Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length.

[BarryM]

By way of an exception I recall a casualty which took place some years ago when a diesel in good condition with a working overspeed governor nevertheless took off and ran to destruction.

The details I cannot remember but it involved a running diesel located in a compartment which for whatever reason started to fill with flammable vapour. The vapour was sucked into the air intake and thence to the cylinders where it combusted and the engine speed increased. To counter this the overspeed governor gradually reduced the fuel until the point was reached where the fuel was shut off completely but the engine continued to run - and revs increase - because it was sucking in the flammable vapour/air mixture. With no effective governor, engine revs increased to the point where something gave way and everybody's day was thoroughly spoiled.

If I remember right, when all the bits were put back together, a modification was applied which resulted in unsafe speeds operating a damper on the air intake as well as closing the fuel rack.

Barry M

[steamboatmodel]

Years back I did building maintenance, the one building I had just started at (17 floor Office building) had an Natural Gas powered emergency generator, 16 cylinder truck style engine. At the time I started this engine had nor been run for some time ( It was scheduled to be tested weekly), I posted in the logbook that the engine should be serviced and checked out by the manufacturer before we tested it. The fellow on the next shift disagreed and started the engine, It ran fine for a couple of minutes and he decided to let it run to charge the batteries and wont off to do something else. Five minutes later the lube pump ran dry, then the generator bearings and then as the engine tore loose from its mounting the engine seized. The Generator set being in the mechanical floor at the top of the building had to be taken out through the side of the building which was not fun.
Regards,
Gerald.

[Inkmark]

Hi Guys
Having 40+ years on steam engines, mostly turbines, a couple of thoughts on overspeed.
Turbines have two sets of valves, throttles and emergency valves. The throttle should be able to control the
speed but if these fail the machine will activate the emergency valves. These are usually latched open by oil
pressure and when the oil is dumped the valves slam shut under springs. If this fails then the electrically driven
boiler stop valves are used.
I am surprised by comments from flashtwo, the rotor is a heavy beast and would have to smash its way through the casing to escape.
What usually happens is the blades begin to come off and these do sometimes go through the casing.
My only experience was debris under the valve seats that prevented the machine from overspeeding.
It shed LP blades that ended up in the condenser, they could not penetrate the casing. Boiler stop valves ended that exciting incident.
On reciprocating engines as they go faster the steam port is open for less time so perhaps they could be viewed as self limiting.
Just an idea, I've never seen a recip. overspeed.

On a point back on the original thread, even when using clean water and chemicals it is still required to operate the blow down
valves depending on water analysis to remove the solids from the boiler. Sometimes several times per shift.
Cheers
Mark

[boatmadman]

Regarding your comment on blowdowns, Mark, it is also common practise to keep TDS (total dissolved solids) under control by use of continuous blowdown at a lower flow rate.

Ian

[BarryM]

Hi Guys
Having 40+ years on steam engines, mostly turbines, a couple of thoughts on overspeed.
Turbines have two sets of valves, throttles and emergency valves. The throttle should be able to control the
speed but if these fail the machine will activate the emergency valves. These are usually latched open by oil
pressure and when the oil is dumped the valves slam shut under springs. If this fails then the electrically driven
boiler stop valves are used.
[unquote]



My experience over a similar time at sea is that he who puts his trust in the emergency stop valve may well end up regretting it. As for "electrically driven boiler stop valves", most of my time was spent on vessels where shutting the boiler Main Stops involved big wheel keys and a lot of sweaty effort in awkward places. By the time the stops were shut the damage would be well past and the Chief already filling in the Casualty Report. As the manoeuvring platform was never left unmanned, a swing of the throttles was preferable.
Regards
Barry M

[flashtwo]

Hi Mark,

Your surprise at my remarks regarding turbine overspeed made me refresh my memory regarding the Woolwich incident and I found the following website:-

http://homepage.ntlworld.com/chris.mansfield1/power.htm,  which states it was only part of the turbine that was projected across the Thames into North Woolwich on the Essex side. I had forgotten about the poor "xxxxx" in the bus!

Certainly one of the turbine rotors (LP?) landed in the Uskmouth marsh - devoid of blades.  It was the sort of incident that was drummed into young engineers when getting involved with turbine protection systems, as you most likely have experienced.

One of the problems with boiler feed pump turbines was demonstrating that the overspeed systems would work. Unlike a turbo-generator, which could be run unloaded, feed pumps always (or should have!) had a water load in the pump. The only way to overspeed was to uncouple the turbine from the pump and run it up (nominally 7000RPM). This exercise would take a week of removing and replacing the coupling bolts, and a week without the steam pump was a lot of money.

Barry..

I too remember the non-electric valves - the boiler interceptor being the worst for the number of turns it required and being a two man job.

Ian.