Variable speed cold water boosters

The consumption of energy within residential, commercial and industrial settings is the subject of ever-increasing scrutiny. Many businesses and homeowners rely on a water booster set to pump water throughout expansive or multi-storey buildings, but traditional booster sets work at a fixed speed, meaning the level of energy consumed remains the same even when the demand for water is very low.

A pressure booster system maintains the required domestic water flow and pressure throughout an entire building. However, as cities become more heavily populated, buildings are getting higher and the demand placed on pressurised water supplied is increasing.

In an attempt to reduce leakage, pressures have been dropped at reservoirs, but millions of litres of water are still being wasted. This means it is less about supply, and more about the solution, looking at improving how water is transported around a building in order to meet user needs. Legally, water companies have to supply a minimum water pressure of 1 bar, but in an ideal situation, this would actually be 6 bar. This is where a booster set can help to ensure those accessing water at higher points of a building don’t lose pressure.

The benefits

The key benefit is that large energy savings can be made compared to a fixed speed system. Over 85% of the lifecycle cost of a pump is through energy consumption. Water consumption is not uniform during the day in a building, so a variable speed solution that meets these demand fluctuations will harbour great energy reductions.

According to the Third Affinity Law, a 20% reduction in pump speed reduces the energy consumption of the pump motor by nearly 50%. These savings are possible because of fundamental rules for centrifugal pumps: the reduction of the speed provides a lowering of the flow according to the linear function, a reduction of the head according to a quadratic function, and a reduction of the power consumption according to a cubic function. Compared with other common control systems, such as bypass valves, reduction valves or interrupted operation, the speed reduction provides significant energy savings at partial load.

Size is another significant benefit. Fixed speed booster sets consist of large storage or pressure tanks, which take up a lot of space. However, with variable speed booster sets, only small vessels are required to help provide a positive pressure for the electronics to work against.

A variable speed booster also offers a soft-start function, which is much kinder on the system fittings and pipework, reducing the risk of water hammer and in turn decreasing maintenance costs. It avoids pressure peaks and prevents pipe fractures. In addition, should the power supply fail, occupiers of the building will be able to continue to use what water they can, without realising.

Getting started: Sizing

When sizing a booster set, there are a number of factors to consider:

1 Building type

Usage patterns and the types of appliances being operated can vary greatly. For example, in a school, it is unlikely every toilet or tap will be in use at the same time, but in a sports stadium this is common.

2 Plumbing fixture type

Different fixtures have different flow and pressure requirements and engineers should check the local plumbing regulations for fixture flow requirements.

3 Friction losses

As flow decreases, friction losses in the piping system also decrease. Constant speed pressure boosters cannot take this into account.

Next steps: Selection

Consider these factors:

1 Flow rate

There are three main UK standards for calculating flow requirement, one being BS EN 806-3-2006 (see p21 for the method published in Annex C of BS EN 8558:2015). Using this specification, you can work out the loading unit and flow value for different appliances. Once you have added the loading units of all the necessary appliances together, there is a graph within the standard that can be used to obtain the diversity factor. By reading from the chart, you can equate this loading unit to the required flow rate.

2 Head requirement

To work out the total head requirement, add the static lift, residual pressure and friction losses together. The discharge pipe should be sized using a velocity of approximately 2.5 m/s, and once the flow rate has been calculated, pipework sizes can be ascertained using the accepted velocity. Friction losses can then be calculated using flow rate and pipe size.

3 Booster set location

The location of the plant room is important in relation to the break tanks. It determines the head available at the pump suction, enabling correct selection to be made without danger of cavitation. It should also be located away from accommodation areas to reduce noise and disturbance. Where this is not an option, anti-vibration kits should be used

4 Application

Power and water supply are critical to some applications, such as hospitals, so a fail-safe solution is vital. If this cannot be guaranteed, consider the use of diesel-drive pumps and roof-level storage. Ideally, all variable speed pumps should also have their own independent inverter complete with transducer. This ensures that if an inverter fails the variable speed control is not lost and the set continues to operate as a variable speed pump set. For sets with only one inverter, if it fails, the set either stops completely or operates as fixed speed. Likewise, some pump sets have inverters on all pumps but only one transducer. If the transducer fails the pump set is out of action.

5 Communication

Most variable speed products have capabilities enabling the pump set to talk to a business management system (BMS). For the BMS system to understand the language of the pump set, the protocol of the BMS and the pump set has to match. It is important to make sure the variable speed drives chosen have the same protocol as the BMS they are connecting to. Otherwise, a gateway will need adding to convert from one protocol to another.

Finally, when the above has been assessed and water requirements considered, then the booster set can be selected. Depending on the importance of supply you may want a duty and standby pump to give 100% back up, or the duty may be split into three pumps with standby, for example: 3 x 50% pumps, meaning that if one fails you will still have a 100% duty available. Where the supply is not so critical it may be more cost effective to have a Duty Assist pump, sized to 50/60% of duty.

About Xylem

Xylem is a leading water technology company committed to ‘solving water’ by creating innovative and smart technology solutions to meet the world’s water, wastewater and energy needs.

This article first appeared in the Nov/Dec 2019 issue of P&H Engineering, the magazine for members of the CIPHE. Find out how to join here.

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