What is a Mechanical Seal?
In centrifugal pump systems, mechanical seals are essential components. By excluding impurities and preventing fluid leaks, these devices maintain the integrity of the pump systems. On various seal designs, mechanical seal systems are employed to check for leaks, regulate the sealed environment, and lubricate secondary seals. There are different mechanical seal kinds to choose from depending on the type of pump and the process variables. Each type of seal is distinct in appearance and features, making it appropriate for a particular use.
How does a Mechanical seal work?
A stuffing box or seal chamber allows a centrifugal pump’s rotating shaft to enter the casing from the back. A mechanical seal’s main function in a centrifugal pump is to stop fluid from leaking along this revolving shaft and into the atmosphere. Considerations for safety and dependability must be considered when choosing the appropriate mechanical seal for each pump application. One of the main causes of pump failure is frequently mechanical seal failure. The professionals in the industry usually demonstrate in this guide how to install a mechanical seal in the centrifugal pump with the least amount of downtime.
Two extremely flat and smooth (lapped) seal faces must be in contact for a mechanical seal; one must rotate with the shaft, and the other must be stationary with the casing. Through the application of secondary seals, these seal faces are attached to their proper holders (o-rings or gaskets). The faces can be brought into contact and moved to correct for static and dynamic misalignments and wear because they are mechanically powered and flexible. When fluid tries to escape through the seal faces, it is broken down by the seal faces, causing a small amount of vapor to be released. All mechanical seals, when chosen and sized properly, leak vapor.
Fundamentals of Mechanical Seal
The goal of mechanical seals is to stop fluid (liquid or even gaseous) leakage through the space between a shaft and the fluid container. The seal rings on which a mechanical force produced by springs or bellows and a hydraulic force produced by the pressure of the process fluid are acting are the major parts of a mechanical seal. The seal ring connected to the machinery’s casing is referred to as the “stationary ring,” The seal ring that rotates with the shaft is known as the “rotary ring.” Secondary seals are necessary to perform static sealing between stationary rings and the machinery’s casing, as well as between rotating rings and shafts.
Although alternate solutions, such as those detailed in the following sections, can be employed, elastomeric O-Rings are typically used as secondary seals. On mixers and pumps, mechanical seals are typically installed. As mentioned above, an appropriate mechanism must be installed to seal the fluid inside the case on both of the setups.
The sheet of liquid An effective lubrication is needed to reduce the amount of friction between the seal rings. With twin mechanical seals, an appropriate auxiliary fluid can lubricate the seal faces and the process fluid. A stable and full layer of lubrication significantly impacts a mechanical seal’s performance and lifespan.
The proper selection of a mechanical seal must take into account the following factors to ensure enough cooling and efficient lubrication of the seal rings:
- The temperature of the process fluid
- The pressure of vaporization at operational temperature
Features of process fluid
The ideas and guidelines previously covered apply to any mechanical seal that uses a liquid fluid. The differences in operation between dry-running and gas seals are something to keep in mind.
Every mechanical seal results in leakage. The explanation is found in the lubrication theory that was previously mentioned; it is evident that a continuous lubrication layer implies some degree of leakage. Calculating leakage is possible and depends on several variables, including rotating speed, fluid pressure and properties, and balancing ratio. However, the machinery on which the mechanical seal is mounted may also have some bearing. Frequently, leakage is so minimal that it is not visible (vaporization).
The extent of freedom readily available
For a mechanical seal to function properly, the elastic parts (spring or even bellow gaskets) are crucial.
Because it must follow the movement of the ring caused by unavoidable occurrences like vibrations, misalignment, and shaft run-out, the gasket installed on the seal ring driven by the bellow or spring (often the rotary ring) is known as a “dynamic” gasket. It follows that for proper application of a mechanical seal, factors, including working length, gasket compatibility with the process fluid, dimension, and shaft finishing, must be properly considered.
When consistent pressure is given to a piston, it is known that the force generated will be proportional to the area of the piston. In mechanical seals, the fluid pressure creates a hydrostatic force that operates on the seal ring in addition to the closing force produced by the springs or bellow. As was already mentioned, the hydrostatic flow also creates a lubricating film between the seal faces and generates an opening force. The balancing ratio is the proportion between the forces closing the seal ring and those opening it.
Different Types of Mechanical Seals
Different mechanical seal types have different configurations and methods for distributing the hydraulic forces that operate on their faces. The following are some of the most typical seal varieties:
A system where the forces exerted at the seal faces are balanced in a mechanical seal arrangement. Due to the lower face loading, the seal faces are lubricated more evenly, which extends the seal life. Today, find out more about our lubricating systems for mechanical seals.
Higher operating pressures, often exceeding 200 PSIG, are where balanced mechanical seals excel. They are also a wise choice when working with liquids high in volatility and low in lubricity.
Unbalanced mechanical seal varieties are frequently used as a more cost-effective substitute for balance seals, which are more intricate. Due to greater control over the face film, unbalanced seals may also show less product leakage but, as a result, may have a substantially lower mean time between failures. Unbalanced seals are not advised for high-pressure applications or the majority of hydrocarbon uses.
Pusher seals use one or more springs to sustain the seal’s closing forces. The mechanical seal’s revolving or stationary component may have springs. The elastomer behind the primary seal face of pusher-type seals, which can be worn out as the face advances down the shaft or sleeve during operation, is a problem that prevents them from sealing at very high pressures.
A metal or elastomeric bellows is used by non-pusher seals to maintain seal closing forces. Applications involving dirt and high temperatures are best suited for these seals. Bellows seals can only be used in applications with medium to low pressure. Both balanced and unbalanced variants are available for pusher and non-pusher designs.
Ordinarily less expensive and frequently put on standard service equipment are conventional seals. As they are installed as separate components, these seals demand a greater level of operator skill to service.
Mechanical cartridge seals combine all seal components into a single assembly. As a result, the possibility of assembly errors and the time needed for seal changes are significantly decreased. Find out more about the distinction between mechanical seals that use cartridges and those that do not.
How to choose a Mechanical Seal?
Operators must choose the seal system for a centrifugal pump based on the specifics of their application. Failure to choose the right seal type can result in lost pump integrity, breakdowns, and expensive repairs. Before making a choice, all operators must consider the following to avoid these negative outcomes.
The Fluid to Be Pumped Type
The most crucial element to consider when choosing a seal type is the fluid one is pumping. Elements including cleanliness, lubricity, and volatility will greatly impact the mechanical seal’s design and the seal support system.
Fluid Pump Pressure
The performance of a mechanical seal is significantly influenced by force applied to its faces. An imbalanced mechanical seal will work well if a pump is to be run at low pressures. However, balanced seals will be a more dependable solution in circumstances where higher pressures are predicted.
Temperature Considerations to be taken into account
When working temperatures are usually higher than usual, balanced mechanical seals outperform their unbalanced counterparts. Compared to other seal types, balanced mechanical seals retain heat-sensitive components better.
Operator Safety Concerns
Operator safety is considered the main consideration, as it is for all machines. Double mechanical seals, which have a greater sealing capacity and are generally more dependable, are used in centrifugal pumps to give additional protection.
How to Replace a Mechanical Seal?
Using a centrifugal pump causes water to flow through the plumbing system due to the principle of centrifugal force. In this pump, an impeller is rotated by a shaft that spins incredibly quickly. As a result, centrifugal force draws water into a vacuum, which is then formed. Behind the centrifugal pump’s impeller is a mechanical seal that keeps water out of the electrical system.
It becomes necessary to repair the seal when wear and tear result in damage. One must access the pump system and remove the impeller from the shaft to complete the work. Understand the procedures for changing a mechanical seal on a water pump.
Remove the power
The centrifugal motor must be turned off, so it is not in motion. Close the main power source and make sure there is no danger of the device restarting. After this is finished, it’s time to start working again.
Keep the Fluid Separate from the Pump
Close the pump’s inlet and outlet isolation valves before removing the casing drain plug to drain the pump’s casing.
Disconnect the Centrifugal Pump
Remove the spacer piece from the pump coupling if the pump has a “back pull-out” configuration. Slide the remaining pump portion away from the casing after removing the casing bolts. The casing is no longer connected to the inlet and outlet pipework, making it possible to access the mechanical seal without doing so. If the pump is not a “back pull-out” design, one must first remove the coupling that connects the pump and motor shaft before removing the entire pump. One will need to remove the complete pump and motor if the pump is a close-coupled design, which means that the motor’s shaft serves as the pump’s shaft. Bolts holding the casing in place should be removed.
Get rid of the impeller
On the pump shaft, the mechanical seal is situated behind the impeller. Both screws and bolts are used to secure impellers to the shaft. With the shaft securely held with a wrench, spin the screwed-on impeller counterclockwise until it is entirely unscrewed. A fastened impeller can be removed by holding the shaft in place while removing the bolt.
Remove the Seal
Now that direct access has been reached to the stationary and rotating seal components. Set screws are often used to secure the rotational components along the shaft. Slide the rotating seal components off after removing the set screw. Remove the stationary seal component from the seal chamber bore or casing.
Change the Seal
It’s time to attach a fresh mechanical seal to the shaft. Slide the replacement seal components along the shaft with caution. Press the stationary component into the casing or seal chamber bore using a fresh o-ring or gasket material. To properly reattach the rotary component to the shaft, according to the instructions. This is an important action.
It is important to note that one should always operate in a spotless environment when installing mechanical seals. Avoid touching the front of the seal faces since they are vulnerable to body oils and could get compromised, leading to poor performance. Before installing the seal, keep it in its box.
Putting in the Impeller
As one screws the impeller onto the pump’s shaft using a fresh impeller o-ring or gasket, they may use the wrench to maintain the shaft in place. Alternatively, one can fasten the impeller to the shaft’s end using the impeller bolt and a fresh o-ring or gasket.
Use the existing casing bolts to reattach the casing
Slide the back pull-out component against the fitted casing in back pull-out designs and secure it with a bolt. After the step below, it will be important to check the alignment of the pump. Use the casing bolts to reinstall the casing for closely linked designs or do not have a back pull-out. Always follow the pump installation, operation, and maintenance (IOM) instructions when tightening the casing nuts.
Connect the pump again
The spacer element must be reinstalled to the existing coupling hubs along the pump and motor shafts for back pull-out designs, and the mounting feet of the assembly must be secured with bolts to the pump baseplate. Reposition the motor and pump. Reinstall the pump and re-connect the inlet and output pipework for close-coupled pumps. Replace the pump on the baseplate, connect the inlet and outlet pipes, re-bolt the pump to the baseplate, connect the coupling, and then realign the pump and motor for non-back pull-out pump and motor designs.
Restart the apparatus
By releasing the isolation valves at the inlet and output, confirm that the pump casing has been filled again. Refer to the pump IOM to determine whether venting is necessary for some pump designs. It is usually a good idea to double-check the motor’s rotation to ensure it is accurate before starting the pump. Before re-connecting the pump and motor coupling, this is done. A motor bump will demonstrate spinning. The coupling should then be re-connected before the pump may operate.
Exercise the necessary caution
Always go over the safety measures listed in the pump IOM. When working on any pump, utilize the Pump IOM at all times. Following the manufacturer’s detailed directions, install the mechanical seals. Last but not least, always ensure that the motor and pump are adjusted between 001″ and 002″. Mechanical seals will fail early as a result of misalignment.
Materials used in making Mechanical Seal
Perfect planarity of the seal faces is necessary for effective sealing, even in constant temperature gradients. Additionally, seal faces must be lubricated and cooled at an optimal level due to the high relative speed and pressure at which they must work. Combining the elements above results in selecting suitable materials that are easily developed and manufactured (lapping).
The first and most crucial step to ensuring extended wear and positive outcomes is selecting the right seal face material. One seal face made of graphite and the other made of silicon carbide, tungsten carbide, or ceramic is the most durable material combination. The primary benefit of a graphite seal ring is its ability to quickly and flawlessly complement the counterface.
It is advised to install two hard faces, such as silicon carbide or tungsten carbide, when the fluid to be sealed is abrasive. In the latter scenario, extra caution should be used to avoid the potential of temporary dry running, which might result in long-term damage to the seal.
This material is the top option for the seal face due to its self-lubricating qualities. There are many different types on the market, and each one is made by sintering carbon and graphite powder that has been properly linked with resins or metals. Bonding is necessary to close the microporosity caused by the high temperature (over 1000°C) required for the sintering process.
The main typical varieties of graphite are
- With a high level of chemical resistance and suitability for most chemical applications, resin-impregnated graphite.
- Graphite with a metal impurity (usually antimony or bronze) can withstand greater operating temperatures and pressures.
- High-temperature sintered electrographite, 2500 °C. Suited to extremely abrasive fluids and high temperatures.
- The ability of graphite to quickly remove modest initial planarity flaws is one of its main advantages. Good self-lubricating qualities that allow for brief dry running
Graphite-like qualities are shared by PTFE, except for low mechanical strength. Various bonding materials are used to boost the wearing resistance, with glass being the most popular. When mated to a silicon carbide or ceramic counterface, PTFE is inert and suited for any hostile fluid. Stellite and Chromium steel are inappropriate partners.
Cobalt, tungsten, and chromium, which contribute to this alloy’s exceptional surface hardness, are the main ingredients. Usually used as a coating on rings made of stainless steel to create a firm sliding surface. Inability to withstand thermal dilatation
This stainless steel has a high percentage of chromium, which provides a great balance of hardness and corrosion resistance. Stellite’s disadvantages in thermal dilatation are not present in seal rings manufactured of this substance. Graphite counterfaces are typically coupled with chrome steel.
This substance, called aluminum oxide, is made by sintering powders and is machined by grinding. The amount of material purity helps distinguish between the various sorts offered on the market. Fluiten uses a highly chemical and wear resistant 99.7% pure Al2O3. Ceramics is quite hard, making it a good material for abrasive items. The main drawback is a lack of thermal shock resistance. Typically, it is mated with counterfaces made of reinforced PTFE or graphite saturated with resin.
This material has a high degree of mechanical resistance, making it acceptable for use with abrasive fluids. It also has a small but extremely useful capacity to survive brief transitory conditions of inadequate lubrication. Tungsten can be combined with nickel or cobalt to form the alloy. The bonding materials provide different qualities.
Cobalt is frequently used in machining tools because of its great mechanical strength. Nickel is preferred for making seal faces because it provides slightly less mechanical and better chemical resistance. The production is accomplished through sinterization in a vacuum environment, followed by grounding machining. The material is perfect for seal faces because of its extremely low porosity.
Sinterization in a vacuum environment creates the product, which is ground to shape. Its extremely low porosity is the perfect material for seal faces. When working with highly abrasive goods, installing a counterface in tungsten or silicon carbide is customary, always ensuring effective lubrication. The standard mating material is resin or antimony-impregnated graphite.
This substance is created by sintering silicon carbide particles, often with pure silicon. There are two main varieties of silicon carbide on the market, depending on the production method.
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