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Overall Installation of mechanical seals（1）

Preparation before installation of mechanical seals

1. Take out the mechanical seal to be installed from the packing box and place it on a clean surface, protect the sealing surface, and arrange all labels and certificates for inspection;

2. Check the model, specification and material certificate of the mechanical seal, the number of parts, whether the pair of rotary face and stationary seat meet the requirements of the drawings, and whether there are identification marks of the manufacturer on the parts;

3. Check the surface quality of each part, especially whether the mechanical seal rotary face and stationary seat face are damaged or scratched. If damaged, it must be repaired or replaced. The surface of the parts must be cleaned. Especially the mechanical seal rotary face and stationary seat face can be applied by a layer of oil;

4. Check the water pressure test certificate of the graphite ring, the glass-filled PTFE ring, and the assembled rotary face and stationary seat . If there is no pressure test certificate, a water pressure test must be done before installation. The test must be in accordance with the first section of Chapter VI. Regulations;

5. For mechanical seals driven by parallel coil springs, pay special attention to the direction of rotation of the spring. The direction of rotation should be the same as that of the shaft so that when we rotates it in the right direction,the spring will be the tighter and tighter .That is, in the angle from stationary seat to the rotary face according to the shaft rotation direction,a clockwise direction requires a right-handed maneuver, and vice versa.

When using multiple springs, check the number of and free length of each group of springs;

6 Check the axical displacement, shaft radial play, related perpendicularity, concentricity, surface roughness, matching size and requirements for the shaft coupling and so on. They are all technical requirements for the host machine installed with the mechanical seal.

axical displacement value：no more than 0.1mm, generally between 0.025~0.1mm. If too big, the specific pressure of the mechanical seal spring will be unstable, causing the wear or leakage of the seal face due to the vibrating of the shaft. The seal is damaged, which is more obvious in the graphite ring. In addition, because of the too big axical displacement value, partial corrosion, erosion or wearing will take place at the contacting part between the mechanical seal and the shaft (or sleeve). PTFE sealing ring are less elastic than rubber counterparts and thus more affected. Therefore, the axical displacement value needs to be changed from the original ±0.5mm to no more than 0.1mm, which can reduce the damage caused by the excessively big deviation. On the other hand, if the axical displacement value is too small, less than 0.025mm for example, it will have an adverse effect on the bearing.

In fact, the axical displacement value is also related to the shaft speed. The following figure picture shows the relationship between the axical displacement and the shaft speed, which can be used for reference during installation.

（2）Allowable value of shaft radial play: Excessive shaft radial play will cause the jittering or deviation of the shaft,thus increase the seal contact surface, reduce lubricating film, and accelerate wear, which is detrimental to the sealing performance and service life of the mechanical seal. Therefore, JB4 127-85 stipulates：

shaft radial play is not only related to shaft diameter, but also related to shaft speed of the mechanical seal.

The verticality between the shaft and the stuffing box: no more than 0.1mm, generally 0.08mm. If the value is too big, the cover will tilt up and the mechanical seal will vibrate, and the wall surface will have uneven specific pressure and liquid film loss. Then erosion and abrasion will occur. The vibration of the mechanical seal will also wear out the driving pin, and finally make the sealing performance and service life unsatisfactory.

According to the inspection of the mechanical seal, the verticality between the shaft and the stuffing box is related to the shaft diameter and the shaft speed. It is also related to the compression method of the gasket used for sealing when the cover is installed. The use of unilateral compression will cause leakage.

The shaft radial play value is required to be measured at two points on the impeller wall and the driving wall, and also measured when the impeller is installed.

Concentricity between the shaft and the stuffing box of the mechanical seal : within the sliding speed V=25m/s, make sure the concentricity does not exceed 0.2mm, generally 0.125-0.2mm.

The surface roughness of the shaft of the mechanical seal is not less than Ra1.6, and the outer diameter tolerance of the shaft is h6.

Pump coupling requirements: The centering of the drive part is extremely important, because the side face runout and radial play of the coupling, the concentricity and the size of the cross-sectional gap will affect the life of the mechanical seal, thus requires checking regularly. The installation requirements of the coupling are usually stated in the instructions provided by the machine manufacturer.

The inspection required for couplingment should be carried out before the installation of all pipeline joints, otherwise, the centering work will be incomplete.

Regarding the gap between the rear end faces of the two half couplings, refer to the relevant regulations（try out the keywords“mechanical seals regulations regarding couplings” to find them）.

Generally, the end face gap is slightly larger than the axial displacement of the mechanical seal.

For there are many types of couplings, the installation of the mechanical seals should be carried out according to different requirements.

Background knowledge: Coupling types: 1. Flange coupling: on no impact and good centering occasions. 2. Slider coupling: for low-speed transmission. 3. Flexible coupling: suitable for rotary encoders and stepping motors. 4. Safety coupling: overload warning. 5. Rigid coupling. 6. Plum blossom coupling: for servo and stepping motors.