The geometry of the housing keys where they engage the pipe groove.

On a flexible coupling the housing key is narrower than the groove, leaving clearance so the pipe can rotate slightly and move a little axially. On a rigid coupling the key is profiled to fill the groove and to bear on its flanks as the bolts are tightened, so the pipe ends are drawn together and clamped. Some designs use an angled pad or offset that wedges as it tightens.

With the clearance removed there is nowhere for the joint to move, and the connection resists bending and torsion as well as tension.

The practical consequence for the installer is that rigid couplings are less forgiving. A flexible coupling will take up a small misalignment; a rigid one will fight it, and forcing it loads both the coupling and the pipe. Align properly before assembling.

Wherever the pipework must hold its position, transmit load, or behave as a continuous structural member.

Typical cases are vertical risers, where the coupling has to carry the weight of pipe below and must not allow the stack to settle; connections at pumps, chillers and other equipment where movement in the pipe would be transmitted into the machine; systems designed with rigid-pipe hanger spacing, since a flexible joint between supports changes the span behaviour; and locations where the pipe would otherwise need additional bracing to stop it rotating.

They are also used simply to reduce the number of movement points in a system, so that the movement that does occur happens where the designer intended.

Use flexible couplings where movement must be permitted, and at the points chosen to absorb expansion, settlement or vibration. Most real systems use both.

It gives comparable performance with far less site work, which is the whole reason grooved systems are used.

Against welding: no hot work, no permit, no welder, no purging, no radiography, and no destruction of galvanising. A joint that would take a welder a significant time is made in minutes with hand tools, and it can be dismantled later.

Against flanging: fewer components, much less weight, no bolt circle to align, and a fraction of the assembly time. A flanged joint needs a flange at each end, a gasket and a full set of bolts; a grooved joint needs two housing halves, one gasket and two bolts.

The requirement grooved systems add is the groove itself, cut or rolled into the pipe end, which needs a grooving machine and a pipe wall thick enough to take it. On site that is normally far less trouble than the alternatives.

A groove of the correct profile and depth, a specified distance from a square-cut end, on a clean pipe.

Grooves are either roll-grooved, where the pipe wall is cold-formed inward by a rolling machine, or cut-grooved, where material is machined away. Roll grooving is quicker and removes no metal, so it suits standard wall pipe. Cut grooving is used on heavier walls and where the pipe is too thick to roll.

The dimensions matter precisely. Groove depth, width and the distance from the pipe end are specified by the coupling manufacturer, and a groove outside tolerance either lets the joint leak or prevents the housings closing. So does an out-of-round or badly cut end.

Check the pipe wall thickness is adequate for the groove type before specifying - roll grooving a wall that is too thin leaves insufficient material and the joint is not rated.

Only as part of a design that handles the expansion somewhere else. A rigid coupling does not absorb it.

A hot water, chilled water or steam system changes length as it heats and cools. If every joint is rigid, that movement has to go somewhere, and it goes into the supports, the anchors and the equipment connections as force. The result is distorted hangers, cracked brackets, noise, and strain on pump and chiller flanges.

The usual approach in a grooved system is to run rigid couplings through the lengths that must stay put, and introduce flexible couplings in planned groups where movement is to be taken up - three flexible couplings close together will accommodate expansion and act as a flexible connector. Alternatively use expansion loops or purpose-made expansion joints.

Decide this at design stage. Retrofitting movement capability into a fully rigid installation is disruptive.

Both - the gasket is compressed on assembly, and line pressure then acts to improve the seal rather than to break it.

The C-shaped gasket sits around the pipe ends inside the housing. Tightening the bolts compresses its lips onto the pipe outside surfaces, which gives the initial seal. When the system is pressurised, the fluid enters the gasket's internal cavity and pushes the lips harder against the pipe. The higher the pressure, the tighter it seals.

This pressure-responsive design is why grooved joints hold reliably across a wide pressure range and why they can also work under vacuum with the appropriate gasket.

The practical requirement is that the pipe surface under the gasket must be clean, undamaged and free of weld spatter, deep scores or corrosion pitting, since the seal is made against that surface. Lubricate the gasket as the manufacturer specifies and check it is seated before closing the housings.

The one matched to the fluid and its temperature, and the choice is more consequential than it looks because the wrong grade fails slowly.

EPDM is the standard grade for hot and cold water, including heating systems, and for many chemicals - but it is degraded by petroleum products and must not be used on oil or fuel. Nitrile suits petroleum-based fluids, oils and compressed air with oil carry-over, but is poor with hot water. Silicone and fluoroelastomer grades cover high temperature and aggressive chemical service.

Dry pipe and pre-action fire systems, air service and vacuum all have specific gasket recommendations that differ from wet service, so check the application rather than the fluid alone.

For potable water the gasket must hold the drinking-water approval applicable in the market. Approvals are national, so confirm against the local requirement rather than a general claim.