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Sealless pump | Simple Comparison of Canned and Magnet

canned magnet
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Canned pump and magnet pump, which are sealless pump, are very often used in batch chemical plants.

Sealless pump are synonymous with process pump.

Speaking of sealless pump, canned pump and magnet pump.

Many people get confused about which one to use.

Some plants use only magnet pump to the extreme.

We have summarized the proper use of canned pump and magnet pump.

The process pump is determined by the canned pump and magnet pump!

overview

Canned pump and magnet pump are generally used differently depending on the material.

  • canned pump; stainless steel; Used for general-purpose processes
  • magnet pump; PTFE; Acid-resistant process made of resin

For non-mechanical engineers, this is enough.

As a mechanical engineer, I would like to know more about this.

principle

First, let us introduce the principle of canned pump.

Main structure of canned pump

Introduce the main structure of the canned pump.

See the diagram below.

Structure of canned pump

It is OK if only three parts are kept at a minimum.

  • coil
  • impeller
  • bearing

Let’s look at each role.

coil

Coils are there to transfer energy to the impeller shaft.

See the diagram below.

coil movement

This is an image that conveys the energy of the coil to the shaft.

It’s exactly the same as a motor coil.

  1. current flows in the coil
  2. The magnetic field changes with the coil
  3. A current flows in the shaft that receives the change in the magnetic field
  4. Shaft receives force due to current flowing in the magnetic field
  5. the shaft turns.

impeller

The impeller is where the pump is the pump.

The rotation of the impeller transfers energy to the pump.

impeller rotates

As a pump, the impeller is of course important.

There are several patterns, such as closed and semi-open, depending on the shape of the impeller.

bearing

Bearings are especially important in sealless pumps.

This is a minor component for centrifugal pump, but critical for canned pump.

This is because the bearings come into contact with the process fluid.

See the diagram below.

bearing

Since sealless pumps literally have no shaft seal, the bearings are eaten by the process fluid.

Conversely, centrifugal pump with “shaft seals” can have their motors and bearings outside.

In the old days, there were no bearings that could be used for process liquids, so I think centrifugal pump that were outside the market were the mainstream.

Most of the bearings used in canned pump are made of ceramics.

Ordinary bearings used in centrifugal pump are just metal. Bearing itself used for general machine parts.

Drive system

Canned pump and magnet pump have distinctly different drive systems.

  • Canned pump; Rotate the shaft by changing the magnetic field of the canned pump coil
  • Magnet pump; Rotating magnets turn the shaft

The decisive difference is whether the magnet part physically moves or not.

Motor principle

First, let me briefly explain the principle of the motor.

Among the motors, we will describe the three-phase squirrel cage induction motor.

  1. electricity flows in the coil
  2. The magnetic field around the coil changes
  3. The shaft that receives the change in the magnetic field begins to move

Physically, it uses the principle of electromagnetic induction .

It is an electromagnetic mechanism in which the magnetic field changes when electricity is passed through it, and electricity flows when the magnetic field changes.

It is possible to spatially separate the “physically immovable” electric coil and the “physically moving” movable part.

By doing so, it is a device that can convert electrical power into mechanical power.

canned pump is the motor itself

Canned pump uses the principle of the motor itself.

Since there is a bulkhead between the coil and the shaft, there is no risk of leakage.

The bulkhead provides the greatest advantage of not requiring a mechanical seal.

magnet pump rotates magnet

Magne pump is one cushion into the principle of the motor.

  1. electricity flows in the coil
  2. The magnetic field around the coil changes
  3. The shaft that receives the change in the magnetic field begins to move
  4. A magnet connected to a shaft rotates
  5. The magnetic field around the magnet changes
  6. A magnet attached to the shaft on the impeller side rotates

As with canned pumps, there is a diaphragm, so there is no leakage.

You can think of it as a two-stage motor that rotates the magnet.

Material

The decisive difference in specifications between canned pump and magnet pump is the material.

metal canned pump

The wetted parts of canned pump are made of metal. In particular, the main body material is often SUS304.

This is because steel will rust.

When the generated rust clogs between the impeller and the casing,

This is because it becomes an obstacle when transmitting the electromagnetic force of the can to the impeller side.

When using canned pump, we want to keep the inside of the process clean.

Batch-type chemical plants have versatility because the process liquids handled are not fixed.

Whether SUS304 has versatility is a delicate question, but…

Of course, SUS316L and Hastelloy can also be used, but they are of course more expensive.

Magnet pump is resin type

The wetted parts of magnet pump are made of resinPTFE lining is possible.

A pump with low cost and high corrosion resistance can be realized.

This is an advantage that canned pump do not have.

The idea is to improve corrosion resistance, even if it means sacrificing heat resistance and conductivity.

sticker

Canned pump and magnet pump have slightly different sealing properties.

gasket for canned pump

Canned pump seals are basically gaskets.

Gaskets have two characteristics: high sealing performance and low cost.

As a user, I would like to choose the gasket type if nothing happens.

In terms of material, if it is a fluororesin gasket, it is almost no care.

Gaskets have a larger sealing area than O-rings, so they last longer.

Gasket is right.

Gaskets are convenient anyway

O-ring for magnet pump

Magnet pump seals may be of the O-ring type.

Of course, there is also a gasket type, but if you forgive yourself and buy an O-ring type, it will be quite troublesome.

Unfortunately, O-rings are not very good if they are expensive, have a long delivery time, and have a short life.

Fluoropolymer materials are used for O-rings used in chemical plant equipment.

If it is a magnetic pump that requires fluororesin corrosion resistance,

O-rings are also required to be of equivalent material.

The number of cases that can be handled by FKM is gradually decreasing.

However, in FFKM, it is very expensive.

It’s ironic that the O-ring is expensive for equipment that seeks cheapness, such as a magnet pump.

This is the reason why gaskets are preferred for magnet pump.

There is a big difference between manufacturers who are aware of this and those who are not.

O-rings are brittle

Other differences

Canned pump and magnet pump have different drive systems, and the crucial difference is their materials.

There are other subtle differences, so let’s introduce them.

There are many variations of canned pump

Canned pump are characterized by a wide variety of variations.

In addition to materials, there are applications similar to centrifugal pump, such as types such as vertical and horizontal types, and jacketed and self-priming types.

On the other hand, magnet pump, are mostly limited to horizontal types.

Magnet pump requires parts

Magnet pump consists of a combination of several parts.

It is easy to deal with failures if parts are secured.

It looks like an advantage when written like this, but doesn’t it look like a disadvantage when written like this?

I have to prepare the parts.

Parts are useful for highly corrosive liquids that can only be handled by magnet pump.

But if it is a system that can be used for both canned pump and magnet pump, can parts be an advantage?

Isn’t it better to use canned pump that can be used without worrying about parts?

It’s not strange that there are users who think like this.

Usually, it’s troublesome to take care of parts. As a security guard, I feel the same way.

Pump heat input

Magnet pump have lower pump heat input than canned pump.

Low pump heat input is related to process safety.

The lower the heat input to the pump, the less likely the contents will warm up.

Canned pump and magnet pump are the same up to the point that magnetic force is generated by electric current.

Let’s assume that the energy loss that occurs up there is approximately equal.

The problem is until the generated magnetic force is transmitted to the shaft.

magnet pump is cooled by outside air

magnet pump is in the direction of being cooled by the outside air.

Rather, it is easier to understand if you think of it as the opposite of a canned pump.

It is also possible to interpret that heat is difficult to transfer to the process liquid.

canned pump is cooled by the internal solution

canned pump is cooled with its contents.

When canned pump is operated, heat is transferred to the process liquid.

This is because the outside of the motor coil of the canned pump is in contact with the atmosphere , and the inside is in contact with the liquid inside.

Liquids are denser than gases, so they conduct heat more easily.

As a result, the amount of heat transmitted to canned pump is greater than that of magnet pump, which tends to warm the process liquid.

reference

Pumps are basic equipment, but that’s why it’s difficult to select.

It will take more than 5 years to be able to select a pump properly, so it is a good idea to take your time and work on it.

Related article

Related information

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lastly

I explained the difference between canned pumps and magnetic pumps from the perspective of a batch chemical plant.

Although there are large differences in materials and drive methods, there are also subtle differences in sealability and heat input.

Magnetic pumps are the most versatile.

I want to be an engineer who knows the difference.

Please feel free to post your worries, questions, and questions about the design, maintenance, and operation of chemical plants in the comments section. (Comments are at the bottom of this article.)

*We will read all the comments and reply seriously.