Doodlebug Pump -- FAQ

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Pressure Tanks

How do pressure tanks work?

Pressure (Pneumatic) tanks store the energy the pump has expanded in the form of pressure.

It is based on the fact that air can be compressed but water cannot. The pressure to force the water from the tank thru the pipes to the point of service, is obtained by compressing air in the tank. And by the pump forcing water against this air pocket. The air is forced to occupy less and less space, and so exerts more and more pressure on the incoming water.

The water filling the tank compresses the air in a pressure tank. This air cushion acts like an enormous spring maintaining a constant pressure on the water in the tank. which is then transmitted throughout the entire system.

When a faucet is opened the air expands to replace the water that is forced thru the pipes by the air pressure.

When the pump starts and forces additional water into the tank, the air is compressed at a higher pressure and occupies less space.

Pressure (pneumatic) tanks are divided into three general classifications. Galvanized or Steel, Bladder and Diaphragm. The oldest type of tank is the galvanized or steel pressure tank. They can vary in size from 20 gallons to several hundred gallons.

For purposes of this discussion, we will limit our discussion on galvanized tanks to the three most popular sizes: 82, 120 and 220 gallons.

None of these tanks have any pre-charged air. The tank is empty when the pump first starts and slowly builds up air pressure as the tank fills.

In the case of a pump with a 20/40 pressure switch, a little over 50% of the tank is filled with water before the pressure reaches 20 PSI. By the time 40 PSI is reached the tank is over 2/3 full of water. The water available is that amount between 2/3 (40 PSI shut-off) and 1/2 (20 PSI pump start pressure). In the case of a 220-gallon tank this represents approximately 42 gallons (19%) If we increase the switch setting to 30/50 PSI, the available water drops to 33 gallons (15%). At a 40/60 PSI switch setting the available water is only 26 gallons (12%)

Keep in mind an 82-gallon galvanized tank only has 8 gallons of available water assuming a 20/40 pressure switch setting.

Besides its physical size, the major down side to this type of tank is that it will "water log" as the pump continues to cycle on and off. The air in the tank is gradually absorbed into the water. This requires the user to periodiclly drain down the tank or inject air in order for it to function properly. If this is not done, the pump will begin to "short-cycle", causing excessive electric power use and premature motor failure.

From a pricing point of view, an 82- gallon galvanized tank is slightly cheaper than a similar size captive air tank, however, the 120 and 220 galvanized tanks are more expensive. This is particularly important when freight costs are taken into consideration. A 220 gallon captive air tank of the same pressure water storage capacity as a 220 gallon galivanized tank, but is only physically the same size as a 82 gallon galvanized tank

Modern tanks are referred to as "captive air". This means they need no maintenance, they are pre-charged at the factory with air pressure.

This allows a captive air tank to be physially much smaller than a galvanized tank.

For Example:With a 220 gallon galvanized tank, over half of the tank capacity is unusable. Remember from our earlier discussion, when the tank is empty and the pump starts to fill, approximately 50% will fill with water until a 20-PSI reading is reached (assuming a 20/40 PSI pressure switch setting). This is unusable water! By the time the pump reaches 40 PSI, over 2/3 of the tank is filled with water. In a Bladder or Diaphragm tank air is compressed in the top chamber and it is separated from the water by a diaphragm or rubber bladder. When you turn on the tap the pressure is releases in the form of water flow. After about 30 or so gallons of flow a device called a pressure switch turns the pump on to rebuild the pressure. As soon as the system builds 50 psi the pump turns off. Again the pressure is stored in the tank until it is needed once more.

The important thing to note is all of the water in the captive air tank is available. Because of the air pre-charge the equivalent tank can be 2/3 smaller in physical size and still do the same job!

For pump sizing purposes a 1/2HP pump requires a 82-gallon tank. A 3/4HP pump requires a 120-gallon tank, and a 1HP pump requires a 220-gallon tank. Using the above sizing will allow a minimum one minute run time for the above pump.

If you size your tank smaller than recommended, the pump will short cycle. The result? It will use more electric power and shorten the pump life.

Diaphragm Tanks have proven their superiority to bladder tanks for the following reasons; bladder tanks use a balloon with a large surface and with a large surface it's more vulnerable to tears also the bladder rubs on the inside of the tank walls causing wear on the bladder not to mention the moister that gathers inside tank walls and causing rust.

Diaphragm Tanks have a sealed diaphragm in the center of the tank and they are made so as to move in the center of the tank only. Diaphragm Tanks have a standard 5-year warranty from the manufacturer and (more importantly a very low failure rate.

We don't like to replace tanks, therefore we only sell Diaphragm Tanks that we feel will last. It's cheaper in the long run for both you and us!

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From: paul@tonernet.com... I have a bladder holding tank. Last week it started to appear to "run out of water" and stop running while I was watering some plants. Thinking the pressure had dropped I added pressure and since the gauge was incorrect I increased the pressure to approx. 25/45. This gave me great pressure but still, even in the shower, the water will stop and waits about 30-45 seconds for the pump to kick back on. The water stays clear so I don't beleive its the well. Any ideas? Paul

This problem is not a result of a failure (or incorrect pressure setting) in in bladder tank. It is related to the pump/well delivery. Review the FAQ section on our web site as to how to set the precharge in the tank with respect to the pressure switch setting. Remember with the system depressurised (pump not on and no pressure against the tank) the precharge should be 2 psi below the low setting on the pressure switch. Example: if your pressure switch setting is 30/50, then the precharge should be 28 psi. The cause of the "pressure drop" is related to the pump or water level in the well. Normally this condition occurs when the water level in the well is too low for the pump to remain submerged. This assumes the electrical side of the pump is supplying sufficient power & the motor is functioning properly.

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		Doodlebug Pump -- FAQ Welcome to the FAQ page. This is where you will find the answers to frequently asked questions. We are constantly adding to this section, it covers submersible pumps, jet pumps, jet pump trouble shooting, pressure tanks, pressure tank sizing,level control, Reverse Osmosis Filter Systems, Questions About UV Sterilizers & Water Softeners If you have a question - please include your e-mail address. We will not answer unknown requests. here Pressure Tanks General Questions About Pressure Tanks How do pressure tanks work? From: paul@tonernet.com... I have a bladder holding tank. Last week it started to appear to "run out of water" and stop running while I was watering some plants. Thinking the pressure had dropped I added pressure and since the gauge was incorrect I increased the pressure to approx. 25/45. This gave me great pressure but still, even in the shower, the water will stop and waits about 30-45 seconds for the pump to kick back on. The water stays clear so I don't beleive its the well. Any ideas? Paul Pressure Tanks How do pressure tanks work? Pressure (Pneumatic) tanks store the energy the pump has expanded in the form of pressure. It is based on the fact that air can be compressed but water cannot. The pressure to force the water from the tank thru the pipes to the point of service, is obtained by compressing air in the tank. And by the pump forcing water against this air pocket. The air is forced to occupy less and less space, and so exerts more and more pressure on the incoming water. The water filling the tank compresses the air in a pressure tank. This air cushion acts like an enormous spring maintaining a constant pressure on the water in the tank. which is then transmitted throughout the entire system. When a faucet is opened the air expands to replace the water that is forced thru the pipes by the air pressure. When the pump starts and forces additional water into the tank, the air is compressed at a higher pressure and occupies less space. Pressure (pneumatic) tanks are divided into three general classifications. Galvanized or Steel, Bladder and Diaphragm. The oldest type of tank is the galvanized or steel pressure tank. They can vary in size from 20 gallons to several hundred gallons. For purposes of this discussion, we will limit our discussion on galvanized tanks to the three most popular sizes: 82, 120 and 220 gallons. None of these tanks have any pre-charged air. The tank is empty when the pump first starts and slowly builds up air pressure as the tank fills. In the case of a pump with a 20/40 pressure switch, a little over 50% of the tank is filled with water before the pressure reaches 20 PSI. By the time 40 PSI is reached the tank is over 2/3 full of water. The water available is that amount between 2/3 (40 PSI shut-off) and 1/2 (20 PSI pump start pressure). In the case of a 220-gallon tank this represents approximately 42 gallons (19%) If we increase the switch setting to 30/50 PSI, the available water drops to 33 gallons (15%). At a 40/60 PSI switch setting the available water is only 26 gallons (12%) Keep in mind an 82-gallon galvanized tank only has 8 gallons of available water assuming a 20/40 pressure switch setting. Besides its physical size, the major down side to this type of tank is that it will "water log" as the pump continues to cycle on and off. The air in the tank is gradually absorbed into the water. This requires the user to periodiclly drain down the tank or inject air in order for it to function properly. If this is not done, the pump will begin to "short-cycle", causing excessive electric power use and premature motor failure. From a pricing point of view, an 82- gallon galvanized tank is slightly cheaper than a similar size captive air tank, however, the 120 and 220 galvanized tanks are more expensive. This is particularly important when freight costs are taken into consideration. A 220 gallon captive air tank of the same pressure water storage capacity as a 220 gallon galivanized tank, but is only physically the same size as a 82 gallon galvanized tank Modern tanks are referred to as "captive air". This means they need no maintenance, they are pre-charged at the factory with air pressure. This allows a captive air tank to be physially much smaller than a galvanized tank. For Example:With a 220 gallon galvanized tank, over half of the tank capacity is unusable. Remember from our earlier discussion, when the tank is empty and the pump starts to fill, approximately 50% will fill with water until a 20-PSI reading is reached (assuming a 20/40 PSI pressure switch setting). This is unusable water! By the time the pump reaches 40 PSI, over 2/3 of the tank is filled with water. In a Bladder or Diaphragm tank air is compressed in the top chamber and it is separated from the water by a diaphragm or rubber bladder. When you turn on the tap the pressure is releases in the form of water flow. After about 30 or so gallons of flow a device called a pressure switch turns the pump on to rebuild the pressure. As soon as the system builds 50 psi the pump turns off. Again the pressure is stored in the tank until it is needed once more. The important thing to note is all of the water in the captive air tank is available. Because of the air pre-charge the equivalent tank can be 2/3 smaller in physical size and still do the same job! For pump sizing purposes a 1/2HP pump requires a 82-gallon tank. A 3/4HP pump requires a 120-gallon tank, and a 1HP pump requires a 220-gallon tank. Using the above sizing will allow a minimum one minute run time for the above pump. If you size your tank smaller than recommended, the pump will short cycle. The result? It will use more electric power and shorten the pump life. Diaphragm Tanks have proven their superiority to bladder tanks for the following reasons; bladder tanks use a balloon with a large surface and with a large surface it's more vulnerable to tears also the bladder rubs on the inside of the tank walls causing wear on the bladder not to mention the moister that gathers inside tank walls and causing rust. Diaphragm Tanks have a sealed diaphragm in the center of the tank and they are made so as to move in the center of the tank only. Diaphragm Tanks have a standard 5-year warranty from the manufacturer and (more importantly a *very low* failure rate. We don't like to replace tanks, therefore we only sell Diaphragm Tanks that we feel will last. It's cheaper in the long run for both you and us! Return to top From: paul@tonernet.com... I have a bladder holding tank. Last week it started to appear to "run out of water" and stop running while I was watering some plants. Thinking the pressure had dropped I added pressure and since the gauge was incorrect I increased the pressure to approx. 25/45. This gave me great pressure but still, even in the shower, the water will stop and waits about 30-45 seconds for the pump to kick back on. The water stays clear so I don't beleive its the well. Any ideas? Paul This problem is not a result of a failure (or incorrect pressure setting) in in bladder tank. It is related to the pump/well delivery. Review the FAQ section on our web site as to how to set the precharge in the tank with respect to the pressure switch setting. Remember with the system depressurised (pump not on and no pressure against the tank) the precharge should be 2 psi below the low setting on the pressure switch. Example: if your pressure switch setting is 30/50, then the precharge should be 28 psi. The cause of the "pressure drop" is related to the pump or water level in the well. Normally this condition occurs when the water level in the well is too low for the pump to remain submerged. This assumes the electrical side of the pump is supplying sufficient power & the motor is functioning properly. Return to top

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