4 Centrifugal Pump

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CENTRIFUGAL PUMPS CHARACTERISTICS 1 GOALS This paper presents the experimental procedure used to establish the internal characteristic for a

centrifugal pump. The pumps are hydraulic generators that are working with liquids. In this way,

they transform mechanical energy supplied by an electrical motor in hydraulic energy. Generaly

speaking, between inlet and outlet, there will be an increase of pressure.

2 GENERAL DESCRIPTION Centrifugal pumps consist substantially of one or several impellers rotating in a suitably shaped

casing. In single stage pumps (see figure 1) a single impeller 1 rotates in a casing 2 of spiral or

volute form, whilst in multistage pumps two or more impellers are fitted to a common shaft. Fluid

enters the impeller axially through the eye, the flow continuing radially and discharging around the

entire circumference into the casing. In passing through the impeller the fluid receives energy from

vanes 3 incorporated in the impeller, resulting in an increase of both pressure and velocity. The

kinetic energy of the fluid leaving the impeller is partially transformed into pressure inside the

casing.

Fig. 1 Single stage, single inlet centrifugal pump According to requirements, a variety of pump designs exist in practice such as single and double

suction pumps, turbine pumps, mixed flow pumps, and so on. If diffuser vanes 4 surround the

impeller, we speak of turbine pumps probably because its construction is similar to that of turbines

having guide vanes. The function of the diffuser is to guide the fluid, whereas in volute type pumps

the fluid, after leaving the impeller, discharges freely into the casing. In double inlet pumps (see

figure 11.2), fluid enters from two sides as if two impellers were placed back to back, thus doubling

discharge for the same head.

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3 PUMP CHARACTERISTICS. DESIGN POINT

Pump characteristics (curves) generally relate to the performance of a pump under varying

conditions. Usually, lift (also called pressure head) H , power P (useful and consumed) and

efficiency are plotted against discharge Q for a constant speed. A typical set of curves is shown in Figure 2. It appears that lift H decreases and power

consumption cP increases with increasing discharge. The overall efficiency increases from zero to

a maximum value, then decreases again. The value of the design discharge dQ and the design lift

dH are corresponding for efficiency peak (design point).

Fig. 2 Pump characteristics Pump pressure head, represents the change of liquid specific energy difference between the outlet

"o" and inlet "i" section of the pump:

)z(zpp2

eeH ioio

2i

2o

io ++==

g g [m] (1)

Useful or hydraulic power uP is computed with the Equation:

HQPu = [W] (2) It represents the part of the consumed power cP (the power at pump shaft) converted in hydraulic

(useful) power.

MPc = [W] (3) where: angular speed of the impeller; M torque at the pump shaft.

In this way, the overall efficiency can be written as:

c

uPP= [-] (4)

Generally, the pump characteristics are determined experimentally.

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The laboratory principle

The installation (see Figure 3), is composed of: water tank 1 with the discharge pipeline DP,

centrifugal pumps 2, 4, put in action by the electric motors 3, 5 and the pipeline network 6. Also, we

have represented the experimental set-up 8 of the action forces of a water jet on solid surfaces (for

details see laboratory Acting Force on Different Shaped Solid Surfaces). For this practical work it will be used the centrifugal pump 2, its electric motor being connected to

the power network through the watt-metric bridge 7. According to relation (1), in order to compute the head pump H, is necessary to know the water

parameters in the pump entry section, respectively in the pump exit section. The mean velocities in the two calculus sections are determined with the discharge equation for

incompressible fluids:

2o,i

o,i d Q 4

= [m/s], (5)

tQ V= [m3/s], (6)

where t is the necessary time for passing of the volume V (pre-established) of water

through flow meter CV. For used pump the two characteristics diameters are equal, so the

corresponding speeds are equal too.

The pressure in the inlet section ip is measured using a manovacuumeter M1, and in the outlet

section op with a manometer M2.

Experimental procedure Check if the water level from tank 1 exceeds the exit section of the return pipeline. Check the pipeline networks valves: for this work V1, V2, V4, V5 must be closed and V3 and

V6 open; for parallel coupling of the pumps V2, V4, V5 must be closed and V1, V3 and V6

open; for series coupling V1, V4, V5 must be closed and V2, V3 and V6 opened.

Start the installation; are done measurements at the two manometers and at the wattmetric bridge;

Open tap V1 establishing a flowing regime; is timed t in which a pre-established water volume V, passes through the flow meter; simultaneously, are made readings at the

manometers and at the wattmetric bridge;

Are repeated the previous operations for at least six flowing regimes; the installation is stopped;

Compute the discharge (flow rates) Q with equation (6), the velocities oi vv = with relation (5) for mm50dd oi == , head pump H with equationn (1) for m 57.0z -z io = and the useful powers uP with equation (2).

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Is determined the consumed power cP as function of the electric power in the network eP indicated by wattmeter, according to the dependency ), (P fP ec = shown in figure 4 and is computed the pump efficiency with the equation (4);

Are plotted the pump characteristic curves.

Fig. 4 Dependency ), f(PP ec = Table 1

V t ip op eP [m3] [s] [kgf/cm2] [kgf/cm2] [W]

1. - - 2. 3. 4. 5. 6. 7.

Table 2

Q oi = ip op g io pp H uP cP

[m3/s] [m/s] [N/m2] [N/m2] [m] [m] [kW] [kW] [-] 1. 0 0 0 0 2. 3. 4. 5. 6. 7.