Why does the size of your pipe matter?

Why does the size of your pipe matter?

One of the most common misconceptions that both normal customers and also highly experienced trade customers have when they come to us is about the pipe size they need to use. It can be very frustrating trying to tell a 50 year old plumber (that knows everything) that no you can’t increase the pressure in a house magically by using smaller pipe.

Important parts of water.

When you are pumping water there are a number of things that you need to take into account to ensure that you end up with the water flow and pressure that you desire. To start with, let me explain the basics:

  1. Flow rate. The flow rate (usually measured in L/min, m3/hr or L/sec) is the volume of water that is travelling past a fixed point in the pipe. You measure flow rate with a paddle or electronic water meter. As an equivalent in electricity is the Amps or current.
  2. Pressure (measured in kilopascals or pounds per square inch) is the amount of mechanical force being applied to the water at a certain point in a closed pipe. You measure pressure either with a pressure gauge or pressure sensor (transducer) screwed into a fitting in the pipe. An equivalent in electricity is voltage or volts.
  3. Volume (measured in litres, cubic meters or gallons) is the measure of how much water is moved over a period or stored in a particular vessel.
Pressure gauge reading pipe pressure

Pressure gauge reading pipe pressure

These three measurements of a flowing pipeline or water make up the different things that determine what we use the water for. Now we get a little bit trickier. You need water pressure (be it under gravity or being pumped) to get water to move from its source (a tank, a bore or a dam) to what you are using it for (irrigation, washing something etc.). Pressure is simply a measure at a certain place in a pipe, if you move to a different part of your system, then the pressure will change.

 

 

 

How does pressure and flow work?

Pressure is created by a couple of different forces and is destroyed by a couple of others. The main sources of pressure are:

  1. This is known as positive head, when the source of the water is elevated above where you are going to use it. For every meter above your use this creates 98kpa in static pressure (if you turn the tap off a pressure gauge will read 98kpa).
  2. A pump is used to create pressure by applying a mechanical force (from an impeller, rotor or piston) into the water and this increases the pressure leaving the pump forcing the water up the pipeline. The maximum pressure a pump can create is called the shut off head and this is measured by shutting a valve off after the pump and showing what pressure is created when there is no water moving.

static-head

There is also 2 ways in which pressure is lost too:

  1. Head loss. As with having the water source above your use can create pressure, if the source is at the bottom of the slope and you want it to be used at the top, then there is a loss of pressure. An example of your pump is creating around 50m head (490kpa) at the bottom of a 30m high hill, then at the top of the hill when you shut the tap off, the pipe will only have 20m (196kpa) showing on the gauge.
  2. Friction loss. When you put water (or air or any fluid) down a pipeline, the flowing fluid rubs up against the walls of the pipe. As it does this it creates friction and to allow the water to keep flowing at the same rate, a little bit of pressure is consumed. This is called friction loss and over a distance, this can have a very large effect on how much water comes out and what the final pressure is.

Friction loss ?

Now friction loss gets into pretty high end engineering to both explain and calculate it properly. The basic concept though is that if your pipe is too small when you are pumping water, then over a distance you will find that your system doesn’t have as much pressure out as you put in. An example is in your garden irrigation system. Say you are using normal 25mm (1 inch) PVC pipe to run to the sprinklers. You are running a large station of sprinklers a long way from the pump (in this example we are running 75m) and the sprinklers are using 100L /min. Now your really expensive new pump is running along at 100L/min and the pressure gauge on the outlet is showing a nice 500kpa, more than enough to be running big garden sprinklers.  But for some strange reason, the sprinklers on your station are just not working properly, the water is barely dribbling out and when you test the pressure at them its only reading 150kpa. What is going on? This is friction loss, as the pipe is a bit too small to pump 100L/min over a distance like 75m, most of the pressure (about 35m or 345kpa) is consumed by the pipe walls.

friction-loss

There is an easy fix for this though (except if you have already buries the pipe) and that is to replace the pipe with a larger diameter. If you swapped your 25mm pipe for a still reasonably cheap 40mm PVC pipe, then your friction loss would drop to only 3m and magically your sprinklers will start to work properly.

One of the other big places in a pump system where we see problems occur a fair bit is in the suction pipe prior to the pump. Unlike in the pressure side of the system, this side of a pump can be very sensitive to the size of the pipe, the meters of head from the source to the pump and any valves or fittings that are attached in between them. Its quite common for customers to come in with their pump either losing performance or losing prime constantly and all we do is replace the foot valve and the suction pipe and the pump starts working like new again. Like when you try and drink a milkshake through a straw it involves a lot of effort to get a mouthful to drink. Well a pump has the same problem and if the size (or length) of the suction pipe is too small or there is a restriction (like a stuck check valve) then the pump has to work twice as hard and therefore it won’t deliver the same flow rate or pressure it was designed to.

We have a number of people on staff who are very experienced in the sizing and selection of pipework and they are more than happy to discuss with you how to make your pump system more efficient and help solve any problems that you are experiencing. Give us a call today on (08) 9721 3577 or sales@southernswater.com.au.

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