How do control valves direct the air

How do control valves direct the air?

By Martin Lissenburg, Product Specialist

As outlined in our article on valve terminology, a control valve consists of a number of ports (inlets and outlets) and a number of switching positions, but what mechanism allows different inlets and outlets to be connected together, or blocked?

This article will review two basic methods - the use of a spool, either soft-seal or glandless, which slides inside a bore and the poppet style valve.

What is a spool type valve?

A spool valve is part of a more general family called shear-action valves; they are called this because the spool mechanism slides across the inlets and outlets, its position controlling the routing of the air flow.

The profiled spool fits inside a machined metal bore and, clearly, the seal between the surfaces is critical in minimising leakage. In one system (soft seal) resilient but softer (compared to metal) sealing rings are employed. The seals are fixed to the moving spool in one option and this is called dynamic sealing; in static sealing, the seals are held in the valve body and the spool moves through them.

A second system (glandless) relies on the tolerance between the metal spool and metal bore surfaces being so precise that softer seals are unecessary (an "air bearing" principle). In all cases, the profiling on the spool allows connection (or blocking) between ports as the spool moves.

What is a poppet valve?

The poppet valve has a 'simpler' configuration compared to a spool valve. An annular face seal acts against a flat poppet seat, the seal being maintained by a spring force. Actuation overcomes the spring force and opens the valve seal allowing flow downstream. 

What is the relative performance of each type?

Poppet valves are usually only available for 2/2 or 3/2 applications and cannot be reverse ported; however, they have low leakage and are very tolerant to dirty or harsh environments, the sealing surface being cleaned every cycle. Spool valves offer a greater variety of functions, for example 5/2 and 5/3.

The soft-seal option offers higher flow and lower leak rates compared to glandless; however, glandless has a significantly longer life - soft-seal typically 30 to 50 million cycles compared to 250 to 300 miilion cycles for glandless - the latter also able to work at faster cycle rates and higher operating pressures.

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