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Author Topic: Why overspeed tests are done!  (Read 2120 times)

boatmadman

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gwa84

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Re: Why overspeed tests are done!
« Reply #1 on: March 27, 2011, 12:00:58 AM »

all you can say to that is oops  <*<
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CGAux26

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Re: Why overspeed tests are done!
« Reply #2 on: March 27, 2011, 02:25:39 AM »

In my career as a machinery engineer I saw several destructive overspeeds of steam turbines.  Fortunately no one hurt in any of them.  None of ours are anywhere the size of this one.  Stress in rotating parts is proportional to the square of the speed, so 120% speed equates to 144% stress.  Typically turbines are tested to their overspeed trip limits of 110% uncoupled.  The steam inlet block valve is opened very slowly, just enough to bring the machine to trip speed.  If the shutdowns don't work, you stop and find out why, then test again.
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flashtwo

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Re: Why overspeed tests are done!
« Reply #3 on: March 27, 2011, 10:20:00 AM »

Hi,

I had a colleague who was at the Uskmouth Power Station in Wales in the 1960s, when the turbine stop valves failed to close on overspeed due to a disconnection from the National Grid. The cause was found to be sea water contamination in the valves hydraulic system.

He had been standing with a workmate by the turbine only 5 minutes before the turbine broke apart. He , luckily, had walked to another machine when all hell broke loose but his mate was killed.

He always carried a photo in his wallet of the H.P. turbine laying in the mud a mile away in the estuary!

With the large base-load turbo-generators, which run for years without stopping, provision is made for on-load valve testing, where typically there are four pairs of stop/governor valves. This enables the valve mechanisms to be tested one by one with only a partial load loss and gives confidence that they would close in an emergency.

I used to work on electronic turbine speed governors and, amazingly, the 7-tonne springs on each valve were compressed, via the hydraulics, with only a 26mA signal. Part of the requirement was for the valves to slam shut in 250ms, boy did that make us jump! Those 26mA signals controlled a million horse-power turbine.

Despite modern computer controls, which can be triplicated, the last bit of protection is the mechanical overspeed bolts on the main shaft; if they jam then you're in trouble.

Ian.


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boatmadman

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Re: Why overspeed tests are done!
« Reply #4 on: March 27, 2011, 05:23:43 PM »

Ian,

Interesting comments, I  always think its difficult to appreciate just how much energy is involved in these large machines, even after 35yrs of working with them.

With respect, I would point out that in this country at least, these turbines do not now run for years without without stopping. They are all subject to rigorous inspections at intervals determined by experience, design, manufacturers recommendations and insurance requirements.

The test regime at the power station I work at involves weekly part stroke testing of the turbine control valves, weekly testing of the electronic overspeed protection and 2 yearly real overspeed testing.

Actually doing the real overspeed test is something approached with real caution and a clear procedure of what to do, hopefully to avoid disaster.

Having said that, I know of one power station in this country that never did a real overspeed for 10 years or more, they thought that testing the electronic protection was enough. They have been put right now.

Ian
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CGAux26

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Re: Why overspeed tests are done!
« Reply #5 on: March 28, 2011, 04:17:51 AM »

In the US refining industry, in which I worked, there was a lively debate about O/S testing.  Some only test the electronics, online, some like us eliminated the mechanical overspeed bolt when we went to redundant electronic governors.  And we had excercisers on the trip/throttle (stop) valves of large turbines.  These were required to be stroked weekly.  I too have seen sludge in the hydraulic system.  It's a constant battle.
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ZZ56

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Re: Why overspeed tests are done!
« Reply #6 on: March 29, 2011, 02:40:02 AM »

As a stationary engineer in training, this isn't exactly comforting news... :p  Can't understand why they didn't ensure that the manual trip switch was in fact manned by an alert and competant human being before ever firing the turbine up. 

We have a small turbine in our college shop, around 30 horse, but it has been offline all year due to rebuilding and the coupling being misplaced. 
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CGAux26

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Re: Why overspeed tests are done!
« Reply #7 on: March 29, 2011, 03:07:28 AM »

ZZ56, turbine overspeed testing CAN be done safely.  The big thing to remember is that, if the turbine is uncoupled for the test, all that horsepower is still available in the machine.  But instead of it driving the generator or compressor, the horsepower is now available for ACCELERATION, if it is not carefully controlled.  Only be around a turbine while it is being O/S tested is you are part of the crew doing the test, and NEVER stand alongside the turbine, perpendicular to the shaft.  If it overspeeds to destruction, most of the pieces will fly radially.
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