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Rotary valve failure is fortunately a very rare event, but it does not have to mean significant loss of revenue for manufacturers, according to Rota Val’s managing director Ian Blackmore.

Eliminating contamination to ensure product purity is a constant challenge, particularly for the food and pharmaceutical industries.

Standards of equipment are constantly being driven upwards with stainless steel being the choice of material for manufacture.

Also, when needed to meet extreme hygiene requirements, highly polished surface finishes allied to easy clean and CIP features are required to minimise the build up of pathogens.

Unfortunately, contact between moving parts almost invariably results in damage due to the high tendency of stainless steel to gall and seize.

Rotary valves are particularly vulnerable due to the necessarily close-running clearances needed for efficient performance.

Such contact may be due to poor valve design, inadequate maintenance, incorrect strip-down and reassembly during cleaning, bearing failure, unusual temperature variations or the action of certain materials on rotor to housing clearances.

Rota Val Easy-Clean valves are specifically designed to reduce the possibilities of incorrect reassembly, but over time operator errors or other problems may still occur.

The consequences of such an event are normally severe, often rendering the valve unusable.

At best, any limited contact will leave the surface finish compromised and is likely to shed particles of stainless steel, contaminating the product.

Thereafter, any continued contact of the surfaces will start to gall/score and shortly after will try to seize.

Normally noise and or valve distress is the only indication of problems, which by then will be too late to avoid damage.

If the valve concerned is deeply buried in a plant installation, or surrounded by otherwise noisy equipment, then operators are unlikely to notice warning signs early enough to prevent significant damage.

A further disadvantage is that most commonly used grades of stainless steel are non-magnetic, avoiding normal metal detection or collection means.

Thus, if particles of stainless steel are shed into the product, then it becomes very difficult to identify and isolate contaminated batches with absolute precision.

Food and pharmaceutical manufacturers always take the most pessimistic view, wherever possible never allowing suspect products to reach the customer.

The result is that, for safety reasons, a substantial amount of expensive product may be scrapped, including material that may not have been affected by valve failure.

Where the valve is designated under Atex as an ‘Autonomous Safety Device’ for flame and explosion containment, then even light scoring will breach critical clearances, rendering the valve certification void, as will any attempt to repair it outside of a certified and licensed procedure.

This, of course, always assumes that the valve finishes in a repairable condition.

Damage to the valve will, therefore, compromise the customer’s product, create a potentially dangerous situation with regard to explosion containment and, inevitably, increase costs.

To overcome these difficulties, several valve manufacturers have been developing contact detection systems with the objective that the system can virtually instantly stop the valve in the event of contact being sensed.

All known systems at this time on the market are arranged to detect contact by detecting a fall in electrical resistance between an electrically isolated rotor component and the valve housing.

The circuits invariably employ Intrinsically Safe Barriers with switching capability to avoid conflict with any Atex certification.

While such systems can and will stop the valve drive motor on contact fault detection, they also monitor the circuit and isolate the power if there is a circuit fault.

With the exception of the Rota Val Rotasafe RM2 detection system, others are believed to function on analogue control, which has limitations, being a simple method with an inherently wide tolerance.

This results in high detection resistance values where ‘tripping’ is possible under some 12,000ohms and definitely under 3,700ohms, meaning product conductivity issues often result in false or ‘nuisance’ tripping.

Further, there is no means to filter out nondescript electrical currents from a range of causes or sources that have been identified as often present and which the barrier sees as a contact fault, again resulting in unnecessary tripping.

Barrier tripping without a defined and clear cause is a major problem as users have to assume that there has been contact and stop production to investigate the integrity of the valve and the purity of the product passing through at the time.

This leads to a substantial loss of production, possible unnecessary waste and increased cost of maintenance.

To try to overcome these issues some manufacturers recommend that a time-delay control is introduced to give an integration time of 3s from initial contact and only if detection remains after this time will it stop the valve.

In practice, while this may avoid some short-term transient influences, it does mean that in a genuine contact fault condition, at a typical 25rpm, the rotor will be in full contact for 1.25 revolutions and severe damage will almost invariably occur.

The Rotasafe RM2 deals with all of these issues through employing digital technology and a specially designed circuit with configurable settings that provides for very accurate settings.

The system has the ability to set contact resistance detection values to as low as 10ohms, thereby eliminating any product conductivity issues.

The system has been designed to provide variable tolerance zones for filtering out common electrical interferences.

The differentiation of false from genuine signals has been a major problem with other systems where their design exhibits an innate inability to adequately deal with electrical noise.

Although factory-set default settings meet the needs of most installations, it is essential to be able to quantify the factors affecting detection and to determine system settings if tripping is experienced without due cause.

Rota Val has developed computerised test/monitoring equipment specifically to measure the Rotasafe RM2 circuit condition and activity that overcomes the difficulty of not otherwise knowing whether a contact fault trip is genuine or spurious, as well as to identify and quantify specific causes.

According to Blackmore, this has been a major development in rotor monitoring technology that provides productivity improvements allied to cost and time savings for the food and pharmaceutical industries.

The results of extensive in-plant testing and application experiences are said to have shown the Rotasafe RM2 to be a very reliable rotor monitoring system in a variety of manufacturing installations.

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