Many cyclic hydraulic systems only need to provide flow at pressure during specific stages in the machine's cycle. This is even more common in the theme park industry, where we require time to reset a show or load and unload guests.
Because of how these control systems typically operate, it is expected that most cyclic hydraulic systems that move ride or display equipment are always 'ready to go.'
This leads us to pressure-compensated pump delivery systems that can quickly come into action. And during periods when low or no flow is required, the adverse compensating effects (dead-heading) can have little to no detrimental effects.
Although this is an advantage for a system that requires a quick response, it is not the most energy-efficient design.
Consider this situation and see if you agree:
Cyclic Hydraulic Equipment:
- A 500-gallon tank feeds three variable displacement pumps rated at 75 gallons per minute, with two pumps operating and one installed as a standby.
- The pump compensators are simple pressure compensators with three units set to cascading compensation of 1700 psi, 1650 psi, and 1625 psi.
- The manifolds and valves are located 75 feet away and control six hydraulic actuators connected to a 6DOF motion simulator.
- Each actuator has its own 15-gallon bladder accumulator to assist in making large movements at high flows.
Cyclic Hydraulic Efficiency:
- The maximum system pressure is 1700 psi. The minimum pressure for the simulator is 1200 psi, at which point a warning message is received.
- The average flow rate is 60-70 gallons per minute for the two operating pumps.
- The maximum flow is approximately 130 gallons per minute, but only for short durations of 2-4 seconds.
- In-flight leakage is 12.5 gallons per minute due to leakage from the servo valve and actuator.
- The flight duration is 4 minutes, with 3 minutes between flights.
- Not all simulators fly every 7 minutes. Demand can be sporadic, and some simulators may operate only during peak times, collectively being compensated for a total of 6-7 hours per day at 1700 psi.
Cyclic Hydraulic Energy Consumption:
- An energy consumption study revealed that for 2-3 hours each morning and 1.5 hours each evening, the simulators remain in compensated mode due to maintenance and operational procedures.
- Using the amperage data from each electric motor, we found that during compensation periods, the 'lead' pump draws 32.5 amps, and the 'lag' pump draws 19.5 amps at 480 VAC 3-phase (note the lead pump supplies 12.5 gpm to always compensate for system leakage). This amounts to 15600 watts for the lead pump plus 9360 watts for the lag, totaling 24960 watts.
- Multiply this by the average of 4.5 hours in compensated mode per day, and you will have 112.3 kilowatt-hours for each simulator per day.
- With six simulators operating 365 days a year, this amount totals 245937 kilowatt-hours annually at a cost of $0.125 per kilowatt-hour.
This practice shows us that operating pumps at 1700 psi for 4.5 hours per day incurs an annual cost of $30742. This cost only accounts for the times in the morning and evening when the pumps are compensated. Additional savings can be achieved with other incremental controls available on most pumps.
Hydraulic Pump Control Solution:
These six hydraulic power units are each controlled by standard PLCs (programmable logic controllers) and currently receive pressure feedback from the simulator that indicates the pressure of each system. Given that the system must wait for flights, a new pump control was devised that operates under the following parameters:
- The capability to discharge both pumps
- Individually
- The ability to return the pumps individually with proportional control
- The ability to turn on and/or off the electric motor for each pump individually
- The ability to recover from a completely dormant (off) HPU within 30 seconds
- We then began creating control scenarios wherein the HPU would respond to the simulator’s needs without requiring the simulator control system for decision-making. Through a communication link between processors, we utilize information from the motion control computer to tell us when the cabin doors open for people to exit the vehicles, when the doors are blocked, and when the seat belt is blocked to aid in knowing when we need to prepare for flight.
If there is no need for the simulator, an inactivity timer starts timing at the end of the previous flight. A normal dwell time between flights is 3 minutes, so at 5 minutes of inactivity, we begin systematic shutdown of the HPU, which occurs in stages. Each of these stages has a different recovery scenario, as seen in Figure 2.
Since we never know when a flight request might come in, we need to be able to recover from any step in the inactivity process as quickly as possible. Recovery time varies from 9 seconds for the first stage to 30 seconds for a completely off HPU.
Notably, at the point of discharging the primary (lead) pump to 250 psi, the total system pressure also drops to 250 psi. The oil stored in the accumulators begins discharging through all six servo valves at 12.5 gpm. After 2 to 3 minutes under these conditions, the accumulators discharge to 250 psi and require an additional 22 seconds of recovery time to refill completely.
Conclusion of the Hydraulic Pump Control Solution:
The energy savings that can be achieved from this type of control should be substantiated by collecting data after installing a prototype system. However, conservative estimates tell us that if at least four simulators utilize the inactivity process four times, we could save an additional 155490 kilowatt-hours daily.
This is equivalent to $19436 annually. When added to the $30742 that can be saved by discharging and shutting down HPUs at both ends of the day, we will see a total savings of $50178 in annual electricity costs.
There are many new solutions for energy savings that are excellent additions to new designs. However, sometimes those newer solutions do not integrate well with older systems. In our case, variable frequency drives were considered to enhance our design, only to find that the total cost of implementing them was prohibitively high, with a return on investment exceeding 10 years.
Sometimes when seeking energy savings, you may find that the old tools of the trade that we have been using more intelligently for years could provide significant savings in energy consumption.
This type of pump control may not be suitable for your applications, but its components may be beneficial in your industry.
I hope you enjoyed reading the article (Hydraulic Pump Control Solutions: Energy Savings in Hydraulic Systems).
For any questions or suggestions, please contact the Pargarān team.
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