Through a deliberate clearance between the housing key and the pipe groove.

On a flexible coupling the key that engages the groove is narrower than the groove itself. Once the housings are bolted up, that gap remains, and it allows the pipe end a small amount of freedom - it can slide a little along its axis, and it can tilt through a small angle, and it can rotate.

Throughout that movement the C-shaped gasket stays compressed against the pipe outside surface, because the gasket seals on a length of pipe rather than at a single machined interface. The seal simply travels with the movement.

Each coupling contributes only a few millimetres of axial movement and a few degrees of deflection. Substantial movement is accommodated by providing several couplings, which is why they are often grouped in threes where a flexible connection is needed.

At the points where the designer has decided the system should move, with anchors and rigid couplings defining where it should not.

The usual locations are either side of building expansion joints; at connections to pumps, chillers and other rotating or vibrating equipment; at the base and head of risers where differential movement occurs; at intervals along long straight runs to absorb thermal change; and where the pipeline crosses from one structure to another.

The critical companion decision is anchoring. Movement is only controlled if the system is fixed somewhere - anchors define the points from which expansion is measured and force it to occur where the flexible couplings are.

A system with flexible couplings and no anchors does not absorb movement in a controlled way; it drifts. Design the anchors and the flexible points together.

Only a little - typically a few millimetres of axial movement and a small angular deflection - and the published figures should be used rather than estimated.

The figures vary by coupling size, and they behave differently from what people expect: angular deflection in degrees decreases as pipe diameter increases, because the same groove clearance subtends a smaller angle on a bigger pipe. So a large-diameter system needs proportionally more couplings to absorb the same angular movement.

Manufacturers publish deflection per coupling and the resulting offset over a given pipe length, which is the practical figure for calculating how many are required.

Two cautions. Deflection used to correct installation misalignment is no longer available for service movement. And couplings should not be installed at their deflection limit, since there must be capacity in both directions.

Yes, and it is one of the most common reasons for specifying them at equipment connections.

Vibration from a pump or chiller travels along rigid pipework as structure-borne sound and can be heard a long way from the plant room. Each flexible coupling interrupts that path: the clearance in the joint and the elastomeric gasket between the pipe ends absorb and damp the transmitted energy rather than passing it straight through.

A single coupling helps a little. Three flexible couplings installed close together adjacent to the equipment act as a flexible connector, isolating the equipment from the pipework and also accommodating the small misalignments that occur when a machine settles on its mounts.

This does not replace a properly selected flexible connector or inertia base where the vibration is severe, and it does nothing for airborne noise. It addresses the transmission path along the pipe.

They are designed to be - a well-engineered grooved system uses both deliberately.

Rigid couplings define the sections that must hold alignment and carry load: risers, equipment connections that must not move, and spans designed to rigid-pipe hanger tables. Flexible couplings are then placed where movement is wanted.

The two also share the same grooves, gaskets and bolt patterns in most manufacturers' ranges, so the pipe preparation is identical and the decision is which housing to fit. That makes it practical to change the design late, and to correct a system that turns out to need more or fewer movement points.

What does not work is choosing them at random on site. The distinction between rigid and flexible is a design decision about where the system moves, and swapping one for the other during installation quietly changes the behaviour the designer intended.

Yes, and grooved systems are widely used precisely because of it.

During an earthquake a building distorts and its parts move relative to one another. Rigid pipework resists that movement and the resulting forces break joints, tear out supports and damage equipment connections. A grooved system with flexible couplings placed at planned intervals allows the pipework to articulate with the structure rather than fight it.

Seismic design also requires bracing, and the two work together: braces restrain the pipe against sway and prevent it striking the structure, while flexible couplings permit the controlled movement between braced sections.

Where the design is being carried out to a seismic code, the coupling arrangement, the brace positions and the anchor points are all specified together, and manufacturers publish guidance for their own products. Follow that rather than treating the couplings as a general-purpose flexible element.

With the pipe ends properly aligned and correctly gapped, and without pulling the joint into service to correct an error.

Cut the pipe square and groove it to the manufacturer's dimensions. Set the gap between the pipe ends within the specified range - this gap is what provides the axial movement, so closing it up removes the flexibility the coupling was chosen for, and opening it too far can let the gasket bulge. Lubricate the gasket as specified, seat it over both pipe ends, and close the housings evenly, tightening the bolts alternately until metal-to-metal contact is achieved at the bolt pads.

That metal-to-metal contact is the visual check that the coupling is properly closed, and it should be confirmed all round.

Do not use the coupling's deflection allowance to take up pipework that does not line up. That misalignment stays in the system and consumes movement capacity that service conditions will need.