Effect of Pump Suction and Discharge
Orientations on the Selection of
By TK NG
When selecting vibration isolators for water pump installation, the first few things come to our minds would be the loading of the pumpset and inertia block imposed on the isolators, and the isolation efficiency to be achieved. However, the orientations of the pump suction and discharge may add significant “weight” to the pumpset and inertia block assembly, resulting in the need of employing vibration isolators of heavier duty.
2. Taking a high-rise close water circulation system as an example, Figure 1 shows an end suction circulation pump with the suction inlet connected horizontally to a flexible connector. Employing the control volume concept of fluid mechanics, the external vertical downward force acting on the pumpset and inertia block assembly (dotted box in red) is that due to the discharge pressure P1, while P2 at the suction inlet has a sideway thrust effect only.
3. For a discharge pressure (P1) of 14 bar serving a circulation system of 120m high and a 150mm dia. Pump discharge connection to the flexible connector, the downward force acting on the pumpset and inertia block assembly is around 24.7 kN (Pi x (0.150/2)2 x 1,400 kN/m2) or 2,470 kgf, i.e. two and a half tonnes. This would add to the load on the vibration isolators besides that of the pumpset / inertia block assembly. P1 would return to a static pressure of 12 bar when the pump is not operating since the height of the circulation system is 120m.
4. In the case of Figure 2, the difference as compared with Figure 1 is that the suction inlet is connected vertically but not horizontally to a flexible connector, hence increasing the total downward force acting on the pumpset / inertia block assembly.
5. For a suction side running pressure (P2) of 10 bar Note 1 at the 150mm dia. suction inlet / flexible connector interface, the additional downward force due to P2 is around 17.7 kN or 1,770 kgf (Pi x (0.150/2)2 x 1,000 kN/m2). The total vertical downward force is then 42.4 kN (24.7 kN + 17.7 kN) or 4,240 kgf, i.e. more than four tonnes. This means the load on the vibration isolators is substantially increased as compared with the case in Figure 1, not to mention the additional load due to the suction elbow and the larger inertia block.
6. When the pump is not operating, both P1 and P2 would return to around 12 bar but the total downward force acting on the pumpset / inertia block assembly remains unchanged at around 42.4 kN (2 x Pi x (0.150/2)2 x 1,200 kN/m2) or 4,240kgf.
7. It is essential that the configuration of the pump suction/discharge connections and the pressure of the fluid being handled, inter alia, are taken into account when determining the vibration isolator duties, while provision has also to be made to cater for the side thrust created by P2 as in the case of Figure 1 Note 2.
Taking the pump discharge pressure P1 and total friction loss of the close circulation system as 14 bar and 4 bar respectively, the running pressure P2 at the suction inlet is around 10 bar (14 – 4) since there is minimal level difference between the pump discharge and suction flexible connectors.
Spring vibration isolators with the housing capable of restraining side movement are used in the example.