Figuring out the right tank for your water system.
By David Kill, PE (retired)
A pressure tank is added to a water system to maximize the life of a water well pump.
The pressure tank (Figure 1) serves three purposes. It can:
- Prevent rapid cycling of the pump
- Provide water between pump run cycles
- Provide additional water if the system demand exceeds the pump capacity.
Protecting Against Heat
Rapid cycling of the pump using a submersible pump motor is if the starts exceed 300 per a 24-hour period for motors less than 1 HP or as few as 50 per 24 hours for HPs greater that 5. See Table 1.
This is assuming that the starts are uniformly spread over 24 hours and not all occurring in a much shorter period of time.
Heat in the motor is the main cause of shortened motor life. Minimizing the pump starts allows the heat from starting amperage to be properly dissipated. Keeping the pump starts to a minimum also requires pumps have a longer runtime each cycle. The longer runtimes help dissipate the motor’s heat. The runtime is also dependent on the pumping rate of the system pump.
Providing water between run cycles is the function of the pressure change in the water system. System pressure change is typically 20 psi from pump off to pump on time. Many systems today are run at 60 psi pump off and 40 psi for pump on. The pressure tank volume determines the volume of water available in this 20 psi pressure change. This volume of water available is called drawdown capacity.
Let’s quickly recap the key definitions above:
- Minimum runtime: dependent on motor HP
- Cut in pressure: pump starts
- Cut out pressure: pump stops
- Drawdown capacity: volume of water between pump on and pump off
- GPM: nominal pump capacity in gallons per minute.
The most frequent guideline applied to these definitions is the minimum runtime, which is one minute for a horsepower less than 2 and two minutes for 2 HP and above. Knowing the pumping rate then determines the tank volume.
So, let’s go over all of this in an example:
A 10 GPM pump with a ½ HP motor operating on a 40-60 pressure switch would require a tank with 10 gallons drawdown capacity. This pump requires a one-minute runtime; thus the tank drawdown capacity is based on 10 GPM.
The pumping rate is also not exactly 10 GPM as it varies with the 20 psi difference on the discharge of the pump. Back pressure on a centrifugal pump always determines where it is operating on its H-Q curve.
From Table 2, the V100 would be the recommended tank as it comes closest to the 10-gallon drawdown required.
A 25 GPM pump with a 3 HP motor would require a tank with 50 gallons of drawdown capacity. This pump requires a two-minute runtime, thus the 50 gallons. Using Table 2 and a 40-60 psi pressure switch, we do not find a tank of this capacity. The correct design is a two-tank system with 25 gallons drawdown capacity in each tank. The appropriate tank is then either the V250 or V260.
Another method of tank selection is to calculate pressure factor for any given tank. This is useful if the pressure settings are not standard as shown in Table 2.
Pressure factor (PF) is 1 minus acceptance factor (AF). It looks like this: PF = 1 – AF.
Acceptance factor = cut in psi + 14.7 divided by cut out psi + 14.7.
The total volume tank required is: drawdown capacity required divided by pressure factor.
So, let’s work through another example:
18 GPM pump with a 2 HP motor operating on a 20 psi pressure differential but set at 45 psi cut in and 65 psi cut out. Based on the 2 HP motor, the runtime should be two minutes, which means the tank drawdown capacity
should be 36 gallons. The calculation looks like this:
PF = 1 – 59.7/79.7 = .25
Table 3 also gives this number.
Total volume tank recommended is 36/.25 = 144 gallons.
From Table 3, the minimum recommended tanks would be two V200.
All these examples are based on captive air pressure tanks (Figure 2) which have either a diaphragm or bladder separating the air and water. It also assumes that the air pressure is 2 psi below the operating cut in pressure.
When troubleshooting the operation of a water system tank issue, the first item to check is the air pressure. If the diaphragm or bladder has failed and water has escaped to the air side, it will not be at the desired pressure.
Another indication of failed diaphragm is if the air pressure cannot be maintained at the desired setting.
Non-captive air tanks are used if the pumped water is gaseous, and allowing some of that gas to escape in the pressure tank can improve the water quality. This tank requires close monitoring of the air pressure as the air/water interface will not be separated by a nonporous material.
The discussion to this point is for conventional water systems with typically a 20 psi operating pressure differential. Many captive air tanks are now being used on constant pressure water systems. The sizing guidelines for these are different.
The most common guideline for constant pressure systems is: the total tank volume is 20% of the nominal pump capacity. In other words, a 10 GPM pump would require a 2-gallon total volume tank. A much smaller tank, and thus a definite advantage of the variable speed constant pressure water system.
The air pressure in the tank on a constant pressure water well system is typically 10-20 psi below the operating pressure of the water system. With the constant pressure, little water is exchanged in the tank, so new designs of mounting fittings are being used to get flow into the tank.
Selecting the proper water system pressure tank can go a long way in enhancing the operating life of all your pumping systems.
David L. Kill, PE, has been active in the groundwater and water well industry for more than 50 years. He has been a lecturer at programs on groundwater, water well design, and pump selection and application, including several courses given by the National Ground Water Association. Kill received the 2021 NGWA Robert Storm Intersectional Cooperation Award for promoting collaboration, enhancing cooperation, and fostering community among all groundwater professionals. He retired from Goulds Pumps ITT Corp. in 2011, where he was the regional market development manager.