Maintenance Tips
Pressure Tank
Captive Air Type
Quick Procedure
1. Determine pump start pressure by observing at what pressure the pump starts as pressure drops slowly on the HIGH SIDE pressure gauge.
2. Remove all SYSTEM pressure from the pressure tank.
3. Check tank pre-charge pressure at the schraeder valve with pressure gauge.
4. Air pre-charge pressure should be set to 2 psi below the pump start pressure.
5. Add or remove air as required through the schraeder valve.
6. Learn more about pressure tanks by reading further.
Explanation
1. The pressure tank is an energy storage device. The tank stores energy in the form of compressed air. This stored energy is needed for the following reasons:
a. The compressed air pushes water into the system to supply small flows and thereby prevents the pumps from having to start to feed these small needs. Low flow rates result from small system leaks, faucets or hose bibs operating, small drip zones operating, etc.
b. If a piping system is constructed of metal pipes such as galvanized steel or copper, the pipe system itself will not store any energy, and the water itself cannot store energy because water is non-compressible. Therefore, with no pressure tank, with metal pipes, and with non-compressible water, system pressure would immediately drop to zero when the pump was turned off because no energy is stored anywhere in the system.
c. PVC, CPVC, Polypropylene, HDPE, and all other plastic pipe systems will flex or stretch in response to pressure. Therefore these systems store energy in the pipe system. Long large plastic pipe systems may in fact store sufficient energy so as to dwarf the storage capacity of the pressure tank.
2. The type of pressure tank most often used on irrigation systems is called a Captive Air Tank. This means that there is either a plastic bag inside the pressure tank containing the water, or there is a diaphragm separating the water from the air.
3. BLADDER or BAG TANKS - In most bladder tanks the water is inside the bag, and the air surrounds the bag, the air is trapped between the bag and the side walls of the tank. This prevents the water from making contact with the tank itself, and on metal tanks there is the obvious advantage of no corrosion from water on metal. These tanks will often allow the owner to replace the bag and thereby save money.
4. DIAPHRAGM or FIXED BLADDER TANKS - This type separates air from the water by means of a diaphragm. Typically diaphragms cannot be replaced so the whole tank must be replaced when the diaphragm fails.
5. There is a valve on the tank (usually near the top) allowing air pre-charge pressure to be checked, and air added when required. This valve is exactly the same type of valve used on automobile tires and is called a Schraeder Valve.
6. Water enters the pressure tank through a pipe connection to the tank, often on the bottom of the tank.
7. With no water pressure on the tank through the water connection, then the air pressure measured at the Schraeder valve is called Pre-Charge pressure. If water pressure is present and this pressure is greater than the pre-charge pressure, then the pressure measured at the schraeder valve will be the same as the system pressure.
Checking and Correcting the Pre-Charge Pressure
1. First, determine the pump starting pressure. Watch a pressure gauge while dropping system pressure VERY SLOWLY. Record the pressure at which the pump starts. This is the Start pressure. Remember to drop the pressure VERY SLOWLY to determine the start pressure because if system pressure is dropped rapidly when taking this reading you will obtain a false reading (you will record a start pressure that is too low).
2. Next, Turn off the pumps and remove all water pressure from the pressure tank feed line.
3. Once the tank is completely drained of water pressure then the only pressure left inside the tank will be AIR PRE-CHARGE PRESSURE.
4. Using an accurate tire pressure gauge, check the air pre-charge pressure at the schraeder valve. Add or remove air as needed through the Schraeder valve. Typically you will always be adding air in maintenance operations.
5. Correct pre-charge pressure is about 2 psi below the start pressure of a system. For example; if a pump is starting at 65 psi, then the air pre-charge pressure should be set to 63 psi.
6. On Bag or Bladder type tanks the pressure tank may make noise just before the pump starts. If the pressure tank makes noises just before the pump starts, then lower the pre-set pressure a few psi at a time until the noise stops.
Principles of Operation
Imagine the system fully pressurized and the pumps off. The pressure tank is pushing small amounts of water into the system to feed small leaks or water flows by means of the air expanding in the pressure tank. This air expansion either crushes the bag or pushes against the bladder forcing the water contained in the tank into the system.
At some point the pressure tank will become “exhausted”, no additional air pressure will be available to push water into the system. When air pre-charge pressure exactly equals system pressure then the air tank is EXHAUSTED.
On Bag type tanks the bag collapses as air pressure forces water out of the bag into the system. The bag wrinkles and forms small pockets of trapped water. As system pressure drops closer to the point of tank exhaustion, the pockets of water trapped in the bag or bladder can be violently expelled into the system and cause severe noises. If noises are heard in the pressure tank just before the pump starts the diagnosis is simple: AIR PRE-CHARGE PRESSURE IS TOO HIGH.
Remember this principle, it is desirable that the air pre-charge pressure be set to the highest possible amount without noise in the pressure tank occurring just before the pump starts. The pump should start just before the pressure tank is exhausted, or before noise occurs.
There are two important reasons why air pre-charge should be as close as possible to the start pressure. First, air pre-charge typically leaks out over time requiring someone to add air to the tank; and by putting in the maximum amount of air we can add air less often. Secondly, the higher the air pre-charge, the more energy the pressure tank can store.
If the pressure tank will not hold air reliably, or if water is expelled from the schraeder valve when the pressure tank is under system pressure, then the bladder has failed and must be replaced. When there is either no air pre-charge at all, or if the tank bladder has failed, the pressure tank is said to be WATER LOGGED.
Symptoms of Low Pre-Charge Air Pressure
or
Symptoms of Failed Bladder (Water Logged Tank)
1. Pump starts more frequently.
2. Pump does not run as long once started.
3. On Irrigation Craft systems a RAPID CYCLE alarm may indicate a pressure tank problem.
Symptoms of High Pre-Charge Pressure
1. Pressure tank makes noise just before the pump starts. There will be no noise on diaphragm or fixed bladder type tanks.
2. If air pre-charge is very much too high then the pumps will start more often and also the pumps will not run as long.
3. On Irrigation Craft systems a RAPID CYCLE alarm may indicate that air pre-charge pressure is far too high.
Notice that the symptoms for excessively high pre-charge pressure and no pre-charge pressure are the same. This is because in both cases (no pre-charge and extremely high pre-charge) no energy is being stored in the system, and therefore the only source of energy is the pump.
