Another Reason For Noise In Your System That Can Give You Problems

There is another intermittent and problematic source of noise in hydraulic systems – decompression.

This problem arises because hydraulic oil is NOT incompressible. The ratio of a fluid’s decrease in volume as a result of increase in pressure is given by its bulk modulus of elasticity.

The bulk modulus for hydrocarbon-based hydraulic fluids is approximately 250,000 PSI, (17,240 bar) which results in a volume change of around 0.4% per 1,000 PSI (70 bar).

When the change in volume exceeds 10 cubic inches (160 cubic centimeters) decompression must be controlled.

The compression of hydraulic fluid results in storage of energy, similar to the potential energy stored in a compressed spring. Like a compressed spring, compressed fluid has the ability to do work.

If decompression is not controlled, the stored energy dissipates instantaneously. This sudden release of energy accelerates the fluid, which does work on anything in its path.

Uncontrolled decompression stresses hydraulic hose, pipe and fittings. It creates noise and can cause pressure transients that can damage hydraulic components.

Decompression is an inherent problem in hydraulic presses for example, due to the large volume cylinders operating at high pressures.

Although hydrocarbon-based hydraulic fluids compress 0.4% – 0.5% by volume per 1,000 PSI, in actual application it is wise to calculate compression at 1% per 1,000 PSI. This compensates for the elasticity of the cylinder and conductors and a possible increase in the volume of air entrained in the fluid.

For example, if the combined captive volume of the hydraulic cylinder and conductors on a press was 10 gallons and operating pressure was 5,000 PSI, the volume of compressed fluid would be 0.5 gallons (10 x 0.01 x 5).

This equates to potential energy of around 33,000 watt-seconds. If the release of this amount of energy is not controlled, you can expect to hear a bang!

Decompression is controlled by converting the potential energy of the compressed fluid into heat. This is achieved by metering the compressed volume of fluid across an orifice.

Craig Cook

Noisy Hydraulics

Noise level in the workplace is always an issue. The high power density and corresponding high noise emission of hydraulic components means that industrial hydraulic systems are often the target of efforts to reduce noise levels in the workplace.

The dominant source of noise in hydraulic systems is the pump. The hydraulic pump transmits structure-borne and fluid-borne noise into the system and radiates air-borne noise.

All positive-displacement hydraulic pumps have a specific number of pumping chambers, which operate in a continuous cycle of opening to be filled (inlet), closing to prevent back flow, opening to expel contents (outlet), and closing to prevent back flow.

These separate but superimposed flows result in a pulsating delivery, which causes a corresponding sequence of pressure pulsations. These pulsations create fluid-borne noise, which causes all downstream components to vibrate.

The pump also creates structure-borne noise by exciting vibration in any component with which it is mechanically linked, e.g. tank lid.

The transfer of fluid and structure induced vibration to the adjacent air mass resulting in air-borne noise.

Reducing fluid-borne noise

While fluid-borne noise attributable to pressure pulsation can be minimized through hydraulicpump design, it cannot be completely eliminated. In large hydraulic systems or noise-sensitive applications, the propagation of fluid-borne noise can be reduced by the installation of a silencer.

The simplest type of silencer used in hydraulic applications is the reflection silencer, which eliminates sound waves by superimposing a second sound wave of the same amplitude and frequency at a 180-degree phase angle to the first.

Reducing structure-borne noise

The propagation of structure-borne noise created by the vibrating mass of the power unit (thehydraulic pump and its prime mover) can be minimized through the elimination of sound bridges between the power unit and tank, and the power unit and valves.

This is normally achieved through the use of flexible connections i.e. rubber mounting blocks and flexible hoses, but in some situations it is necessary to introduce additional mass, the inertia of which reduces the transmission of vibration at bridging points.

Reducing air-borne noise

The magnitude of noise radiation from an object is proportional to its area and inversely proportional to its mass. Reducing an object’s surface area or increasing its mass can therefore reduce its noise radiation.

For example, constructing the hydraulic reservoir from thicker plate (increased mass) will reduce its noise radiation.

The magnitude of air-borne noise radiated directly from the hydraulic pump can be reduced by mounting the pump inside the tank. For full effectiveness, there must be a clearance of 0.5 meter between the pump and the sides of tank, and the mounting arrangement must incorporate decoupling between the power unit and tank to insulate against structure-borne noise.

The obvious disadvantage of mounting the hydraulic pump inside the tank is that it restricts access for maintenance and adjustment.

If hydraulic system noise remains outside the required level after all of the above noise propagation countermeasures have been exhausted, encapsulation or screening must be considered.

Craig Cook

Hydraulic Filter Condition Monitoring

Continuous monitoring of the filter elements in a hydraulic system can provide valuable clues to the performance of the filter and the condition of the system.

Before I discuss this, let’s consider some of the advantages and disadvantages of common filter locations.

Pressure filtration

Locating filtering media in the pressure line provides maximum protection for components located immediately downstream. Filtration rates of two microns or less are possible, due to the positive pressure (in comparison to an intake line filter) available to force fluid through the media (in comparison to an intake line filter). Filter efficiency may be reduced by the presence of high flow velocities, and pressure and flow transients, which disturb trapped particles. The major disadvantage of pressure filtration is economic. Because the housings and elements (high-collapse type) must be designed to withstand system operating pressure, pressure filtration has the highest initial and ongoing cost.

Return filtration

The rationale for locating filtering media in the return line is this: if the reservoir, and the fluid it contains, start out clean and all air entering the reservoir and returning fluid is adequately filtered, then fluid cleanliness will be maintained. The other advantage of the return line as a filter location is that sufficient pressure is available to force fluid through fine media – typically 10 microns, but pressure is not high enough to complicate filter or housing design. This combined with relatively low flow velocity, means that a high degree of filtering efficiency can be achieved at an economical cost. For these reasons, return filtration is a feature of most hydraulic systems.

Off-line filtration

Off-line filtration enables continuous, multi-pass filtration at a controlled flow velocity and pressure drop, which results in high filtering efficiency. Filtration rates of two microns or less are possible, and water absorbent filters and heat exchangers can be included in the circuit for total fluid conditioning. Off-line filtration has a high initial cost, although this can often be justified on a life-of-machine cost basis.

Filter condition monitoring

Warning of filter-bypass is typically afforded by visual or electric clogging-indicators. These devices indicate when pressure drop (delta P) across the element is approaching the opening pressure of the bypass valve (where fitted). In the case of a return filter for example, if the bypass valve opens at a delta P of 3 Bar, the clogging indicator will typically switch at 2 Bar.

Advanced filter condition monitoring

Replacing standard clogging-indicators with differential pressure gauges or transducers enables continuous, condition monitoring of the filter element. This permits trending of fluid cleanliness against filter element pressure-drop, which may be used to optimize oil sample and filter change intervals. For example, the optimal change for a return filter in a particular system could be higher or lower than the clogging indicator switching pressure of 2 Bar.

Continuous monitoring of filter pressure drop can also provide early warning of component failures and element rupture. For example, if the delta P across a pressure filter suddenly increased from 1 to 3 Bar (all other things equal), this could be an indication of an imminent failure of a component upstream. Similarly, a sudden decrease in delta P could indicate a rupture in the element – something that a standard clogging indicator will not warn of.

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

Symptoms of Common Hydraulic Problems and Their Root Causes

Abnormal Noise

Abnormal noise in hydraulic systems is often caused by aeration or cavitation. Aeration occurs when air contaminates the hydraulic fluid. Air in the hydraulic fluid makes an alarming banging or knocking noise when it compresses and decompresses, as it circulates through the system. Other symptoms include foaming of the fluid and erratic actuator movement. Aeration accelerates degradation of the fluid and causes damage to system components through loss of lubrication, overheating and burning of seals.

Air usually enters the hydraulic system through the pump’s inlet. For this reason, it is important to make sure pump intake lines are in good condition and all clamps and fittings are tight. Flexible intake lines can become porous with age; therefore, replace old or suspect intake lines. If the fluid level in the reservoir is low, a vortex can develop, allowing air to enter the pump intake. Check the fluid level in the reservoir, and if low, fill to the correct level. In some systems, air can enter the pump through its shaft seal. Check the condition of the pump shaft seal and if it is leaking, replace it.

Cavitation occurs when the volume of fluid demanded by any part of a hydraulic circuit exceeds the volume of fluid being supplied. This causes the absolute pressure in that part of the circuit to fall below the vapor pressure of the hydraulic fluid. This results in the formation of vapor cavities within the fluid, which implode when compressed, causing a characteristic knocking noise.

The consequences of cavitation in a hydraulic system can be serious. Cavitation causes metal erosion, which damages hydraulic components and contaminates the fluid. In extreme cases, cavitation can cause mechanical failure of system components.

While cavitation can occur just about anywhere within a hydraulic circuit, it commonly occurs at the pump. A clogged inlet strainer or restricted intake line will cause the fluid in the intake line to vaporize. If the pump has an inlet strainer or filter, it is important for it not to become clogged. If a gate-type isolation valve is fitted to the intake line, it must be fully open. This type of isolation device is prone to vibrating closed. The intake line between the reservoir and pump should not be restricted. Flexible intake lines are prone to collapsing with age; therefore, replace old or suspect intake lines.

High Fluid Temperature

Fluid temperatures above 180°F (82°C) can damage seals and accelerate degradation of the fluid. This means that the operation of any hydraulic system at temperatures above 180°F is detrimental and should be avoided. Fluid temperature is too high when viscosity falls below the optimum value for the system’s components. The temperature at which this occurs is dependent on the viscosity grade of the fluid in the system and can be well below 180°F.

High fluid temperature can be caused by anything that either reduces the system’s capacity to dissipate heat or increases its heat load. Hydraulic systems dissipate heat through the reservoir. Therefore, the reservoir fluid level should be monitored and maintained at the correct level. Check that there are no obstructions to airflow around the reservoir, such as a build up of dirt or debris.

It is important to inspect the heat exchanger and ensure that the core is not blocked. The ability of the heat exchanger to dissipate heat is dependent on the flow rate of both the hydraulic fluid and the cooling air or water circulating through the exchanger. Therefore, check the performance of all cooling circuit components and replace as necessary.

When fluid moves from an area of high pressure to an area of low pressure without performing useful work (pressure drop), heat is generated. This means that any component that has abnormal internal leakage will increase the heat load on the system. This could be anything from a cylinder that is leaking high-pressure fluid past its piston seal, to an incorrectly adjusted relief valve. Identify and change-out any heat-generating components.

Air generates heat when compressed. This means that aeration increases the heat load on the hydraulic system. As already explained, cavitation is the formation of vapor cavities within the fluid. These cavities generate heat when compressed. Like aeration, cavitation increases heat load. Therefore, inspect the system for possible causes of aeration and cavitation.

In addition to damaging seals and reducing the service life of the hydraulic fluid, high fluid temperature can cause damage to system components through inadequate lubrication as a result of excessive thinning of the oil film (low viscosity). To prevent damage caused by high fluid temperature, a fluid temperature alarm should be installed in the system and all high temperature indications investigated and rectified immediately.

Slow Operation

A reduction in machine performance is often the first indication that there is something wrong with a hydraulic system. This usually manifests itself in longer cycle times or slow operation. It is important to remember that in a hydraulic system, flow determines actuator speed and response. Therefore, a loss of speed indicates a loss of flow.

Flow can escape from a hydraulic circuit through external or internal leakage. External leakage such as a burst hose is usually obvious and therefore easy to find. Internal leakage can occur in the pump, valves or actuators, and unless you are gifted with X-ray vision, is more difficult to isolate.

As previously noted, where there is internal leakage there is a pressure drop, and where there is a pressure drop heat is generated. This makes an infrared thermometer a useful tool for identifying components with abnormal internal leakage. However, temperature measurement is not always conclusive in isolating internal leakage and in these cases the use of a hydraulic flow-tester will be required.

The influence of internal leakage on heat load means that slow operation and high fluid temperature often appear together. This can be a vicious circle. When fluid temperature increases, viscosity decreases. When viscosity decreases, internal leakage increases. When internal leakage increases, heat load increases, resulting in a further increase in fluid temperature and so the cycle continues.

Proactively monitoring noise, fluid temperature and cycle times is an effective way to detect conditions that can lead to costly component failures and unscheduled downtime of hydraulic equipment. In most cases, informed observation is all that is required.

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

Understanding Valve Leakage

Let’s talk about valve leakage.

There are two main types of valve designs used in hydraulic systems.

Spool-type
Poppet-type

In a spool design, a spool is positioned in its bore to connect the various ports in the valve. The most common type of spool valve we’re all familiar with is the directional control valve.

Because radial clearance is required for the spool to slide in its bore, this valve design in not leakless. To say this another way, even when a port in a spool valve is closed off – a small amount of leakage is possible and should be expected.

In a poppet design, the valve ‘poppet’ closes against a seat. This design is generally considered leakless. That is, if the valve is closed and the poppet and its seat are in good condition – there is no leakage across the valve’s ports.

BUT there’s an important exception to this rule you should be aware of. Slip-in cartridge valves, also called logic elements are a type of poppet valve commonly found in today’s hydraulic systems.

Even though a logic element can be configured for flow in two directions, it is only ‘leakless’ in one direction.

Craig Cook

Hydraulic Troubleshooting – Basics

Hydraulic Troubleshooting Basics

Here is a hypothetical hydraulic problem and how to trouble shoot it.

“We have a hydraulic system that operates two cylinders. The pump (piston-type) has failed – for reasons unknown at this time. The tank, valves and cylinders were cleaned and a replacement pump installed. The new pump is delivering a maximum pressure of 1,000 PSI and appears to be creating heat.”

In any troubleshooting situation, no matter how simple or complex the hydraulic system, always start with the basics. This ensures that the obvious is never overlooked. In order for the ‘obvious’ to be obvious, the fundamental laws of hydraulics must be kept in mind:

Hydraulic pumps create flow – not pressure.
Resistance to flow creates pressure.
Flow determines actuator speed.
Pressure determines actuator force.
Fluid under pressure takes the path of least resistance.
When fluid moves from an area of high pressure to an area of low pressure (pressure drop) without performing useful work, heat is generated.

Theory is great, but it always makes more sense when put into practice. So let’s apply these fundamentals to the above situation in a way that ensures the obvious things are not overlooked.

“The new pump is delivering a maximum pressure of 1,000 PSI…”

We know that a hydraulic pump can only produce flow (pressure is created by resistance to flow). It follows that if the pump can’t get oil it can’t produce flow. So, check that the reservoir is filled to the correct level, the breather is not clogged, the suction strainer or filter (if fitted) is not clogged, the pump intake isolation valve is fully open, and the pump intake line is otherwise unrestricted.

If the pump is producing flow, then an absence of pressure indicates an absence of resistance to flow. Knowing this, and that fluid under pressure always takes the path of least resistance, the task now is to find the point at which pump flow is escaping from the circuit. If you’re skilled in reading and interpreting hydraulic symbols, the system’s schematic diagram (if available) can be useful in identifying possible locations.

“The new pump… appears to be creating heat.”

Because heat is generated when there is a pressure drop, using an infrared thermometer to check the temperature of individual components will quickly lead us to the hottest part of the system – and the probable location of the internal leakage. Note that in a properly functioning system fitted with a piston pump, it is not unusual for the pump case to be the hottest part of the circuit.

The above checks should have taken less than 10 minutes. If nothing conclusive was revealed, I would continue the process of elimination using a flow-tester to conduct a direct pump test.

Fitting Selection: The Key to Leak-Free Hydraulic Plumbing

Reliable Connections

Leak-free reliability begins at the design stage, when the type of hydraulic fitting is selected for port, tube-end, and hose-end connections.

Ports

Connectors that incorporate an elastomeric seal such as UNO, BSPP, and SAE 4-bolt flange offer the highest seal reliability. NPT is the least reliable type of connector for high-pressure hydraulic systems because the thread itself provides a leak path. The threads are deformed when tightened and, as a result, any subsequent loosening or tightening increases the potential for leaks. In existing systems, pipe thread connections should be replaced with UNO or BSPP for leak-free reliability.

Tube and Hose Ends

ORFS tube and hose end connections feature the high seal reliability afforded by an elastomeric seal, but due to its cost, ORFS is not as widely used as compression fittings and JIC 37-degree flare.

Flared connections have gained widespread acceptance due to their simplicity and low cost. However, the metal-to-metal seal of the flare means that a permanent, leak-free joint is not always achieved, particularly in the case of tube-end connections.

Leaking flare joints can be eliminated using a purpose-built seal developed by Flaretite. The Flaretite seal is a stainless steel stamping shaped like a JIC nose, with concentric ribs that contain pre-applied sealant. When tightened, the ribs crush between the two faces of the joint, eliminating any misalignment and surface imperfections. The combination of the crush on the ribs and the sealant ensure that a leak-free joint is achieved.

Incorrect Torque

A common cause of leaks from flare joints is incorrect torque. Insufficient torque results in inadequate seat contact, while excessive torque can result in damage to the tube and fitting through cold working. The following is a simple method to ensure flare joints are correctly torqued:

Finger tighten the nut until it bottoms on the seat.
Using a permanent marker, draw a line lengthwise across the nut and fitting.
Wrench tighten the nut until it has been rotated the number of hex flats listed in the following table:

Vibration

Vibration can stress plumbing, affecting hydraulic fitting torque and causing fatigue. Tube is more susceptible than hose. If vibration is excessive, the root cause should be addressed. Ensure all conductors are adequately supported and if necessary, replace problematic tubes with hose.

Seal Damage

Having outlined the benefits of hydraulic fittings that incorporate an elastomeric seal, it is important to note that their reliability is contingent on fluid temperature being maintained within acceptable limits. A single over-temperature event of sufficient magnitude can damage all the seals in a hydraulic system, resulting in numerous leaks.

A leak-free hydraulic system should be considered the norm for modern hydraulic machines – not the exception. But the proper selection, installation and maintenance of hydraulic plumbing are essential to ensure leak-free reliability.

Craig Cook