Introduction to WRAS Approved Expansion Bellows
Thermal expansion and contraction of pipework is an unavoidable consequence of temperature changes in any water supply system. As water temperature rises and falls, the pipes carrying it expand and contract accordingly, creating potentially significant mechanical stresses at fixed connection points, valves, and equipment. WRAS approved expansion bellows are specifically engineered to absorb this thermal movement, protecting pipework and connected equipment from damage while maintaining the integrity of the potable water containment. This guide explains how WRAS approved expansion bellows work, where they are used, and how to select and install them correctly in UK potable water systems.
What Are WRAS Approved Expansion Bellows?
WRAS approved expansion bellows, also known as flexible connectors or compensators, are flexible pipe sections designed to absorb axial compression and extension, lateral offset, angular deflection, and vibration between fixed pipework connections. They consist of a corrugated inner element (the bellows) that flexes to accommodate movement, typically enclosed within a braided outer sleeve for pressure containment, and terminating in flanged or screwed end connections to attach to the pipework system.
The WRAS approval of expansion bellows confirms that all materials in contact with the potable water, including the inner bellows material, any internal liners, and end fittings, have been independently tested and certified as safe for drinking water service in the UK. This certification is essential for expansion bellows installed in potable water systems, ensuring compliance with the Water Supply (Water Fittings) Regulations 1999.
Types of WRAS Approved Expansion Bellows
WRAS approved expansion bellows are available in several construction types, each suited to different movement requirements and operating conditions.
Rubber WRAS Approved Expansion Bellows
Rubber WRAS approved expansion bellows are the most widely used type in building services and commercial water supply applications. The flexible rubber body provides excellent vibration absorption and can accommodate axial, lateral, and angular movement. WRAS approved rubber expansion bellows are typically constructed with an EPDM inner layer for potable water compatibility, reinforced with multiple plies of fabric or wire braid, and finished with an outer weather-resistant rubber cover. They are available with flanged end connections in sizes from 32mm to 300mm and larger.
Rubber WRAS approved expansion bellows are the standard specification for isolating pump vibration from connected pipework, for connecting pipework to equipment such as boilers, heat exchangers, and water treatment plant, and for accommodating differential settlement between structures. Their WRAS approved EPDM inner construction ensures they are safe for potable water service in cold water systems and low-temperature hot water systems up to approximately 80°C.
Stainless Steel WRAS Approved Expansion Bellows
Stainless steel WRAS approved expansion bellows use a corrugated stainless steel bellows element to accommodate thermal movement. Unlike rubber types, stainless steel WRAS approved expansion bellows can handle higher temperatures and pressures, making them suitable for hot water systems and steam applications. The stainless steel construction provides excellent corrosion resistance and long service life in aggressive water conditions. WRAS approved stainless steel expansion bellows are available in flanged and threaded connections and are used in hospital hot water systems, industrial process water pipework, and high-temperature building services applications.
PTFE-Lined WRAS Approved Expansion Bellows
PTFE-lined WRAS approved expansion bellows incorporate a smooth PTFE inner liner within a rubber or stainless steel outer bellows. The PTFE lining provides exceptional chemical resistance and a hygienic, non-stick surface that prevents biological growth and facilitates cleaning. PTFE-lined WRAS approved expansion bellows are specified in food and beverage production facilities, pharmaceutical water systems, and other applications where hygiene is paramount and water chemistry may be aggressive.
Key Applications of WRAS Approved Expansion Bellows
WRAS approved expansion bellows fulfil several important functions in potable water systems.
Pump Vibration Isolation
The most common application for WRAS approved expansion bellows in potable water systems is the isolation of pump vibration from connected pipework and building structure. Pumps generate vibration through motor and impeller imbalance, cavitation, and hydraulic forces. Without WRAS approved expansion bellows on both the inlet and outlet connections, this vibration transmits through the rigid pipework and can cause fatigue failures at pipe joints, noise complaints in occupied buildings, and accelerated wear of pump bearings. WRAS approved expansion bellows absorb vibration energy and significantly reduce transmission to the building structure and pipework.
Thermal Movement Accommodation
Hot water supply and heating system pipework experiences significant thermal expansion as system temperatures vary. Without WRAS approved expansion bellows or expansion loops, this movement creates mechanical stress at fixed points such as boiler connections, pump flanges, and pipe anchors. Over time, these stresses can cause joint failures, valve damage, and pipe distortion. WRAS approved expansion bellows installed at appropriate intervals absorb this thermal movement, protecting the entire pipework system from thermally induced stress.
Equipment Connection Flexibility
Connecting rigid pipework directly to equipment such as water treatment plant, heat exchangers, and storage tanks creates problems when the equipment requires replacement or maintenance, as the rigid pipe cannot flex to accommodate movement during disconnection. WRAS approved expansion bellows installed in these connections provide the flexibility needed for easy equipment removal and replacement. They also accommodate the minor misalignment that often exists between equipment connections and pipework, preventing stress concentration at connection points.
Seismic and Settlement Protection
In areas prone to seismic activity or where differential settlement between building elements is anticipated, WRAS approved expansion bellows provide critical protection for pipework connections. The flexibility of the bellows element accommodates the relative movement between connected building elements without transferring the resulting forces into the pipework system. This prevents the catastrophic pipe failures that can result from earthquake-induced movement in rigid pipework systems.
Selection Criteria for WRAS Approved Expansion Bellows
Selecting the correct WRAS approved expansion bellows for a given application requires careful analysis of the movement requirements and service conditions.
Movement type and magnitude must be accurately determined before selecting WRAS approved expansion bellows. Axial compression and extension, lateral offset, and angular deflection each impose different stresses on the bellows element, and the ratings for each movement type must not be exceeded. Combining multiple movement types simultaneously further reduces the allowable movement in each axis. Refer to the manufacturer's published movement ratings for WRAS approved expansion bellows and calculate the required movement in each axis based on the pipe system thermal analysis.
Operating pressure and temperature must be within the rated performance envelope of the selected WRAS approved expansion bellows. Rubber WRAS approved expansion bellows are typically rated to PN16 (16 bar) at temperatures up to 80°C for EPDM construction. Stainless steel WRAS approved expansion bellows can accommodate higher pressures and temperatures up to 300°C or beyond depending on design. Always confirm that the selected WRAS approved expansion bellows can meet the system's maximum pressure and temperature requirements with appropriate safety margins.
Pipe size and connection type must match the existing or planned pipework system. WRAS approved expansion bellows are typically available with flanged connections conforming to PN10, PN16, or ANSI standards, and in screwed connections for smaller sizes. Confirm the flange standard and bolt circle dimensions before ordering WRAS approved expansion bellows to ensure compatibility with existing flanges.
Chemical compatibility of the bellows material with the water quality and any treatment chemicals used in the system must be confirmed. EPDM rubber is resistant to most water treatment chemicals including chlorine, but may be affected by certain aggressive chemicals used in specialist water treatment. Confirm compatibility with the manufacturer of the WRAS approved expansion bellows for applications involving unusual water chemistry.
Installing WRAS Approved Expansion Bellows
Correct installation of WRAS approved expansion bellows is essential to ensure they perform as intended and achieve their rated service life.
Install WRAS approved expansion bellows in the neutral (undeflected) position wherever possible. Pre-compressing or pre-extending the bellows during installation reduces the available movement capacity and may invalidate the manufacturer's warranty. Ensure that the connected pipework is properly supported and guided so that all movement is directed through the WRAS approved expansion bellows rather than creating stress concentrations elsewhere in the system.
For pump vibration isolation applications, install WRAS approved expansion bellows as close to the pump connections as possible. Excessive pipe runs between the pump and the WRAS approved expansion bellows allow vibration to transmit through the rigid pipe section, reducing isolation effectiveness. Ideally, the WRAS approved expansion bellows should be the first fitting on both the suction and discharge connections of the pump.
Do not use WRAS approved expansion bellows as the sole means of restraining the thrust forces generated by system pressure. Internal pressure in a flexible bellows creates axial thrust forces that will extend the bellows and may cause system distortion or failure if not properly restrained. Use tie rods, anchor points, and pipe guides to control the thrust forces generated by system pressure, allowing the WRAS approved expansion bellows to accommodate only movement and vibration.
Maintenance of WRAS Approved Expansion Bellows
WRAS approved expansion bellows require periodic inspection to confirm they remain in good condition and continue to provide effective movement accommodation and vibration isolation.
Inspect WRAS approved expansion bellows annually for signs of external deterioration. Check rubber types for surface cracking, blistering, or evidence of oil contamination from external sources. Check stainless steel types for corrosion staining, particularly at the bellows convolutions where crevice corrosion can initiate. Inspect the end flanges and all connections for leakage. Any visible damage or leakage indicates that the WRAS approved expansion bellows should be replaced promptly.
Replace rubber WRAS approved expansion bellows proactively at the end of their expected service life, typically 10 to 15 years depending on operating conditions. Rubber bellows that remain in service beyond this period are at increased risk of sudden failure, particularly in hot water applications where elevated temperatures accelerate rubber degradation. Planned replacement of WRAS approved expansion bellows is far less disruptive than emergency repair following an in-service failure.
WRAS Approved Expansion Bellows from Wras Valves
Wras Valves stocks WRAS approved expansion bellows for potable water system applications, including rubber flanged types for pump vibration isolation and thermal movement accommodation in cold and low-temperature hot water systems. All expansion bellows in our range carry WRAS approval documentation confirming their suitability for contact with drinking water in UK potable water systems. Contact our technical team to discuss your specific movement accommodation requirements and receive guidance on the most appropriate WRAS approved expansion bellows specification for your application.
Conclusion
WRAS approved expansion bellows play a vital but often underappreciated role in protecting potable water systems from the damaging effects of thermal movement, vibration, and differential settlement. By absorbing these forces and movements before they can damage pipework, valves, and connected equipment, WRAS approved expansion bellows extend the service life of the entire water supply system and reduce maintenance costs over its operational life. Their WRAS approved status ensures full compliance with UK water regulations and provides the documented evidence required for building control sign-off. For the full range of WRAS approved expansion bellows available from Wras Valves, visit our website today.