Metal Rectangular Expansion Joints That Outperform

A publication of Oakridge Bellows

They don’t make ‘em like they used to

Your original metal boiler ducting expansion joints were 25+ years old, so you replaced them, or lagged over them with PTFE coated fabric belts. That was 7 years ago, and after numerous patch repairs the entire belt needs to be replaced again. So much for the inexpensive option.

If the above sounds familiar, then you are not alone. Many boiler engineers are longing for the original metal expansion joints that were there when the switch was first flipped on and have withstood the test of time.

Fabric belts – good but not perfect

For the record – I am not an anti-fabricite. Some of my closest friends are fabric expansion joints. Fabric joints are the cure for many highly corrosive applications such as scrubbers, and they are great for large lateral movements. The fabric belts also make for an easy retro-fit by lagging over the existing metal joints.

For some duct applications the temperatures are too hot for the PTFE coated fabric belts. Even well insulated belts will have some leaks and hot gases will eventually cut an ever increasing swath through the temperature limited PTFE barrier. Fly ash also fills up the lagged over cavity causing the joint to lockup and damage ducting and equipment during start-up and shut-downs.

(original graphics not shown)

Bullet – proof metal designs of yester-year

Plant operators looked for less expensive metal duct joint options when presented with the boiler OEM prices. They found lots of ‘as equal’ metal expansion joint suppliers who quietly slipped in welded mitered corners that barely made it past warranty. That’s why, in some applications, fabric belts became the grudging default option for replacing the aging metal joints.

The original equipment expansion joints supplied by the boiler companies have a very good track record for performance with many still in operation after 30+ years. The OEM boiler expansion joints all had one thing in common – round corners. And the convolutions with straight walls that are the deepest have the most flexibility.

(original graphics not shown)

I believe these designs are still available from the boiler OEM’s above…in exchange for your first-born. A better choice: Oakridge Bellows has developed the tooling to duplicate all these exact profiles with round corners at competitive prices (yes, I know, this is supposed to be an unbiased technical publication free from flagrant marketing…).

The bottom line – go back to old faithful

If any of the above rings true for your boiler duct joints, and fabric belts are not living up to all the hype – then go back to the vintage metal designs.

Nothing beats the test of time.

What works

The most common metal rectangle expansion joint – the V-span profile with a single miter corner, doesn’t work. They’re lousy, lame. They have a fundamental design flaw. I hate them. Phew, now that I’ve gotten that off my chest, I feel much better.

The world would be a much better place if all metal rectangle expansion joints had a U-profile with rounded corners.

V-shapes verses U-shapes

V-span profile convolutions (if you can call those convolutions) are prolific because they can be easily made with a 60 ton break-press, which every self respecting man has in his garage. The design flaw involves the angle of the side wall. When the ‘convolution’ compresses it wants to grow taller, but can’t. The rail is fixed at the corner to the next side, which wants to grow taller and pull away. The corner bears the brunt of this displacement. And, unfortunately, that’s right where the weld resides.

(original graphics not shown)

The U-profile has side walls that are parallel that start in a neutral position so when they swing inward the change in height is reduced by HALF. That makes a huge reduction in the stress levels. Pre-set that same U-profile 50% and the height change is only 15% of a V-span profile.

A round corner is the cure

The weld on a mitered corner is where the stresses are focused. A round corner spreads out that stress AND gets the welds out of the high stress area.

The published stress equations in the EJMA standards do not adequately address the design flaw of a V-span profile or the inadequacies of the corner welds.

You say your V-spans are working well for you? Then they are not seeing the full design conditions. If the V-spans really compressed the full rated motion, they’ll crack in the corners. When I say that while giving seminars to maintenance engineers I get more amen’s than a Sunday morning sermon. Sure, a V-span is still more flexible than straight ducting but is WAY less flexible than a round corner U-profile.

The cost is close

V-spans are inexpensive to fabricate because they do not require special tooling. If an expansion joint company has already invested in the tooling (obligatory marketing pitch – Oakridge Bellows has got ‘em) then the cost comparisons are surprisingly close.

U-span profiles with round corners work. They work very well. They’re superior, dependable. It’s an excellent design. I love them.

Seed Newsvine

Bookmark this article in your preferred program

Comments

Add Comment

Popular Articles in this cathegory

This article is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 License.