How Do You Calculate The Power Needed For A Hydraulic Pump?

How do you calculate the power needed for a hydraulic pump?

A hydraulic pump is a mechanical device that converts mechanical power into hydraulic energy. It generates flow and pressure to move and control hydraulic fluid in hydraulic systems. Hydraulic pumps are used in many applications such as manufacturing machinery, construction equipment, aircraft, and more. Proper pump sizing and selection is critical for hydraulic systems to function efficiently. Knowing how to calculate the required power ensures you choose a pump that can meet the system’s demands.

Calculating hydraulic pump power requirements involves determining the system’s flow rate and pressure needs. This enables you to select a pump that can deliver adequate flow at the necessary pressures. The flow rate and pressure, along with pump efficiency, are used to calculate the pump’s power requirements using a simple power formula. Understanding how to make these calculations is an essential skill for designing and maintaining hydraulic systems.

Hydraulic Pump Basics

Hydraulic pumps convert mechanical energy into hydraulic fluid flow and pressure [1]. They use rotary or reciprocating motion to move fluid from the reservoir to the hydraulic system. The key components of a hydraulic pump include [2]:

  • Gears – used in gear pumps to move fluid
  • Pistons – used in piston pumps to pressurize fluid
  • Rotors – used in vane and screw pumps to move fluid
  • Valves – check valves control flow direction, relief valves limit pressure

Common types of hydraulic pumps include [1]:

  • Gear pumps – two meshing gears push fluid
  • Vane pumps – rotating vanes trap fluid between them and the casing
  • Piston pumps – reciprocating pistons pressurize fluid
  • Screw pumps – rotating screws convey fluid

Hydraulic pumps build pressure to move fluid through the system and operate hydraulic actuators. Selecting the right pump involves matching the pump’s flow rate and pressure capabilities to the system requirements.

[1] https://www.whitehouseproductsltd.com/blog/basic-fundamentals-of-hydraulic-pumps-you-should-know
[2] https://gearpumpus.com/products-services/hydraulic-pump-components/

Determining Flow Rate

The flow rate required for a hydraulic pump depends on several factors related to the application. Some key considerations when determining flow rate include:

  • Weight and speed of moving parts – Heavier loads and faster cycle times require greater flow.
  • Number and size of hydraulic cylinders – More and larger cylinders require higher flow rates.
  • Type of valves and accessories – Certain valve configurations and accessories like accumulators impact flow.
  • System pressure – Higher pressure systems need increased flow.

To calculate the required flow rate, you must first understand the velocity and volume requirements of each hydraulic actuator in the system. This is calculated based on the bore diameter, stroke length, and cycling time of the cylinders. The flow rates of all actuators are added together to determine the system’s total flow rate requirement.

The basic formula for calculating flow rate is:

Q = VA

Where:

  • Q = Flow Rate (GPM or L/min)
  • V = Velocity of actuator (in/sec or m/sec)
  • A = Area of actuator (sq.in or sq.m)

By understanding the application parameters and using the flow rate formula, you can accurately calculate the required flow rate for proper hydraulic pump selection.

Source

Determining Pressure Requirements

Hydraulic systems operate at a wide range of pressures depending on the application. As per API Standard 17F, typical pressure ranges include:

  • Low pressure – Up to 5000 psi
  • Medium pressure – 5,000 to 10,000 psi
  • High pressure – 10,000 to 15,000 psi
  • Ultrahigh pressure – Above 15,000 psi

To determine the specific pressure requirement for your hydraulic pump application, consider factors like:

  • The pressure needed for the hydraulic actuator or cylinder to generate the required force or torque
  • Flow resistance from valves, fittings, and hydraulic hose or pipe
  • The weight or force that needs to be lifted, moved or controlled
  • A safety factor, usually 1.5x to 2x the expected operating pressure

Perform calculations using the force, area, and pressure relationship: Pressure = Force / Area. Increase the pressure as needed to overcome flow resistance losses that occur. Select a pump that can reliably generate adequate pressure for the entire cycle.

Power Formula

The basic formula for calculating hydraulic power is:

Ph = Q x P x ηmech

Where:

  • Ph = Hydraulic power (kW)
  • Q = Flow rate (L/min)
  • P = Pressure (bar)
  • ηmech = Mechanical efficiency of the pump (typically 0.8 – 0.9)

This formula shows that hydraulic power depends on both the flow rate and pressure generated by the pump. Flow rate is the volume of fluid being pumped per unit time (e.g. Liters/min). Pressure is the force per unit area the pump generates to move the fluid (e.g. bar). The efficiency accounts for mechanical losses in the pump. Multiplying flow, pressure, and efficiency determines the hydraulic pump’s power requirements in kilowatts.

Sources:
https://powerturbines.eu/en/blog/calculate-the-power-of-a-water-turbine/
https://www.piping-designer.com/index.php/properties/fluid-mechanics/3144-hydraulic-power

Estimating Efficiency

Pump efficiency is an important factor in determining the power required. Typical pump efficiency can range from 60% to 90% depending on the pump type, design, and operating conditions.

Efficiency is impacted by factors like flow rate, pressure, viscosity, temperature, and impeller clearance. At optimal operating conditions, centrifugal pump efficiency is normally between 70-85%. Positive displacement pumps like piston and gear pumps usually have slightly higher efficiency around 85-95%.

As pressure increases, efficiency tends to decrease due to greater fluid slip, turbulence and mechanical losses. Operating at too low of a flow rate can also negatively impact efficiency. Liquid properties like high viscosity can impair efficiency as well. Following recommended operation ranges and maintaining clearances is key.

According to one industry source, “Increasing system pressure also reduces hydraulic pump efficiency. Higher pressures generally lead to both increased mechanical losses (there are higher shear forces and more surface area in contact with fluid) as well as volumetric losses (increased internal leakage as clearances open up under pressure).” (Source)

Applying a Safety Factor

A safety factor is crucial when sizing a hydraulic pump to account for variables and to prevent component failure. Hydraulic systems often undergo pressure spikes, fatigue over time, misuse, and demands exceeding design limits. Applying an adequate safety factor provides a margin of safety against these risks.

Industry standards recommend a safety factor of at least 2:1 for hydraulic components. This means the component should be sized to withstand double the expected operating pressure. According to the National Fluid Power Association (NFPA), hydraulic hoses require a safety factor of 4:1 (Ryco, n.d.). High pressure systems or critical applications may use safety factors of 5:1 or even 10:1.

The main goal is to prevent the maximum anticipated working pressure from exceeding the component’s burst pressure rating. With an appropriate safety cushion, the system can withstand pressure spikes, fatigue, and misuse while avoiding catastrophic failure.

Power Conversion

To determine the power requirement at the shaft, you must convert the hydraulic power into shaft horsepower (hp). There are some key considerations in this conversion:

First, the hydraulic power must be divided by the pump efficiency to determine the shaft hp. As an example, if the hydraulic power required is 50 hp and the pump efficiency is 80%, the shaft hp would be calculated as follows:

Shaft hp = Hydraulic hp / Pump efficiency

Shaft hp = 50 hp / 0.80 = 62.5 hp

This accounts for frictional and other losses in the pump requiring more input shaft power. Typical pump efficiencies range from 70-90% depending on the pump type and size [1].

Second, an additional allowance must be made for the motor or engine driving the pump shaft. This accounts for losses in power transmission and the motor itself. A typical allowance is 10-15% beyond the calculated shaft hp [2].

For example, with a required shaft hp of 62.5 hp, adding 15% would give a drive motor size of 72 hp (1.15 x 62.5 hp) to sufficiently power the pump.

Pump Selection

Once the required pump power is calculated, the next step is selecting a pump that meets the power requirements. The power calculation provides the minimum power rating needed for the pump. When reviewing pump specifications, choose a pump that matches or exceeds the required power.

In addition to power, there are other factors to consider in pump selection:

  • Flow rate – The pump must be able to deliver the needed flow rate.
  • Pressure rating – Select a pump that meets the maximum system pressure needed.
  • Pump type – Gear, vane, piston, and centrifugal pumps are common options.
  • Fluid compatibility – Choose a pump designed to work with the hydraulic fluid used in the system.
  • Mounting – Ensure the pump design fits the available mounting space.
  • Cost – Compare pump costs to find the most economical choice.

Consulting the pump curves from manufacturers is helpful for comparing power, flow rate, pressure ratings, and efficiency across different models. Proper pump selection is critical for effective and reliable hydraulic system performance. The calculated power requirements guide selection, but all the factors above should be evaluated.

As an example, if the power calculation determined a 50 hp pump is needed, choosing a 55 hp or 60 hp pump would provide a safety margin. Within those options, comparing flow rates, pressures, efficiency ratings, and costs would narrow down the decision to the optimal pump for the application.

Conclusion

In summary, the key steps in determining the required power for a hydraulic pump are: calculating the flow rate needed based on cylinder size and speed, determining the pressure requirements of the system, using the power formula to calculate theoretical hydraulic power required, estimating pump efficiency to find actual power needed, and applying a safety factor. Properly sizing a hydraulic pump is crucial to ensure the pump can provide the required flow and pressure for the hydraulic system to function as intended. An undersized pump may lead to insufficient flow or pressure, while an oversized pump wastes energy and money. By following the best practices outlined here for calculating hydraulic power requirements, you can select the optimally sized pump for your application.

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