Hydraulics

The penstock is controlled by a hydraulic system to raise and lower it.

When the control system wants to open the penstock it instructs the hydraulic system to turn on the hydraulic pump.
This pump forces oil into the lower part of the hydraulic ram and this opens the penstock.
Oil displaced from the upper part of the ram is forced into the accumulator.

When the control system wants to close the penstock it instructs the hydraulic system to open the “close valve”.
This allows the oil pressurised by the accumulator to push oil into the upper part of the hydraulic ram and this forces the penstock to close downwards.
The accumulator operates like a giant spring which is constantly pushing down on top of the penstock continuously trying to close it.

Try the simulation here to see for yourself how this works

The only information the control system receives from the hydraulic system is from limit switches on the gate itself.
This means it only knows if the penstock is “open”, “closed” or “transit” (somewhere in between).
Therefore there is no information about the health of the hydraulics available to the operations staff.

In July 2019 we installed a pressure sensor to monitor the accumulator pressure. This enables real-time monitoring of the health of the system and enables us to know how much the penstock is open. In theory this knowledge could be used to improve the accuracy of the calculation of the rate water is flowing through penstock using the Bernoulli equation adapted for a drowned/submerged sluice/penstock.

The data logger now records the accumulator pressure and this allows us to monitor (through calculation) the accumulator precharge pressure and the range of pressures caused by opening and closing the penstock. A drift in the range would imply either an oil leak or gas leak.

The data logger will email a member of the operations staff if there is a problem, such as:

  • Gas leakage from the accumulator
  • Oil leakage
  • Penstock ram internal leakages
  • Priming valve failure or incorrect setting

The maximum and minimum pressures are derived by saving the accumulator pressure when the penstock is known to be open and when it is known to be closed. The “ideal gas equation“ is then applied using these pressures, the temperature and the actual pressure, to derive the original Nitrogen pressure and the amount the penstock is open.

In October 2019 we installed a more accurate temperature sensor directly on the accumulator.
If you look closely at the Accumulator Temperature graph you can see it dips every time the penstock closes.
This is an Isentropic (reversible adiabatic) process in action where energy is extracted from the accumulator and used to close the penstock.
The generalised equation is T2∝T1(P2/P1) so increasing the gas volume (by letting oil out and decreasing the pressure) the temperature decreases – in our case by about 0.7°C.