Skip to main content


Go Search
Welcome to the John Wood CompanyThe John Wood Company
About Us
Products & Markets
Plumbing & Heating Tanks
ASME Tanks & Accessories
Custom Tanks & Pressure Vessels
Fluid Handling Systems
Customer Support
The John Wood Company > FAQs  


Modify settings and columns
Frequently Asked Questions
What are hydro-pneumatic tanks?
These tanks store water prior to distribution in a water supply system, working with the pumps to maintain a stable water system pressure.  The system pressure is controlled by a pressure switch set for minimum and maximum pressures – giving you a cut-in and a cut-out pressure for the pumps.  When the pumps cut-out or stop running, water demand is met by the water volume in the piping and the tank.  As water is drawn down, the system pressure starts to drop.  When it reaches the minimum system pressure, the pump cuts back in and runs until the system pressure reaches the normal maximum pressure.
What are some typical applications for hydro-pneumatic tanks?

Applications include residential water systems and commercial pump systems.  Pump booster systems provide consistent water pressure for:

  1. Commercial buildings - multi-story towers, hotels, apartments, schools, hospitals ...
  2. Industry - food processing, breweries, dairies, water treatment plants, petrochem, pharmaceutical, metal industries, tourism and recreation, water parks, fountains ...
  3. Irrigation - commercial landscaping, golf courses, agriculture, sports arenas, parks ...
What are the advantages of using a bladder type hydro-pneumatic tank?
The bladder tank separates the system water from the air precharge, preventing air loss due to absorption.  Maintaining a stable air precharge pressure ensures that the system will run at peak efficiency, preventing the pumps from short-cycling. 
How does the hydro-pneumatic tank operate in the system?
The tank is precharged with air to a pressure that is close to the minimum system pressure (the cut-in pressure).  As the system fills, water is pumped into the bladder, compressing the air cushion in the tank.  This causes an increase in system pressure.  The pumps continue to run until the system pressure reaches the maximum setting (the cut-out pressure for the pump).  Water is held in the tank until needed.  As water is drawn from the system, the pressure will begin to decrease.  When the system pressure reaches the minimum setting (the cut-in pressure), the pumps will turn back on and restart the cycle.
How do I determine what size hydro-pneumatic bladder type tank is required for my system?

To properly size a hydro-pneumatic bladder tank, you need the following information:

  1. the pump delivery rate in GPM
  2. the recommended minimum run time for the pump
  3. the minimum (cut-in) and maximum (cut-out) system pressures
How do I pick the correct air precharge pressure for my hydro-pneumatic bladder tank?
The tank should be set approximately 2 PSI below the lowest pressure switch setting (pump cut-in pressure).  For example, the air charge pressure should be set at 28 PSI for a pressure switch setting of 30 PSI minimum and 50 PSI maximum.  For surge applications, the tank should be set approximately 10% to 20% below the system pressure.
How do I adjust the air precharge pressure?
To accurately set the air charge pressure, the tank must be locked out or disconnected from the system piping.  Drain all of the water from the bladder.  Locate the Schrader air charge valve.  Check the air pressure with an accurate pressure gauge and charge the tank to the desired set pressure.  Use a leak detector solution or soapy water to check the Schrader valve for signs of leakage.  Prior to turning on the system, leak check all connections including the system connection and cover plate seals, including the studs and nuts.
Do I need an expansion tank for my heating or cooling closed loop system?
The heating and cooling expansion tanks allow for the expansion of system water during normal operation.  The expansion tank provides additional space in the system for the exapnded volume of water as it heats up.
How does an air separator work?

Air separators allow for the release of air that enters a closed loop system when make-up water is occasionally added.  The air separator provides a convenient point in the system to collect and vent the air to atmosphere.  In the tangential type air separators, the vortex action allows the heavier air-free water to move to the vessel wall area while forcing the separated air into the center where it is vented out of the top of the unit.  In the air purger units, the flow across the integral flow baffle allows the separated air to collect and be vented out of the top.

How do buffer tanks improve the performance of a chilled water system?

Having as much water volume as possible increases the thermal mass and flywheel effect within the system.  Thermal mass can store and even out fluctuations in temperature.  Buildings with a low thermal mass are very responsive to changes in internal temperature.  They are often subject to wide variances.


Water has a very high thermal capacity.  Meeting or exceeding minimum system water volume recommendations will result in satisfactory water temperature control.  Some small tonnage chillers have short water loops with less water.  Adding buffer tanks to the system will increase the water volume, ensuring stable water temperature control and increasing reliability by reducing compressor cycling. 


Our sizing guidelines follow the chiller manufacturers' recommendations for effective water loops -- US gallons/ton and liters/cooling kW:

For air conditioning: minimum 3.0 gallons/ton (3.3 liters/KW); preferred 5.0 to 8.0 gallons/ton (5.4 to 8.6 liters/KW)

For process applications: minimum 6.0 gallons/ton (6.5 liters/KW); preferred 7.0 to 11.0 gallons/ton (7.6 to 11.9 liters/KW)

Is it important to periodically check the air charge pressure in a bladder tank?

The bladder tank is just like every other piece of equipment in your requires routine inspection.  Monitoring or checking the air pressure in the tank is critical to ensure proper system operation and a reliable and long service life for the tank bladder.

The bladder tank needs to maintain the initial air charge pressure.  Periodic inspection of the tank is required.  One of the easiest ways to provide a point of inspection is to install a pressure gauge on the tank.  This provides not only a local inspection point but also allows for documentation of the inspection with a reading from the gauge.  Local gauges are most common, but transmitters for remote monitoring will provide an even higher level of visibility to the air charge on the vessel. The gauges and transmitters can be field installed on a tee along with the air charge valve on the charging valve opening.  The standard air charge valve opening is 1/4" NPT.

So don't forget to check the air charge pressure in your tank.  Remember that during operation, the air charge pressure will normally be higher than your initial air pressure.  That's because of the relationship of the pressure to the volume -- if air is held in a sealed container at constant temperature, the air pressure increases at the same rate as the volume of the air is reduced.  As water fills the bladder, the volume of the air cushion is reduced.  As the volume is reduced, the pressure will rise.  So since your tank will typically have some water in it, the pressure must be higher than the initial air charge pressure. 

Make it easy on your maintenance staff and install a pressure gauge on your bladder tank.  If you'd like to take it to the next level and install a transmitter for remote monitoring, you may want to start by considering products available from Dwyer Instruments.  Recommendations include the Series 628-LED pressure transmitter with LED display and 32B controller and the Series EDA electronic pressure controller.


John Wood Company Privacy Policy - Web Site Terms of Use