Frame 6B Trip on High Lube Oil Temp

D

Thread Starter

davidmcc

Hi All,

Following an oil leak with no ratcheting when coming off load and 3 hrs oil circulation, machine came back with minimal vibrations. however after a few hours running, tripped on high lube oil temp on load gearbox. Any ideas?

I've been assured that the oil is the same spec, but this was my first suspicion - seems unlikely to me that any gearbox damage has occurred while all the bearings temperatures are ok. Declared available at 32MW instead of normal 42MW to allow continuous running without hitting limit. Thanks for any advice.

Regards,
David
 
davidmcc,

It's not clear how the oil could change specifications. Do you mean it has suddenly developed a bad pH, or the total dissolved solids suddenly changed, or something like that?

While it's common for there to be a slight increase in bearing metal temperature when vibrations increase, it doesn't always occur. And, conversely when bearing metal temperatures increase that doesn't always mean that there has been an increase in vibration, though the two also commonly occur simultaneously.

What is the bearing header temperature (usually signal name LTTH1)? Has it changed? When the bearing header temperature increases then the oil doesn't remove as much heat from the bearings as before and that usually causes bearing metal temperatures to increase as well--but that happens across the unit because all the bearings are fed from the same header at the same oil temperature.

Have you verified the sensor in the load gear box (reduction gear box) drain is working correctly?

Where was the leak that caused the lube oil to have to be shut off? Was it in the Load Compartment where the load gear box (reduction gear box) is located?

And lastly, there are usually multiple bearings in the load gear box but only one common drain. Did all the load gear bearings increase in temperature, or just one or two? In other words, what would cause the common oil drain temperature to increase to the trip setpoint.

And, by the way, it's not common for there to be a trip on bearing drain temperature--from just about any bearing, including thrust bearings. There were some packagers of GE design heavy duty gas turbines which added such trips to the units they sold, or there were some Customers which requested such trips--but they are not, by any measure, common.

It's hard to imagine how a common oil drain could increase in temperature <b><i>based on the information provided</b></i> unless there was some kind of restriction in the return flow path--which is the reason for the question of where the oil leak was. If the load gear box bearing metal temperaures didn't increase very much AND the bearing header temperature was fairly constant it's hard to understand how the drain temperature could increase to a trip setpoint.

Please write back with your findings!
 
After the oil leak, about 3500 litres was replaced. I was thinking maybe the spec was different, and it's cooling capabilities not as good as the original oil - maybe unlikely.

We were seeing an increase in temperature across all measurements. It just so happened that the gearbox drain hit the trip temperature first. So we don't suspect a faulty temperature sensor because they were all showing a raised temperature.

The leak was at the DC ratchet pump small bore pipework which fractured as the pump mountings came loose. It's in the accessory compartment where the starting diesel engine is.

All drain temperatures increased. We do have a trip on the gearbox drain temperature, but we have found some non-standard programming on our machines in the past. (we had to modify the program where the a.c. lube oil pump was still running to maintain temperature when at sync speed. But the sync permissive needed to see it stopped so if it happened to be running for temperature maintenance then sync and breaker close wouldn't happen).

We're looking to replace the cooling water flow valve through a process of elimination as it seems to have been known amongst users to cause these kinds of symptoms. It may be just a coincidence that it happened after the oil leak. We don't have any vibration issues. So we seem to have got away without bearing damage due to 3 hrs oil circulation after stopping before the oil was all lost.

Will post findings in due course.
 
davidmcc,

Thanks for the feedback!

So, it seems the L.O. Bearing Header temperature was not being monitored and controlled correctly. The typical Robertshaw temperature regulating valve used by GE and packagers of GE-design heavy duty gas turbines for DECADES was extremely misunderstood and commonly misadjusted. This is one of those parameters (L.O. Bearing Header temperature) that was NOT controlled by Speedtronic turbine control systems, but was controlled "locally" by the temperature regulating valve actuator.

Many people believe the hand-crank on the side of the valve's actuator was to be used for adjusting temperature, but that is incorrect. The manufacturer's instructions were almost always provided in the Service manuals supplied with the units, and are easily found on the World Wide Web. There is a small triangular-shaped "wheel" at the top of the adjusting mechanism on the shaft that is to be used to set the temperature. It's a LONG process, usually about 15-30 minutes between adjustments of the triangular-shaped wheel to see the steady-state effects of a change.

The valve has an internal hole drilled in the plug which prevents the comnplete shut-off of cooling water to the L.O. heat exchangers in the event of a failure of the actuator (which is common because many people walk on/damage/cut the capillary tube which connects the actuator to the sensor because it's usually never properly installed/protected during commissioning and maintenance activities). This is a key attribute of any replacement regulating valve.

Many new GE-design heavy duty gas turbines are shipping with pneumatically-actuated L.O. temperature regulating valves, which receive a 4-20 mA signal from the Speedtronic turbine control system to an I/P actuator that controls L.O. Bearing Header temperature by looking at the header temperature T/Cs (usually at least two of them, for redundancy) as feedback. (No; I don't have any information on the manufacturer of these valves, nor do I know if they are fail-safe (meaning they will not completely shut off cooling water flow in the event of loss of control signal/air supply--which is another consideration which must be taken into account when choosing a new controller/actuator.)

I know of several sites which have replaced the as-shipped actuator on the Robertshaw valve with either pneumatic or electraulic (a very small, self-contained electric (125 VDC) hydraulic pump and cylinder and control) which receive a 4-20 mA signal from the Speedtronic turbine control system. A couple of the electraulic actuators seem to have failed prematurely, but mostly, I believe, because--again--the manual adjustment was used to control temperature which caused the pump motor to burn out or the pump to leak.

Robertshaw might be also selling a conversion kit for these actuators; there's really nothing wrong with the valve itself--it's perfect for the application. It's really just the actuator--or, more correctly, the operators and maintenance people who don't understand, or take the time to understand, how the actuator works and how to adjust it, and how to properly protect it from damage during maintenance activities. It's a really good self-contained, mechanical actuator which has worked at thousands of sites for decades with proper knowledge of how it works.
 
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