If you've got a booster set, there's a fair chance you've got a pressure vessel. And if you haven't got a pressure vessel, you probably should. But a lot of people don't fully understand their function, and they're often overlooked for maintenance.
In case you haven't come across them, they're the spaceship-like canisters that are usually either floor mounted or located directly on top of the pump discharge pipework. A straightforward device which is pretty much just a single inlet/outlet connection, a rubber diaphragm, some air, and a pressure release valve.
The diaphragm is fixed halfway along the vessel (you can see the join in the above pictures) and seals the two halves off from each other. The top half is then filled with air and pressurised to the chosen pressure. When the pressure vessel has been installed on the system, the half next to the inlet/outlet will fill with the pumped liquid (usually water). Now it's ready to go.
So, what does a booster set pressure vessel do? In one word: it's a buffer. A booster set is designed to supply a variable volume of water at a constant pressure. For example in a large office building there may be anything from a single tap running, needing a tiny quantity of water, to a commercial kitchen running at full speed, needing lots of water. In either case they need reasonable water pressure. The way most modern booster sets achieve this is by having a pressure sensor on the pipework, so as soon as the demand starts (eg. the tap is turned on) and the water pressure in the system drops, the pump/s ramp up until the pressure is maintained. When the demand stops (eg. the tap is turned off again) and the water pressure starts to climb, the pump/s ramp down and stop. The problem with this is that there can be lag after turning the tap on, waiting for the pump to start. There can also be a surge in pressure when the tap is closed, potentially bursting a joint. The pressure vessel provides instant pressure when the tap is opened, and takes up the slack again when the tap is closed.
If that description is a bit long-winded, think of it this way - imagine taking a dog for a walk with a pole instead of a lead. When you start it jerks the dog forward, and when you stop the dog gets another jolt. Not too comfortable for you or the dog. The pressure vessel is the equivalent of a lead with slack.
When specifying a pressure vessel for a booster set, there's a few things to take into consideration. The most important ones are:
Temperature of liquid - if it's too hot then some diaphragms will perish and split
Type of liquid - if it's potable water, you'll need a WRAS approved pressure vessel
Size of vessel - these are designed according to the specification of the pump system
Pre-charge pressure - the air charge pressure is usually set to 10% below the pump cut in pressure
On average, pressure vessels last about 5-7 years. Eventually they will fail, and the first part to go is usually the diaphragm. When this happens the top half fills with water as well as the bottom half, so effectively there is no pressurised air to act as a buffer. While the system will keep working, it will act as if there is no pressure vessel on there. It is also potentially dangerous, as being a 'dead leg' where legionella can occur.
Checking the pressure vessel is a standard part of our booster set servicing. It's a simple job to do, but it's really important for all of the reasons talked about above. Other than checking it's still working, there's not a lot to worry about with maintenance of pressure vessels. Just remember that it will be used probably many more times per day than you will walk your dog, so don't overlook it!