Double wall tank - Interstitial layer doubts 1

[JohnBC]

Hello everyone!

I'm working on a construction of a storage tank of 10,000L, this is a double wall tank and here is where I have some trouble. I can't find any information regarding the interstitial layer that shall be vacuumed.

My questions are:

* What is the commercial name for this interstitial layer?
* Is it possible to buy this interstitial layer and install it yourself or it is required to be done by a tech or company specilzied in tanks?
* Doc / info about how to install this interstitial layer?

If anyone can enlight me with this I'd really appreciate it!

Thanks everyone!

Best regards,

John.

 

[RVAmeche]

First of all the outer wall of the tank is typically just called the outer wall/outer tank. I've never heard anyone refer to it as "the interstitial layer", primarily because the space between the inner and outer tank is the interstitial space. There is no "interstitial layer", unless you mean some sort of coating in the interstitial space.

Is this steel or fiberglass? Aboveground or underground? I don't think it'll really make a different; as far as I know you have to buy a double wall tank from the manufacturer. There are no "add-ons" where you can attach an outer wall to the existing tank. Doing so would be seem to be awkward and clumsy (would you lift the existing tank from its saddle supports or base? how do you make sure it's centered? etc)

If you need a double wall tank, you also have to think about the nozzles depending on the state and EPA regulations. Some areas require the nozzles (underground) to also be monitored, thus they need to be double wall and share the same interstitial space.

Long story short...if you have an existing single wall tank and need a double wall...just buy a new double wall tank.

 

[MJCronin]

Double walled tanks (in both steel and fiberglass) require double bottoms. How are you going to do this ?

What does the "vacuuming" requirement mean ?..... I have never heard this before. Can you explain ?

Did the client force this requirement on you ? ........

MJCronin
Sr. Process Engineer

 

[RVAmeche]

MJCronin,

I believe he's referring to the need to draw vacuum on the interstitial space for leak detection purposes. Or since he mentioned construction, possibly the vacuum that is typically applied prior to shipment to prove the double-wall is intact upon receipt.

 

[JohnBC]

Hello RVAmeche and MJCronin,

RVAmeche Yes! You are right, that is exactly why I need to draw vacuum from the interstitial space. I read about it in "Handbook of storage tank system", where it says this:

"A jacketed design employs a UL 58 primary tank encapsulated by a nonmetallic layer of protection. The secondary containment may be made of an FRP combination, a high-density polyethylene or a polymer—all of which provide a durable outer layer resistant to corrosion and compatible with a wide range of petroleum and chemical products. One of the most popular features is vacuum testing—holding 1 in. of Hg vacuum, which equals 2.036 psi—a typical method of showing that a jacketed tank is tight."

It also makes a reference to UL-1746 where it says the procedure for the vacuum testing, but obviously this vaccum test has to be performed on something (some sort of layer?) that is in contact with the primary tank inside the interstitial space.

MJCronin, about the double bottoms I haven't figured it out yet, I'm still stuck with this vacuum testing and yes it is a requirement.

RVAmeche, the primary tank is already build up, so buying a new tank is not an option for me.

Thank you both for your responses.

Best regards,

John.

 

[RVAmeche]

If buying a new tank is not an option, then frankly neither is a double wall tank. You don't just slap together walls around an existing tank. I'd review your other methods of secondary containment such as concrete containment, spill containment structure, etc.

The vacuum test is not performed on just the outer tank. The space between the two tanks (the interstitial space) is hooked to a vacuum pump and evacuated. You typically have at least one nozzle that is only connected to the interstitial space for things like a leak detection sensor (located at the bottom for liquid contact) or to plumb tubing for a continuous vacuum system.

 

[MJCronin]

JohnBC ... I agree with RVAmeche

If you want a double walled tank with an interstital space that can be vacuum tested, then you must buy a shop built, UL-58 steel wall, horizontal axis tank.

You cannot re-fit these requirements onto an existing tank

MJCronin
Sr. Process Engineer

 

[JohnBC]

Hi MJCronin and RVAmeche,

Thanks RVAmeche for the explanation, this really gave me an idea on how to do this.

MJCronin, the primary tank was indeed fabricated based on UL-58, using A36 and horizontal axis.

Now I see factible creating the vacuum in the interstitial space with a double steel wall tank, but I have read that this vacuum can be done in a jacketed tank, Modern Welding Company uses their own method called Glasteel II. I know this is an impossible shot but does anyone knows how to do this vacuum with a jacketed (FPR - Fibre-reinforced plastic) double wall tank?

Best regards,

John.

 

[RVAmeche]

Note your mixing up terminology by calling a double wall tank a "jacketed" tank; tank jackets are typically used for heat transfer (dimple jackets, etc). I'm not familiar with the Glasteel tank line but the manufacturer will tell you how much vacuum the proposed tank should be able to withstand in the interstitial space.

The "how" of drawing a vacuum on the interstitial space is up to you/the design, while ensuring you meet the applicable state & federal regulations. For USTs involved in fuel dispensing applications, a siphon port off the submersible turbine pump is a common method to draw vacuum. Otherwise you need a dedicated vacuum pump and control system. If your regulations require continuous vacuum monitoring, there's other requirements.