What should we choose Mark VIe Simplex, Dual or TMR configuration?


Thread Starter


Dear folks,

I would like to know or I would like to get different view about the subject of choosing the appropriate configuration of the Mark VIe developed by GE.

GE propose simplex, dual and TMR, which is the more suitable configuration for my application. Before choosing should I cheek the most reliable and cheapest one. I would like to have opinion and experience of engineer who has dealt with this situation.

Thanks in advance
Speaking as a former GE engineer, for new gas turbine and steam turbine applications, GE will propose TMR Mark VIe controls. These controls are very reliable when the controls and instrumentation are maintained according to GE's recommendations. Most of the problems I have seen occur when users treat the triple redundant sensors as wired spares. If you don't replace failed sensors, you don't have TMR! The purpose of any redundant system is to permit you to continue operation with a single failure, but you have to correct that failure at the earliest opportunity. In general, failed components of a TMR control system can be replaced while the unit is running.

For combined cycle plants, GE will propose TMR controls for the turbines and dual redundant controls for the HRSG and BOP equipment. This is consistent with industry practices. Critical (protective) sensors are still triple redundant. (Some, like drum level, are used for both protection and control.) Closed loop control sensors are dual redundant (where there are dual redundant pumps, each pump may have simplex sensors, but since there are 2 pump systems, there is still a backup). Generally, the control valves are not redundant, due to cost considerations. A failed actuator can usually be manually controlled at least until a "normal" shutdown can be made.

For replacement of old (Mark I, Mark II or older) controls where redundant sensors are not currently installed, simplex controls are usually proposed, again for cost considerations. It would make little sense to provide redundant controls without redundant process sensors, and installation of redundant process sensors on older turbines is generally cost prohibitive.
> GE propose simplex, dual and TMR, which is the more suitable configuration for my application. <

What is your application?

What control system do you currently have?

Are you considering any control systems from other vendors? If so, what are they or what have they proposed?

The answer, as usual, is not a simple one. Many people find Speedtronic turbine control systems to be very reliable. So, for may part reliability is not the issue (I'm sure others will have other opinions about this statement).

SIMPLEX control systems make sense for turbines which don't run day and day out, or are "cycled" on a daily basis. It's likely that a failure of any component in a SIMPLEX control system will require a shutdown of the turbine to replace the component. So, if running reliability for a turbine that runs for long periods between shutdowns, then a SIMPLEX probably isn't the best choice.

Any time a control system is made to be redundant through the use of redundant components, the number of component failures increases. I didn't say the component failure rate increases, I said the number of failures increases. More components equals more failures even at the same failure rate, right? So, redundancy requires one to have more spares (redundant spares) to properly support operations. I can't tell you the number of sites that have one of each card (when three or four or more of the same card are used in the panel) and when that card fails it seems that another one of the same cards fails before the replacement is received! It's a very frustrating situation to be in, so if you buy a redundant system, make sure you order and stock sufficient replacement parts to cover the possibility that more than one of a particular card will fail in a short period of time.

If your application requires the ability to replace components while the unit continues to run, then a redundant system is the better choice. Whether or not you choose DUAL redundant or TMR, one of the consideration would depend on your application and any future plans for the unit. For example, if you plan to possibly upgrade the combustion system to a DLN system, GE likely won't do that with anything other than a TMR system. So, if you by a DUAL Redundant system now and in a couple of years want to convert to DLN combustion, you'll likely have to replace the control system with a TMR system.

I don't know if GE has any data on MTBF (Mean Time Between Failures) or MTTR (Mean Time To Repair) for DUAL Redundant and TMR systems; if they do, it would behoove you to look it over and factor it into your considerations.

Hopefully this will get you started in your deliberations.