Rigidly coupling an electric motor with a compressor or pump will have to deal with the problem of starting torque which can be overcome when a soft coupling such as a hydrodynamic transmission is utilized to couple the output shaft of the motor and the input shaft of the compressor. Hydrodynamic transmissions such as hydraulic couplings and torque converters are fluid couplings that typically use oil as a process liquid to transmit power from an output shaft (driver) pump to an input shaft (driven) turbine.
This article contains excerpts from the paper, “Hydrodynamic torque converters for oil and gas compression and pumping applications: basic principles, performance characteristics and applications” presented at the 2016 Asia Turbomachinery & Pump Symposium by Klaus Brun, then with Southwest Research, Christoph Meyenberg of Voith Turbo, and Joseph Thorpe, engineering consultant.
Here a pump is used to energize the working fluid (oil) which then drives a turbine and returns the oil to the pump in a closed circuit. The pump is connected to the driver shaft and the turbine to the driven equipment shaft. The more power the pump introduces into the working fluid, the more is available for the turbine to drive the driven equipment. Since power equals rotating speed multiplied by torque (P=Mω), the available power for the driven equipment shaft can either be utilized as a high torque at low speeds or as low torque at high speeds. Typical drivers are electric motors, gas turbines, steam turbines, and internal combustion engines and driven equipment can be variable speed pumps, centrifugal compressors, and reciprocating compressors. Hydrodynamic transmissions have the following common technical functions and features:
- They provide step-less variable speed and variable torque output.
- Because they are soft coupled, their output shaft speed is load dependent.
- Torque amplification at low speeds is possible since, for a given available hydraulic power from the pump, the turbine can exchange torque for speed.
- Speed amplification or reduction is possible based on the selection of the pump and turbine performance characteristics.
- Torsional vibrations between the driver and driven equipment are decoupled.
- Because there is no hard interface between the output and input shaft (clutch), there is no direct mechanical wear.
During starting, the pump and the turbine are not rigidly coupled and, therefore, their speeds do not have to be equal. When trying to start the driven equipment, the pump can rotate at a very high speed and produce sufficient power for the required high torque at low speeds of the turbine and driven equipment.
Specifically, for the torque required by the turbine, MT, at low turbine speed, ωT, the pump produces sufficient torque (MP≥MT) at a high pump speed ωP. Additionally the curves demonstrate that the driven equipment could operate over its full speed range from ω/ωN=0 to ω/ωN=1 since the pump power (and torque) will exceed required turbine hydraulic power (and torque) for all conditions.
The hydrodynamic transmission allows for variable speed and torque output which is only limited by its input shaft power and internal fluid losses. It is important to recognize that hydrodynamic transmission is fundamentally different than other types of power transmissions.
Rigidly coupled transmissions such as a belt-drive or a gearbox have fixed speed and torque ratios. Similarly a hydrostatic drive utilizes positive displacement pumps and turbines and effectively acts like a rigid transmission. Only hydrodynamic transmissions allow for slip between the input and output shafts and can provide varying speed and torque ratio power transmission. However, because of internal fluid dynamic losses, hydrodynamic transmissions tend to have a lower transmission efficiency (= Power Out / Power In) than rigidly coupled transmissions
In a typical hydrodynamic coupling with a mixed radial flow pump driving a mixed radial flow turbine the fluid enters the pump axially and is redirected toward the outward radial direction by the rotating blades which impart tangential momentum and thus flow head to the circulating fluid. The high flow head fluid then enters the turbine in the axial direction and is turned toward the inward radial direction and then circulates back into the pump in the axial direction. The turbine converts the flow head which is mostly in the form of angular momentum into mechanical rotation of the turbine to drive the output shaft. The hydraulic coupling thus consists, as a minimum, of two flow components: A pump and a turbine. Torque converters also utilize a stator.