Moulded rubber bellows with fabric reinforcement, held between two metal flanges, used to absorb pipe movement and to isolate pump vibration from building pipework. Construction and test requirements for the type are given in ASTM F1123.
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A rubber expansion joint is a moulded elastomeric bellows, reinforced with fabric plies, clamped between two metal flanges. The arch flexes to absorb axial movement, lateral offset and angular deflection, and because rubber transmits far less structural noise than steel it is the usual choice for isolating pump vibration from building pipework.
ASTM F1123, the published specification for arch-type non-metallic expansion joints, requires an elastomeric tube reinforced with multiple plies of woven cloth or tyre cord - nylon, polyester, fibreglass or aramid, and not cotton - under a cover of 100 % Hypalon (CSM) or chloroprene (CR) at Shore A 50 to 75 durometer. It sets the maximum allowable working pressure at not more than one quarter of the measured burst pressure, and requires each production joint to be hydrostatically tested at one and a half times that working pressure at the rated maximum operating temperature.
The elastomer decides the service. EPDM suits water, hot water and glycol; nitrile (NBR) suits mineral oil and fuel. EPDM swells badly in mineral oil and must never be substituted for nitrile on an oil line, however good its temperature range looks.
When ordering, state the nominal bore; the flange drilling - EN 1092-1 PN10/16, ASME B16.5 Class 150 or ASME B16.1 Class 125, which are different systems and will not bolt together; the working pressure and whether full vacuum has to be held; the fluid and its temperature; and the axial, lateral and angular movements required. A rubber joint is pressure-limited far below the pipe it sits in, must be installed within the manufacturer's stated movement limits, and needs control rods where the line is not anchored either side of it.
| Type | Arch-type non-metallic (rubber) expansion joint, flanged both ends |
|---|---|
| Construction | Elastomeric inner tube reinforced with multiple plies of woven cloth or tyre cord, under a synthetic rubber cover |
| Reinforcement Material | Nylon, polyester, fibreglass or aramid; cotton is not permitted |
| Cover Material | 100 % Hypalon (CSM) or chloroprene (CR), not blended with other elastomers; Shore A 50 to 75 |
| Pressure Rating Basis | Maximum allowable working pressure not more than one quarter of the measured burst pressure; the figure depends on bore and build and is stated by the manufacturer for each size |
| Production Test | Hydrostatic test at 1,5 x maximum allowable working pressure at the rated maximum operating temperature, with no leakage |
| Flange Drilling | ASTM F1123 calls for metallic flanges to ANSI/ASME B16.1, B16.5 or B16.24; European-pattern stock is normally drilled to EN 1092-1 PN10/16. EN and ASME drillings are not interchangeable - at DN50 / NPS 2 the bolt circles are 125 mm and 120,7 mm |
| Retaining Rings | Minimum 9,5 mm (3/8 in) thick steel or ductile iron, drilled to match the flange drilling |
| Elastomer Selection | EPDM for water, hot water and glycol; NBR (nitrile) for mineral oil and fuel. EPDM swells in mineral oil and must not be substituted for nitrile |
| Construction of the Unit Photographed | Single arch, with loose (rotating) backing flanges drilled for 8 bolts |
