04/06/2020
Overview
Water pressure is the force that drives the water through the water fitting (pipe), which is measured in bar. 1 bar equates to 10m in height.
Water pressure can vary throughout the day; typically it will be lowest in the morning before people go to work and as they come home from work, and will be highest overnight. Static pressure refers to the pressure in a water pipe with all fittings closed, dynamic pressure is pressure recorded as the water flows through the pipework and will vary depending on the size of the orifice that is open. (We would see a drop in static pressure as a tap is opened)
Water flow refers to the amount of water coming out of a hose or a water fitting, and is measured in Litres/sec.
Velocity of water is the speed that water moves through a pipe fitting, measured in Meters/sec.
When we put our finger over the end of a hose to create a jet, we are not changing the water pressure, we are just increasing the speed of the water.
Pipe friction (material), joints, valves, diameter of pipes can all add up to reduced performance as the water travels through a building. If mains water pressure is unable to overcome these restrictions, we will notice a reduced flow from taps and showers. Combination boilers and other specific equipment that require a set pressure will go into fault mode.
A typical kitchen tap is 0.13 Litre/sec. Depending on the restrictions set about above, it will need “X” amount of pressure to drive the water through the fitting at this flow rate.
To overcome these restrictions, we fit a cold-water booster set on the incoming main, which increases the pressure and the flow rate. The 1999 water regulations allow us to fit a pump directly to the mains supply as long as it does not exceed 12 litres/min (0.2 Litres/sec) as anything greater than this has the potential to draw the water from the mains supply and surrounding properties.
While this is sufficient for a shower, it will not cover a whole house problem.
For larger flow-rates we must fit a break tank between the mains supply and the booster set, as this will remove the potential for the pump to suck the mains supply dry.
Traditionally break tanks are quite large and filled slowly via a float valve, the connected pump would draw the water from the tank and boost around the house. Besides taking up space, we have the risk of stored water arming up and promoting bacterial growth.
Arrow Valves “Pent-A-Boost” units combine the break tank and booster pump in one small unique package. The break tank is fed by a full-bore solenoid valve(s) which, unlike a restrictive float valve does not hamper the flow through its body. Because we have lost these restrictions, the break tank can be down sized considerably. Our break tanks are 24 litres, if the pipework feeding the tank is the recommended size and a statutory 1 bar is provided the tanks will stay flooded, and the pump(s) will boost accordingly.
Sizing
Sizing pumps can vary between properties – important factors being the height of the building (bar) and the fittings within the building. 3 bar at any appliance is more than sufficient in most situations, typically a 3-4 bedroom properties require 1.0 litre/sec at 3 bar and 4-6 bedrooms 2.0 litre/sec at 3 bar. We have a unit capable of 3.5 litres/sec @ 3 bar for larger properties.
Diversity plays an important factor when sizing pump sets, as if we were to add up all of the fittings within a house the loading would be significantly higher than we actually require. We would never have a situation where baths, showers, washing machines, WC’s outside taps were all running at once. There are loading guides and equations to predict the exact requirement. (Institute of Plumbing. (2002) Plumbing Engineering Services Design Guide.) One source of said information.
High efficiency variable speed motor drives allow quite operation and consume less energy than their equivalent fixed speed pump versions.
The table below shows depicts various flow rates and pressures, and the pumps that will attain this duty.
