, the pressure will fluctuate considerably depending upon its load. In general, most systems with variable displacement pumps should maintain a somewhat stable system pressure. Significant pressure swings usually indicate that the system is being starved of flow, warranting further investigation.

Causes of Pressure Swings

Common causes of pressure swings are worn pumps, bypassing components, large leaks, and accumulator failure.

Sometimes, pressures are increased by operators or technicians in a misguided attempt to speed up the system. While this may indeed work, it is not the correct way to increase speed and will result in excess force being generated. This excess force will attack the weak points of the system resulting in shock, leaks and overheating.

Minimum & Maximum Current Draw

Monthly records of the minimum and maximum current draw of the drive motor can be a valuable reference when problems occur. This is the best single indication of how hard the system is working. Drops in system speed may not be immediately noticeable until they cause a problem, but the current draw will immediately change with any difference in pressure or flow.

While a worn pump is not the only cause of a drop in normal current draw, a pump is certainly easier to turn when it is badly worn than when it is brand new. A drop in current draw coupled with an increase in case flow is therefore a very strong indication of a pump that is worn. This, of course, presupposes that we have a good idea of what “normal” is.

5. Component Temperature Checks

Whenever a hydraulic component fails, it will almost always leak. Sometimes they leak out onto the floor, which is an obvious indication of impending failure. Most of the time however, the leakage is internal and is called “bypassing”. The more the component wears, the more it will bypass. Most components bypass to some degree even when brand new and some components can bypass significantly before they cause any production issues. Bypassing always results in a pressure drop that does no useful work. Whenever energy is put into the system and not used to do useful work, the energy doesn’t just “go away”, it gets converted to some other form. The vast bulk of energy that doesn’t move a load is converted to heat. We can thus reliably predict an impending component failure by tracking the temperature gain across it. Again, this presupposes we have some idea of what “normal” temperature gain across critical components should be at different times of the year, so we recommend monthly checks of temperature gain across valves, pressure controls, and flow dividers.

6. Heat Exchangers

No matter how efficiently our system is designed, there must necessarily be some heat generation. Whenever more heat is generated by design than can be dissipated to atmosphere by the surface area of the reservoir, a heat exchanger must also be installed. There are two types – water and air.

The water heat exchanger is more efficient than the air type, and correspondingly more expensive. The added expense is not so much the heat exchanger itself, but the associated infrastructure of a chiller tower since we usually cannot use a nearby body of water or municipal water. In a canister type heat exchanger, the heat in the oil is transmitted to water circulating through the coils. In a plate-type heat exchanger, the most efficient type, oil and water pass through the heat exchanger in opposite directions, separated by plates and heat transfers from the oil to the water.

In both types, the amount of water flow is critical to the efficiency. The maximum heat transfer will occur when the water flow is about one-fourth to one-third that of the oil flow. Many heat exchangers control the water flow with a modulating valve that will open and close to cause the maximum heat transfer based on the heat measured by a capillary tube in the reservoir. If manual adjustments are made, it is easiest to adjust for the maximum temperature gain across the inlet and outlet water lines. Water quality is critical, particularly in the plate type heat exchanger. Corrosion can cause either oil in the water or water in the oil. The air heat exchanger works similarly to the radiator in a car. Oil flows through tubing that has been bonded to fins. When the oil temperature reaches the thermostat setting, usually approximately 115º F (46º C), an electric fan blows air through the fins to dissipate heat to atmosphere.

Good air flow is critical to the efficiency of the air heat exchanger. Daylight should always be visible through the fins. The fins should be cleaned regularly to remove dust, dirt and greasy deposits. A stiff brush or air nozzle can be used for loose dirt removal.  Be careful not to bend the fins when cleaning.  If the fins are bent use a “comb” for straightening to insure a good flow of air.  Use a mild alkaline cleaning solution with a brush for removing solid and greasy deposits. To clean deposits from the outside of the core, remove the core and tank assembly and plug all openings. Use a mildly organic solution such as Fine Organics® 2223 or Keychem® 06000. Mix 10% of the solution with water and if possible heat to 160 – 180º F (71 – 82º C). Agitating the core will help in removing the contaminants.  Ultrasonic equipment is effective in breaking up the deposits.  Once a year the piping should be disconnected and either of the above-mentioned solutions used to clean the internal tubes. When flushing, circulate the oil in the opposite direction of the normal oil flow.  Once cleaning is complete, flush the unit with oil to avoid rust formation on the internal surfaces.

7. Accumulators

Hydraulic accumulators can be used for two different purposes, depending upon their pre-charge. The pre-charge is the nitrogen pressure with no oil in the accumulator:

  • Add additional volume to the system at a very fast rate (or maintain pressure in the event of a power failure)
  • Absorb shock

Volume accumulators are typically much larger than shock accumulators. They are to be pre-charged well below system pressure so that oil can enter the accumulator. In the absence of designer specifications, we recommend that it be pre-charged to one-half of the maximum system pressure. If it is not specifically used to maintain pressure in the event of a power failure and regularly discharges to the system during high volume requirement portions of the machine cycle, more heat should be felt on the bottom one-half to two-thirds of the accumulator shell than on top. If heat is only felt on the bottom of the accumulator shell, it is overcharged. If heat is felt all over the shell, it is undercharged.

Overcharged

Piston accumulators will overcharge during day-to-day use as oil bypasses the piston and collects on top, displacing nitrogen. The diminished capacity can be regained by installing the charging rig and cracking open the bleeder valve while the system is pressurized. System pressure will push the piston to the top, removing all the nitrogen and the bypassed oil. When oil stops coming out of the bleeder valve, the piston is at the top. The accumulator can then be isolated from the system, the dump valve opened and the accumulator can be properly pre-charged, regaining its diminished capacity. If the oil never stops coming out, the piston is badly worn and must be replaced.

Undercharged

If the system is shut down for a long period of time, nitrogen can bypass the piston and go out into the system. This does not necessarily mean that the piston is badly worn. The nitrogen can simply be replaced to bring the pre-charge back up to normal. Nitrogen can also be lost to atmosphere through a leaky valve core. Always use a high-pressure valve core replacement, not the low-pressure car tire type.

Using A Charging Rig

Bladder accumulators have a rubber bladder that conforms to the shape of the shell. If no heat is felt on a bladder accumulator that is being used for volume purposes other than maintaining pressure for emergency use, one of three things has happened:

  • The accumulator has been pre-charged above the maximum system pressure.
  • The accumulator has lost its pre-charge.
  • The bladder has ruptured.

We recommend that the pre-charge of any accumulator should be checked at least twice per year. On volume accumulators, the pre-charge can be checked either with the charging rig or by watching the gauge drop at shutdown. If checking with the charging rig, the accumulator must either be isolated from the system or the system must be shut down. The dump valve must also be open so there is no oil in the accumulator. Turning the gas chuck handle clockwise to compress the pin in the Schrader valve and read the nitrogen pressure on the gauge. The system must be shut down to read the nitrogen pre-charge hydraulically. When first shut down, pressure will be locked in the system unless there is an automatic dump valve. The pressure reading on the system gauge will drop very slowly when the dump valve opens because of the downsized dump line (the downsized dump line keeps the fluid in the accumulator from dumping too rapidly into the tank, stirring up sludge and plugging the suction strainer) until it reaches the current pre-charge. Once the needle drops to the current pre-charge, it will drop immediately to 0 PSI. A shock accumulator will have little or no oil in it, so no heat should be felt on its shell. The pre-charge can only be checked by attaching the charging rig. Also, there is typically no isolation or dump valve, so the pre-charge can only be checked when the system is shut down and pressure is bled from the line.

8. Leaks

Most industrial plants measure the cost of a leak by the amount of oil that is lost. This is only one of the many costs of a leak and is not even the most expensive one. Whenever oil has a way out of the machine, contaminants have a way in and leaks can get bad enough to impact a system’s performance.

Products such as Oil-Dri® and absorbent pads used to clean up leaked oil cost more than the oil they pick up, so the cost of the oil can be more than doubled once this cost is included, not to mention the time spent cleaning it up.

Oil on the floor is a serious safety hazard. According to OSHA, over one million American workers per year are injured by falling on slippery surfaces and they are the cause of over 15% of accidental deaths. As reported in 2013 by National Safety Council, “fall from the same level” ($7.94 billion) and “fall to lower level” ($5.35 billion) were the second and third highest injury causes of disabling workplace injuries in 2011.

Leaks should be tagged immediately upon discovery and repaired during the next scheduled outage.

Most industrial plants measure the cost of a leak by the amount of oil that is lost. This is only one of the many costs of a leak and is not even the most expensive one. Whenever oil has a way out of the machine, contaminants have a way in and leaks can get bad enough to impact a system’s performance.

Products such as Oil-Dri® and absorbent pads used to clean up leaked oil cost more than the oil they pick up, so the cost of the oil can be more than doubled once this cost is included, not to mention the time spent cleaning it up.

Oil on the floor is a serious safety hazard. According to OSHA, over one million American workers per year are injured by falling on slippery surfaces and they are the cause of over 15% of accidental deaths. As reported in 2013 by National Safety Council, “fall from the same level” ($7.94 billion) and “fall to lower level” ($5.35 billion) were the second and third highest injury causes of disabling workplace injuries in 2011.

Leaks should be tagged immediately upon discovery and repaired during the next scheduled outage.

9. Pipe Clamping

Only designated hydraulic clamps should be used to anchor hydraulic pipes, never electrical conduit clamps or other types of clamping. A proper hydraulic pipe clamp will be fashioned of a semi-flexible material such as santoprene or polypropylene braced by steel top and bottom. They should be welded, not bolted, spaced no further apart than five feet and within six inches of any termination. Clamping should be inspected quarterly, tagged immediately and corrected as soon as possible.

10. Hose Length

When determining the correct hose length, remember that the length will change 2 – 4% under pressure through expansion or contraction. Machine vibration and motion should be considered when planning hose routing. For components that move, such as a traveling cylinder, be sure to allow enough length so that the fittings are not subjected to any sort of pull-off force when the limit of travel is reached.

Never stretch a hose. This can restrict fluid flow. Hoses should not be long enough to require clamping, but if it cannot be avoided, never clamp a hose at its bend. Allow for length changes when the hose is pressurized. High- and low-pressure lines should not be clamped together. When replacing a hose, always cut the new hose the same length as the one being removed. Remember that hydraulic hoses should usually not exceed four to six feet.

Hose Protection

Ideally, hoses should not touch each other or anything else. When it is not possible to avoid this, use protective sleeves to protect the hose. Hoses rubbing against other hoses or any surface are simply leaks waiting to happen, but it is a rare plant indeed where we don’t find several examples of this. There are a number of excellent products available to protect hoses and all of them that we have seen do a superb job.