Magnetic Drive Centrifugal Pump Working Principle

In this article, we learn about the Magnetic Drive Centrifugal Pump Working Principle. Magnetic drive centrifugal pumps are used in both Chemical and Petrochemical industries for pumping hazardous liquids.

The need for Magnetic Drive Centrifugal pump

Environmental concerns and recurring mechanical seal problems have created a need for sealless pumps in the chemical and petrochemical industries. In some cases, more stringent regulations by EPA, OSHA, and local agencies are mandating the use of sealless pumps. Magnetic drive pump comes under the category of Sealless pump.

The magnetic drive pump which uses a permanent magnetic coupling to transmit torque to the impeller without the need for a mechanical seal for shaft sealing.

Magnetic Drive Centrifugal Pumps Working Principle

The working principle of Magnetic drive centrifugal pump is similar to a standard centrifugal pump except the shaft seal is eliminated. The Magnetic drive pumps are designed to isolate the pump body from the motor driving an impeller and magnet assembly with a drive magnet attached to the motor shaft. Refer below figure.

The coaxial synchronous torque coupling consists of two rings of permanent magnets as shown in below figure. A magnetic force field is established between the north and south pole of magnets in the drive and drive assemblies.  The magnetic field is shown as dashed lines and shaded areas. Refer below figure

Motor torque is transmitted to the impeller assembly by mean of magnetic coupling.

Magnetic coupling consisting of a permanent outer magnet ring (in the picture shown in RED colour) and an inner torque ring or inner magnetic ring (in the picture shown in Green colour) containing a network of copper rods supported on a mild steel core. The rotating outer magnet ring generates eddy currents in the copper rods which converts the core to an electromagnet. The electromagnet follows the rotating outer magnet ring, but at a slightly slower speed due to slip.

Containment shell

It is a pressure containing part located within the drive end that separates the inner and outer magnet ring of a magnetic drive pump. The shell must contain the full working pressure of the pump since it isolates the pumped liquid from the atmosphere. Since the magnetic force field must pass through the shell, it must be made of a non-magnetic material. Non-magnetic metals such as Hastelloy and 316SS are typical choices for the containment shell.

The motion of magnets past an electrically conductive containment shell produces eddy currents, which generate heat and must be removed by a process fluid recirculation circuit. The eddy currents also create a horsepower loss, which reduces the efficiency of the pump. Metals with low electrical conductivity have lower eddy current losses, providing superior pump efficiency.

Hastelloy has a relatively low electrical conductivity and good corrosion resistance thus it is an excellent choice for containment shells. Electrically non-conductive materials such as plastic and ceramics are also good choices for containment shells since the eddy current losses are eliminated. Thereby sealless pump efficiency is equal to convention centrifugal pumps. Plastic containment shells are generally limited to lower pressure and temperature.

Bearing

Magnetic drive pumps utilize process lubricated bearings to support the inner drive rotor. These bearings are subject to corrosive nature of the liquid being pumped. Hence the bearing made up of corrosion-resistant materials.

Two commonly used materials are hard carbon and silicon carbide (SIC). Pure sintered SIC is superior to reaction bonded SIC since reaction bonded SIC has free silicon left in the matrix, resulting in lower chemical resistance and lower strength.

Hard Carbon

Hard carbon against silicon carbide offers excellent service life for many chemical applications and also offers the advantage of short-term operation in marginal lubrication conditions.

Silicon Carbide

Silicon carbide against silicon carbide offers excellent service life for nearly all chemical applications. Its hardness, high thermal conductivity, and strength make it an excellent bearing material. Silicon carbide must be handled carefully to prevent chipping. Silicon carbide against silicon carbide has very limited capability in marginal lubrication conditions.

As similar to normal centrifugal pumps these pumps also consist of Impeller, wearing, throat bushings, auxiliary impeller, and/or flushing-line arrangements. Also, maintain the rotor chamber pressure greater than the suction pressure. The pump design shall also ensure that the temperature and pressure in the rotor chamber prevent vaporization at all operating conditions, including minimum flow while providing continuous flow through the rotor chamber for cooling and bearing lubrication.

Important Factor For Magnetic Drive Pump Selection

1. Vaporization is the bearing area of sealless pumps may occur at flows above minimum flow in the rotor area. To avoid this the temperature versus pressure profile throughout the pump must be above the vapour pressure curve at all locations in the flow path for all conditions. This is to ensure that the fluid does not flash.
2. We should consider an appropriate NPSH margin in addition to the NPSH required specified by pump vendor. It is desirable to have an operating NPSH margin that is sufficient at all flows (from minimum continuous stable flow to a maximum expected operating flow) to protect the pump from damage caused by flow recirculation, separation, and cavitation. Consideration should be given to the effects of heated fluid when recirculated back to the pump suction in establishing the NPSH margin.
3. Magnetic drive pump shall be not used to high temperature pumping liquid.

Do not Install Magnetic Drive Pump to Overcome System Problem

Magnetic drive pumps should not be installed to solve a maintenance problem, such as a troublesome mechanical seal, without first determining the real reason for the problem. Once the problem has been identified, ensure that installation of Magnetic drive pumps will not create a ripple effect. Typical pump and system problems to watch for are:

• Cavitation
• Operating too far from the best efficiency point (BEP)
• Net positive suction head available (NPSHA) too low
• Slurries
• Pump operating without liquid in the unit.