17. Pipe flow V (11.5-11.7) Pump types Pump systems Pumps in series and in parallel Exercises: D35-36, and D38
Pump types centrifugal pump VVR 120 Fluid Mechanics
Pump types axial flow pumps VVR 120 Fluid Mechanics
Rotating movement by centrifugal impeller or propeller Pressure increases over the pump Pressure increases over pump: (p out p in ) / ρg = H p = pump head Specific energy consumption pump = (w Q H p )/(3600 Q η) kwh/m 3 Efficiency η = power output/power input
Energy line H valve Energy line H p = function(q p )
A pump is characterized by a so-called pump curve η = power output/power input
Calculation of flowrate and pressure in a pump system Water is pumped from reservoir A to reservoir B What will the flowrate, Q p, be if the pipe characteristics, L, D, k s are known as well as static head, Δz, and pump curve? The pressure increase over pump (energy supply from pump to water) should achieve two things with respect to lifting water from reservoir A to B: 1) overcome geometric height, Δz 2) overcome head losses h f1 + h f2
The hydraulic characteristics for the pipe system, H syst, is obtained from the energy equation L Q 2 H syst = Δz +Σh = Δz + f losses D 2gA 2 (local losses neglected in this case) H syst states how much energy that is needed to transport 1 kg of water from A to B H p states how much energy the pump can provide to the water When the pump is introduced in the pipe system the flowrate and pump head will adjust so that H syst = H p Pressure increase over pump Q p in pipe
D36 Water is pumped between two reservoirs with the same water surface elevation z o. Total pipe length is L = 2500 m, diameter D = 0.1 m and equivalent sand roughness k = 0.0001 m. The pump characteristics is given by the figure. What is the maximum permissible distance x from the upstream reservoir to the suction side of the pump, if the pressure must not be less than atmospheric. The local losses may be neglected. The temperature is 20 C. 1 2 3
PARALLEL PUMPING Pumps operating in parallel are replaced by a fictive equivalent pump with a pump curve obtained by horizontal addition of the single pumps pump curves Equivalent pump 2 pumps in parallel
PUMPS IN SERIES Pumps operating in series are replaced by a fictive equivalent pump with a pump curve obtained by vertical addition of the single pumps pump curves 2 pumps 1 pump
EXAMPLES OF SYSTEM CURVES 1) Two pumps operating in parallel System curve (independent of number of pumps) If one pump runs If two pumps run
2) Heat pump systems Heat pump system
3) Increase of natural flow rate 2 2 L Q L Q Without pump: Δz = f H = Δz + f = 0 2 syst 2 D 2gA D 2gA with pump H pump without pump H syst Q without Q with
4) Flow control using a valve H syst = Δz + ( Kvalve + f L D ) 2 Q 2gA 2 h valve Increasing K valve (choking) Valve
5) Time-varying reservoir surface H syst = Δz + f L D 2 Q 2gA 2 (Δz varies)
6) Flow regulation using speed-adjustable pumps Pump curves Speedadjustable pump (rotation per min)
D35 Water is to be pumped by two centrifugal pumps through a pipeline connecting two reservoirs. The pumps can be operated in parallel or one at a time. The pipeline is 2000 m long, diameter 250 mm and equivalent sand roughness is 0.2 mm. The static lift is 25.0 m. Calculate the specific energy consumption (kwh/m 3 ) both for single pump operation and when both pumps operate. The pump characteristics are Discharge Q (m 3 /s) 0.020 0.030 0.040 0.050 Head (m) 55 47 35 20 Total efficiency (%) 78 80 72 60
D38 Water (20 C) is pumped between two reservoirs through two identical, parallel pipes each with a diameter of 0.2 m, length 1000 m, and equivalent sand roughness of 4 10-4 m. a) What flow is expected through the pump? b) How much energy (kwh) is needed to pump 1 m 3 of water? The efficiency of the pump η = 0.75