Expansion Joints; Measuring Instruments - Grundfos NB Original Installation And Operating Instructions

7.8 Expansion joints

Expansion joints provide these advantages:
• Absorption of thermal expansion and contraction of pipework
caused by variations in liquid temperature.
• Reduction of mechanical influences in connection with
pressure surges in the pipework.
• Isolation of structure-borne noise in the pipework (only rubber
bellows expansion joints).
Do not install expansion joints to make up for
inaccuracies in the pipework, such as centre
Note
displacement or misalignment of flanges.
The expansion joints should be fitted at a minimum distance of 1
to 1 1/2 pipe diameters (DN) away from the pump on the suction
and the discharge side. This prevents turbulence in the joints,
thus ensuring optimum suction conditions and minimum pressure
loss on the discharge side. At flow velocities > 5 m/s, we
recommend you to fit larger expansion joints matching to the
pipework.
Figures 20 and 21 show examples of rubber bellows expansion
joints with or without limiting rods.
Fig. 20 Rubber bellows expansion joint with limiting rods
Fig. 21 Rubber bellows expansion joint without limiting rods
Expansion joints with limiting rods can be used to reduce the
effects of the expansion/contraction forces on the pipework.
We always recommend expansion joints with limiting rods for
flanges larger than DN 100.
Anchor the pipes in such a way that they do not stress the
expansion joints and the pump. Follow the supplier's instructions
and pass them on to advisers or pipe installers.
Figure 22 shows an example of a metal bellows expansion joint
with limiting rods.
Fig. 22 Metal bellows expansion joint with limiting rods
Due to the risk of rupture of the rubber bellows, metal bellows
expansion joints may be preferred at temperatures above
+100 °C combined with high pressure.

7.9 Measuring instruments

To ensure continuous monitoring of operation, it is recommended
to install a pressure gauge (on the discharge side) and a mano-
vacuum gauge (on the suction side). The pressure gauge taps
should only be opened for test purposes. The measuring range of
the gauges should be 20 % above the maximum pump discharge
pressure.
When measuring with pressure gauges on the pump flanges, it
should be noted that a pressure gauge does not register dynamic
pressure (velocity pressure). On all NB pumps, the diameters of
the suction and discharge flanges are different which results in
different flow velocities at the two flanges. Consequently, the
pressure gauge on the discharge flange will not show the
pressure stated in the technical documentation, but a value which
may be up to 1.5 bar (approx. 15 metres) lower.
To check the motor load, it is recommended to connect an
ammeter.
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