A diaphragm pressure tank holds a cushion of compressed air behind a flexible diaphragm. When the pump runs, water pushes into the tank and compresses the air. When a tap opens, that air pushes water back out, so small draws come from the tank instead of starting the pump. A correctly sized and charged tank means fewer pump starts, steadier pressure and less water hammer.
The tanks in the Pressure Tanks range are the Hyjet HPT vertical diaphragm tanks.
The Hyjet HPT
Per the listing, the HPT has a potable grade butyl diaphragm and a polypropylene liner, so water doesn't touch the steel shell. It has a stainless steel connection, a brass stem air valve, an epoxy primer and two-part polyurethane finish, a 10 bar (150 psi) maximum pressure and a 90°C maximum temperature. It's listed for stabilising pump booster systems, reducing pump cycling and water hammer, and absorbing thermal expansion.
| Size | Price | Size | Price |
|---|---|---|---|
| 2L (HPT02) | $72.55 | 60L (HPT60) | $276.05 |
| 8L (HPT08) | $111.05 | 80L (HPT80) | $463.05 |
| 18L (HPT18) | $149.55 | 100L (HPT100) | $529.05 |
| 24L (HPT24) | $171.55 | 150L (HPT150) | $793.05 |
Why the size matters
Only part of a tank's volume is usable water. The rest is air. The usable amount, called the drawdown, depends on the tank size and the gap between the pump's cut-in and cut-out pressures. A tank that's too small for the pump gives very little drawdown, so the pump starts and stops constantly. Every start puts wear on the motor and the switch, and most pump makers set a maximum number of starts per hour.
As a general guide:
- Pressure switch systems need the biggest tanks, because the tank is what stops the pump cycling. Larger pumps need larger tanks. Check the pump maker's maximum starts per hour and size the tank's drawdown to keep below it.
- Electronic pressure controller systems usually need only a small tank, mainly to cushion pressure spikes and cover tiny draws.
- VSD systems also use small tanks, because the drive slows the pump instead of switching it off and on. Hyjet's own VSD pumps come with 3L or 8L tanks, and its booster sets with 8L or 18L.
Setting the pre-charge
The pre-charge is the air pressure in the tank when it's empty of water. It has to be set to suit the system, and checked with the pump off and the water side drained. The HPT listing gives two settings:
- Pressure switch systems: set the pre-charge 20 kPa below the cut-in pressure.
- VSD or electronic controller systems: pre-charge to 65% of maximum system pressure.
Too low and the tank holds less usable water. Too high and the diaphragm may not fill at all, so the pump cycles as if the tank wasn't there.
Looking after it
- Check the pre-charge once or twice a year with a tyre gauge on the air valve, with the pump off and the water pressure released.
- Sign of a failed diaphragm: water comes out of the air valve, or the pump starts on every small draw even with correct pre-charge.
- Install an isolating valve so the tank can be checked or replaced without draining the system.
For the switch that goes with the tank, see our guide to pressure controllers vs pressure switches. The HPT range is available to order on backorder; contact PSA to confirm lead time.
Frequently Asked Questions
What should I set my pressure tank pre-charge to?
For a pressure switch system, 20 kPa below the cut-in pressure. For a VSD or electronic controller system, 65% of maximum system pressure, per the Hyjet HPT listing.
Why does my pump keep switching on and off?
Common causes are a pressure tank that's too small, a wrong or lost pre-charge, a failed diaphragm, or a leak somewhere in the system.
Does a VSD pump need a pressure tank?
Usually a small one, to cushion pressure and cover small draws. Hyjet supplies its VSD pumps with 3L or 8L tanks.
Does water touch the steel inside a Hyjet HPT tank?
No. The water sits inside a butyl diaphragm with a polypropylene liner, so it doesn't contact the steel shell.