Water tank cutaway showing air and water spaces (C) Daniel FriedmanWater Pressure Tank Bladder Construction
What are the parts of an internal-bladder water pressure tank?

  • WATER TANK BLADDER CONSTRUCTION - CONTENTS: What are the parts of an internal-bladder water pressure tank? How an internal bladder works in a water tank, which parts contain water and which contain air? What goes wrong? Broken, burst, busted or leaky water tank bladder diagnosis & repair procedures
  • POST a QUESTION or READ FAQs about water tanks that use an internal bladder

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

Internal bladder type water pressure tank construction details:

This article describes the parts that you might find inside of a typical internal-bladder type water tank if you could cut it apart. We using photographs, sketches & text we point out and explain the functions and features of each of the internal parts of the water tank, and we answer the question of where the air goes and where the water goes in these systems.

Green links show where you are. © Copyright 2017, All Rights Reserved.

Water Tank Bladder Construction & Parts

Water tank cutaway showing air and water spaces (C) Daniel FriedmanReader comment: is the air in the bladder or is the water in the bladder in a water pressure tank?


" If the water tank bladder has developed a leak or has burst WATER WILL HAVE LEFT THE BLADDER and entered the air space in the tank making it difficult or even impossible to drain the tank completely.

That's because water that leaks into the tank's air chamber from a small hole in the tank bladder will not readily return to the bladder (water space) as you are draining the tank. "

Comment: There is nor supposed to be any water IN the bladder. The water comes in to the bottom of the steel tank and
See diagrams here:

Reply: the water is contained in a combined plastic liner (tank base) and bladder-underside (upper area of water reservoir) and air is above that space

[Click to enlarge any image]

Thanks for the head's up, James. In this article I have expanded the illustrations and clarified where water and air will be found, elaborating the explanation to include the case you cite. You're right that I needed to clarify the discussion.

Still, if there is any leak in the bladder, water can push through that leak, be it a pinhole or a tear, and then can enter the air space in the tank. Water will not readily return into the proper water area and the tank will both become water logged and have a shortened draw-down volume or ycle even though the system pressures will remain.

At the Ferreteria Don Pedro in San Miguel de Allende, I have just photographed a cutaway water pressure tank that will allow illustration of how these tanks work. There are, of course, other water pressure tank designs. But the case you cite is pretty standard.

Our image above shows the air and water space inside of a typical internal-bladder type water pressure tank. This tank actually has three distinct areas that I'll name from bottom-up:

  1. The Base Liner, pvc plastic-lined lower water tank portion into which water flows via a port in the tank bottom (not visible in the photo)
  2. The Bladder, a flexible rubber bladder that is pressed upwards by water entering the tank at tank bottom and is pressed downwards by air pressure in the two upper chambers of the tank.

    The rubber bladder and plastic base liner are sealed together by a ring that holds the bladder in place in this design. Other water pressure tank designs may eschew the plastic base liner, sending water into or allowing water to drain out of a flexible rubber bladder alone.
  3. Air Space, including a space below as well as space above the limiter dome; above the dome is an upper air-only chamber into which air may be forced when the bladder is fully-extended with water. Air moves through a vent opening to pass between the "air only" space above the limiter dome and the bladder expansion area (below the limiter dome) as water passes into or out of the water reservoir area in the tank bottom.

Water tank cutaway showing air and water spaces (C) Daniel Friedman

Above: I'm lifting the edge of the flexible rubber tank bladder that is currently in its fully collapsed or "concave" position. In this position the tank will be virtually empty of water. Below my hand you can see the light-colored edge of the plastic liner that will contains water in the bottom area of the tank when the pump sends water in.

As water enters the pressure tank it will push the rubber bladder upwards into a convex position. When the tank is completely full, water will be contained in the lower 2/3 of the tank in the space that is lined in its lower half by plastic and in its upper half by the now convex rubber bladder.

When the water tank is full, air above the bladder will have been pushed into the upper area of the tank. The purpose of the convex metal shell at the bottom of the upper tank area is to prevent over-expansion and tearing of the rubber bladder as water is sent into the tank by the water pump. The black button in the center of that convex metal shell is an air port that allows air to move between the space above the bladder when the tank is empty of water and the space above the shell as the bladder pushes upwards while water is entering the pressure tank.

In the photo below I've lifted the rubber tank bladder from its empty position to show both the white plastic liner of the water tank base and the round opening in the tank bottom thorough which water will enter the pressure tank.

Water tank cutaway showing air and water spaces (C) Daniel Friedman Schematic of a bladder type captive air water pressure tank (C) Carson Dunlop Associates

To be clear, if an internal bladder collapses and tears this can prevent water from entering the pressure tank. And when the bladder is punctured or torn, water can enter the space that normally contains only air.

The water pressure tank sketches shown here were provided courtesy of Carson Dunlop Associates, a Toronto home inspection & education company.

Question: is the water in the bladder or is it air that's in the bladder

Water tank construction details showing bladder (C) Daniel Friedman(Aug 3, 2016) James said:
Your article says:

f the water tank bladder has developed a leak or has burst WATER WILL HAVE LEFT THE BLADDER and entered the air space in the tank making it difficult or even impossible to drain the tank completely. That's because water that leaks into the tank's air chamber from a small hole in the tank bladder will not readily return to the bladder (water space) as you are draining the tank.

Comment: There is nor supposed to be any water IN the bladder. The water comes in to the bottom of the steel tank and PUSHES AGAINST the AIR BLADDER above it.

See diagrams here: WATER TANK BLADDERS & CAPTIVE AIR of water coming in bottom and pushing against the BLADDER which has charge of air in it.

This question was originally posted at WATER TANK PRESSURE ADJUSTMENT

Reply: designs vary. In this design water is in a half-plastic, half-rubber container and air is in a half rubber-half steel container of the pressure tank

Thank you James, you're right. We've clarified and illustrated your point.

Water tank designs vary on the point of whether it's the air or the water that is contained inside of a rubber bladder.

In the illustration above, water enters at the bottom of this pressure tank and is contained in a hybrid chamber: the chamber bottom consists of a plastic liner. The chamber upper half consists of a rubber bladder, shown collapsed in my photo. When the water chamber is filled the water is contained in a half rigid-plastic and half flexible rubber container bonded together at their center meeting point.

Air is contained above the rubber bladder - more properly a diaphragm in this case, and is thus in a space that is half coated steel and half rubber.

Diagnose Recurrent Water Pressure Tank Bladder Failures vs Tank Construction & Tank Selection

Question: Elbi water pressure tank failure investigation: is there a tank construction, selection, installation, or use problem?

Elbi water tank mounted on a water pump system in India (C) InspectapediaI have read with interest your treatise on failure management of booster pump pressure tanks with bladder. I am approaching you for guidance regarding the frequent premature failure of pressure tank in a 1 HP booster pump used in my residential home with 3 bathroom s. The make of pump is Crompton India and the make of pressure tank is Elbi-Italy. Picture of Pump is provided below:

I installed the pump in Jan 2016 which is on the side of the overhead tank in the open sun but covered with a FRP shelter which prevents direct sunlight and the green house cover in the terrace reduces the heat too by 50%. Max Atmospheric temperatur is 38 C during April 15-Jun 15 and 28-35 C for balance 10 months.

Earlier in 2009 I had purchased similar pump of Espa Spain make with another pressure tank, also an Italian make. Like the earlier pressure tank which failed every 2 years due to leak/burst, the new 2016 tank also failed but in 14 months, just 2 months after warranty expired.

I attach a picture of the failed tank bottom which shows 2 pin holes on the body of the tank 2 inches away from the male connector. Presumably water has collected inside vessel at the joint and the MS has got rusted and resulted in leak or as I read in your treatise there has been a pin hole in the bladder which has caused the hole in MS shell. This was accompanied by rapid drop in pressure and more frequent restart of the pump.

Happily the supplier replaced with a new pressure tank which cost me nearly 30% of pump cost- they always fleece customers while selling spares. Based on previous experience over 7 years I had taken care to install an online filter ( 5 PPM 3M Filter-as expensive as the pump !) to ensure water quality is good and the fine suspended particles which can blast-puncture the bladder are eliminated. Yet the tank failed in 14 months while the other Italian brand lasted 2 years.

I would be grateful if you can advise what causes such frequent failure? Is it the wrong choice of bladder material ? Frankly I do not know the material used. I have read EPDM and Butyl Synth Rubber bladders withstand 90-100C ( in any case it is not hot water here but only water from OHT heated by Sunshine at 30-38C). I have also read FRP tanks do not rust and are preferred over MS

Thanks in advance. - Anonymous by private email 2017/04/07

Reply: things to check: tank selection of type, model, size vs water pressure tank requirements & type of water system in use

Elbi offers replacement water pressure tank bladders and INSTRUCTIONS for ANK BLADDER REPLACEMENT for 8-45L tanks - for which we provide a copy of the company's PDF document, retrievedf 2017/04/08, original source: - we give contact information for Elbi below.

But that doesn't address the early failure of your system.

I proceed by asking for more information that might help us diagnose the problem?

1. I'd like to see some photos of your water system with tank and pump as it is installed.

2. Can you tell me the operating pressure range of your water system?

3. Have you made a magnified examination of the area of tank bladder failure (the pinholes you cite) AND of the corresponding tank surfaces against which the bladder is forced during use? I'd look for welding spatter or any other sharp surface properties or even just debris in the tank.

4. Can you tell me the specific model of your Elbi water tank. It's possible that the tank you are using is either not the correct model, OR as a second line of thought it may be too small for the water volume you are using.

Looking at the company's website for residential diaphraghm-type pressure tanks, you should be using a DWT-series pressure tank. Your photos don't look like any of those, but I may not be finding the company's full product line. A tank the size of the one in your photo would be perhaps a 2-gallon DWT-8-RO8

I'd also like to know how your water system works. Here in Mexico we use a pressure-boosting water pump that boosts pressure from a rooftop water storage tank or cistern; those systems often use a very small pressure tank, sometimes one built right into the pump.

More-conventional pump and well systems use a larger water pressure tank in part because the pump and tank are not designed for very frequent on-off cycling nor for very long continuous pump-on cycles.

Or using the wrong tank could be a cause of tank bladder failure. Not suitable, for example would be their XT / XTV series are intended for use in hot water solar, glycol-protected systems (not that you probably need that in India) and are not intended for domestic water systems.

Contact Elbi Water Tanks

5. Have you tried asking Elbi technical support what is the anticipated life of the tank you are using? Try sending an e-mail to .

U.S. readers are also given this Texas contact for Elbi


Continue reading at WATER TANK BLADDERS - home, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.





Suggested citation for this web page

WATER TANK BLADDER CONSTRUCTION at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia


Frequently Asked Questions (FAQs)

Click to Show or Hide FAQs

Ask a Question or Search InspectApedia

Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Search the InspectApedia website

Comment Box is loading comments...

Technical Reviewers & References

Click to Show or Hide Citations & References

Publisher's Google+ Page by Daniel Friedman