Hydraulic Speed Governor with Starting and Load Limiting Device

Hydraulic Speed Governor with Starting and Load Limiting Device

Function

• The function of the hydraulic speed governor is to operate the control valves to give the appropriate turbine steam throughput for the particular load condition. 
• The arrangement and functioning of the governor within the overall governing system is described in the section on governing.

Construction

• The principal components of the speed governor are the bellows (8), the link (11), the speed setting spring (13), the sleeve (5) and the follow-up piston (4). 
• The primary oil supply from the hydraulic speed transmitter is available at connection ‘a1‘. 
• A fire resistant fluid is used as the hydraulic fluid in the governing system.
•  An additional bellows (9) prevents primary oil getting into the control fluid circuit if there be a leakage in the governor bellows (8). 
• In this case, the leakage oil can be drained off via connection ‘c1 ‘. 
• In case a leak in the bellows (9) occur, the control fluid that has leaked in will also be drained off via connection ‘c1’.
• The primary oil pressure (connection ‘a1‘) is dependent on the speed and determines the position of the link (11) via the bellows (8) and the push rod (10). 
• The speed setting spring (13) opposes the primary oil pressure. Its pre-compression can be varied either by hand or remotely by the motor (16). 
• The sleeve (5) which can slide on the bottom end of the follow-up piston (4) is attached to the link (11). 
• The follow-up piston is held against the auxiliary secondary fluid pressure (connection ‘b’) by the tension spring (3). 
• The follow-up piston and the sleeve have ports, which at normal overlap allow sufficient fluid to escape to produce equilibrium between the auxiliary secondary fluid pressure and the force of the tension spring (3).
• Each steady-state position of the link (11) and hence of the sleeve (5) corresponds to a specific force from the tension spring (3) and hence to a specific secondary fluid pressure which in turn determines the position of the control valves.

Mode of Operation

• If the primary oil pressure falls (as a result of increasing load and the resulting drop in speed), the link (11) and the sleeve (5) sliding on the follow-up piston (4) are moved downwards by the speed setting spring (13) so that the overlap of the ports in the sleeve and the follow-up piston is reduced. 
• This causes the pressure in the auxiliary secondary fluid circuit to rise and the follow- up piston follows the movement of the sleeve against the increasing force of the tension spring (3) until normal overlap of the ports and equilibrium are restored. 
• The lift of the control valves is increased in this manner by the increased secondary fluid pressure. 
• Conversely, a rise in primary oil pressure causes the lift of the control valves to be reduced. 
• When the pre-compression of the speed setting spring (13) is varied with the reference speed setter it changes the relationship between the primary oil pressure and the secondary fluid pressure and hence the relationship between speed and power output. 
• Lever (12) allows the link (11) to be depressed by hand to give a lift signal to the governor, e.g. to provide a second means of overspeeding the machine for testing the overspeed trips in addition to the overspeed trip tester.

Starting and load limiting device

• Before start-up, the pilot valve (21) is brought to its bottom limit position either by hand or remotely by the motor (20). 
• This causes the bellows to be compressed via the lever (6) and the pin (7) until the governor assumes the position “Control valves closed”. 
• With the pilot valve (21) in the bottom limit position, control fluid from connection ‘a’ can flow simultaneously to the auxiliary start-up fluid circuit (connection ‘u1‘) and as start-up fluid via connection ‘u’  to the stop valve to prepare these for opening. 
• When the pilot valve (21) is moved back the auxiliary start-up fluid circuit is depressurized and subsequently the start-up fluid connection ‘u’ is opened to the return ‘c’. 
• This opens the stop valves. Further upward movement of the pilot valve (21) causes the pin (7) to release the bellows as with falling primary oil pressure and the control valves are opened.
• The release of the bellows can be limited by the pin (7) so that the control valves do not open any further despite a further reduction in primary oil pressure.
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