Separate lube oil and control oil systems

[bicycledisciple]

In an API steam turbine driven centrifugal gas compressor application, what could be some of the potential reasons for a buyer to insist on separate lube oil and control oil systems ? As in, the control oil for turbine is not from a common reservoir (as is standard practice with some OEMS). Is the added complexity and cost worth it for a 20-30 year life period? Thank you for the insights.

 

[rotw]

Typically most common is to have both combined; separate systems would be rather the exception in my view.
Potential reason is probably different oil specification requirement for each of lube oil / control oil system (special application?).
Possibly the user wants that the two systems are kept separate or isolated in order to address various scenario/consequences (view points of safety, oil contamination/degradation, etc). Maybe there are some functional and operational tests that are imposed on the control oil system that would require the system to undergo a dedicated test (unlikely...).
Maybe operational reasons necessitate otherwise a "delayed trip" in case of loss of oil pumps, whereby the turbine continues to be operated for a limited duration due to process reasons (such as plant safety) while it is accepted that the machinery bearings would be fed via the oil from the rundown tank (not an ordinary case nor something I've seen).

By the way, I think that the control oil system size should generally be small when compared to the lube oil duty (driven/ driver equipment) so its a bit difficult to justify a dedicated system, unless there is a real bottom line behind it.

 

[JJPellin]

We have systems set up both ways. In order to get the quick response and dependable operation needed for control oil, this system typically runs well above 100 psi. If the systems are combined, then the entire lube oil system has to be capable of providing this pressure. As noted, the lube oil system is likely much larger. If the lube oil pumps, filters, coolers, temperature and pressure controls must be designed to provide 150 psi oil, this will increase installation and operating cost.

Once the systems are separated they can both be optimized. Each would end up using a different oil, different pressure, different filtration, different run-down/accumulator design, etc. This would tend to show that combining the systems will involve compromise in each of these areas.


Johnny Pellin

 

[romke]

in the past some turbine designs used a separate control oil system for safety reasons. in that case the control system used to be filled with a "fire resistant" fluid. nowadays the fire hazard is not regarded as a big problem and thus the standard turbine oil is used from the turbine lubrication system.

another reason might be due to the fact that the control fluid does not circulate that much and thus might be subjected to a higher thermal load in terms of more units of heat per unit of volume. that may lead to deposit formation negatively influencing the control mechanisms involved due to sticking valves etc. a fluid with higher oxidation stability might then be a solution. however, with suitable filtration of the circulating bulk turbine oil deposit stimulating components of the fluid may well be kept to a safe minimum. especially bypass filters where the oil is cooled down considerably necessitating deposit formation components to fall out of the solution and being trapped in the filter are quite good at that.

 

[Hoxton]

Reminds me of my gas turbine days. Yes, you could have a common system for turbine lube oil and hydraulic start oil. Look how much capital cost you save..... And how much grief you bequeath to the maintenance personnel for the next 20 years!

Same case with a control air system, some engineer though it was a bright idea to tap into the control air system to supply air to blast clean the turbine combustion air filters. We actually ordered a $250 000 control air package for a 10 MW Gas Turbine, before someone realized that we normally order a $ 10 000 system! The review meeting was interesting!

 

[bicycledisciple]

Thank you all for the responses. Greatly appreciated and sounds like I would need to stand guard of such customer requests before finding out specific reasons for it.