Diesel Generator Overspeed 2

I do recall a similar frightening experience, many years ago as a trainee in the infernal combustion engine business. I was working next to a big V8 on dyno test when I heard a loud crack and the revs began to build rapidly, the operator pulled the injector pump's control lever to the shut-off position but the revs continued to increase. He grabbed a couple of 7lb tins of grease from a rack next to him and threw one to me, indicating what he wanted to do as it was way too noisy to shout. We offered the bases of the tins up to the inductiom manifold faces, they fitted well and suffocated the engine before it flew apart. All the big-end bolts had stretched and there were marks on the top of the pistons where the valves were beginning to float and touch. It's over forty years ago now and I've forgotten the details, but we were lucky that the tins bottoms were the right size and survived the suction, albeit well bowed!

It turned out that the rack on the (New) pump had a faulty heat treatment and had broken at the shut-off lever end, allowing the spring bias on the rack to move all the delivery cams to Max Fuel.

Do you think it might be better to move your post to the "Automotive Engineers" forum on this site?

Trevor Clarke. (R & D) Scientific Instruments.Somerset. UK

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How exactly does the overspeed device shut off the engine? If it's relying on the same governor mechanism then possibly it could not overcome the mechanical failure. If it is an independent fuel shutoff, perhaps there is enough fuel inside the fuel system (between the shutoff and the main injection plungers) for the engine to blow itself apart.

But, there is another possibility. Diesel engines can suffer from a "runaway" condition if they ingest lubricating oil, for example from failed valve stem seals or piston rings or crankcase vent or turbocharger seals. Look for evidence of excess lubricating oil in the intake system where there shouldn't be any.

A similar run away cause a huge explosion and fire a few years back on a gasoline barge when the fumes made it into the induction system.

Staying with the theme of unexpected sources of energy going into the system (IFR's example is by no means the first time a flammable atmosphere has made a DG run away to destruction), it's probably worth considering briefly the possibility of an electrical fault having caused the generator to motor and backdrive the diesel.

A.

How would a motoring event lead to an overspeed? They lead to all sorts of bad things but an overspeed is definitely a new one to me.


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If we learn from our mistakes I'm getting a great education!

"The overspeed switch is mounted on the suction side of the turbo blower and was proved to be operational after the rebuild".

Is it possible the blower is faulty? If the overspeed switch is on the suction side, and the blower is kaput, cant the engine still over rev if the switch isnt reading enough vacuum as generated by the turbo?

I would guess a good place to start asking questions would be with the motor manufacturer.

Scotty,

I think the mechanism for a motoring event would need to have something to do with a multi-pole machine losing some windings - unlikely enough to be able to be eliminated pretty quickly, but maybe worth a moment's thought.

A.

clientsarealwaysrite,

In what capacity are you operating for your client over this matter?

- Steve

The old 2-cycle Detriot diesels or Jimmys as they were known were famous for running away. A part of the design was an air flap in the air intake for emergency shutdown. These machines used a scavenging blower and a lube oil leak in a blower seal would fuel the engine to destruction.
Many contemporary gen sets have the over speed device close the fuel solenoid. Some move the rack to the zero speed position.
The old "Jimmys" used a centrifugal trip to release the air flap and cut off the combustion air on gen sets. Truck ad equipment engines had a manual release controlled by the operator.
If the engine has been rebuilt and the shutdown switch is working now, it is probably too late to determine the cause of the wreck.
A sticking rack is more probable than a rack going full open.
Had a unit with a sticking rack. The set would pick up the load fine. When the electrical load was reduced the rack would not return. The over speed device would close the fuel solenoid to stop the engine. Had the overspeed device failed the engine would probably self destructed.
Summary.
It is obvious that the motor overspeeded.
A combination of a rack problem and a failed overspeed device could be the cause.
However, that takes two simultaneous failures. It may happen but the odds favor one failure. That could be an alternate source of fuel that was not under the control of the rack or the fuel shutoff valve.
A common source of alternate fuel is a failed seal in a turbo or blower that allows lube oil to enter the air inlet and fuel the engine.
I doubt that you will ever find out, but I also believe that some one or two does have a good idea of the cause.
Too late now for much investigation.



Bill
--------------------
"Why not the best?"
Jimmy Carter

The thing that would usually be mounted on the suction side of the blower is an air shutoff valve, biased to the closed position by a serious spring, and held open except in emergency by a mechanical latch that can be triggered manually or by means of a solenoid... that must in turn be energized by an overspeed switch that measures crank speed by means of a magnetic pickup.

Some people have told me that overspeed valves that autonomously measure or sense the air flow and trip themselves closed on excess flow exist, but I do not recall ever seeing or touching one.

An air shutoff so positioned could not stop a runaway if the turbo blower case were substantially breached, e.g. by a turbo blower explosion, which could potentially also provide enough lube oil to fuel the engine to a runaway, if the turbo's bearing housing were also substantially breached. Such a scenario would seem plausible, but good luck proving it from the remains of the engine...



Mike Halloran
Pembroke Pines, FL, USA

Thanks Folks,

To further explain the engine was undergoing routine maintenance when the failure occurred. It is coupled to an alternator as standby power to a 600kVA UPS system.

The engine was running at 1100rpm (normal running speed 1500rpm) and as the engineer was checking why it wasnt at normal speed it ran away and failed. It is suspected that the fuel rack stuck and then "unstuck" going to full fuel.

My client has asked me to investigate as an independent engineer with the remit to finding out why and more importantly what he should do to prevent a re-occurrence.

The engine was rebuilt and everything checks out correctly (ie overspeed switch ok, turbo ok etc).
The o/speed trip is pneumo-electric in that it senses the air pressure and sends a trip signal to a solenoid valve on the main fuel line vie the control panel.

Hence my query ... any one got any ideas as to how this could occur, are these overspeed trips reliable and any thoughts as to means of preventing this happening again ?

Thanks for all your contributions.

David

... I just wondered whether you might have been a lawyer. Or worse, some insurance claim investigator.

- Steve

Eeeeek ! A lawyer or claims adjuster !! Nope lube oil in the veins, bones made from stainless steel and skin formed from thistlebond patches ... trained as a marine engineer, now trying to get clients to understand that engineers spend money to make their lives easier !!

That overspeed trip setup is, um, not completely right.

You can't reliably stop a runaway by cutting off the fuel supply.

You _can_ damage the injection pump by doing so.

Have they been using that fuel shutoff solenoid valve as the normal shutdown means also? If so, bad idea.



Mike Halloran
Pembroke Pines, FL, USA

The fuel rack could have stuck and stayed stuck. At 1100 RPM your frequency would have been below 40 Hz. The UPS may have interpreted this as a failure and went to batteries. Or some protection may have tripped the generator breaker.The AVR would have probably dropped the voltage to about 75%. That could have triggered a load loss.
A load loss with a stuck rack would certainly be capable of over speeding an engine.
The old Jimmys of the 40's had an air flap and a mechanical centrifugal device to release the air flap. Manifold vacuum then held the flap closed. They were as dependable and rugged as a ball pein hammer.
I would be wary of any device that did not monitor RPM. I wouldn't trust an expensive engine to an intake pressure sensor. How is the pressure affected by dirty filters? How high does the set point have to be to accommodate both dirty and clean filters?


Bill
--------------------
"Why not the best?"
Jimmy Carter

"I would be wary of any device that did not monitor RPM. I wouldn't trust an expensive engine to an intake pressure sensor."

I would agree with this point - appears to be a poor way to monitor o/speed which might have proven correct in this instance.

All of our diesel engine overspeed shutdowns are like MikeHalloran describes - flywheel mag pickup and air inlet valve (e.g. Chalwyn or Amot) with pneumatic, oil, or solenoid actuator. They are all spring to close and air, oil, or power to open.

While we do this on all our engines, it is especially important in hazardous areas where the ingestion of gas in the air can cause the engine to runaway.

djv hit on one of two possibilities that i was thinking about. The gas ingestion, mainly propane, on boats has caused many a runaway. I know of this happening on one stationary engine at a hospital.
Another possibility is that someone mixed fuels, like gasoline in the diesel. That's also happens occasssionally around the docks here.
You mention the GMs and we run a lot of the old surplus Grays on the older boats. On the most of the surplus Grays there is no overspeed trip of any sort only the governor. Several years ago I ask a friend who works on diesels and was coxswain during WWII and percipitation in several invasion landings. His short answer was that during the first few runs to the beach that it wasn't uncommon to add a little gas to fuel tanks to get a little more speed to get to and off the beach. He also told me many years ago if I was around a runaway GM to use one pant leg and a cap to kill it.