Avoid tubing failures

06/01/2001


A walk across any manufacturing shop floor or around any process plant will reveal hundreds or even thousands of feet of metallic and plastic tubing transporting gases and liquids to and from sample stations, valve actuators, pump seals, and hundreds of seemingly ho-hum installations.

But when any of that tubing starts leaking suddenly the tubing and its associated connectors attract attention in the form of connector "snugging."

Tighter is not always better

When someone discovers a leaky tube fitting, the first action is to tighten it up just a little. But often the slightest additional tightening results in the tube snapping and what was a small leak becomes a gusher.

Seldom is such a situation examined for root cause, but if it were, tubing failure is unlikely to be the root cause; improperly installed connectors are. Tubing often fails because the connector was over tightened, thus weakening the tubing.

When a connector is properly installed, it is possible to disconnect and reconnect the connector many times without jeopardizing the integrity of the tubing or connector.

Installation and re-tightening procedures remain essentially the same for all tubing connectors with the only variable being the amount of revolutions applied to the retaining nut, depending on tubing size.

Initial installation

Connector assembly for 1-in. (25 mm) diameter tubing and smaller can be successfully accomplished using standard handtools. (Special tools are required for tubing diameters greater than 1-in. and are not being discussed in this article.)

Once the tubing has been properly fitted to the installation and tubing ends squared and deburred, the tubing and connector are ready to be made up.

Step 1 -Inspect the connector body, ferrule, and nut to ensure all pieces are present, undamaged, and of the correct size and material for the installation. Note: if the tubing can be pinched together using your fingers, a tubing insert is required, regardless of the tubing size.

Step 2 -Assemble the tube connector body, ferrule, and nut to finger tight and insert the tubing into the connector assembly, ensuring the tubing completely bottoms out in the connector body.

Step 3 -Before tightening, mark the nut at the 6 o'clock or bottom-most position.

Step 4 -While holding the fitting body steady with a backup wrench, tighten the nut 3/4 of a turn for tubing up to 3/16-in. diameter (4 mm) and 1 1/4 turn for tubing 1/4-in. to 1-in. diameter (6 to 25 mm).

That's it; that's all there is to properly assemble a tubing connector.

Tube connector manufacturers, such as Parker Hannifin (Cleveland, O.) and Swagelok (Cleveland, O.), highly recommend using a "go/no-go" gap inspection tool to verify the connector is properly made up.

Disassembly and reassembly

Most people find connector disassembly and reassembly as straightforward and easy as initial assembly.

Step 1 -Ensure the media flowing through the tubing has been shut off and the pressure on the line removed.

Step 2 -Establish the "original make up" position by marking the connector and nut.

Step 3 -Using a backup wrench on the connector body and a wrench on the nut, turn the nut counter-clockwise until it is loose.

Step 4 -Unscrew the nut, remove the tubing from the body, and perform what service is needed.

Step 5 -Insert the tubing into the body of the connector.

Step 6 -Tighten the nut to the original position plus 1/12 of a turn or 1/2 of a flat on the hex nut.

By following these simple assembly and reassemble steps the tubing running above, below, in, and around manufacturing processes is more likely to provide an uninterrupted flow of the gases and liquids critical to keeping equipment operational.




Dave Harrold, senior editor
dharrold@cahners.com





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