Bladder / Diaphragm Tank Question

Mike_Sheppard

Everything I have ever read says to set a bladder / diaphragm tank to whatever your system pressure is. If you have your system pressure set to 40psi, charge the tank to 40psi. (Assuming that is the proper system pressure for the application).

I was told today that the proper way to do it is to set the tank air pressure 5psi lower than system pressure. I have never read or heard of this before.

Is there a specific manufacturer that wants it done that way, or is it not correct?

Mark Eatherton

The reason for this is to keep the diaphragm from sticking to the tank and not allowing expanding water to come into the tank. By keeping it partially distended, it won't stick to the face of the tank and keep it from flexing.

One of the major tank manufacturers used to put talcum power in the can to keep this from happening. In reality, if you've done a proper job of sizing the tank, it will be slightly oversized, so this minor intrusion into the "acceptance" volume will not (should not) have any major consequences. The safety factor is that not all fluid within the system will ever make it to the maximum estimated operating temperature. It can't. If it did, you've just experienced a nuclear thermal event, and at that point, it just don't matter any more...

Here's a piece of expansion tank trivia. What method was originally used to charge the gas on the gas side of expansion tanks before they all started getting Schrader valves on them? (Think Amtrol, early producers of diaphragm style expansion tanks)

ME

Mike_Sheppard

@Mark Eatherton Dry ice!

But the pressure was eventually lost through the diaphragm and there was no way to recharge it.

So by setting the air pressure 5 psi lower, it allows some water in to prevent it from sticking to the tank. So there's some water between the diaphragm and the wall of the tank. Did I understand that properly?

Mark Eatherton

Correct. The diaphragm is not in a fully rested state, but instead is taking on just a little water, and BINGO, you are correct, a small chunk of dry ice was tossed into the air side of the tank right before it was welded shut. Dan Holohan taught me that!

I believe it's a good idea to treat a potable water pressure tank the same way. This will keep the tank from running completely out of pressure before the pump that fills it kicks back on. I use a rule of 80. I pressurize the tank to 80% of the required operating pressure. So If my make/break are centered around a 50 PSI operation, then 50 times .8 = 40 PSI. That's what I pressurize my Well-X-Trol tank to.

But I wouldn't use the same formula for a thermal fluid expansion tank. you'd take up too much of the tanks "acceptance" volume in doing so.

Your milage (and experience of others) may vary.

ME

Mike_Sheppard

@Mark Eatherton

Thanks a ton for the info. Learn something new every day. Now I hope all the ones I set at operating pressure are still operating!!!!

Dan Holohan taught me that as well. Through his books!

Mark, do you still teach any classes?

Mark Eatherton

Yes, I now do custom hydronic training programs.

I'm working with/for property management people who have hydronic systems in their buildings, as well as training hotel employees who also have hydronics in their systems, and I am putting together a training program for hydronics contractors to show them what administration has to go through to get the job, and what their job is with each performance. I'm working smarter, not harder, headed for eventual retirement.

And I will occasionally get out and teach a full days class for wholesalers and manufacturers reps.

I'm also in the process of finishing a book that I've been working on for years. I'd originally started out with the intention of writing a "Best Practices" manual, then I realized that what's best for me may not be best for you, so decided instead to write a Recommended Practices Manual, showing ALL of the different ways of doing hydronic.radiant heating, and let the reader make the decision about what works BEST for them , their employees and their customers.

Have bags, will travel

ME

Mike_Sheppard

I call first dibs on the book!!!!! That will be awesome to have.

The company I work for now is not the greatest for providing education. They do what they can but it gets expensive with so many people. If I have the opportunity one day I’d love to partake in something if you’re not retired.

SuperTech

I'd be interested in the book as well. I never get tired of learning!

Larry Weingarten

Mark, I want one too!!! Perhaps Erin could get you into her virtual bookstore.

Yours, Larry

Erin Holohan Haskell

Larry Weingarten said:

Mark, I want one too!!! Perhaps Erin could get you into her virtual bookstore.

Absolutely!

RayWohlfarth

Good luck Mark with the book. I know it will be good. I look forward to reading it. Hoping you have a version for dummies like me