By J.D. Doyle, CFPHS
Reducing horsepower lost in hydraulic systems has been a hot topic for the last few years due to both increased energy cost as well as awareness of the environmental effects. Reducing hydraulic system horsepower loss continues to challenge both equipment design engineers and hydraulic system design engineers. The desired reduction is a balancing act between the higher costs of a lower horsepower hydraulic system, compared to energy savings over the life of the equipment.
Horsepower can be reduced the following three ways:
Considering the input pump source is primarily a comparison of variable displacement pumps with fixed displacement pumps. The higher initial equipment cost for the variable system is immediately apparent. The fixed displacement pumps can be a fraction of the cost of a variable displacement pump. However, the single most common cost evaluation error is failing to correctly examine the additional valving required in the fixed displacement system. The fixed pump system usually requires both additional valving as well as additional cooling costs. Yet, there are many applications where the fixed displacement system can be operated without excessive horsepower loss. Additionally, the duty cycle of the application can make the fixed system the more cost-effective approach, especially when energy consumption is also considered.
The variable displacement pump can be the more cost effective approach when all factors are considered. The energy consumption by the hydraulic systems with more complex duty cycles and multiple machine functions can be significant. Load-sensing variable displacement pump systems are often the most energy efficient and may require a network of sensing checks or shuttle valves. The sensing checks or shuttle valve must be sized properly. If the valves are too small, a problem is created where their pressure drop causes a horsepower loss. Merely changing the system’s pump to a variable displacement load sensing pump does not necessarily make the system more efficient. Proper valve sizing is also critically important even with the load sensing valves.
The second topic area for horsepower reduction is the system control valving. This area can be further divided into directional control and motion control-type valves or the combination the two. The directional valve is the part of the hydraulic system which turns functions on and off along with switching the direction of motion. In some cases they are just on/off valves and other cases they are proportional type valves. In either case the pressure drop for the full flow required by the actuator should be evaluated and again the initial component cost versus operating cost with energy consumption should be considered. This is a very tough comparison to evaluate as in some cases especially with proportional control where the valve needs to operate with high pressure drops to get the control required of the actuator or machine movement. In either case it’s important to look at the pressure drop of the directional control valve and look at the next size larger capacity valve to review the increase cost compared to the horsepower loss. The other area of concern is with motion control valves where sometimes a different type of circuitry or a different type motion control valve can make a big difference in horsepower loss without the large increase in initial component cost. In some cases it’s more efficient to have the load-induced pressure do the work instead of having to power down a suspended load which will reduce the horsepower consumption with a load lowering function.
The last area for horsepower loss reduction concerns the fluid conveying lines of the hydraulic system: the hydraulic fittings, hoses, tubing and piping of the hydraulic system. As with the other two areas, it is a balancing act between the initial cost and line sizes compared to the energy consumption savings by reducing pressure drops in the system. This area of design seems to have been the most neglected over the years because it is hard to do an accurate cost comparison. As with the fixed displacement pumps, the wasted horsepower goes directly to heat. This added heat needs to be removed to maintain proper operating temperature and this has a hidden cost. As engineers and designers, we should all make the effort to size both the fluid conveying lines and the system component connection port sizes larger in order to reduce the horsepower lost in hydraulic systems. This helps reduce the energy consumption of equipment powered by hydraulics.