The Choice of Materials For Water Turbines

Photo by Ahmad Suhendra

Determination of material for water turbine is very important, because turbines work on conditions such as high pressure, abrasion caused by sand carried by the flow, and also of pH levels and salinity that can cause corrosion, so that the appropriate materials needed to treat the condition. There are several types of water turbines and materials used in their manufacture.
Penentuan bahan untuk membuat turbin air sangatlah penting, karena turbin bekerja pada tekanan tinggi, adanya pengikisan karena pasir yang terbawa air , juga derajat keasaman (pH air) dan kadar garam dalam air yang dapat menyebabkan korosi, maka untuk itu diperlukan bahan yang tepat untuk mengatasi hal tersebut.
Disini diperlihatkan beberapa jenis turbin air dan bahan yang digunakan untuk membuatnya.

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Ref:
http://www.ivt.ntnu.no/ept/fag/tep4200/innhold/The%20choice%20of%20materials.pdf

Shaft Diameter and Shaft Critical Speed

By.Ahmad Suhendra
The size of the diameter of the shaft is very influential at critical speed. If the shaft is rotating close to or exceed the critical speed, the shaft becomes very fast vibrate and cause the bearings become damaged, so the correct practice should be working below the shaft critical speed.

Example calculations.
A Pelton turbine disc has a weight of 30 kg , and planned to have a nominal rotational speed is 1500 rpm and Power = 100 kilowatts. Shaft material (solid cylinder shaft) to be used is a standard steel, with the number: AISI-SAE 1040, which has Ultimate tensile stress (UTS) 90.000 psi and Young's modulus (E) 29 10^6 psi .
What is the diameter (D,mm) of the shaft is used?
If the distance between the bearings and disc is L= 50 mm (see picture),
what is the critical rotational speed (Nc)?
Did diameter shaft that is used safely?

Given :
N = 1500 rpm
P =100 kW
m = 30 kg
UTS = 90,000 psi (620.5 N/mm^2)
E = 29*10^6 psi (199948 N/mm^2)

I.) Torsional or twisting moment (T, N.mm).
T = 9.55 10^6 P/N................................(1)
= 9.55 10^6 * 100 / 1500
= 636667 N.mm

II.) Ultimate shear stress (UST, N/mm^2).
UST = 0.75 *UTS........................................(2)
= 0.75 * 620.5
= 465 N/mm^2

The allowable stresses (Ss) that are generally used in practice are:
4000 psi ( 28 N/mm^2) for main power-transmitting shafts (Ss < UST).

III.) Polar Section Modulus (Zp, mm^3).
Zp =T / Ss............................................(3)
= 636667/28
=22738 mm^3
and,
for circular solid shaft is :
Zp = (phi D^3) / 16................................(4)
D^3 = Zp *16 /phi
= 22738 * 16 / 3.14
D = 115863^(1/3)
= 49 mm or 2 inchi. (Shaft diameter)

IV.) Polar moment of inertia (I ,mm^4) of a circular solid shaft.
I=1/4* R^4.............................................(5)
I = phi D^4 / 64
= 3.14 ( 49^4)/64
=282836 mm^4

V.) Critical speed.
shaft weight is negligible.
E =199948 N/mm^2
I=282836 mm^4
L=50 mm
m=30 kg

Nc =[ 3 * E * I / (m *L^3)]^0.5 / (2*phi)..............(6)
= [ 3 * 199948 * 282836 / (30 * 50^3)] ^0.5 / ( 6.28)
= 45242^0.5 / 6.28
=213 /6.28
=34 rev/s
=2040 rpm

Nc > N, 1500 rpm (OK, can be used safely).

N(1500) / Nc(2040) = 0.735
0.735 < 0.8 (subcritical operation)

Note:
lower than 0.8 * Critical speed - subcritical operation
higher than 1.25 * Critical speed - above critical operation


Ref:
http://www.tribology-abc.com/calculators/uts.htm
http://www.roymech.co.uk/Useful_Tables/Matter/shear_tensile.htm
http://www.engineeringtoolbox.com/torsion-shafts-d_947.html
http://www.roymech.co.uk/Useful_Tables/Drive/Shaft_Critical_Speed.html
http://jimnevins.net/Stuff/Machinery%27s%20%20handbook%2026th%20Edition/MH26/yc.pdf

An example design of the crossflow turbine, type BYS-T3

Picture : www.boutiquepower.com.au

You can use an example design of the crossflow turbine type BYS/T3 by Ueli Meier as a teaching or a comparison with the turbine you plan, if you look at and study the sample design,it will open your insights about how to design a crossflow turbine. For those just learning about the turbines may be found in many less obvious things but do not worry , a lot of reading on the internet resources that can help you to understand it. We hope you become part of people who care about the future of the planet by using renewable energy . Thanks

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Ref : Ueli Meier , " Design of Crossflow Turbine BYS/T3 " , Swiss Center Appropriate Technology - Varnbuelstrasse 14 CH-9000 St.Gallen Switzerland

Resistance Coefficient ( Ke ) For Entrance

Inward projecting----Sharp edged---Slightly Rounded---Well Rounded

he = Ke * V₂ ² / (2*g)

he = Entrance losses (m)
Ke = Depends upon the shape of the intake opening
V2 = The average velocity (m/s) of water in penstock
g = Gravitational constant (9.8 m/s^2)

Ref :
ESHA (European Small Hydropower Association),”Layman’s Handbook on How To Develop a Small Hydro Site,”2nd ed, 1998 http://www.scribd.com/doc/8885765/Layman-Handbook-for-hydro-electric-power-plants

Resistance Coefficient K for Sudden Expansion-Contraction

The losses through these fitting are generally evaluated by first obtaining

ß = d2 / d1

Important Note:

the resulting K values as tabled below are based on the flow velocity in the larger pipe,

if the flow velocity in the small pipe is used to evaluate the head loss then the K values tabled below should be multiplied by 

( ß)^4 = (d2 / d1) ^4

Head loss (h):

h_expansion(he) = Ke*(v2)^2 / (2*g)

h_contraction(hc) = Kc*(v2)^2 / (2*g)

v2= Average velocity (m/s) of water in small pipe

g =  Constant of gravity 9.8 m/s^2

Table of Ke & Kc against β = d2 / d1

β	Ke	Kc
0.15	1887.42	965.43
0.2	576	300
0.25	225	120
0.3	102.23	56.17
0.35	51.31	29.24
0.4	27.56	16.41
0.45	15.51	9.72
0.5	9	6
0.55	5.32	3.81
0.6	3.16	2.47
0.65	1.87	1.62
0.7	1.08	1.06
0.75	0.6	0.69
0.8	0.32	0.44
0.85	0.15	0.27
0.9	0.06	0.14
0.95	0.01	0.06
1	0	0

Ref : http://www.roymech.co.uk/Related/Fluids/Fluids_Pipe.html

Trash rack (or screen) losses

By Ahmad Suhendra

A screen is always required at the entrance of a pressure pipe . The function of screen (trash rack) is :

• keep debris away from the entrance to the outlet works where the debris will not clog the critical portions of the structure;
  
• capture debris in such a way that relatively easy removal is possible;

The flow of water through the rack also gives rise to a head loss. Though usually small, it can be calculated by a formula due to Kirchmer .

ht = [Kt * (t / b)^(4 / 3) * (Vo ^2 * Sin α )] / (2 * g)

ht = Screen head loss (m)
Kt = Resistance coefficient
t = Bar thickness (mm)
b = Width between bars (mm)
Vo= Approach velocity (m/s)
g = Gravitational constant (9.8 m/s^2)
α = Angle of inclination from horizontal


Ref :
ESHA (European Small Hydropower Association),”Layman’s Handbook on How To Develop a Small Hydro Site,”2nd ed, 1998 http://www.scribd.com/doc/8885765/Layman-Handbook-for-hydro-electric-power-plants

http://www.knoxcounty.org/stormwater/pdfs/vol2/3-3-5%20Trash%20Racks%20and%20Safety%20Grates.pdf

The microhydro plant

By Manfred Mornhinweg (manfred@ludens.cl)

My little paradise has a stream that provides enough water flow and head to run a small turbine, to provide electricity to my home. While writing this, the microhydro plant is being implemented, and here are some photos of the process.

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Walsh River Micro-Hydro Turbine Contruction Guide

Prepared by Max Enfield, December 2007, revised March 2010

These notes are intended as a guide to those wishing the construct a Banki-crossflow turbine like that were previously available and used in the Walsh River Micro-Hydro Systems. The focus of the notes is on construction details, rather than design.

The Walsh River Micro-Hydro System derived its name from the Walsh River in Far North Queensland, where the prototype system was installed. The system concept was developed and prototyped in 1991 by Jerry Jeffress and features the integration of custom made Banki-crossflow turbines, with Baldor DC generators and AERL Hydromax DC:DC step down controllers. It can also be used to pump water, indeed it has been used to pump water and generate electricity simultaneously.

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