Expansion tank failures

[Teto]

All,
We've had repeated expansion tank bladder failures in our facility and I need some advice as to where to look for problems. It is a large volume tank (57 gallon, 215 L) on a hot water hydronic system. It is sitting on the floor and connected on the down stream of the hot water boiler and upstream of the pump. It is connected to the bottom drain of the air separator and after the 2nd failure we installed a braided 3/4" hose rather than the 1" line it had originally. It sees 20-30 psi at 200 degrees F. It is rated for 125 psi at 240 F max. It was precharded to 25 psi on the air side. The boiler's relief valve is set at 45 PSI. We've had 4 failures in the last 3 years. This last one lasted 5 months. The bladder looks like a giant hot water bottle and the tears always occur at the "neck". No sharp edges in the tank. The tears appear like the material is stretched to a failing point. One last item, the factory has no answers. It does not appear that the seams are coming undone. Anyone had similar experiences? Any ideas?

 

[imok2]

If your charging wirh air then that could be the problem, try using nitrogen

 

[Teto]

This system has been in operation since 1997. All of a sudden we're losing bladders. No sign of oxidation on the air side of the bladder.

 

[HEC]

Check for any changes to the bladder design. Although still from the OEM, they may have altered the design. It sounds like the bladder is creeping up into the expansion vessel, hence the necking at the inlet. The bladder is possibly "walking" up the sides of the vessel as it expands and contracts. Try a lubricant between the bladder and shell.
Cheers

Mark Hutton

 

[HeshamM]

I discussed this matter with proffessional manufacturers and most of them confirmed that although they design the bladder type expansion tank to be vertically mounted, for the large sized they prefer if it is horizontally mounted. Of course, you will still have the space problem.

 

[knagus]

The exspansion tank should be connected on the suction side of the system pump.

 

[russmech]

Three things:

1) Review your system pressure calc's. Ensure you take into consideration the vertical height of your system (in feet), convert to psi, then add 5 psi to ensure a minimum pressure at the highest point of the system.

2) Check to make sure no one has opened a bypass valve accross the make-up water pressure regulator (if you have one) or that the glycol make-up tank pressure switch/regulator isn't overpressurizing your system. Just becuase the boiler pressure relief valve is set for 45 psi doesn't mean it's actually working (they tend to stick if not excersized occassionally).

3) Change the expansion tank to a "full acceptance" type so the bladder can expand into the full tank volume. It sounds like you have a "limited acceptance" tank, which will not allow the bladder as much expansion.

Good luck.

 

[Jabba007]

I'd have the hydronic system's chemistry checked as well. Just to rule out chemistry changes that may be driving the failures.

Jabba

 

[atlas06]

Have you considered installing a tank by another manufacturer? any cast iron material where you are having cracks? thermal chocks may be? can you consider running at 180F instead of 200F, etc..
Definitely redo your pressure calcs as russmech suggests.
Did you go out and see your manufcaturer's other installations similar to yours?

Atlas

 

[lilliput1]

You should have a water makeup pressure reducing valve PRV(or pressure regulator)with discharge pressure set to the expansion tank correct fill presure (see russmech item 1).
When the water heats up, it will expand and increase the pressure. The PRV will shut down when the pressure exceeds its set point.The expansion tank bladder will then expand to accomodate the expansion. If the tank is properly sized the operating pressure will be between the fill pressure and the proper relief valve setpoint. The relief valve should be set such that at no point in the piping, allowing for the pump heads, static head and expansion pressure will the piping be subjected to its limit pressure. Usually this is the 125 psig for the expansion tank bladder limitwhile the piping limit is 150 psig. For for safety allowance the RV is set at around 100 psig. If the expansion tank is undersized or if higher pressure (above design PRV setpoint) makeup water is introduced or if the relief valve setting is instead too high the expansion tank bladder may be overstressed.

 

[lilliput1]

Ops you did say the boiler relief valve was set at 45 psig. Better check the sizing of the expansion tank. Because it would have an operating range of only 45-25 = 20 psig, it may be undersized. You may have to add another one in parallel with it.

 

[Teto]

All,
Thanks for the input. Last week I started checking the size of the tank compared to the system volume. Using one manufacture's work sheet I calculated the size I would need. I've gotten one estimate for a replacement as the present tank came in low, in capacity. I'd like to use this forum as a sounding board, If I may. I have a total system volume of 1642 gallons. What size would I need in a full acceptance tank? Is this the problem? I failed to include the following: There are 2 boilers and they were not installed at the same time, years apart. Aparently the same is true of the some of the heating system. Oh and the present tank's capacity is 57 gallons. I came up with a tank with 140 gallons. I could have added another smaller tank to make up th difference, but the difference in cost was not considerable.
Thoughts?
Thanks,
Teto

 

[Teto]

A couple more items to add to my last posting: There are 4 relief valves on the system, all set at 45 psi. They can't all be failing shut. The present tank is a Taco CA215 full acceptance.
Another question: In the worksheet I used, 5 parameters were required. Total system volume, minimum system temperature, maximum system temperature, minimum system pressure at tank and maximum system pressure at tank. All were straight forward. However, what value would be used for minimum system temperature? I found myself considering either makeup water temperature (55 F to 75 F) or the lowest possible temperature (say 100 F), perhaps due to a system failure shutdown.
Any thoughts,
Thanks
Teto

 

[atlas06]

Min. temperature should be your fill water temprature. A design with 40F fill temp is common with expansion to 200F.
Try to send your concerns to Amtrol, they have very knowlegeable reps in the expansion tank business. I would stay away from any TACO product.

http://www.amtrol.com/

Atlas