The application of the mechanical seal in the pump products is very broad, with products that improve the technical level and the requirement of energy saving, application of mechanical seal will be more widely. The mechanical seal effect will directly affect the overall operation, especially in the petrochemical field, due to the presence of flammable and explosive, volatile, toxic and other media, the mechanical seal leakage, will seriously affect the normal production, serious will have serious safety accidents. People in the reason of quality failure, are often used in mechanical seal itself.

One Mechanical seal The principle and requirements

Mechanical seal is by a A end to the relative motion of the ring (a fixed, another with the shaft to rotate together, see Figure 1) small axial clearance fit between the formation of the sealing function, this device is called a mechanical seal.

Mechanical seals are usually made of a movable ring and a static ring, a pressing element and sealing element. The dynamic ring and the static ring end face of a friction ring on the seal chamber fluid pressure in the face of pressure on the static ring end face, and generate the appropriate pressure and keep a very thin liquid film and achieve the purpose of sealing ring on the end in two. Compressioncomponents generate pressure can make the pump in the operating condition, maintain end fitting, ensure the sealing medium does not leak, and prevent the impurities from entering the seal face. The sealing element for sealing the gap between B and ring and the static ring and shaft clearance C gland function, at the same time, the impact of the pump vibration dampening effect. Mechanical seal in the actual operation is not an isolated component, it is with the other components of the pump combined operation, and through the basic principle can be seen, the normal operation of the mechanical seal is conditional, for example: clearance of the pump shaft is not too large, otherwise the friction faces cannot form normal requirements the pressure; mechanical seal pump shaft not at the deflection is too large, otherwise it will face pressure is not uniform and so on. Only to meet such external conditions, their performance coupled with good mechanical seal, in order to achieve the ideal sealing effect.

Analysis of the influencing factors of 2 external conditions

The amount of axial pump shaft 2.1 big

Mechanical seal The sealing surface has a certain pressure, so as to play a role in sealing, spring which requires mechanical seal to a certain amount of compression, a thrust to seal face rotation so that the sealing surface sealing the required pressure. In order to ensure that a specific pressure, mechanical seal pump shaft is not required clearance is too large, generally to ensure that within 0.5 mm. But in the actual design, due to the unreasonable design, pump shaft oftenproduce a large quantity, which is unfavorable for the use of mechanical seals. This phenomenon often appears in multistage centrifugal pump, especially in the process of pump starting, channeling a large amount of.

The working principle is to balance axial force balance disc method. Balance disc work automatically change the axial clearance between the balance disc and the equalizing ring B, thereby changing the balance disc before and after the pressure difference on both sides, a force and axial force in the opposite direction to balance the axial force. Due to the movement of the rotor inertia and transient pump fluctuations, the rotor will not run in a stationary equilibrium axial position. The balance disc is always around the moving state. The amount of axial balance disc in normal operation only 0105 ~ 011 mm, meet the mechanical seal allows axial amount of 015 mm, but the balance disc, startup and shutdown in the pump axial load condition changes may greatly exceed the amount of the axial mechanical seal allowed.

The pump after a long time running, the balance disc and the equalizing ring friction and wear, gap of B increases with the increasing amount of axial mechanical seal. Due to the axial force of the suction side of the sealing surface of the pressing force is increased, the sealing surface wear, until the sealing surface damage, loss of sealing effect. The out side of the mechanical seal, with the balance disc wear, axial amount of rotor axial amount is greater than the sealing requirements, reduce the sealing surface of the pressing force, not up to the requirements of sealing, the pump mechanical seal on both sides of the total loss of sealing effect.

2.2 axial force is too large

The mechanical seal used in the process is not able to bear the axial force, if there is axial force, serious impact on the mechanical seal. Sometimes due to the design of the pump axial force balance mechanism is not reasonable and the manufacture, installation and use, caused by the axial force is not balanced. Mechanical seal under an axial force, running gland temperature will be high, the polypropylene medium at high temperature will be melting, so the pump start soon after losing the sealing effect, the sealing face discontinuous spray leakage phenomenon of stationary pump.

2.3 the pump shaft large deflection

Mechanical end face seal, contact a rotating axial seal, it is in the fluid medium and the function of the elastic element, two perpendicular to the axis line of the seal tightly, the relative rotation, so as to achieve the sealing effect, thus requiring two to seal between the uniform stress. But because of the unreasonable design of pump, pump shaft running, large deflection produced at the installation of mechanical seal, the sealing surface between the uneven stress, leading to the sealing effect is not good.

2.4 no auxiliary flushing system or auxiliary flushing system is not reasonable

The auxiliary mechanical seal flushing system is very important, it can effectively protect the sealing surface to cooling, lubrication, wash away the sundries etc.. Sometimes the designer not reasonable allocation of auxiliary flushing system, not up to the sealing effect; although sometimes has designed auxiliary system, but because of the impurity washing liquid, washing fluid flow, the pressure is not enough, rinse mouth position design unreasonable, also did not reach the sealing effect.

2.5 large vibration

Mechanical seal The vibration is too large, resulting in loss of sealing effect. But the mechanical seal excessive vibration causes are often not the mechanical seal itself and other parts of the pump are the origin of the vibration of the pump shaft, such as unreasonable design, machining precision and bearing, coupling the parallel degree of difference, radial force and other reasons.

2.6 pump cavitation

Because of the unreasonable operation system and an inlet of the pump cavitation performance is not good, the pump speed is high, the local occurrence of cavitation in the pump inlet, cavitation, water bubbles will, on the one hand it will impact the mechanical sealing surface of the outer surface, so that the surface is damaged; on the other hand will make the flow of dynamic and static ring membrane anastomosis the bubble also contains, can not form a stable film flow, caused by dynamic and static ring anastomosis surface dry friction, mechanical seal device damage.

2.7 of machining precision is not enough

Not the machining precision, there are many reasons, there is not enough precision mechanical seal itself, the reason for this is easy to arouse people's attention, it is easy to find. But sometimes it is not enough accuracy of the pump of the other parts. The reasons for this, not easy to arouse people's attention. For example: a pump shaft, a shaft sleeve, a pump body, sealing cavity to increase the accuracy and other reasons. The existence of these reasons is very disadvantageous to the mechanical seal effect.

3 measures to be taken

3.1 the elimination of large amount of pump shaft channeling measures

The reasonable design of axial force balance device, eliminating the axial volume. In order to meet this requirement, the multistage centrifugal pump, design ideal to have two: one is the balancedisc axial thrust bearing, the balance disc balance the axial force, the axial thrust bearing on the pump shaft axial spacing; the other is a balancing drum and axial thrust bearing. By balancing drum balance out most of the axial force, the axial force of the thrust bearing by the remaining bear, while the axial thrust bearing on the pump shaft axial spacing. The second key scheme is reasonable design of balancing drum, so that it can really balance out most of the axial force. For other single-stage pump, pump and other products, in the design to take some measures to ensure the pump shaft clearance within the required range of mechanical seal.

3.2 measures to eliminate the axial force is too large

The reasonable design of the axial force balance mechanism, so that it can really take off the balance of axial force, mechanical seal to create a good condition. For some important application in power plants, petroleum, chemical and other fields, in the factory before, must do bench test and find out and solve the problems. In the rotor design of an axial force measuring ring can be some important pump at any time, monitoring of the size of the axial force, find and solve problems timely.

The pump shaft 3.3 to eliminate the large deflection measures

This phenomenon is mostly horizontal multistage centrifugal pump, take the following measures in design:

(1) to reduce the distance between the two ends of the bearing. Not too many series of pump impeller, the pump lift requirements higher, to improve the level of each impeller head, reducing series.

(2) increase the diameter of the pump shaft. In the design of the pump shaft diameter, not simply consider the transmission power, and to consider the mechanical seal, shaft deflection, starting method and relevant factors such as inertia load, radial force. Many designers are not fully aware of this point.

(3) to improve the material level of the pump shaft.

(4) to complete the design of the pump shaft, pump shaft deflection to check test calculation.

3.4 auxiliary flushing system

Where conditions permit, as far as possible the design of auxiliary flushing system. General requirements higher than the flushing pressure seal chamber pressure of 0107 ~ 011 MPa, if the transmission medium is easy to be vaporized, it should be higher than the vaporization pressure 01175 ~ 012 MPa. The pressure in the seal cavity according to the structure, each pump system pressure and other factors to calculate. The seal cavity of high pressure or pressure near the seal using the highest limit, but also by the liquid seal chamber to lead the region of low pressure, the seal liquid flow to frictional heat away. Rinse the recommended amount as shown in table 1.

According to each pump operating conditions, a reasonable allocation of piping and accessories. Such as Kong Ban, cooler, filter, valve, flow indicator, pressure gauge, temperature etc.. In fact, the life and reliability of the seal, depends largely on the seal auxiliary system configuration.

3.5 the elimination of import pump cavitation measures

(1) improve the cavitation performance of the pump, to meet the requirements of the cavitation performance of field device.

(2) the field test device is required to match the pump cavitation performance level.

(3) installation and field condition adjustment to create favorable conditions for pump.

3.6 the elimination of pump vibration measures

(1) pump products in the design process, we must fully analyze the vibration source, to eliminate the vibration source.

(2) the manufacturing assembly process of pump products, strictly according to the implementation standards and procedures, to eliminate the vibration source.

(3) pump motor base, on-site pipeline and other auxiliary equipment in the on-site installation, should strictly eliminate the vibration source.

(4) the production, operation, maintenance, adjustment, strictly eliminate the vibration source.

3.7 strict implementation of design standard

The design of mechanical products and pump sealing products to implement the relevant standards at home and abroad, in the process of product design, designers should earnestly implement specific standards, meaning deep understanding of every standard of content, the content of the standard will be executed to the process of product design. So far, there are many who do not understand the actual meaning of the standard, not strictly to the implementation of the new standards, but blindly copying the old drawings and old design experience. This practice in China to improve the level of product technology and enter the international market is very bad. To improve the standardization of knowledge, is the machinery industry designer an urgent need to solve the problem.