How to size a booster pump for a building?

How to select a pressure boosting pump

 

USE OF PRESSURE BOOSTER SET

Water is usually supplied from the main public water pressure line which is normally sufficient for a building (water and sanitary utility equipment).

 

When this pressure is not sufficient, booster sets are used to increase water pressure and ensure an acceptable minimum pressure is available at the furthest water point.

 

Water can be supplied to a booster set in two ways:

  1. By installing a water storage with a booster set (indirect connection). It requires pumps with greater head because the indirect connection does not allow the main water system pressure to be used.
  2. By connecting the booster set directly to the main water system (direct connection).
  1. Residential buildings:

The main data needed for calculation of the quantity of water required is given in the following list:

  • The number of outlets
  • Consumption per each type of outlet (Table 1)
  • The contemporaneity factor (Fc)

Table 1: Maximum consumption at points of demand

Non-residential buildings

For calculation of quantities of water required, the following types of building are considered:

  • Offices
  • Shopping centers
  • Hospitals
  • Hotels

 

These buildings require quantities of water greater than those needed in residential buildings.

Diagram B below shows the values of actual delivery for the main types of building. The values are based on hypothetical numbers of persons present in these buildings.

The outlet pressure required for proper operation of electrical appliances must not be lower than 1.5 bars and not greater than 4-5 bars.

When the pressure level is insufficient and to such a degree that it impedes operation of domestic appliances, a pressure-boosting system must be installed to ensure adequate pressure also at the more unfavorably located points of demand.

fig 5

Hr = the minimum residual pressure at the highest outlet (normally 1.5 bar)

fig 7
fig 8

When the pumps draw water from a well, the dynamic height difference (Ha), when pumps are operating, should not exceed 4 m. A greater suction head or erroneous sizing of the suction pipe may cause improper operation of the pumps-e.g. cavitation’s and priming loss. The pumps are installed with a positive suction head when they are connected to a raised tank or pressurized tank.

The pumps therefore have an initial pressure at the suction port which can vary from 0.1 bar (suction with a collection tank) to 2-3 bars (with suction from a pressurized primary collection tank).

When choosing a pressure-boosting system, the positive value of the initial pressure (Pi) must be considered as a value to be subtracted from the height (Hg).

The system head loss(Pc) are given by the sum of the losses of the pipes(including the suction pipe) added to the losses due to gate valves, non-return valves, water purifiers, counters, filters, elbows etc.…

Head loss in the tubes, caused by friction of the water against the inner surface of the pipelines, may be quantified as 0.5 m per floor in the case of new systems and 1 m per floor in case of old systems.

To avoid pressure levels greater than 4-5 bar arriving at outlets on the lower floors of apartment blocks and other buildings with a height greater than 30 m (about 10 floors), pressure reducers must be installed at the off take point of the lower floors or otherwise two pressure-boosting units can be installed: one for the lower floors and one for the upper floors.

 

Selecting a pressure vessel for a booster set

The two main purpose of a pressure vessel are: -

  1. To protect against water surge due to water hammering.
  2. To retain certain amount of water under pressure to prevent continuous running of water pumps.

The selection of the vessel must be made in-line with the pump flow and pressure and number of starts allowed by the motor. This is given in the table below (for Calpeda water pumps):-

The surge vessel may be of the following type:

  • Air cushion vessels
  • Membrane vessels

 

  • Air cushion vessels
  • Membrane vessels

These vessels are fitted with an inner membrane separating the water and air. When installed, they must be pre-charged at a pressure in-line with the pressure switch settings.

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