Issue No. 252〔Seminar〕
Fluid Coupling for Boiler Feed Pump
Author
Yoshinori KATAYA
Fluid Machinery & Systems Company
1. Introduction
2. Overview of fluid couplings
(1)Reducing the power consumption by controlling the output rotating speed according to the plant load;
(2)Absorbing shocks to the driver and driven machine because they transmit power through oil; and
(3)No need for special driver design or electric equipment even for a turbo machine with a large inertial moment, because they allow the driven machine to be started with almost no load applied to it
2.1 Basic structure
A fluid coupling basically consists of two components: an impeller and a runner. The impeller is coupled to the drive shaft and is also called the pump vane. The runner is coupled to the driven shaft and is also called the turbine vane.
As Figure 1 shows, the impeller and runner are wheels that have radially-arranged straight blades, face each other, and provide an impeller casing to contain oil inside.

Fig. 1 Basic structure of fluid coupling
2.2 Operation principles

Fig. 2 Operation principle of fluid coupling
2.3 Speed variation principles
As described above, a fluid coupling uses velocity energy of oil to transmit power. With an increase in the amount of oil flow from the impeller to the runner, the power to be transmitted increases, resulting in a higher output rotating speed. On the other hand, with a decrease in the amount of oil flow, the power to b transmitted decreases, resulting in a lower output rotating speed. The amount of oil flow from the impeller to the runner is determined by the amount of oil filled inside the coupling. As Figure 3 shows, a fluid coupling, which operates at a given output rotating speed, controls the output rotating speed by scooping a certain amount of oil inside the coupling using a scoop tube to regulate the oil amount.

Fig. 3 Speed variation principles of fluid coupling
3. Fluid coupling for motor-driven boiler feed pump
3.1 Structure

Fig. 4 Structure of fluid coupling for boiler feed pumps

Fig. 5 General view of fluid coupling for boiler feed pumps
[1] Speed increasing gear
[2] Impeller and runner
[3] Bearings
[4] Scoop tube
Operating at stepless output rotating speed, the fluid coupling for motor-driven boiler feed pump is provided with a scoop tube.
Since the scoop tube is used to scoop oil circulating at a high speed, the tube tip is made of a special alloy for higher erosion resistance.
[5] Oil pumps
3.2 Scoop tube operation mechanism
As a mechanism for operating the scoop tube, an electro-hydraulic servo mechanism, which is our proprietary technology, is used to achieve a compact and highly reliable system.
Operation principles of the electro-hydraulic servo mechanism are described below.
3.2.1 Speed reducing mechanism (Figure 6)

Fig. 6 Speed reducing mechanism
3.2.2 Speed increasing mechanism (Figure 7)

Fig. 7 Speed increasing mechanism
3.3 Working oil system

Fig. 8 Working oil system
3.4 Lubricating oil system

Fig. 9 Lubricating oil system
4. Trends of fluid couplings for boiler feed pumps

Fig. 10 Changes in rated transmitted power
5. Conclusion
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