Richter SCK-S - Manual (Page 6)

Series MNK-S, MNK-SB, SCK-S, “Self-priming“ design Page 6
9230-062-en Revision 02
TM 6566 Edition 08/2006
The enclosed air is now transported with the medium
by the impeller through the volute housing to the
pump housing. Here, the air and medium separate
again; the air can escape through the discharge noz-
zle and the medium can collect at the bottom of the
pump housing. In this operating mode no medium is
conveyed out of the pump, it circulates in an internal
circuit and only the drawn-in air leaves the pump
through the discharge nozzle.
Once the suction line has been completely drained,
conveyance of the medium commences. The suction
and discharge nozzles are filled with medium. The
pressure generated by the impeller rises as it is now
completely filled with medium.
The self-priming pump has already been optimally
modified by the manufacturer to suit the operating
conditions required. The internal circuit in the pump is
maintained throughout conveyance.
3.2 Field of application
The possible applications of the self-priming pump
can be divided into two groups, which can be desig-
nated "emptying waste-water basins" and "emptying
with overflow".
In the case of emptying waste-water basins the task
of the pump is to convey the medium out of a con-
tainer which is situated lower than the pump into a
collecting container. With the suction line free of me-
dium, the pump must raise the medium by the geo-
detic suction lift H
S,geo
. This is to be differentiated from
the length of the immersion pipe L1.
In the case of emptying with overflow, e.g. out of a
tank truck into an upright tank, the pump has in gen-
eral a supply, but the medium in this case must over-
come the overflow with the geodetic suction lift H
S,geo
.
When the previously filled pump (see Section 6.1.1)
is switched on for the first time, the discharge line
still contains no medium, i.e. it can be vented without
back pressure.
However, once the container on the suction side has
been emptied and then the shut-off valve on the dis-
charge side closed and the pump switched off, the
pump must overcome in both applications a static
medium column, designated by the geodetic back
pressure H
D,geo
, each time venting takes place again.
The self-priming pump can only vent up to a certain
maximum back pressure. If this pressure is ex-
ceeded, a separate pressure-free venting line has to
be provided.
The temperature of the medium in the pump in-
creases as a function of the venting time. It must be
guaranteed that the vapour pressure of the medium,
which increases as the temperature rises, always
remains lower than the pressure actually in the suc-
tion line.
3.3 Application limits
The application limits of the self-priming pump are
governed by the density of the medium.
The following table indicates the maximum possible
geodetic suction lift as a function of the density.
Density
[kg/dm
3 ]
H
s, geo
max.
[m]
1 6
1.1 5.4
1.2 4.7
1.3 4.2
1.4 3.7
1.5 3.3
1.6 2.8
1.7 2.3
1.8 1.9
1.9 1.5
2 1
The product of the medium density in kg/dm 3
and the geodetic back pressure H
D,geo
in m must
not exceed 6 at 1450 and 1750 rpm and not ex-
ceed 18 at 2900 and 3500 rpm.
In order to keep the venting time as short as possi-
ble, the temperature rise in the pump as low as pos-
sible and therefore the difference to the vapour
pressure of the medium as large as possible, the
suction line should be as short as possible. An
adequate difference to the vapour pressure is ob-
served for the particular application for which the
pump was supplied.