Mechanical Joint vs. Flange

Sarah
DeGuzman

Determining when to use a mechanical joint vs. a flange is an essential consideration for piping engineers. In this article, you’ll learn the fundamentals of a mechanical joint connection, the use of flanges, and receive guidance on when to use a mechanical joint vs. a flange.

Mechanical Joint

Mechanical Joint
Courtesy: Plant Services

A mechanical joint, sometimes referred to as MJ, is a compression joint designed for underground connections for pipes, valves, and fittings

Function

Mechanical joints allow for some joint deflection while maintaining a seal under pressure. This type of connection allows for some misalignment of the two connecting pieces due to ground settling or other installation difficulties.

Installation

Installation does not necessitate any mechanical expertise. The following steps ensure a secure connection.

  • First, obtain the components that require fastening. This may include a mechanical joint bell on the pipe, valve, or fitting, and a mechanical joint gland, gasket, and tee-head bolts and nuts.
  • Place the gland and gasket into the MJ bell of the connecting pipe, valve, or fitting at the spigot end of the line.
  • Insert the tee bolts into the gland and bell. The gland and bell come together as the bolts tighten, squeezing the rubber gasket between them. The gasket compresses, causing it to push tightly against the pipe and form a seal. For 2′′ to 24′′ fittings, an MJ seal rates for 350 psi, and for 30′′ to 36′′ fittings, rates for 250 psi.

Application

Composite structure assemblies always use mechanical joints because of their cost efficiency and accessibility. However, the use of mechanical joints involves the drilling of holes and installation of fasteners.

Flange

Stainless steel round bar
Courtesy: Viha Steel

Piping components bolt together between flanges

Function

Flanges connect pipes and connect valves, fittings, and specialist objects like strainers and pressure vessels. In fact, flanges are the second most used joining method after welding. They’re well-known, dependable, cost-effective, and widely available from a variety of vendors.

Furthermore, when compared to other mechanical connectors, flanges have an extensive moment-carrying capability. This is a critical feature for systems that experience pipe wandering or lateral buckling due to temperature and pressure changes (e.g., deep water lines). Flanges fulfill a variety of criteria, including high-temperature and corrosion resistance. 

Installation

Many different configurations of flanges exist – from socket weld to threaded and slip-on welded connections. Most commonly, flanges possess a weld neck that allows the flange to weld directly to the in-line pipe.

ZORO SELECT 120-050-000 Weldneck Raise Face,Carbon Steel,285 psi
Courtesy: Grainger

A flanged joint bolts to another flange and seal with a gasket. The gasket is specially chosen to ensure a tight seal. Flanged joints produce by bolting two flanges together and sealing them with a gasket.

Most frequently flanges are fabricated to published standards. For rare custom jobs that involve high pressures, flanges may be fabricated to custom specified dimensions. ANSI (American National Standards Institue) provides that most commonly used dimensional, pressure, and temperature standards for flanges.

The temperature and pressure that a connection sees during operation determines the flange size.

Applications

Many engineering and plumbing applications require the use of flanges. Flanges and their associated piping are common in the transport, manufacturing, power generation, and chemical industry, to name a few. Flanges come in various types and shapes, and their materials vary depending on the fluid and service environment (low carbon steel, low alloy steel, stainless steel, high alloy steel, non-ferrous steel, and so on).

When To Use Mechanical Joint vs. Flange

Underground connections for pipes, valves, and fittings provide the primary use case for mechanical joints. . The mechanical joint, functioning either as a non-restrained or retrained joint, uses a wedge action retaining gland. The compression of the gland against the bell wedges the gasket, establishing a watertight seal.

Advantages of Mechanical Joint

  • No thermal or structural transformation of workpieces.
  • Numerous choices of material.
  • High strength capacity.
  • Mobility of material after joining.
  • Easy to control the quality.
  • Good environmental behavior – no emission or pollution.

Disadvantages of Mechanical Joint

  • It can be difficult or impossible to repair joints.
  • Many rivets may mean higher costs.
  • Typically not finanically viable for above-ground applications.

Meanwhile, the flange provides a stiff joint that does not deflect or expand in any way. This property does not bode well for underground installations where shifting occurs quite frequently. A ground shift could damage the integration of the joint. The watertight seal forms by tightening the bolts and compressing the gasket between the two flanges.

Advantages of Flanges

  • Painting and blasting are not necessary, although usually required by specficiation.
  • Hot work permitting may not apply in the case of threaded connections.
  • Numerous piping spools can be manufactured and shipped loose if necessary.

Disadvantages of Flanges

  • It is expensive to insulate flanged pipes.
  • More space is needed for flanged pipe systems.
  • A flange connection can leak.

The choice of whether to use a mechanical joint vs. a flange depends on client specifications, project requirements, and the application. Engineers should carefully consider the advantages and disadvantages of each type of connection prior to installation.