Seal selection and general knowledge about mechanical seals

For the selection of seals, the working conditions must be considered, so as to select the appropriate seal form, material and installation structure.

The working conditions that need to be considered generally include: temperature, pressure, and medium. Others also need to consider the selection of the equipment in combination with the characteristics of the equipment itself.

Regarding temperature, for example, the operating temperature range of NBR materials is generally -40 to +120°C. If the temperature exceeds 120°C, FKM or even PTFE should be considered. Low temperature conditions are relatively rare. Generally speaking, cold-resistant NBR should be considered at -20 to -40°C. General NBR materials will harden the material and cause leakage in low temperature environments, such as railway locomotives in winter in the north. Such problems often arise.

As for the pressure, it affects the form of the seal to be selected. Generally speaking, there will be no big problem in choosing rubber or PU seals for low and medium pressure. However, there is a problem of impact pressure to be considered. For example, when construction machinery is started, its impact pressure is much greater than the normal working pressure when it is closed, so HBY or SPGW with a pressure resistance of up to 70MPA and impact and crush resistance are often selected as primary seal. PTFE combination seals are also a popular choice.

The question of media is relatively simple. Just need to pay attention to some special small problems, such as oil cylinders in the metallurgical industry often use water-glycol and phosphate ester hydraulic oil. Sometimes although the working temperature is not high, you still have to be very careful in choosing the material. When the hydraulic oil uses the latter, you still need to choose FKM.

The selection of seals is very important, and it must be selected in combination with the conditions of the equipment itself. For example, if responsiveness is required, precise control is of course the choice of the Strovengle circle. High reliability is required, and SPGW is often selected. For another example, construction machinery working in the field often has shocks and low-temperature weather when working, and its dust-proof rings will use exposed metal skeletons to avoid falling off.

The seals of cylinders, compressor piston rings, high-pressure oil seals, and oil pumps in various scientific instruments all need special structures to meet the requirements.

All of these are different, welcome to communicate with me about the selection of seals

What is common sense about mechanical seals?

Mechanical seals are one of the basic components of machinery with precision and complex structures, and are key components of various pumps, reaction synthesis kettles, turbo compressors, submersible motors and other equipment. Its sealing performance and service life depend on many factors, such as type selection, machine precision, correct installation and use, etc.

1. Selection method

According to different working conditions and medium properties, mechanical seals include high temperature resistant and low temperature resistant mechanical seals, high pressure resistant and corrosion resistant mechanical seals, mechanical seals resistant to granular media and mechanical seals suitable for light hydrocarbon media that are easy to vaporize, etc. Different purposes choose mechanical seals of different structures and materials.

The main parameters for type selection are: seal cavity pressure (MPA), fluid temperature (°C), working speed (M/S), fluid characteristics, and effective space for installing the seal, etc.

What is the common sense of mechanical seals? The basic principles of selection are:

1. According to the pressure of the sealing cavity, determine whether the sealing structure adopts balanced or unbalanced type, single end face or double end face, etc.

2. According to the working speed, it is determined to adopt the rotary or static type, hydrodynamic pressure type or non-contact type.

3. According to the temperature and fluid properties, determine the friction pair and auxiliary sealing material, and correctly select the mechanical seal cycle protection system such as lubrication, flushing, heat preservation, cooling, etc.

4. According to the effective space of the installation seal, it is determined to use multiple springs or single springs or wave springs, built-in or external.

2. Requirements for the installation and use of mechanical seals

Common sense of mechanical seals 1. Requirements of mechanical seals for machine accuracy (taking mechanical seals for pumps as an example)

(1) The radial runout tolerance of the shaft (or shaft sleeve) where the mechanical seal is installed should not exceed 0.04-0.06MM at most.

(2) The axial movement of the rotor does not exceed 0.3MM.

(3) The maximum run-out tolerance of the positioning end of the sealing cavity and the sealing end cover to the surface of the shaft (or sleeve) shall not exceed 0.04-0.06MM.

Common sense of mechanical seals 

2. Confirmation of seals

(1) Confirm whether the installed seal is consistent with the required model.

(2) Before installation, carefully compare with the general assembly drawing to see if the number of parts is complete.

(3) For mechanical seals driven by double-coil springs, the springs can be divided into left-handed and right-handed, which must be selected according to the rotational direction of the rotating shaft.

3. Installation

The installation method varies with the type of mechanical seal and the type of machine, but the installation essentials are almost the same. The installation steps and precautions are as follows:

(1) Determination of installation size

When installing, the installation dimensions of the mechanical seal should be guaranteed according to the product instruction manual or sample.

(2) Before loading, the shaft (shaft sleeve) and gland should be free of burrs, and the bearings should be in good condition; the seals, shaft, seal cavity, and gland should all be cleaned. In order to reduce frictional resistance, the part where the mechanical seal is installed on the shaft should be coated with a thin layer of oil for lubrication. Considering the compatibility of the rubber O-ring, if oil is not suitable, soapy water can be applied. The floating static ring does not have an anti-rotation pin structure, so it is not suitable to be oiled, and it should be installed in the gland dry.

(3) Install the stationary ring and gland on the shaft first, taking care not to touch the shaft, and then install the moving ring assembly. The set screws of the spring seat or drive seat should be tightened evenly several times.

Before the gland is fixed, push the compensating ring by hand for axial compression. After loosening, the compensating ring can automatically spring back without stagnation, and then lock the gland bolts evenly.

4. Use

(1) When the temperature of the conveying medium is too high, too low, or contains impurity particles, is flammable, explosive, or toxic, corresponding measures such as blocking, flushing, cooling, and filtering must be taken.

(2) Turn the car by hand before running, and pay attention to whether the torque is too large, whether there is rubbing or abnormal sound.

(3) Pay attention to the direction of rotation, whether the coupling is centered, whether the lubricating oil at the bearing part is properly added, and whether the piping is correct.

(4) Before operation, firstly open the medium and cooling water valves, check whether the gas in the sealed cavity is completely discharged to prevent leakage caused by static pressure, and then start the operation.

(5) Whether the work is normal and stable after driving, whether there is abnormal torque caused by shaft rotation, abnormal noise and overheating.