Pumps Flashcards
Bernoulli’s equation across a pump, pump head (hp)
Pumping head = suction lift + friction loss in suction pipe+ delivery lift + friction loss in delivery pipe
hp = P / rhoQg
suction lift: elevation change between the sump water level and the pump
delivery lift: elevation change between the pump and the point of supply
friction losses: in suction and
pipeline will vary with discharge
minor losses: in inlet and other components are usually small and
it is common practice to add 10% to the friction loss in the pipeline in the initial design stage
Centrifugal pumps
fluid enters the pump impeller along or near to the rotating axis and is accelerated by the impeller, flowing radially outward
* small discharges at high pressures
Axial flow pumps
- fluid is driven parallel to the shaft of the impeller, their flow does not change their radial locations. It allows the fluid to enter the impeller axially and discharge the fluid nearly axially.
- large volumes of water at
low pressure
Mixed flow pumps
- The impeller is so shaped that the fluid enters axially but leaves axially and radially.
- more efficient with larger quantities of water than centrifugal pumps
- more efficient at higher pressures than axial pumps
pump flow Q if radial velocity Vr is known
Q = 2pi r b Vr
radius of the blade (r)
width of the blade (b)
angle of blade beta
cot beta = U - Vtheta / Vr
radial velocity (Vr)
velocity of whirl (Vtheta)
absolute velocity (V)
relative velocity (W)
sqrt(Vr^2 + (U - Vtheta)^2)
taken from velocity triangle
blade speed (U)
maximum power that can be
passed to fluid for a pump at a given
speed
Pmax = 1/2πρrb tanβ U2^3
scaling law between two pumps
hp / hpM = λ^2 (n/nM)^2
Q / QM = λ^3 n / nM
pump head (hp)
rotational speed of pump rotor (n)
second pump (M)
size ratio (λ) = L/LM
pump selection
*10~70ns = Centrifugal, high head and low discharge
*70~170ns = Mixed flow, medium head and medium discharge
*>170ns = Axial flow, low head and large discharge
Specific speed (ns)
