How to Select the Correct Expansion Joint Design for a Fixed Tubesheet PTFE Exchanger Shell?

A fixed-tubesheet PTFE heat exchanger is mechanically simple and frequently economically attractive, but the design involves a basic thermal conflict. The PTFE tubes expand strongly at increasing temperature, but the surrounding steel shell expands very little. If movement is limited, the mismatch can cause tremendous axial stress. Some part of the exchanger must bend to absorb this differential expansion. The answer is a metal bellows expansion joint welded into the shell - a flexible, accordion-like segment that expands and contracts as the PTFE tube bundle thermally breaths during operation.

The bellows assembly is the single most important mechanical component in the whole exchanger system for any expansion joint fixed tubesheet PTFE exchanger selection.

Why Expansion Joints are Needed on PTFE Exchangers
PTFE's Extreme Thermal Expansion
The thermal expansion coefficient of PTFE is much higher than carbon steel or stainless steel. The PTFE tubes can expand several times the magnitude of the shell construction around them with changes in operation temperature.

Without a flexible compensation system, significant issues may arise:

Distortions in the tubesheet

Tube rupture

Excessive axial forces

Cracking the shell

Weld fatigue

Leakage failures

The expansion joint is the zone of controlled movements and does not allow the concentration of the thermal stresses in the exchanger body.

The Bellows as a Flexible Pressure Boundary
The bellows are the metal lungs of the exchanger, breathing in and out with each thermal cycle, yet maintaining the integrity of the pressure barrier. As long as there is proper flexibility, the PTFE tube bundle can expand and contract naturally without over-stressing the shell.

This adaptability needs to be engineered deliberately, rather than mimicked with generic piping gear.

Key Design Criteria for the Choice of Expansion Joints
Full travel on the axial
The most critical parameter in expansion joint sizing is the ability to move axially. The bellows must accommodate safely the total estimated thermal expansion due to the PTFE tubes over the entire operational temperature range.

Typical design calculations take into account:

Length of tube

Thermal Expansion Coefficient of PTFE

Startup temperature differences

Contraction at shutdown

Process upset situation

Maximum Design Temp (MDT)

Too short a journey may overstretch the bellows or produce failure through compression fatigue.

Why We Like Thick-Walled Single-Ply Bellows
Requirements for resistance to pressure
For fixed-tubesheet PTFE exchangers, the bellows are subjected to internal shell pressure but are also capable of movement. Thin, multi-ply bellows, as often employed in low pressure piping systems, may not be sufficiently strong for this purpose.

For this reason often thick-wall single-ply bellows designs are adopted.

Benefits include:

Increased structural strength

Better Pressure Containment

Increased corrosion allowance

Better damage resistance handling

Lower susceptibility to ply separation

The bellows assembly is not just a flexible connector. It is a section of a pressure vessel that is simultaneously subjected to heat, pressure and fatigue loads.

Selection of Bellows Material
Why 316L stainless steel is so common
The most popular material for exchanger shell bellows is 316L stainless steel since it offers a mix of:

Resistance to corrosion

Welding skills

Fatigue characteristics

Availability:

Mechanical bendability

Atmospheric corrosion resistance is then a crucial factor as the bellows are normally on the external shell side rather than in direct contact with aggressive process chemistry.

External Corrosion Hazards
The plant environment where the bellows are located may expose them to:

Acid fumes

Salt-heavy humidity

Wash down chemicals

* Spills Procedure

Contamination with chloride

In particular trace chlorides are very harmful as they can cause external stress corrosion cracking of stainless steel bellows under tensile stress.

The material selection process so needs to consider both the working process and the environmental exposure around the exchanger installation.

Design of Fatigue Life and Convolution
The Role of Convolution Geometry
Convolutions are the corrugations that compress and extend with heat movement to create the bellows flexibility.

The design shall consist of:

Number of convolutions

Convolution height

Pitch shape

Thickness of walls

Axial spring constant

Too few convolutions may cause too much stiffness and too many convolutions may cause less pressure capability and fatigue resistance.

Thermal Cycling Problems
Exchangers that undergo numerous start-ups, shutdowns, or batch process conditions are subjected to repeated temperature cycling causing cyclic strain to the bellows material.

For high-cycle applications, fatigue resistant bellows designs are necessary.

Other engineering measures may include:

Optimized shape of the convolution

Reduced stress concentration areas

Improved control of weld quality

Restrictions on movement

Sections of reinforced collars

Fatigue failure usually occurs progressively by crack initiation at the severely stressed convolution roots.

Conformity to EJMA and ASME Standards
EJMA Design Criteria
Expansion joints in pressure retaining exchanger shells shall be designed in accordance with the criteria specified by the Expansion Joint Manufacturers Association (EJMA).

EJMA approach provides suggestions regarding:

Stress calculation

Analysis of fatigue

Pressure rating

Restrictions on Movement

Prediction of spring force

Life cycle prediction

ASME Appendix 26
In pressure vessel applications, ASME Appendix 26 may also guide the bellows design and allowed stress evaluation.

Use of established design codes helps to assure that the expansion joint is considered a fully engineered pressure component and not an accessory item.

Protection of Bellows from Mechanical Damage
The significance of external shrouds
External mechanical damage to bellows convolutions from tools, foot traffic, falling items or maintenance work might cause vulnerability.

Therefore the bellows group should be fitted with an external protective shroud.

The shroud protects against:

Damage from impact

Accidental deformation

Accumulation of debris

Weather exposure

Personnel contact.

Physical protection is vital for long term reliability as small denting or deformation can greatly diminish fatigue life.

Why Off-The-Shelf Expansion Joints Don't Work
Engineering Requirements Customised
A fixed-tubesheet PTFE exchanger leads to very specialized loading conditions of:

High thermal displacement

Pressure »

Cyclical loading

Corrosive environment exposure

Mechanical shaking

Off-the-shelf piping expansion joints seldom are adequate in service because they are not intended to meet the combined movement and pressure requirements of PTFE exchanger service.

Instead, each bellows assembly should be constructed to tailor suit the exchanger geometry and operating conditions.

Summary
The expansion joint is the ingenious constructed flexible link that overcomes the basic thermal stress dilemma with a fixed-tubesheet PTFE heat exchanger. The bellows compensates in a safe manner for the large temperature growth difference between the PTFE tubes and the surrounding steel shell and prevents damaging mechanical loading on the exchanger whilst maintaining pressure integrity.

The choice of a fixed tubesheet PTFE exchanger with a properly sized expansion joint is the result of careful consideration of axial travel capacity, wall thickness, convolution geometry, fatigue resistance, corrosion resistance and adherence to EJMA or ASME design standards. Equally crucial to long-term service life are proper external protection and environmental compatibility.

One of the most dependable machines in thermal equipment engineering is often the one that is intended to accommodate its own internal movement with grace, rather of resisting it stiffly.\