Wholesome Water Supply within High-Rise Buildings

In this technical paper submitted to the Building Engineering Services Association (BESA), David Rhodes, Technical Manager of Albion Valves, explores the challenges facing the designers of water distribution systems throughout high-rise buildings, made even more important as the numbers of these buildings increase.

Therefore, in designing a water distribution system the designer needs to be mindful that to get the water to the top of the building he must overcome the force of the static head pressure created by the water within the pipework system. Static head is the height difference between the water source and the point of use.

The main issue is that for every 1m change in height, the pressure changes by approximately 9.81kPa (due to Earth's gravity). As water goes up the building, the pressure decreases by 9.81kPa per metre. Roughly 1 bar for every 10m head pressure (100kPa).

Another aspect to consider is the pressure drops accumulated throughout the pipework as the water travels through various bends and other valves and the frictional losses caused by the pipework itself. These losses must be considered when sizing the pipework as it cannot be purely based on the pressure which is being delivered from the initial source.

The design of such systems should aim to maintain an even distribution of water to all floors whilst combatting the effect of gravity ensuring the higher floors still receive adequate water pressure. If the pipe is adequately sized to incorporate these pressure losses and the designer has taken these into account, then the PRVs should be fitted into the line at the current pipe size and not require the pipe to be reduced or bushed up to incorporate the PRV.

UK water authorities are obliged to supply water at a minimum of 0.7 bar (70kPa) which equates to roughly 7m head (height) of water. This water pressure decreases against the geodetic height of the building to such an extent that it is only sufficient to provide water to 2-stories, due to the sheer height of many high-rise buildings, the municipal water supply pressure is therefore not enough to meet the requirements of the high-rise water distribution system, these type of buildings are therefore required to have a ‘pressure boosted water distribution system’.

To achieve this, they use water pumps, which are the mainstay of achieving the water pressure requirements throughout tall buildings. Without a correctly pressure-boosted system, there is a risk that the higher floors within the system will receive a lower flow pressure, particularly during peak times.

These water pumps, typically located in the building's basement or a dedicated pump room, are responsible for elevating the water pressure to ensure it reaches the upper floors of the building. They effectively compensate for the pressure drop caused by the height of the building.

The addition of water pumps will increase the pressure of the water to a level that ensures all floors within the high-rise receive water at an adequate pressure, however, whilst solving this issue, it creates another in that it will also increase the water pressure within the pipework significantly, particularly at the lower floors.

High-pressure media flowing through pipework within a building can also cause issues for the occupants whether this is from noisy pipework due to excessive vibrations caused by cavitation/water hammer, or a more rapid deterioration in valve seals causing terminal units to be replaced more frequently.

These issues surrounding excessive pressure are relatively easy to resolve by adding Pressure Reducing Valves (PRVs) in the system where excessive pressure exists, but it is not an exact science as it also depends upon the diversity that occurs with the use of water outlets/water consumption within a building.

The designed water system must comply with the current UK Building Regulations, that limit the designed daily water consumption. Some designers currently size pipework based on full flow conditions in which they assume that every outlet such as toilets, sinks, showers etc. will always be in operation, as they try to size the pipework based on worst-case scenarios, hence the assumption of full flow which leads to oversized pipework.

However, this is not true for real-world applications as not every outlet will be in use at all times, and therefore sizing the pipework adequately becomes increasingly more important.

Therefore, systems should not be designed to accommodate all outlets being supplied simultaneously and should instead be calculated using loaded units.

According to CIPHE, loaded units can be defined as:

The loading units (LU) within BS EN 806-3 take into account the flow rates at the draw-off point, the length of time the appliance is in use and the frequency of use. One loading unit (1 LU) equates to a draw-off flow rate QA of 0,1 l/s.

These loaded units in combination with a series of pipework tables can help calculate the correct pipe size required.

PRVs are designed to restrict pressure as the water moves from the valve inlet across the valve to the outlet port. It does this by passing the water across a spring and diaphragm arrangement that creates a resistance and therefore lowers the pressure to a manual pre-set determinate level, this can be verified by the gauge.

In regulating the water flow to a specific pressure, any sudden spikes in pressure will be eradicated as the spring and diaphragm respond to the changes in upstream pressure, thus maintaining a more constant downstream water supply, thus adding additional protection to any downstream pipework or equipment.

The design of such systems can be summarised into 3 main options as described below;

Remedy with Pressure Reducing Valves

There are several ways in which to address this challenge.

Option 1 - The Division Into Pressure Zones

In this case, several floors are protected collectively via a pressure reducer. In this case, the Pressure Reducing Valves are placed on the lower level of the zone and set to a high-pressure range. This type of installation is a more cost-effective solution approach, but has the disadvantage in the event of maintenance that the individual zone must be supplied with emergency pressure during this period and the pressure control takes place in a much wider spread area.

Option 2 - Floor Control

Each floor that requires water pressure reduction is equipped with its own pressure reducer. Each Pressure Reducing Valve can then be adjusted individually. Result: Optimum pressure for each floor, flat, or office.

• A constant supply to all floors is guaranteed at all times in case of maintenance.

• Savings in operating costs due to resource-saving water consumption.

• Initially higher project costs.

Option 3 - Use A Reservoir

The water is pumped into an elevated tank on the roof of the building to be supplied. The water then flows by gravity to the individual floors that are too high to be adequately supplied with the urban pressure. However, the floors supplied with this top-down supply model have the opposite problem on the lower floors, as the water pressure increases with each floor downwards due to the geodesic height. So, while the water pressure is sufficient on the upper floors of a tall building, there is usually too much pressure on the lower floors.

For over forty years, Albion Valves (UK) Limited has helped installers deliver optimised water-based systems. The company is continually developing range of products and their in-house expertise enables Albion to supply a complete, quality valve solution alongside excellent cradle-to-grave technical support and service for the industry.

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