Industrialized society is reliant on mechanical seal in nearly every aspect. Anywhere a rotating shaft moves fluid, mechanical seals are essential for keeping contaminants out and process fluids in. To the rotating shaft and the stationary housing of the equipment, there must be a sealing element. By using gaskets and O-rings, leaks can be prevented.
What is Mechanical Seal in Pump? Definition & Function
Mechanical seals differ from other types of seals in that they involve rotating and stationary components, as well as a spring. Seals act as the interface between moving and stationary components. To say the least, sealing the junction of a moving and a stationary part is a challenging goal, and a spring ensures that the parts remain in contact enough to maintain a seal despite movement.
Define Mechanical Seal in Pump
Mechanical end-face seals are devices used on rotating shafts to keep fluids in and contaminants out. Fluids pumped through an asset, usually a centrifugal pump, are prevented from leaking. The seals are in the stuffing chamber or seal box of these assets. The pump shaft connects to the drive to this pump area.
Functions of Material Seals
The purpose of mechanical seals is to prevent the fluid used inside a machine from leaking into the environment. By preventing environmental contamination, improving machine efficiency, and improving machine safety, mechanical seals play a role in reducing energy consumption.
Faces of Mechanical seals attach to rotary seals & are retained by glands or pumps. Stuffing box covers and tension assemblies maintain a tight seal between the stationary face and rotary face, preventing contamination and leakage. The shaft spins both when the pump is not operating and when the pump is not in operation. To complete the seal assembly, elastomeric bellows, or static sealing gaskets are strategically positioned.
Materials of Mechanical Seals
Choosing the right seal material can also affect seal life. Chemistry and process compatibility are just a few factors to consider. Materials with greater abrasion resistance are generally preferred in nonabrasive applications. However, this may result in undesirable wear characteristics if both sealing elements are hard materials.
For starting and incidental contact, using one sealing element made of a softer material one containing lubricating components, such as graphite, reduces friction. In addition to reducing friction, composite hard faces provide microscopic reservoirs of system fluid at the interface.
The thermal conductivity of a particular material will help the sealing interface to radiate away the heat. Seal durability can also be influenced by the material toughness. It is also possible that the inherent surface texture of the material may contribute to the desired film thickness.
Many seal failures are the result of chemical or pressure compatibility, or temperature limits being exceeded by secondary seals. Spring and other hardware must function despite corrosion, and metal parts must be compatible.
Parts of Mechanical Seals
The design of mechanical seals requires basic components and principles at the interface between a rotating shaft and a stationary pump or mixer or seal-chamber housing. The mechanical seal will normally consist of an end face or a rotating face, however in some designs. As a result, it may consist of the circumferential face or even a lip-type seal hybrid.
Closing Forces
For minimum leakage, the gap at the sealing interface must be kept at a functional minimum, since leakage is a function of the thickness of the film. The sealing force is optimized throughout the operating range of a mechanical seal by considering both opening and closing forces.
Seals function properly from start-up with the initial closing force. A coil spring, multiple coil springs, a bellows unit, or flat or formed springs are available for use as spring components in end- or rotating-face mechanical seals. Magnetic forces, compressed elastomers, and other means of applying a closing force can also create initial biasing forces. Lip-type mechanical seals employ a deflected polymer or a garter spring for less resilient materials to provide their initial force.
Stationary Shafts
The rotating shaft of the pump, mixer, or other equipment passes through the stationary housing and seals against the rotating primary sealing element.
Rotating Shafts
Machines with rotating shafts, such as pumps and compressors, are generally known as rotating machines. Mechanical seals are packing installed in the power transmission shaft of rotating machines. In addition to automobiles
, ships, rockets, industrial plant equipment, and residential devices, these components are used widely in a variety of solutions.
Sealing Points
There are three sealing points on a mechanical seal. The seal between rotating and stationary members need more consideration than the static seals between two of the seal points. All seals are designed around this primary seal and it is essential to their effectiveness.
Static Secondary Seals
Static secondary seals stop leakage between components that appear to be stationary. Consider the interface between the shaft and the sleeve, in which both rotate but do not shift as a result. Due to the deflection of the lip-type mechanical seal, static secondary seals may be all that’s needed for lip-type mechanical seals.
Dynamic Secondary Seals
Furthermore, a dynamic secondary seal provides an effective means of stopping leaks between components that move relative to one another. With a spring-mounted seal face, the face can be moved as the spring is deflected, and the secondary seal will prevent leakage between the seal face and the component upon which the seal face is resiliently mounted.
Mechanical seals can be used efficiently on the rotary face which needs just one dynamic secondary seal.
It is because the mating faces of the sealing interface are rigid materials that cannot conform to shaft misalignments, thermal expansion, and shaft play. At least one of the seal faces and the component to which it is mounted will be able to accommodate relative motion when the dynamic secondary seal is used.
Design of Mechanical Seals
When process pressure increases, sealing design can affect film thickness if it balances closing forces on the sealing interface in such a manner that the interface does not become overloaded. If the closing force is too high, it will result in a thin film of fluid at the sealing interface, causing significant heat buildup.
Surface features at the sealing interface that enhance the effect of hydrodynamic lifting between rotary and stationary sealing elements are also a way of influencing film thickness. The result can be a thicker fluid film that reduces wear on the face and provides cooling at the sealing interface.
In this scenario, the stationary part of the seal may be secured with an o-ring or gasket that is clamped between the stationary part and the pump housing. It is usually an O ring that seals the rotary portion of the seal to the shaft. As this part of the seal rotates with the shaft, this sealing point can also be considered static.
This mechanical seal is the interface between the static and rotary components of the seal. Static or rotary seals always have an element that is resiliently mounted and spring-loaded to accommodate small shaft deflections, shaft movement due to bearing tolerances, and out-of-perpendicular alignment due to manufacturing tolerances.
Process of Mechanical Seals
Mechanical seals are highly effective devices. In order to keep fluid or gas in the wet end of a centrifugal pump, it must accommodate a spinning shaft. Essentially, this consists of a rotary seal face connected to a motor that rotates at the same speed as the shaft of the pump.
Process considerations other than abrasion may hinder the formation of a cool, clean lubricating film at the sealing interface. For instance, flashing may occur if the sealing fluid has a low vapor point. A flash occurs when sealed fluid changes from liquid to gas and expands rapidly at the sealing interface, forcing the sealing elements apart until pressure is increased or temperature drops, only to have the sealing elements collapse back into contact a short time later.
Fluid Film
It is common for mechanical seals to maintain a thin film of fluid between their faces to keep them lubricated. The film can originate either from the process fluid being pumped or from an external source.
There is a challenge in design with the presence of a fluid film between the faces of the seal. As a result, this allows sufficient lubricant to flow between the faces without leaking an unacceptable amount of process fluid or allowing contaminants into the film that could damage the seal. By maintaining a precise gap between the seal faces, a small amount of clean liquid lubricating can enter the gap between the seal face to make sure that contaminants do not enter.
Environmental Control
Environmental controls at the sealing interface greatly contribute to the formation of a cool, clean lubricating film. Process pressure will force particles-laden liquid into the sealing interface if the process fluid is a slurry mixture, resulting in abrasion and accelerated wear.
By using environmental controls, such as a restriction bushing and clean flush, the mechanical seal is isolated from the harsh process, allowing the seal to seal the cleaner, cooler flush fluid. Other times, the pump product crystallizes, causing premature wear at the sealing interface. A double seal with a buffer or barrier fluid can be used to prevent product crystallization by controlling the temperature, quenching the atmospheric side of the sealing interface, or quenching the atmospheric side.
Types of Mechanical Seals
Normally, there are three types of seals which is known as Balanced seals, Unbalanced seals, and Pusher seals. Mechanical seals must be manufactured with precise measurements and made of materials that can withstand extreme temperatures or chemicals, or the seal will fail. There are many types of mechanical seals, so we can group them into general types.
Pusher Seals
The seal closing force is maintained by one or more springs in a pusher seal. A mechanical seal’s springs can be located in one of two locations: in the rotating or stationary elements.
It is possible to seal very high pressures with pusher type seals. It has one drawback, however: the elastomer underneath the primary seal face can wear out when the seal face moves along the shaft or sleeve during operation.
Non Pusher Seals
To maintain closure forces of non-pusher seals, metal or elastomeric bellows are employed. They are especially suitable for dirty, high-temperature applications. In contrast, belt seals are only suitable for medium and lower pressures.
Water Pump Seals
For applications such as shower pumps, irrigation, heating systems, swimming pools and spas, it is of utmost importance that water pump seals utilize excellent materials because water is such a poor lubricant for seal faces.
Diaphragm Seals
In addition to agitators and mixers, diaphragm seals can also be found in pumps and other power plants that use rotary shafts. Diaphragms in rotary heads seal on the shaft. As a result, they are unaffected by rotational direction.
Parallel Mechanical Seals
They are made up of two heavy-duty springs. They are available with either left-hand wound springs or right-hand wound springs, depending on how the seal is provided to the face. In marine pump applications, waste water and pulp and paper applications as well as refrigeration compressors, they offer a good general-purpose seal.
Bellows Mechanical Seals
Bellows Mechanical Seals are compact, versatile, and extremely reliable seals ideal for applications across a wide range. Available with either O Ring Mounted Stationeries or Boot Mounted Stationeries in both DIN and Non-Din sizes. Pumps, mixers, compressors, etc. commonly use these for solid-containing media.
Cartridge Seals
All of the seal elements are contained within a cartridge type mechanical seal. Consequently, the possibility of assembly error is dramatically reduced as well as the time required for seal replacements.
Balanced Seals
A balanced mechanical seal arrangement refers to a system where the forces acting at the seal faces are equal. Low faces loading results in even lubrication of the seal faces and longer seals life. Mechanical seals with balanced configurations are suitable for higher operating pressures. Moreover, they are an excellent choice for liquids with low lubricity and high volatility.
Unbalanced Seals
Mechanical seals that are unbalanced are an economical alternative to balance seals that are complex. Due to tighter control of the face film, unbalanced seals may also exhibit less product leakage, but also have a much shorter mean time between failures. It is not recommended to use unbalanced seals for high pressure or hydrocarbon applications.
Conventional Seals
Seals that are conventional tend to be cheaper and are often installed on general service equipment. Due to their installation as individual components, these seals require higher operator skill levels.
Wave Spring Mechanical Seals
Wave spring mechanical seals are commonly used for pharmaceutical, dairy, brewery, and food processing liquids of high viscosity. They are usually attached to the shaft with grub screws, and in the case of media with high viscosity, they would also have stationeries with anti-rotation pins. There are some designs that are specially constructed for shallow, confined seal housings.
Conical Mechanical Seals
Mechanical seals like this are extremely versatile and popular. Designed to withstand tough conditions, they are available in a variety of materials. Although these seals come with either left- or right-handed springs, they are rotation-dependent.
Air Seals
Sealing rotating shafts are noncontact, pneumatic air seals. Seals of this type are typically used in dry powder or slurry applications. A small amount of air or inert gas is used to prevent product loss, emissions, and contamination. The air is throttled to create positive pressure and ensure a tight seal.
Failure of Mechanical Seals
In the case of grooves or notches on the seal, it’s a sign that your mechanical seal might be under excessive force, which can lead to failure. Insufficient lubrication or abrasive materials can cause the seal face to become grooved. Seal failure is caused by mechanical damage to the sealing contact surfaces. There is no lubricating film.
Chemical Attack
There is the weakness of every material. An incompatible liquid may cause internal parts such as rubber seals, gaskets, impellers, hoses, pump casings, diffusers to crack, expand, contract or deteriorate.
Tables describing chemical compatibility show which materials are compatible with which liquids based on their level of compatibility. A pump that wears out frequently or fails after a short period of time is one of the first items to check.
Overheating
A high fluid temperature can cause elastomers to swell or melt, causing leaks by restricting their effectiveness.
It is not only high temperatures caused by the process, but if fluid is re-circulated for an extended period of time, either due to a closed valve or from recirculation through a relief valve, energy is transferred to the fluid causing it to heat up.
Wearing
Certain elastomers can lead to increased shaft wear, and when a seal is replaced, if the shaft has a smaller OD as a result of friction over time, then the seal will not fit tightly enough, and fluid will leak. In the case of an unbalanced shaft, the lateral movements will force the seal to absorb these forces rather than the bearings, something in which it is not designed. The seal can tear under high torsional forces.
Running Dry
The process of running a pump without fluid is known as dry running. Thus, parts that often rely on pumping liquids for cooling and lubrication are susceptible to high levels of friction and heat, leading to immediate failure. A pump can fail in seconds, so make sure the pump is completely filled with fluid before starting it up.
Cavity
The pump is controlled by the system in which it operates. The motors can operate at speeds that harm the internals if they are not restrained like driving downhill in first gear at high speeds when the rev counter is in the red. Running a pump at the wrong part of a performance curve can accelerate the wear of its parts, including its seal.
Solid Waste
When a seal encounters solid particles such as sand or silt that it is not designed to handle, it can form grooves in the soft seal faces, causing the seal to leak and drip process media. As long as the concentration of solids does not exceed those specified at the enquiry stage, a hard faced mechanical seal can limit and prevent this issue from recurring.
Fitting Issue
It is designed to move along with the shaft when a seal is installed. The shaft will move laterally and vertically. Excessive play can damage the seal if improperly installed. If the seal is too restricted, the springs may snap or the faces may wear out.
Solution of Mechanical Seals Failure
Operators should prevent such upsets by selecting the proper mechanical seals and incorporating process controls. In addition, additional process conditions may require special attention, such as fluids that harden, are toxic, require anaerobic conditions, are part of a food source or water supply, or present another specific constraint.
With no Mechanical Seals
Liquid leaks through the clearance between the shaft and body if no seal is used or gland packing is installed.
Gland Packing Method
It is made up of a braided, rope-like material that is wrapped around the shaft and physically stuffs the gap between the shaft and the housing. The use of gland packing is still prevalent in many applications, however mechanical seals are becoming more popular.
Workflow
Friction from the shaft rotating wears down the packing over time, resulting in greater leakage until the packing is maintained. Due to the friction of the shaft, it is also important to flush the packing with large volumes of water in order to keep it cool.
As the packing must press against the shaft in order to reduce leakage, the pump must use more power to turn the shaft, wasting energy. Eventually, packing will wear a groove into the shaft since it has to contact it, which is costly to repair or replace.
With Mechanical Seal
In order to ensure minimal leakage of the liquid used by the machine without affecting the rotation force of the shaft, two separate rings are installed on the shaft and on the machine housing.
To ensure this, each part is made according to a precise design. Even hazardous substances like acids, salts, and alkalis that are difficult to handle mechanically or under high pressure and rotating speeds, mechanical seals prevent leakage.
Uses of Mechanical Seals
Mechanical seals must be customized to fit the application where they are used for maximum efficiency. They can be used in a wide range of applications.
Install in proper pump location
The impeller rotating shaft is sealed with a mechanical seal. A clearance between the pump shaft and body prevents liquid from leaking out.
Stationary & Rotary Rings
Rotation of the rotary ring is coupled to rotation of the shaft. A clearance in the order of micrometers is maintained between the stationary and rotary rings as they rub against each other. They are called face materials when they rub against each other.
Pump’s Internal Structure
Pumps are machines that move fluids, such as lifting liquids from the outside to the inside. Their impellers rotate to move liquids to their designated structure.
Shaft Location
Seals are often made up of two rings: a rotary ring mounted on the shaft and a stationary one mounted on the pump housing.
Leakage Control
Material on the stationary ring and the rotary ring where they rub together is the most important barrier to fluid. The clearance must be sufficient to prevent friction from increasing, hindering shaft motion or causing seal failure. The liquid will leak if the clearance is too large. As a result, it is necessary to maintain clearances in the order of micrometers to prevent leaks, while also lubricating the surfaces to reduce the sliding torque and avoid hindrances to the machines.
Merits of Mechanical Seals
- Fluids such as acids, salts, and rough particles can be handled by the device.
- Despite its long working life, it does not require maintenance.
- Inconsistent or marginally misaligned mechanical seal handle
- Wear on the shaft is not caused by the operation.
- The condition of the shaft cannot be considered basic
- Take on bi-directional shaft revolutions, huge weights, and temperature and speed shifts.
- It is a positive fixing for food preparation, dangerous chemicals, and radioactive liquids.
Demerits of Mechanical Seals
- Outspread lip seals require more space.
- The pivotal end of mechanical seals normally cannot handle this fact.
- Faces should be sealed smooth so they cannot be damaged.
- It has high initial cost.
Technology of Mechanical Seals
Considering the above-mentioned functions and applications, mechanical seal technology is both engineering and a physical technology. Mechanical seal technology is based on the friction, wear and lubrication method, which controls how the surface of stationary and rotary rings rubs against one another.
By improving the functionality of mechanical seals, not only will a machine’s liquids and gases be prevented from leaking to the outside, but it will be more energy-efficient and prevent environmental contamination as well. Moreover, certain rotating machines handle media that, in the event of leaks, may result in dangerous accidents. Therefore, mechanical seals should be manufactured with solid engineering expertise so that they are highly reliable.
Seals will have increasingly important functions and roles as a result as time goes on. They will continue to be innovative as a result If you want to meet these expectations positively.
Difference between Single & Double Mechanical Seals
Nearly nine of the ten single mechanical seals are installed on a regular basis. Most mechanical seals depend on the liquid being cool, not unpredictable, lubricious, and not containing any broken down or rough solids. Mechanical seals have the least number of parts, so the single seal is the most conservative way of fixing them. For sealing grease, the help of a large pumping liquid is utilized. The seal confront can be infused with a helper liquid if the pumped liquid isn’t tasteful as a fixing liquid.
With a double mechanical seal pump, some liquids may not flow perfectly. This fluid typically conveys a coarse suspension of material that would quickly wear the seal faces, or that could be destructive and dangerous. Two solutions exist for this problem. There are two methods for controlling leakages like utilizing natural controls and providing multiple seals or double mechanical seals.
One limitation of single mechanical seals is there is no move down available in case of seal spills. In the same way, framework disturbance results in glistening fluid and blazing harms appearances.
Its low weight is one of the drawbacks of a double mechanical seal pump. The greasing up film between the faces is crushed and soon the appearances are crushed if the end compels applied on the seal confront surpass that farthest point. Fortunately, that problem can be dealt with by balancing the seals.
Lifespan of Mechanical Seals
Depending on factors such as operating conditions and environment, a mechanical seal of this type is typical to last between three and five years. Leaks from a mechanical seal can be an obvious sign that it needs to be repaired or replaced. Water spraying from the shaft seal indicates that the mechanical seal needs to be inspected.
FAQs for Mechanical Seals
Why spring is used in Mechanical Seals?
Seals use spring mechanisms of several types to supply the pressure necessary to keep the stationary and rotating faces of a seal in alignment. Four general types of springs can be distinguished: single spring, multiple spring, wave spring, and metal bellows.
How can you measure a Mechanical Seal?
The inside diameter of the seal head is determined by measuring the end of the head facing the spring. In this case, the seal diameter will be either the shaft size or the sleeve size of the seal, whichever is the larger of the two. The seal’s front side is usually coated with black carbon.
How do you measure Mechanical Seal?
Measuring the Seal head’s inside diameter should be done on its backside. The seal size diameter is determined by the shaft diameter or the sleeve diameter of the seal. It usually has a black carbon face on the front.
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