WK
Fig. 35 Characteristics for constant pump rotational speed
As can be seen in the Fig. 35 the power absorbed by the pump
does not decrease proportionately with decreasing rate of flow,
but remains relatively high at the pump shut-off point (capacity
Q = 0).
This absorbed power is almost wholly converted into heat inside
the pump and this heating up process can lead to rapid
evaporation of the fluid inside the pump, particularly if the driving
motor is powerful and the fluid pumped is hot; this happens at
the pump shut-off point (Q = 0) and at very low rate of flow.
In order to avoid such evaporation which might damage the
pump, It is necessary to ensure a given minimum rate of flow
through the pump at all times, which removes the heat
generated.
For this purpose, an automatic recirculation valve (combined
with a non-return valve) is provided (see section 9.3.1); this
valve automatically opens a by-pass line when the rate of flow
drops below a given preset value. If such a valve is not
incorporated in the plant, the pump must not be operated below
a given minimum rate of flow, nor must it be allowed to run
against a closed discharge valve. After start up against a closed
discharge valve, the latter should be opened immediately. If
the pump handles a hot fluid or a fluid with a low boiling point
(highly volatile), or if it operates on suction lift, steps must be
taken to ensure that the fluid at the pump inlet nozzles has
attained the pressure prescribed in the Confirmation of Order,
in order to prevent vapour formation and the resulting damage
caused by cavitation particularly the disintegration of the first
stage impeller). If the back pressure is too low, the capacity of
the pump will increase unduly, and the danger than arise of
overloading and overheating of the driving motor.
3.
Drive
The driver is usually connected to the stub shaft at the suction
end of the pump. The direction of rotation is clockwise viewed
from the driver into the pump. On request, the drive can be
arranged at the discharge end of the pump (direction of rotation
anticlockwise) or the pump can be provided with a stub shaft
at both ends.
There are too many different types of drivers to allow them to
be described in detail here, and we would therefore refer you
to the operating instructions for the driver, published by the
driver manufacturer, which are attached.
4.
Transport
If the pump is supplied as a unit bolted onto a baseplate, the
ropes for handling and transport should be slung under the
pump and driver as illustrated in Fig. 36.
Fig. 36 Slinging the ropes under the pump and driver
mounted on a combine baseplate
5.
Painting
Standard construction pumps operating at temperatures below
140
0
C are provided with a coat of primer and a top coat of
synthetic resin base blue enamel paint (RAL 5001). If the
operating temperature exceeds 140
0
C, the ‘‘hot’’ pump
components, i.e. casing, pressure gauge piping and connection
rods are provided with a coat of aluminium bronze paint (RAL
9006 silver bronze). All the ‘‘Cold’’ pump components, viz. the
base-plate, bearing brackets etc. receive a coat of primer and
a top coat of blue enamel paint (RAL 5001).
Special painting to customer’s specification can be carried out
on request in accordance with the Confirmation of Order. All
bright aprts and surfaces on the pump are coated with oil
grease.
6.
Condition of Equipment as Supplied
The following constructions can be supplied on request (see
Confirmation of Order for certified and binding data) :
1. Pump without baseplate (Fig. 0).
2. Pump mounted on short baseplate (Fig. 4).
(designed to accomodate pump only)
3. Pump and driver mounted on combbined baseplate
(Fig. 3).
If the pump is supplied with a short baseplate or without a
baseplate, the ropes should be slung under the connection
rods as illustrated in Fig. 37.
Caution :
When slinging the ropes for transport, never sling
them under the pump stub shafts or under the bearing brackets.
The internal interconnecting piping for the pressure relief of
the shaft seal, and any cooling liquid supply and drain lines or
sealing liquid lines, in so far as required, are already laid at
our works prior to despatch, up to the limit of the Extent of
Supply. The coupling and coupling guard are already mounted
on the pump.
When a pump is supplied mounted on a combined baseplate,
only the pump is dowelled to the baseplate, after having been
aligned with the driver.
The driver is dowelled on site with cylindrical dowel pins after
the final alignment on site. The necessary cylindrical dowel
pins are supplied loose with the pump.
8
