To change or not to change ...

In July, my Powerspout will have run for 4 years on the same set of bearings; in that time I've not greased them once; they've run only on their factory fill of grease and, having run continuously, the total hours will be just over 35,000.

EcoInnovation's recommendation is to grease regularly and to change the bearings every year (8,760 hours); I've not followed that advice, not because it's bad advice, but because I wanted to see how long I could get bearings to last.

Mind you, ...the bearings I'm using are a premium type from SKF, designated E2 Energy efficient and more expensive than their standard Explorer series; the claims for them are: "longer service life", "longer grease life", "reduced frictional loss" and "lower cost of ownership";

...and I've modified the sealing arrangement at the 'wet end' of the shaft to reduce the risk of water getting in and causing the grease there to be degraded; see here.

So the title of this post: "To change or not to change..." looks at whether after 4 years, the time is ripe for a new set.

According to the SKF leaflet, bearing service life is almost always limited by grease life; for their E2 bearings, SKF give the following chart to estimate how many hours the grease in E2 bearings should last; the estimate is based on an L10 grease life, and that is defined as the period of time at the end of which 90% of a sufficiently large group of seemingly identical bearings are still reliably lubricated.

The red arrow on the chart is placed to indicate the 'life curve' for an A value of 40,000 and that 'life curve' can be seen to intersect the logarithmic y axis at around 80,000 to 100,000 hours, the same as if the A value was 100,000. This means the L10 grease life should be at least 80,000 hours, - or 9 years of continuous running.

This being the case, I am not going to change the bearings this year; I'll hold out at least for another 12 months and think again when the next anniversary comes around in July 2022.

For the technically minded: 

The diagram shows 'grease life curves' for various values of A, under varying bearing operating temperatures, and for bearings working with a load value P = 0.05 * C; this value of P signifies the lowest loading and was assumed to be the value most likely to be applicable for the load experienced by the bearings operating in a Powerspout; the leaflet gives de-rating factors to lower the L10 hours for bearings subject to a higher load value.

the operating temperature of bearings in a Powerspout is almost certainly less than the lowest value given in the diagram (50 deg C); as the dashed lines of the life curves show, this will mean an arbitrary reduction in the value of L10 hours for any given A value.

the A value for the size of bearing used and the particular conditions under which the bearing is operating is calculated from:

A = n * dm

where: n   = rotational speed in revolutions / min (for my turbine 1000 r/m)

            dm = mean diameter, mm, of smallest bearing, 6005 size, ( [25 + 47] * 0.5 = 36 )

Thus: A = 1000 * 36 = 36,000 (which is near enough 40,000)

Post script added 11 September 2022.

One year on from when the above was written, the bearings are still sounding perfectly OK, so I have again decided not to change them and wait another 12 months.

Grease ...the story !

End of the month again and not a bad month generation-wise: 437 kWh clocked up which, for a month with only 28 days in it, isn't too bad. It makes the capacity factor for the month 87% whereas the corresponding month last year saw 100%.  Ah well, you can't have a bonanza February every year !

End of the month is greasing time and this morning, just after 8am, I gave the Powerspout its usual 2 squeezes of the grease gun which delivers a meagre 0.7 mls ( 0.4g) of grease.

In its first life, the shaft and bearing housing of a Powerspout did service in a Whirlpool washing machine.  In such an application, the bearings didn't get the same 24/7 operational use so there was no need to re-grease them. When they failed, that was the signal for the washing machine to go to the scrap heap. But, if it was a lucky washing machine, its bearing housing, shaft and motor might get salvaged for a second life in a Powerspout.

In this second life, re-greasing becomes essential, not only because of the continuous use but because of the damp environment the bearings find themselves in.  So a greasing point has been added to the bearing housing which connects with a grease nipple on the turbine casing.

The point of relating all this is that when grease is pumped in, it has to envelop the rotating shaft and to travel left and right along the shaft to reach the bearings at each end.  New grease contributes significant drag on the shaft which can be seen as a reduction in power output immediately after greasing. After a few hours, the effect disappears. 

Here is the record of power out to the grid immediately after my greasing at 8am this morning, showing a drop of about 5 w.

Another point arising from this arrangement of delivering grease centrally between the two bearings is that significant hydraulic pressure can be produced by a grease gun especially if it is operated forcefully.  The dust shields, whether metal or rubber, are not designed to have grease forced against them and could conceivably be deformed by the pressure achieved.  (This paragraph was amended 14/4/2016)

To prevent this, I operate the grease gun very slowly. Also, I have removed the inner dust shields of the two bearings so they are more open to let grease enter the ball race, pass through it and out the other side.  It is quite easy to remove the shield by gently bending inward the tabs of the shield where it locates in a groove in the outer race.

So that's Grease...the story !  I don't think it's likely to be shown in a cinema near you anytime soon !

Upkeep

How demanding is it having a Powerspout and keeping it operational to maximum advantage ? - that's what I'm setting out to answer in this post. Every site is different though, and others may have very different levels of upkeep: I'd be interested to hear.

1. Trash clearing is seasonal: every day when leaves are falling, but once per day is always enough.  In times of strong winds, clearance is also needed daily because of the large fall of twigs and branches that rain down onto my abstraction site. For the rest of the time, weekly clearing is enough, although I often find myself up there for other reasons and brush off the screen anyway:

As my scheme has a good head but low flow, the nozzles have small orifices: diameters of between 3.64 and 8.90 mm.  Being so small, it would be easy for them to get blocked so having a screen taking out all the trash before the water drops into the header tank is crucial. The screen is of perforated stainless steel with 3mm holes at 5mm centres, and its value has been proved in that I have only ever had one nozzle blockage from trash.

Whilst only having had one blockage from trash, I used to have a more frequent problem with blockage from another cause: toads finding their way into the tank and thence down the penstock. The old galvanised sheets now covering the tank have put an end to that, which is probably of equal relief to both me and the toads.

2. Greasing is every ~500 hours, which is about every three weeks.  I have opted to retain the manual method using a grease gun, rather than going for the self-dispensing GreaseMax canisters recommended by EcoInnovation because I fear too much grease can be as bad as too little.  The canisters contain 125 mls grease which is dispensed over 12 months.  My turbine only runs for 9 months each year and the greasing schedule I work to only delivers 20 mls in that time.

3. Nozzle changes have been necessary 16 times between Nov 5th 2014, when generation started for this year, and today, 7th Feb 2015. They are necessary in order to keep the flow delivered to the turbine matched to the flow available from the spring source.
  
Each change takes about 10 mins and rather than being a chore, is something I enjoy.  In this second "water year", the shortest time between changes has been 1 day and the longest 23 days.  Last year I had a spell at full flow which lasted 75 days which is the record so far.  It all depends on what the rainfall is.  Designing your installation for good access does make this job very much easier if it's something you're likely to have to do as often as I do.

4. General oversight is something which goes on all the time: a water turbine is NOT a generating source like pv panels which can be said to be "fit and forget".  Having said that,  there has not been one instance of malfunction from my turbine apart from some initial teething problems after commissioning over a year ago: it just keeps on going.  In the 3+ months which have been completed in this second 'water year' of its operation,  it has not ceased generating once, other than for nozzle changes.

Occasionally I notice from the continuous record of power output to the grid that the inverter has disconnected from the grid on account of a transient grid failure or voltage fluctuation. When this happens, the Powerspout copes with it seamlessly by diverting output to its inboard dump element until the inverter has satisfied itself that the grid parameters are back to being OK again.

Oversight is helped in my situation by the turbine being sited close to the track to our house.  Every time we come and go, it is easy to glance at it to check that all is well.  If the season is a time when water availability is changing, whilst passing a check can also be made on the fullness of the header tank using the display housed in the pillar by the turbine. More about that in a future post.

If we go away for a few days, I now have enough confidence in everything to leave it running.  Sometimes this will mean putting in smaller nozzles before going away to be sure the system will not drain down while I'm away.  The 'on-line visibility' of its power output also means I can give myself peace of mind by checking on its wellbeing when I'm away.

In summary, for someone who needs to be around their property most of the time, a Powerspout is very little extra work.  In truth, I keep a closer eye on mine than is needful: I walk with the dog down to the turbine last thing every night, - mostly because it is so nice to hear it humming away to itself !