Effect of Pump Suction and Discharge
Orientations on the Selection of
Vibration Isolators
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.
Note 1:
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. 
Note 2:
Spring vibration isolators with the housing capable of restraining side movement are used in the example.
I do not quite agree with the pressure at the pump suction is higher than the discharge. Since, force is equal to pressure x area, therefore, force acting on component at the suction also cannot be higher than that at the discharge.
For end suction pumps, and when the pump is not in operation the pressure at the suction can be higher by virtual of its level from datum is lower than the discharge point. It is due to the static head by the column of water above that point and its value will depend on the levels of that connection point and expansion tank above the discharge point level.
Once the pump starts running the pressure at suction will go lower and discharge go higher. The difference between the suction and discharge pressures will be the “Operating” pump head, which is equal to the total system pressure drop due to friction of pipe runs and components.
The total pressure at the suction will be the combination of the static pressure and pressure drops. The value of the pressure will depend on the point where the expansion tank is connected – as this will be the constant pressure point due to the constant water level in the tank.
For example:
(i) if the expansion tank (say, 2 m head) is connected at the high point of the circuit. Then, that particular point (P-point) will be maintained at 2 m (reference). Any points upstream of the P-point will have pressure above 2 m and working backward with additional of pressure drops due to friction and gain in static head till the pump discharge point. Likewise, any points downstream of P-point will have lower pressure than 2 m working forward will deduction of pressures and gain in static head till the pump suction.
(ii) if the expansion tank of the same height is connected directly at suction point, then the reference static pressure will be at the suction point. The pressures at various points of the whole system will be raised. the pressure raised will be the friction losses between the original point of connection and the new point of connection.
Thanks very much for the comment.
The original article does mean what you say but the presentation may be a bit confusing and misleading. It has now been fine tuned and re-posted. Please see if you have any further comment. Thank you again for your contribution.
By the way, please note that we have migrated to a new domain “learnerthon.org”. Some articles are still in the process of migration and may not be available temporarily. However, the revised “pump” article is ready for viewing.