How Does a Booster Set Work?
A booster set might look straightforward: a group of pumps, pipework, a control panel and a pressure vessel. But behind that relatively simple arrangement is a system designed to maintain the required pressure as demand changes.
How it responds to those changes depends on the type of booster set. Fixed-speed systems can start and stop pumps as pressure changes, while variable-speed systems can adjust pump speed to match demand more closely.
So, what happens when demand increases?
Why do buildings need booster sets?
Mains water pressure isn’t always enough to deliver the required flow and pressure throughout a building.
The problem becomes more pronounced as buildings get taller or larger. Water must overcome static head as it travels upwards, while pressure is also lost through pipework, valves, fittings and other equipment.
Then there’s demand.
A building doesn’t use water at a constant rate. A hotel may go from relatively low overnight demand to a significant morning peak. An apartment block will have its own demand profile, while an office building may experience peaks around the start and end of the working day.
A booster set needs to accommodate these changing conditions while maintaining the required pressure.
What happens when demand increases?
Let’s say the system is operating at its target pressure and a tap is opened.
When system pressure drops, our booster sets detect the change and respond according to their control arrangement. Our variable-speed systems use a pressure sensor to monitor system pressure and adjust pump speed to suit demand, while our fixed-speed units use a pressure switch to control the pumps.
If demand continues to increase and one pump can no longer provide the required flow at the set pressure, another pump is brought into operation.
On a variable-speed system, pump speed can be reduced as demand falls. On a fixed-speed system, pumps will stop when demand ceases, and they reach their closed-valve pressure.
The control system manages the available pump capacity to maintain the required pressure as demand changes.
Why use multiple pumps?
This is where duty, assist and standby arrangements come into play.
At low demand, one pump may be enough. As demand increases, an assist pump can start to provide additional flow. A standby pump can provide resilience, depending on how the system has been specified.
Many sets also use pump alternation, rotating the lead position between pumps. This distributes running hours rather than allowing one pump to continually do the majority of the work. It’s a relatively simple control function, but it can make a big difference over the operating life of the system.
Variable speed and the affinity laws
Variable-speed drives are widely used on modern booster sets because they allow pump output to be adjusted to suit demand.
Rather than a pump simply running at full speed and switching off when pressure is reached, its speed can be continuously adjusted.
The pump affinity laws describe the relationship between pump speed and performance:
Flow: Q₂ / Q₁ = N₂ / N₁
Head: H₂ / H₁ = (N₂ / N₁)²
Power: P₂ / P₁ = (N₂ / N₁)³
Where:
- Q = flow rate
- H = pump head
- P = power
- N = rotational speed
In practical terms, flow changes roughly in proportion to speed, head changes with the square of speed, and power changes with the cube of speed.
For example, reducing pump speed by 20% means the pump is operating at 80% of its original speed. Under the assumptions of the affinity laws, flow reduces to approximately 80%, head to 64%, and power to around 51% of the original level. Actual power consumption will also be influenced by changes in pump efficiency, motor efficiency and the variable-speed drive.
For a system that spends a significant amount of time operating below peak demand, this can have a meaningful impact on energy use.
The actual saving will depend on the pump, system and operating profile, but the principle is simple: don’t run at full capacity when the building doesn’t need it.
This is particularly relevant to energy efficiency.
Buildings rarely operate at their peak water demand continuously, so a booster
set that can reduce pump speed during periods of lower demand can avoid running
pumps harder than necessary.
Variable-speed operation can therefore help reduce energy consumption, particularly on systems with significant variation in demand. The potential saving depends on the pump selection, characteristics and operating profile, but the relationship between speed and power explains why even relatively small reductions in speed can have a significant effect.
How is the pressure setpoint determined?
A booster set isn’t designed to produce as much pressure as possible. It is selected to maintain a specified pressure at the required point in the system. The inlet pressure available to the booster set also needs to be considered, as this forms part of the calculation used to determine the required pump duty.
That means looking at factors including:
- Required flow and peak demand
- Building height and static head
- Pressure losses through pipework and fittings
- Available inlet pressure
- Required pressure at the most demanding outlet
These determine the system duty point.
Selecting a pump with a very high maximum head isn’t necessarily the answer. Too little pressure and the system won’t perform. Too much can result in unnecessary energy consumption and excessive pressure on downstream components.
The aim is to provide the right pressure at the right flow, across the expected operating range.
What does the pressure vessel do?
The pressure vessel is an important part of a booster set. It provides a volume of pressurised water that helps accommodate smaller changes in demand and supports more stable system operation.
It also helps protect the system from pressure fluctuations and water hammer – the banging or knocking that can occur in pipework when water flow changes suddenly.
By providing a degree of hydraulic buffering, the vessel can help reduce unnecessary pump cycling and contribute to smoother operation.
The size and configuration of the pressure vessel depend on the application, but it plays an important role in maintaining stable system performance.
What happens at peak demand?
At peak demand, the duty pump may reach the point where it cannot provide any more flow while maintaining the required pressure.
The controller can then bring an additional pump online.
Booster-set pumps are connected in parallel, so when multiple pumps operate at the same time, they work together to increase the available flow while maintaining the required pressure.
Multiple pumps can also provide redundancy, meaning the system can continue to operate if one pump is unavailable. The level of redundancy required will depend on the application and how the booster set has been specified.
When demand falls again, pumps can be taken out of operation according to the control arrangement.
This ability to stage pumps in and out allows a multi-pump booster set to cover a wide range of demand rather than being designed around one fixed operating condition.
It’s more than just a group of pumps
It’s easy to look at a booster set and think of it as several pumps working together.
In reality, the control system is just as important.
The pressure transducer provides feedback. The controller interprets that information. Variable-speed drives adjust motor speed. The pumps provide the hydraulic performance, while the sequencing logic determines when pumps start, stop or alternate.
All of those elements need to work together.
A correctly sized pump in the wrong configuration won’t necessarily deliver the expected result, and sophisticated controls can’t compensate for an incorrectly selected duty point.
That’s why booster-set selection needs to start with the building and its demand profile, rather than simply choosing a pump based on maximum head or flow.
Booster sets and BMS integration
Modern booster sets can also form part of a building’s wider control and monitoring system. Depending on the specification, a booster set can communicate with a Building Management System (BMS), allowing information such as operating status, pressure, alarms and pump condition to be monitored remotely.
Communication protocols such as BACnet and Modbus can be used to integrate the booster set with other building systems, giving facilities and building-services teams greater visibility of system performance.
For larger commercial and building-services applications, this connectivity can make it easier to monitor operation, identify faults and manage equipment as part of the wider building infrastructure.
Looking for a booster set?
At Trebles, we manufacture our own range of booster sets, giving us the flexibility to build systems around the requirements of the application. From standard configurations to bespoke arrangements, our team can help with pump selection and system design for the project.
Trebles can also supply and assist with the associated cold water storage tank, helping provide a complete solution.
Want to know more? Take a look at our booster sets or get in touch with the Trebles team to discuss your requirements.