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Hydraulic Fittings in The Chemical Industry

Hydraulic fittings, used in conjunction with couplings, connect straight pipe or tubing segments. Fittings are secured to the segment ends, while couplings mate the segments together. In the chemical industry, hydraulic fittings are designed for much greater pressures and temperatures as well as more demanding environments than those fittings applied in non-industrial piping, tubing or residential plumbing systems. Specifically, hydraulic fittings for the chemical and petrochemical industry are usually resistant to corrosion, chemicals, blowouts, static charges and, in some cases, explosions. They are constructed with precise dimensions, high tensile strength and are considered to be low maintenance devices.

Function and Types

Beyond serving as a simple, piping or tubing connecting device, fittings fulfill several functions during the construction of piping systems.

These functions include:

  1. Enabling the connection of different types of fittings together, such as o-ring, flared/JIC, threaded, among others
  2. Assist in connecting parts of different orientations, such as, 90o, 45o or straight
  3. Connect bulkhead fasteners, fixtures or other hardware and finally
  4. Function as a fast disconnect.

There are three general categories of fittings: flared, compression (or flareless) and permanent. Flared fittings are commonly used in tube-type, hydraulic systems, but are also used in underground gas piping systems. They offer good protection against leakage since they are fitted against the flared end of a pipe, hose or tube, to provide holding tension. Compression fittings, such as bite-style, are common in the chemical industry for pressure piping with high stresses. The o-ring fitting is a compression fitting that creates an impermeable watertight seal by using a small, rubber o-ring gasket that is compressed when the fitting is assembled on the pipe or tubing. O-ring fittings avoid the problem of over-tightening, which can create leakage problems as the tension between components relaxes over time. Permanent fittings are brazed, sweated or welded. Brazed fittings are typically applied when piping temperatures do not go over 425oF and cold, line pressures don’t go over 3,000 psi.  Sweated or soldered fittings are generally used in residential plumbing systems to connect copper pipes; they are not a strong joint, therefore, should not be used in high pressure or high temperature applications. Welded fittings are generally used on large piping systems and are most suited for high temperature, high pressure and applications where vibrations exist. 

Materials: Special Requirements of the Chemical Industry

To adapt to the special requirements of the chemical industry, such as high temperature, pressure and hazardous environments, hydraulic fittings are manufactured with materials that can withstand these specific requirements to ensure good performance, low maintenance and a long service life.

Carbon steel, stainless steel and brass are the most common materials used for hydraulic fittings in this industry. For most hydraulic oil applications, carbon steel fittings with a parkerized-finish are a common way to combat corrosive environments while aiding the assembly process. Parkerizing, sometimes called phosphating and phosphatizing, is a process that creates a very thin, corrosion-resistant external layer that has a fine amount of sealant oil enclosed within the fitting’s skin. This process both resists corrosion as well as lubricates the parts when the fitting is connected on a pipe.

For corrosive environments, brass or stainless steel is used instead of carbon steel. Brass fittings are a common choice for medium-corrosive environments. For a combination of corrosion resistance and high tensile strength, stainless steel fittings with nickel-plating are common in fluid systems of the chemical industry, process engineering and offshore applications.

Due to the excessive pressures endured in the chemical industry, the type of fitting varies. Flared fittings are used in medium pressure applications. O-ring, compression-style fittings are used in high pressure environments for leak-free purposes. But for excessively high pressures of over 20,000 psi, stainless steel combined with autoclave-type, metal-to-metal, cone and threaded connections are presently used.