Norgren designs in brass for a hot application

On occasions customers wish to use control valves, actuators, drive cylinders and other components for purposes that could potentially push these devices beyond their design limits.

On occasions customers wish to use control valves, actuators, drive cylinders and other components for purposes that could potentially push these devices beyond their design limits.

The engineers and designers at IMI Norgren occasionally find themselves faced with special requirements where customers wish to use control valves, actuators, drive cylinders and other components for purposes that could potentially push these devices beyond their design limits.

In such cases Norgren's clients obviously represent the centre of expertise on the actual process or application.

The request for Norgren's help, therefore, is usually specified by defining functional parameters in the context of the operating environment.

These can vary to include, for example, high or low temperatures, corrosive atmospheres, high humidity, vibration, dual-circuitry for security in remote applications and even, on a recent occasion, the control system for a novel, variable severity, traffic calming device nick-named "the friendly bump" which reacts to a speed/load signal.

A typical high temperature application however is chosen here as an illustration of the use of brass for a redesigned control valve to operate at temperatures in excess of 200C, in this case in the context of a metal refining environment.

At the start of the project, Norgren was briefed that hard crusts can form on the surface of molten metals in induction furnaces and, in this particular case, a client wanted to break up this crust by the impact of a pneumatic ram.

Calculating the power required, and selecting the appropriate drive cylinder from Norgren's extensive range in order to design a plunger, was the easy part.

The conceptual choice was for a ram driven by a 200mm bore pneumatic cylinder to be controlled by a standard, free machining brass, signalling poppet seat valve.

The custom assembly was bench tested at Norgren's works and the mechanical results were 100% positive.

Live tests were then conducted by the refiner on a mock-up basis at the elevated operating temperature of the refining plant.

Nigel Fox, Design Engineer with IMI Norgren, picks up the story at this point: "The 'killer-factor' on this project was known from the outset.

The operating temperature, an elevated 200-220C, was expected to give the brass signalling valve some problems.

What we did not know, however, was the extent of these and how correctable they would be without a material change.

In other words, we had a typical R and D situation on our hands.

A live test conducted under furnace conditions gave us the feedback we required in order to work on the design modifications and overcome any problems.

For pneumatic control valves and components we frequently work with brass, particularly in rugged, heavy-duty and corrosion-resistant applications.

The reason for this is very evident when you study the metallurgy; brasses generally exhibit a degree of self-lubricity which, together with good corrosion-resistance and machinability, adds to their attraction.

Also, since induction heating was the method used to refine metals in this case, the nonmagnetic property of the valve's brass outer casing confers additional advantage.

It was our belief, based on many years of designing in brass, that our material choice would be correct as long as the design could be fine-tuned to provide for this more severe environment.

In fact the combination of points we had to address following initial live trials included: steel spring material; elastomer seal material; grade of seal lubricant; and finally some marginal stress damage to the body on nonfunctional surfaces due to thermal and mechanical stresses locally exceeding the top limit for this chosen grade of brass.

This latter point was interesting from a design point of view in that thoughtful redesign of specific areas significantly lowered these stress-raising effects.

Eliminating sharp corners, sharp edges and rapid changes in section was a thoroughly worthwhile product design exercise even without the thermal challenge".

Norgren found that correcting the other points by upgrading the spring, seal and lubricant was possible by re-specification.

Combined with the redesign this has resulted in the successful development of a brass-based pneumatic product that is ideally suited to perform in the arduous and relatively severe, high-temperature operating environment of an induction heated, refining furnace.

This unique product has now been added to the Norgren range.

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