How Does a Booster Set Work?
Ever wondered how a booster set works? A booster set might look straightforward: a group of pumps, pipework, a control panel and, depending on the application, 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.
On a system using pressure feedback, a pressure transducer detects the change and sends information to the booster set’s controller. On a variable-speed system, the controller can increase the speed of the running pump to compensate.
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.
When demand falls, the booster set responds according to its control arrangement. On a variable-speed system, pump speed can be reduced; on a fixed-speed system, pumps can be stopped as the required capacity falls.
The booster set is therefore not simply trying to produce maximum pressure. Its 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.
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 available inlet pressure also needs to be considered. The booster set may only need to provide the additional pressure required to bring the incoming supply up to the system duty.
That means looking at factors including:
- Required flow and peak demand
- Building height and static head
- Pressure losses through pipework and fittings
- Available mains 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 another important part of the system, although it isn’t simply there to store water. It provides a volume of pressurised water that can help accommodate smaller changes in demand and support more stable system operation. Depending on the system design, it can also help reduce unnecessary pump cycling.
Its size and configuration depend on the application, but its role is closely linked to maintaining stable system operation.
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.
The pumps operate together to provide the required flow. When demand falls again, the assist pump can stop and, depending on the control arrangement, the remaining pump can either continue at its operating speed or reduce speed.
This ability to stage pumps in and out is one of the key advantages of a multi-pump booster set. The system can 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.
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, system design and the control strategy for the project.
Want to know more? Take a look at our booster sets or get in touch with the Trebles team to discuss your requirements.