I am looking for some help out there or any guidance if this has been seen before:

Our unit is a 1955 Lynn machine -15megawatts. We just went through a full overhaul and we still have a mysterious issue of the MSV closing on its own erratically. The over speed is not activating it, and the solenoid trip valve is not activating, and the emergency trip valve is not activating it. The data that we collect is showing that the MSV is the first to activate, but all of the levers stay in the reset position. In order to restart the machine the levers need to be tripped prior to re opening the MSV.

Anyone that can help, it is greatly appreciated.

Thanks

 

Gary H. Smith,

Without being able to see the hydraulic system/line-up instruction drawings for the turbine it's very difficult to recommend a possible cause, but if the valve is hydraulically-actuated against spring pressure then it would seem that something is causing a loss of hydraulic pressure to keep the valve open. My recommendations presume hydraulic pressure is used to open the MSV and keep it open, and that when hydraulic pressure is relieved a large spring will close the MSV.

You've eliminated the mechanical overspeed bolt and the trip solenoid, but are there any other devices in the hydraulic "supply" to the actuator that might have worn or scored surfaces (o-rings; cylinder walls; etc.)? Perhaps the hydraulic actuator itself?

You say you just completed a full overhaul; did that include the hydraulic actuator for the MSV? What was done to the MSV actuator during the overhaul? If the MSV and actuator was worked on during the overhaul, when was the last time they were worked on?

Did this problem exist BEFORE the overhaul?

Have any components of the hydraulic system been replaced recently?

What kind of control system is in use on the turbine? Are you confident the outputs of the control system are working properly and not intermittently failing?

How are you able to gather data? Can you monitor MSV actuator pressure--perhaps by temporarily installing a pressure transmitter? How do you know the MSV is first to actuate?

If the MSV is mechanically latched open, is there any vibration that could be unlatching the mechanism? Is the latching mechanism worn?

Again, without more information about the MSV and how it's opened and held open and the control system/scheme we can only make a presumption and recommendations on the presumption. A lot of these older machines have been modified through the years, and even when they were manufactured differences were common as smaller machines were kind of forgotten step-children to the larger steam turbines (though I'm sure the Lynn, MA, personnel manufacturing the turbines wouldn't completely agree).

Please write back to let us know how you fare!