ENGLISH
17
phenomena which occur when, at the impeller intake, the absolute pressure falls to values that allow the formation of vapour bubbles in
the fluid, so that the pump works irregularly with a fall in head. The pump must not cavitate because, as well as producing considerable
noise similar to metallic hammering, it would cause irreparable damage to the impeller.
To determine the suction level Z1, the following formula must be applied:
Z1 = pb - rqd. N.P.S.H. - Hr - correct pV
where:
Z1
= difference in level in metres between the axis of the pump and the free surface of the liquid to be pumped
Pb
= barometric pressure in mcw of the place of installation
(fig. 6 , page 93)
NPSH
= net load at intake of the place of work
(page 94-96)
Hr
= load loss in metres on the whole intake duct (pipe - curves - foot valves)
pV
= vapour tension in metres of the liquid in relation to the temperature expressed in °C
(see fig. 7 , page 93)
Example 1: installation at sea level and fluid at t = 20°C
required N.P.S.H.:
3,25 m
pb :
10,33 mcw
(fig. 6, page 93)
Hr:
2,04 m
t:
20°C
pV:
0.22 m
(fig. 7, page 93)
Z1
10,33
–
3,25
–
2,04
–
0,22 = 4,82 approx.
Example 2: installation at a height of 1500 m and fluid at t = 50°C
required N.P.S.H.:
3,25 m
pb :
8,6 mcw
(fig. 6, page 93)
Hr:
2,04 m
t:
50°C
pV:
1,147 m
(fig. 7, page 93)
Z1
8,6
–
3,25
–
2,04
–
1,147 = 2,16 approx.
Example 3: installation at sea level and fluid at t = 90°C
required N.P.S.H. :
3,25 m
pb :
10,33 mcw
(fig. 6, page 93)
Hr:
2,04 m
t:
90°C
pV:
7,035 m
(fig. 7, page 93)
Z1
10,33
–
3,25
–
2,04
–
7,035 = -1,99 approx.
In the last case, in order to operate correctly the pump must be fed with a positive head of 1.99 - 2 m, that is the free surface of the
water must be 2 m higher than the axis of the pump.
N.B.: it is always good practice to leave a safety margin (0.5 m in the case of cold water) to allow for errors or
unexpected variations in the estimated data. This margin becomes especially important with liquids at a temperature
close to boiling point, because slight temperature variations cause considerable differences in the working
conditions. For example in the third case, if instead of 90°C the water temperature reaches 95°C at any time, the head required
by the pump would no longer be 1.99 but 3.51 metres.
8. ELECTRICAL CONNECTION
Scrupulously follow the wiring diagrams inside the terminal board box and those on page 1 of this manual.
The requirements of the electric energy supply company must be scrupulously complied with.
In the case of three-phase motors with star-delta start, ensure that the switch-over time from star to delta is as short as possible
and that it falls within table 2 on page 91.
In particular the earth terminal must be connected to the yellow/green lead of the power cable. The earth lead used must be
longer than the phase leads so that it does not disconnect first when subject to traction.
−
Before opening the terminal board and working on the pump, ensure that the
power has been switched off.
−
Check the mains voltage before making any connection. If it is the same as the voltage on the data plate, proceed to connect
the wires to the terminal board,
giving priority to the earth lead. (Fig. D)
−
The pumps must always be connected to an external switch
.
−
Three-phase motors must be protected with special remote-control motor-protectors calibrated for the current shown on the plate
or with fuses of the size indicated in chapter 4.
9. STARTING UP
Do not start the pump unless it has been completely filled with fluid.
Before starting up, check that the pump is properly primed; fill it completely with clean water by means of the hole provided after having
removed the filler cap on the discharge body. This ensures that the mechanical seal is well lubricated and that the pump immediately
starts to work regularly.
(Fig. E)
The filler cap must then be put back in place.
Dry operation causes irreparable damage to the mechanical
seal and the stuffing box seal.
−
Fully open the gate valve on intake and keep the one on delivery almost closed.
−
Switch on the power and check that the motor is turning in the right direction, that is clockwise when viewed from the fan side,
Fig. F
(indicated also by the arrow on the fan cover). Otherwise invert any two phase leads, after having disconnected the pump from the mains.
