By way of example, we recently supplied a turbine on a site with a 135m of head, flow
0.5-1 L/s.
The turbine the clients been using for the last 2 years had a bronze Pelton rotor
made in the USA using a car alternator. The bronze rotor has been all but worn away
after 2 years of continuous use. Also the car alternator needed the brushes replacing
every 6 months and overhauling every 12 months. With a small PCD rotor it was running
at about 2500 rpm. After 2 years of running the bronze rotor is very badly worn and
unusable.
We supplied this client a plastic Pelton rotor turbine on a trial basis to see what
would happen. The rotor PCD is much larger as it is only running at 1000 rpm on a
smart drive PMG.
After 50 days on test the client has decided to purchase the turbine, he has indicated
that some wear has occurred but is happy with the unit. After talking to the client
most of the wear occurred when he was doing high flow tests. We has a 100mm line (way
to large for the low flow he has most of the time). When he opened up the line with a
larger jet accumulated grit/slit in the line started to move and sand blasted the
rotor for may hours (the line is long and it took time to clean out). His view is that
most of the wear occurred at this time as up to this point there was no noticeable
wear on the spoons.
We normally only approve them up to 100m of head. You can see from this example that
135m might be pushing it. Even if the spoons only last 6 months, 4 sets are still only
a fraction of the price of the bronze unit. In fact he could buy over 10 sets of
replacement spoons for the price of one bronze rotor. If you always have a spare set
on hand then it is not a problem. It will not be long before we see Chinese injection
moulded Pelton spoons that will make even our ones look expensive.
Metal spoons have advantages but the labour component required to polish and balance
them means that there are expensive to make in developed countries with high labour
rates. Injection moulding on the other hand takes about 10 seconds a shot.
For most clients in the 20-70m range (that we commonly supply) we have not had any
reports of wear problem and we have enough of them out there to have heard of a
problem if it existed. We wouldn't give a 2 year warrantee if we thought they wouldn't
do the job.
Regards
Michael Lawley
Renewable Energy Engineer
EcoInnovation
671 Kent Road
R.D.1
New Plymouth
New Zealand
Phone: (NZ) 06 752 2765
Web site: www.ecoinn.co.nz
email: [EMAIL PROTECTED]
----- Original Message -----
From: Marc de Piolenc
To: [EMAIL PROTECTED]
Sent: Friday, October 29, 2004 1:52 PM
Subject: [microhydro] Abrasion and surface hardness
I'm a little disturbed at the assumption, evident in
many recent messages, that a hard surface is
necessarily more resistant to abrasion than a soft
one. This is not correct, and people on this list in
particular need to be aware of that.
When a hard abrasive particle hits a hard surface, it
loses nearly none of its kinetic energy, and in a
turbulent stream may strike the surface again and
again, each time spalling or scraping off a small
amount of material.
When it hits a soft surface, it tends to lose energy -
essentially embedding itself temporarily. This can
often result in LESS wear on a soft surface than on a
hard one. The ultimate useful application of this
property is in an industrial process called lapping,
in which a soft tool (the lap) charged with an
abrasive, is used to remove material from a much
harder workpiece.
It is therefore incorrect to assume that a plastic
turbine bucket will necessarily have less wear in it
than a metal bucket of the same profile. It MAY be so
- not all soft surfaces have the property of resisting
wear - but that must be substantiated by test. Surface
hardness is only one of the relevant parameters.
Best,
Marc de Piolenc
Iligan City, Philippines
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