STEAM TURBINE OVER SPEED TRIP SYSTEMS

Prem Baboo

Sr. Manager (Prod)

National Fertilizers Ltd., Vijaipur, India

TRIPPING DEVICE

• Whenever the turbine is to be tripped, the governing oil pressure (trip oil line

after tripping device) is being drained by tripping device.

• Thus pressure in front of stop valve piston disc secondary oil pressure falls

resulting in closure of stop valve piston disc and control valves.

• In addition to a speed control system, steam turbines are fitted with a shutdown

system to prevent damage to the machine.

• In the event the speed governor fails to control the speed, the over speed trip

actuates to shut down the machine.

• When shaft speed exceeds a desired safe level, generally 10% over speed, a

latching device or oil dump mechanism is actuated to close a special emergency

stop valve.

• This system is totally independent of the governor

OVERSPEED GOVERNOR

� Over speed governor protect the turbine against speeds higher than the safe

value for turbine operation.

� Over speed governor consists of an eccentric pin located inside the turbine shaft

and held in position with a spring.

� At a preset speed (trip speed) centrifugal force of eccentric pin will move out of

shaft.

� The out ward movement of the pin actuates a lever of tripping device and

thereby tripping the turbine.

ECCENTRIC PIN

The governor and over speed systems vary from machine to machine and may be

mechanical, hydraulic, electrical, and pneumatic or combinations.

Governor control systems consist of three basic elements.

Sensing ↦ Transmitting ↦ Correcting.

Eccentric Pin

• A mechanical system that is completely separate from the speed governing

systems.

• A trip pin or plunger is mounted in the turbine shaft with its center of gravity

slightly off center.

• In the event the speed regulating governor fails to control the speed, the

unbalanced plunger overcomes a spring force at a preset trip speed.

• As it moves outward, it strikes the trip-lever, causing release of a spring dump

valve that releases the trip circuit oil pressure.

• This unbalances a piston-spring combination and causes the trip and throttle

valve to slam shut by the force of a spring and the steam pressure above the

valve disk.

• TROUBLE SHOOTING FOR GOVERNOR

Sr.

No. POSSIBLE CAUSE

SOLUTION

1 Compensation needle valve adjustment of the

Adjust the compensation needle valve.

governor is incorrect

2 Dirty oil or sludge in governor

Drain and clean the PG-PL governor and refill with

new or filtered oil. Bleed air and adjust the

Compensation needle valve.

3 Low oil level which permits air to enter and 1 Add oil to the governor and check oil level against

causes foaming the mark on the sight glass.

2 If the oil level decreases and you don’t see

external leaks on the governor, then check

the drive shaft for oil leaks.

3 If foaming continues, drain the oil and refill with a

different type of oil.

4 Lost motion in engine linkage or fuel pumps Repair linkage and/or pumps.

5 Governor output shaft travel is too short Adjust the travel until the PG-PL governor has the

to provide full fuel.

right travel.

6 Spring too weak in telescopic link Install heavier spring so that the link stays solid at

all times

7 Fuel linkage incorrectly set

Adjust the linkage from governor to engine

to get linear relationship.

8 Faulty linkage.

Check yield links, shutdown arrangements, etc., to

be sure that the engine torque changes for very

Small increments of governor output shaft travel.

9 Rough engine drive or rough governor drive 1 Inspect the drive mechanism.

2 Check the alignment of gears.

3 Inspect for rough gear teeth, eccentric gears or

Excessive backlash in gear train.

4 Check gear keys and nuts or set screws holding

Drive gears to shafts.

5 Check for bent drive shaft.

6 Check serrated or spline coupling for wear and

Alignment.

7 Check the backlash and the gear shimmed so

that there is no binding, if you use a keyed drive

10 Improper governor alignment

Loosen the governor mounting screws and

move the governor slightly on its mounting

Pad to align the drive shaft with its coupling.

11 Failure of flexible drive in flyweight head

Return the governor to Licenser for repair,

Dirty or worn ballhead bearing.

if one of these causes appear.

Rough or worn gear teeth.

Bent speeder spring.

12 Air in the governor which causes a jiggle

Check that the governor has the right oil

during start up or transient.

13 Internal speed droop adjustment incorrect 1 Check if the governor travel is at least 50% of the

total travel, if necessary shorten the terminal shaft

lever.

2 Return the governor to Licenser for readjustment.

14 Speed setting of the governors are not the same 1 Check the control air pressure at the two

Governors with accurate pressure gauges.

2 Check that the manual speed setting knobs are at

Minimum.

15 Governor too slow. 1 Adjust the compensation needle valve for

Maximum opening.

2 Install lighter buffer spring, if possible.

16 Speed setting too high Decrease the start speed setting.