17. pipe flow v (11.5-11.7) - lth · • pumps in series and in parallel exercises: d35-36, and...

VVR 120 Fluid VVR 120 Fluid MechanicsMechanics

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


Pump types – axial flow pumps


Rotating movement by centrifugal impeller or propeller ⇒Pressure increases over the pump

Pressure increases over pump: (pout – pin) / ρg = Hp = pump head

Specific energy consumption pump = (w Q Hp)/(3600 Q η) kWh/m3

Efficiency η = power output/power input


Energy line

Energy line

Hvalve

Hp = function(Qp)


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 toreservoir B

What will the flowrate, Qp, be if the pipecharacteristics, L, D, ks are known as wellas static head, Δz, and pump curve?

The pressure increase over pump (energysupply from pump to water) shouldachieve two things with respect to liftingwater from reservoir A to B: 1) overcome geometric height, Δz2) overcome head losses hf1 + hf2


The hydraulic characteristics for thepipe system, Hsyst, is obtained fromthe energy equation ⇒

(local losses neglected in this case)

Hsyst states how much energy that is needed to transport 1 kg of waterfrom A to B

Hp states how much energy thepump can provide to the water

When the pump is introduced in thepipe system the flowrate and pumphead will adjust so that Hsyst = Hp

22

2

gAQ

DLfzlosseshzsystH +Δ=Σ+Δ=

Qp in pipe

Pressureincreaseover pump


D36 Water is pumped between two reservoirs with the same water surface elevation zo. 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.

1122

33


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

2 pumps in parallel

Equivalent pump


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

If one pump runs If two pumps run

System curve (independent of number of pumps)


2) Heat pump systems

Heat pump system


3) Increase of natural flow rate

Without pump: 022 2

2

2

2

=+Δ=⇒=Δ−gAQ

DLfzH

gAQ

DLfz syst

with pumpwithout pump

Hsyst

Hpump

QwithoutQwith


4) Flow control using a valve

2

2

2)(

gAQ

DLfKzH valvesyst ++Δ=

hvalve

Valve

Increasing

Kvalve

(choking)


5) Time-varying reservoir surface

)variesΔz(2 2

2

gAQ

DLfzH syst +Δ=


6) Flow regulation using speed-adjustable pumps

Speed-adjustablepump

Pump curves

(rotation per min)


D35 Water is to be pumped by two centrifugal pumps through a pipeline connecting two reservoirs. The pumps can be operated inparallel 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/m3) both for single pump operation and when both pumps operate.

The pump characteristics are

Discharge Q (m3/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 m3 of water? The efficiency of the pump η = 0.75

17. pipe flow v (11.5-11.7) - lth · • pumps in series and in parallel exercises: d35-36, and...

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