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Why are ball valves commonly used for high viscosity media (e.g. heavy oil)?

一 Introduction

In the field of industrial fluid transfer, the control of high viscosity media (such as heavy oil) has always been a challenging subject.

This type of medium has high viscosity, poor fluidity, easy to solidify and other characteristics, the valve in the delivery system puts forward very high requirements. As an important cut-off valve, ball valves are widely used in the control of high viscosity media.

二、the characteristics of high-viscosity media analysis

(一) characteristics

high viscosity: heavy oil and other media viscosity is much higher than ordinary liquids, for example, at room temperature, the viscosity of heavy oil can reach hundreds or even thousands of centipoises, which makes its flow resistance in the pipeline is very great, and is likely to result in energy loss and reduced transport efficiency.

    Poor mobility: due to the high viscosity, the medium's flow rate is slow, in the pipeline is easy to form a stagnant phenomenon, especially in the valve and other components, may lead to media accumulation, affecting the normal operation of the system.

   Temperature sensitivity: the viscosity of high-viscosity media with temperature changes significantly. Generally speaking, the temperature rises, the viscosity decreases; the temperature decreases, the viscosity rises sharply, and even solidification occurs. For example, heavy oil in the low temperature environment is easy to solidify into a semi-solid or solid, to transport and control bring great difficulties.

Corrosivity: some high-viscosity media may contain corrosive components, such as heavy oil may contain sulfides, etc., the valve material has a certain degree of corrosivity, the valve is required to have good corrosion resistance.

(二) the special requirements of the valve

sealing performance: high viscosity media is easy to form a leakage path at the valve sealing surface, so the valve needs to have excellent sealing performance to prevent media leakage, to avoid causing environmental pollution and safety hazards.

Flow capacity: In order to reduce the flow resistance of the medium in the valve, the valve should have a larger flow cross-sectional area and reasonable flow path design, to ensure that high-viscosity media can pass smoothly.

Anti-blocking ability: the high viscosity and poor fluidity of the medium is likely to lead to internal blockage of the valve, so the valve structure should be as simple as possible, to avoid the existence of dead ends and crevices that are easy to accumulate media.

Wear resistance and corrosion resistance: high viscosity media in the flow process may cause wear and tear on the valve components, while corrosive components will also cause erosion of the valve material, so the valve material needs to have good wear resistance and corrosion resistance.

Convenience of operation: because the delivery system of high viscosity media usually need to operate the valve frequently, so the valve should have a light operating torque and reliable operating mechanism to reduce the operator's labour intensity and improve work efficiency.

三、the ball valve is suitable for high-viscosity media key factors

(一) the advantages of the flow channel structure

Straight-through flow channel: most ball valves use straight-through flow channel design, this flow channel structure is simple, the medium in the valve flow path is short, resistance is small, can effectively reduce the high viscosity medium in the valve energy loss.

Compared with other types of valves (such as gate valves, globe valves, etc.), the ball valve flow path is smoother, especially suitable for high viscosity media flow.

Full bore design: many ball valves use full bore structure, that is, the diameter of the through hole of the ball body is equal to the internal diameter of the pipeline. This design makes the medium through the valve, the cross-sectional area of the flow path will not change abruptly, to avoid the contraction or expansion of the flow path caused by the medium turbulence and pressure loss, further reducing the flow resistance, improve the circulation of the medium.

For high-viscosity media, the full bore design is particularly important, it can reduce the media in the valve stagnation and accumulation, reduce the risk of clogging.

Runner surface finish: The runner surface of the ball valve is usually precision machined, with a high degree of finish. Smooth runner surface can reduce the friction resistance between the medium and the inner wall of the valve, so that high-viscosity media can pass through the valve more smoothly.

At the same time, the smooth surface is not easy to adhere to the medium, reducing the residue of the medium in the valve, which is conducive to maintaining the cleanliness and normal operation of the valve.

(二) sealing performance characteristics

1. sealing form

Metal hard seal: in the application of high viscosity media, metal hard seal ball valve is widely used. Metal hard seal is usually made of stainless steel, hard alloy and other materials, with high hardness and wear resistance.

When the valve is closed, the seal between the ball and the valve seat is achieved through the close contact of the metal surface, which can withstand high pressure and temperature, and has a certain degree of resistance to impurities such as solid particles in high-viscosity media. Metal hard seal sealing performance is reliable, especially for high temperature, high pressure, high viscosity and contains impurities in the media conditions.

Elastic seal: elastic seal ball valve usually use rubber, PTFE and other elastic materials as the seal. These elastic materials have good elasticity and sealing, when the valve is closed, the sealing ring can closely fit the surface of the ball to form a reliable seal.

    The sealing performance of the elastic sealing ball valve is excellent, especially in low-pressure conditions, and can effectively prevent the leakage of high-viscosity media. In addition, the elastic sealing material also has a certain degree of corrosion resistance, can adapt to some of the corrosive high viscosity media.

    2. sealing surface design

    Sealing surface angle: the sealing surface of the ball valve is usually designed into a certain angle, such as the common 45 degrees, 60 degrees, etc.. Reasonable sealing surface angle design can make the ball in the rotation process, the sealing surface gradually contact and separation, reduce the friction between the sealing surface and wear, but also conducive to the flow of media in the sealing surface, to avoid the accumulation of media in the sealing surface.

    For high-viscosity media, the appropriate sealing surface angle design can reduce the media resistance at the sealing surface, improve the sealing performance and service life of the valve.

    Sealing surface width: the width of the sealing surface is also one of the important factors affecting the sealing performance of the ball valve. Wider sealing surface can provide a larger sealing contact area, increase the reliability of the seal, but also increase the frictional resistance between the sealing surface, so that the valve operating torque increases.

In the application of high-viscosity media, according to the characteristics of the medium and working condition requirements, need to reasonably design the width of the sealing surface, in order to balance the relationship between sealing performance and operating torque.

(三) anti-clogging and self-cleaning ability

Structural simplicity: the structure of the ball valve is relatively simple, there are no complex internal parts and easy to block the channel. The sealing surface between the ball and the valve seat is usually a smooth curved surface, the flow path of the medium in the valve is smooth, not easy to form dead ends and vortex, thus reducing the retention and accumulation of media in the valve, reducing the risk of clogging.

Compared with gate valves, globe valves and other valves, the structure of the ball valve is more suitable for the flow of high-viscosity media, can effectively avoid the media due to stagnation and solidification or deposition.

Self-cleaning function: In the opening and closing process of the ball valve, the ball and the valve seat will produce relative motion. This relative motion can wipe the sealing surface, remove the media and impurities attached to the sealing surface, play a self-cleaning role.

For high-viscosity media, because it is easy to adhere to the sealing surface, the self-cleaning function is particularly important.

Through the rotation of the ball, the sealing surface of the media can be removed in a timely manner to maintain the sealing surface of the clean and smooth, so as to ensure the sealing performance and normal operation of the valve.

In addition, some specially designed ball valves (such as orbital ball valves) in the opening, the ball first off the valve seat, and then rotate, further reducing the friction and wear between the ball and the valve seat, while also enhancing the self-cleaning effect.