I would like to better understand the criteria for sizing the turbo machinery lube oil system, with separate lube oil console (i.e. no shaft driven main lube oil pump).

It is my understanding, that the units equipped with API 614 lube oil console can support a failure in one of the two AC lube oil pumps, as the system is sized for allowing the time of re-acceleration and pressure build-up of the stand-by pump, through an accumulator or a rundown tank. Vice versa, the large units, e.g. the F gas turbines, do not have API 614 lube oil consoles, because large enough accumulators can not reasonably be installed so that the sudden failure of one lube oil pump brings to unit's trip. Therefore the standby pump is used to share the operation, but must be started before the stop of the operating pump. In these units is commonly installed a DC emergency pump for safe coast down only.

I would appreciate your advising me whether this interpretation is correct.

Thanks and best regards

 

Giovanni,

All manufacturers warrants their F-class machines, and we all know how expensive they are. So, wouldn't it be reasonable to assume that the manufacturers have "done their homework" to try to assess the likelihood of a failure of one of the AC motor-driven L.O. pumps and taken that into account in the design of their turbine and auxiliary equipment?

What is the predominant failure mode of the type of L.O. pump (and motor!) used by manufacturers on their F-class units? Would it be likely to presume that in the event of most common types of failures that the bearing header pressure would not decay to zero so quickly so as to trip the unit on low L.O. pressure, but rather to decay slowly and allow time for the standby L.O. pump to be started and brought into service?

Many of the manufacturers also have operate-and-maintain contracts or contractual service agreements or long-term service agreements on quite a number of the F-class units in service around the world. In many of these contracts, the manufacturers assume some of the liability for cost of failure scenarios. Would it not be reasonable to presume that if they had experienced a high number of failures of a type that resulted in a sudden loss of L.O. pressure and a trip of the unit from load (which GE really dislikes on their F-class machines, by the way!) that they would be working on a solution to the problem?

There are many, many considerations that are taken into account when designing any piece of equipment. There are engineering decisions, and there are economic decisions. Most major manufacturers now employ some kind of fault analysis and choose components to satisfy their engineering and their economic criteria. Would it make sense to design a system to withstand every possible scenario, when it could not reasonably be expected to experience some of those scenarios even once during its projected life?

Couldn't many of the types of failures be avoided with sound and proper preventive maintenance practices?

One of the truest sayings I ever heard a colleague tell me when I was just starting to work in engineering was: Engineering is a series of compromises.

Compromises don't always work out to everyone's satisfaction, and if you've experienced a trip from load because of a L.O. pump/motor failure, it would be really helpful if you would explain all the details of the event. Many times, the details can be the most important part of the circumstances, even if they are discounted by some during the analysis.

I believe your interpretation sounds reasonable for what I know of F-class gas turbines.

One last thing, the start-up of standy pumps on the F-class units I'm familiar with is done on decreasing pump discharge pressure, not on decreasing bearing header pressure. For all but the most sudden loss of pressure events (failed motor bearing, or broken pump shaft, etc.), the L.O. pump pressure will start to decrease well before the regulated bearing header pressure will start to decrease, affording some time for the standby pump to be started and provide "relief" for the lead pump.

Ciao