Throughout all areas of fluids handling there are demands for pumps to deliver leak-free performance. Primarily, these demands are to be found in sectors where the fluids being handled pose a threat to health, safety, the environment or where the intrinsic value of the fluid is very high.
Where the fluid handling process requires the use of centrifugal pumps, preventing both vapour and fluid leakage has to be a serious consideration…
Inherent risk of shaft seals
Centrifugal pumps containing rotary mechanical seals can and do provide very high levels of fluid and vapour containment, and are used successfully in a great many difficult applications. However, an element of seal failure will always remain should external influences impinge on pump or system performance. A centrifugal pump that does not contain mechanical seals and delivers far higher levels of leak-free performance is the magnetic- drive pump. More commonly known as the mag-drive pump, all the driven elements are enclosed within the pump housing, so there is no impeller shaft connection to an external motor. Most significantly, they offer high levels of liquid and vapour containment and are a highly effective solution for a great many challenging applications.
Mag-drive pumps remove the need for rotary seals and as a result, when applied correctly, they eliminate up to 90% of the problems associated with seal downtime.
A volute casing with the end removed exposing the impeller. The casing sections are sealed with an o-ring system, the material of which is specified to be chemically and temperature compatible with the transfer media. There is no frictional wear on any o-rings within the pump so the fluid will be 100% contained.
Providing the pump handling the fluid as specified when ordering, the operation should be flawless for a long service life compared to other pump types.
Working principle
Mag-drive pumps use a series of magnets attached to the pump shaft which is contained within the pump housing. Forming a seal-less unit, the pump shaft is driven by magnets on the motor output shaft which is rotated by the motor acting on the pump shaft magnets through the housing. Like conventional centrifugal pumps fitted with mechanical seals, the magnetic drive pump is driven by a standard motor which is connected to the magnetic drive via the coupling.
The magnetic coupling consists of two cylindrical magnet assemblies, these being the repulsion. This force is used to transfer torque from the motor to the impeller.
The permanent magnetic coupling creates neither slippage nor induction currents during rotation, factors that will contribute to increased energy efficiency. Should excessive torque be applied, the magnets will de-couple and not re-couple unless the pump is stopped. There is no energy loss in a permanent coupling unless an electrically conductive containment is placed between the outer and inner magnets. If an electrically conductive material is used for containment, eddy currents will be generated, resulting in some energy loss.
Mag-drive pumps remove the need for rotary seals and as a result, when applied correctly, they eliminate up to 90% of the problems associated with seal downtime.
Verdermag units installed in a fast-loop system ready to be shipped to a petrochemical company. The fast-loop system requires high pressures to maintain the homogeneity of the oil necessary for accurate monitoring. The two Verdermag units were both specified due to their ability to handle the high system pressure and reliability.
Many of these systems are in remote working environments where downtime carries an exorbitant cost. The pumps provide peace of mind to the end user and contractor that the heart of the system is always working.
Applications
The leakage that occurs in traditional mechanically sealed pumps can result in unwanted costs for plant operators: maintenance and environmental damage being prime examples. Up to as much as fifty per cent of the cost of owning a pump can be spent in the year after the pump is purchased as a result of maintenance issues. The leading causes of high maintenance in conventionally sealed pumps include those associated with mechanical seal replacement and the premature wear of the bushings and close-fitting metal parts due to insufficient support of the pumping elements.
Then there are the environmental costs of leakage, which include cleanup, possible system or plant shut-down and environment hazard fines and penalties. And if there are wider environmental issues to contend with, there is the damage to a company’s reputation.
It would, however, be wrong to single out the mechanically sealed centrifugal pump as being unsuited to pumping media that require high levels of containment. This pump type has a dominant position within the pump industry and has nothing to prove when it comes down to delivering excellent performance in some of the most exacting applications and environments. What the mag-drive centrifugal pump offers is an alternative option that needs to be considered when product containment is the key issue. Its design also means that it can come into contention where floor space is at a premium, for it is a relatively compact pump particularly where flow rates are in the small to medium range and pressures are modest.
The applications for the Verdermag mag-drive pumps are varied as the range is readily available in metallic and non-metallic construction and can accommodate flows ranging between 100lt/min and 200³m/h, pressures up to 200bar and temperatures as high as 205oC.
Industry sectors where these pumps have a proven track record and are used extensively include chemicals, electronics plating, food and drink processing, mining, galvanic, pharmaceuticals, steel production, tanker offloading, and offshore oil and gas production.
Verder Global stainless steel pumps are a good option when it comes to handling HTF. In this application the fluid can be at a high temperature and pressure, so if there was a leak in the gland area of a mechanically sealed pump the fluid would pose a serious danger to the operator. Designed for high temperature and high pressure applications, the build quality of the components makes them compatible with the most aggressive fluids.
Where acids are concerned, the Verdermag ETFE lined pump has a proven track record. ETFE is similar to PTFE but harder wearing. In the case of the Verdermag U series, the ETFE lined components such as the impeller, front and rear casings, liner and internal magnet are reinforced with 20% carbon fibre to strengthen and increase temperature capabilities to 90oC. Caustic is a good example of where the ETFE lined pump is a sound option and Verder’s customers have used this type of pump on caustic as a gland leak would cause a concrete base to rot.
Hot water can be a challenge for the mechanically sealed pump and from a maintenance point, pumping this far less exotic and low value fluid could still call for a mag-drive pump, with a heat exchanger being a good example of its suitability.
Above: A cross-section of a Verdermag non-metallic model. The Impeller and all wet-side surfaces are encapsulated in ETFE which is compatible with many aggressive fluids. The O-ring is also visible, providing absolute containment of the fluid.
Below: A Verdermag high pressure metallic model, commonly used in fast-loop systems, power stations and nanofiltration processes.<
Summary
In the past mag-drive pumps may well have been expensive to operate, as the impeller would rotate more slowly than the motor speed. This could add heat to the media being pumped, but today’s generation of pumps through the use of rare earth magnets can be just as efficient as mechanically sealed pumps. Mag-drive pumps come into their own where there are sealing problems, so when users talk about balanced seals, utilities, flushing, double seals or cartridges, then it is time to start looking at the mag-drive pump.
