DESIGN CONSIDERATION FOR PRESSURE BOSSTING SETS

There is an increase in the need to boost water pressures in recent times. This increase has resulted from increasing numbers of taller buildings, more condensed living accommodation (increasing popularity of apartments), unequal pressure on site and an increasing demand for high water pressure by plumbing fixtures and during industrial processing. However, another major contributory factor is due to water authorities reducing mains pressure to reduce leakage from their distribution systems or as a result of losses in distribution networks.

Pressure boosting is therefore used in many different applications. The aim is always is to ensure that sufficient water reaches where it is needed. In high-rise buildings, for example, water pressure needs to be same on the top floor as it is on the ground.

An important aspect to remember is that current water authority legislation will not allow booster sets to be installed directly on the incoming mains supply, in order to prevent back siphonage from any terminal outlet. In most cases it will be necessary to install a break (storage) tank.

There are certain key aspects that must be taken into the equation when sizing a booster set. The first is to size the break tank, and this can be established through a simple calculation using the table below.

(Table 1).Provision of Cold Water to cover 24-hour interruption of supply C.P 310 Water Supply.

Type of Building	Storage (Litres)
Dwelling houses & flats per residents Hostels per residents Hotels per residents Offices without canteens per head Offices with canteens per head Restaurants per head/per meal Day schools per head Boarding schools per head Nurses’ homes & medical quarter per residents	91 91 136 37 45 7 27 91 114

The next step is to correctly size the booster pump or pumps by establishing:

• Required flow rate at peak demand;
• Required outlet pressure at appliance;
• Static height;
• Pipework friction losses;
• Suction conditions;
• Voltage.

A common error when sizing a booster set is to over-estimate the system demand by assuming that all appliances will run simultaneously, which is very rarely the case.

Therefore we should calculate peak demand as a percentage of the maximum, using either actual usage figures, loading units or flow rates required at the appliances as shown below.

Table 2. Recommended minimum rate of flow rate of appliances.

Type of Appliances	Rate of Flow (Liters/s)
W.C flushing cistern Wash basin Wash basin with spray taps Bath (private) Bath (public) Shower (with nozzle) Sink with 13mm taps Sink with 19mm taps Sink with 25mm taps	0.12 0.15 0.04 0.30 0.60 0.12 0.20 0.30 0.60

There are at least five different ways of calculating the required flow rate for water-booster systems.  However, using loading units as the calculation vehicle, maybe an easier method (Table 3).

Table 3: Sizing a break tank using loading units immediately allows for the fact that not all outlets will operate at the same time.

Another benefit of loading units is that it allows for the diversification factor that not all outlets will operate at the same time. There are some exceptions, such as irrigation systems, but the above will hold true in most instances.

Having determined the total demand flow rate, we now need to consider the pressure needed to achieve duty. Factors to be considered include:

• Static height of the building;
• Friction losses through pipework system (calculated at peak demand);
• End pressure required.

Once the required pressure has been determined, it is recommended that you refer to pump selection catalog for selecting both the most suitable number of pumps and the control method.

A pressure boosting system may consist of one or more vertical multistage pumps. Pumps have a common manifold on both suction side and discharge side, non-return valves, shut-off valves, manometer and pressure switch for each pump. A system is delivered on a common base frame (hence the ‘packaged’ title). In most applications, a membrane tank will also be included.

A Note On Variable speed Pumps

There is increasing awareness that the most cost-effective solutions will be achieved through selecting variable-speed pumps. These pumps have the following advantages

• Large energy savings, as the pump only uses the power required to meet the duty and changes in the system.
• Matching the duty of the pump to the system needs imposes less wear and tear on individual parts in the system — extending their life
• Many system problems such as water hammer and noisy valves can be resolved using inverter pumps.
• The need for valves in systems such as final commissioning, bypass valves and starters in control panels can be reduced.
• A more controlled system provides greater user comfort.
• The wider duty range covered by inverter pumps makes selection simpler.

Advanced control facilities:

A number of increasingly sophisticated control features are available that can control up to six pumps connected in parallel to ensure constant pressure in the system. This can be supplemented by pipe-loss compensation, which improves comfort levels and contributes to energy savings.

Other features available include timer programme, alternative set point, pump priority and bus communication.

REFERENCES AND FURTHER READING

1.   Building Services and Equipment. Volume 1, P.4,  F. Hall M.I.O.B, M.I.P.H.E., Published by Longman Group Limited London

2.    Environmental Engineering And Sanitation. 3^rd edition. 1982. P.1000, Joseph A. Salvato, P.E., Publisher: John Wiley & Sons, New York.

3.   CHADDERTON, DAVID, V.; BUILDING SERVICES ENGINEERING, 5^TH ED.,  Published by Taylor & Francis, London, 2000

4.   BARRY, R.; THE CONSTRUCTION OF BUILDINGS VOL. 5 BUILDING SERVICES. Published by Blackwell Science, UK, 1998

5.   MERRITT, FREDERICK S.; BUILDING AND ENGINEERING SYSTEMS DESIGN. Published by Van Nostrand Reinhold Company, NY,1979


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