The set up

The set up
5.36mm jet delivering 0.63 l/s to the pelton which is rotating at 870 rpm and generating 127 watts into the grid.

Friday, 3 July 2020

The Doble notch

Although the pelton wheel is named after Lester Pelton, - the man who saw the benefit of the splitter ridge in each cup, - another man, William A. Doble, invented a modification which was possibly of greater significance.

In 1907, Doble obtained a US patent for the notch which is seen at the outer edge of the cups of all modern pelton wheels. He realised two things: first, that as the cups entered the jet, they disrupted it, causing what was meant to be a clean, solid shaft of water to break up and fail to transfer maximum energy to the wheel; second, that the first bit of water entering each cup was deflected upward, where it hit the underside of the cup next to arrive. By hitting the underside of that cup, the force of the water was acting to make the wheel turn the opposite way, - and that was clearly a bad thing. 

By designing a notch which allowed the jet to pass through one cup and continue onward to the cup which preceded it, his invention ensured the next cup entered the jet when the wheel had rotated a bit further round. This meant the jet struck the cup more perpendicularly and the direction of the deflected exhaust water was no longer upward toward the following cup, but outward to the sides.

His design also described the shape and inclination of the splitter ridge so it was the sharp tip, the extreme tip, of the splitter ridge which was first to enter the jet. The effect of this was to make it as if a knife was cutting cleanly into the jet, disturbing the integrity of the solid column of water as little as possible. 

The sequence of events happening when a jet hits cups which have a Doble notch is seen in this photo from my last blog post:

  • the jet can be seen passing through the notch of cup 1...
  • ... and striking the splitter ridge of cup 2 almost perpendicularly, sending water to the sides and not back toward cup 1
  • cup 3 is cut off from the jet by cup 2, but water which was entering it before cup 2 got in the way, is seen to be still passing across the floor of the cup and up its side wall where it is just beginning to exit 
  • within a fraction of a second, the wheel will rotate a tiny amount clockwise bringing the tip of the splitter ridge of cup 1 cleanly into the path of the jet

Doble's patent can be viewed as he originally wrote it here.  It is worth reading in its entirety for its conciseness.

These are two extracts from it:


William Doble became chief engineer in Lester Pelton's company from 1912, but history has ensured Pelton's name rather than Doble's has been tied to the impulse turbine they both worked to develop.   
It just goes to show that having a brilliant idea doesn't ensure history will remember you !

Tuesday, 16 June 2020

Frozen action.

Last night, after dark, I went down to my turbine with a camera, which was loaned to me, to take pictures I've been longing to take for ages.

The reason for it being after dark was so the speed of the flash, rather than the speed of the shutter, froze the action.

Inevitably, a lot of water sprayed onto the camera lens and that has taken away some of the quality I was hoping for, but the pictures do show nicely how: 
  • the cups cut into the jet and chop it up 
  • the jet is carved in two by the splitter ridge
  • the notch in each cup ensures the splitter ridge is the first part to enter the jet 
  • water then passes down and around the floor of the cup 
  • water is thrown up and away by the side walls
  • pulses of water are created exiting to the side

The photos were made possible only by choosing when to capture them. I use an SMA Windy Boy inverter to connect to the grid, and it takes 4 minutes from the time it first receives power before connecting to the grid. During this time, the pelton overspeeds and causes exhaust water to exit in a different way to when it's at its operational speed. A line of approach relatively free of spray is created which gives a good view of the jet.

The rotational speed in the photos was 1270 rpm, giving a linear velocity for the runner at the pcd (pitch circle diameter) of about 15 m/s. 

At this speed, the time taken for a cup to move to the position of the cup ahead of it is about one 400th of a second.

The nozzle orifice was 6.48 mm diameter, the flow 0.91 l/s and the jet velocity about 31 m/s.

The camera was a Sony Cybershot DSG TX10.  It is water proof, - and it needed to be !

Monday, 18 May 2020

An engineering solution to cover fixing.

Owners and operators of Powerspout turbines, especially the pelton sort, will know that the front and back covers are held on by self-tapping screws which screw into the plastic of the turbine carcass.

The method has something of the wood-worker about it.

Repeated screwing and unscrewing of these fixtures inevitably causes them to de-thread eventually, whereupon one solution is to drill a new hole and continue to use a self tapping screw in the new hole.

Although the holes have not yet de-threaded for me, they're beginning to feel that way, and since that is after nearly 7 years of fairly frequent removal of both front and back covers, the self tapping screw method does have some merit in lasting for quite a while.

But I am not enthusiastic about drilling new holes and continuing with self tappers so for some time I have been looking around for a better solution, - some way of fixing that comes more from the toolbox of an engineer than a wood-worker.

During this past week of continuing Covid lockdown, I have received the parts I'd decided on and have put them in: this post tells the story.

I've opted to use M5 stainless steel flanged screws and have these screwing into brass inserts fixed into the holes previously used by the self tapping screws.

Links to where I purchased the bits are at the bottom and are correct at the time of writing.
Brass inserts with M5 internal thread to accept M5 threaded stainless steel, flanged, screw.
Close up of brass insert to show coarse outer thread with longitudinal tracks to aid it to lock into the plastic and not unscrew.
Method used for placing insert by mounting it on an M5 socket headed screw with two lock nuts so it can be screwed into the plastic; a socket headed screw was used for placement because quite a bit of axial force is needed to get the coarse outer threads to bite.
Screwing the insert into place taking care to keep perpendicular to the face.
The insert in place with its surface flush with the plastic; no prior drilling out of the hole was done; it was found that the size of the hole after it had been used by self-tapping screws was about right to get a really tight fit of the brass insert into the plastic; a round file used for sharpening a chain saw (7/32", 5.5 mm size) was used to tidy up the hole beforehand but the hole should not be enlarged by the use of the file.
To finish off, a new neoprene self adhesive strip was placed; to make holes in it for the screws, a hot nail poked through does the job neatly.

The completed job, - no leaks ! The turbine here is running on the bottom jet only, rotating at 940 rpm, generating 322 W into the grid at a water-to-grid efficiency of exactly 50%.

And to finish off, a neat tool for dealing with the 8 mm hex-headed screws.
- M5, flanged, A2 stainless steel screws, 16 mm long were purchased here (£2.90 for 20, Free postage to UK) 
- Threaded brass, double ended, self tapped, screw fit inserts, M5 internal thread were purchased here (£7.30 for 25, Free postage to UK)
- Black Neoprene self-adhesive sponge, 6 mm thick x 15 mm wide x 5 m long purchased here (£5.50 Free postage in UK)
- Britool 8 mm nut-spinner, available while stocks last,here 
(£3.85 Free postage in UK)