Why Use Bently Nevada in Tandem with Mark VI?

The Bently Nevada controls may be for other components like pumps and motors as part of the control strategy, it may also present data info at specialized access points....

http://www.ccj-online.com/4q-2014/control-systems-cogen-plant-in-the-oil-patch-gets-a-controls-makeover/

The Bently Nevada is used solely to monitor the vibration on the engine, high vibrations will trip the turbine, the Mark VI system to control the engine and the ABB DCS to control and monitor all the auxiliaries for the HRSG boiler feed pumps, steam valves etc.

Does the Bentley have any S/D controls? Are the output relays shutting anything down directly or do they just tell the Mark VI to shut things down? You seemed to suggest that the Bentley is keyed to the shaft rotation and can see much finer detail than the Mark VI, maybe it is there for just that reason. The next question would be can anyone drag data out of it if you need to. I agree that the DCS is just there to handle the auxiliary stuff; it would be rare for GE to let somebody else control their turbine.

I am pretty certain, the Bentley Nevada equipment does not have SD capability, however it does transmit vibration readings to the Mark VI system, and that will produce a shut-down in high high limit.

I am just waiting for turbines to all be equipped with the latest active vibration suppression technology in the bearings (mag/other bearings that are actively controlled), and see how that "flies". I heard about this technology being tried a few years back.

I was at a pump facility recently where they ran a bunch of stuff including bearing temperatures into the Bentley and then ran a signal out of that direct to the Multilin on the switchgear bypassing the PLC and shutting down the pump. It wasn't clear to us why they did that other than to make it difficult for us to troubleshoot what the hell was going on in the unit.

It's called "defense in depth", you can't rely on a single point of measurement to protect your equipment. The Bently Nevada is there to provide an independent, albeit redundant, monitoring point. Since it's a dedicated unit it's also faster and more reliable than the multi-tasking Mark IV, and can provide a calibrating/comparison signal to check the "sanity" of the Mark IV's readings; if they're not reading the same then it's time to find out why.