Pump Impeller Screw Failure

Could be a problem of backspin/restart with high head and no check valve on the discharge.

Look for damage on the key and note which side of the key this damage occurs.

There is no damage to the key or key ways. The units incorporate a deep key seat which haven't allowed the impeller to spin on the shaft which would have caused catastrophic damage to the impeller or shaft.

You mentioned impeller/suction cover contact, yet there is no damage or even slight marks on the key?

Luckily for the customer no. There is very little clearance so impeller rubs against the cover before is completely comes off shaft.

Is it possible that the bolt is bottoming out in the threaded hole? Could there have been a thicker washer that was replaced with a thinner one? Would you consider a stud that could be set into the impeller shaft, and then using a washer/nut to secure the impeller? Welcome to the insanity.

I think I will try this on one of the pumps we have in the shop right now, although there has been no change in washer thickness and hole is certainly deep enough for cap screw.

It would be nice to know what size pump. And what are the low flow conditions?

Duration / severity?

Materials of construction?

Can you record the difference in vibration during low flow?

Pump is a 3" suction, 2 1/2" discharge. Impeller is 10 1/2" diameter. Impeller is 316ss, casing is cast iron, shaft is 1040 steel. Pumps are running at 3600 rpm. The pumps are located in a truck wash with no controls and 50 or 60 year old system. When they open in the morning they turn the pumps on and run them until they close. Various times during the day they certainly have near "dead head" no flow conditions which I believe is the culprit for the radial load issues. They always have some flow or I would notice a discoloration in the paint from the heat that would build up from no flow.

Go buy a brass 2" relief valve, install it on the discharge piping and pipe it back to the liquid source. This will help with the shock on the system

Set the valve just below the deadhead pressure of the pump, based on that pressure gauge you have on the discharge piping.

You do have a gauge on the discharge piping.....right ?

I've recommended this to them along with a vfd. They just keep paying us to repair the pumps and I want to solve the problem. Since their piping is really old and they have numerous leaks, the drive would require a lot of repiping to allow it to work. They pull their supply straight from city water so they would need to add a supply tank for the relief/bypass to return to right?

We did this very thing, pumping a solution of nitrilotriacetonitrile (NTAN) in dimethylacetamide into a hydrogenation reactor at 1450psig. Steam-traced lines, and many problems due to a lazy maintenance crew (i.e. not making sure the steam-tracing and insulation are restored after a piping change or problem such as a clog). If the line has cool spots, the NTAN comes out of solution and clogs the line.

That relief valve saved us many times when clogs occurred.

What about the impeller design? What type of impellar pickup do you have? Maybe the supply is not constant enough and sets up unneccessary cavitation which will cause intermitant "pull forces" on the bolts.

Couple of quick questions. When you say the Impeller shears off the shaft, is the shaft breaking or the bolts? Have you recorded the amp loads on the motor for a couple of days run time. If so what abnormal indications are you seeing if any. Can you up load a picture of a sheared impeller/shaft. (not sure that's possible on this forum)

I have seen several condition over the years that will cause similar conditions to appear. The one that sticks out from your description above, is the material used to make the shaft. My choice might have been 4140. But, I do not have enough information on the unit in question at this time to help much.

Any way to put a relief valve somewhere in there?

Sounds like lots of shock loads, some modulation might help. But with good tight clearances, between the shaft and impeller? I don't know.

bobc has a point. What has changed?

Municipalities tend to add (excessive) new users to their system without upgrading. The pressures on the suction side may therefore fluctuate (almost approaching water hammer).

A pressure gauge on the suction side may just provide you a different reason for the failure.

What about fitting a large water hammer arrestor?

Just a thought. Is the incoming (suction) water coming directly into the face of the impellor or is it entering via a bend pipe elbow? If there is a bend pipe close to the mounting face of the pump the water actually impacts offset the centre line of the impellor causing a 'Slug' loading that creates a dynamic loading that the bolts cannot resist and they break. This has happened on larger dredge pumping units I have worked on and the vibration can become detrimental to the surrounding structure. The impellor can also be subject to dis-intergration over time due to uneven loading on the lead-in section of the vanes.

In car turbo charger units that have a carburetor set off-centre the impellor via a bend pipe, they destroy themselves in firery fashion as seen in the earlier American oval racing cars as a slug of fuel hits the 100,000 rpm spinning impellor

Further to problem, a similar situation was encountered in a pump design some years ago which I was troubleshooting, and it was discovered that with continuous operation, any significant drop in pressure caused the impellor to "vibrate" (or as one respondant put - cavitate), and this effectively dislodged the impeller to such a degree that the impeller rubbed against the casing, momentarily jamming the impeller but not causing failure of the key or keyway. Subsequently the end location boss became loose and effectively freed the impeller.

In the end the solution was to protect the pump from surges (hence non-return valves and pressure relief valves) and introduction of a solenoid valve to regulate flow under low or high pressure conditions. A change of material was considered, but 316S Stainless Steel was considered the most cost effective anyway so nothing was done there.

What was achieved from that point on, was that the centrigual pump working with grit/untreated water never had a problem afterwards, and all subsequent intallations were done similarly.

Hope this is help to you.

Definitely VFD with controls (PLC) is the way to go. Tell your customer you can cut their energy bill from this setup by 80% by upgrading the system. Even if the trucks are going through the wash bumper to bumper the whole day, high pressure demand might be less than 50%. Add in the dead time between trucks and your talking about a lot of wasted energy (and money). Your could easily do some energy calculations and show them how much they are spending now and how much you could save them.

Also the reduced maintenance and down time for a more reliable system would be a big savings as well.

Jim

Shaft is a straight fit. Bore is in good condition and fit. We repair and assemble about 15 to 20 pumps a week and this is the only customer we have ever had this problem with.

If it is the "only" one then some questions:

- how does he use the pump ? circuit, fluid, cycle.

- is the pump a special design or same as other you maintain?

You have to find the "singularity" of this application how does it differ from all other and then you got the solution to your problem. You have to find the origin.

You mentioned that it was the bolt head that breaks off, not at the shaft face or end of the threads. So my first thought was that the surface underneath the bolt head was not flat, though I didn't explain that well. Is there perhaps a sharp corner on a tight bolt hole? Is there a washer/spacer and is it flat?

What type of impellor? Are there balance holes or pump out vanes? I can't imagine the pressure differential being so great, but?

It is keyed to the shaft, and the key is not shearing? How long does it run before breaking?

Because it's the head that breaks off the bolt, I'm still thinking it's to do with bolt seating.

Or perhaps the threaded hole int the shaft is not parallel to the shaft so that when the bolt is done up tight there is bending applied to the bolt shaft at the head.

Have a look at some of the broken bolts, head and shank. look for evidence of bending, scoring as Rick mentioned, fatigue crack propagation, etc

Also are you using High Tensile bolts? or mild steel.

If there is scoring look for the cause, is the assembly as originally manufactured? If there are changes, ask what and why.

If there is evidence of bending, perhaps a heavy Nylon washer between the bolt head and the steel washer may help.

Regards

Bill

After reading all the quality responses I would suggest hiring an ultra sound technician to listen to the pump and report his findings. An experienced operator will be able to give you some insight as to what is happening. I have worked on a lot of pumps and never had a keyed impeller cause the hold down bolt to break. Loosen off yes but not broken. Seems like a lot of end force being applied for some reason.

I have seen similar situations when the bolts are being torqued to much. Check the spec for your bolts.

Could you make a picture of a broken bolt head and show it on the site?

It is different if the head alone breaks or the shaft at the last thread. Loads can be different and reasons as well.

The main cause of screw failure are detailed below.

1- there is no NRV on delivery line.

2- screw should be locked properly with the help of locking washer.

3- Delivery valve should be closed firmly prior to switch off the pump.

Did your custumor placed a new impeller and forgot to drill the hydraulic balancing holes in to the rear wall of the impeller. Has the custumor also problems with the shaft sealings??

To reduce the axial load of the impeller often so called hydralulic balacing holes are drilled in the rear wall of the impeller. Sinze the pressure on the back side of the impeller is sometimes very high (and specially when they running to the cut off situation) the axial force on the impeller is also very high and forces the impeller towards the suction. Drilling these balacing holes will have almost no influence on the pump capacity since they are very small (holes of f.i 5 mm will do the job)

Hi, may i know whether did you finally find out what is causing the impeller screw failures?

What is the impeller shouldered against on the thrust washer side of the shaft? If there is any compliance there it could end up with a tiny amount of axial movement which could cause fatigue issues on the bolt.

It does sound like dead heading the pump is not doing it any favours too. Is this the only client who is dead heading it? I'd try eliminating all the variables one by one and as Sherlock Holmes put it, "when you have eliminated the impossible, whatever remains, however improbable, must be the truth."

Get a metallurgist to look at the fracture surface of the failed capscrew and tell you what the failure mechanism is. Once you know the mechanism then finding the cause is usually pretty simple. There is almost no hydraulic phenomenon that can put an axial load (tensile stress) on the impeller bolt of enough magnitude for it to fail. Fatigue failure is more likely. but again most of the stresses are in the radial direction and have no effect on the impeeler bolt.

I may have missed it in all the responses, but what is the pumped fluid? Is there the possibility of things like chloride induced stress corrosion cracking?

Are you really interested in an answer?

If yes you should give more information as for instance at least a snapshot of the broken surfaces and if possible not for only one bolt head/shaft.