INTRODUCTION

 In the case of pumps with a conventional drive, it is impossible to seal the rotating part against the static housing without any residual leakage  Even minor quantities of liquid can create safety problems if the media is hazardous.  The statutory regulations set stringent limits in many cases.  Excessive mechanical seal leakage or failures represent over 50% of pump repair incidents.

 Nevertheless, there is one possibility which enables pumps to be operated without any leakage whatsoever – the contact free drive by means of magnets.  The design features an outer rotor, equipped with magnets on the inside and powered by a motor, respectively a driving shaft, and an inner rotor fitted to pump shaft, which has magnets on its outer surface. In between, there is a can which seals off the interior of the pump.  The torque is transmitted contact-free by means of magnetic field generated by permanent magnets.

The Importance of Liquid Containment  Pumps are generally considered to be more expensive than the piping, valves, and related accessories surrounding them, pumps are higher profile when they leak.  To prevent the loss of money in case of costly pumpage.  To maintain the statutory regulations.  To prevent the pump from failure due to highly corrosive liquids.

CONVENTIONAL SHAFT SEALING

Function of the sealing device  Seals are designed as controlled leakage devices  They must be permitted to leak in the order of 10 to 20 drops per minute to stay lubricated and to minimize shaft or sleeve wear and damage.

 The sealing faces cannot be permitted to contact in a dry environment. Some liquid must pass between the rotating and stationary face in order to minimize friction and wear and to carry away the heat generated. SEAL FACE SEPARATIOIN IN MICRO INCHES

Energy consumption  It comes as no surprise that power is needed to make the seal rotate. Noise  The choice of seal face materials is decisive for the function and the life of the mechanical shaft seal.  Noise is generated as a result of the poor lubricating conditions in seals handling low viscosity liquids.

Leakage  The pumped liquid lubricates the seal face of a mechanical shaft seal.  Better lubrication means less friction and increased leakage.  Less leakage means worse lubricating conditions and increased friction.

SEALLESS PUMPS

 Each of the sealing technologies utilizes dynamic sealing, whether it be a shaft rotating in a stationary packing or a seal face rotating against a stationary face.  Sealless pumps use no dynamic seals. They have zero leakage and provide the most reliable seal for hazardous or difficult-to-contain liquids.

Types of Sealless pumps 1) Magnetic Drive Pump 2) Canned Motor Pump  On a magnetic drive pump, the impeller is mounted to an inner magnet carrier. The inner and outer magnetic carriers are sealed by shell.  On a canned motor pump the impeller is mounted directly to the motor rotor. The atmospheric sealing element between the motor stator and rotor is called a liner or “can.”

PARTS OF MAG-DRIVE PUMP AND CONSTRUCTIONAL DETAILS

 Magnetic Circuit  Driven (Inner) Magnets Assembly  Driver (Outer) Magnets Assembly  Containment Shell  Bearings  Casing  Impeller  Shaft

Magnetic Circuit  A magnetic circuit usually consists of two sets of permanent magnets and inner and outer conducting rings. The conducting rings can be cast iron, ductile iron, or a 400 series stainless steel.

Magnet Materials  The two types of rare earth materials commonly used are Neodymium Iron Boron (NdFeB) and Samarium Cobalt (SmCo). The SmCo is four times stronger than AlNiCo. NdFeB, at 21 °C, is 20% stronger than SmCo. The advantage of SmCo is the maximum service temperature of 288 °C, almost twice that of NdFeB, which is 149 °C.

Gap  The “overall gap” is made up of the air gap, containment shell thickness, liquid gap, and encapsulation. Gap dimensions are based on the pressure requirement for the shell, the number of magnets and the material of the shell (metallic or non-metallic) in the gap.

Inner and Outer Carriers

Encapsulation of Inner Carrier Magnets  Encapsulation can be accomplished with either metallic or polymer materials. It is probably the most expensive and extensive process in a magnetic drive sealless pump. After encapsulation, the carrier should be non- destructively tested to confirm 100% effectiveness Encapsulation of Outer Carrier Magnets  The magnets for the outer carriers do not have to be encapsulated. However, it is highly recommended that they should be coated with an epoxy or metal sheathing for atmospheric protection and handling.

Containment Shell  The containment shell divides the inner magnets from the outer magnets.  Shape and thickness depends on working pressure, material, and temperature.

 Materials used for shell are,  Metals  Reinforced polymer  Ceramic Eddy Currents  Metallic or metallic-lined shells will produce eddy currents. Depending on the thickness of the shell, the eddy current losses (P L ) can amount to as much as 20% of the total power. P L = (K x T x L x N 2 x B g 2 D 3 M) / R

Bearings  The internal shaft system of the pump is supported by one or more bearings.  The bearing, which consists of a journal and bushing, is made of various materials, depending on the loads and pumpage (used for product lubrication).  The bearing loads are from the weight of the components and hydraulic forces from the impeller and inner carrier. The impeller forces are both radial and axial.

 The radial load on the bearing, is almost equal to that of the impeller, because there is no overhang from impeller to first bearing.  The axial load depends on whether the impeller is enclosed or semi-open. Flow Path  The amount and direction of flow for cooling of the magnets and lubrication of the bearings is critical to the operation of a sealless pump.

 It is preferable for the liquid to lubricate the bearings before being heated by the magnets.  The temperature rise of the liquid  With a non-metallic shell a typical temperature rise might be 0.5 to 1 °C.  For a metallic shell, with its much higher eddy current losses, the temperature rise may be as much as 4 to 7 °C.

torque

Transmittal Torque  Torque is determined from the following relationship, with appropriate units. T = (K x B g 2 x L x M x r)/g  For a given design type and configuration, the torque varies inversely as the square of the overall gap. Torque Capability  The ultimate torque is the static “breakaway torque.”. The torque value at the starting, at which the two carrier assemblies break loose from each other radially-or “decouple”-is called the “breakaway torque.”

INNOVATIONS IN MAG DRIVE DESIGN

Use of More Than Two Rows of Magnets

Dual Containment

High Performance Magnetic Couplings  The most recent innovation is a new generation of high performance magnetic couplings. The new couplings combine the strength of the Samarium Cobalt magnets with high temperature capabilities of AlNiCo magnet systems.

CONSIDERATIONS FOR TROUBLE-FREE MAG DRIVE SELECTION AND OPERATION

 The Effects of Heat  The Problem of Decoupling  Solids Handling Capability  Bearings  Water Hammer

Advantages disadvantages and applications

Advantages  No leakage to the environment.  No loss of valuable liquids.  Lower noise levels.  High suction pressure does not affect the axial thrust.  Can handle toxic liquids.  Because of no leakage, there is much less chance of a fire.  Easier to obtain construction permits and permits for continued operation.  Low Maintenance.  High suction pressures possible (standard to 25 bar, in special designs up to 200 bar possible).

Disadvantages  Higher Cost.  Higher replacement cost of parts in case of damage.  Monitoring of the pump deserves special attention.  Not suitable for fluids having solid particles.  The magnetic coupling and the product-wetted bearings have weaknesses and limitations that must be designed and operated around to get reliable performance.

Applications  Where zero leakage must be achieved  When pumpage is  Toxic / Lethal  Poisonous  Carcinogenic  Contaminated  Aggressive  Risk of fire  Cryogenic.  At High pressure  Explosive  Acid  Hot / Cold