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Fire pumps shaft shear

Fire pumps shaft shear

Fire pumps shaft shear

(OP)
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.



RE: Fire pumps shaft shear

Quote:

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)' ?

RE: Fire pumps shaft shear

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

Muthu
www.edison.co.in

RE: Fire pumps shaft shear

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

RE: Fire pumps shaft shear

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
www.edison.co.in

RE: Fire pumps shaft shear

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

RE: Fire pumps shaft shear

(OP)
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.

RE: Fire pumps shaft shear

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.

RE: Fire pumps shaft shear

(OP)
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

RE: Fire pumps shaft shear

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

RE: Fire pumps shaft shear

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]

RE: Fire pumps shaft shear

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

RE: Fire pumps shaft shear

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

RE: Fire pumps shaft shear

(OP)
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)

RE: Fire pumps shaft shear

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.

RE: Fire pumps shaft shear

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

RE: Fire pumps shaft shear

Do the pumps have fairly long straight discharge piping?

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

RE: Fire pumps shaft shear

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

RE: Fire pumps shaft shear

The fractures look fairly brittle. I'm not reading too much into the corrosion as I don't know whether it was exposed to seawater post-fracture but it has the appearance of a crack which has propagated over time followed by abrupt failure. Maybe a post, or a link to this thread, in forum338: Corrosion engineering would elicit some comments from the materials guys who do this stuff for a living while we play with electrons?

RE: Fire pumps shaft shear

(OP)
My mech colleague now thinks the stressing factor whilst normally consider ample in the case is border line in our application and keyway should have more radius.

Since finding out soft starter, i was beginning to think for mechanical/application/material design issue than an issue could be linked to two different type of motor controls. although was going to get on board next week to look at the acceleration ramp profiles out of interest for both softstarter and also settings for vsd.

"The barriers to entry for shipbuilding are not huge" - its only january but that could already be a contender for understatement of 2016.

RE: Fire pumps shaft shear

Reflected pressure pulses are known to cause mechanical damage at critical speeds/frequencies. I would expect the pressure transients would also cause torque transients.

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

RE: Fire pumps shaft shear

The barriers to working in a shipyard do not generally include pre-existing skills.
A statistically significant number of similar failures with two different drive systems suggests that something other than the drive systems may be involved.
The next thing I'd check is who actually installed the pumps and couplings, in an attempt to find whether they were installed and aligned correctly.
I'd also take a look at whether the correct couplings were selected at the design stage, and if substitutions were made after that.


Mike Halloran
Pembroke Pines, FL, USA

RE: Fire pumps shaft shear

Another thought: as this looks increasingly like a pump / shaft material problem consider a post in forum407: Pump engineering

RE: Fire pumps shaft shear

Although rarely easy, you might consider strain gauging the shaft and monitoring the torsional strain. The gauging isn't so bad - it's getting the signal out from a rotating shaft.

RE: Fire pumps shaft shear

The photos indicate two possible failures modes.

1.Overload-
(High impact loading [quick stop or jam])

2.Fatigue, mechanical or dynamic-
(Excessive rotary bending, such as an overhung load or damage causing stress raisers.)

Shaft fractures are either “ductile” or “brittle”.
A ductile failure has a smooth appearance, like represented in the photographs.
A brittle failure has a rough surface. And described as torsional or twisting in nature.

Metals Handbook (various volumes) can be an excellent source for learning about shaft failures.

Other failure modes related to shaft failures are:
Corrosion/environmental, Thermal, Residual, and Electromagnetic... which based on the photos would seem to be ruled out.

John

RE: Fire pumps shaft shear

(OP)
Thanks all, there are quite a few suggestions and thoughts - I will pass on to my colleague next week. I will recommend he also gets himself an account and continues/links the thread on the pump/material design groups.

He mentioned stress raisers quite a bit in our recent conversation since he made some calculations yesterday. He has also just been given info that OEM/shipbuilder had changed the arrangement of shaft on two replacement pumps (motor ever so slighly closer to pump) and to date he has been told this may have resolved the issue. But he is still conducting his own indepedent investigation and too early to confirm that conclusively.

RE: Fire pumps shaft shear

Jimmy

Can you advise details on the 150KW motor - IE frame size, speed, enclosure. Nameplate data would help.
It appears the motor shaft is breaking in the pump impeller. Are the pumps/motor package integral with each other (flanged motor mounted onto pump)
IE: what is called a close-coupled pump. You have noted 6 - 8 shaft breaks over a period of time, are all motors the same brand?
If so have you talked to motor mfg.?

thanks
Mac

RE: Fire pumps shaft shear

Check for high pressure transients that match the impellor blades passing the discharge opening.
Is the piping to all the failed pumps similar? Of particular concern will be straight sections to the first elbow.

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

RE: Fire pumps shaft shear

My attention is in a similar direction as Muthu (edison)

It looks to me as if the shaft failed inside the coupling hub.

If I'm understanding the location correctly (shaft failure in the hub), that's a very odd place to fail. No change in diameter. And more importantly, shaft is ordinarily not transmitting torque along it's length here... the torque is transferred from shaft to hub to other hub to other shaft. There should be nothing past the coupling hub that would grab the "free" end of the shaft to transmit torque (unless maybe the shaft is inserted all the way through one hub into the other, which would be an incorrect assembly). If for some reason shaft breaks inside the coupling hub (as I'm imagining), and the key/keyway remain intact enough to assure shaft and hub rotate at same speed (as shown in photos), then the driven machine should continue to turn!

What exactly happens when shaft breaks? Does driven machine in fact stop turning while driver continues? Or some other symptom.
Can you describe what type of coupling you have and where the shaft failed in relation to the coupling hub:
  1. How long is the piece of shaft that broke free?
  2. how much distance from shaft break to each end of the hub?
Any idea what interference of clearance between hub and shaft? (and also shaft diameter).
Is the alignment ever checked?

It doesn't make a lot of sense to me. Maybe if you can answer some of these questions it will make more sense (or I will realize what it is that I'm completely misunderstanding) .


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

RE: Fire pumps shaft shear

(OP)
So I passed on the numerous suggestions, comments, questions to my mech colleague and he said he was on top of the mechanical stuff, oh well.

anyway i looked back at some data I had captured from power analyser back in november, I wasn't aware of these fire pump failures at that time. Anyway in the data was a number of the fire pumps that used softstarters that triggered the analyser back then. See attached.

The waveforms look like the softstarter was doing little in terms of "softstart" so I decided to get on board one of our ships yesterday to get the softstarter settings (see attached also where I penned on the settings). Unfortunately the settings I recorded are off a different ship to those that I obtained the analyser data, all ships are experiencing fire pump failures though (both VSD and softstarter types). Also it looks like the settings of the softstarter I obtained yesterday are factory default! So can't categorically say yet but I suspect all softstarters have default factory settings.

the motor nameplate current is 240A, the softstarter has the default 320A setting. Current limit is 400% (default factory), so our current limit is 1280A. Looking at the traces it seems the motor never actually experiences this current limit it is up and away in 0.5-1secs.

To recap, both VSD and softstart pumps are experiencing mechanical failure. Normal operation is to have one VSD and one DoL (softstarted) fire pump running 24hrs.

In previous response most dismissed the likelihood of two independent type electrical driven motors suffering same mechanical failure.


I'd be interested to see what people think of attached.

RE: Fire pumps shaft shear

Your attachment seems to show a peak current of 1400A at 0.75s for every pump, unless I am misreading something. Can you correlate that current to a torque value, and compare that with the ratings of the mechanical components?

Never mind.
You apparently have a fleet of ships, all plagued by the same failure.
That seems to point to a system design issue.

... such as, maybe the people who supplied your Fire Pumps do not share your understanding of a Fire Pump's design duty cycle.
This would be a good time to contact the pumps' manufacturer, not a distributor, not a sales person, not a marketing person, but an actual engineer who works on/with the pumps you are killing, and ask for their help in figuring out why the pumps are not meeting your expectations, or why your expectations are unreasonable.





Mike Halloran
Pembroke Pines, FL, USA

RE: Fire pumps shaft shear

(OP)
OEM is engaged via Prime Contractor. Subject to warranty and no doubt a bun fight down the road as to where blame lies (OEM, requirements definition, operation). I'm taking interest in electrical side of things as to my knowledge no one is looking at that.

Not sure if this is correct approach ....


Rated torque for this motor is 808Nm

From datasheet DoL locked rotor starting torque is given as 210% so 1696Nm is available on starting this motor in direct on line mode.

From the softstarter setting the current limit appears to be 400% and rated current is programmed as 320A, so softstart current is limited to 1280 amps.

The motor data sheet tells us rated current for this motor is 240A (not 320A), the datasheet also says the locked rotor starting current is 7.7 times rated current. So starting this motor DoL would draw 7.7 x 240 = 1848 locked rotor amps.

So the actual softstart current as a proportion of DoL current is 1280 / 1848 = 70%


starting torque developed on softstarting would be a squared function of the ratio of the softstart to DoL locked rotor current? so our softstarting current is 70% of rated DoL locked rotor current, meaning that the starting locked rotor torque developed would be 0.7x0.7 x 1696 Nm = 724Nm ?

(should have made explained in earlier post, the monitoring equipment was looking at our shore power incomer. not individual fire pumps. so the signature of the motor start is on top of the ship base load of 350A.)

RE: Fire pumps shaft shear

Hi jimmy2times

Possible failure modes are given in the link I posted earlier, however a good rule of thumb for your mechanical dept to determine whether it's a fatigue failure or not is:- if the principle tensile stresses as calculated and including stress raisers fall below a quarter of the ultimate tensile stress of the material then a fatigue failure is unlikely.

That said I think the shafts are suffering fatigue but I haven't got enough information to prove it from your posts.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Fire pumps shaft shear

Quote (OP)

My mechanical colleague asked me today if this could be electrical problem.
After reading through the posts again, I would answer probably not, but possibly indirectly.
At critical speeds centrifugal pumps may generate high pressure transients with possible high torque transients.
The effect is greater at low flow rates.
The pressure/torque transients may affect all pumps discharging into a common manifold or header.
Anecdote:
The first time I saw this, a 60 HP pump broke. It blew out a section of the pump case about the size of a mans hand.
The pump was replaced.
The second pump experienced a similar failure.
The cause was determined to be a critical speed and reflected pressure transients.
The solution was to determine the critical speed and to program the VFD to avoid extended running at the critical speed.
Anecdote off.
I believe that your VFDs are inadvertently finding a critical speed where mechanical problems occur.

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

RE: Fire pumps shaft shear

Take a look at some of the later posts in this thread.
DC motor risers break 2
thread237-401562: DC motor risers break
The information on torsional vibration may be of use.

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

RE: Fire pumps shaft shear

Can you discreetly obtain details and pictures about the original and "new" configuration? Mechanical layout, coupling make and model, etc?
Can you confirm if the coupling is an interference fit? Those keys look like they are way overloaded.

Are the motor shafts similar diameter?

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