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Centrifugal pump sealing device leakage, packing sealing and mechanical sealing knowledge

2021-10-27
Centrifugal pump is the use of external seal seal form, centrifugal pump leakage is usually the sealing position leakage, centrifugal pump sealing is common mechanical seal and packing seal, centrifugal pump shaft sealing device role: because there is a certain gap between the rotor and the pump shell, so in the pump shaft out of the pump shell parts to seal device. The seal at the suction end of the pump is used to prevent air leakage, damage vacuum and affect water absorption, and the seal at the outlet end can prevent high-pressure water leakage.


Mechanical seal

I. Form and working principle of mechanical seal
Mechanical seal is a limit working fluid leakage along the shaft, no packing end seal device, mainly composed of static ring, moving ring, elastic (or magnetic) components, transmission components and auxiliary sealing ring.
Mechanical seal work is fixed on the shaft of the moving ring and fixed on the pump shell of the static ring, and the use of elastic force of the elastic element and sealing fluid pressure, to promote the dynamic and static ring end face close to achieve sealing function. In the mechanical seal device, the pressure shaft seal water on the one hand to prevent high pressure drainage water, on the other hand, squeeze into the dynamic and static ring to maintain a layer of flowing lubrication film, so that the dynamic and static ring end face do not contact. Because the flow film is very thin and is acted upon by high-pressure water, there is very little leakage. Between the static ring and the sealing gland; Between the moving ring and the rotating shaft and between the sealing gland and the shell, the auxiliary sealing ring is used to solve the sealing problem of these leakage points.
Two, the working process of mechanical seal:
Between the static ring and the sealing gland, the static ring is sealed by the static ring sealing rubber ring and the elastic tightness of the sealing rubber ring is used to fix the static ring on the sealing gland, and the use of 1 anti-swivel pin to prevent the static ring rotating; The moving ring uses the elastic force of the elastic element and the static ring to closely fit together, the moving ring and the shaft are sealed by the moving ring sealing apron, and connected with the elastic element through the transmission pin, and the elastic element rotates together with the elastic element, which is fixed on the shaft by fastening screws. In this way, when the shaft rotates, the rotating shaft drives the elastic element to rotate through the fastening screw, and the elastic element drives the moving ring to rotate together through the drive pin, so that the relative rotary movement and good fitting contact between the moving ring and the static ring are generated, so as to achieve the purpose of sealing.


Three, the type of mechanical seal:

1, safety end is divided into: single end face, double end face mechanical seal.
Single face mechanical seal: a mechanical seal consisting of a pair of seal faces. Simple structure, easy to manufacture, easy to install, generally used in the medium itself good lubricity and allow trace leakage conditions.
Double face mechanical seal: a mechanical seal consisting of two pairs of seal faces. When the medium itself poor lubricity, toxic, flammable, explosive, easy to play and leakage has strict requirements. A sealing cooling liquid higher than the medium pressure is introduced between the two ends for sealing and cooling. It is possible to achieve "zero leakage" of the medium. Also divided into axial and radial double face.
2, according to the layout of the spring is divided into: spring built-in mechanical seal and spring external mechanical seal.
Spring built - in mechanical seal: the spring in contact with the medium. Easy to be corroded, easy to be blocked by debris in the medium, if the spring rotates with the shaft, it is not suitable for use in high viscosity media.
Spring external mechanical seal: the spring is not in contact with the medium. Used in high corrosion, high viscosity and easy to crystallize medium equipment.
3, according to the leakage direction of sealing fluid (medium) : internal flow mechanical seal and external flow mechanical seal.
The direction of leakage between the ends of the seal fluid (medium) is opposite to the direction of the centrifugal force. Small leakage and reliable sealing.
The direction of leakage between the ends of the seal fluid (medium) is the same as that of the centrifugal force. When the speed is very high, it is suitable for strengthening the end face lubrication, but the medium pressure is not easy to be too high, generally 1-2mpa.
4. According to the contact mode of seal face, it can be divided into contact type and non-contact mechanical seal.
Contact mechanical seal: seal face in boundary or semi-liquid lubrication state. Simple structure, less leakage, but wear, power consumption, heat are large, the use of high speed and high pressure under certain restrictions.
Non-contact mechanical seal: seal face in full liquid lubrication state. Low heat, power consumption, no wear during normal operation, can work under harsh conditions such as high pressure and high speed, but the leakage is large. Also divided into: hydrostatic pressure non-contact and fluid dynamic pressure non-contact mechanical seal.
A. Hydrostatic non-contact mechanical seal: the hydrostatic effect is produced by the pressure drop of the sealing end face through the introduction of external pressure fluid or the sealed medium itself.
B. Fluid dynamic pressure non-contact mechanical seal: the seal that produces fluid dynamic pressure effect by the relative rotation of the end face, such as spiral groove end face seal.
Other bellows type mechanical seals and unipolar, bipolar (multipolar) mechanical seals.
5, according to the number of springs: single spring mechanical seal and multi-spring mechanical seal.
An elastic element (seal compensator) having only one spring. Spring wire is thick, corrosion resistant, solid particles are not easy to accumulate in the spring, but the force on the end face is not uniform.
An elastic element (seal compensator) having a set of springs. The force on the end face is uniform, and it is easy to increase or decrease the number of springs to adjust the spring force. The axial length is short, but the spring wire is fine, the corrosion resistance life is short, and the installation dimension is strict.
6, safety balance mode points: balance type and non-balance mechanical seal.
Balanced mechanical seal: can make the medium acting on the seal face pressure unloading. The unloading degree is divided into partial balance type (partial unloading) and over balance type (all unloading). It can reduce the friction and wear on the end face, reduce friction heat, bearing capacity is large, but the structure is more complex, generally need to process a step on the shaft or shaft sleeve, the cost is higher.
Unbalanced mechanical seal: can not make the medium acting on the seal face pressure unloading. Simple structure, widely used when the medium pressure is less than 0.7mpa.
7, according to the form of elastic components (seal compensator) : rotary mechanical seal and static mechanical seal.
Rotary mechanical seal: the elastic element (seal compensator) rotates with the shaft. The application is wide, because the centrifugal force on the spring is easy to affect the pressure of the seal end face. Not suitable for high speed.
Static mechanical seal: the elastic element (seal compensator) does not rotate with the shaft. Suitable for high speed. Suitable for high speed.


Packing type sealing device :

the structure is mainly composed of: shaft sleeve, packing culvert, packing, water seal and so on.
1, shaft sleeve: is used to protect the shaft, prevent liquid corrosion of the shaft and the shaft does not directly produce friction with the packing.
2. Water seal: the water seal ring (page 191 at the beginning) is added to the spice inclusion, and is externally connected to the water seal pipe. When working, the small holes and grooves around the water seal ring form water rings, so as to prevent air from leaking into the pump. It can also lubricate and cool the packing and bushing, preventing a lot of wear of the packing and bushing.
3, packing culvert and packing: packing box and packing) plays the role of the external and internal partition of the pump shell, to reduce the amount of leakage.
 leakage treatment:
1. Working process: In order to reduce the leakage, the filler should be installed in the right way first.
1) First, the inside of the packing culvert should be thoroughly cleaned, and check whether the outer surface of the shaft sleeve and packing culvert is intact, and whether there is obvious wear and tear.
2) The specifications of the coil should be selected according to the provisions, the performance should be adapted to the liquid, and the size should meet the requirements. Too fine will leak.
3) When cutting the coiling root, the knife edge should be sharp, the interface should be cut into a bevel Angle of 30o ~ 45O, and the cutting surface should be smooth. The cut coil must be a full circle after it is installed in the packing culvert, and must not be short or too long.
4) After packing the packing culvert, the two adjacent circles shall stagger 90O. If it is equipped with water cooling structure, attention should be paid to the winding stagger the cooling water inlet of the packing culvert, and the circular chamber of the water sealing ring is just opposite the positive water inlet.
5) After installing a circle of packing, install the packing gland and tighten it evenly until it is confirmed that the packing is in place. Before loosening packing gland, tighten again to proper tightness. (After the general packing is not tight or slightly tight force, after the pump water injection in tight packing, but to let the packing has a trace of leakage. After the pump starts, tighten the coil according to the temperature and leakage of the coil. Neither too much leakage nor too high temperature.)
6) After tightening the upper packing root, check the clearance between the packing gland and the shaft, and the clearance around should be the same; Check that pressure is the same around the gland. Prevent friction between gland and shaft.
Check after packing root:

Check whether the tightening force of the packing gland fastening nut is appropriate. If the tightening force is too large, although the leakage is reduced, it will cause the friction of the disk and the bushing surface to increase. When it is serious, it will heat up and smoke until the disk and the bushing are burned. If the tightness is too small, the leakage will be large. Therefore, the tightening force must be appropriate, should make the liquid through the disc root and the shaft sleeve gap gradually reduce pressure and form a layer of water film, in order to increase lubrication, reduce friction and cooling of the shaft sleeve. After the pump is started, it is better to keep a small amount of liquid flowing out of the packing culvert. The gland tightening force can be adjusted after the pump starts.