Seal-less pumps?

Published:  27 May, 2007

Is there such a thing as a seal-less pump? Certainly there are rotary motion pumps that are designated seal-less by their manufacturers in that they do not contain rotating seals.  However, these pumps do contain static seals, typically in the form of 'O' rings and gaskets, even when designated seal-less. So the answer is yes, and no. Tony Peters, managing director of AxFlow explains.

If we start by looking at static seals, their sole purpose is to fill a gap and, because they are between two fixed surfaces, reliability is fairly assured. However, their performance over the long term can be affected by movement, abrasive erosion, corrosives and attack from chemicals that can lead to softening or swelling of the seal material. They can also be ‘blown out" by over pressure.

There are some simple rules that can be applied when designing a static seal arrangement. These include:

  • Choosing the most appropriate seal material
  • Selecting the safest and most practical seal type or design
  • Ensuring that the machined surfaces that house or mate with the seal are in total accordance with the seal manufacturer’s instructions
  • Making sure that the surfaces to be sealed are aligned correctly and fixed together either using bolts or clamps.

 

Reciprocating positive displacement, rotary positive displacement and centrifugal are the most commonly used pumping principles. Whereas designing a static seal arrangement is relatively straight forward, it is more challenging to form and maintain a seal between moving surfaces. For this reason, the designers of rotary dynamic pumps have always sought to find ways of eliminating rotating seals. Their efforts have been imaginative and have resulted in the emergence of submersible pumps, sump pumps, tank pumps, canned motor centrifugal pumps and magnetic drive pumps.

 

Seal-less pump types

Submersible pumps are fully submerged in the liquid to be pumped so there are no pressurised seals - however there is a seal between the pump shaft and motor.  Sump pumps use a centrifugal impeller and volute mounted on a long vertical shaft, with the pumping elements located in a wet pit. The motor and shaft bearings are mounted in a dry position at the top of the pit. Tank pumps are of a similar design, but a wet pit is replaced by a purpose-built tank that houses the vertical pump.

Canned motor pumps are characterised by their compact and integrated design, with the motor and pump forming a single unit with the rotor and impeller fitted on to a common shaft. The rotor is guided by two slide bearings and the drive motor stator is separated from the rotor using a cylindrical can. The rotor compartment, together with the hydraulic section of the pump, is filled with the pumped medium during operation. The heat loss from the motor is carried off by a partial flow between the motor and stator. At the same time the partial flow lubricates both slide bearings in the rotor compartment. The can, which is a hermetically sealed component and the motor casing offer primary and secondary containment. It is this, together with the compact design and low noise level, which differentiates the canned motor pump from the magnetically coupled pump.

The double containment design free of a shaft sealing device makes the canned motor pump one hundred percent leak free. Even in the unlikely event of a stator line rupturing there is no leakage to atmosphere. Furthermore, the absence of mechanical seals and the employment of maintenance free slide bearings, as opposed to roller bearings, optimised MTBF (Mean Time Between Failure) are ensured. These factors make the canned motor pump ideally suited to the safe handling of volatile organic compounds.

Canned motor pumps satisfy the highest safety standards, because the liquid compartment is hermetically sealed off from the environment.

Magnetically driven pumps use a series of magnets attached to the pump shaft contained within a bell 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 bell housing.

Submersible, sump, tank, canned and mag-drive pumps remove the need for rotary seals and as a result, when applied correctly, they eliminate up to 90% of sealing downtime problems.

There is another seal-less pump type that falls between rotary and reciprocating pumps, this being the peristaltic principle, whereby a tube containing the pumped liquid is housed within a semi-circular casing. Rollers, or shoes, driven a by a geared motor pass over the tube producing a pulsating squeezing action that pumps the liquid in the tube.

Where process liquids need gentle handling and leak-free operation, these requirements can be satisfied by Blackmer’s Mouvex technology. Here the pump unit employs an oscillating eccentric piston to produce the pumping performance of a rotary positive displacement pump, so mechanical seals are not required. The medium is pumped by the eccentric movement of the oscillating piston within the pump chamber. Self-priming and capable of dry-running, this type of pump will clear both the intake and discharge lines of any residual liquid. Lubrication of all moving parts is provided by the pumped medium.

Blackmer Mouvex pumps provide a constant product flow that is independent of both pressure and viscosity.  This, together with the presence of automatic piston/cylinder wear adjustment, ensures full flow efficiency and high levels of product stability at all times.

 

Diaphragm pumps

Mechanically coupled and hydraulically actuated diaphragm pumpheads are one of the areas of focus for new developments in metering pump technology. They are typically specified where toxic materials are being handled and leak-proof performance is required. Available in a wide variety of materials, diaphragm pumpheads are now capable of accommodating hazardous, toxic and dangerous liquids at variable flow rates and pressures. As greater emphasis is placed on safety and environmental consideration and further legislation is introduced, the market is moving towards diaphragm pumps and away from the traditional plunger design.

Piston and plunger reciprocating positive displacement pumps present particularly unique challenges with regard to the development of seal-less designs. Many piston and plunger pumping applications can in fact be performed by diaphragm pumps. In mechanical diaphragm pump the typical reciprocating action of the piston/plunger via a connecting rod and crankshaft is applied to the centre of a flexible diaphragm. The pulsating action is the same as for the piston or plunger pump, but significantly the pump does not contain reciprocating seals.

The performance of the mechanically actuated diaphragm pump is limited by the mechanical strength of the diaphragm, which must be able to ‘hold’ the differential pressure between the pumped liquid and the atmosphere. With the hydraulically actuated diaphragm pumphead, the diaphragm is used to pump the liquid, the diaphragm being actuated by the movement of a plunger in hydraulic oil.  The diaphragm acts in pressure equilibrium providing extreme reliability. The diaphragm acts as a barrier permitting the pumped liquid to come into contact only with materials that have been specified for their resistance to chemical attack or corrosion.

 

Air operated double diaphragm pumps

When it comes to handling fluids that may have high solids content or where hazardous environments demand a leak-free ATEX compliant pump technology, then the air-operated double diaphragm pump comes into its own. It can transfer heavy, abrasive liquids and slurries equally as efficiently and effectively as fluids that can be damaged by high shear forces, such as those in food processing.

When running, the dynamic components that come into contact with the fluid are the two diaphragms which are connected by a common shaft, the two inlet valve balls and the two discharge valve balls. The diaphragms act as a separation membrane between the compressed air supply and the fluid. Driving the diaphragms with compressed air, rather than the shaft balances the load on the diaphragm which removes mechanical stress and therefore extends diaphragm life. The valve balls open and close on the valve seats to direct the flow.

The absence of close fitting parts allows solids to pass through the pump without harm, whilst abrasive and viscous products can be pumped without any damage to the fluid characteristics. Portable, straightforward to use and easy to maintain this pump type pump can run dry, self-prime and ‘deadhead’ without any adverse implications.

For further information please visit: www.axflow.co.uk

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