The tubing connects to the base rather than to the valve, so the valve can be replaced without disturbing any connections.

On a direct-ported valve, the working lines, the supply and the exhaust all screw into the valve body. Replacing it means disconnecting all of them, fitting them to the new valve, and getting each one into the right port - which takes time and is an opportunity to reconnect something wrongly.

With a sub-base, all those connections are made into the base plate once, at installation. The valve mounts on top through a machined face with seals, retained by a few screws.

So a valve change is: isolate, exhaust, remove screws, lift off, fit new valve, replace screws. The tubing never moves and the ports cannot be swapped.

On machines where valves are a wearing item or where downtime is costly, that difference is significant - and it also means a less-experienced person can safely make the change.

It consolidates the pneumatics but not the electrics - the valves still wire individually.

A manifold rail carries a common supply and exhaust along its length and presents mounting stations for several valves, so one supply connection feeds them all and one exhaust collects from them. That is the same pneumatic saving a valve terminal provides.

What it does not do is consolidate the electrical connections. Each valve still has its own coil connector and its own cable back to the control system, exactly as individually mounted valves would.

That suits situations where the pneumatic tidiness is wanted but a terminal is not appropriate - a small number of valves, a machine with no network, valves of different makes or types that will not sit on a common terminal, or a retrofit where the existing wiring is staying.

It is also generally cheaper than a terminal for small counts, since there is no node or multi-pin interface to buy.

Side, bottom and top porting, chosen by how the assembly must fit and how the tubing is to be routed.

Side porting brings the working lines out horizontally, which suits a base mounted on a vertical surface with tubing running away to each side.

Bottom porting takes the connections down through the mounting face, so the tubes disappear behind the panel the base is fixed to - the tidiest arrangement where there is space behind, and the usual choice for control panels.

Top porting brings everything out on the same face as the valve, which suits a base fixed flat to a machine bed where there is no access underneath.

The choice affects the space envelope and the tube bend radius required, which is worth checking against the actual installation rather than deciding on appearance.

Some ranges allow the exhaust to be piped away rather than vented at the base, which matters in cleanrooms and where the exhaust is noisy or carries oil mist.

Broadly no, though certain interface standards create partial interchangeability within size ranges.

The mounting interface - the pattern of ports, the seal positions and the fixing holes - is specific to a valve family. A valve from another maker will generally not seal or port correctly on a sub-base designed for a different range.

There are exceptions: some valve interface dimensions are standardised, so valves from different manufacturers conforming to the same interface can be mounted on the same sub-base. That is useful for second-sourcing and for maintaining older machines.

Where it applies it is worth knowing, because it means a valve can be replaced with an equivalent from another supplier without changing the base.

Do not assume it. Confirm the interface designation on both the base and the intended valve, and record it in the machine documentation - it is exactly the information that is unavailable when a valve fails years later.

On some designs yes, through isolation modules or individual supply arrangements - and it is worth specifying where downtime matters.

A plain manifold rail shares one supply passage, so exhausting it to change one valve depressurises every station on the rail. On a machine where the other valves are holding something in position, that may be unacceptable.

Some ranges provide isolation at each station, or allow a supply separator that divides the rail into independently fed sections.

Where neither is available, the practical approach is to plan the layout so that valves likely to need attention are grouped, and so that the machine can be brought to a safe state with the rail depressurised.

This is a design consideration rather than a component choice, and it is easier to address when the machine is being designed than after a valve fails at an inconvenient moment.

Seal compatibility with the air and the environment, and body material suited to the surroundings - with washdown being the usual driver.

Standard sub-bases are aluminium or a technical polymer with nitrile seals, which suits ordinary plant air in a dry factory.

Washdown and food environments call for corrosion-resistant materials and seals compatible with the cleaning chemicals - which are frequently more aggressive than anything the process itself involves. High-pressure hosing also drives the ingress protection requirement at the valve mounted on top.

Where the air is lubricated, the seals must tolerate the oil used; where it is oil-free, seals intended for lubricated service may harden.

Temperature extremes affect seal choice at both ends, and outdoor installations need attention to the exhaust, which is a route for water and insects to enter.

Specify the environment including cleaning method, not just the air quality.

On a flat, rigid surface with the ports accessible and the exhaust unobstructed - and with thought given to where the exhaust air goes.

Sub-bases and rails need a flat mounting face, since distortion can affect the seal between valve and base. Fixing to thin sheet that flexes is a common cause of intermittent leaks.

Orientation matters for drainage: any condensate that reaches the valves should be able to drain rather than collect, and a base mounted so that a port faces upwards can accumulate water.

The exhaust needs space. A silencer that is pressed against a panel is restricted, which slows the valve and the actuator, and this is an easy thing to do accidentally in a crowded installation.

Finally, mount where the assembly can be reached. A rail installed behind a fixed guard saves space at build time and costs it back every time a valve needs changing.