Download - Steam turbine over speed trip systems (6)
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.