Problems and correction methods of mechanical seal for submersible sewage pump

Usually, in the process of fluid sealing occasions there is pollution, mainly adopts the contact type ordinary double mechanical seal, the seal between the two fluid assisted by blocking system is higher than the obstruction sealing fluid medium pressure injection pressure, prevent leakage to the outside of the sealed medium. But in practical application, often due to various reasons of failure and can effectively prevent the leakage of the sealed medium, especially for conveying fluid medium containing solid particles, the frequent pumping process and easy to stop, the sealing reliability is far from satisfactory. The sewage discharge pump of a single plant of a Petrochemical Industries Co is not ideal because of the sealing effect, and the leakage of the pump is an urgent problem to be solved.
1, submersible sewage pump mechanical sealing technology parameters of the pump model is WQ30-35-7.5, in normal work, the inlet pressure is close to the normal pressure, the outlet pressure is 0.55mpa, the conveying temperature is room temperature. The mechanical seal is a double end face with a multi spring balance structure, the inner diameter of the sealing surface is 35.2mm, the outside diameter is 44.2mm, the width of the end face is 4.5mm, and the balance diameter is 36.8mm.

2, under the conditions of the existing problems
(1) isolation fluid pressure is less than the sealed medium pressure in order to ensure the sealing function of the double face theory, isolation fluid pressure must be higher than the sealed medium pressure 0.1-0.15mpa, to prevent the leakage of the medium containing certain toxicity. But in practical engineering, the isolation fluid pressure is not greater than 0.3MPa, less than the pressure of the sealing medium 0.55mpa, so the use of sealing effect is not ideal.
(2) transport medium containing solid particles due to a large number of sewage pump is put in process unit discharges to various process fluid in the tank (including a large number of cooling water) timing transported to the sewage treatment station for purification treatment, impurity class which contains a certain proportion of the solid particles, easily into the sealing between severe abrasive wear the early failure of the seal. If the isolated fluid auxiliary system can not work, the pressure of the isolated fluid is less than the pressure of the sealed medium, the solid particles will enter the inner seal between the end face, the probability of failure is greatly increased.
(3) in the frequent start stop state when the liquid in the tank after the completion of the sewage pump, the pump will stop automatically when the tank fluid to the specified level, the pump is automatically switched to the running state, it is because the pump is in frequent on-off state, so that the mechanical seal appears periodic instability dry friction state, this is one of the main causes of the frequent failure of seal.
(4) pump because the pump is prone to frequent on-off state, when the liquid level of tank drops to a certain extent, it is easy to make the pump evacuation phenomenon, vibration and noise intensity, this phenomenon will inevitably lead to seal the unstable state, the possibility of seal failure greatly increased. Based on the above analysis that the double end common contacting mechanical seal sealing principle because of its particularity restrictions and conditions, and ensure the long period operation of the seal in this difficult environment. Moreover, in a stable condition, although the use of the double seal or seal series can basically eliminate the leakage of the sealed medium, but seal auxiliary system is complex, operating costs increased, and the reliability of auxiliary system directly affects the use effect of sealing.

3, mechanical seal of the submersible sewage pump transformation due to conic mechanical seal is especially suitable for containing solid suspended pumping equipment, so the sealing surface to cone seal. The static ring material as tetrafuoroethylene ring material, stainless steel, tungsten carbide spray cone. Its main features are as follows.
(1) mechanical seal ring cone is a conical ring, with static ring wear, contact the bus length of L growth, the static ring diameter increases.
(2) the sealing contact surface generatrix length is very short (stable at the beginning of operation for the 0.3-0.5mm), so the contact surface temperature difference is small, low temperature rise, reducing the degree of adhesive wear. The static ring has a large compression cross section, and the pressure deformation is small.
(3) the sealing sleeve and the moving ring have a certain float, so the sealing cover can always keep a good contact state, and the suspended particles in the medium are not easy to enter the sealing surface. The direction of the leakage of the medium is in a certain angle with the centrifugal force direction, which is favorable for reducing the leakage.