Technical issues with mechanical seals for pumps

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file:///C:/DOCUME~1/ADMINI~1/LOCALS~1/Temp/msohtmlclip1/01/clip_image002.jpg Mechanical seals are classified into assembled and non-assembled types based on their common structure. • 1 Main components of the assembled type； It consists of seven components: gland, shaft sleeve, moving ring assembly, stationary ring, water jacket, seal gasket or O-ring, and screws. • 2 Non-containerized main components ； Moving ring assembly – Stationary ring [includes gasket or O-ring and screws]. Mechanical seals are categorized into moving seals and stationary seals. (I) Moving seals ； A mechanical seal is a device in which the sealing element is mounted vertically on a shaft or sleeve and rotates together with them, thereby preventing liquids from leaking out; this is known as a dynamic seal. (ii) Static seal ； A seal installed on a stationary device is called a static seal. All solid objects are statically sealed. Common materials include metal, rubber, asbestos, etc. What is a mechanical seal? • To understand what a mechanical seal is, it is first necessary to have a basic understanding of water pump equipment. Most water pumps rely on a motor as the driving force to rotate the pump impeller. The water pump impeller contains a certain number of flow channels; as the liquid medium passes through these channels, it is forced out of them by centrifugal force. By providing an outlet at an appropriate location to guide the liquid medium ejected by the impeller, the liquid is accelerated and pressurized as it is pumped out from the inlet of the pump; this is the basic principle of a conventional centrifugal pump. The water pump is driven by a motor that causes the impeller to rotate at high speed; however, the pump casing remains stationary, and its interior is filled with a liquid medium. How to prevent the liquid medium from leaking out through the gap between the pump body and the pump shaft. Sealing mechanism • A mechanical seal is a shaft sealing device used in rotating fluid machinery. (Used in equipment such as centrifugal pumps, centrifuges, reaction vessels, and compressors; there is a circumferential gap between the shaft and the equipment chamber, through which the medium inside the equipment leaks, so a leak-blocking device must be installed.) Due to its advantages such as low leakage and long service life, the mechanical seal has become the primary type of shaft seal, also known as a face seal. ) • **Definition in relevant standards:** A device that prevents fluid leakage; it consists of at least one pair of end faces perpendicular to the rotation axis. Under the combined action of fluid pressure and the elastic force (or magnetic force) of a compensation mechanism, and with the assistance of auxiliary sealing rings, these end faces remain in contact and can slide relative to each other. Technical issues of mechanical seal systems for pumps • 1 ； Overview of Mechanical Seals for Pumps • 2 ； Five common problems in the maintenance of mechanical seals for pumps • 3 ； Analysis and Judgment of Mechanical Seal Leakage Causes • 4 ； Misconceptions in the maintenance of mechanical seals • 5 ； Requirements for installing mechanical seals: 1. A mechanical seal for pumps refers to a device that is installed and operates within pump equipment to provide dynamic sealing. A key component that prevents the fluid medium inside the pump chamber from flowing out of the pump through the pump shaft ； Since pumps transfer fluid media and are characterized by horizontal installation, high rotational speeds, high pressure, the presence of solid particles, and strong corrosivity, the material used for the mechanical seal components of pumps must be able to withstand the abrasive impact of these solid particles. Factors such as high rotational speeds, high-temperature corrosion, and unstable sealing parameters due to high pressure pose challenges, resulting in high requirements for the materials used in mechanical seals. Select the sealing material based on factors such as the medium pumped by the pump and the temperature. The type of sealing structure also needs to be selected based on the pump design. A mechanical seal is one of the high-precision, complexly structured mechanical components that combine metals and non-metals; it is a key part of pump machinery. Its sealing performance and service life depend on many factors. Factors such as the selection of seals, mechanical precision, careful installation, proper use, and the operating conditions at the site. Mechanical seals come in a wide variety, with complex structures and different models; it is also very difficult to identify and determine how to repair them. 2. There are mainly five reasons for leakage in common mechanical seals: • The seal between the shaft sleeve and the shaft; • The seal between the moving ring and the shaft sleeve; • The seal at the interface between the moving ring and the stationary ring; • The seal between the stationary ring and its seat; • The seal between the sealing gland and the pump cover. 3. When there is a seal leakage, it is necessary to first determine which of the above issues is at fault before carrying out repairs. Visually determine the cause of seal leakage when removing the pump, and judge for yourself whether it is correct. • Leakage at static pressure A ； After installation and commissioning, mechanical seals are generally subjected to a hydrostatic test. Observe the leakage; if it occurs between the shaft sleeve and the pump shaft, it indicates a problem with the shaft seal ring or gasket. Observe the leakage volume; if it is relatively small, it is often due to problems with the sealing ring of the rotating or stationary ring. If the leakage volume is relatively large, it indicates that there is a problem between the rotating and stationary ring friction pairs. 【If there is a significant change in the leakage rate of the turntable, it can be determined that the issue lies between the moving and stationary ring friction pairs】 ； 】If the leaking medium leaks along the outside of the sleeve, it is usually due to a problem with the rotating ring O-ring. If the leaking medium sprays out in all directions or leaks from the water cooling holes, it is usually a problem with the static ring seal. If the bellows seal suddenly leaks during normal operation, it is due to a broken bellows. As long as one is familiar with the sealing structure and conducts a careful on-site inspection, it is definitely possible to make the correct judgment. • Leaks occur when pump B is in operation. Although the seals have undergone hydrostatic testing, after ruling out inter-shaft and stuffing box seal failures, such leaks during operation are generally caused by damage to the dynamic and static ring friction pairs. The main factors causing seal failure in friction pairs are ； • 【1】During operation, abnormal conditions such as pipeline evacuation, cavitation, and pressure buildup can generate significant axial forces, causing the contact surfaces of the moving and stationary rings to separate or suffer severe wear ； • [2] The selected compression amount for the mechanical seal is too large, resulting in severe wear and scuffing of the end faces of the sealing friction pair. • 【3】The rotating ring seal is too tight, preventing the spring from adjusting the axial float of the rotating ring. • [4] The stationary ring seal is too loose; when the moving ring undergoes axial movement, the stationary ring detaches from the stationary ring seat. • A sudden leak occurs during normal operation of C; the friction pair of the spring-type seal’s moving and stationary rings is damaged, or the sealing ring has reached the end of its useful life. Or it may be due to changes in operating conditions. • 【1】Unstable pipeline medium can cause evacuation, cavitation, or prolonged pressure buildup, leading to seal damage. • [2] The pump output is too low; a large amount of medium circulates within the pump, causing heat to accumulate. This leads to vaporization of the medium and subsequent seal failure. • 【3】The flushing volume is too high. Add a shutoff control plate at the flushing end or modify the direction of the flushing hole. • [4] For pumps that have been shut down for an extended period, manual cranking was not performed upon restart. The sealing is damaged due to adhesion at the end faces of the friction pair. • 【5】An increase in corrosive, polymerizing, and gelling substances in the medium. • 【6】Changes or adjustments in operating conditions. 4. The greater the compression amount of Spring A, the better the sealing effect. In fact, this is not the case; excessive compression of the spring can lead to rapid wear of the friction pair, resulting in instant burnout. Excessive compression deprives the spring of its ability to adjust the end face of the moving ring, resulting in seal failure. • For the B moving ring seal, the tighter it is, the better. In fact, having an overly tight dynamic ring seal is more harmful than beneficial. First, it exacerbates the wear between the sealing ring and the shaft sleeve, leading to premature leakage. Second, it enhances the ability of axial adjustment and compensation for the moving ring; however, when operating conditions change frequently, timely adjustments cannot be made. Third, excessive fatigue of the spring leads to easy wear ； Fourth, it causes the rotating ring seal to deform excessively, thereby affecting the sealing performance. • The tighter the C static ring seal, the better. The static ring seal ring remains essentially at rest, and it also provides radial and axial sealing. Too tight makes installation difficult. The rigid installation causes the static ring to deform and damages the sealing ring. Mechanical seal structures can generally be divided into • 1 Metal wave-tube mechanical seals. • 2 PTFE bellows mechanical seal • 3 Rubber bellows mechanical seal. • 4 sets of [large] spring mechanical seals • 5 sets of [small] spring mechanical seals. 6-stack spring mechanical seal ㈥ Functions of various components when installed in different devices – 1 Compression gland ； Its main function is to provide a geometric installation space for the stationary ring. 【Easy maintenance and cost savings】 2-axis sleeve ； It mainly enables the rotating ring assembly to be connected to the shaft via a sleeve, allowing it to rotate synchronously with the shaft 【which also facilitates maintenance and reduces maintenance costs】 – 3 rotating ring assemblies ； Its main function is to connect the moving ring assembly to the shaft sleeve using screws, and through bellows or springs, it pushes the moving ring to make close contact with the two end surfaces of the stationary ring during operation. – 4 Static ring ； It is mainly connected to the stationary part [cover] by means of screws or O-rings through interference fit, and it has an end face that makes precise contact with the moving ring – 5 water jackets ； Its main function is to serve as a high-temperature medium; through the cooling water, it enables the directed flow of cooling water within the sealed chamber for cooling purposes. – 6 screws and gaskets. – The materials of the above components are determined based on the composition of the medium. ㈦ Inspection of the mechanical seal before installation • 1 First, check the quantity of each component of the seal that you are about to install. • 2 Conduct a visual inspection of the quality of the unsealed parts. 3 There are several key points to note during visual inspection ； ①Check whether the roughness at the contact point of the static ring O-ring in the inner hole of the gland meets the requirements, and whether there are any scratches on the end face. Check whether the size of the gland gasket installed at the end-face rabbet of the gland is appropriate, and whether there are any scratches on the end face. • ②bushing ； It mainly checks whether an O-ring or shaft seal gasket is present. Check again the roughness of the contact points between the shaft sleeve and the dynamic ring O-rings, as well as whether there are any scratches. • ③ Dynamic ring assembly ； Check whether it moves smoothly when pressed, and whether there are any cracks, scratches, or dust on the sealing surface. • ④static ring ； The main check is to verify whether there are any cracks, scratches, or dust on the sealing surface of the stationary ring, as well as to ensure that the O-rings of the stationary ring are in good condition. • After the inspection, perform a trial assembly first; the main purpose is to check whether the geometric tolerance dimensions of all components are appropriate. Then wipe each component clean with a clean cloth. Do not allow unauthorized personnel to handle it – take measures to prevent dust. ㈧ Precautions during installation: 1 First, place the seal in a safe location to prevent it from being damaged or broken. • 2 Handle with care during installation. Install in order. • 3. When installing the rotating and stationary rings, wipe the sealing surfaces with a particularly clean cloth once more, apply a little clean engine oil to the surfaces, and then bring the two surfaces into contact with each other. • 4 Tighten all screws; do not use a hammer or subject to severe vibrations. • 5 After all installations are complete, is manual cranking smooth? • 6. While driving, is the surface temperature of the hand-feel seal gland normal? If the temperature is too high, there are two possible reasons ； The sealing shrinkage is too large. Removing the seal reduces the compression amount. • Cavitation occurs in the pipes and sealing chambers. The vehicle should be stopped immediately to eliminate the hazard.