A Simple Guide to Master Bellows and Expansion Joints, and Why we need them?

The Bellows and Expansion Joints for Piping and Equipment are one of the foremost factors in designing a safe operation of any plant, refinery, or processing unit. Moreover, the very existence of the bellows is to take into consideration several common factors like low or high pressure, extreme temperature, and flow characteristics during operation. If expansion joints are removed from the piping systems, high stresses would be created by the thermal expansion. This can cause huge pressure resulting in the crack of the welds and bending of the pipes.

Additionally, the bellows are designed to be able to withstand the pressure and temperature changes for the life of the plant.

What is an Expansion Joint?

Expansion joints are structural components that separate adjacent pipes or sections of piping or parts of equipment that are exposed to different pressure or temperature conditions.

The expansion joints are a crucial component for any piping or equipment, whether that be any process media, air , water, or steam.

They can be universal bellow, lateral bellow, angular bellow, wall seals, or Special types.

What is the purpose of Bellow Expansion Joints?

The Bellows are the flexible element of the Expansion Joint. 

They are installed in the expansion gap and are used to absorb the movement between the two elements. They are installed to absorb the circumferential pressure and longitudinal deflections for the designed operations.

Plays a vital role in safeguarding equipment or piping. Depending on design requirements, bellows can be Single Bellow Joints, Universal Bellow Joint, Hinged Angular Bellow and Angular Bellow types.

Additionally, bellow joints provide a safe space for the piping and equipment to move with the least amount of friction. The bellows and expansion joint must be compatible with the pressure and temperature range.

Moreover, the bellows and expansion joint must be capable of providing a leak-free joint and not create any unwanted vibration.

Furthermore, they address the main concerns in any industry, like abnormal dimensions, high temperatures, corrosive media, and large movements.

What are the Type of Bellows?

There are three main types of Bellows, formed bellows, welded bellows, and electroformed bellows. We will be discussing the Formed Types basically the Convoluted/Sylphons in this article.

In general, for piping or any equipment, the Bellows are manufactured based on requirements such as Metallic Bellows, Fabric Bellows, Rubber Bellows, and Flexible Corrugated Hose assemblies with wire braiding.

Furthermore, the Metallic Bellow Expansion Joints are the most recommended in Oil and Gas Industries, though other types are also used based on service requirements. Moreover, they should follow recommendations stated in the referenced codes like ASME 31.3, API or EJMA standards.

In brief, there are three main types of bellows used in construction/design:

Axial Bellows for longitudinal axial compression and expansion. Angular Bellows for angular expansion joints, Lateral Bellows for lateral movements. They are generally installed at the junction of two pipes or sections of piping.

What are the requirements for a bellow and the basis for bellow selection?

Additionally, based on design requirements, Metallic Bellows can be thin wall Axial Bellows, Universal Bellows, Pressure balanced bellows, Hinge Bellow, Gimbal bellows, Dismantling joints, or slip type bellows.

What is the material of construction of Bellows?

Also, the Material of Construction for metallic bellows may be Stainless Steel, Hastelloy, Incoloy, Inconel, Monel, Duplex Stainless Steel, rubber, nitrile, PVC/EDPM -based on service requirements.

Furthermore, the main factors for consideration of applicable materials include:

1. Process Requirements like corrosiveness, temperature

2. Flexibility, Yield Strength and Fatigue Resistance

3. Tightness and Pressure Resistance

4. Service Life

Moreover, metallic bellow joints are mostly manufactured by hydraulic forming, roll forming or any other method specified in latest edition of EJMA Standards.

What are the bellow parts, and how do bellows function?

Let’s start by understanding the function of metal corrugated bellow, in general. 

Mostly, metal bellows are thin-walled cylindrical components with a corrugated structure that is perpendicular to the cylinder axis. The design varies based on equipment to be safeguarded, like the HSRG Boilers, where rectangular type bellows are normally preferred.

The inside diameter is generally the cylinder internal diameter and the outside diameter includes the whole convolution height.

The corrugations as a whole are completely convoluted in length. The top of corrugate is denoted as Crest, with the side as Sidewall and connecting member to the cylinder at neck, as the Root. 

With this unique corrugated structure, the bellows are highly flexible during axial, lateral, and/or angular deformation.

Bellow designs are mostly pressure-resistant, pressure-tight, temperature, and corrosion-resistant, as well as torsion-resistant.

What are the common defects and how to repair bellow expansion joints?

Well, in most cases, below expansion joints are replaced as is. Expansion Joints are mostly prohibited from repairs or any hot works and mechanical activities. Hence, they are mostly in the bolted arrangements.

However, due to some limitations and requirements, procedures are developed by Industries for repairs to the Expansion Joints or Bellows. These involve mild repair welds to the corrugations without damaging the sheet or the expansion joint connections.

What are the Damages and Testing for Expansion Bellows?

Damages on the Expansion Joint bellows may be due to the wrong design/process changes that need to be evaluated before purchasing a replacement. Furthermore, each bellow has a Service life beyond which Bellows are not

Also, during the handling and installation of expansion joints, great care should be taken while slinging. An expansion joint should never be slung with bellows corrugations/external shrouds, tie rods, angles, etc. An expansion joint should preferably be slung on the end pipes/flanges or on the middle pipe.

For metallic bellows, normally, the NDT requirements like DPT and UT thickness gauging are advised. Rubber/Nitrile or PVC/EDPM are visually inspected and are mostly replaced in case of any damage. Hydrostatic Tests are also done as per job specification criteria norms.

Additionally, Stifness Tests and Type Tests like Cycle Life, SQUIRM AND Yield Rupture Tests are also considered based on the design requirements.

Reference Code & standard for Bellows

STANDARDS OF THE EXPANSION JOINT MANUFACTURERS ASSOCIATION, INC. (EJMA)