Magnetic Pigs

Magnetic pigs are mandrel bodied pigs fitted with high strength neodymium magnet boxes and are used for pig tracking or cleaning of ferrous debris in the pipeline. Magnets can also be moulded into solid flexible polyurethane pigs or moulded into polyurethane discs if space is a restricting factor.

About: Magnetic Pigs

Magnets are attached to pigs for a number of reasons:

The ones used to operate valves are generally bonded into small diameter pigs and are used in the food industry to open or close valves during the manufacturing process. They are also used in the soft drinks industry for the same purpose, to open and close valves, in order to change from one process to another.

Magnetic non-intrusive pig signaller detection devises are looking for a change in magnetic field for them to operate. Magnets for this purpose can be bonded into the polyurethane spacers on smaller diameter pigs or they can be mounted directly onto the pig body in cassette bracelets.

The design, power, location and orientation should always be considered in order to provide the highest gauss/magnetic field possible to ensure the easiest detection. Magnets used for this purpose are generally neodymium iron boron rare earth composite magnets and can be quite small but quite powerful. It is important to remember that the magnets are not housed inside the body of a carbon steel pig, carbon steel pig bodies can shroud the magnetic field being generated by the magnet(s) therefore minimising their power.

If they are mounted onto the body, they should either mounted onto the rear spider plate of the pig, therefore protruding from the back or mounted directly to the outside of the pig body centrally between the mounting flanges.

The only time they can be mounted inside the body is if the body is manufacture from stainless steel as stainless steel does not have the same shrouding effect as carbon steel as it is an alloy and therefore nonferrous. Note: this should also be considered when mounting electro-magnetic transmitters into the pig body as the same design phenomenon is pertinent.

A magnetic cleaning pig is used to remove ferrous debris from pipeline. In this case it is more effective if the magnets are attached to the body of the pig as they are more effective in collecting debris. However when space is critical, particularly in small diameter pigs up to 12”, the neodymium magnets can be moulded into the polyurethane spacers on small diameter pigs.

It has to be remembered that should the magnets be placed too close to the pipe wall there is a chance that the centre of the pig, between the steel mounting flanges, could become compacted with ferrous debris. This could cause the pig could become lodged in the pipeline.

Once again, it is important that all the details of the pipeline are known before the pig is designed and manufactured. Brushers can often used in conjunction with magnets to assist in the ferrous debris being removed from the pipe wall. The resulting ferrous debris can then be collected with the magnets mounted on the pig.

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