Two years ago, the concept of a totally leakproof industrial pump owed more to wishful thinking than reality. But today, using magnetic fluid sealing, the technology to produce such a pump is available and affordable.
Ferrofluidics is the company pushing magnetic fluid sealing technology into a wider industrial arena. Standard applications for ferrofluids include product testing by makers of magnetic media such as disc drives and CD players and the damping of oscillating motion in precision stepper motors. The company has been developing products with sealing applications in the semiconductor, optical and vacuum coating industries.
Its move into industrial applications has been pushed by environmental concerns: ‘Why continue to use a seal that leaks when there is one that does not?’ says Joe Wadsworth, an environmental seals specialist with Ferrofluidics.
The company saw it as a logical step to fit fluid seals to fans and pumps used in industries where emissions cannot be tolerated.
It is now auditing its technology for inclusion in the Government’s environmental technology programme.
Magnetic fluid seals are classified as zero leakage to 10-8 mbar/litre/sec by test body TUV. In everyday terms, leakage from such a seal would take 100,000 years to fill a matchbox. Such seals also prevent ingress of air into the process.
Industry’s response has been enthusiastic. Last year, Ferrofluidics, which claims to be the only producer of magnetic fluid seals for industrial process equipment, saw its business grow 3,000%.
Fan Systems in Halifax builds fans for customers such as BNFL, where there is a risk that tiny quantities of nuclear contamination could leak into the atmosphere. It is fitting double acting mechanical seals with Ferrofluidics’ magnetic fluid cartridge seals on nuclear industry fans with a working life of between 15 and 20 years, which come into a category classified as gas tight.
Engineering manager Martin Crouch says magnetic fluid seals do wear. But while some friction is generated in the form of shear forces as the shaft rotates, they produce only around 1/50th the resistance of a usual mechanical seal, which needs replacing every 40,000 hours.
A 90mm magnetic fluid cartridge seal costs £1,000 but Crouch expects them to be cheaper than mechanical seals in the long term. ‘People who need gas-tight seals can afford them,’ he says.
Ferrofluids provide perfect seals for rotating shafts because, performing like a liquid O-ring, they accommodate eccentric running of out-of-tolerance shafts, filling any gaps and staying in place with limpet-like tenacity. The fluid seal is kept in place by magnetic particles in the fluid.
A ferrofluid seal system consists of the magnetic fluid, a ring of permanent magnets, two pole pieces and a magnetically permeable shaft or shaft sleeve. The magnetic circuit created by the magnets, pole pieces and shaft concentrates magnetic flux lines through the gap between the shaft and the pole pieces. When the ferrofluid is added to the system it is attracted to this gap and assumes the shape of a liquid O-ring around the rotating shaft.
The ferrofluid has three components: very small magnetically permeable ‘sub magnetic domain’ particles, measuring around 100 anstroms diameter, which have no inherent magnetic behaviour so act homogenously under a strong magnetic field; a surfactant that maintains particle distribution; and, accounting for up to 95% of the fluid, a hydrocarbon (typically an ester) or fluorocarbon (mineral oil) carrier that holds the particles in a colloidal suspension.
Ferrofluids are effective for shaft speeds up to 20m/ sec the average industrial equipment rotates at 3 8m/sec.
The company is working on a new application for submarine and other turbines which require a ferrofluid that can tolerate shear forces created by a shaft running at 100m/sec.