Pressure

1 bar = 1000mbar = 10m head of water = 14.7 psi

Mains water pressure is typically between 2 and 6 bar.

PVC, ABS and other industrial thermoplastic pipe systems are designed to operate at between 10 and 16 bar.

What pressure is your pipe system running at? If you don't know then I can pretty much guarantee it will be a lot less than you think. With the exception of high pressure jet wash systems and ultra fine filtration of less than 5 microns (0.005mm) or so, there are very few systems that operate above 3 or 4 bar pressure. So, how do you know what pressure your system is operating at? The simple option is to fit a pressure gauge; but in the absence of this, or if you are in the planning stage to a new system, the following may help.

Before describing the ways to measure pressure, you should be aware that there are two types of pressure in a system, both of which need to be understood and factored in:

The first of these is STATIC PRESSURE - The static pressure (sometimes also called the closed valve pressure) is the pressure of a system when nothing is flowing through it. Imagine you have a header tank supplying your plastic pipe and all the valves in the system are shut, or a pump that is running with, once again, all the vales shut off.

The "static pressure" is the maximum amount of pressure your system will ever experience without adding other physical factors to the system (such as a second or larger pump, or extending and increasing the height of the header tank). As soon as you open a valve the pressure in the pipework will fall.

The second is the DYNAMIC PRESSURE - This is the pressure in your system when it is running. As the name "dynamic" suggests, it is a pressure that can and will change. From a closed system which is sitting with its full static pressure (see above), open one valve and the pressure will fall to a new lower level; this is the dynamic pressure of the system with just that one valve open. Open a second valve and the pressure will fall again, the dynamic pressure has changed.

The dynamic pressure is dependent on a combination of the size and position of any outlets and also the friction losses of the fluid flowing through the system: the more outlets and the larger they are, the lower the dynamic pressure; the higher the velocity of the fluid in the pipework, the more friction loss there is and the lower the dynamic pressure will be, as the fluid is losing energy through friction.

Ideally, any plastic pipe system (be it PVC [PVCu uPVC], ABS, PE, PP) should be able to withstand the static pressure, as this protects the system against damage in the event of all the outlets being shut off.

Pressure in pipelines and networks is typically provided in one of two ways:

• Header tanks - Using a header tank gives us a simple way of calculating the pressure. At any point in the system, the "static pressure" of the system is the difference in height between that point in the system and the water level in the header tank. Measure this in metres, divide by 10, and this will give your static head in "bars"
  • Example - We have a pipe coming out of a header tank, and going round a building. The header tank water level is 4.5m off the ground, and a point in the pipework that we want to know the pressure at (for example a valve supplying water) is 1m off the ground. The difference between the two is 3.5m. Divide this by 10 and we have a pressure, at that point in the pipe, of 0.35bar
  • In a header tank scenario, you should always have the same level in your header tank no matter how much water you are taking in or out of the system. Header tanks are typically kept constant by ball cock type valves, electrically modulating valves or simply by being allowed to always run slightly to overflow.
• Pumped systems - The pressure delivered by pumps typically varies with the flow rate that is coming out of them, and pumps typically have a curve such as the one below.
  • The graph above represents a typical centrifugal pump curve. (Centrifugals are the most common type of pump and probably account for >95% of all pumps). As the flow increases the pressure declines. Contrary to popular belief, however, if you shut off all the valves from a pump, the pressure will not build up and up and eventually explode the pipework or pump. This is because they are nowhere near 100% efficient. They are usually around 65 - 70% efficient and the higher the pressure, the less efficient they become, as it becomes harder for them to push fluid against the pressure. This results in all centrifugal pumps having a "dead head" pressure point. The point where they simply cannot pump any fluid against the existing pressure. This is the "Static Head" or "Dead Head" figure we are looking for on a pump curve.
  • As an aside, you may be asking "what happens when a pump is allowed to run against a static or dead head?" The answer is that the temperature of the water in the pump builds up. It's basic physics - the pump is still rotating, throwing water around inside, so that energy must go somewhere - it can't go up the pipe as the pressure is too great, so it converts to heat energy. The pump gets hotter and hotter and often results in either the plastic pipework connected to the pump softening and bursting, or some component in the pump such as a seal, failing as a result of the heat.
  • If you don't have a curve for your pump (you can usually find it in the manual), pumps will often have a specification plate attached which will often give a maximum figure in pressure and a maximum figure in flow rate. The first is the pressure at no flow (the STATIC HEAD) and the second the flow rate at maximum flow. So, in other words, the two values at either end of the graph
  • Examples of typical pump static heads:
    • Pool and spa pumps - 3.5 bar
    • Waste water pumps - 2 bar
    • Multistage Process pumps - 6-8 bar
    • Submersibles - 1-4 bar
    • Small petrol or diesel pumps - 3 bar

I hope this has helped explain why, for the most part, 9 or 10 bar plastic pipe and fittings are more than adequate for most systems. If you have any queries, please do not hesitate to contact us for free and impartial advice.