volumetric efficiency hydraulic pump factory

In a condition-based maintenance environment, the decision to change out a hydraulic pump or motor is usually based on remaining bearing life or deteriorating efficiency, whichever occurs first.

Despite recent advances in predictive maintenance technologies, the maintenance professional’s ability to determine the remaining bearing life of a pump or motor, with a high degree of accuracy, remains elusive.

Deteriorating efficiency on the other hand is easy to detect, because it typically shows itself through increased cycle times. In other words, the machine slows down. When this occurs, quantification of the efficiency loss isn’t always necessary. If the machine slows to the point where its cycle time is unacceptably slow, the pump or motor is replaced. End of story.

In certain situations, however, it can be helpful, even necessary, to quantify the pump or motor’s actual efficiency and compare it to the component’s native efficiency. For this, an understanding of hydraulic pump and motor efficiency ratings is essential.

There are three categories of efficiency used to describe hydraulic pumps (and motors): volumetric efficiency, mechanical/hydraulic efficiency and overall efficiency.

Volumetric efficiency is determined by dividing the actual flow delivered by a pump at a given pressure by its theoretical flow. Theoreticalflow is calculated by multiplying the pump’s displacement per revolution by its driven speed. So if the pump has a displacement of 100 cc/rev and is being driven at 1000 RPM, its theoretical flow is 100 liters/minute.

Actualflow has to be measured using a flow meter. If when tested, the above pump had an actual flow of 90 liters/minute at 207 bar (3000 PSI), we can say the pump has a volumetric efficiency of 90% at 207 bar (90 / 100 x 100 = 90%).

Its volumetric efficiency used most in the field to determine the condition of a hydraulic pump - based on its increase in internal leakage through wear or damage. But without reference to theoretical flow, the actual flow measured by the flow meter would be meaningless.

A pump’s mechanical/hydraulic efficiency is determined by dividing thetheoretical torque required to drive it by the actual torque required to drive it. A mechanical/hydraulic efficiency of 100 percent would mean if the pump was delivering flow at zero pressure, no force or torque would be required to drive it. Intuitively, we know this is not possible, due to mechanical and fluid friction.

Table 1. The typical overall efficiencies of hydraulic pumps, as shown above, are simply the product of volumetric and mechanical/hydraulic efficiency.Source: Bosch Rexroth

Like theoretical flow, theoretical drive torque can be calculated. For the above pump, in SI units: 100 cc/rev x 207 bar / 20 x p = 329 Newton meters. But like actual flow, actual drive torque must be measured and this requires the use of a dynamometer. Not something we can - or need - to do in the field. For the purposes of this example though, assume the actual drive torque was 360 Nm. Mechanical efficiency would be 91% (329 / 360 x 100 = 91%).

Overall efficiency is simply the product of volumetric and mechanical/hydraulic efficiency. Continuing with the above example, the overall efficiency of the pump is 0.9 x 0.91 x 100 = 82%. Typical overall efficiencies for different types of hydraulic pumps are shown in the Table 1.

System designers use the pump manufacturers’ volumetric efficiency value to calculate the actual flow a pump of a given displacement, operating at a particular pressure, will deliver.

As already mentioned, volumetric efficiency is used in the field to assess the condition of a pump, based on the increase in internal leakage due to wear or damage.

When calculating volumetric efficiency based on actual flow testing, it’s important to be aware that the various leakage paths within the pump are usually constant. This means if pump flow is tested at less than full displacement (or maximum RPM) this will skew the calculated efficiency - unless leakage is treated as a constant and a necessary adjustment made.

For example, consider a variable displacement pump with a maximum flow rate of 100 liters/minute. If it was flow tested at full displacement and the measured flow rate was 90 liters/minute, the calculated volumetric efficiency would be 90 percent (90/100 x 100). But if the same pump was flow tested at the same pressure and oil temperature but at half displacement (50 L/min), the leakage losses would still be 10 liters/minute, and so the calculated volumetric efficiency would be 80 percent (40/50 x 100).

The second calculation is not actually wrong, but it requires qualification: this pump is 80 percent efficient at half displacement. Because the leakage losses of 10 liters/minute are nearly constant, the same pump tested under the same conditions will be 90 percent efficient at 100 percent displacement (100 L/min) - and 0 percent efficient at 10 percent displacement (10 L/min).

To help understand why pump leakage at a given pressure and temperature is virtually constant, think of the various leakage paths as fixed orifices. The rate of flow through an orifice is dependant on the diameter (and shape) of the orifice, the pressure drop across it and fluid viscosity. This means that if these variables remain constant, the rate of internal leakage remains constant, independent of the pump"s displacement or shaft speed.

Overall efficiency is used to calculate the drive power required by a pump at a given flow and pressure. For example, using the overall efficiencies from the table above, let us calculate the required drive power for an external gear pump and a bent axis piston pump at a flow of 90 liters/minute at 207 bar:

As you’d expect, the more efficient pump requires less drive power for the same output flow and pressure. With a little more math, we can quickly calculate the heat load of each pump:

No surprise that a system with gear pumps and motors requires a bigger heat exchanger than an equivalent (all other things equal) system comprising piston pumps and motors.

Hydraulic pumps convert mechanical energy into hydraulic energy. A high-performance piston pump can convert mechanical energy into hydraulic energy with an efficiency of 92 percent.

If the pump drives a piston motor, the motor is able to convert this hydraulic energy back into mechanical energy with an efficiency of 92 percent. The overall efficiency of this hydraulic drive, without considering flow losses, is 85 percent (0.92 x 0.92 x 100 = 85).

The inefficiencies or losses in a hydraulic drive can be divided into two categories: hydraulic-mechanical, which comprise flow and mechanical friction losses, and volumetric, which comprise leakage and compressibility losses (Figure 1).

The advantages of a hydraulic drive, which include high-power density (high-power output per unit mass), variable speed control, simple overload protection and both rotary and linear motion, are possible from a single system.

As Table 1 shows, a key disadvantage of a hydraulic drive is that it is far less efficient than a mechanical drive. What’s worse, the wear process decreases a hydraulic drive’s volumetric efficiency (and therefore total efficiency) causing the drive to slow down and more energy to be given up to heat.

The hydraulic pump is usually the hardest working component of a hydraulic system. As the pump wears in service, internal leakage increases and therefore the percentage of theoretical flow available to do useful work (volumetric efficiency) decreases. If volumetric efficiency falls below a level considered acceptable for the application, the pump will need to be overhauled.

In a condition-based maintenance environment, the decision to change-out the pump is often based on remaining bearing life or deterioration in volumetric efficiency, whichever occurs first.

Volumetric efficiency is the percentage of theoretical pump flow available to do useful work. In other words, it is a measure of a hydraulic pump’s volumetric losses through internal leakage and fluid compression. It is calculated by dividing the pump’s actual output in liters or gallons per minute by its theoretical output, expressed as a percentage. Actual output is determined using a flow-tester to load the pump and measure its flow rate.

Because internal leakage increases as operating pressure increases and fluid viscosity decreases, these variables should be included when stating volumetric efficiency. For example, a hydraulic pump with a theoretical output of 100 GPM, and an actual output of 94 GPM at 5,000 PSI and 46 cSt is said to have a volumetric efficiency of 94 percent at 5,000 PSI and 46 cSt.

In practice, fluid viscosity is established by noting the fluid temperature at which actual pump output is measured and reading the viscosity off the temperature/viscosity graph for the grade of fluid in the hydraulic system.

When calculating the volumetric efficiency of a variable displacement pump, internal leakage must be expressed as a constant. Consider this example: I was recently asked to give a second opinion on the condition of a large, variable displacement pump. My client had been advised that its volumetric efficiency was down to 80 percent and based on this advice, he was considering having the pump overhauled.

The hydraulic pump in question had a theoretical output of 1,000 liters per minute at full displacement and maximum rpm. Its actual output was 920 liters per minute at 4,350 PSI and 46 cSt. When I advised my client that the pump’s volumetric efficiency was in fact 92 percent he was alarmed by the conflicting assessments. To explain the disparity, I asked to see the first technician’s test report.

The technician had limited the pump’s displacement to give an output of 400 liters per minute (presumably the maximum capacity of his flow-tester) at maximum rpm and no load. At 4,350 PSI the recorded output was 320 liters per minute. From these results, volumetric efficiency had been calculated to be 80 percent (320/400 x 100 = 80).

To help understand why this interpretation is incorrect, think of the various leakage paths within a hydraulic pump as fixed orifices. The rate of flow through an orifice is dependent on the diameter (and shape) of the orifice, the pressure drop across it and fluid viscosity. This means that if these variables remain constant, the rate of internal leakage remains constant, independent of the pump’s displacement.

Note that in the above example, the internal leakage in both tests was 80 liters per minute. If the same test was conducted with pump displacement set to 100 liters per minute at no load, pump output would be 20 liters per minute at 4,350 PSI - all other things equal.

This means that this pump has a volumetric efficiency of 20 percent at 10 percent displacement, 80 percent at 40 percent displacement and 92 percent at 100 percent displacement. As you can see, if actual pump output is measured at less-than-full displacement (or maximum rpm), an adjustment needs to be made when calculating volumetric efficiency.

In considering whether it is necessary to have this hydraulic pump overhauled, the important number is volumetric efficiency at 100 percent displacement, which is within acceptable limits. If my client had based his decision on volumetric efficiency at 40 percent displacement, his company would have paid thousands of dollars for unnecessary repairs.

Volumetric efficiency is the actual amount of fluid flowing through a pump, rather than its theoretical maximum. Put another way, it is the measure of volumetric losses of a reciprocal pump through internal leakage and fluid compression; this calculation is the value that should be used to evaluate your current pumping mechanism.

Common causes of loss in volumetric efficiency include worn valves, seats, liners, piston rings, or plungers, pockets of air or vapor in the inlet line or trapped above the inlet manifold, or loose belts, valve covers, cylinder heads, or bolts in the pump inlet manifold. Routine maintenance and inspection in these areas can greatly increase output over time.

Obstructions such as a safety relief valve partially held open or failing to maintain pressure, foreign objects preventing the pump inlet or discharge valve from closing or blocking liquid passage, or a vortex in the supply tank are some of the other more easily remedied factors reducing efficiency.

There are many potential hazards that can hamper volumetric efficiency and cause your components to operate at less than full strength — but there are also a number of methods to increase it.

Your final drive includes a hydraulic motor and that motor has a certain level of efficiency associated with it. Over time, that efficiency can drop -- so find out how efficiency is measured, what the source of losses are, and how to minimize them.

No system, no matter how well it’s designed, is going to be 100% efficient. High-quality, well-maintained radial piston motors are about 95% efficient while axial piston motors are about 90% efficient--which is likely why you see these two types of hydraulic motors used in the vast majority of final drive motors.

The definition of efficiency depends on what type of system you’re talking about, and even then there can be some variations. For a hydraulic motor, there are three ways efficiency can be measured or estimated: volumetric, mechanical/hydraulic, and overall efficiency.

Volumetric efficiency looks at the theoretical flow rate and the actual flow rate and provides information about leakage and wear. The theoretical flow rate is pretty easy to calculate: theoretical flow = (pump displacement per revolution) x (revolution speed).

This works much better in SI units, too. If the displacement is in cc/rev and the speed is in rpm, the results will be in liters/minute. Actual flow is then measured using a flow meter. The efficiency is then actual flow / theoretical flow x 100 to get efficiency as a percent.

Mechanical efficiency is based on actual work done and theoretical work done, both per revolution. This is based on theoretical torque and the actual torque, and in most hydraulic motors it’s about 0.9 (or 90%). Actual torque can be measured with a dynamometer, but is rarely done. The losses related to mechanical efficiency are directly tied to mechanical friction between mating parts.

Overall efficiency combines volumetric and mechanical efficiency. It"s simply the product of these two values: overall efficiency = mechanical efficiency x volumetric efficiency, and gives you an overall idea of how efficient your hydraulic motor is.

Some degree of internal leakage is normal and actually beneficial, but past a certain point it becomes a problem. Excess internal leakage most often results from wear. For example, the size of key clearances in a hydraulic motor can, over time, become larger because of abrasive wear and lead to internal leakage. That type of wear usually results from contaminated hydraulic fluid but can also result from normal wear and tear.

Friction is another major source of losses. Rough surfaces where they should be smooth cause friction issues with the hydraulic fluid, reducing the amount of power that can be transferred. There are other ways that friction can be introduced, however. For example, anti-friction bearings or plane bearings that are wearing out will be a source of friction.

One of the keys to preventing hydraulic motor losses relates to good maintenance practices, such as keeping the hydraulic fluid clean, replacing hydraulic filters, and not ignoring hydraulic leaks. It"s also important to look for symptoms of potential problems with the bearings, such as new noises, excessive vibration, and overheating.

Before purchasing just any type of pump, it’s essential to have some understanding of both the volumetric and mechanical efficiency of pumps. Of course, when working with a reputable source, a company representative will gladly provide whatever information you need. However, it’s still a good idea to build your knowledge base ahead of time.

For the efficiency of centrifugal pumps, there are four distinct kinds. Along with volumetric and mechanical efficiencies, you should also learn the basics of the hydraulic and overall efficiencies. In layman’s terms, these formulas, whether for multistate centrifugal pumps or centrifugal vacuum pumps, ensure the most efficient operation possible.

Volumetric Efficiency – The official definition for this particular efficiency is the ratio of the actual flow rate that the pump delivers to the theoretical discharge flow rate. The latter is the flow rate void of any leakage. Typically, you would use this formula to determine the volume of liquid lost during the flow because of a leak.

Mechanical Efficiency – For the various types, including centrifugal vacuum pumps and others, the formula for this efficiency is the ratio of theoretical power the pump needs to operate to the actual power delivered to the pump itself. In this case, you would use this efficiency formula to identify power lost in specific moving parts such as the bearings. Ultimately, mechanical efficiency determines the amount of power a pump must have to perform optimally.

Hydraulic Efficiency – The mechanical energy of multistate centrifugal pumps and other types converts into hydraulic energy. This consists of flow, pressure, and velocity. The ratio formula for hydraulic efficiency is useful hydrodynamic energy in the form of fluid to the amount of mechanical energy delivered to the rotor. Smaller centrifugal pumps usually land in the 50 to 70 percent range. In comparison, larger ones typically reach efficiencies of 75 to 93 percent.

Overall Efficiency – The ratio formula for this is the output of actual power of centrifugal pumps to the input of actual power. The overall efficiency is what you would rely on to determine the amount of energy lost overall.

Whether you’re in the market for one of the types of centrifugal pumps mentioned or you already have one that needs servicing, you can always count on the experts at PFS Pumps. With years of experience in both areas, we guarantee quality and affordability. Contact us today for more information.

Volumetric efficiency accounts for the leakage of fluid in the pump that doesn"t do any work.  Mechanical/hydraulic efficiency accounts for friction losses.  Total efficiency is volumetric efficiency X mechanical/hydraulic efficiency.

Depending on what you"re calculating, one of these efficiencies will be important.  For flow rate and speed, volumetric efficiency is important.  For displacement, pressure rise, and torque, mechanical efficiency is important.  Finally, for input power, total efficiency is important.

While it’s not usually possible to see inside a hydraulic pump and motor assembly and observe the wear taking place, there is one surefire clue that makes it possible to know if these important components are on their way out. Declining efficiency is a sign of leakage and/or increased internal friction, which is why keeping an eye on the efficiency of your pumps is a good way to monitor the health of your equipment. In this article, we’ll explore two aspects of  hydraulic pump and motor efficiency and what they can tell you about what’s going on inside. Allowing you to repair and replace worn units in time to avoid costly failures.

Every new pump or motor has a specified ‘theoretical’ flow and torque rating. These are the numbers the unit should be able to achieve in a perfect world. However, the real world is never perfect and in reality, things like friction will mean that your actual performance will always be somewhere below the ideal theoretical number.

In addition, over the lifespan of the unit, various components such as bearings, pump elements, fluid, and other internals begin to deteriorate and as this happens, the pump gradually becomes less efficient. This deterioration can be measured in relation to the specified performance of the pump. This is useful to understand because it’s a clear indication of pump wear and gives us a valuable clue to alert us to repair or replace these units before its too late.

We can measure this loss of efficiency by looking at two aspects of efficiency, which are hydro-mechanical efficiency and volumetric efficiency. Each of these tells us something about the condition of the pump/motor and whether it is likely to fail in the near future.

In layman’s terms, Volumetric Efficiency refers to the amount of fluid a pump delivers and it is usually measured in litres per minute. In ideal conditions, a positive-displacement pump should deliver the same amount of liquid for each rotating cycle. As the unit wears, fluid slippage slowly increases and the amount of liquid delivered per cycle decreases. Thus, a decrease in volumetric efficiency indicates losses due to leakage or bypass. The decline in volumetric efficiency is accompanied by an increase in the cycle time of actuators such as hydraulic cylinders as the flow has been slowed. If the degradation is allowed to continue, the hydraulic system may become completely inoperable, requiring the repair or replacement of the worn pumps and motors to get up and running again.

The second category to look at is hydro-mechanical efficiency, which indicates the amount of fluid and mechanical friction within the system. To determine a motor’s hydro-mechanical efficiency, look at the actual torque output compared to the unit’s rated torque output. If a motor’s real-world output is 20% below its theoretical torque rating, then the motor can be said to be 80% efficient. A reduction in actual torque is, therefore, a sign that bearings and other mechanical internals are becoming worn and generating more friction.

Hydraulic pump and motor efficiency can have a significant effect on your hydraulic system. Inefficient components draw more power and drive up the running costs of your operation, so always a good idea to be aware of how your equipment is performing. However, declining efficiency is also an important way to monitor the health of your hydraulic pumps and motors. Warning signs such as slowing hydraulic actuators and loss of torque are a symptom of decreasing efficiency that – if left unchecked – will eventually lead to failure of hydraulic equipment. These components will eventually need to be repaired or replaced, but in the long term it’s more cost-effective to do so before they fail or start to inflate operating expenses.

Volumetric efficiency is the actual flow produced by a pump at a certain pressure divided by the theoretical flow. Theoretical flow is the predominate category used to determine a hydraulic pump’s condition in terms of internal leakage, either by design or through wear and/or damage. While mechanical and hydraulic efficiency are sometimes grouped into a single category (hydromechanical efficiency), mechanical friction involves energy losses that occur among mechanical seals, the bearing frame, and stuffing box while hydraulic efficiency takes into account factors such as liquid friction and other losses that occur within the volute (diffuser) and impeller. Pressure losses and friction losses among those various components can negatively impact a system’s hydromechanical, or mechanical/hydraulic, efficiency.