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Basic Knowledge of Mechanical seal for plant engineer

mechanical seal basic
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Mechanical seal are, in a sense, a must-know if you are an electromechanical engineer in a chemical plant.

The mechanical seal itself sounds cool.

A mass of technology. It feels like a machine.

Although it is a part that appears in a centrifugal pump, the design of the mechanical seal is so important that it can be said to be the design of the centrifugal pump itself.

There are various types and it is difficult to understand, but it is easy to cause problems in the field.

With this knowledge alone, it is possible to propose countermeasures and gain trust from the site.


Mechanical seals are one of the few “mechanical” machines in chemical plants.

Structure of mechanical seal

Various types of mechanical seals have been developed.

  • rotary ring
  • fixed ring
  • secondary seal
  • spring
  • metal pin

Although there are many models, the five basic points listed here are common.

The model is expressed in terms such as 1st generation, 2nd generation, etc., and the name “mechanical seal” makes it a part with a strong mechanical image.

Material of sliding surface

There are two major types of materials for the sliding surfaces of mechanical seals: SiC and carbon.

The sliding surface refers to the surface where the rotating ring and stationary ring face each other.

Here, the process objects such as liquids and gases are sealed to prevent them from leaking to the outside.

There are various requirements for the material of the sliding surface.

Mechanical properties, self-lubricating properties, wear resistance, thermal conductivity, heat resistance, coefficient of linear expansion, corrosion resistance 

There is no material in the world that satisfies all of these properties, and there are various candidate materials that have advantages and disadvantages.

Carbon, cemented carbide, alumina, SiC…

Of these, knowing only SiC and carbon is sufficient for batch-type chemical plants.

SiC

SiC is a ceramic called silicon carbide.

It has high thermal conductivity, high hardness, and high corrosion resistance.

High thermal conductivity allows heat generated on sliding surfaces to escape to the outside.

Hardness makes it resistant to friction that occurs in sliding parts.

High corrosion resistance is very important in chemical plants.

Because it has excellent overall performance, it is used as the basic material for sliding surfaces.

carbon

Carbon is created by sintering carbon.

It conducts heat easily, is porous, and is extremely soft and resistant to corrosion.

Its characteristic is that it is softer than SiC.

Combination of sliding surfaces

Let’s take a look at the combination of sliding surfaces.

  • SiC / carbon
  • SiC / SiC

These two patterns are common.

This distinction is “hardness“.

SiC / carbon has a certain degree of clear difference in hardness, and the image is that it is used while cutting carbon.

You would think that SiC / SiC would have the same hardness because it is the same SiC, but even SiC has different hardness.

It is characterized by being difficult to chip because there is little difference in hardness.

SiC / SiC is suitable for slurry, etc.

If slurry gets caught in the sliding surface, the sliding surface will wear down as the machine continues to operate.

SiC/SiC is a good choice for mechanical seals for slurry pumps .

We use SiC / SiC with the idea that we want to make it as hard as possible to alleviate this abrasion.

Material of sliding surface.

SiC / carbon is common.

Inside type/Outside type

There are two types, an inside type and an outside type, which are a combination of a fixed ring and a rotating ring.

inside type

The image of the inside type is as follows.

Inside (mechanical seal)

Inside-side is a structure in which the rotating ring is inside, that is, inside the process liquid.

The inside type is more commonly used.

The reason is simple: even if leakage occurs from sliding surfaces, damage is unlikely to spread .

This is because centrifugal force causes the liquid to move away from the axis of rotation.

It would be faster to look at the outside type.

Position of rotating ring/fixed ring

Inside type is common

outside type

The image of the outside type is as follows.

Outside (mechanical seal)

The outside side is a structure in which the rotating ring is outside the process liquid.

If there is a leak from the sliding surface of the outside type, the direction of the leak and the direction of the centrifugal force will match.

The liquid that receives the centrifugal force actively moves in the direction of leakage.

This is the exact opposite of safety.

I don’t want to use it as much as possible.

The reason for choosing the outside is the problem of the material of the spring.

Corrosion resistance of materials is an issue for mechanical seals for centrifugal pumps that require high corrosion resistance.

Springs are the most important here.

For springs that are subjected to large forces in rotating equipment, in addition to corrosion resistance at room temperature and as a static device, we must also consider corrosion resistance when subjected to force or at elevated temperatures.

I feel anxious.

In that case , the idea is that it would be safer to use an outside type so that the spring does not come into contact with the process.

Rotating type/Stationary type

The difference between rotary type and stationary type is where the spring is attached.

See the diagram below. Rotating type on the left, stationary type on the right.

Rotating stationary (mechanical seal)

The rotary type has a spring on the rotating ring side, and the stationary type has a spring on the stationary ring side.

The rotating type is a simpler idea.

Mechanical seals use springs, etc., to maintain the seal surface without being affected by vibrations generated by rotational motion.

You want to place the spring exactly where the vibration occurs.

However, the stationary type has greater advantages.

  • Strong against high speed rotation
  • Resistant to slurry

If the structure is such that a spring is attached to the rotating shaft, such as a rotating type, the spring itself will deform during high-speed rotation.

This is the direction that prevents vibration absorption.

With a stationary type, the spring is not affected by centrifugal force, which makes it more stable.

If it is a stationary type, it is also possible to take the spring out of the process.

If the spring does not come into contact with the process liquid, there is no possibility of slurry liquid getting into the spring, and spring failure is less likely to occur.

Rotating type/Stationary type.

Stationary type is common.

Balanced type/Unbalanced type

Mechanical seals come in balanced and unbalanced types.

Balanced/Unbalanced (mechanical seal)

The left side is the unbalanced type and the right side is the balanced type.

Balanced type and unbalanced type refer to the balance of pressure applied to the rotating ring.

If it is an unbalanced type, it will be difficult to absorb the pressure of the process liquid (imbalanced).

This causes the sealing surface to vibrate and lose stability, increasing the possibility of leakage.

The rotating ring receives two types of pressure : the process fluid and the seal surface.

In the case of a single mechanism, the process fluid and atmospheric pressure act on the rotating ring.

Naturally, the process liquid has a higher pressure, so the higher the pressure receiving area of ​​the high pressure part, the worse the balance will be.

The balanced type attempts to force balance by increasing the pressure receiving area on the seal side (atmospheric pressure side).

The unbalanced type is more basic, and the unbalanced type is sufficient for batch chemical plants.

There are some special circumstances that you must consider when choosing a balanced type.

  • operate at high pressure
  • add a step to the shaft

The balanced type is selected only when the pressure on the process liquid side is high.

Batch chemical plants operate at low pressures that can be handled by unbalanced plants.

If you choose a balanced type , you will need to add a step to the shaft.

This increases the possibility of shaft breakage and increases the size of the mechanical seal box, creating unfavorable factors for the pump.

Even so, high pressure operation cannot be avoided, so the idea is to use a balanced type.

balance/unbalance

Imbalance is common

Single spring/Multi spring

There are two types of mechanical seal springs: single springs and multi springs.

  • single spring 1 piece spring
  • Multi-spring  multiple springs

Although this is the only difference, there are many other differences.

Single springs are resistant to slurry, and multi-springs are suitable for high-speed rotation.

Single springs have only one spring, so they are larger.

The gap inside the spring is large, making it difficult for slurry to accumulate even if it gets in.

Multi-springs have a large number of springs and are small in size, so they tend to get clogged easily.

It is best to avoid using multi-springs for slurry.

On the other hand, multi-springs have a large number of springs, so they can easily absorb vibrations caused by high-speed rotation.

For example, if you use three springs to press the sliding surface, it will be very easy to balance. That’s a three point instruction.

secondary seal

A secondary seal is a seal that is not a primary seal.

The primary seal is the sliding surface of the mechanical seal itself.

Secondary seals refer to seals in the following locations.

  • Fixed ring and mechanical seal box
  • Rotation intuition and shaft

Common sealing materials such as O-rings, V-rings, and bellows are used.

O-ring

A mechanical seal with an O-ring is shown below.

O-ring (mechanical seal)

An O-ring is used for the seal between the rotating ring and the sleeve.

O-ring seals are not often used in chemical plants.

This is because the O-ring has high corrosion resistance.

If the chemical solution cannot be handled by general FKM, expensive FEP or FFKM will be required.

V ring

V-rings may be used instead of O-rings.

V-rings are specialized for dynamic seals. A type of packing.

PTFE V-rings don’t look very good, but they function normally.

Since it is a V-ring, the idea is to stack multiple pieces to extend its life.

bellows

Bellows may be used instead of O-rings or V-rings.

They have the advantage of reducing the number of parts.

Can serve as both a spring and a secondary seal.

But there are some caveats.

  • Cannot be used with highly corrosive liquids
  • Not suitable for slurry liquid

The problem is the same as when choosing the outside type due to the spring issue.

Of course, since bellows are used instead of springs.

hysteresis

Hysteresis is a hot topic for secondary seals.

It means that the conditions are different between the forward and backward movements of the reciprocating motion.

Hysteresis (mechanical seal)

This shows the relationship between spring deflection X and spring force F.

According to Hooke’s law of high school physics, F = kx, there should be a linear relationship, but in the diagram above, it is not a linear relationship.

This is because the secondary seal obstructs the movement of the spring.

The shape of O-rings and V-packings is a problem, and the resistance changes when they expand or contract.

On the other hand, bellows doesn’t make much difference when it expands or contracts.

If you compare the lifespan of O-rings, V-rings, and bellows using clean, non-corrosive liquid, the bellows will likely have the longest lifespan.

This is because the hysteresis is small and the fluctuation of the force applied to the spring is the smallest and stable.

How to fix the rotating ring

There are several patterns for fixing the rotating ring.

pin

This is the first method that comes to mind if you have mechanical knowledge.

Pin (mechanical seal)

Connect the mechanical seal to the shaft and casing with pins.

The rotating ring now moves in conjunction with the rotating shaft, and the stationary ring is fixed to the casing without being dragged by the rotating ring.

A very primitive method.

Simple and reliable.

I usually use this pin fixation.

spring bellows

It is intended to give the spring and bellows themselves a function to prevent rotation.

The rotating ring is fixed with a spring or bellows without a pin.

Since it is fixed with a spring or bellows, there is no need for a pin, which has the advantage of reducing the number of parts.

However, springs and bellows require a certain amount of strength.

rubber

The idea is to use rubber instead of using metal parts such as pins.

It is rarely used in chemical plants.

This is because there are various problems such as deterioration of the rubber itself, corrosion resistance, and friction strength.

How to install the fixed ring

There are several ways to attach the fixed ring.

clamp type

The clamp type is as shown below.

Clamp (mechanical seal)

This method securely fixes the fixed ring.

That’s why if the casing or shaft is tilted, the fixed ring will also tilt.

Accuracy during pump manufacturing and centering during installation are important.

press fit

The press fit type is as shown below.

Press fit (mechanical seal)

The casing completely absorbs the pressure applied to the fixed ring.

As introduced with the O-ring and gasket types, the casing does not receive only a portion of the pressure applied to the fixed ring, but the entire surface.

Therefore, press-fit types are used for high pressure applications.

float

The float type is as shown below.

Float (mechanical seal)

It is less balanced than the press-fit type, and does not clamp like the clamp type.

It’s probably called a float because it looks like it’s floating.

Because it is a float, it has the property of easily following vibrations caused by rotational motion.

shaft protection

For mechanical seals, it is necessary to consider the protection of the shaft in practical terms.

sleeve

A mechanical seal secures the rotating ring to the shaft.

At this point, friction occurs between the rotating shaft and the shaft, causing the shaft to wear down.

If the mechanical seal reaches the end of its lifespan and is replaced when the shaft also reaches the end of its lifespan, the damage will be considerable.

Major repairs will be required, including removing and reinstalling various parts such as the impeller.

To prevent this, attach a sleeve to the shaft.

The shaft and sleeve can be separated, and the sleeve and mechanical seal come into contact.

Only the sleeve wears out during operation.

The idea is that when replacing a mechanical seal, you only need to replace the sleeve.

This is a method commonly used in rotating machinery.

If you use a balanced type with a step on the shaft, you will also need to think about the sleeve.

bellows

There is an idea to use bellows in a similar way to the sleeve.

The idea is to protect the shaft with metal bellows or rubber bellows.

Its uses are limited.

Double mechanical seal

There are two types of mechanical seals : single mechanical seals, which use only one seal, and double mechanical seals, which use two mechanical seals side by side.

The double mechanical seal has the following characteristics:

  • Used for high pressure process fluids
  • Prevent direct leakage of process fluid

Double mechanical seals can contain a buffer between the two mechanical seals.

It is also closely related to the flushing plan.

Back (mechanical seal)

Back type

Front (mechanical seal)

front

Tandem (mechanical seal)

Tandem

In the back type, the two sealing surfaces face oppositely, and in the tandem type, they face the same direction.

The rear type is a combination of outside and inside types, and the tandem type is a combination of inside and inside types.

combinationprocess sideatmospheric sidebuffer
backoutsideinsidehigh pressure
Tandeminsideinsidelow pressure

The back type and tandem type are related to the pressure of the buffer solution.

It will be determined by the response to leaks , which was discussed on Inside and Outside.

For backside type systems, it is preferable for the buffer solution to be at a higher pressure than the process solution.

This is because the back type seal is an outside type.

It is used when there is a strong need to prevent the process liquid from leaking outside.

When the seal breaks with an outside type, the process liquid tends to leak into the buffer side due to centrifugal force, but the idea is that the buffer solution suppresses this.

If it is necessary to take care of contamination of the buffer solution into the process liquid side, it is difficult to choose the rear type.

In this case, you will need to choose a front type (the opposite of the back type) or a tandem type, or consider eliminating the double mechanical seal itself in the first place.

The tandem type has the role of stabilizing the operation of the mechanical seal used at high pressure.

It really acts as a buffer.

The idea is to use a buffer solution at a constant pressure to press the high-pressure process fluid against the sliding surface in order to maintain an appropriate balance of pressure on the sliding surfaces, which was discussed as a balanced/unbalanced model.

The front type is unbalanced on the atmosphere side, so the buffer solution leaks outside, so it is not used often.

PV value

Mechanical seals have the concept of PV value.

It is a value determined by pressure P x ​​rotational speed V, and the idea is to select the mechanical seal model based on this value P.

The figure below shows the PV value and the range of applicable mechanical seals.

PV(mechanical seal)

There are many types.

There are 37kW vortex pumps and 22kW stirring blades as large rotating machines in batch chemical plants, but in this table, they are shown in the lower left corner…

With this, you don’t have to be particular about the type of mechanical seal.

reference

Related article

Related information

pump specification

lastly

We have explained the knowledge of mechanical seals as they are used in batch chemical plants.

Material・Inside/Outside・Rotating type/Stationary type・Balanced/Unbalanced・Single/Multi・Secondary seal・Fixing method・Double mechanism・PV

There are many different types, but if you know the basics, you can apply them widely.

Please feel free to post any concerns, questions, or concerns you may have regarding the design, maintenance, and operation of chemical plants in the comments section. (The comment section is at the bottom of this article.)

*We will read all comments received and respond seriously.