There is much to consider for any industrial hydraulic system. The primary determination is what pump type to use. This is especially important today due to the high cost of power and industry’s demand for energy-efficient systems. In recent years, system designers have sought new ways to reduce energy consumption and carbon footprint. They have also created products that adjust flow and pressure to match the demand of the application.
Hydraulic pumps are the backbone of most hydraulic systems and prime energy consumers. A popular misconception is that pumps create pressure. However, pumps create flow. The pressure in the system is determined by the load on the actuators. Hydraulic horsepower is calculated as a multiplication of flow and pressure and divided by a constant. For a given horsepower, as flow increases, pressure must decrease and vice versa. Electric motor sizes are typically determined by the maximum flow and pressure required for the application; however, maximum flow and pressure rarely occur simultaneously. Therefore, electric motors tend to be oversized, increasing system and energy costs plus unwanted heat losses. Tandem Hydraulic Cylinder
There are a variety of hydraulic pumps, benefits, and issues to consider during the selection process. As the complexity of the pump design increases, so does the initial cost impact.
The most common version of the hydraulic pump is the fixed displacement, which is also the most cost-effective. This pump can be of gear, vane, geroler, or piston design. These pumps are coupled to fixed-speed electric motors, providing constant flow to a hydraulic circuit. The relief valve setting limits the pressure in the system. However, most pumps in these systems are sized for the maximum flow requirement. Any reduction in the flow requirement for a particular machine function means excess flow would bypass the relief valve setting, creating heat and wasting energy.
The variable volume, pressure-compensated pump was a notable change for the hydraulic pump industry. These pumps vary the flow to match the system demand, reducing the heat generated as no flow bypasses the system relief valve. In addition, the pump’s internal pressure compensator determines the system’s maximum pressure. When there is no flow demand on the pump, it reduces output flow to zero but maintains maximum pressure in the system.
Pressure-compensated pumps have decreased wasted energy and improved performance, but the motor selected still tends to be oversized for the application. The development of load-sense variable volume pumps, primarily for the mobile hydraulics industry, improves this situation. Allowing the pump to sense the actual load (or pressure) in a particular function causes the pump to deliver the required flow at load pressure only. This typically requires less energy from the electric motor or engine, reducing operating costs. Mobile equipment engine speed varies with engine control systems, regulated to provide optimum power and torque and minimize fuel consumption. However, industrial applications still use fixed-speed electric motors, and the motor sizing criteria are like standard pressure-compensated pumps.
Recently, hydraulic pump systems have improved with the addition of electronic control with variable frequency drives (VFDs) and utilizing IIoT, leading to transformative reductions in power consumption, noise, and heat loss.
Varying the electric motor speed with a VFD controlling pump flow rates is new, especially with drive costs dropping in recent years. Many power utilities have promoted using VFDs to reduce power consumption and operational costs. While using a VFD in simple applications such as air compressors or fan drives may be adequate, hydraulic system dynamics are much different and require faster response times reacting to machine functions.
Using a VFD with either a fixed or variable pump allows adjustment of input speed to the pump to vary the flow, within the limitations of the pump speed range. However, hydraulic systems move at high frequency, so changing the input speed to adjust flow will not meet the requirements. Another consideration is that squirrel-cage induction motors controlled by VFD only provide constant torque and not constant horsepower. As the speed changes, so does the horsepower to maintain the constant torque requirement (i.e., a 10-hp motor running at half speed is only a 5-hp motor).
Current trends toward conn