Technical, operational and practical analysis

Modern industrial, power and district heating installations use two basic methods of joining pipelines and fittings: welded connections and flanged connections.

Both solutions have their own advantages, limitations and areas of application. In engineering practice, there is no universal solution for all cases. However, in modern district heating systems, especially in pre-insulated networks laid directly in the ground, welded solutions and fully welded valves are increasingly preferred.

This is primarily due to requirements regarding: durability, tightness, reducing service work, operational safety, and minimising the cost of maintaining the infrastructure.

Characteristics of flanged connections

A flanged connection consists of:

• two flanges
• bolts and nuts
• a gasket
• appropriate mechanical clamping force
  

The biggest advantage of a flanged connection is the ability to:

• install it easily
• disassemble it quickly
• replace equipment
• carry out service work without cutting the pipeline
  

That’s why flanged connections remain essential in many parts of an installation, particularly for:

• pumps
• heat exchangers
• filters
• fittings requiring periodic servicing
• measuring devices
• process installations with frequent upgrades

Limitations and risks of flanged connections

Despite their widespread use, flanged connections carry a number of potential sources of operational problems.
Risk of leakage:

The tightness of a flanged connection depends simultaneously on:

• gasket quality
• the quality of the sealing surfaces
• correct flange alignment
• correct bolt tightening torque
• installation quality

Even small installation errors can cause:

• seepage of the medium
• micro-leaks
• loss of tightness during temperature and pressure changes
• degradation of the thermal insulation

Material creep and stress relaxation

In high-temperature installations, the following are of key importance:

• bolt creep
• stress relaxation
• gasket settling
  

These phenomena cause a gradual loss of gasket clamping force, which can lead to the connection losing its seal after a longer period of operation.

This problem is particularly significant in:

• district heating
• power generation
• steam installations
• large DN diameters
• systems operating cyclically

Greater service requirements

Flanged connections often require:

• periodic inspection
• checking tightening torques
• gasket replacement
• bolt condition checks
• checking sealing surfaces for corrosion
  

In practice, this means:

• higher operating costs
• a greater number of potential failure points
• the need for periodic service work
  

Corrosion and environmental impact

Flanged connections are more susceptible to:

• crevice corrosion
• bolt degradation
• insulation moisture ingress
• corrosion under insulation
• bolts seizing after many years of operation
  

In pre-insulated systems, every additional external element also makes it more difficult to achieve effective thermal insulation.

Characteristics of welded connections

Unlike a flanged connection, a welded connection:

• does not use a gasket
• does not require bolts
• does not require maintaining mechanical clamping force
• has significantly fewer components that can wear out
  

As a result, welded connections work very well in installations that are:

• high-temperature
• high-pressure
• hard to access
• intended for many years of operation without service intervention
  

Advantages of welded connections

Well-executed welded connections are characterised by very high operational reliability.

A correctly made weld:

• eliminates the risk of losing clamping force
• reduces the possibility of micro-leaks forming
• ensures very high connection durability

Lower service requirements

Welded connections:

• do not require periodic retightening
• do not require gasket replacement
• reduce the amount of maintenance work
• reduce the risk of service errors
  

This is particularly important in underground installations, where access to the pipeline after backfilling is difficult and costly.

Better compatibility with pre-insulated systems

In modern pre-insulated networks, welded connections:

• are easier to insulate
• reduce thermal bridges
• reduce the risk of insulation moisture ingress
• improve the durability of the entire system

That’s why modern district heating systems are increasingly using:

• welded pipelines
• fully welded valves
• a minimal number of flanged connections
  

Limitations of welded connections

Welded connections also have their own requirements and limitations.

They require:

• qualified personnel
• appropriate welding technology
• weld quality control
• non-destructive testing (NDT)
• proper material preparation

Additionally:

• disassembling the connection is difficult
• repairs usually require cutting out a section of the installation
• upgrading the system is more labour-intensive
  

That’s why welded connections do not completely replace flanged connections.

Where flanged connections are more advantageous?

Flanged connections have a very important advantage in situations where components made of different materials are being joined.

This applies, among others, to connections between:

• pump housings
• heat exchangers
• cast components
• specialist fittings
• cast iron or cast steel elements with typical pipelines made, for example, of P235GH steel

In such cases, a flanged connection is often the solution that is:

• safer
• more practical
• technologically easier

A flanged connection:

• eliminates problems associated with welding different materials together
• reduces the risk of cracks caused by differing thermal expansion
• reduces the risk of damaging the device housing during welding
• allows for easier disassembly of the device
• helps limit thermal stresses in castings

For example: a pump housing can be made of cast iron or cast steel, while the pipeline and valve are made of P235GH steel. Making a direct welded connection between such components can be technologically difficult or not recommended by the equipment manufacturer. That’s why in industrial practice the following rule is very often applied:

• linear installation – mainly welded connections,
• process and serviceable equipment – flanged connections.

Conclusions:

Welded and flanged connections are not competing solutions – they complement each other.

However, in modern district heating and pre-insulated installations, where the following are of key importance:

• durability
• tightness
• reducing the number of failures
• minimising servicing
• operational safety

welded connections are currently the preferred solution for the main pipeline sections and linear fittings.

Flanged connections, in turn, remain essential:

• for equipment requiring disassembly
• for connections between different materials
• in places requiring easy service access

In practice, the best results are achieved by consciously combining both technologies and using each of them exactly where it performs best.