Hydropneumatic Tank Sizing

Hydropneumatic tanks are primarily used in a domestic water system for draw down purposes when the pressure booster system is off on no-flow shutdown (NFSD). The NFSD circuitry turns the lead pump off when there is no demand on the system. While the system is off in this condition, the hydropneumatic tank will satisfy small demands on the system. Without the tank, the booster would restart upon the slightest call for flow such as a single toilet being flushed or even a minute leak in the piping system.

Hydropneumatic tank sizing is dependent on two factors:

1. Length of time you wish the pumps to remain off in a no-flow situation.
2. The tank location in relation to the pressure booster.

Any given building will have a low demand rate for various times of the day. Leaky faucets or someone getting a glass of water in the middle of the night are factors which prevent this low demand period from being a no demand period. It is not often that a system will have periods of zero demand.

The estimated low demand GPM should be multiplied by the minimum number of minutes you want your booster to stay off on no-flow shutdown to determine draw down volume of the tank. Due to the time delays built into most no-flow shutdown circuits, three minutes is generally the minimum off time considered. Typically, the maximum amount of time is 30 minutes. The longer the unit is off the more energy we save but the larger our tank must be. Therefore, a compromise must be made between tank size and minimum shutdown time.

TANK DRAW DOWN CALCULATION

The tank size is not equal to the amount of water which can actually be drawn from the tank. The usable volume of the tank is dependent upon the normal system pressure, minimum allowable system pressure and the drawdown coefficient of the tank. This drawdown coefficient can be obtained from the tank manufacturer’s published data.

HYDROPNEUMATIC TANK PLACEMENT

There are several places where a hydropneumatic tank can be connected to the system. The most common connection point is to the discharge header of the booster package. Some tanks are connected just after the discharge of the pump but before the PRV. Another fairly common location is farther out in the system, usually on the roof of the building. There are pros and cons to each location.

Locating the tank near the top of the system as shown in Figure 1 usually results in the smallest tank. It also eliminates concerns about high working pressures which can occur at the bottom of a multi-story system. This is normally the best overall location for the tank. However, not all buildings have room for a tank on the upper floors and you must be sure to have a means of transporting the tank through the building.

The tank can also be located at the discharge of the pressure booster package as shown in Figure 2 . In most buildings, it is considerably easier to install a tank in the equipment room than on an upper floor which makes this location the most common. If locating a tank at the bottom of the system, it is important to make sure that the static height of the building plus the discharge pressure of the package does not exceed the maximum allowable working pressure of the tank.

For an even higher final pressure, the tank can be connected prior to the lead pump PRV (Figure 3). This is a higher pressure point because the pump TDH and suction pressure have not yet been reduced by the PRV. This again helps us to reduce the size of the tank. If this approach is taken the tank must be connected to the discharge of the lead pump at all times.

If your booster is equipped with pump alternation and the tank’s pump is moved to the 2nd or 3rd in sequence, the tank will not charge. An uncharged tank cannot provide any draw down volume so it will be of no use during a low flow shut down condition. Since this location will see higher pressures than either of the first two examples, it is particularly important to make sure you don’t exceed the maximum working pressure of the tank.

HYDROPNEUMATIC TANK SIZING

First we must determine the tank acceptance volume. Refer to Table, below, for a guide to typical acceptance volumes for various facilities. These figures are estimates based on 30 minute shutdown time and should be viewed accordingly.

Use this table for estimating purposes only. Final determination of the acceptance volume is the responsibility of the design engineer. Remember to consult local codes!

The thirty minute shut down time can be adjusted for different times by using the following formula:

ACCEPTANCE VOLUME (from Table) X DESIRED SHUTDOWN TIME / 30 MINUTES = ADJUSTED ACCEPTANCE VOLUME

Once we have determined the required acceptance volume, we can calculate the tank size based on draw down capabilities. Consult your hydropneumatic tank supplier for information on draw down volume of their tanks. A typical data sheet is shown in Figure 5. Since different manufacturer’s tanks have different draw down capabilities, it is imperative that you use the data supplied by the manufacturer whose tank you plan to use.

The value in the intersection of initial pressure and final pressure is your draw down coefficient. Divide your acceptance volume by this coefficient to obtain the total tank volume.

EXAMPLE #1: TANK ON ROOF

We have a pressure booster sized for 500 GPM at 75 PSIG discharge pressure with a 40 PSIG minimum suction pressure available from the city. The tank will be located on the roof of a 5 story building. Calculate the tank size required for a 15 minute shutdown during low flow conditions and a 65 PSIG booster cut-in pressure:

• From Table above, we can see that a booster sized for 500 GPM in an apartment building to be off for 30 minutes on low flow, an acceptance volume of 75 gallons is required. However, since we only need our booster to be off for 15 minutes, we must adjust this acceptance volume accordingly 75 x 15 / 30 = 37.5.

Therefore, our acceptance volume will be 37.5 gallons.

• Our initial pressure is equal to the pressure at the tank connection point at booster cut in pressure. This value is equal to the cut-in pressure less the static elevation of the tank above the discharge of the booster package. We must also account for the friction loss in the piping between the package discharge and the tank connection point. In this case, we have calculated a friction loss of 10 feet or 4.73 PSIG. The tank is located approximately 70 feet above the booster which equates to 30.3 PSIG.

65 PSIG (CUT IN) – 4.73 PSIG (FRICTION LOSS AT DESIGN FLOW) – 30.3 PSIG (STATIC HEIGHT) = 30 PSIG

• Final pressure is equal to the pressure at the tank connection point when system is fully pressurized.

75 PSIG (SYSTEM PRESSURE) – 4.73 PSIG (FRICTION LOSS AT DESIGN FLOW) – 30.3 PSIG (STATIC HEIGHT) = 40 PSIG

• Using Figure 5, we can determine that our draw down coefficient is .183.
• Divide the acceptance volume by the draw down coefficient to obtain the total tank volume that will give us 75 GPM during Low flow shutdown.

37.5 GPM / .183 = 205

Therefore, we need a minimum tank volume of 205 gallons to meet our shutdown requirements.

EXAMPLE #2: TANK AT DISCHARGE OF BOOSTER PACKAGE

We again have a pressure booster sized for 500 GPM at 75 PSIG discharge pressure with a 40 PSIG minimum suction pressure available from the city. Now, the tank will be located in the basement of a 5 story building and be connected to the discharge header of the package. Calculate the tank size required for a 15 minute shutdown during low flow conditions and a 65 PSIG booster cut-in pressure:

• From Table above, we can see that for a booster sized for 500 GPM in an apartment building to be off for 30 minutes on low flow, an acceptance volume of 75 gallons is required. However, since we only need our booster to be off for 15 minutes, we must adjust this acceptance volume accordingly 75 x 15 / 30 = 37.5.

Therefore, our acceptance volume will be 37.5 gallons.

• Our initial pressure is equal to cut-in pressure less static height and piping losses to the tank. However, since the tank is located at the discharge of the package, static height and friction losses are insignificant. Therefore, we can conclude that the initial pressure is actually equal to cut-in pressure.

INITIAL PRESSURE = CUT-IN PRESSURE = 65 PSIG.

• Likewise, the insignificance of static height and friction losses also apply to our calculation of final pressure. We can conclude that final pressure is equal to the pressure at the tank connection point when the system is fully pressurized.

FINAL PRESSURE = SYSTEM PRESSURE = 75 PSIG

• Using Figure 5, we can determine that our draw down coefficient is .111.

• Divide the acceptance volume by the draw down coefficient to obtain the total tank volume that will give us 75 GPM during Low flow shutdown.

37.5 GPM / .111 = 340

Therefore, we need a minimum tank volume of 340 gallons to meet our shutdown requirements.

EXAMPLE #3: TANK CONNECTION BETWEEN PUMP DISCHARGE AND PRV

Using the same pressure booster as the previous two examples, sized for 500 GPM at 75 PSIG discharge pressure with a 40 PSIG minimum suction pressure available from the city. The tank will be located in the basement as in example #2 but will be connected before the pressure reducing valve. Calculate the tank size required for a 15 minute shutdown during low flow conditions and a 65 PSIG booster cut-in pressure:

• From Table above, we can see that a booster sized for 500 GPM in an apartment building to be off for 30 minutes on low flow, an acceptance volume of 75 gallons is required. However, since we only need our booster to be off for 15 minutes, we must adjust this acceptance volume accordingly.

75 x 15 / 30 = 37.5

Therefore, our acceptance volume will be 37.5 gallons.

• Our initial pressure is still going to be equal to cut-in pressure as in Example 2. We do not need to be concerned with static height and friction loss since the tank will be located adjacent to the pumps. Therefore, our initial pressure will be equal to cut-in pressure.

INITIAL PRESSURE = CUT-IN PRESSURE = 65 PSIG.

Final pressure is going to be significantly higher than in example #2 because our tank is connected to the system prior to the pressure reducing valve. Therefore, we actually have pump TDH at minimal flow plus minimum suction pressure. If our pump has a flow vs. Head curve as shown below in Figure 4, the final pressure is going to be 155. (TDH @) 0 GPM) plus minimum suction pressure of 40 PSIG. Therefore, our final pressure can be calculated by adding these values.

67 PSIG (PUMP TDH @ 0 GPM) + 40 PSIG (MIN. SUCTION PRESSURE) = 107 PSIG

• Using Figure 5, we can determine that our draw down coefficient is .335
• Divide the acceptance volume by the draw down coefficient to obtain the total tank volume that will give us 75 GPM during low flow shutdown.

37.5 GPM / .335 = 1 12

Therefore, we need a minimum tank volume of 112 gallons to meet our shutdown requirements.

HYDROPNEUMATIC TANK CHARGING

Most hydropneumatic tanks ship from the manufacturer pre-charged to a pressure that is usually well below the actual charging requirement for the system. In other words, the air volume in the tank is too small once the tank is installed in the system. Pumps short cycle, draw down is limited, and in some cases, the situation is so severe that the tank could be removed from the system and nobody would know the difference. So, if we’re going to spend the money for a tank, let’s make sure it works by charging it correctly.

The correct tank pre-charge pressure depends upon the following factors:

1. Minimum allowable system pressure
2. Tank elevation relative to the pressure booster package
3. Tank connection point in the system

We will define these variables as follows for our pre-charge calculation:

• Let D = Desired system pressure in PSIG (PRV setting)
• Let M = Maximum allowable pressure depression below PRV setting (D)
• Let H = Tank elevation above pressure booster in PSIG (PSIG = Feet / 2.31)
• Let P = Tank pre-charge pressure (tank empty) in PSIG

We will also estimate a 1 PSIG pressure drop across the PRV at very low flow rates encountered during a low demand period.

If the tank is located above the pressure booster as shown in Figure 1, the pre-charge is calculated like this:

P = D – M – H – 1

Tanks located approximately level to the booster and connected to the system downstream of the PRV (Figure 2) have their pre-charge pressure as follows:

P = D – M – 1

If the tank is approximately level with the booster but connected to the system prior to the PRV (Figure 3), then we do not have to subtract the 1 PSIG drop across the valve. Therefore, the calculation is as follows:

P = D – M

To confirm the pre-charge pressure of an existing tank, the tank must be isolated from the pumping / piping system. Then the water side of the tank is drained and the air pressure read with a gauge at the air charging valve. This reading is the precharge pressure.

By simply taking a little extra time to make sure our tank is pre-charged correctly, we can be certain that it will serve its purpose of keeping the pumps off during periods of low demand.

PRE-CHARGING EXAMPLE

Let’s take a look at the roof tank described in Figure 1. We know that the correct pre-charge pressure is defined as:

P = D – M – H – 1

We know that:

• D = 75
• M = system pressure – cut-in pressure = 75 – 65 = 10
• H = 70 / 2.31 = 30.3

Therefore, our correct pre-charge pressure is:

75 – 10 – 30.3 – 1 = 33.7 PSIG

To ensure correct operation of the tank during the booster’s low-flow shutdown sequence, it must be precharged to 33.7 PSIG.

HYDROPNEUMATIC TANK SUMMARY

As you can see, there are few hard and fast rules to tank sizing. It is predominantly a matter of weighing various factors and compromising on a balance of initial cost and potential energy savings. Locating the tank connection prior to the pressure reducing valve results in the smallest tank but requires its respective pump to always be the lead pump.

A tank connection at the discharge header results in a larger tank but allows you to alternate all pumps. A roof mounted tank seems like a pretty reasonable compromise but you must consider the complications of transporting the tank to the roof. In conclusion, tank location has a significant impact on the tank size and must be addressed on a project by project basis.

initial tank pressure is equal to the minimum allowable pressure of the system (at the point of the tank) where the booster system will come back on line.

Final tank pressure is equal to the maximum system discharge pressure (at the point of the tank) or, the pressure reducing valve setting if the tank is mounted on the booster system.

Actual usable gallons may vary ± 10%.

Xylem Company