Sizing pressure booster systems: The parameters to consider
In larger buildings, the pressure supplied by water utilities is usually insufficient to supply the upper levels with enough water. The solution is pressure booster systems. These pump systems respond to fluctuating demand in water supply and adjust their output accordingly, so all consumer points in the building are supplied with the required minimum flow pressure at all times. KSB explores the factors which influence the sizing of such a system.
Usually, water utilities supply drinking water at an average minimum supply pressure of about 2 to 3.5 bar in the piping. This is generally enough to provide the furthest consumer point in a two-storey building with sufficient water. In higher buildings or when pressure- reducing water meters, filters or water treatment systems are used in the inlet, the pressure is no longer sufficient – and the water only trickles out of the tap. In such cases, a pressure booster system (PBS) has to be installed to provide the pressure required.
In mathematical terms, according to DIN 1988 Part 500 (a standard for PBSs with frequency inverter; cascade-controlled systems are to be avoided in the future) a pressure booster system is required specifically when the sum of the pressure loss from geodetic height difference (Δpgeo), the water meter pressure loss (ΔpWZ), the pipe friction and other individual losses [Σ(R · l + Z)], and the apparatus pressure losses (ΔpAp) (e.g. from filters or drinking water post- treatment systems) exceeds the minimum supply pressure pmin, v (or SPLN according to the latest standard) of the local water utility. Summarised in a formula:
SPLN < Δpgeo + ΔpWZ + [Σ(R · l + Z)] + ΔpAp + pmin, FL
The technical challenge is that the PBS has to provide a constant pressure at all times. This especially applies to times of peak demand. Today, new systems or replacements are designed as pressure booster systems with several variable speed pumps arranged in parallel on a baseplate. This means the electronically speed- controlled pumps start up or stop depending on the demand. These pumps generally work in alternation: The next pump to start up is always the pump with the least operating hours; the next pump to stop is always the pump with the most operating hours. This ensures an even distribution of operating hours and prevents water from stagnating in the pumps.
In buildings with fire-fighting or fire-protection systems, the PBS provides a fast and reliable supply of fire-fighting water. In high-rise buildings it is essential for the wall hydrants on all storeys to be supplied with a suitable flow rate and the required minimum flow pressure. The fire-fighting lines further have to be separated from the drinking water system to protect the drinking water from contamination and other hygienically negative impacts.
THE REQUIREMENTS
Installing a pressure booster system is linked with stringent legal regulations that cover many different aspects. Here is an overview:
Drinking water hygiene
To continuously prevent microbial contamination of the water and ensure a high drinking water quality, the installation of PBSs is governed by the stringent guidelines of the drinking water regulation as well as by generally applicable regulations such as DIN EN 806, DIN EN 1717 and DIN 1988.
Operating reliability
DIN 1988 recommends an additional stand-by pump that is installed and ready for operation. This is why even small pressure booster systems have to be fitted with a minimum of two pumps. (This regulation does not apply to single-family or two-family houses.)
Installation room
According to DIN 1988-500, pressure booster systems have to be installed in suitable rooms, for example in a control room. The surface they are positioned on has to be level and of sufficient load- carrying capacity. The room has to be frost-proof, and the temperature in the room must not cause the drinking water temperature to rise to 25°C or above, during stagnation, as stated in VDI 6023.
Further national and regional regulations apply to PBSs regarding the installation, connection types, commissioning, regular servicing, etc.
THE PARAMETERS
The precise sizing of a PBS for a larger building is far from trivial; it requires comprehensive understanding and know-how. For calculating the discharge pressure, the following basic formula is generally recommended:
∆pP = pdischarge – p“ (in bar)
This means that the required discharge pressure (∆pP) should equal the start-up or set pressure at the peak flow rate downstream of the PBS (pdischarge) minus the available minimum supply pressure upstream of the PBS (pinl). So far, so simple. The complicated part is to determine pdischarge and pinl, which are influenced by a number of parameters:
- The minimum supply pressure of the water utility upstream of the PBS (at the water connection point of the house). This is also referred to as SPLN (lowest normal service pressure) in the standard.
- The minimum flow pressure at the hydraulically least favourable consumer point.
- The pressure loss caused by the height difference between the PBS and the highest consumer point (geodetic height difference) downstream and upstream of the PBS.
- The pressure loss of pipe friction and other individual losses downstream and upstream of the PBS.
- The pressure loss of the water meter.
- The pressure losses of any apparatus installed in-between (e.g. filters, dosing equipment, mixing taps, waterfall shower head, etc.) downstream and upstream of the PBS.
For a rough calculation of the peak flow rate the type of building should also be considered: The water-use behaviour in schools is quite different to that in hotels, for example.
Calculating the exact required pressure (head) and flow rate (peak flow rate) downstream of the PBS, selecting the right PBS variant and determining the total cost of ownership would go well beyond the scope of this article.
SUMMARY AND CONCLUSION
A pressure booster system is required whenever the minimum pressure supplied by the local water provider is insufficient. Pressure booster systems and their ancillary components must be designed and operated in such a way that neither the public water supply nor any other consumer units are interfered with – any degradation in the quality of drinking water must likewise be ruled out.
Given the many options for pressure booster systems (cascade control, variable speed control of one or several pumps, direct or indirect connection) it is particularly important to select the right concept as early as possible in the planning phase of a project.
A detailed guide titled ‘Planning Information for Pressure Booster Systems’ is available to download from KSB via the link below.
