Comprehensive Guide to Screw Pumps

High-viscosity pump pipelines are typically cleaned using two common methods: compressed air pigging and mechanical pigging (using a pig). The compressed air pigging process and its associated equipment are relatively simple, but the cleaning is often incomplete. For example, when cleaning pipelines used by high-viscosity gear pumps to transport palm oil, residual grease often remains adhered to the pipe walls.

A mechanical pigging system consists of a launching barrel, a receiving barrel, a cup-type pig, and pig indicators. To monitor the pig's position within the pipeline in real time, a mobile electronic locator receiver can be configured. The launching and receiving barrels are installed at both ends of the pipeline to dispatch and retrieve the pig, respectively. A cup-type pig comprises a steel frame, a radio transmitter, and two to four leather cups. The transmitter is connected to the pig and continuously emits highly stable low-frequency pulse signals during operation, with a transmission range exceeding 20 meters. Together with the transmitter, the electronic locator receiver forms an electronic positioning system that receives these signals. This allows for accurate tracking and positioning if the pig becomes stuck or blocked in the pipeline. Its positioning accuracy is ±0.1 m, with a detection depth of 6–8 m and a receiving radius of 10 m. Additionally, the pig indicator emits a precise audible and visual alarm when the pig passes a monitoring point, confirming its normal passage.

Because the media conveyed by high-viscosity pumps have high viscosity, heating or insulation is required to reduce flow resistance and enhance the pump's suction capability. Electric heating elements are commonly used to ensure uniform heating of viscous liquids. However, if temperature fluctuations are minimal and the high-viscosity liquid is prone to degradation, fluid heating is recommended, especially for high-displacement gear pumps. Fluid heating systems are divided into internal and external structures. An internal structure features a heating jacket designed within the pump body or end cover, whereas an external structure connects the heating jacket to the pump body via bolts. Depending on the specific medium, steam, heat transfer oil, or cooling water is circulated through the jacket. Internal structures are ideal for applications requiring high temperature uniformity or uniform cooling of high-temperature liquids.

This lubrication method offers several advantages: all the lubricating fluid entering the bearing is low-temperature medium, making it easy for the viscous lubricant to form a hydrodynamic oil film with excellent load-bearing capacity. The continuous circulation of the lubricant effectively removes heat from the bearing, providing superior lubrication and cooling. Furthermore, sufficient liquid fills the root of the gear teeth immediately after disengagement, which enhances the gear pump's self-priming performance and prevents dry running. This not only improves volumetric efficiency but also helps mitigate cavitation and reduce noise.

What are the causes of seal failure in high-viscosity pumps?

Leakage from the compensating ring sealing ring: Main causes include deformation of the gland, uneven pre-tightening force, improper installation, substandard sealing ring quality, or incorrect selection of the sealing ring.

Leakage from the sealing faces of the stationary and rotating rings: Main causes include surface flatness or roughness failing to meet requirements, surface scratches, the presence of particulate matter between the faces preventing uniform operation, or improper installation and incorrect installation methods.

Blockage of the seal flush liquid orifice plate or filter screen: This leads to insufficient oil supply, causing mechanical seal failure.

Excessive seal face clearance during installation: The flush liquid cannot remove the heat generated by the friction pair in time, or the flush liquid leaks through the clearance, causing overheating and damage to the sealing faces.

Poor lubricity of the liquid medium combined with operating pressure overload: This causes asynchronous tracking rotation of the two sealing faces. For instance, in a high-speed pump operating at 20,445 r/min with a seal face center diameter of 7 cm, the linear speed reaches 75 m/s. If one sealing face lags and fails to track the rotation, instantaneous high temperatures will damage the sealing face.

Vaporization and expansion of the liquid medium: This exerts a vaporization expansion force that separates the two sealing faces. When the faces are forced back together, the lubricating film is destroyed, leading to overheating of the sealing face surfaces.

Surface grooves on the sealing faces or gaps during face contact: These cause seal element failure. The primary reasons include:

Contaminated liquid medium: Fine, hard particles enter the sealing faces at high speeds, scratching the surfaces.

Poor coaxiality of pump transmission components: This causes the sealing faces to wobble and rub against each other once per revolution. The rotating ring's trajectory becomes eccentric, leading to vaporization and overheating wear of the sealing faces.

Frequent hydraulic pulsations of the liquid medium: This induces pump set vibration, resulting in seal face misalignment and failure.

Corrosion, stress concentration, erosion, and material incompatibility: Corrosion of seal elements by the liquid medium, improper matching of soft and hard materials, and incompatibility or deformation of auxiliary seals (such as O-rings, V-rings, and concave rings) with the liquid medium all lead to mechanical seal surface damage. Therefore, a comprehensive analysis of the damage patterns is necessary to identify the root cause, ensuring the long-term operation of the mechanical seal in stainless steel rotor pumps.