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.
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.