All machined brass components plus steel springs
and elastomer seals in pre- assembly.
Custom flexibility
The engineers and designers at IMI Norgren, a company world-renowned for their
standard and custom, pneumatic products and systems, 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 re-designed control valve to operate at temperatures in excess of 200°C, in this case in the context of a metal refining environment.
At the start of the project, Norgren were 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.
High-temperature compatible brass poppet
signalling valve 100mm x 40mm diameter.
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 200-220ºC, 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&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 non-magnetic 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 non-functional surfaces due to thermal and
mechanical stresses locally exceeding the top limit for this chosen grade of brass.
Reducing stress-raising features
This latter point was interesting from a design point of view in that thoughtful re-design
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.
Note to editors:
JPEGs @ 300dpi available by email from marilyn.comparetto@copperdev.co.uk
For further information contact:
Marilyn Comparetto
Copper Development Association
5 Grovelands Business Centre, Boundary Way
Hemel Hempstead, Herts HP2 7TE
Tel: 01442 275705, Fax: 01442 275716
Email: marilyn.comparetto@copperdev.co.uk
Websites: www.cda.org.uk & www.brass.org
PR 623
June 2003