- 2 November 2006 -

Can ceramic ports help centrifuges last longer?

Neil Lacey, service manager for Centriquip, the UK's largest decanter manufacturer, looks at how choosing the correct port liner for centrifuges can make all the difference in reducing wear, downtime and maintenance costs.

The cost of maintenance in factories is escalating - perhaps not in real terms, as most companies operate effective maintenance programmes, and modern equipment is inherently reliable - but the cost of a shutdown has never been higher. Even planned maintenance, in these days of high efficiency and optimal production levels, can reduce productivity at an alarming rate.

Modern equipment of all kinds operates at higher stresses and finer tolerances than in the past. People need to be trained to a much higher level and paid accordingly. Health and safety requirements dominate the manufacturing environment, and just-in-time logistics mean that an unscheduled breakdown has a knock-on effect out of proportion to the initial problem. As companies are put under extreme pressure to cut costs and increase efficiency the temptation to keep maintenance as lean as possible must be irresistible. But professional operators know that to skimp on maintenance is a false economy.

Often, cost-cutting programmes have a negative impact on employees' working attitudes as well. This can be directly linked to equipment reliability problems which can increase costs and reduce operating efficiency, causing a vicious cycle of employee dissatisfaction.

Henry Ford, it appears, had the right idea: "Put all machinery in the best possible condition, keep it that way, and insist on absolute cleanliness everywhere in order that a man may learn to respect his tools, his surroundings, and himself," is what he said, all be it way back in 1926!

Today, maintenance is much more sophisticated. Companies operate programmes using a combination of run-to-failure, preventative maintenance and predictive maintenance, balanced to fit the needs of the plant.

Run to failure is a perfectly acceptable principle for non-critical parts; preventative maintenance uses experience to set timed programmes for maintenance to ensure that critical equipment is fixed before it goes wrong; however, predicting when maintenance is necessary is more sophisticated, involving vibration analysis techniques for example, that leaves well alone until intervention is necessary.

But whatever type or mix of programme is employed, there is one underlying principle: maintenance should not be performed until it is needed. To do so wastes money. Not to do so, of course, is potentially worse.

Ceramic port liners
Fr equently the type of material chosen for critical wear parts can have a dramatic effect on the frequency with which maintenance needs to be performed. Five years ago, for example, Centriquip made a design decision to use ceramics for its solid discharge port liners. The effect on the maintenance programmes of those companies using the equipment has been dramatic.

All centrifuges suffer from varying amounts of wear in certain areas, depending on the process of the machine. These areas are particularly the feed chamber, feed chamber discharge ports, scroll flight tips and solids discharge ports. This wear is caused by the coarse and abrasive nature of the feed product. If the feed contains abrasive material it will wear out the 'wear zones' at an accelerated rate. Applications that can cause extreme wear are those that contain materials such as sand, grit, stone and metals. These particles can be common in all sorts of feeds from raw sewage, potato wash water, tannery effluent, wool scouring effluent, environmental waste, waste oil sludges, vegetable effluent and industrial or scrap metal wash water.

The discharge ports on some centrifuges use sacrificial liners, usually made from tungsten carbide, that can be changed as necessary without causing any damage to the body of the machine. But ceramics are now proving to be much more effective. On average, ceramic ports last twice as long as tungsten.

The liners themselves are not high-value items but replacing them can be a costly exercise requiring the whole decanter to be shut dow n, stripped and refitted by skilled labour. The whole proces s can take up to a week. The cost of the maintenance work is minor compared to the cost of having the unit out of commission.

But most manufacturers still use the old-style tungsten liners despite them being more expensive and heavier than ceramics. (Being lighter reduces the weight of the rotating bowl and has a beneficial effect on its balance and, therefore, its reliability.) For this reason, Centriquip has introduced a new service to replace the tungsten carbide liners in any manufacturer's decanters thereby extending the service intervals and saving money. This process is done during planned maintenance so does not cause any additional interruption in production.

Scottish Power
One company that has recently used this service is Scottish Power in Glasgow , Scotland . The company uses 12 Andritz-Guinard decanter centrifuges in continuous operation to dewater sewage sludge. Each machine has 10 discharge ports for evacuating the solid phase from the machine. The ports have sacrificial liners made from tungsten in a stainless steel housing. Until now, these liners have worn quickly requiring the machine to be stripped for maintenance every three months. The new liners from Centriquip, however, are half the cost of the tungsten liners and last three times as long.

Keith Burdon, the maintenance supervisor for Scottish Power, said in a recent interview that th e new liners had made a significant saving in time and maintenance costs. "We shut the machines down for routine checking every three months. We use an endoscope to check the condition of the liners and strip the machines only if they need changing. A strip down takes us a week. The ceramic liners are lasting much better than the old ones so major maintenance is needed less often," he said.

Of course this is not just a money issue. Replacing liners is a time-consuming business, often involving heavy lifting equipment. Whenever this type of heavy engineering work is carried out there is always a health and safety element. The best way to prevent accidents is to avoid doing the work at all; reducing the frequency of replacements has, therefore, benefits all round.

Repair not replace
Updating old equipment in this way can be a most cost-effective way of improving productivity and reliability without incurring the high capital cost of buying new equipment. Replacing an ageing decanter is a costly business, but often their service life can be extended with an effective repair, particularly if it can be back in service within a few weeks.

Frequently original equipment manufacturers are concerned primarily with the supply of the machinery: service and, more significantly, repair, are secondary. Similarly, with some ageing equipment the original equipment manufacturer may no longer stock spares. In these cases it's often best to rebuild a decanter by updating it to include the latest materials (such as ceramic liners or hardened scroll flight tips) and manufacturing replacement parts from scratch. In the UK, this can often be achieved much quicker and less expensively than by returning the equipment to its original manufacturer overseas.