Overspeed Trip Test of Main Turbine

Testing with Turbine under Load Condition

• Overspeed trips MAY10 AA001 and 002 can be tested using test device MAX62AA001 with the turbine running under load or no- load conditions. 
• To operate the test device, pilot valve KA03 is first pushed downwards and held in this position. 
• This isolates the auxiliary trip medium circuit from the overspeed trips and prevents the main trip being initiated by the overspeed trips. 
• Subsequent operation of hand-wheel KA01 moves the center pilot valve downwards. 
• This action blocks the drain and allows the control oil to flow through the center bore of the pump shaft into overspeed trips. 
• The control oil pressure thus builds up and moves the eccentric bolts/strikers outwards against the spring force, releasing the pawls of the overspeed trip releasing device, as a results of which the pilot valve moves rapidly inwards. 
• The pressure in the auxiliary trip medium circuit, up to the over speed trip test device, then collapses. 
• Operation is followed by observing the reading at pressure gauge MAX52CP501. 
• The trip pressure is read off at pressure gauge MAX62CP501. 
• If during operation at rated speed, this pressure should deviate from the baseline value as recorded in the test report, a defect in the overspeed trip may be assumed. 
• If the trip pressure is too high, the bolt may be made to move freely by rapidly operating the pilot valve by means of hand-wheel KA01 several times in succession. 
• If this measure does not have the desired result, the turbine must be shut down and the emergency governor to be inspected. 
• As soon as the auxiliary trip medium pressure drops to 0 at pressure gauge MAX52CP501, the center pilot valve must be returned to its original position using hand-wheel KA01. 
• The pressure in the test line should then return to 0, as can be read off at pressure gauge MAX62CP501. 
• The bolts/strikers of the overspeed trips should return to their original position. 
• When this happened, pilot valve KA02 must be pushed downwards to admit control medium into the auxiliary start-up medium circuit to the differential pilot valve of the overspeed trip device. 
• The pilot valve moves towards the right and latches the overspeed trip device in again. 
• The buildup of pressure in the auxiliary start- up medium circuit between the overseed trip test device and the overspeed trip release device can be followed at pressure gauge MAX48CP501. 
• When pilot valve KA02 is then released, the auxiliary start-up medium pressure returns to 0 pressure. 
• The auxiliary trip medium pressure must then remain at its full value (readable at pressure gauge MAX52CP501). 
• If this is the case, pilot valve KA03 may be released. 
• The test is completed. 
• If, when valve KA02 is released, the auxiliary trip medium pressure collapses, pilot valve KA02 must be pushed downwards again and must be held in this position a little longer. 
• It is essential that the auxiliary trip medium pressure must remain steady before valve KA03 is released.

Testing with Turbine under No-Load Condition

• Overspeed trips MAY10AA001 and 002 must be tested at regular intervals by running the unloaded turbine up to trip speed. 
• This is done by operating lever KA07 of hydraulic speed governor MAX46BY001, which presses linkage KA03 downwards, thus increasing the secondary medium pressures. 
• This causes the control valves to open and the turbine starts to overspeed. 
• The actual speed at which trip occurs can be read off at pressure gauge MAX44CP501.
• Limit switches MAY10CG001&002C of overspeed trip release device MAY10 AA001 and 002 indicate in the control room that main trip valves MAX51AA005 and MAX51AA006 have been actuated by overspeed protective device.