Fire pumps shaft shear

[jimmy2times]

We have some 150kw fire pumps on a ship. Some (6) are vsd fed and some (4) are star/delta starters, same design pump/motor in each case (i think, need to check that though)

This is first i hear of problem today, but apparently catastrophic mechanical failure is occuring on the shaft (shearing near keyway i was told). My mechanical colleague asked me today if this could be electrical problem. Apparently 8 failures over past year mainly VSD types but also has been S/D type failures as well more recently.

I advised that for a start different type of starters but i thought couldnt rule out completely. I gave him the spiel how shoddy S/D starters were, i.e. changeover dwell transient, which could lead to large torque. I them advised that VSD also could cause torque pulsation. Im a bit rusty on my drive theory these days, used to be due to counter rotating 5th and 7th harmonics setting up 6th harmonic mechanical resonance in air gap if my memory is correct ?? Or did i just dream that up! Is this still an issue with more modern drives and control algorithms?

I was going to start helping the guy out proper tomorrow and find out info on drive type etc. just wanted to get some feelers on what to look for/other advice before i delve in to documentation.

Any thoughts on same failure mode but with two independent electromechanical drive dynamics that lead to that failure. Or do people think this sounds more a mechanical/pump/application issue

Could it otherwise be VSD set-up (rather than harmonic torque pulsations) that could lead to mechanical failure, in the vsd failure cases.

 

[electricpete]

Apparently 8 failures over past year mainly VSD types but also has been S/D type failures as well more recently.

With S/D failures, there's no reason to focus on VSD.

However if you want to consider VSD factors, there are at least two:
1 - high frequency torque oscillations originating from drive (dependent on topology) as you referred to.
2 - the fact that speed is variable. That means it's much more likely that a lateral or torsional excitation frequency (typically running speed or harmonic thereof) will coincide with a system lateral or torsional natural frequency. I'd guess this scenario is more likely than #1.

The possibilities are wide open. Trained people can can learn a lot by studying the appearance of the shaft failure area. Also review vib history for clues if you've got any.


=====================================
(2B)+(2B)' ?

 

[edison123]

More likely shaft itself is poorly designed. Stress raisers at the coupling step, inadequate diameter etc.

Muthu

http://www.edison.co.in

 

[jraef]

It's hard to imagine that a torque pulsation from a modern VFD could shear a shaft. Did anyone inspect the bearings? Bearing failure from EDM caused by the VFD could have lead to it seizing, then shearing possibly.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[edison123]

jeff

These are fire pumps and hence not on 24x7. Don't edm damages happen over a long period of operation? Also, S/D starter motors also have suffered.


Muthu

http://www.edison.co.in

 

[MikeHalloran]

Even if the keyways are horribly designed, they shouldn't be seeing enough duty to make it a problem, in a fire pump. How often does the ship catch fire? How often do you conduct drills?

OR, have you changed the duty required of the fire pumps, e.g. to power water cannons to repel pirates? If something like that happened, you may have to evaluate the addition of pumps designed for actual continuous duty, vs. the risks associated with using fire pumps for purposes for which they were not designed.








Mike Halloran
Pembroke Pines, FL, USA

 

[jimmy2times]

Just to confirm 4 fire pumps are vsd and 6 fire pumps are star delta, 10 fire pumps on boards. The sea water fire main is always pressurised so i understand, usually two pumps running (24hrs/7days a week) all the time, one vsd and one direct mains fed (having been s/d started). They rotate fire pumps periodically.

Both s/d and vsd type fire pumps are failing at the shaft.

Jraef, EDM = ? (Electric drive motor?
Are you referring to circulation bearing currents here? From recollection, if i find NDE bearing is insulated could this be ruled out.

 

[ScottyUK]

jimmy,

EDM Same process as a spark erosion machine, just taking place in your bearing races.

How is this system grounded? I don't know a great deal about marine practice but high frequency currents taking the least desirable path through system capacitances seems plausible if the system is ungrounded or high impedance grounded.

 

[jimmy2times]

Yes unearthed network lv 440v 60hz

6 main lv switchboard supply from MV network, so that is 6 isolated LV networks (no interconnectibility between those 6 power systems other than the common MV bus)

Mixture of vsd and s/d pumps Dom some of those lv swbds e.g one board may have one of each. Never running 2 pumps from same board though

 

[itsmoked]

You say some are running 24/7... Lot of fires eh?

Are the shafts shearing on start-up or do running pumps spontaneously shear?

What type of pumps are these? Are they centrifugal or positive displacement?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[crshears]

EDM = electrical discharge machining [right?]

I have the same question as itsmoked; if ANY positive displacement pump were to be left running on the fire system with no back-pressure type relieving valve providing an outlet, the entire system would become overpressurized, plus any centrifugal pump in parallel with the PD pump will be running deadheaded, viz., at zero flow; not a good idea. Wouldn't want to see that situation even with two centrifugal pumps; you'd want to have a slight bleed from the system at all times so there is at least some continuous water flow, however small, through any running pumps.

In my view, running a PD pump under those conditions could, and likely would, stress its pump/motor shaft couplings.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[waross]

Have you concidered forbiden frequencies? In the early days of VFD application to pumping, there were quite a few catastrophic failures. Sometimes the pump case, sometimes the piping.
As each impellor blade passes the discharge port it sends a pressure pulse down the puping. The pressure pulse would reflect from the first elbow or other feature. If the speed of the propogation of the pulses was such that the returning pulse hit an outgoing pulse, pressure buildup would result. The first time I encountered the issue a piece of casting about the size of a large man's hand was blown out of the side of the pump housing.
The pump was replaced and it happened again.
This was about 60 HP.
Jraef is more familiar with VFDs than I. Jeff, may modern VFDs be programmed to avoid continous running at problem or forbiden frequencies or speeds?
It may be that returning pressure pulses are causing torque transients.
The speed or frequency depends on the speed of a pulse in the fluid being pumped and on the distance to the first reflection point.
It also depends on the number of vanes or blades on the impellor.
If the pumps are running 24/7 to maintain pressure, they are probably being speed controlled to hold the pressurea at the set point.
This may not be the cause but it is an issue which should be checked whenever large pumps are driven by VFDs.

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

 

[jimmy2times]

all pumps are centrifugal pumps I'm informed and the system has been arranged with 10% bleed back

 

[jimmy2times]

looks like I was given some duff info to start with. I've just looked at drawings and 4 pumps are VSD and the other 6 pumps are softstarters (not star / delta as original post)

 

[Parchie]

Jimmy,
I have encountered shaft shearing problems on pumps before. The culprit was: check-valve at pump's discharge piping was passing and the pump was back-pedalling before it was started. No amount of big shaft will survive if the impeller was already turning backwards and then suddenly made to run counter to its spin direction! Please check your one-way valves.

 

[MikeHalloran]

Terrestrial fire systems include one or more main fire pumps that are normally not running, but start up to supply the flow when the system has been triggered, and a much smaller, specialized 'jockey pump' that maintains the mains pressure and compensates for leaks when the system is idle.

This sounds a little different; okay, a lot different.
Again, are the pumps centrifugal, or positive displacement?
How old is the ship?
Are the pump failures recent, or chronic?



Mike Halloran
Pembroke Pines, FL, USA

 

[jimmy2times]

new ship, failures over last year.

centrifugal pumps.

piccies attached

 

[jimmy2times]

and another

 

[waross]

Do the pumps have fairly long straight discharge piping?

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

 

[MikeHalloran]

The barriers to entry for shipbuilding are not huge.
There exists the possibility that the fire system was designed by idiots.
... who may not have understood that while centrifugal pumps can run deadheaded for a short time without damage, 24/7 is another matter entirely.




Mike Halloran
Pembroke Pines, FL, USA