Stephen Booth explains how a variable speed cold water booster could be a better option for promoting a greener future.
The consumption of energy within commercial, residential and industrial settings is the subject of ever-increasing scrutiny. Many businesses and residents 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 low.
A pressure booster system maintains the required 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, and in an environmentally friendly way. 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.
There are many reasons why installers choose a variable speed option, however, the key benefit is that large energy savings can be made compared to a fixed speed system. Over 85 per cent of the life cycle cost of a pump is through energy consumption, so reducing this is vital to minimise electricity costs, carbon emissions and environmental impact. Water consumption is not uniform during the day in a building, so a variable speed solution that meets these demand fluctuations, rather than staying at constant, 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 like bypass valves, reduction valves or interrupted operation, the speed reduction provides significant energy savings at partial load.
Size is another benefit. Fixed speed booster sets consist of large storage / pressure tanks, which take up a lot of space. However, only small vessels are required to help provide a positive pressure for the electronics to work against. This space saving aspect is attractive for those building residential or commercial properties in already restricted areas, as the variable speed booster set can be installed in smaller areas, such as under the stairs.
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. Water hammer can occur during the stopping of a fixed speed pump, whereby the fluid velocity in pipe systems suddenly changes. This damages the pipework and will reduce the lifetime of a system.
However, the soft start and stop of a speed-controlled pump, which is much like turning a tap on gradually, avoids pressure peaks and prevents pipe fractures. In addition, should the power supply fail, occupiers of the building will continue to use what water they can, without realising. However, when the power then comes back on, a constant speed pump would normally register the low pressure and speed up to full to get to its set-point. This creates a pressure surge that can cause the pipes to break. With a variable speed solution, the gentle start up and pipe filling function minimises this risk.
When sizing a booster set, there are a number of factors to consider, such as building type, plumbing fixture type and friction losses in the piping system and equipment.
1) Building type
Considering the building type is important as the usage patterns and the types of appliances being operated vary greatly. For example, in a school, it is unlikely that every toilet or tap will be in use at the same time, but in a sport stadium this will be common during the intervals. Therefore, each case will need to be looked at in its own right and assessed accordingly when sizing.
2) Plumbing fixture type
The plumbing fixture type will also need to be looked at as different fixtures have different flow and pressure requirements. It is therefore important that engineers 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, which is why a variable speed booster is ideal because the drive will accommodate fluctuations in demand.
Next steps: Selection
When selecting a booster set, there are also a number of things that need to be considered. For example, it’s important to know the flow rate required, the head required for this flow rate, the booster set location, the application and also what BMS (Building Management System) is being used.
1) Flow rate
The current standard for calculating flow requirement is BS EN 806-3-2006. Using this specification, you can work out the loading unit and flow value for different appliances. For example, a washing machine is rated at two loading units, whereas a handbasin is one loading unit. 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
In order to work out the total head requirement, you must add the static lift, residual pressure and friction losses together. The discharge pipe should be sized using a velocity of approximately 2.5m/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 the flow rate and pipe size.
3) Booster set location
The location of the plant room is particularly important in relation to the break tanks. This will determine the head available at the pump suction, enabling the correct selection to be made without the danger of cavitation. It should also be located away from accommodation areas to reduce any noise and disturbance. Where this is not an option, anti-vibration kits should be used to reduce the structural transmission of noise.
Power and water supply can be critical to some applications, such as hospitals, so a fail-safe solution is of vital importance. If this cannot be guaranteed, the use of diesel drive pumps and roof level storage should be considered. In addition, ideally all variable speed pumps should also have their own independent inverter complete with their own 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 only. Likewise, some pump sets have inverters on all pumps but only one transducer. Again, if the transducer fails the pump set is out of action.
Most variable speed products have communication capabilities enabling the pump set to talk to a building management system (BMS). For the BMS system to understand the language of the pump set that it is talking to, the protocol of the BMS and the pump set has to match. There are many different protocols so 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 the details of the water requirements considered, then the booster set can be selected. Depending on the importance of supply you may wish to have 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% to 60% of duty.
Stephen Booth is Building Services and Industry sales manager at Xylem