Emerson’s Mike McQuade, a member of the Valve Automation business unit, describes the process of storing hydraulic energy for slurry pipeline shutdowns. Slurry pipelines are found in mining operations where ore is mixed with water and pumped from the mining area to the processing area.
Extended slurry pipeline runs need to be protected from total drainage in the event of a catastrophic rupture of the piping. Normally there are valves positioned in the pipeline grid with a specific function to isolate a section of the pipeline. Pump stations, choke stations, and slurry receiving terminals all need to be isolated in an emergency.
The large isolation valves typically have associated hydraulic actuators along with hydraulic power units that provide the force for closing the valves. The valve operating system must have the capability of closing the valves even after the loss of electrical power. To accomplish this requirement, an energy storage device is needed which may be a battery back-up system, or more commonly, a hydraulic accumulator bank.
A critical concern is–does the accumulator bank have enough stored fluid volume at the right pressure for a complete process shutdown? Monitoring the pressure in an accumulator bank is relatively simple but knowing the exact volume of fluid in an accumulator presents a challenge—especially if a bladder-type accumulator is used.
An accumulator charging method known as thermal volume motor control provides an effective method for solving the pipeline operators concern. Explained below is the operating principle.
If you look closely at the thermal volume motor control, a rod extends out the bottom of the cylinder. This rod effectively creates a smaller surface area on the bottom side of the piston than on the top side. In contrast, the piston in the primary accumulator has equal surface areas on both sides. The net effect is that the hydraulic pump will always drive fluid into the accumulator first. As the motor/pump continues to run, the accumulator piston will top-out against the upper head of the accumulator. Only after the accumulator piston can move no further will the thermal volume motor control, piston/rod begin to move. As the piston/rod moves up, it trips a switch and stops the motor.
The pipeline control room can now monitor the pressure in the accumulator and they inherently know that the accumulator is completely topped-out full of hydraulic fluid or the electric motor that charges the accumulator would have never turned off. With this accumulator-charging method, the operator knows that even if electrical power is lost there is enough stored hydraulic fluid at the right pressure for a complete process shutdown.