Page 413 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Steam Turbines and Turboerpanders 391

Clearances

1. Disassembly is not required to check axial clearances (H, L & M, Fig-
ure 8-10).
2. Remove bearing caps to check radial clearances (A, Figure 8-10) with
feeler gauges.

Overspeed Trip System

Warning: Under no circumstances should the trip valve be blocked
or held open to render the trip system inoperative. Over-
riding the trip system, and allowing the turbine to exceed
the rated (nameplate) trip speed, may result in fatal in-
jury to personnel and extensive turbine damage. Always
close all isolating valves and open drains to depressurize
the turbine casing and steam chest before performing
maintenance on the overspeed trip system.

The overspeed trip pin assembly is contained in the trip body mounted
on the turbine rotor shaft. When the turbine speed increases above the
rated operating speed, centrifugal force exerted on the trip pin (1, Figure
9-12) increases. When the centrifugal force overcomes the force of the
trip pin spring (2), the weighted end of the pin protrudes from the trip
body. The pin strikes the plunger assembly (7), forcing it against the ad-
justable jack screw (8) in the bottom of the hand trip lever. The lever
pivots on a shoulder stud; causing the top of the lever to move away from
the resetting lever. This movement disengages the latch from the reset-
ting lever knife edge and allows the closing spring to pull the trip valve
closed. This stops the steam flow through the turbine.
Disassembly, Overspeed Ttlp Mechanism

Note: To check the trip pin for cracks, it is recommended that ei-
ther the zyglo or dye check method be used. The “U” lock
staples should also be examined for bends or cracks. The
overspeed trip pin assembly can be checked by monitoring
the frequency of overspeed trips. Check the assembly at least
every 30 overspeed trips and at two year periods.

1. Remove the steam end bearing cap as outlined earlier.
2. Remove the “U lock staple (3, Figure 8-12), smunding the adjust-
ing nut (4), by prying it out of the trip body.
3. Remove the adjusting nut, trip spring (2) and washers (5), if pro-
vided.