The hydraulic power is supplied to the actuators by an accumulator bank through a blow-down valve. The accumulator bank provides the high flow needed to simulate a transient earthquake signal, and the blow-down valve converts the high-pressure oil from the accumulators (35 MPa) to a constant system pressure of 21 MPa for controlling the actuators. Two standard MTS Hydraulic Power Units (HPUs) are provided to pump oil into the accumulator bank. These HPUs include filters, heat exchangers, and all necessary valves and controls needed for operation. Return flow is directed to an auxiliary reservoir or surge tank. Because of the targeted future upgrade to six degrees-of-freedom, the hydraulic power system construction allows for increasing the flow in the future. The technical specifications for the hydraulic power supply are summarized in the following table.
|Accumulator Swept Volume||7.5 m||295.3 in|
|Accumulator Bank Pressure||35 MPa||5000 psi|
|System Pressure||20.7 MPa||3000 psi|
|Blowdown Flow Rate||38000 lpm||10000 gpm|
|HPU Flow Rate @ 5000 psi||431 lpm||114 gpm|
|HPU Flow Rate @ 3000 psi||718 lpm||190 gpm|
|Surge Tank Capacity||10000 liters||2642 gal|
For the accumulator bank and blow-down valve system, only about 1,900 liter of the 9,500 liter of the accumulator bank capacity are available at or above the nominal pressure of 21 MPa. Therefore, the accumulator bank, when supplemented with the two 720 liter/min hydraulic power units, satisfies the design requirement of providing 2,720 liter at 21 MPa or above in order to simulate the same earthquake record back-to-back during the on-line iterative (OLI) compensation method. The combined pump and accumulator bank configuration of the hydraulic power supply is capable of providing sufficient flow for unlimited tuning time. The accumulator banks (without the pumps) provide a limited tuning time of 2.91 min.
The blow-down valve flow capacity of 38,000 liter/min satisfies the peak flow requirement of 36,000 liter/min. One of the two hydraulic power units with a total continuous flow capacity of 720 liter/min at 21 MPa and 430 liter/min at 35 MPa will be used to charge up the blow-down accumulator bank. The recharge time will be approximately 5 min, which is very satisfactory, leaving the operator just enough time to verify the data acquisition and control system between tests.