Pressure switches, the unsung hero of a pump system

Pressure switches are generally an essential part of your water pump system. So let's kick things off with a crash course on what is a pressure switch,  the unsung heroes of electric water pump systems. If you're new to pumps, don't worry – I'll break it down for you in simple terms.

What is a pressure switch

To start with, lets give a basic idea of how a pump works. Most pumps are centrifugal pumps, there are other types like rotor or piston type but most are centrifugal. A centrifugal pump is made up of 3 main parts:

• The impellor- this is a disc shaped component that is spun by the motor. The water flow and pressure are created by the impellor spinning. The impellor is partially hollow with paths built into the inside of it so that water is drawn into the eye (the front centre part) and slung outwards through the middle (the vanes) and exits through the outer edge.
• The housing- this is a metal or plastic case which contains the water and allows the impellor to work. It does this by sealing the inlet side of the impellor from the outlet side and directing water from the inlet/suction pipe into the eye and then out of the outer edge and directing this pressurised, flowing water to an outlet pipe.
• The motor- this is the device, attached to the pump drive shaft that causes the pump impellor to spin and creates the flow and pressure. A motor is generally electric but can also be a petrol or diesel piston engine. This depends on your location and the use of the pump.

A centrifugal pump is designed to have a certain size motor and the impellor has a fixed size and shape and generally rotation speed. This means that when the pump is turned on, the pump will flow at a certain flow rate and at the same time produce a certain pressure. For example at Davey Hp45-05T house pump is designed to produce 50L/min and 250kpa of pressure. If the flow is restricted by a valve, then it will produce 20L/min at 320kpa and if you shut the water off completely it will “dead head” at about 330kpa.

Now this Davey pump has a very common use in Australia as a pressure pump to provide a house its water pressure from a rainwater tank when there is no mains water. In a house, there is usually very few times in a day when a full 50L/min is required, unless the washing machine is running along with a shower and maybe the kitchen sink. But this is only for a short period. Also unless the pump is a long way down from the house, you may also not want to have 300kpa or more of pressure when the pump is only making 10 or 20L/min of a tap.

So to get around this problem, we use a pressure switch. This is an automatic switch that cuts the power off to the pump motor when the pressure is too high and then starts the motor again when the pressure drops. It also works to shut off the pump when you don’t need water (when the taps are shut) and then start the pump automatically when someone opens a tap.  We have a number of pressure switches available in our webshop. They are all high quality and reliable from the common Square D to the more robust commmercial grade Telemecanique XMP we have options available to suit your pump. 

So how does a pressure switch work? 

So we’ve learned what is a pressure switch, so now how do they work? The motor on your pump, which most of the time is an electric motor and for most domestic applications is single phase (we will go into what a single or three phase motor is in another blog). As most motors are quite small, generally under 3kw (5hp)of power and draw under 15A of current, then they are plugged into a power socket with a standard 3 pin plug and have a insulated cable to run the electricity to the motor.

Now for this blog we are talking about pressure pumps, these are pumps which start and stop automatically with the help of a pressure switch (or sometimes a pressure controller or variable speed drive which we will discuss in other blogs). All pumps produce pressure, some low pressure (under 100kpa) and some very high pressure (over 1000kpa) but a pressure pump is the only type that controls the output pressure automatically.

It does this by turning off the motor when the pump produces enough pressure so the pump stops spinning and the pressure can drop. If the pressure drops to a certain set point the motor can then start again and start to build pressure again.

The pressure switch itself has to be attached to the pipework on the outlet side of the pump, there is a hole in the bottom of the switch to let the water pressure get into a small chamber in the switch. This chamber is made water tight with a rubber diaphragm which also stretches and contracts with the rise and fall of pipe pressure. See the pressure switch is not measuring the pump output, it is measuring the pressure in the pipes just after the pump, sometimes these aren’t the same.

This little rubber diaphragm pushes against a lever or a piston (depending on the type) and this will move and open or shut a set of switch points that turn on or off the electricity to the motor. There is a set of springs and some adjusting nuts which provide tension against the diaphragm and also adjust the place where both the switch turns on and off. For a normal pressure switch we have the two setting points, the high pressure where the pump switches off (this is called the cut out pressure) and the low pressure where the pump switches on (this is called the cut in pressure). The setting points can be adjusted on most switches to better suit your conditions and normally both can be adjusted separately so the pressure differential (the range of pressure in the middle when the pump is running) can be made smaller or larger which is very important sometimes to make a pump work properly.  

Why is a pressure switch important?

So now you have all the info on what a pressure switch is and what it does, you probably want to know also why it is so important. Well pressure switches are important for a couple of reasons.

• They automate the pump function. This means that the pump will turn on and off by itself without you having to go out to the pump shed and turn on the switch whenever you want water running. This also means that the pump can be a distance you’re your house, if the tank or well is not adjacent and can still provide a constant pressure on demand water supply.
• Pumps also don’t like being dead headed. From earlier you learned that dead head is when the outlet tap is shut off and there is not water flowing plus the pump can produce the maximum pressure it is capable of. For a short period (30 seconds to a minute) this is OK generally but if it stays like this for longer the friction of the water inside the moving pump parts causes the water to heat up. If left for a long period of time this water can get near boiling point. In some pumps with mostly metal components or rated for hot water this isn’t a problem, but in most domestic pumps the housings and impellors are made from various plastics and they get soft and can melt. The hot water can also melt the suction or outlet pipe if it is made from PVC or Poly. This can cause bad damage to the pump and if a pipe bursts in your pump shed, then this can cause a flood and damage electrical components too. Very expensive.
• The pressure switch can sometimes assist in stopping your pump running dry. Running dry is when the suction doesn’t have enough water entering it either the tank is empty or the suction pipe has a problem. This causes the pump to blow out all the water inside it and then keep spinning with no water. The water flowing through a pump helps the pump components to spin and also keeps the housing cool. If there is no water then parts like the mechanical seal and the impellor if it is rubbing slightly can get hot and the plastics and rubber start to melt. At the very worst the impellor melts and fuses to the housing and then the motor seizes up. If this is left too long and a circuit breaker doesn’t trip, then the motor windings can melt from too much current and the entire pump and motor are damaged beyond repair. We sell the Square D Loss of Prime pressure switch or the Presscontrol pressure controller to stop running dry.
• Most of the time a pressure switch needs more components with it to stop running dry, these could be a paddle flow switch (which turns the pump off if there is no flow), a “run dry” mechanism in the switch which cut the pressure switch if the motor is on but the inlet pressure drops to zero or the pressure switch is swapped for an electronic pressure controller (or press control) which we will discuss in another blog. We sell the Kelco F60 which has both the pressure switch and flow switch function built in for more rebost applications. 
• Saving energy. A pump motor runs on electricity which is good because it is clean, quiet and powerful. An electric motor is also great because if you don’t want it to cost money from running when you don’]t need it, you can just turn it off. When there is an automatic pressure switch built onto the pump, this means that the pump only runs for a short period of time and then stops so it saves power. When a motor starts there is also a large “inrush current” where the motor uses a lot more power than normal to get spinning for 0.5 to 1 second. This can also add up to a lot of cost if the pump is starting and stopping constantly plus it makes the motor hot and adds to wear and tear. So making sure the pressure switch is adjusted properly is very important and using a pressure tank that is adjusted properly so that the pump doesn’t rapidly cycle.

So there you have it – a beginner's guide to pressure switches in electric water pump systems. These little gadgets may seem simple, but they perform a large and important task in keeping your pump running smoothly. We have a wide range of pressure switches and other components available for delivery Australia wide on our Rural Pumps website. If you have any questions about your pressure switch please email us and someone from Rural Pumps will gladly respond to help you out.