This article will explain what a mechanical seal is and the key design features that make them work.
Basics of fluid pump sealing
A mechanical seal is simply a method of containing fluid within a vessel (typically pumps, mixers, etc.) where a rotating shaft passes through a stationary housing or occasionally, where the housing rotates around the shaft.
When sealing a centrifugal pump, the challenge is to allow a rotating shaft to enter the ‘wet’ area of the pump, without allowing large volumes of pressurized fluid to escape.
To address this challenge there needs to be a seal between the shaft and the pump housing that can contain the pressure of the process being pumped and withstand the friction caused by the shaft rotating.
Before examining how mechanical seals function it is important to understand other methods of forming this seal. One such method still widely used is Gland Packing.
Gland packing is a braided, rope like material that is packed around the shaft - physically stuffing the gap between the shaft and the pump housing.
Gland packing is still commonly used in many applications, however increasingly users are adopting mechanical seals for the following reasons;
- The friction of the shaft rotating wears away at the packing over time, which leads to increased leakage until the packing is adjusted or re-packed.
- The friction of the shaft also means that packing also needs to be flushed with large volumes of water in order to keep it cool.
- Packing needs to press against the shaft in order to reduce leakage – this means that the pump needs more drive power to turn the shaft, wasting energy.
- Because packing needs to contact the shaft it will eventually wear a groove into it, which can be costly to repair or replace.
Mechanical seals are designed to overcome these drawbacks
A basic mechanical seal contains three sealing points.
The stationary part of the seal is fitted to the pump housing with a static seal –this may be sealed with an o-ring or gasket clamped between the stationary part and the pump housing.
(Highlighted in red below, left the stationary part and right the rotary portion)
The rotary portion of the seal is sealed onto the shaft usually with an O ring. This sealing point can also be regarded as static as this part of the seal rotates with the shaft.
The mechanical seal itself is the interface between the static and rotary portions of the seal.
One part of the seal, either to 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 alignment due to manufacturing tolerances.
While two of the sealing points in a seal design are simple static seals, the seal between the rotating and stationary members needs a little more consideration. This primary seal is the basis of all seal design and is essential to its effectiveness.
The primary seal is essentially a spring loaded vertical bearing - consisting of two extremely flat faces, one fixed, one rotating, running against each other. The seal faces are pushed together using a combination of hydraulic force from the sealed fluid and spring force from the seal design. In this way a seal is formed to prevent process leaking between the rotating (shaft) and stationary areas of the pump.
The surfaces of the seal faces are super-lapped to a high degree of flatness; typically 2-3 Helium light-bands (0.00003” / 0.0008mm).
If the seal faces rotated against each other without some form of lubrication they would wear and quickly fail due to face friction and heat generation. For this reason some form of lubrication is required between the rotary and stationary seal face; this is known as the fluid film
The Fluid Film
In most mechanical seals the faces are kept lubricated by maintaining a thin film of fluid between the seal faces. This film can either come from the process fluid being pumped or from an external source.
The need for a fluid film between the faces presents a design challenge – allowing sufficient lubricant to flow between the seal faces without the seal leaking an unacceptable amount of process fluid, or allowing contaminants in between the faces that could damage the seal itself.
This is achieved by maintaining a precise gap between the faces that is large enough to allow in a small amounts of clean lubricating liquid but small enough to prevent contaminants from entering the gap between the seal faces.
The gap between the faces on a typical seal is as little as 1 micron – 75 times narrower than a human hair. Because the gap is so tiny, particles that would otherwise damage the seal faces are unable to enter, and the amount of liquid that leaks through this space is so small that it appears as vapor – around ½ a teaspoon a day on a typical application.
This micro-gap is maintained using springs and hydraulic force to push the seal faces together, while the pressure of the liquid between the faces (the fluid film) acts to push them apart.
Without the pressure pushing them apart the two seal faces would be in full contact, this is known as dry running and would lead to rapid seal failure.
Without the process pressure (and the force of the springs) pushing the faces together the seal faces would separate too far, and allow fluid to leak out.
Mechanical seal engineering focuses on increasing the longevity of the primary seal faces by ensuring a high quality of lubricating fluid, and by selecting appropriate seal face materials for the process being pumped.
When we talk about leakage we are referring to visible leakage of the seal. This is because as detailed above, a very thin fluid film holds the two seal faces apart from each other. By maintaining a micro-gap a leak path is created making it impossible for a mechanical seal to be totally leak free. What we can say, however, is that unlike gland packing, the amount of leakage on a mechanical seal should be so low as to be visually undetectable.
A recent case study indicated - Upgrading from packing reduces water usage and operating costs.
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-adjusting 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.
How to Improve Pump Reliability
If you want to find out more about increasing the operating life of your seals, see our video series...
how to improve pump reliability
Mechanical seals are devices that are used to provide a seal at the point of entry or exit of a rotating shaft. Typically it is used to prevent the leakage of one high pressure fluid into a lower pressure fluid.What is a mechanical seal and how does it work? ›
Mechanical seals are leakage control devices, which are found on rotating equipment such as pumps and mixers to prevent the leakage of liquids and gases from escaping into the environment.What are the three types of mechanical seals? ›
The most common seal types include the following: Balanced seals. Unbalanced seals. Pusher seals.What happens when mechanical seal fails? ›
When mechanical seals fail, they tend to spin on the shaft, or shaft sleeve. Grooves are worn into the metal and require that the shaft or sleeve be replaced. Once they're worn, they shouldn't be re-used.What is the most common mechanical seal? ›
The most common seal is the rotating seal and it is used as a component that holds rotating springs together.Is O-ring a mechanical seal? ›
An O-ring, also known as a packing or a toric joint, is a mechanical gasket in the shape of a torus; it is a loop of elastomer with a round cross-section, designed to be seated in a groove and compressed during assembly between two or more parts, forming a seal at the interface.How long do mechanical seals last? ›
Most seal manufacturers say that operation of the seal at one of these limits will result in a minimum life span of two years; three years with some seal types such as those specified in Standard API 682.How does a mechanical seal prevent leakage? ›
The whole purpose of pump seals – either mechanical seals or packing seals – is to contain the pressure of the pumping process and withstand the friction of the pump shaft rotating to prevent the pump from leaking.How do you identify a mechanical seal? ›
To determine the size of a component mechanical seal, measure the inside diameter of the seal face, the length of the spring, and diameter and thickness of the stationary seat.What is difference between mechanical seal and oil seal? ›
Oil or Rotary shaft seals are vulnerable to extreme temperatures, which can cause them to wear out faster if they are used in very high or low temperatures. A mechanical seal features stationary elements, rotating components and a spring.
Generally speaking, gaskets serve as a static seal between flat surfaces, such as joints, while seals are used in more dynamic environments between active components such as rotating shafts, pumps, and engines.Do mechanical seals need lubrication? ›
Lubrication is necessary for proper mechanical seal installation. Lack thereof can damage o-rings or rubber bellows on the seal, causing them to tear, or roll.How many mechanical seals does a pump have? ›
Double mechanical seal - This is where two mechanical seals are used within a pump. The first seal retains the fluid within the pump as per the standard mechanical seal above. The second seal contains a barrier fluid to ensure the faces are kept clean and are lubricated preventing failure.What is the price of mechanical seal? ›
Mechanical PTFE Seal
Price Range:-Rs. 50 - Rs. 10,000 (per piece).
The repair process may differ slightly but, on the whole, following safe disassembly, some parts can simply be cleaned while others can be reworked—seal faces being relapped, for example—and some, such as elastomers and metalwork, replaced.What causes mechanical seal to leak? ›
Mechanical seal leakage is typically unavoidable as seals age and develop wear. Inadequate flushing and lubrication of mechanical seals can also lead to seal damage, resulting in leaks that may present earlier in a seal's life than anticipated.Do all mechanical seals leak? ›
All mechanical seals leak. They actually have to in order to maintain a fluid film over the entire mechanical seal face. This mandatory leakage is what is referred to as a managed leak.Does a mechanical seal need water? ›
Mechanical seals require clean water, or other compatible liquid, for the lubrication of the seal faces. The faces in a typical mechanical seal are lubricated with a boundary layer of gas or liquid between the faces.What material is used for mechanical seal? ›
For mechanical seals, these include silicon carbide, silicon nitride, tungsten carbide, and alumina oxide.Is a dry gas seal a mechanical seal? ›
Dry gas seals are mechanical seals but use other chemicals and functions so that they do not contaminate a process. These seals are typically used in a harsh working environment such as oil exploration, extraction and refining, petrochemical industries, gas transmission and chemical processing.
O-Rings are the best choice for high-pressure applications as they can withstand extreme amounts without failure. While both sealing devices can function in a wide range of temperatures, gaskets are better suited for extreme temperature environments.Do O-rings stop leaks? ›
O-rings are one of the most common types of mechanical seals. Consisting of a ring-shaped piece of elastomeric material, they are placed around the mating surfaces of two parts. O-rings are designed to prevent leaks from around these mating surfaces.What is the most common cause of seal failure? ›
Improper installation is probably the most common cause of seal failure. Using the right tools is critical to prevent seals from being installed in the wrong direction or becoming damaged during installation.How can you tell if a seal is failing? ›
But detecting seal failure is possible by using a implementing a seal fail alarm into your pump controller box. The seal failure alarm uses probe sensors to detect early failure to the seal and tells the operator the pump needs to be serviced soon.When should mechanical seals be replaced? ›
The most obvious indicator that the mechanical seal on your equipment needs to be repaired or replaced is a spraying leak. If water is spraying by the shaft seal, there is definitely a need to have the mechanical seal checked out.What is the best way to seal a leak? ›
Use epoxy putty or pipe putty as a temporary fix to a leaky pipe. Pipe putty is designed to harden at room temperature and seal the hole or crack.Do mechanical seals need cooling? ›
Packing and mechanical seals used in fluid applications both require constant cooling and lubrication, and water is the most common means for achieving this. Seal water is used for cooling the seal and lubricating the seal faces.How is a mechanical seal lubricated? ›
In most mechanical seals the faces are kept lubricated by maintaining a thin film of fluid between the seal faces. This film can either come from the process fluid being pumped or from an external source.Why are o-rings used on mechanical seals? ›
An O-ring is a mechanical gasket that is shaped like a ring and they are placed within grooves and are designed to handle compression between two parts. This then works to create a seal, making them one of the most common seals seen in machine design.What is the purpose of a seal on a document? ›
Seals are used primarily to authenticate documents, specifically those which carry some legal import. There are two main ways in which a seal may be attached to a document. It may be applied directly to the face of the paper or parchment (an applied seal); or it may hang loose from it (a pendent seal).
A Seal-in circuit is a method of maintaining current flow after a momentary switch has been pressed and released.What is the life expectancy of a mechanical seal? ›
Most seal manufacturers say that operation of the seal at one of these limits will result in a minimum life span of two years; three years with some seal types such as those specified in Standard API 682.What causes a mechanical seal to fail? ›
Improper installation, improper start-up, and lack of maintenance can wear down seals and eventually cause them to fail. Mishandling of seals before installation and introduction of dirt, oil, or any other abrasive material can also cause damage that gets worse as the pump runs.How long does it take to get a seal contract? ›
12+ months of initial training that includes Basic Underwater Demolition/SEAL BUD/S School, Parachute Jump School and SEAL Qualification Training (SQT) 18 months of pre-deployment training and intensive specialized training.Does a seal count as a signature? ›
The word “SEAL” at the end of (or below) a signature line means 'signature'. For example, a mortgage note or Fannie Mae/Freddie Mac Texas Deed of Trust will have “(Seal)” at the end of the line for the borrower's signature.Can I use a seal instead of a signature? ›
Yes, signature stamps are legally binding as long as the stamp represents the user's intention and is validated by you or an authorized party you select. Therefore, if the signature stamp meets these requirements, it is a considered legal signature.Why are seals used in engineering? ›
Seals directly impact equipment performance. Used properly, they prevent fluid leaks, extend cylinder life, minimize friction and energy consumption, and improve actuator positioning accuracy. Seals are engineered to prevent leaks with minimal friction and wear.What are 3 types of motor controls? ›
There are four basic motor controller and drive types: AC, DC, servo, and stepper, each having an input power type modified to the desired output function to match with an application.Why is it called 3 wire control? ›
Three wire control means, one wire for stop command ("NC" Pushbutton) and two wires for drive start command of "Forward and Reverse("NO" Push button)". In three wire control, the stop command is provided separately.