Thread Content
This post was last edited by Sun Yong on 2024-6-25 at 10:01. Function of the shaft seal device: Since a certain gap is required between the rotor and the pump casing, a sealing device is installed at the part where the pump shaft extends out of the casing. The seal at the suction side of the water pump prevents air from entering and disrupting the vacuum, which could affect water absorption, while the seal at the discharge side prevents high-pressure water from leaking out. Packing seal device (packing) I. Structure It is mainly composed of: shaft sleeve, packing box, packing, water seal, etc. 1. Shaft sleeve: It is used to protect the shaft, preventing the shaft from being corroded by liquids and avoiding direct friction between the shaft and the packing. 2. Packing boxes and packing: (gland boxes and packing) serve to separate the outside from the inside of the pump casing, thereby reducing leakage. 3. Water seal: A water seal ring is placed inside the seasoning chamber and aligned with the external water seal tube; during operation, water rings are formed at the small holes and grooves around the water seal ring, thereby preventing air from entering the pump. It can also serve to lubricate and cool the packing and shaft sleeve, preventing excessive wear of them. II. Process for dealing with leaks: To reduce the amount of leakage, the packing must first be installed in the correct manner. (1) First, the stuffing box should be thoroughly cleaned, and it is necessary to check whether the outer surfaces of the shaft sleeve and the stuffing box are in good condition, with no signs of significant wear. (2) The specifications of the packing should be selected in accordance with regulations; its performance must be suitable for the liquid being transported, and its dimensions must meet the required standards. If it’s too thin, it will leak. (3) When cutting the gasket, the blade must be sharp; the joint should be cut at an angle of 30° to 45°, and the cut surface should be smooth. After the cut packing is placed in the packing box, it must form a perfect circle – it cannot be short nor too long. (4) After the packing is installed in the packing box, the joints between adjacent turns should be offset by at least 90°. If it is equipped with a water cooling system, care should be taken to offset the packing gland’s cooling water inlet, and to align the annular chamber of the water seal ring precisely with the water inlet. (5) After installing the last ring of packing, attach the packing gland and tighten it evenly until it is confirmed that the packing is in place. Loosen the packing gland and tighten it again to the appropriate tightness. (Generally, after installing the packing, it is best not to tighten it too much at first, or to tighten it only slightly; tighten the packing after pumping water in, but allow a slight amount of leakage from the packing.) After the pump starts, tighten the packing based on its temperature and leakage rate. That is, it cannot leak too much nor can the temperature be too high. ) (6) After tightening the packing, check the gap between the packing gland and the shaft; the gaps around it should be equal ； Check whether the pressure around the gland is even. Prevent friction between the gland and the shaft. III. Inspection after packing installation: Check whether the tightening torque of the packing gland nuts is appropriate. If the torque is too high, although the leakage rate decreases, it will increase the friction between the packing and the surface of the shaft sleeve; in severe cases, this can lead to heating and smoking, eventually causing the packing and shaft sleeve to be damaged ； If the tension is too low, the leakage will be high. Therefore, the tightening force must be appropriate; it should allow the liquid to gradually reduce pressure through the gap between the packing and the shaft sleeve, thereby forming a water film that enhances lubrication, reduces friction, and cools the shaft sleeve. After the pump is started, it is advisable to have a small amount of liquid continuously flowing out from the packing gland. The gland tightness can be adjusted after the pump is started. Mechanical Seals I. Types and Working Principles of Mechanical Seals A mechanical seal is a gapless end-face sealing device that prevents the leakage of working fluid along the shaft; it consists mainly of a stationary ring, a rotating ring, elastic (or magnetic) elements, transmission elements, and auxiliary sealing rings. During operation, a mechanical seal relies on the rotating ring fixed to the shaft and the stationary ring fixed to the pump casing; the elastic force of elastic elements together with the pressure of the sealing fluid are used to ensure tight contact between the end surfaces of these rings, thereby achieving the sealing function. In mechanical seal devices, the pressure shaft seal water prevents high-pressure fluid from leaking out, and at the same time forces a layer of flowing lubricant film between the rotating and stationary rings, thereby preventing contact between their end surfaces. Since the flow membrane is very thin and under the action of high-pressure water, the leakage amount is minimal. Auxiliary sealing rings are used between the static ring and the sealing gland, the packing between the dynamic ring and the rotating shaft, and between the sealing gland and the housing, thereby solving the sealing problems at these leakage points. II. Working process of the mechanical seal: The stationary ring is sealed between it and the sealing gland by a sealing rubber ring; the elasticity of this rubber ring provides the force needed to hold the stationary ring in place on the sealing gland. Additionally, a anti-rotation pin is used to prevent the stationary ring from rotating ； The moving ring is pressed tightly against the stationary ring by the elastic force of the elastic element; the space between the moving ring and the shaft is sealed using a sealing rubber ring on the moving ring, and it is connected to the elastic element via a transmission pin, rotating together with the elastic element. The elastic element is fixed to the shaft using fastening screws, so as to rotate along with the shaft. In this way, as the shaft rotates, the rotating shaft drives the elastic element to rotate through the fastening screws, and the elastic element in turn drives the moving ring to rotate via the transmission pins. This results in relative rotational motion between the moving ring and the stationary ring, as well as good fitting and contact, thereby achieving sealing. III. Types of mechanical seals 1. Classified by end face: single-end face and double-end face mechanical seals. (1) Single-face mechanical seal: A mechanical seal consisting of a pair of sealing faces. It has a simple structure and is easy to manufacture and install; it is generally used in situations where the medium itself has good lubricity and minor leakage is acceptable. (2) Double-end mechanical seal: A mechanical seal consisting of two pairs of sealing surfaces. When the medium itself has poor lubricity, is toxic, flammable, explosive, volatile, and strict requirements are imposed on the amount of leakage. A sealing cooling fluid at a pressure higher than that of the medium is introduced between the two end faces to achieve sealing and cooling. It is possible to achieve “zero leakage” of the medium. It is further divided into axial and radial double-end faces. (3) Classified by balance method: balanced and unbalanced mechanical seals. a: Balanced mechanical seal: One that allows the pressure exerted by the medium on the sealing surface to be relieved. Depending on the degree of unloading, it is divided into partially balanced (partial unloading) and over-balanced (complete unloading) types. It can reduce friction and wear on the end faces, decrease frictional heat, and has a high load-bearing capacity; however, its structure is relatively complex, usually requiring a step to be machined on the shaft or sleeve, which results in higher costs. b: Unbalanced mechanical seal: One that cannot relieve the pressure exerted by the medium on the sealing surface. It has a simple structure and is widely used when the medium pressure is less than 0.7 Mpa. 2. Classified by the arrangement of the springs: spring-mounted mechanical seals and spring-exposed mechanical seals. (1) Spring-mounted mechanical seal: The spring is placed in the medium and in contact with it. It is prone to corrosion and can get clogged by impurities in the medium; if the spring rotates with the shaft, it is not suitable for use in media with high viscosity. (2) Spring-mounted mechanical seal: The spring is not immersed in the medium and remains in contact with it. Used in equipment operating in highly corrosive, highly viscous, and crystallizable media. 3. Classified by the number of springs: single-spring mechanical seals and multi-spring mechanical seals. (1) Among the elastic elements (seal compensators), there is only one type that uses a spring. The spring wire is thick, corrosion-resistant, and solid particles do not tend to accumulate at the spring, but the stress on the end faces is uneven. (2) In the elastic element (seal compensator), there is a set of springs. The end face is subjected to relatively uniform stress, making it easy to adjust the spring force by changing the number of springs. It has a short axial length, but the spring wires are thin, resulting in a short corrosion resistance life; moreover, strict requirements are placed on the installation dimensions. 4. Classified by the type of elastic element (sealing compensator): rotary mechanical seals and stationary mechanical seals. (1) Rotary mechanical seal: The elastic element (seal compensator) rotates together with the shaft. It is widely used, as the effect of centrifugal force on the spring during assembly can affect the pressure at the sealing surface. Not suitable for high rotational speeds. (2) Stationary mechanical seal: The elastic element (seal compensator) does not rotate with the shaft. Suitable for high-speed applications. Suitable for high-speed applications. 5. Classified by the leakage direction of the sealing fluid (medium): internal flow mechanical seals and external flow mechanical seals. (1) The direction of leakage of the sealing fluid (medium) between the end faces is opposite to the direction of centrifugal force. Low leakage rate and reliable sealing. (2) The direction of leakage of the sealing fluid (medium) between the end faces is the same as the direction of centrifugal force. At very high rotational speeds, it is more suitable for enhancing end-face lubrication, but the medium pressure should not be too high, generally ranging from 1 to 2 Mpa. 6. Classified by the contact method of the sealed end face: contact mechanical seals and non-contact mechanical seals. (1) Contact mechanical seal: The sealing surface is in a boundary or semi-liquid lubrication state. It has a simple structure and low leakage, but it experiences high wear, power consumption, and heat generation, which limits its use under high speed and high pressure conditions. (2) Contactless mechanical seal: The sealing surface is in a fully liquid-lubricated state. It has a low calorific value and power consumption; there is no wear during normal operation, and it can function under harsh conditions such as high pressure and high speed, but its leakage rate is relatively high. It is further divided into hydrostatic non-contact and hydrodynamic non-contact mechanical seals. a. Hydrostatic non-contact mechanical seal: It utilizes externally introduced pressurized fluid or the sealed medium itself, generating a hydrostatic effect through the pressure drop at the sealing surface. b. Hydrodynamic non-contact mechanical seals: Seals that utilize the relative rotation of the end faces to generate a hydrodynamic effect, such as helical groove face seals. Others include bellows-type mechanical seals as well as single-pole, double-pole (multi-pole) mechanical seals.