Ops Tips - Mechanical Overspeed Trip Governor

Mechanical Overspeed Trip Governor

(Worthington & Turbodyne design units only)

Just what is an overspeed trip governor? It is a device which operates to stop the turbine when its speed exceeds a certain predetermined value for which the overspeed governor has been set, but which is inoperative as long as this value is not exceeded.

A trip bolt is set slightly off-center and is held in place by spring force. As speed increases and exceeds the overspeed limit, centrifugal force on the bolt exceeds the spring force, the bolt extends, striking the trip paddle and releases the linkage.

It is important that the entire emergency trip system be properly adjusted and free of binding or lost motion. The operation of the overspeed governor should be tested as often as possible (preferably monthly), especially during times that maintenance is performed. A permanent record of these tests should be kept for future reference. The overspeed governor should function to shut down the turbine three consecutive times within 3% of the trip speed specified on the Turbine Data sheets supplied with your instruction manual. If the operational tests indicate a requirement for adjustment, proceed as follows:

Adjustments

Once a turbine is in service, adjustments to the overspeed governor are uncommon. If adjustments are needed, they are typically minor. Failure of the overspeed governor to trip within the prescribed limit is most often caused by a problem with linkage. Prior to making the following adjustments, check to ensure trip linkage is free of dirt and corrosion, all latch points are sharp and all engagement points have the proper clearance.

Gain access to the overspeed governor; see governor section drawing for location of trip finger and to determine if overspeed governor is in the cup design or in the shaft design.

Manually rotate turbine shaft until setscrew is accessible, then loosen the set screw.

Using a suitable spanner tool, turn adjusting screw clockwise to raise tripping speed or counterclockwise to lower, as necessary.

Tighten lock screw securely to lock adjustment position, then repeat overspeed trip test.

Notes

For 4" and 6" trip and throttle valve systems, the clearance between the overspeed trip lever and the emergency weight, shown in the Governor Section drawing, is properly set at the factory for .060" plus/minus .010", (1.52 mm plus/minus .254 mm).

NEW!! For 4" and 6" Venturi trip systems, the clearance between the overspeed trip lever and the emergency weight is properly set at the factory for .090"/2.286 mm plus or minus .010"/.254 mm.

Warning!

Prior to adjusting the overspeed governor, please refer to the unit O&M manual or contact your D-R service organization.

The overspeed trip assembly should be locked in place before starting the turbine. Three checks to make are as follows:

The overspeed cup must be tightly screwed into the shaft.

One of the tabs of the lockwasher is bent into the milled slot of the overspeed cup.

If the overspeed cup has a drilled and tapped hole for a setscrew – be sure that a screw is installed, tightened and staked into place. The runout of the cup must also be checked and adjusted to within 0.002 inch TIR.

The overspeed cup will have two milled slots for the tabs of the lockwasher, but it will only be possible to line up one of the tabs. This is due to the fact that the tabs are not symmetrically located around the outer diameter. There is no need for concern as long as one of the tabs can be completely bent over into the slot.

At not time during the testing process should the turbine speed exceed 103% of the overspeed trip setting.

Always ensure the driven machine is suitable for the speed of the overspeed limit.

Only small adjustments to the adjusting screw are needed to make large changes to the overspeed setting.