Power Savings in Hydraulic Systems

Hydraulic Power and its conversion

As in all of nature, hydraulic energy cannot be created or destroyed, rather converted from one form to another. Electrical or mechanical energy of the motor or engine is transferred to the pump which in turn develops flow and pressure in hydraulic oil. The oil does work converting hydraulic energy to useful motion (kinetic or potential) which is what we need.

Sadly, no energy conversion is 100% efficient. There is always an amount that is dissipated or lost in translation from one form to another. A good hydraulic engineer does not aim to make a system 100% efficient; rather, he strives to reduce energy wastage as much as possible.

The power conversion in a system is given by the following formula:

P_INPUT = P_OUTPUT x η

Where η is the efficiency of the system.

Alternatively, we can also write it as follows:

P_INPUT = P_OUTPUT - ∑P_LOSS

Where ∑P_LOSS is the sum of all the Power Losses in the system components and is given by:

∑P_LOSS = P_{LOSS PUMP}+P_{LOSS VALVES}+P_{LOSS PIPES}+P_{LOSS FITTINGS}+P_{LOSS ACTUATORS}

Power losses in the system components come in many forms. The two predominant forms of energy waste are heat and noise (component vibrations). While both are caused by varying factors, in this article we aim to reduce hydraulic system heat generation.

Hydraulic Oil Heating

Hydraulic Power Units need effective oil-cooling
mechanisms to maintain fluid temperature.

In any hydraulic system, the oil medium is bound to heat up. Constantly being under pressure for most of the cycle, their molecular friction generates heat when the fluid is sheared. The higher the viscosity of the oil, higher is the friction produced. The oil molecules then radiate their kinetic energy in the form of heat. The chemical composition and grade of oil suggested for a system is such that it allows for normal working up to a temperature higher than that of ambient conditions.

As a generalization, commonly used hydraulic oil is rated for temperatures up to 90°C at the higher end of the spectrum. The problem arises when overheating occurs. When the system capability to dissipate heat from the oil is less than the heat generated, over heating of the oil is imminent.

The heat generated must be dissipated from the oil into the environs in some way or another else temperatures in the system will escalate until there is a heat balance (where the heat dissipated into the surroundings is balanced by the generated heat in the system) at an undesired extreme temperature or the system is damaged by the effects of heat. Temperatures above normal may not only affect the efficiency of the system, but also the working of seals and cause degradation of oil, both of which will be addressed separately.

Shell and Tube Type Heat Exchangers

For example, a system working at 200 bar pressure and having a flow of 90 lpm would produce 30kW of output power. For a system with 80% efficiency, this would mean heat generation of 7.5 kW. Hence, the system would need to be able to dissipate 7.5 kW of heat to be able to function up to standard.

An increase in temperature or a decrease in the dissipation capacity would cause a change in the balance between heat load and heat dissipation.

Hence it is evident that there are only two ways to address overheating issues.

1. Reduce the heat generated by the system
2. Increase the heat dissipation capacity

Although the latter saves the system from overheating, the former has a two fold effect. It reduces overheating as well as improves system efficiency!

Reducing system heat generation

Wasted power can be drastically reduced by designing your systems to develop less heat. Starting from the heart of the system, the pump, using load sensing pumps to reduce excess power consumption can ensure that the system develops only as much pressure as is needed. Valve pressure drops should be kept to a minimum to avoid over-working the pump. Ports and fittings should be sized properly and hoses and pipes should be laid out in a manner that reduces turbulence in fluid flow. Bypass-type flow control valves, which divert excess flow to tank without raising the pressure, are also a great way to decrease the power footprint and in effect heat generation.

Load Sensing Pumps

Load Sensing Pumps
Load sensing is a term used to describe a type of variable pump control used in open circuits. It is so called because the load-induced pressure downstream of an orifice is sensed and pump flow is adjusted to maintain a constant pressure drop (Δp) across the orifice.

The system typically contains a variable displacement pump typically of an axial-piston design which is fitted with a load-sensing controller, directional control valves having proportional characteristics and an integral load-signal gallery. The load signal gallery is a logic system of shuttle valves which returns the highest pressure in the A or B lines of the various directional control valves back to the load-signal port on the pump.

Load Sense Pumps with Mobile Control Valves. Pressure Lines are shown in RED, 
Tank Lines in GREEN and the Load Sense Gallery in BLUE

To understand a load-sensing system, let us take an example where a single rotary actuator is being driven by the pump. If the pump delivers 50 lpm of flow at 100 bar and the pressure drop across the directional control valve is 15 bar, the motor will operate at a particular speed (say 10 rpm). A load acting on the motor will cause the pressure to increase downstream of the directional control valve. This results in a decrease in pressure drop across the valve which in turn reduces the flow across it. Reduced flow to the motor would then decrease its speed which is undesirable.

In a load sensing circuit, the pressure drop (Δp) across the directional control valve is kept constant by giving a feedback to the pump. With an increase in load pressure, the pressure drop across the valve is reduced. The load pressure is sensed by the pump which then slightly increases its output. This increases the upstream pressure and keeps the pressure drop across the valve and hence the flow rate to the motor, constant.

Load-sensing pumps are even capable of providing accurate and constant flow independent of shaft speed deviation. If the electric motor’s speed decreases, the pump will increase its displacement to maintain the same flow rate.

The pressure drop (Δp) maintained across the directional control valve is typically between 10 to 30 bar. When the pump is not in used, the load-signal port is vented to tank and the pump remains at an interim pressure which is equal to or slightly higher than Δp. The load sensing pump only delivers flow as required by the system and operates at pressures only marginally higher than needed, the are extremely energy efficient. Hydraulic systems with a wide range of fluctuations in flow and pressure can save substantial reduction in amounts of input power.

If we were to graphically compare fixed displacement and pressure compensated and load sensing pumps, we could map the amount of power saved in each case. Let us say there is a system with maximum flow requirements of Q lpm and maximum pressure requirements of P bar. The interim flow requirement and load induced pressure is given by ‘q’ and ‘p’ respectively. We would have graphs as shown in the below figure.

In systems where all the available flow (Q lpm) is continuously converted to useful work, the amount of input power lost to heat is only limited by the efficiency of the system. For systems fitted with fixed displacement pumps where the flow required is q lpm at P bar, the flow not required passes over the system relief valve and is converted to heat. This situation is compounded if the load-induced pressure (p) is less than the set relief pressure - resulting in additional power loss due to pressure drop across the directional control valve.

A similar situation occurs in systems fitted with pressure controlled (pressure compensated) variable pumps, when only a portion of available flow is required at less than maximum system pressure. Because this type of control regulates pump flow at the maximum pressure setting, power is lost to heat due to the potentially large pressure drop across the metering orifice.

A load sensing controlled variable pump largely eliminates these inefficiencies. The power lost to heat is limited to the relatively small pressure drop across the metering orifice, which is held constant across the system's operating pressure range

Bypassing Unused Flow
Most Load Sensing Pumps are quite an expensive investment, although they cover the costs fairly quickly. The alternative is using fixed displacement pumps where power can be saved not by the pump itself but by using bypass systems which direct excess flow from pump to tank at low pressures. Although these may not be as effective as Load Sensing pumps since the latter varies its displacement according to the demand and thus has hardly any excess flow, the system is beneficial in its own right when compared to not doing anything at all!

Dumping Pump Flow by sensing pressure requirements

Pump Bypass Valves use logic elements to collect feedback from the system in the same way as a load signal gallery would with load sensing pumps. The feedback then either opens or closes the logic element to dump flow to tank if the flow required is less or more respectively. There are other additions that can be made to the circuit such as combining the system relief valve into the fold or even including a venting solenoid to keep the pump on standby from a remote location.

Dumping unrequired flow by using bypass type flow control valves.

In the same league of bypassing flows from pumps are pressure compensated flow control valves. These can be used to replace simple flow regulation with flow diversion. Restrictive type pressure compensated flow control valves still build up pressure up-stream of the valve forcing excess flow to pass over the relief valve and cause heating of the system. Bypass type pressure compensated valves, on the other hand, divert excess flow to the tank line, dumping flow at a lower pressure and ensuring that the pump never develops pressure more than a few bars of the load pressure. The valves act as flow diverters providing excess flow when the downstream pressure increases to compensate for the reduction in Δp which causes a drop in flow rate and to reduce the flow when Δp is high and is causing and increase in flow rate. This provides an even flow rate regardless of downstream pressure while keeping the pump at a lower pressure level.

Keeping Valve Pressure Drops to a Minimum

The days of selecting a valve based upon the size of the piping are gone. Selecting the correct valve size for a given application requires knowledge of process conditions that the valve will actually see in service. The technique for using this information to size the valve is based upon a combination of theory and experimentation.

Pressure drop is a critical element in valve sizing and valve application. Pressure drops of valves for flow rates must be known by the engineer designing the system to ensure proper valve selection. Critical factors determining pressure drops through valves are the port entry sizes in the valve and the oil’s internal flow path through the valve.

As in all hydraulics, pressure drop across the valve and the flow rate through it are dependant on one another. The higher the flow rates through the valve, the greater the pressure drops. Conversely, the lower the flow rates, the lower the pressure drops.

Improper valve sizing can be both, expensive and inconvenient. A valve that is too small will not pass the required flow, and in the process can either result in large pressure drops and consequently consume more power, or starve the system of oil causing damage. An over sized valve will be more expensive and may lead to some instability. As a rule of thumb, it is always best to use an over sized valve rather than an undersized one.

The graph shows how the same valve can be rated for two
different flow rates depending on the standard pressure drop.

 To understand the pressure drops a valve may face in the field, it is easiest to refer to the manufacturer’s manual. We say easiest since most curves in manuals aren’t very accurate and are usually airbrushed to make the valve more marketable. Although not the ideal solution, it is best to experimentally check pressure drops across valves yourself before finalizing on the product.

An actual pressure drop curve and a catalogue curve are usually quite different. Also, various manufacturers have various methods to rate their valves for flow. While certain manufacturers may give the maximum flow a valve can take before it affects the working of the internal components, others may have flow rates depending upon how much flow effects a standard value of pressure drop across the valve. Some of the North American valve manufacturers rate their valves for 30 bar (420 psi) pressure drops. European and British manufacturers rate them for 7 bar (100 psi).

Ideally, pressure drops should be as low as possible; somewhere between 4 and 7 bar (60-100psi). Since valves connected in series have additive pressure drops, care must be taken to account for the entire pressure drop across a line.

It is best to read the graphs of the valve in various sizes and then determine which is best suited for the application.

Proper Port and Fitting Sizing

Just like in valves, ports and fittings have to be sized effectively. Undersized ports and fittings may cause increased fluid velocity and greater pressure drops while over sized pipes may result in sluggish movement of the system.

As a rule of thumb, for BSP ports, the following chart may be used to determine the flow rates

Port Size	Max. Bore	Flow Rate
1/8” BSP	7 mm	Up to 15 lpm
1/4" BSP	10 mm	Up to 30 lpm
3/8” BSP	12 mm	Up to 60 lpm
1/2" BSP	18 mm	Up to 100 lpm
3/4" BSP	22 mm	Up to 170 lpm
1” BSP	28 mm	Up to 220 lpm
1.1/4” BSP	36 mm	Up to 350 lpm
1.1/2” BSP	42 mm	Up to 500 lpm

Proper Hose Arrangements

Hoses provide flexible connections between two points. Typically a rubber hose is constructed of an extruded inner synthetic rubber tube that has the sole purpose to keep the conveyed fluid in the hose. The elastomeric nature of rubber requires that a reinforcement layer be wound or braided around the tube in order to hold the internal pressure. The reinforcement layer(s) are either textile or steel (or both). To protect these inner layers of the hose from the ambient conditions, an outer synthetic rubber cover is extruded around the reinforcement. Their ends may be permanent or reusable. The latter facilitates fitting over and over again permitting assembling required lengths of hoses at site. Hose ends can be threaded (DIN, NPT, BSP, GAS, SAE, JIS, Metric etc), flanged (SAE3000, SAE 6000, 4 Bolt etc) or even welded.

Out of the threaded connections, NPT is the least reliable type of threaded connection 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. Replacing pipe thread connections (BSPT or NPT) with soft seal connections (UNO or BSPP) greatly improves 'no-leak' reliability since leakages are now prevented with a sealing element (washer, O-ring etc)

Hoses need adequate care to be exercised in their application and installation as regards to operation within permissible limits of pressure, temperature and bend radius. In the case of hoses, manufacturers estimate that 80% of hose failures are attributable to external physical damage through stretching, kinking, crushing or abrasion of the hose. To prevent hose damage, all clamps have to be kept secure, careful attention should be paid to routing whenever a replacement hose is installed and if necessary, polyethylene spiral wrap can be used to protect hoses from external abrasion.

The minimum bend radius of a hose is the minimum radius that the hose may be bent through at the maximum allowable working pressure. Bending radius is not a measurement or indicator of hose flexibility and is usually specified by international or manufacturer specific standards. Bending the hose below the minimum bending radius leads to a loss of mechanical strength and can result in possible hose failures. A minimum straight length of 1.5 times the hose’s outside diameter should be kept between the hose fitting and the point at which the bend starts.

Allowable bending in Hydraulic Pipes

Following the above guidelines will ensure a smooth flow path for fluid through hoses. This results in lesser turbulence of the hydraulic oil and reduced vibrations and heat generation within the system.

Conclusion

Heat generation due to lack of heat quality dissipating apparatus and improper components are sure fire ways to distinguish between a well designed or a badly designed system.

The topics covered above are only a few ways one can increase the efficiency and reduce the heat generation within their hydraulic system. The methods to better your systems performance are limitless and there are always newer technologies being developed to increase efficiency.