Maintenance and Overhaul of Pump Mechanical Seals

First, the mechanical seal operation and maintenance content After the mechanical seal is put into use, it must be properly maintained, so that it can have a better sealing effect and a long service life. Generally pay attention to the following aspects.
1. Leakage due to loose parts should be avoided. Attention should be paid to the phenomenon of heat caused by impurities entering the end face and whether there is any abnormal noise during operation. For continuously operating pumps, care should be taken not only to prevent dry friction during driving, but also to prevent dry friction during operation. Do not empty the pump. If necessary, set an automatic device to prevent the pump from emptying. For the intermittent operation of the pump, attention should be paid to observe the crystallization of the dry material formed after the pump is stopped, or the crystals precipitated when the temperature is lowered. When the pump is started, heating or flushing measures should be taken to prevent the crystallized material from scratching the end surface and affecting the sealing effect.
2. Whether the rinsing and cooling cycle protection system and the instrument work normally and stably. Pay attention to sudden stoppage of water to cause poor cooling, seal failure, or accident due to blockage of cooling pipes, flush pipes, and pressure equalization pipes.
3, centrifugal pump itself vibration, heat and other factors will also affect the sealing performance must be observed regularly. When the bearing is broken, it will also affect the sealing performance. Therefore, it is necessary to pay attention to whether the bearing is hot and the sound during operation is abnormal, so that it can be repaired in time.
Second, mechanical seal end face flatness inspection method China's mechanical industry standard JB/T 7369-2011 "mechanical seal end surface flatness inspection method" provides a mechanical seal face flatness test device, inspection procedures, flatness measurement value interpretation and other content It is applicable to the optical method of using a monochromatic light source to check the flatness of the mechanical seal ring end face.
1. Verification of the device The structure of the recommended test device is shown in Figure 1. The light source should be a monochromatic light source, and optical flat crystals for inspection should be first-order accuracy (the flatness should be between 0.02 and 0.10 μm). The diameter of the optical flat crystal should be larger than the outer diameter of the end face of the inspected seal ring.

Figure 1 Structure of a mechanical seal end face flatness tester
1-case; 2-ring; 3-optical flat crystal;4-active door;
5-hole movable plate; 6-glass mirror; 7-ground glass;
8-sodium light tube; 9-regulator element; 10-light barrier

The device is placed in a dry, clean, vibration-free workplace. The device must have a certain protective element to prevent the light beam from shining directly on the observer's skin or eyes. If you use mirror observation, you should ensure that the mirror is not deformed and distorted.
2. Inspection procedure When testing, the ambient temperature should be controlled at (20 ± 5) °C. Turn on the power switch of the flatness detector and warm it up until the lamp fully glows. Remove the fibers, particles, oil stains, water vapor and other contaminants on the end face of the seal ring to be inspected and the optical flat crystal surface, and protect the end face of the seal ring and the optically flat surface from being damaged, and keep the surfaces from being recontaminated during the inspection process. . Put the sealed sealing ring gently on the optical flat crystal (or gently place the flat crystal on the sealing ring) so that the end face of the sealing ring is in close contact with the optical flat crystal. The interference band appears, and the number of spectral bands should not be read. It is subjected to additional external forces. The interference spectrum band is interpreted by observing the interference pattern of the sealing end surface (or observing the interference pattern on the sealing end surface through optical flat crystal) through mirror observation.
3. Interpretation of the measured value of flatness (1) The interpretation of the measured value of flatness is as shown in Fig. 2 to Fig. 5. The second and third interferograms for each side of the map are the wedge-shaped interferograms for the side plane and the flat crystal, and the remaining four are the interferograms for the parallel plane contact between the plane to be measured and the flat crystal. The number of light bands is the number of line segments AB passing through the dark band. For graphics that are not included, the reader should interpret it with reference to the legend based on the correct understanding of the principle of light interference.

Figure 2 An illustration of a light strip

Figure 3 Two light bands

Figure 4 Three light bands

Figure 5 Multiple light bands (more than three light bands) legend

(2) The spherical convex surface and spherical concave surface are determined as follows.
a. When observing the interferogram, the eye moves from the top to the bottom, and several spectral bands move toward the center of the circle, which is a spherical concave surface. Some spectral bands move toward the outer diameter, which is a spherical convex surface.
b. Press gently on the outside of the flat crystal or the seal ring with a finger. Several spectral bands are curved around the finger. The spherical convex surface. Some spectral bands are curved toward the outside of the finger. It is a spherical concave surface.
(3) The calculation formula for the measured value of flatness is: Δ = 0.5Nλ
Where △ - the measured value of flatness, μm;
N—the number of interference bands;
λ - wavelength of monochromatic light, μm.
For example, a commonly used monochromatic light source, sodium light, has a wavelength of λ=0.6 μm, and a band of light has a Δ of 0.3 μm, ie, a flatness of 0.3 μm.
Third, the mechanical seal overhaul several misunderstandings 1, the greater the spring pressure, the better the sealing effect Although there is no leakage during the test run, but with high-speed continuous operation, the spring compression is too large, it will lead to rapid wear of the friction pair Even an instant burnout, excessive compression causes the spring to lose its ability to adjust the face of the moving ring, causing the seal to fail or directly causing the spring to fail.
2. The tighter the dynamic ring sealing ring, the better the dynamic ring sealing ring is, the harmful and unfavorable will not only increase the wear between the seal ring and the shaft sleeve (axis), but also increase the resistance of the dynamic adjustment of the axial movement of the dynamic ring. Frequently, it can not be adjusted timely, and it can easily lead to excessive fatigue and damage to the spring, so that the dynamic ring seal ring deformation, causing leakage, affecting the sealing effect.
3, the static ring seal ring as tight as possible The static ring seal ring is basically in a static state, relatively tight seal effect will be better, but over-tight is also harmful: First, due to static ring seal due to excessive deformation, affect the sealing effect; The material of the stationary ring is mostly graphite, which is generally brittle. It is easy to cause fragmentation due to excessive stress. Third, it is difficult to install and disassemble, and it is very easy to damage the stationary ring.
4. The tighter the impeller lock nut, the better. Sometimes there is leakage (shaft leakage) between the shaft sleeve and the shaft. It is generally believed that the leakage between shafts is that the impeller lock nut is not locked. In fact, there are many factors that cause shaft-to-shaft leakage. For example, the inter-axis mats are damaged, misaligned, there are impurities in the shafts, and there is a large form error in the fitting between the shaft sleeves and the shafts, the contact surfaces are damaged, and there are gaps between shaft components. If the head thread is too long, it will cause shaft leakage. Excessive locking of the lock nut will only lead to premature failure (internal aging, damage, etc.) of the inter-liner pad. On the contrary, a proper lock of the nut will keep the inter-axis pad always compressively elastic and will automatically lock in time during operation. The shaft is always in a good sealed condition.
5. Dismantling maintenance is better than not dismantling. Once a mechanical seal leaks, it is eager to repair it. In fact, sometimes the seal is not damaged. Simply adjust the working conditions or adjust the seal properly to eliminate the leak. In this way, waste can be avoided (due to the possibility of static ring damage or failure of the seal ring when dismantling), and it can also verify its own fault diagnosis capabilities, accumulate maintenance experience, improve the quality of inspections, and reduce the maintenance cost of mechanical seals.
6, the new is relatively good than the old, the effect of using a new mechanical seal better than the old, but the new mechanical seal quality or material selection is not appropriate, with a larger size error will affect the sealing effect; in the polymerization and permeability In the medium, the static ring is not excessively worn. Still not replaced as well. Because the stationary ring is in a stationary state for a long time in the stationary ring seat, the polymer and the impurities are deposited as a whole, and a good sealing effect is achieved.
Fourth, the maintenance of mechanical seal parts 1, friction pair The mechanical seal of the friction ring should be taken down every time the maintenance of a serious inspection, the end surface shall not have scratches, grooves, flatness to meet the requirements. Otherwise, it should be reground and polished according to the technical requirements of the friction sub ring. However, when it comes to repairs, the following specific rules are usually followed. (1) The end face of the friction secondary ring must not have scratches and grooves that communicate with the inner and outer edges, otherwise it will not be repaired.
(2) Thermal cracking of the frictional face is generally not repaired.
(3) Corrosion marks on the friction secondary ring are generally not repaired.
(4) Soft materials are liable to cause chipping and scratching during use and installation. They generally do not allow scribes with internal and external communication. The soft material seal rings allow collapse. Figure 6 shows that b/a ≤ 1/5 is required.

Figure 6 Allowable collapse of soft material seal rings

(5) When the wear amount of the end face of the friction secondary ring exceeds the following value, it is generally not repaired, and when the amount of wear is less than the value shown below, the re-grinding repair can be performed and can be reused when the technical requirements are met.
a. The wear amount of the end face of the surfacing alloy is set to 0.8mm.
5mm。 b, surfacing super hard alloy or Hastelloy alloy face wear amount of 0. 5mm.
c. The wear amount of the sprayed ceramics is 1.0mm.
d, Carbide or ceramic face wear is 1. 8mm.
e. The wear amount of the face of the graphite ring is 3mm and the wear amount of the graphite ring is 1.0mm, and the wear amount of the face of the graphite ring is 4mm, which is 1.5mm.
When repairing the end face of the friction secondary ring, it can be ground on a surface grinder, then ground and polished on the flat plate to repair. Different moving and static ring materials should use different abrasives and grinding tools.
(a) When coarsely grinding cemented carbide and ceramic rings, use 100-200 SiC silicon carbide abrasive powder and kerosene to mix evenly; when grinding, use M20 boron carbide or 240 to 300 silicon carbide corundum and kerosene to mix well. When grinding, place the ring on the plate, place the abrasive in the ring hole, and then grind it by hand with the "8"-shaped motion path (Fig. 7). This will avoid the directionality of the grooves on the ring surface until it is seen. No scratches. Bellows-type shaft seals must be positioned with tools when grinding the sealing surfaces and the bottom plate. After grinding, it is washed with gasoline, wiped with a cloth, and polished. When polishing with M2 ~ M3 emery paste with industrial glycerin (about 1:18) and stir, a small amount of abrasive brush on the grinding disc, still according to "8" shape grinding, the surface roughness up to Ra0.1μm.

Figure 7 Shaft seal grinding method

(b) When coarsely grinding stainless steel, cast iron, and PTFE, use M20 white fused alumina powder and mixed lubricant (2 parts of kerosene, 1 part of gasoline, and 1 part of spindle oil), mix well, and grind on a flat plate; When using M10 white corundum powder plus the above-mentioned mixed lubricant, put it on a flat plate with a certain hardness (240~280HB); when polishing, use M1~M3 white corundum powder or M10 chromium oxide plus the same mixed lubricant to put it on the liner. The white spinning cloth is ground on the plate. During the grinding process, if the lubricant dries out, you only need to add gasoline.
(c) When the coarse and fine-grained graphite rings are used, abrasives are not needed, and only aerospace gasoline is used as a lubricant for grinding on a flat plate, and dry grinding may be used for polishing.
1~0.2μm之间。 After repair, dynamic and static ring surface roughness Ra value between 0. 1 ~ 0.2μm. 04mm。 The surface flatness requirement is not greater than 1μm, the plane of the centerline perpendicularity allowable deviation is 0. 04mm, the dynamic ring and spring contact end facing the centerline perpendicularity allowable deviation is 0. 04mm. Checking the grinding quality of the moving ring and the stationary ring can be done in an easy way. Even if the two ring surfaces of the moving ring and the stationary ring are close to each other, if they cannot be sucked off, the grinding is qualified.
In the case of on-site inspection, if there is no plate or grinding machine, the “viewing mirror” glass on the reaction vessel can be used as a grinding plate for the soft material ring, and then it can be inspected with the knife edge ruler. Or use the coloring method to study the sealing rings. When researching, the contact path must be closed and continuous. The contact area must be greater than 80% of the area of ​​the sealing ring.
2. Sealing ring After a certain period of time, the sealing ring often swells or deteriorates. Therefore, a new seal ring is generally replaced during maintenance.
3, Springs Springs are mostly damaged due to corrosion or use, so that the spring loses its elasticity and affects the seal. Replace the spring after it has been damaged. When the spring is cleaned during maintenance, the elastic force shall be measured and the change of the spring force shall be less than 20%.
a. Permanent deformation The spring is compressed three times with the test load, and the free height variation values ​​after the second and third compressions are measured. This value is used as the permanent deformation of the spring. The permanent deformation should not be greater than 0.3% of free height.
b. Spring characteristics Measurement of spring characteristics is performed on spring testing machines with an accuracy of not less than 1%. The spring characteristic is determined by compressing the spring once until the test load. The test load is calculated according to the test stress specified in Table 1. When the calculated load is larger than the compressive load, the compressive load is used as the test load. Test load type Calculations.

Table 1 Test stress (unit: MPa)

material

Stainless steel wire

Bronze wire

Test stress τs

Tensile strength × 0.45

Tensile strength × 0.4

Where Ps - test load, N;
Τs - test stress, MPa;
d - wire diameter, mm;
D—spring diameter, mm.
c. Outer diameter (or inner diameter), free height, verticality Outer diameter (or inner diameter) is measured with a universal or special gauge. The free height is measured with a universal or special gauge and the highest point of the spring is measured. The squareness is measured with a flat plate and a wide square, as shown in FIG. In the no-load condition, place the tested spring vertically on the flat plate, abut on the wide seat square, rotate one revolution, measure the maximum value of the tip gap; then measure the other end face of the spring according to this method (end to 1 / 2 circle adjacent to the second round of assessment), the larger of the two measured values ​​as the verticality of the spring error.

Figure 8 Measurement of the verticality of the spring

d, pitch, end surface roughness, appearance In the corresponding spring testing machine, the spring is pressed to 80% of the total deformation, and the spring should not contact within the normal pitch circle.
The roughness of the end face is compared with that of a roughness sample.
The quality of the appearance of the spring is checked visually or with a 5x magnifying glass.
4. Shaft or shaft sleeve After the shaft or bushing is operated for a period of time, the surface of the shaft or shaft sleeve will be grooved due to corrosion or wear. At this time, the surface of the shaft or sleeve should be polished to restore the original surface roughness. If after grinding, its diameter size decreases, resulting in too much clearance between the spring seat, the moving ring, and the stationary ring, the shaft sleeve should be replaced or the pump shaft repaired or turned.