Precautions for Stainless Steel Double Ferrule Tube Fittings Used in Gas Piping Systems
During the installation of secondary process gas piping for process equipment or instrument gas tubing in cleanroom facilities, stainless steel double ferrule tube fittings are commonly used to connect process equipment or instrumentation. These fittings do not require welding, are easy to install and disassemble, and can withstand gas pressures up to 8 bar (the highest gas pressure encountered in previous projects was 8 bar) while maintaining a stable, leak-tight connection. The following introduces the construction, installation procedures, and installation precautions of double ferrule tube fittings in detail.
I. Construction and Sealing Principle of Double Ferrule Tube Fittings
(1) Construction
A complete double ferrule tube fitting consists of four major components:
Fitting Body: Manufactured from 304 or 316L stainless steel, with a precision-machined tapered sealing seat.
Nut: The nut is the primary axial compression component. When tightened, it applies axial force to compress the ferrules, causing them to grip the outside surface of the tubing.
Front Ferrule: Provides the primary metal-to-metal gas-tight seal.
Back Ferrule: Its primary function is to absorb vibration and mechanical loading, preventing the fitting from shifting or becoming misaligned.
(2) Sealing Principle
When the nut is tightened, it compresses the front and back ferrules against the tapered seat of the fitting body, forming a metal-to-metal seal between the nut, front ferrule, back ferrule, and fitting body. However, the tightening process must strictly follow the recommended installation procedures and installation guidelines. Otherwise, damage to the sealing surfaces may occur, resulting in sealing failure, gas leakage, or excessive tightening that may crack the fitting components or tubing.
II. Tubing Requirements Before Installation
Double ferrule tube fittings are mainly used with seamless stainless steel tubing. In previous projects, stainless steel AP 5S tubing and BA 5S tubing made of 304 or 316 stainless steel were commonly used. The tube end shall be cut square, free of burrs, and without any angled or uneven cuts.
III. Installation Procedure
Tube Preparation
Use a dedicated tube cutter to cut the stainless steel tubing. After cutting, remove all internal and external burrs from the tube end and blow out any debris inside the tubing to prevent contaminants from entering the sealing area and causing damage or leakage.
Install the Ferrule Components
Install the components onto the tubing in the following order: Nut → Back Ferrule → Front Ferrule.
Pay special attention to the orientation of the ferrules. The cutting edge of the front ferrule must face the fitting body. Never install the ferrules in the wrong direction (Figure 1).
Tube Insertion
Insert the tubing completely into the fitting body until it firmly contacts the shoulder stop. Ensure that the tubing is fully inserted without any offset or misalignment (Figures 2 and 3).
Tightening the Fitting
First, tighten the nut by hand until resistance is felt. Then hold the fitting body with one wrench while tightening the nut with another wrench. For tubing with an outside diameter of 4 mm or smaller, tighten an additional 3/4 turn after finger-tight. For standard tubing with an outside diameter of 6 mm or larger, tighten an additional 1-1/4 turns (Figure 4).
Pressure Holding Test
After tightening, gently move the tubing by hand to verify that it does not loosen. If the tubing remains secure, perform the pressure holding test. Maintain the system pressure for 30 minutes. If there is no pressure drop and no bubbles appear after applying clean water to the fitting connection, the installation is considered acceptable.
IV. Installation Precautions
Never disassemble or tighten tube fittings while the system is under pressure. Doing so can easily damage the metal sealing surfaces, and high-pressure gas may escape and cause personal injury. Always remember that pressurized gas is more dangerous than water.
Since tube fittings and components from different manufacturers have different dimensions and designs, components from different brands must never be mixed.
Do not apply excessive force or over-tighten the nut, as this may crack the ferrules or tubing wall and result in gas leakage.
When tightening, use two wrenches to apply balanced force. Never apply force from only one side, otherwise the sealing surfaces may be damaged and the fitting may lose its sealing performance.
V. Basic Troubleshooting
The following are common installation problems:
Minor Gas Leakage: Check whether burrs remain on the tube end or whether the fitting has been tightened sufficiently to achieve the required seal. If leakage is caused by these issues, rework the tube end and slightly retighten the nut.
Severe Gas Leakage: Check whether the ferrules have been installed in the wrong direction or whether the tubing has been fully inserted into the fitting body. If necessary, disassemble the fitting, inspect all components, and reinstall them after confirming that everything is correct.
Cracked Tubing: Check whether excessive tightening force was applied when tightening the nut. If this occurs, the tubing and both ferrules must be replaced with new components before reinstalling the connection.
Although double ferrule tube fittings eliminate the need for welding and other complicated installation procedures, it is still essential to follow the correct installation procedures and precautions. Proper installation helps prevent gas leakage, tubing damage, and cracking caused by improper assembly.
