Created on:2021-11-24 10:06

Failure analysis and troubleshooting of screw pump

Failure analysis and troubleshooting of screw pump

1. The discharge pressure fails to meet the specified requirements at the user's site (when the delivery test is qualified)

(1) Problems with user system devices.

1) There is air leakage in the suction system pipeline. At this time, the medium suction is insufficient, resulting in the vacuum gauge degree not going up, so the discharge flow decreases, so the pressure does not go up.

2) The screen of the suction system unit is blocked. At this time, the medium suction will also be insufficient, and the value of the vacuum gauge will exceed the specified value range, the pump will vibrate and make noise, the pointer of the pressure gauge and vacuum gauge will swing violently, and the pressure will not go up.

3) The resistance of the suction system device is too large, which affects the pumping performance.

4) The pressure of the user's discharge system does not match the discharge pressure of the pump, which affects the discharge pressure of the pump, that is, the discharge pressure does not go up due to the change of the load of the user's pressure system.

(2) Improper use and installation by users.

1) When the new pump is used for the first time, the pump chamber shall be filled with working liquid and air shall be discharged. When the flow is insufficient, the discharge pressure of the pump can not go up, and due to temporary dry friction, it is easy to cause damage to the mechanical seal, screw and bushing, bearing and other parts.

2) Install and remove the regulating safety valve at will, resulting in leakage and the pressure of the pump can not go up.

3) If the pipeline installed on the pump is too long, there are too many elbow valves, or the oil suction height of the pump exceeds the specified value, the flow of the pump will decrease, and the discharge pressure of the pump will not go up.

4) If the viscosity of the input liquid medium is lower than the specified viscosity range, the leakage will increase and the pressure will decrease.

5) When the mechanical seal of double suction pump works under negative pressure, once the mechanical seal is damaged, it is easy to cause air leakage and affect the pressure fluctuation of the pump.

6) When the bushing, sliding bearing and balance piston of screw pump are worn too much, the flow decreases and the pressure decreases. According to regulations, when the flow is 15% lower than the original, replace the worn parts to ensure the economy of the pump.

2. The pump vibration and noise exceed the specified index

Causes of excessive vibration and noise during pump and motor operation:

(l) Factors in pump.

1) Vibration noise caused by unreasonable screw length. Generally, the working screw length of single-stage pump is L = (1.2 ~ 1.5) h, and that of multi-stage pump is L = Z × H + 0.09h (z is the number of sealing cavities).

Note: Although the screw pump operates stably without pulsation and agitation, it is pulsating in fact, but its pulsating frequency f = (n) relative to piston pump,, etc × H) / 60, so try to stagger the frequency of the pump itself from the natural frequency of the base, or take vibration isolation measures to avoid resonance.

2) During the structural design of the pump, try to improve the stiffness of the shell, screw and other main parts and reduce the deflection. In general, cast iron shall be used for the shell and base, and cast steel, copper and weldments shall be used for military products, which will help to reduce vibration and noise.

3) The flow passage of the pump body into and out of the mouth shall be designed smoothly and the flow rate shall be stable. It shall not be designed to shrink and expand for a while, which will produce fluid impact, vibration and noise. The flow rate shall be as low as possible.

4) For the screw, the inlet end and outlet end should be 180 ° to make its mass as balanced as possible and split evenly, so as to reduce the unbalanced mass and make its operation stable. If necessary, the action balance test can be carried out.

5) Reasonably select the tolerance fit and geometric tolerance of rotating parts to improve the accuracy of concentricity and symmetry.

6) Improve the screw accuracy and the accuracy level of the processing equipment, add some tooling to improve the product quality, and achieve the exchange of parts at the same time.

7) Try to avoid the pump working under cavitation. In order to prolong the service life of the pump, regular inspection and maintenance should be carried out.

8) If the viscosity of the liquid delivered by the pump exceeds the specified requirements, the transmission viscosity resistance will increase, resulting in cavitation and increased vibration and noise. Heating of the transmission medium and pipeline insulation measures can be taken to reduce the transmission resistance.

(2) Factors caused by unit installation and system layout.

1) The motor and coupling of the pump shall be aligned and concentric during installation. With the increase of the speed of the unit, the installation accuracy shall be improved accordingly.

2) The system pipeline layout shall be reasonable, the length of elbow valve shall be minimized, and the system pipeline resistance shall be calculated when necessary. Its value shall not exceed the allowable suction capacity of the pump. At the same time, the relative geometric height between the original center and the liquid level of the oil tank or oil pool shall be calculated. This value shall be considered together with the pipeline resistance and shall not exceed the specified requirements.

3) The oil tank of the system device shall have sufficient oil volume to ensure sufficient liquid level height. For the oil tank with changed oil level, the oil tank shall be equipped with a minimum liquid level alarm device or protective equipment to avoid cavitation or evacuation of the pump, especially for the three screw pump.

4) The pump suction pipeline shall be arranged without bends higher than the height of the pump suction port (except the pressurized pipeline), and there shall be no air leakage at the pipeline connections and valves.

5) The filter on the suction pipe of the pump shall be cleaned regularly. Once it is blocked or the flow section is too small, vibration and noise will also be generated. Generally, the flow area of the filter screen is 6 ~ 10 times that of the suction pipe. If the viscosity of the transmission medium is very large, the multiple will increase. For the three screw pump, more than 80 mesh and the twin screw pump, 40 ~ 60 mesh.

6) For high viscosity medium liquid, heating equipment shall be set at the pump suction, and thermal insulation measures shall be set for pipelines and pumps, especially in places used in northern winter, in which steam, electric heating or heat transfer oil can be used.

7) The pipelines of suction system and discharge system shall be fixed with supports to avoid vibration.

8) When starting the motor unit installed for the first time, observe whether the rotation direction of the motor is consistent with the rotation direction required by the pump. Click to start and correct it in time to ensure the safety of the unit.

3. Faults in other aspects and handling methods

(1) The temperature at the bearing of the pump is too high.

1) The concentricity of pump, motor and coupling is poor, so it shall be calibrated again.

2) The diameter of the oil return pipeline of the bearing and sealing chamber is large or blocked.

3) The axial force of the main rod is not well balanced, and the residual axial force is too large.

(2) The bushing, screw, bearing and other main rotating parts of the pump wear too fast.

1) If there are impurities in the liquid medium delivered by the pump, increase the mesh number and overflow area of the filter.

2) Increase the hardness of the screw surface and select materials with good wear resistance.

3) Improve the concentricity of rotating parts and assembly alignment accuracy.

4) Reduce the unbalanced mass and heat treatment bending deformation of the screw.

(3) The mechanical seal leaks oil.

1) When assembling the machine, the sealing face is not perpendicular to the axis, so it should be re assembled and grinded.

2) If there is a quality problem with the mechanical seal itself, replace the mechanical seal again.

3) If the pressure in the mechanical seal chamber is too high, take measures to reduce the pressure in the chamber (increase the oil return or reduce the leakage in the high-pressure chamber).

4) If the oil temperature in the mechanical seal cavity is too high, the oil viscosity becomes thin, the oil return can be increased, and the preload of the mechanical seal spring can be adjusted.

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