Pressure switch
A pressure switch is a pressure-sensing device that starts and stops a pump to hold a system between two set points. It closes and starts the pump when the pressure falls to the cut-in setting, and it opens and stops the pump when the pressure rises to the cut-out setting.
In more detail
A pressure switch senses pressure. A float switch senses level. That is the whole difference between the two, and it decides which one belongs on a duty: pressure control on a booster set or a borehole system, level control on a wet well, sump or tank. A pressure switch knows nothing about how much water is left in the tank it draws from, which is why a pressure system that can run out of water needs a second device watching the suction side.
Cut-in, cut-out and the differential between them
A pressure switch is set by three related numbers, and only two of them are set independently. Cut-in is the falling pressure at which the switch makes and the pump starts. Cut-out is the rising pressure at which it breaks and the pump stops. The differential, sometimes called the deadband, is the gap between them.
| Setting | What it does |
|---|---|
| Cut-in | Starts the pump as the pressure falls to this point |
| Cut-out | Stops the pump as the pressure rises to this point |
| Differential | The gap between the two, and what decides how often the pump runs |
The differential is the trade. Narrow it and the system holds a tighter pressure, but the pump starts more often and runs for shorter periods. Widen it and the pump runs longer and less often, but the pressure at the outlet swings more between starts. Neither is right in the abstract. It depends on what the system serves and what the pump can take.
There is one fault worth knowing before you set anything. The cut-out has to sit below the pressure the pump can actually generate at the flow the system is drawing. Set it above, and the pump reaches its shut-off head, never gets to cut-out, and runs on. That is a sizing problem presenting as a control problem, and it is exactly the pattern where the pump gets blamed for the system it sits in.
Without a pressure vessel, the switch chatters
A pressure switch on its own has nothing to smooth the system, so a diaphragm pressure vessel goes with it. The vessel holds a small cushion of water under pressure. When someone opens a single tap, the vessel supplies that draw without starting the pump at all.
Take the vessel away and the pressure collapses the moment a tap opens and recovers the moment it closes. The switch then makes and breaks continuously, which is the chattering people hear from the plant room and take for a faulty switch. The switch is usually fine. The system has nothing holding the pressure between starts.
Rapid cycling is what wears the pump out. Every start is a current surge through the contactor and a thermal load on the motor, and manufacturers cap it. Calpeda limits starts per hour on the NM by motor rating: 60 up to 2.2 kW, 40 up to 7.5 kW and 20 up to 30 kW. A pressure switch cycling a pump harder than its nameplate allows will cost you the seal, and the seal is the weak point in almost any pump.
A vessel that has lost its air charge is the same fault in a different guise. A waterlogged vessel has no cushion left, so the set behaves as though there is no vessel at all. Check the precharge against the cut-in setting before you go anywhere near the switch adjustment.
A variable-speed set replaces the switch with a transducer
A pressure switch is a two-state device. It knows above cut-out and below cut-in, and nothing in between, so the pressure in the system is always moving between the two settings.
A modern booster set does not switch at all. A pressure transducer reads the discharge pressure continuously and feeds a controller, which ramps the pump speed to hold one setpoint rather than starting and stopping against two. The pressure at the outlet stays where it was specified, and the pump starts far less often. How a cold water booster set holds pressure covers that behaviour and the staging that goes with it.
The same idea has reached single pumps. Calpeda's MÈTA e-idos units carry an integrated pressure transducer and a non-return valve on the pump itself, so the pump starts and stops as outlets open and close with no separate pressure switch or panel.
The trade is cost and complexity. A switch and a vessel are cheap, they are understood by everybody who will ever touch the installation, and a replacement switch is an off-the-shelf part. A transducer and a drive give you constant pressure and gentler starts, and they give you electronics in a wet plant room. On a commercial building where the pressure at the index outlet has to hold, specify the transducer. On a single pump filling a tank or supplying one dwelling, a pressure switch and a properly sized vessel is still the right answer.
The low-pressure cut-out exists because a mechanical seal needs liquid
A low-pressure or dry-running cut-out is a second pressure setting, below cut-in, that stops the pump when there is nothing on the suction side to draw. It is not a refinement. It is what stands between a shut valve and a new mechanical seal.
The mechanism is simple enough. A mechanical seal is lubricated by the liquid it runs in. Take the liquid away and the film between the faces goes with it, and the faces follow. On a centrifugal pump that is the usual result of dry running, and it does not take long.
The trigger we see most often carries no design fault at all. Somebody closes an isolating valve during work on the system and forgets it is shut, and the pump then runs against a closed suction. The cut-out is what stops that costing you a pump. It only does that job if it was commissioned properly and left enabled, which is worth confirming rather than assuming.
Note what the cut-out cannot do. It reads pressure, so it protects against a loss of supply, not against a tank running down to a level the pump should not be drawing from. That is a job for a level device. A float switch or a well probe on the suction tank does the job a pressure switch cannot.
What to check before you adjust the switch
The switch is rarely the part at fault. Work through the system first, because adjusting the setting to mask a system fault buys you a week and costs you the pump.
Start with the vessel precharge, since a waterlogged vessel produces every symptom people attribute to a worn switch. Then check the suction: a partly shut valve or a blocked strainer starves the pump and shows up as a set that cannot reach cut-out. Then check whether the duty has changed, because a system asked for more flow than it was sized for will not make cut-out either.
Only then look at the settings themselves, and set the cut-out against what the pump can deliver at the flow the system draws rather than against the pressure you would like. If the pump cannot make the pressure the building needs, the switch is not the part to change. That is a sizing question, and it is worth answering properly before anybody quotes you a replacement.
Frequently asked
What is a pressure switch on a pump?
A pressure switch is a pressure-sensing device that starts and stops a pump to hold a system between two set points. It starts the pump when the pressure falls to the cut-in setting and stops it when the pressure rises to the cut-out setting.
How does a pressure switch work?
System pressure acts on a diaphragm or bellows working against a spring. As the pressure falls to the cut-in setting the spring wins and the contacts close, starting the pump. As the pressure rises to the cut-out setting the diaphragm wins and the contacts open, stopping it.
What is the difference between a pressure switch and a float switch?
A pressure switch senses pressure and a float switch senses level. Use a pressure switch for pressure control on a booster set or borehole system, and a float switch for level control on a wet well, sump or tank. A pressure switch cannot tell you how much water is left.
What is cut-in and cut-out pressure?
Cut-in is the falling pressure at which the switch starts the pump. Cut-out is the rising pressure at which it stops the pump. The gap between them is the differential, and it decides how far the pressure swings and how often the pump runs.
Why does my pressure switch keep clicking on and off?
Almost always the pressure vessel, not the switch. A vessel that has lost its air charge leaves no cushion, so pressure collapses the instant a tap opens and recovers the instant it closes, and the switch follows it. Check the precharge before adjusting anything.
Do you need a pressure vessel with a pressure switch?
Yes. Without one, every small draw starts the pump, and the cycling wears out the motor and contactor. Calpeda caps starts per hour on the NM at 60 up to 2.2 kW, 40 up to 7.5 kW and 20 up to 30 kW, and a switch with no vessel behind it can pass that inside an hour of normal use.