Dear gentlemen,

In our thermal power plant, the steam extracting turbine MSV Main stop valve and GV governing valve servo valve were damaged and lose functionality. in the same period: within 2 months 2 MSV and 2GV servo valves lost their functionality and were changed.

It was expected that Lube oil quality is the cause of this incident but it was not sure.

Could you please give me some RCA Root cause analysis about what happened and what will be the main causes of this failure.

Thank you for all

 

Would it be possible to have the manufacture of the servo, or a repair company perform a failure inspection to provide you with the exact failure mechanism to then lead you towards what needs to be corrected.

for example is there
a plugged strainer
an eroded jet of flapper
a broken feedback spring
varnished spool sticking
damaged spool
etc

for the beginning of a RCA matrix, I would look at what is common to all 4 of these servos

the first would be the fluid, besides particles/cleanliness, there are proprieties of fluid that can cause erosion or electrolysis of the internals. a small amount of water or the additives
the environment the servo valves are operating in, like could the background heat have increased. this could also apply to the fluid supply tubing.

the control system could have undesirable noise on the control current causing fatigue.
another would be the manufacturer sequence or if they were remanufactured. I have picked that many servos from the warehouse that failed and once had all that return from being serviced failed.

Getting a failure analysis of the failed valves would be my first recommendation to lead you in the direction.

 

Lube oil quality, or, rather, the lack thereof, is usually the cause of most electro-hydraulic servo-valve failures. The internal passages of most servo-valves are pretty small, and what tends to happen is that particulates and varnish tend to collect and "plate" in these passages which then prevent proper flows and functioning of the servo-valve.

Here's a link to a useful document:

http://www.servovalve.com/technical/newtb_141.pdf

Figure 6 on Pg. 4 is the closest to most types of servo-valves used in power generation applications.

Here's a link to a thread you might also find useful:

http://www.control.com/thread/1351780247#1351831993

It's useful to note that many heavy duty gas turbine packagers and manufacturers use the same oil for both lubrication and hydraulic oil systems. That's what I refer to--but even with dedicated hydraulic oil systems, oil quality is very, very important. I've been to more than one site that had ruptured oil filters, and they had been ruptured for years in some cases (no dp means good filters, right? not for years, though). And, these sites only changed filters when indicated by high dp, and did no oil testing/monitoring at all. They also were complaining of excessive servo-valve failures, which, was understandable once all the facts were known.

There is a LOT of literature and documentation and whinging about servo-valve failures all over the World Wide Web--but the upshot is, if oil condition isn't monitored (usually with periodic testing) it's usually going to degrade. There are additives, filters, centrifuges, and all manner of methods to control varnish and particulate formation. A good oil vendor can also help to determine the cause of the problem (excessive temperatures (high and low); improper filtration; low flow-rates; improper oil selection; etc.).

Servo-valves get a lot of blame for failing, but the most common cause is poor oil quality. The other thing that causes servo-valve failures is exposure to high temperatures. There is generally little flow through a servo-valve when the device it is positioning is in a relatively stable position (changing position requires flow; stable position requires no flow). And, high temperatures combined with "stagnant" flow can cause all manner of problems for servo-valves. This seems to be a problem for a lot of steam turbine applications, where, after maintenance outages the insulation or heat shields around servo-valves are not reinstalled or are not properly reinstalled.

Again, oil quality and high temperatures are the most common causes of servo-valve issues. Given that you've experienced failures of three servo-valves in a short period of time, in two locations, it seems most likely that oil quality is the most likely issue. And, again, there has been a lot written on the World Wide Web about oil quality and servo-valve problems.

Hope this helps!

 

Thanks to all for the help.

Yes that's correct, it was noted that Lube oil quality is very bad.

Thank you

 

Thanks for the feedback!