Eliminate the influence of external conditions on mechanical seals for pumps

When the mechanical seal for pump is used, not only the influencing factors of the mechanical seal itself, but also various influencing factors outside the mechanical seal should be considered.

1. Measures to eliminate the large amount of pump shaft channeling

Reasonably design the balance device of axial force to eliminate axial channeling. In order to meet this requirement, for multi-stage centrifugal pumps, there are two ideal design schemes: one is a balance plate plus an axial thrust bearing, the axial force is balanced by the balance plate, and the axial thrust bearing is used for the pump shaft. Axial limit; the other is the balance drum plus axial thrust bearing, most of the axial force is balanced by the balance drum, and the remaining axial force is borne by the thrust bearing, and the axial thrust bearing is used for the shaft of the pump shaft. to the limit. The key to the second solution is to properly design the balance drum so that it can truly balance out most of the axial force. For other single-stage pumps, split pumps and other products, some measures are taken in the design to ensure that the channeling of the pump shaft is within the range required by the mechanical seal.

2. Measures to eliminate excessive axial force

Reasonably design the axial force balance mechanism, so that it can fully balance the axial force and create a good condition for the mechanical seal. For some important products used in power plants, petroleum, chemical and other fields, before the products leave the factory, it is necessary to perform bench-to-bench tests to detect and find problems and solve problems. For some important pumps, an axial force measuring ring can be designed on the rotor to monitor the magnitude of the axial force at any time, and solve problems in time.

3. Measures to eliminate the large deflection of the pump shaft

This phenomenon mostly exists in horizontal multi-stage centrifugal pumps, and the following measures are taken in the design:

1. Reduce the distance between the bearings at both ends. The number of stages of the pump impeller should not be too many. When the total pump lift is required to be high, try to increase the lift of each stage of the impeller and reduce the number of stages.

2. Increase the diameter of the pump shaft. When designing the diameter of the pump shaft, do not simply consider the size of the transmitted power, but consider the mechanical seal, shaft deflection, starting method and related inertial load, radial force and other factors. Many designers do not fully realize this.

3. Improve the grade of pump shaft material.

4. Before installing the pump shaft, check the deflection of the pump shaft.

4. Measures to eliminate cavitation at the pump inlet

1. Improve the cavitation performance level of the pump and meet the requirements of the cavitation performance of the field device.

2. The requirements of the field test device should match the level of pump cavitation performance.

3. On-site installation and working condition adjustment should create favorable conditions for the pump.

Five, measures to eliminate pump vibration

1. During the assembly process of the pump, strictly follow the standards and operating procedures to eliminate the vibration source.

2. When auxiliary equipment such as pumps, motors, bases, and on-site pipelines are installed on site, strict checks should be made to eliminate vibration sources.

3. During on-site production, operation, maintenance and adjustment, strictly control and eliminate vibration sources.

6. Increase the auxiliary flushing system

When conditions permit, try to design an auxiliary flushing system. The flushing pressure is generally required to be 0.107-0.11 MPa higher than the sealing chamber pressure. If the conveying medium is easily vaporized, it should be 0.1175-0.12 MPa higher than the vaporization pressure. The pressure of the sealing chamber should be calculated according to the structural type of each pump, system pressure and other factors. When the pressure of the shaft seal cavity is very high or the pressure is almost close to the maximum limit of the seal, the liquid can also be drawn from the seal cavity to the low pressure area, so that the shaft seal liquid can flow to take away the frictional heat. Recommended flush volumes are shown in Table 1.

Pump mechanical seal table

According to the operating conditions of each pump, rationally configure piping and accessories. Such as coolers, orifice plates, filters, valves, flow indicators, pressure gauges, temperature, etc. In fact, the reliability and life of the seal depend to a large extent on the configuration of the seal auxiliary system.