Hydraulic power recovery turbine (HPRT) is a machine used to recover energy from a liquid stream by reducing its pressure. A common type of HPRT is a reverse-rotating centrifugal pump that recovers energy from a high pressure process liquid by reducing its pressure that will otherwise be wasted across throttle valves.
Many types of centrifugal pumps can be used as HPRT with few design changes - horizontal or vertical, single stage or multistage, overhung or between-bearing type. They do not require special metallurgy - carbon steel, stainless steel, or chrome, are some commonly used materials.
Some liquids that have been used with hydraulic power recovery turbine include hydrocarbon, rich oil, lean oil, mea solution, rich catacarb solution, selexol, monoethonolamine, triethanelamine, amine solution, solvent, and many others.
A process liquid may not be available always , or its pressure may be variable, thus HPRT is typically not used as standalone driver, rather, it is commonly used in a train arrangement that includes the driven equipment, an electric motor as primary driver, and an HPRT as secondary driver to unload the electric motor to reduce its power consumption.
A typical train arrangement would consist of this sequence: the driven equipment, an electric motor with double-extended shaft, a clutch, and an HPRT. A clutch is included so the HPRT can be disconnected from the motor in case the process liquid becomes unavailable, or its pressure becomes too low, to avoid the HPRT from becoming a drag to the system.
Some train may have the following arrangement: an HPRT, a clutch, the driven equipment with double extended shaft, and a motor. In this arrangement, it must be ensured that the driven equipment is accessible without physically removing the clutch. In one such train arrangement, a company supplied a radial split case pump (driven equipment) between a clutch and a motor. It turns out that the rotor of the radial split case pump cannot be removed for servicing without removing the clutch – a big error that tarnished the reputation of the company that designed the train arrangement.
The design of HPRT are basically the same as a pump except for some changes due to its reverse rotation and high inlet pressure. Example, its threaded parts are threaded in reverse direction to prevent the parts from becoming loose during operation. Its bearing type, and the manner of its hydraulic thrust balancing, may require some changes depending on its pressure. The nozzles of an HPRT are the reverse of that of a pump - the inlet and outlet of an HPRT are the discharge and suction nozzles, respectively, of a pump.
The performance of a hydraulic power recovery turbine is different from that of a pump. Whereas a pump can operate from its minimum to maximum flow range, an HPRT must operate near its best efficiency point (BEP). At some point below its BEP, the capability of an HPRT to recover energy is greatly diminished and, farther below that, the HPRT becomes a drag to the system.
The power recovered by HPRT is calculated from the equation:
Q x H x SG x E HP = ----------------------- 3960
HP - energy recovered by turbine, horsepower Q - turbine capacity, gallons per minute H - differential head across turbine, feet SG - specific gravity of liquid E - turbine efficiency, decimal
A typical hydraulic turbine performance curve is shown above [ * ]
The second part of this article [ * ] provides the answers to these questions:
How is a pump selected as hydraulic power recovery turbine?
How is a pump curve converted into an HPRT performance curve?
In controlling the operation of HPRT with throttle valve, should the valve be installed upstream, or downstream, of the HPRT, and why?
[ * ] Some information are excluded in this article. Read more.