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How To Selection a Proper Tube Fitting

There are a variety of tube fittings.Selecting the proper fitting is important. This selection process is driven by the following fluid system parameters: size, temperature, application, media, and pressure, often referred to as the acronym STAMP. For example:Metal fittings are used for high temperature and pressures, plastic fittings for low.

Temperature and pressure decide which type of tube fitting is needed. Metal fittings are used for high temperature and pressures, plastic fittings for low. Typically, metal fittings are used with metal tubes, but plastic fittings are sometimes used with metal tubes to reduce galvanic corrosion. The fittings used on hydraulic tubing for high-pressure applications are categorized as either permanent or separable. Most separable fittings are threaded.So fitting type, fitting ports, tube size, material, and options and features are important specifications to consider when searching for tube fittings.

Major Advantages of Tubing vs. Pipe

Bending Quality - Tubing has strong but relatively thinner walls; is easy to bend. Tube fabrication is simple.
Greater Strength - Tubing is stronger. No weakened sections from reduction of wall thickness by threading.
Less Turbulence - Smooth bends result in streamlined flow passage and less pressure drop.
Economy of Space and Weight - With its better bending qualities and a smaller outside diameter, tubing saves space and permits working in close quarters. Tube fittings are smaller and also weigh less.
Flexibility - Tubing is less rigid, has less tendency to transmit vibration from one connection to another.
Fewer Fittings - Tubing bends substitute for elbows. Fewer fittings mean fewer joints, fewer leak paths.
Tighter Joints - Quality tube fittings, correctly assembled, give better assurance of leak-free systems.
Better Appearance - Tubing permits smoother contours with fewer fittings for a professional look to tubing systems.
Cleaner Fabrication - No sealing compounds on tube connections. Again no threading; minimum chance of scale, metal chips, foreign particles in system.
Easier Assembly and Disassembly - Every tube connection serves as a union. Tube connections can be reassembled repeatedly with easy wrench action.
Less Maintenance - Advantages of tubing and tube fittings add up to dependable, trouble-free installations.

Types of Tube Fitting

There are a variety of connection methods for tubing applications. Brief descriptions of the most common tube fitting connections are below.

Compression Tube Fittings
The compression tube fitting, which was the first tube fitting to be introduced, is typically made up of three components: nut, body, and gasket ring or ferrule.The disadvantages of this type of connection are that it can withstand only minimal pressure and remake is extremely difficult due to the construction of the sealing ring.

Flare Type Fittings
The world’s most widely used flare fitting is the 37° flare. It consists of a body, sleeve, and nut. The tube end is flared at a 37° angle (from centerline) and held captive between the fitting nose and sleeve with a tube nut. When tightened, a metal-to-metal sealing line forms between the fitting nose and tube end. This sealing method can be used across a wide range of media and temperatures. However, metal-to-metal seals are not as reliable as elastomeric seals found, for example, on O-ring seal fittings.

Plastic fittings
Plastic fittings cannot tolerate extreme heat or high pressures. But they cost less than metal, stop galvanic corrosion, deaden noise, and withstand assault by a wide range of corrosive fluids.Generally, plastic fittings are well suited to low-temperature fluid-transfer applications. At pressures below 500 psi and temperatures below 250°F, plastic fittings are highly competitive with steel, stainless-steel, or brass fittings. Plastic fittings have a pullout resistance ranging from 20 to 300 lb, depending on size, tubing material, and locking devices. Plastic or metal grippers are used to increase pullout resistance. Plastic grippers are used only with plastic tubes, and are so strong that the tubing tears before it pulls out of the fitting. Metal grippers are used with metal tubes, and can resist pullout forces over 300 lb.

Bite-Type Tube Fittings
Bite-type tube fittings may be single or twin ferrule in designs. In the single ferrule bite, the nose of the ferrule performs two functions: bite into the tube to hold it and provide a sealing element for the coupling body. The twin ferrule bite utilizes grip by the back ferrule and seal by the front ferrule.

Mechanical Grip-Type Tube Fittings
Mechanical grip-type tube fittings are typically two-ferrule in design. Fitting pull-up spring loads the front ferrule as it seals by coining the surfaces of the tubing and coupling body.

Proper Way of Tubing Selection

1. Always Match Materials - I.E., S.S. Tubing should be used only with S.S. Fittings. The only exception to this rule is copper tubing with brass fittings. Mixing materials can cause galvanic corrosion.
2. Select proper tubing hardness - Remember Parker Instrumentation Tube Fittings are designed to work within specific hardness ranges. Rb 90 max. for S.S., Rb 80 recommended.
3. Select proper tubing wall thickness - Proper wall thickness is necessary to accommodate accepted safety factors relative to desired working pressures. For details on items 2 & 3 note “Instrumentation Tubing Selection Guide” shown on the following pages.
4. Tubing surface finish - Always select tubing free of visible drawmarks or surface scratches. If possible, cut off any undesirable sections. These “deep” scratches can cause leaks when attempting to seal low-density gases such as argon, nitrogen, or helium.

Choices for tube fitting types include adapter, bulkhead fitting, cap, compression fitting, coupling or connector or hose mender, flexible coupling, 4-way cross, crimp socket, 90 degree elbow, 45 degree elbow, other angle elbow, reducing elbow, expansion joint coupling, ferrule, gland, nipple, offset, plug, concentric reducer, eccentric reducer, return or trap, side outlet elbow, side outlet tee, equal tee, reducing tee, bullhead tee, union, equal wye, and reducing wye. Swivel connections allow one or more fitting ends to rotate or swivel. Fitting ports specifications to consider when searching for tube fittings include configuration, and tube port type selection. The configuration can be all ports are of the same type or multiple port type or adapter. Tube port type choices include NPT male, NPT female, BSP male, BSP female, English straight thread male, English straight thread female, Metric thread male, Metric thread female, barb or rib, compression fitting, face seal, flange, flare, push-to-connect, and weld or braze.

Tube size specifications to consider for tube fittings include fitting size specification, English or fractional size selection, and Metric size selection. The fitting size specification can be based on the inside diameter or the outside diameter. English size selections range from 1/16 to 12 and greater. Metric sizes range from 1mm to 42 mm and greater. Common materials of construction for tube fittings include aluminum, brass, bronze, copper, carbon and alloy steel, stainless steel, and thermoplastic. Common options and features include chromatography fittings, snubber fittings, vacuum tube fittings, biomedical or pharmaceutical fittings, gas fittings, refrigerant fittings, pneumatic tube fittings, hydraulic fittings, and air brake fittings. Sanitary or food grade rated tube fittings are designed and constructed for use with food and beverage applications. Platings such as nickel and chrome are sometimes used on fittings for appearance or corrosion resistance. Pre-insulated fittings include a variety of insulating materials and jacket construction.

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