What Is a Mechanical Seal? Working Principle, Components and Applications

A mechanical seal is a shaft sealing element that prevents process fluid from escaping where the shaft of rotating equipment such as a pump, mixer or compressor leaves the housing, and where needed keeps the outside environment from getting in. It does this by pressing together two precision-machined rings, one rotating with the shaft and the other fixed to the housing; only a fluid film on the order of a micron remains between the two faces.

In the field it is also called a mechanical face seal or simply a pump seal. Whatever the name, the core idea is the same: the sealing line does not run along the shaft surface but forms between two flat faces positioned perpendicular to the shaft. Springs and process pressure push these faces toward each other, and the film between them both limits leakage and lubricates the faces.

This article is the introductory guide to what a mechanical seal is, why it is needed and how it works. For a detailed classification of component, cartridge, bellows and double seals, see the mechanical seal types guide.

For a rotating shaft to pass through a stationary housing, there has to be a clearance between them. While a pump runs, pressurized fluid sits on one side of that clearance and the atmosphere on the other. If the gap is left open, fluid escapes; in equipment operating under vacuum, air is drawn in. The job of a shaft seal is to close this gap while still letting the shaft turn.

A gasket cannot do this, because it is designed to seal two surfaces that do not move relative to each other, such as two flanges, by being compressed between them. Gland packing works by compressing braided rings around the shaft with a gland; the rings rub directly on the shaft or shaft sleeve, and a controlled drip is usually allowed to carry away frictional heat. Lip seals, for their part, are mostly used for low-pressure oil and grease retention.

A mechanical seal moves the dynamic sealing away from the shaft surface to the interface between two rings perpendicular to the shaft. As a result there is usually no visible drip, the shaft or sleeve does not carry a continuous rubbing zone as it does with packing, and friction losses are lower in most applications. In return, the initial cost is higher and installation requires care. A detailed comparison of the two technologies is in the mechanical seal vs gland packing article, and the energy and total cost impact is covered in the pump sealing efficiency guide.

Following a mechanical seal from installation to steady operation is the easiest way to understand the principle. The steps in this section describe a general sequence for a liquid-lubricated single seal; details vary with the design.

When any link in the chain of preload, pressurization, film formation, equilibrium, heat management and wear compensation breaks, the seal starts to leak: if preload is insufficient the faces open, if the film vaporizes the faces rub dry, and if a secondary element hangs up the ring can no longer follow wear.

• Installation and preload: The rotating ring is mounted on the shaft and the stationary ring in the gland or housing. A spring or bellows creates a preload that presses the two faces together before the pump even starts; without it, fluid could escape between the faces when there is no pressure.
• Pressurization: The seal chamber fills with fluid and is pressurized. Pressure acts on the area behind the axially movable ring and creates a second closing force that adds to the spring force.
• Film formation and rotation: Driven by the pressure difference, fluid enters the gap between the faces from the pressurized side toward the atmospheric side and forms a thin film; once the shaft turns, this film lubricates the faces. The pressure inside the film generates an opening force that tries to separate the faces.
• Equilibrium: When the closing forces and the opening force of the film pressure balance, the faces run with a gap on the order of a micron, where solid contact and friction stay limited and leakage remains very low.
• Heat management: Friction at the faces and shear in the film generate heat. This heat is removed by the fluid, by circulation in the seal chamber or by an external flush; if the film approaches its boiling point it becomes unstable.
• Wear compensation: The faces wear very slowly over time. The spring or bellows pushes the axially movable ring forward by the amount of wear to maintain contact; the same mechanism absorbs small axial shaft movements and thermal expansion.

The film between the faces is both the lubricant and the sealing medium of a mechanical seal. Its thickness is typically on the order of a micron or less, tens of times thinner than a human hair. For such a thin film to be stable, the faces must be extremely flat: seal faces are lapped, and flatness is checked in helium light bands using an optical flat under monochromatic light. One helium light band corresponds to roughly 0.3 µm.

The closing force comes from two sources: the spring force and the hydraulic force that process pressure applies to the back of the movable ring. The spring force is relatively small and its main job is to keep the faces in contact when pressure is low or zero; as pressure rises, the hydraulic force makes up most of the closing force. The opening force comes from the film pressure between the faces.

How much of the pressure turns into face load is set by the balance ratio: the area on which pressure acts in the closing direction divided by the face contact area. In unbalanced seals this ratio is typically above 1, and face load rises quickly with pressure. In balanced seals a step on the shaft or sleeve brings the ratio below 1, so face load stays limited even at high pressure.

The duty a face pair can carry is assessed with the PV value, the product of face pressure and sliding velocity. As PV rises, frictional heat increases; if the fluid cannot carry the heat away, the film vaporizes, the faces rub dry and damage such as heat checking, excessive wear or elastomer hardening begins. That is why cooling the seal chamber and, where needed, feeding it with a clean flush fluid is part of the design.

Designs differ, but almost every mechanical seal contains component groups that perform the same functions. In industrial pump seals the spring-loaded, axially movable ring is usually on the rotating side; at high peripheral speeds and in some compact unitized designs, the spring assembly is placed on the stationary side. The secondary element material often sets the temperature and chemical limit of the seal before the faces do.

The component table summarizes the function and typical materials of each part. For a comparison of face materials by hardness, heat conduction and toughness, see the silicon carbide vs tungsten carbide article.

The gap between the faces is not the only place fluid can escape from a mechanical seal. Every point where components meet is a potential leakage path, and each one is closed by a separate element.

Zero leakage? In a liquid-lubricated single mechanical seal, the film flows very slowly from the pressurized side toward the atmospheric side, so leakage is technically not zero. The amount is often so small that it mixes into the surroundings as vapor at the atmospheric-side edge of the faces, helped by frictional heat, and cannot be seen. That is why “no visible leakage” or “very low, controlled leakage” is more accurate than “zero leakage”.

In toxic or volatile services where process fluid must not reach the atmosphere, a common solution is a pressurized double seal with a barrier fluid; depending on the application, unpressurized (buffered) double arrangements that route leakage to a collection system are also used. In the pressurized arrangement a very small amount of barrier fluid passes into the process and to the atmosphere; but as long as barrier pressure is maintained, what escapes is a selected clean barrier fluid, not the process fluid. Possible causes of leak symptoms such as drips, crystal buildup or vapor are covered in the mechanical seal failure and maintenance guide. The first step in diagnosing a leak is to find which of the five paths listed in this section it comes from.

• Primary seal (between the faces): The dynamic line between the rotating and stationary rings; by design the only path that allows a very small film flow.
• Between the rotating unit and the shaft or sleeve: Closed by an O-ring, bellows or wedge; in pusher designs with the spring assembly on the rotating side, this element must be able to slide axially to follow wear.
• Between the stationary ring and the gland: In most designs closed by a static O-ring, wedge or elastomer cup.
• Between the gland and the seal chamber: A static joint closed by a gland gasket or O-ring.
• Between the sleeve and the shaft in cartridge seals: Closed by an O-ring in the sleeve bore.

A mechanical seal can be used in almost any equipment where a rotating shaft has to separate a pressurized liquid or gas from the environment. The difference between applications lies in what is expected first from the seal: compactness and cost in mass-produced home appliances, emission control in a chemical pump, hygiene in a food pump.

Meccanotecnica Umbra holds roughly 35% of the world market for automotive water pump seals and more than 50% of the dishwasher seal market. These two fields are typical examples of mass-production applications where a seal is expected to run long and maintenance-free in a small envelope.

Seal selection starts from the application data, not from the catalog. The questions in this section cover the information to gather before talking to a supplier and the basis for decisions on seal type and materials. Once the answers are clear, suitable types often narrow down to a few options.

• What is the fluid? Chemical composition, viscosity, solids content, tendency to crystallize and hazard class.
• What are the pressure and temperature in the seal chamber? The pressure the seal sees is what counts, not the pump discharge pressure; the margin to the fluid's vapor pressure also matters.
• What are the shaft diameter and speed? Together they set the peripheral speed at the faces, which, together with face load, drives heat generation.
• What are the seal chamber dimensions and available axial length? Does the equipment follow standard dimensions such as DIN 24960 / EN 12756?
• Is the direction of rotation fixed? Direction-dependent designs can cause problems in reverse rotation.
• Is there a risk of dry running, cavitation or frequent start-stop?
• Is there a legal or site limit for leakage to atmosphere, an explosive atmosphere or a hygiene requirement?
• Can cooling, flush or barrier fluid be provided?
• What does downtime cost? The choice of a cartridge seal and the spare parts strategy largely follow from this answer.

Types: Mechanical seals are classified by construction (component and cartridge), number of seals (single and double), pressure balance (balanced and unbalanced), secondary sealing (pusher and bellows), spring design and direction of rotation. The engineering rationale and an application table for each axis are in the mechanical seal types guide.

Materials: The face pair and elastomer selection set the chemical, thermal and wear limits of the seal. For the general framework see material selection for sealing products, and for media with solid particles see mechanical seals for abrasive fluids.

Failures: Dry running, inadequate cooling, secondary element damage, incorrect installation and vibration are frequent causes of early leakage; heat checking is often a result of dry running or inadequate cooling. Diagnostic methods from symptom to root cause and a preventive maintenance program are explained in the mechanical seal failure and maintenance guide.

Standards describe in a common language how a seal is specified and which dimensions it follows. The information in this section is for general guidance; which standard an application must meet is determined by the project specification.

API 682 covers shaft sealing systems for centrifugal and rotary pumps in the petroleum, natural gas and chemical industries. It defines seals by category, arrangement and type: Arrangement 1 is a single seal, Arrangement 2 an unpressurized double seal with a liquid or gas buffer held below seal chamber pressure, and Arrangement 3 a pressurized double seal whose barrier fluid is held above seal chamber pressure.

The standard also describes seal support systems by plan number. For example, API Plan 32 defines injecting clean flush fluid into the seal chamber from an external source, and API Plan 53B a circuit in which the pressurized barrier fluid is kept under pressure by a bladder accumulator.

DIN 24960 and the European standard EN 12756, which has largely replaced it, standardize the main installation dimensions of mechanical seals by shaft diameter; EN 12756 also defines a coding system for seal materials. Seals that follow the same dimensional series are dimensionally interchangeable. Dimensional compatibility, however, does not mean equivalent materials or performance; face and elastomer materials must be checked separately when replacing a seal.

A significant share of mechanical seal failures comes from installation and operating errors rather than materials. The rules in this section apply regardless of seal type; for numerical limits, always rely on the product's technical documentation.

• Keep the faces clean: Do not touch lapped faces with bare hands; protect the seal from dust and chips from the package to installation.
• Check the shaft and seal chamber: Shaft runout, axial play, bearing condition and shaft surface finish must be within the manufacturer's limits.
• Protect the elastomers: Use an installation sleeve when passing sharp edges, keyways and threads; choose a lubricant compatible with the elastomer (for example, mineral oil-based lubricants are not suitable for EPDM).
• Set the preload correctly: On a component seal, set the spring working length to the dimension; on a cartridge seal, remove the setting clips only after tightening the gland bolts and the drive collar set screws.
• Do not run dry: Fill the seal chamber with fluid and vent it before the first start; bring the flush or barrier system into service before the pump.
• Run the pump close to its design point: Cavitation, prolonged operation against a closed valve and operation far from the best efficiency point shorten seal life through heating, vibration and shaft deflection.
• Think of the seal as a whole when replacing: Renew the secondary elements along with the faces; keep the failed seal for root cause analysis.

Meccanotecnica Umbra Turkey is the Turkish company of the Italian mechanical seal manufacturer Meccanotecnica Umbra S.p.A. From Ikitelli OSB, Istanbul, it manufactures and supplies mechanical seals, gland packings, gaskets and rotary joints for automotive, home appliance and industrial pump applications.

Building on the experience of Meccanotecnica Umbra S.p.A., founded in 1966 in Campello sul Clitunno (Umbria, Italy), the catalog offers 189 products, 126 technical PDFs and STEP/DWG drawing support. With your fluid, pressure, temperature and dimension data you can browse the product catalog or contact our technical team for selection support.