Appendix 5: Instructions for isolating a pressure tank
Many groundwater delivery systems utilise pressure tanks. These tanks are designed to:
- maintain water pressure within the delivery system
- provide a minimum storage of useable water to prevent frequent recycling of the pump.
Tanks work by containing a volume of air that is compressed by water in the system thereby maintaining pressure within the system. Draw-off causes water to be released from the tank which allows the air to expand and the pressure falls. Reduction of pressure below a certain point will cause the pump to start which will refill the tank and deliver water to the rest of the system.
There are two types of pressure tanks:
- plain steel - in these tanks there is a water-air contact. Automatic air volume valves are required to maintain the air volume otherwise the air may dissolve into the water. This causes the air volume of the tank and, consequently, its effectiveness to decrease
- diaphragm - in these tanks there is no water-air contact; the two are separated by a rubber diaphragm. These tanks may be identified by the air valve at the top of the tank.
The figures below show a diaphragm tank with its sides cut away. In the first picture the rubber bladder can be seen flush against the tank inlet. In the second picture the bladder has been lifted away from the inlet as it would be when filled with water.
Tanks pose a problem for groundwater sampling because their storage function means additional flushing is required to ensure a sample collected after the tank has been drained does not contain any water which may have been stored for an unknown period of time. Consequently, the collection of a groundwater sample from a system with a pressure tank should be done only if the tank can be isolated from the delivery system. There are a number of ways of achieving this situation:
- sample from a point upstream of the pressure tank
- if the tank is sufficiently small, drain the tank, flush the well, re-drain the tank, re-start the pump and sample
- drain the tank and add air pressure to push the diaphragm against the inlet/outlet point thereby isolating the tank space from the delivery system. Flush the well and sample and then return tank to its operating air pressure.
The following procedure is used by the USGS' Denver office to isolate pressure tanks (J Bails personal communication):
- Note the water pressure in-line (there should be a gauge right next to the tank) and in the tank. Pressure tanks have a valve on top, usually under a plastic screw cap.
- Run the water in the house and note the pressures (from the in-line gauge) where the pump kicks in and when it shuts off.
- Double check the pressure in the tank, right after the pump runs. The pressure is usually a bit higher than when the initial check is done.
- Turn off power to the pump. There is usually a fuse box with a big switch on it near the pump, or it may be controlled through the main breaker box.
- Open up a few taps and release the water pressure from the plumbing.
- With the plumbing drained, check the pressure in the tank again. This is the pressure you need to return the tank to when you are finished, so write the value down.
- With the taps still open, pressurise the tank with the air compressor through the valve on top; 80 psi should be sufficient. Make sure it is more than the maximum pressure you recorded in step 3.
- Close the inside taps used to drain the system.
- Make sure that you have at least two outside taps that you can turn on with hoses on them. Running inside taps while sampling may overload the septic system if one is fitted, so do not use them. If needed, you can use a splitter on the taps you plan to sample from.
- Make sure you are ready to purge: meters calibrated, outside taps open, etc.
- Turn on the power to the pump.
- Make sure that the pump is not cycling on and off; you should be able to see this in the in-line pressure gauge. It will swing wildly, and you will hear the pressure switch for the pump going on and off. If the pump is cycling on and off, you do not have enough taps open. You will know when things are okay when the in-line pressure gauge holds steady at a pressure less than the shut-off pressure noted in step 2. (If there is a tap in the plumbing near the pressure tank, and you can attach a hose and run it to waste outside, this almost guarantees the pump will not cycle on and off. Check this a few times during the sampling.)
- When you are finished with the sampling turn off the power to the pump again. Leave taps open.
- Release the pressure on the tank until you are back to the pressure you recorded in step 6.
- Close all taps.
- Turn on the power to the pump, open and close a few taps to remove the air from the system.
- Note the in-line pressure as the pump pressurises the plumbing and turns itself off - it should be what you noted before.
- Check the pressure in the tank. It should be what you noted before.
- Turn on some taps and watch the pressure gauge as the pump goes through a cycle. Make sure that the pump is turning on and shutting off at the same levels noted before. This setting is controlled by the switch and should not be affected at all by what you have done. This is just a check to make sure, and it usually is right-on.
- Replace the plastic cap back on the valve and you are finished.
Equipment required to remove the pressure tank from the sample pathway:
- air compressor capable of delivering at least 90 psi
- air compressor hose with schraeder valve fitting (car tyre valve)
- low-range air pressure gauge (0-50 psi)
- high-range air pressure gauge (0-100 psi).
