What is a Mechanical Seal?

As its name suggests a mechanical seal is simply a method of containing the fluid within a vessel (pump, mixer etc.) where a rotating shaft passes through a stationary housing or occasionally, where the housing rotates around the shaft.

It contains three seals:

There is a static portion fitted into a housing with a static seal. This may be an O-ring or similar, a shroud or it may be clamped in position.

The rotary portion of the seal is sealed onto the shaft by similar means. This seal can also be regarded as static as it rotates with the shaft at the same speed. As the relative velocity between the rotary part of the shaft and the seal is zero, it can thus be regarded as static.

The mechanical seal itself is the interface between the static and rotary portions of the seal.

One part of the seal, either static or rotary portion, is always resiliently mounted and spring loaded to accommodate any small shaft deflections, shaft movement due to bearing tolerances and out-of-perpendicular due to manufacturing tolerances, etc.

How does a Mechanical Seal work?

From the simplest single-spring unbalanced seal to the most complex balanced multi-spring seal, all mechanical seals operate on the same principle. Once this principle is understood it can be applied to any seal and the reasons for the differences in design and causes of premature seal failure become easily understood.

Seal Life

The life span of a mechanical seal is dependent upon several factors, some within the control of the seal designer and some due to operating conditions under which the seal must work.

A seal, which operates at slow speed in ambient temperature, low-pressure and clean water will last longer than one at high temperature, high pressure, and high rotational speed.

Mechanical seals usually contain two faces in contact. One of the faces is generally made of a softer material and is therefore sacrificial. Carbon is the most common soft-face material, but tungsten carbide can be considered a soft-face if it is run against silicon carbide.

But what is good seal life and how do I know if I have it?

This is simple. As the soft face is deemed to be sacrificial, and if this material has not completely worn away, it must be considered as a premature failure.

Mechanical seals only fail for two reasons. Either the two contacting faces opened or one of the seal parts became damaged. If on inspection of the seal faces it is discovered that the hard face is scored, the seal faces must have opened and allowed debris between them as the soft face is not capable of damaging it. If you think that it is, why do we replace carbon/graphite motor brushes and not the copper armature?

When you are using a well-designed seal most failures are due to some outside influence, the art is knowing what they are. If at home you plugged in the toaster and it blew a fuse, you would replace the fuse and plug it in again. If you then blew another fuse, you would check out the toaster. Mechanical seals are fuses in the system and must be treated as such, yet we regularly replace them every day, blame the seal, curse the seal salesman, and stick pins in the boxes of the manufacturer. Worse still, because usage is high, purchasing now looks elsewhere for something similar but cheaper, sometimes returning to packing. and opens a whole new can of worms.

Correctly chosen and fitted, mechanical seals will repay the initial seal and equipment costs many times over.

Basic Principles

The mechanical seal interface is basically a spring-loaded vertical bearing, the seal face being super-lapped to a high degree of flatness (typically 2-3 Helium light bands (0.00003” / 0.0008mm). The mechanical seal itself is formed by the interface of the faces.

Using a combination of hydraulic force from the sealed fluid and spring force from the seal design, leakage is eliminated. Should the seal faces remain in contact, the only possible way it can visibly leak is if the product particulate size is smaller than 0.00003” / 0.0008mm. This can happen but it is extremely unusual.

What happens between the two seal faces can vary on the individual application i.e.:

· There can be a film of lubricant between the seal faces.

· There can be a film of gas between the faces.

· The seals can be contact dry running.

· Most often it is assumed the seals run with a combination of all three conditions between the faces – boundary lubrication.

The ideal condition under boundary lubrication conditions is when the fluid film thickness between the faces is enough to reduce to a minimum friction loss but thin enough to prevent any visible product leakage.

Too thick and the seal will be running hydrodynamically I.e., friction will be very low, seal life will be high but there would be constant visible leakage.

Too thin or no lubrication film present and the seal will run dry with the consequence of very low seal life due to the friction levels. There can be the added consequence of excessive temperatures conducted to the elastomers again leading to premature failure.

Having now designed two faces that are to run in contact it is possible that the sealed fluid can be acting on the inside or outside diameters of the faces, the choice is made by the seal company.

Fluids acting on the inside diameter of the seal face create two major problems:

a.    Carbon as a material is extremely strong when acting in compression; unfortunately, this design puts it under tension where it is at its weakest.

b.    As centrifugal force created by the rotating element acts on the fluid, any solids however small are forced to the outside. These solids can now build up underneath the dynamic seal face and prevent it from moving freely with the shaft. This would then result in the seal faces opening causing the seal to leak.

Designing the seal with the fluid acting on the outside diameter is a far superior design as this now uses the centrifugal force to throw any solids away from the seal faces, plus the carbon material is now allowed to operate under compression.

Leakage

Leakage has always been referred to as “visible leakage”. This is because as seen above, a very thin fluid film holds the two seal faces apart from each other. As we have a gap here, we have a leak path and it is thus impossible for a mechanical seal to be totally leak-free. What we can say is unlike gland packing, the amount should be so low as to be visually undetectable.

Seal Types

Seals can be broken down into several basic groups and then further subdivided into design variations

The basic groups are:

• Single / Double
• Unbalanced / Balanced

The main variations being:

• Single / Multiple
• Component / Cartridge
• Rotary / Stationary spring
• Inside / Outside

In Summary - Why Do We Use Mechanical Seals?

• No “visible” leak - Seals do leak vapour as the fluid film on the faces reaches the atmospheric side of the seal faces
• This would approximate to 1/2 teaspoon a day at normal operating pressures and temperatures if it were captured and condensed
• Modern cartridge seal designs do not damage the pump shaft or sleeve
• Day-to-day maintenance is reduced as seals have inboard springs which make them self-adjust as the faces wear
• Seals have lightly loaded faces which consume less power than gland packing
• Bearing contamination is reduced in normal operation as the lubricant does not become affected by seal leakage and wash out
• Plant equipment also suffers less from corrosion if the product is contained in the pump
• Vacuum can also be sealed with this technology, a problem for packing as air was drawn into the pump
• Less wasted product will save money, even water is an expensive commodity and less clean-up of the area will be needed

If you want to find out more about increasing the operating life of your seals, see our video series: www.sealselection.com/pump-reliability

Written by Chris Dean