Pumps. Irrigation pumps lift water from an existing source, such as surface or groundwater to a higher level. They have to overcome friction losses during transport of the water and provide pressure for sprinkler and drip irrigation.
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Irrigation pumps lift water from an existing source, such as surface or groundwater to a higher level.
They have to overcome friction losses during transport of the water and provide pressure for sprinkler and drip irrigation.
Irrigation pumps are mechanical devices which use energy from electrical or combustion motors to increase the potential and (or) kinetic energy of the irrigation water.
Pumps are used in irrigation systems to impart a head to the water so it may be distributed to different locations on the farm and used effectively in application systems.
The key requirement in pump selection and design of pump systems for typical irrigation installations is that there is a correspondence between the requirements of the irrigation system and the maximum operating efficiency of the pump.
Requirements of irrigationsystem are:
- flow rates
- pressure out put necessary to operate the system.
physically lifting water in a container
This involves utilizing the fact that water is (effectively) incompressible and can therefore be 'pushed' or displaced.
- Rotary positive displacement pumps, which use gears, vanes, lobes or screws to move, discrete quantities of water from the inlet to the outlet of the pump.
- Reciprocating positive displacement pumps: piston pumps, plunger pumps, diaphragm pumps.
– Gravity operated systems, Siphons
4. Creating a velocity head (velocity pumps
Characteristics of centrifugal , turbine and propeller pumps is given as below.
Where: Ns = specific speed , dimensionless
N = Revolutionary speed of pump , rpm
Q = Pump discharge , L /min
H = discharge pressure head , m
10, 000 (propeller pump).
The swept volume per stroke will be V= AS
The discharge per stroke will be q = V η vol
The pumping rate (per minute) is Q = nq
Water is pushed into the center or eye of the impeller by atmospheric or water pressure and set into a rotary motion by the impeller.
-The rotating movement causes a centrifugal force to act upon the water, which drives the water outward, between the vanes of the impeller, into the surrounding casing from where it moves to the pump outlet.
-Different types of casing: a)Single volute, (b) Double volute, and (c). Diffuser turbine casing.
The capacity of a pump is the volume of water (Q) which the pump can deliver per unit of time, e.g. in litters per second (lt/s) or cubic meters per hour.
2. Pumping Head
The actual pumping head imposed on a pump, gross working head, will be somewhat greater than the actual vertical distance, or static head, water has to be raised.
P = Water pressure in (kpa or meters water column)
= density of the fluid in (kg/m3)
g = acceleration due to gravity in (m/S2)
V = Water velocity in (m/s)
Z = Elevation head in meters relative to a reference level or datum.
g = =specific weight of the fluid (kN/m3)
Phydr = g H Q =
Phydr=hydraulicor water power in Watt.
Q = pumped volume in m3/s.
Phydr = 1000 x 9.81 x 120 x 180/3600 = 4, 905 watt = 4.9 kw
The actual power and energy needs are always greater
than the hydraulic energy needed
hydr = (Phydr / Pmotor)x 100
hydr = pump efficiency
Phydr = water power (kw, hp) Pmotor = break power (kw, hp)
WHP = Q x TDH
where: WHP = Water Horse Power
Q = Flow rate in gallons per minute (GPM)
TDH = Total Dynamic Head (feet)
BHP = WHP
Pump Eff. x Drive Eff.
BHP -- Brake Horsepower (continuous horsepower rating of the power unit).
Pump Eff. -- Efficiency of the pump usually read from a pump curve and having a value between 0 and 1.
Drive Eff. -- Efficiency of the drive unit between the power source and the pump. For direct connection this value is 1, for right angle drives the value is 0.95 and for belt drives it can
vary from 0.7 to 0.85.
Ha = atmospheric pressure on the surface of the water (in m)
Hs = elevation of the water above or below the impeller eye while pumping (in m) (if the level is above the eye, Hs is positive, if the level is below the eye, Hs is negative)
Hf = friction-head losses in the suction piping (in m)
Hvp = Vapor pressure of the water at the pumping temperature (in m).
Q2= Q1 x (N2/N1)
H2 = H1 x (N2/N1) 2
BP2= BP1 x (N2/N1) 3
NPSH2 = NPSH1 x (N2/N1) 2
For constant N ( Rotation per minute)
Q2 = Q1 x (D2/D1)
H2 = H1 x (D2/D1) 2
BP2 = BP1 x (D2/D1) 3
NPSH2 = NPSH1 x (D2/D1)2
Q = discharge
N=number of Revolution per minute
BP = Break power
NPSH = Net positive suction head
D = diameter
H = Available head
Hs = SL+ DL+ DD+ H1 + M1 +HO + VH
HS = System head (m)
SL = Suction lift from static water level (m)
DL = discharge lift from pump to highest discharge point (m)
DD = draw down in water source (m)
H1 = head loss in delivery pipes (m)
M1 = minor losses in fittings (m)
Ho = operating head (m)
VH = velocity head (m)
TDH =Z +Hs + hv + hf
WHP = Q H
WHP = the energy pump produces to move the water
BHP = Input power to the pump given by the motor
= out put of the motor
Input power for the motor is from electricity.
P = Q H Sg
P = power , metric horse power
Q = Pump discharge, L/min
H= Discharge pressure head, m
Sg =specific gravity of fluid, dimensionless
E = pump efficiency , fraction
P = Q H Sg
P = Q H Sg
Where P = power , KW
Q = discharge , m3/s
P = Q x TDH Sg
Where P = power, brake horse power (bhp)
Q = pump discharge , (gpm)
TDH or H = Discharge pressure head , ft
Ep = WHP /BHP
Em = BHP/ input
EPP = WHP/ input = Ep . Em
Where Em = Efficiency of motor
Ep = efficiency of pump
EPP = Efficiency of pumping plant
- To provide more Q and not more head
Q = Q1 + Q2 + Q3
H = H1 + H2+ H3
In Submersible pump a number of impellers are connected in series
NPSHavial. = Patm - Zs- PV – hfs
atm. Pressure - static suction head - vapor pr/ head - friction head loss
Process of choosing the most suitable pump for the irrigation system.
It involves the specification of the discharge and pressure requirements of the irrigation system, selecting the required pumping method and identifying the different pumps (within the chosen method), which can meet the requirements of the irrigation system.
Increase the depth of submergence….
2. Determine the design month
Size the pump and Power source
4.Determine the installed capital cost of the whole
5. Determine the present worth of the recurrent costs, sub-divided into
a. Replacement costs
b. Maintenance costs
6.Life cycle costs
7.Unit water costStep-by-step procedure of costing irrigation pumps
the intake or borehole,
personnel costs, etc
-AVAILABILITY OF TECHNOLOGY AND SPARES
-OPERATIONAL CONVENIENCE- skilled man
Must be included in the cost estimation
permanent pumping installations.
Some guidelines are:
1. by reducing the normal flow velocity through the system by choosing larger diameter pipes;
2. Reducing the speed with which the flow in the system can be changed by:
=>using slow control valves
=>using air pressure tanks –as buffer