Fluid Power Journal

Begin at the End

I was recently at a conference addressing the issue of efficiency in fluid power systems, specifically in hydraulics.  The conference was focused on existing systems rather than designing new ones.  The question was asked, “How do we know if an existing hydraulic system is efficient?”  Now, in all my years (I won’t say how many) of working with fluid power, I had never been asked that question before.  I design systems, I have fixed systems, and I have improved the efficiency of systems, but have never had to explain to others the best process of making the determination of the relative efficiency of a system.

The audience was a group of degreed educators from universities across the nation.   Part of their responsibility is to assist industries in becoming more efficient and therefore more competitive.  When it came to hydraulic systems within a plant, they were at a loss as to what to do. They were not looking for Hydraulics 101 but for tools they could use to determine whether or not an existing hydraulic system was sufficiently inefficient to make it a worthwhile project to address.

I know, I know.  Some of you are screaming, “They need education and certification!”  That was my first thought as well.  But that does not really address the problem.  They are not looking to immediately fix the system, but need to assess the system and then, if appropriate, hand the project over to a fluid power professional for evaluation and possible solutions.  The fluid power professionals certainly need education and certification, especially in the area of energy efficiency, but that is an internal issue within our industry.

I am going to try to accomplish two things as I write this article; I will answer the question regarding assessing the efficiency of a system as an informed observer, and then, look at areas that gnaw away at efficiency and that may provide potential savings, not only in terms of energy, but also in maintenance costs, and productivity.

I had stated in my presentation that, to me, a big heat exchanger is like a carpenter who uses 12” trim.  It is a cover up for the hidden inefficienies of the system.  After my brief presentation, one educator quipped, “So, when we have to evaluate a hydraulic system, we should look for a big heat exchanger.”  I told him to be careful.  A big heat exchanger is a clue but is not exactly a litmus test.  There may be compelling reasons for the heat exchanger due to ambient temperatures or proximity to other heat sources. It may be an oversized heat exchanger that is not being used to its capacity, or it may be used in energy recovery, extracting the heat to be used in another process. In regard to energy recovery, while it is true that energy is wasted it is already being recovered and so time and energy does not need to be spent trying to make the system more efficient.

In another conversation with professionals within the fluid power industry, we were discussing the criteria for establishing standards for efficiency.  We agreed that there has to be an objective way to assess a system that will be a baseline for determining efficiency. We agreed that this baseline needs to be the actual work being accomplished.  This is one of those things that seems so obvious and yet is so often overlooked.

In yet another conversation with one of our maintenance foremen, I asked him how he would determine if a hydraulic system was efficient.  He quickly responded, “Does it do what you need it to do?”  This related to the function, reliability and repeatability of the system.

Hence the title, “Begin at the End.”  What is the actual work to be done and is the system doing it?

When I talk about the work being accomplished, I am not speaking of the maximum power draw, but the average work being done throughout a cycle.   A quick refresher on the meaning of effort, work, and power will be helpful here.  Effort is simply the force applied, whether as torque or linear force.  Work is done when that effort is effective in producing movement.  Power is how fast the work is completed.

For example, last winter, after a rain, the weather turned cold.  I went out to the car and tried to open the door and found that it was frozen shut.  I pulled on it with my normal amount of effort, but it did not move.  I pulled a little harder and it still did not move.  It was not until a provided enough force to break free from the ice that I was able to open the door.  While I was pulling without moving, I was providing effort, but no work was being done.  Only when the door broke free did I actually do work.  When the door finally did break free, all my effort was turned to acceleration and the door opened very fast.  The speed with which the work of opening the door was accomplished is a measurement of power.

When assessing the relative efficiency of a hydraulic system, it is important to keep these simple rules of physics in mind.  When an object is being pushed, pulled, or rotated, work is being done at some rate of speed which determines the power requirement.  When an object is clamped or an actuator is stalled, no work is being done even though a great deal of effort may be applied.  In addition, and this may be a little hard to get into our thinking, when we lift something and lower it back down to its original position, no work is accomplished.  Ok, ok, technically work was done when we lifted the load but then the work was undone when the load was returned; done plus undone equals not done.  In an earlier article, I mentioned a 32,000 pound platen that had to be lifted and lowered for each cycle of the machine.  When the machine was started in the morning, the platen was at its lowest point.  At the end of the day, when the machine was turned off, the platen was again at its lowest point.  From an energy perspective, no work was accomplished.  Everything that was done was later undone.  Potential energy was imparted to the platen at each lift and then that energy was dissipated in heat as the platen was lowered.

In an industrial setting where the prime mover is an electric motor, the relative efficiency of a system, be it hydraulic or mechanical, can easily be determined by comparing the actual work being done at the output of the actuator(s) with the energy input to the prime mover.

In a mobile setting where the prime mover is a combustion engine that is the energy source for a variety of functions, the same assessment can be made at the actuators to determine work accomplished, but the energy imparted to the input shaft of the pump will be where the comparison should be made.

Similarly, if a machine builder wants to determine the efficiency of his system and is tapping into an existing energy source over which he has no control, the work done would be compared to the energy input to the machine.


So, what are some of the indicators of an inefficient hydraulic system?

A large heat exchanger would certainly be a clue, but as mentioned earlier, it is not absolute.

The temperature of the fluid:

The temperature of the fluid in the reservoir can be an indicator, certainly if it is above 45° C.  However, the reservoir temperature can be deceiving.  If it is a nice cool 35° C, it may only indicate an efficient heat exchanger.  The fluid may be screaming across relief valves, through restrictive flow controls, and undersized conductors just before it passes through the heat exchanger.

Dark fluid:

A nastier indicator would be dark fluid.  This is likely fluid that has been oxidized due to high heat.  If the fluid is dark, even if the reservoir is cool, you need to hunt down the heat source in the circuit.  Knowledge of component symbols is important here. It may allow you to determine the heat generating culprits while looking at the circuit diagram.  If you are not comfortable with your circuit reading ability or if the culprit is illusive, a heat gun is a handy tool for finding the hot spots.  The source could turn out to be an inefficient pump, motor, or leaking cylinder which would not be obvious in the circuit diagram.

Frequent fluid replacement:

How often is the hydraulic fluid being changed and why is it being changed?  Changing the fluid on a regular basis could be a symptom of a problem.  If it is simply the habit of the company or the recommendation of the fluid supplier, it may be a waste of resources and a challenge to the environment.  If it is being changed because it has lost its additives, is dark, or is dirty, these are symptoms of another problem.  Heat sources need to be located and addressed, fluid samples taken to determine if additives are needed and to check fluid cleanliness and a proper filtration protocol needs to be put in place.

Oversized electric motors:

This is one of the dirty little secrets of the inefficiency of many of our hydraulic systems. Once you have determined the amount of work that is being accomplished and the time it takes to do the work, you can easily calculate the required kW from the electric motor.  Compare the data on the nameplate of the motor to the calculated kW requirement.  If the nameplate kW is more than 15% higher than the calculated requirement, you may have an opportunity for energy savings.  An electric motor is most efficient when it runs between 85% and 115% of its rated power.  While an inefficient electric motor does not affect the efficiency of our hydraulic circuit, it is an integral part of our system and is adding an unnecessary cost to the owner.

Improper plumbing:

A system with conductors that are too small will waste energy due to the frictional lose through the conductors.  A proper design would be where the fluid velocity is no more than 4.5 liters/second in the pressure line, 3 meters/second in the return line, and 1.2 meters/second in the suction line.  If there are multiple elbows and many fittings, there will also be energy lost in moving the fluid from the pump to the work site.

Multiple hydraulic power units:

When there are a number of hydraulically operated machines in one general area, it will often be the case that each one is oversized or at least overpowered for the machine it controls.  Supplying the systems from one central hydraulic power unit will usually reduce the energy draw, floor space, maintenance costs, and noise.

Noise:

Hydraulic systems do not have to be noisy.  Excess noise may be the sign of a poor installation.  There is a distinct sound accompanying pump cavitation or aeration.  Vibration from misalignment or bearing failure is also an indicator of a potentially costly problem. It is relatively easy to build a hydraulic power unit to be quiet, but it can be very difficult to retrofit a unit to make it quiet.  If you have the luxury of purchasing a new HPU, you should specify a sound specification.  This may challenge the manufacturer but it can be done without a significant price increase as long as it is planned from the beginning.

A reasonably quiet power unit is easier to work around and diagnose the various sounds that can be heard.  The normal drone of the piston pump, the whine of the gear pump, and the whir of the electric motor can be easily recognized against the sound of a relief valve opening or the rush of fluid through the plumbing or across the flow controls.  These latter sounds are an indication of inefficiencies that should be examined.  If flow velocity is too high, there is an energy loss due to friction which will be seen as heat.  High velocity across a restrictive flow control indicates a high pressure differential which again will be inefficiency turned to heat.

Kidney loop with electric motors: 

Many well designed hydraulic power units have a kidney loop that draws fluid from the reservoir and directs it across a filter and a heat exchanger to condition the fluid.  This is performed at relatively low pressure and an electric motor is usually used to drive a fixed displacement pump.  Another electric motor is often used to drive the fan when there is an air cooled heat exchanger.  Both of these electric motors add cost to the installation and operation of the system.  They can be replaced with hydraulic motors using less than 1% of the main pump flow.

An oversized reservoir:

I mention this, not because a large reservoir is in itself an energy waster, but it may carry with it some hidden costs.  First, is the obvious issue of real estate.  In many facilities, space is very valuable and if an oversized reservoir is taking up that space, collecting debris and radiating heat, it can be a maintenance and housekeeping problem.  If there were to be a broken line, the larger reservoir would be likely to lose a substantial amount of fluid before any warning signals were given.  This would have an economic as well as an environmental impact.  An oversized reservoir indicates an excessive installed cost due to the amount of fluid that was purchased.  When the fluid needs to be replaced there will obviously be extra cost.  If the fluid is reconditioned with additives or provided with extra filtration, there will be additional costs in proportion to the amount of unneeded volume.  It may be that a cylindrical reservoir or a new variable volume reservoir could be used instead to make the system more efficient.

Inertia loads:

This may be a hidden energy consumer.  When we have accelerated a load and then need to decelerate it, we need to add a resistive force to control the load. This applies to over-center loads as well as to suspended loads.  An inertia load is usually controlled by adding a restriction to the port on the actuator that is seeing the load.  This restriction is either a flow control or some type of relief valve.  In either case, energy is being dissipated as heat to control the inertia load.  This is energy that was put into the system for work and now is being removed for control.  Many times this energy can be captured and stored for release later.

So, what can we do?

If one or more of these indicators is present, we need to begin at the end and ask if the machine is reliably and consistently doing the job it is needed to do. If not, there is an opportunity to improve productivity by addressing these issues as part of the evaluation.  Continuing to begin at the end, determine the work desired and calculate the actual energy used for that work.  This will provide a baseline to determine the relative efficiency of the system.

Look for parasitic losses:

Many hydraulic systems have been, and continue to be, designed with the idea that the parasitic losses are simply the cost of control that gives hydraulics the wonderful flexibility for which it is known.  Each parasitic component needs to be reviewed to see if there is another, more efficient way to gain the same control.  In this section we will look at some of these components to see just how they consume energy without doing useful work.

Flow control:

Any time we have a fixed displacement pump with restrictive flow controls, energy is being dissipated as heat.  If the flow control is a flow divider, energy is being wasted, even if all the flow is being used. The pressure upstream from the flow divider will have to be at least 6 bar above the highest pressure required by any part of the circuit.  This is because it requires about 6 bar to overcome the internal spring force of the flow divider.  The system relief valve will have to be set 10 to 15 bar above that so the relief valve does not crack open prematurely.  If the circuit allows for the system to stall, all flow will be exhausted across the relief valve at this high pressure.  Even though no work is being accomplished, the system will be at its highest energy usage and at 100% inefficiency.

If each actuator function has a restrictive flow control, energy is being lost in two ways.  First, by definition, more flow is being supplied than is required and the excess will either be dumped across the relief or the by-pass at system pressure.  In addition, the pressure upstream from a restrictive flow control will always be higher than the maximum actuator requirement and the energy will be converted to heat as the pressure is reduced by the restriction in the fluid stream.

The least expensive way to improve the efficiency of a system using a restrictive flow divider is to replace the restrictive divider with a displacement type.  This will not affect the logic of the circuit but will reduce the system pressure.  The pressure upstream from a displacement divider will be some average of the output pressures.  It will be lower than the maximum pressure requirement instead of 6 bar higher.  This will reduce the energy usage during operation and will also reduce the wasted energy during a stall. With a restrictive flow divider there is the danger of pressure intensification, but this can be handled with the relief valve that is usually built into the device.

If the system is driven with a pressure compensated pump, there will still be energy lost when there are restrictive flow controls. The pressure compensator will have to be set at a pressure that is at least six bar higher than the maximum requirement for any actuator. While the system is likely more efficient than with a fixed displacement pump, it is still consuming energy beyond the work requirement.

Accumulators:

We usually think of accumulators as energy saving devices, and correctly so.  However, from another perspective, an accumulator is also an energy waster in the way it is typically used.  Now, before you get mad and stomp off, let me explain.  Let’s say I have a constant speed hydraulic motor that requires 20 lpm at a pressure of 200 bar for 3 seconds to start and then runs at 150 bar for 57 seconds.  It then dwells for one minute.  This is an excellent and typical application for an accumulator.  We can use a pump that produces 10 lpm and store the required volume during the dwell time.  If I use a relatively large accumulator, I can set the maximum pressure at 250 bar.  The savings is clear; I will need 50% of the flow at a 20% increase in pressure, so I have a savings of about 42%.  Not bad.  But from that other annoying perspective, we have also squandered some energy.

As you know, I cannot direct the accumulator flow directly to the motor.  If I did, the motor would go too fast.  I need to add a pressure compensated flow control in the line to the motor to maintain a constant speed.  This flow control has at least a 6 bar pressure drop as discussed earlier.  This makes my minimum pressure requirement 206 bar.  With a fixed displacement pump, the average pressure will be 228 bar.  Using the formula kW = lpm x bar / 600, I find the average power requirement to be 3.8 kW.

If my maximum pressure is 200 bar for three seconds and then 150 bar for 57 seconds, my average pressure will be 152.5 bar. Remember, the cycle time is two minutes, so my average power requirement will be 152.5 bar × 20 lpm ÷ 600÷2 = 2 .54 kW. So, my average power requirement for the two-minute cycle is 2.54 kW.  So you see that with my accumulator, I have an average power input of 3.8 kW. But my actual requirement is only 2.54 kW for an energy loss of 33%. Even if my accumulator were large enough so that my minimum pressure and my maximum pressure were nearly the same, I would still have to operate at 206 bar which would require an output of 3.4 kW; a loss of 26%.

Pressure reducing valves:

The pressure reducing valve by definition is an energy consumer. The pressure reducing valve functions as a variable restriction in the line reducing the pressure from whatever the upstream pressure is to whatever the downstream pressure requirement is. The reduction in pressure in the fluid stream is a power loss that is given off in heat.

Load sensing pumps:

We usually think of a load sensing pump as being a real energy saver. Many times it is. But remember, the load sensing pump is delivering its flow at least 14 bar above the actuator requirements. If more than one actuator is being used, the one with the highest pressure requirement will be controlling the load sensing pump. Actuators with lower pressure requirements will no longer be sensed and will function as though they were being controlled by a pressure compensated pump. Pressure compensated flow controls will have to be installed to control their speed.

Pressure compensated pumps:

Pressure compensated, variable displacement pumps are a common and very useful component in many of our hydraulic systems. But they do have a parasitic draw on the available energy.  The compensator has to be set to match the maximum system pressure requirement.  When this setting is for force limiting to an actuator or for storing energy in an accumulator, and otherwise allows the pump to provide all its flow at system pressure, then we have a pretty efficient system.  But if there are restrictive flow controls in the system, some of the benefits are diminished.  The pressure compensator will have to be set at a pressure that is at least 6 bar higher than the maximum requirement for any actuator.

Servo valves:

This is going to be a little touchy, but it needs to be said.  Servo valves do a remarkable job of controlling speed, force and position.  They have enabled hydraulics to be the muscle married to electronic controls.  They have certainly earned their place in the tool kit for precision control, and I do not suggest that they be eliminated or replaced.  But sometimes, those of us who design systems tend to do a little overkill.  The question of what work actually needs to be done (begin at the end) has to be addressed.  By design, most servo valves operate at a 70 bar ΔP.  This means that for every liter of flow, .12kW is being consumed for control.  This may be perfectly acceptable in some instances where precise control is critical but consider the fact that the ΔP is used primarily for rapid spool movement.  Each system should be evaluated to see if this rapid spool movement is necessary.  It may be that zero hysteresis is more desirable than rapid movement of the spool.  This can be accomplished with a servo valve that is driven by a stepper motor.  It is not nearly as fast, but is extremely repeatable and can have a ΔP that is 15 times less than the traditional valve.

One more thing:

There is one more area where we need to take a look, not just at an existing hydraulic system, but also at the facility as a whole to see where hydraulics might replace electro-mechanical systems and be more efficient.  If there is an electric motor performing one function and operating continually near its nameplate rating, it is likely to be the most efficient method of performing the work.  We should look for electric motors that are under varying loads and/or using gearboxes for speed reduction.   This can include motors of equal or varying sizes. A single motor or a group of motors like these can be replaced by a single, properly designed hydraulic power unit supplying efficient, variable displacement, hydraulic motors with a substantial reduction in power consumption.  The hydraulic motors have a much greater power density, and have full torque throughout their speed range.  The motors can be configured so that each will draw only the energy it needs without the use of restrictive flow control.

In conclusion:

When approaching a hydraulic system for the first time and trying to make a determination if it is sufficiently inefficient to be considered for some type of modification, have a check list of the symptoms mentioned above.  When these symptoms are present, there is a good chance that there would be a substantial economic benefit to making some changes in the components of the system.  Then, begin at the end.  Compare the actual work required and the desired function of the system to the energy consumed and the productivity of the equipment.  Have the system reviewed by a competent fluid power professional; taking into consideration the list of parasitic devices listed here, to see what possible changes could be made that would make good economic sense.

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