Sanitary Butterfly Valve Precautions for use in marine environment

In marine environment, the corrosion of sanitary butterfly valves is much greater than that on land, because the evaporation of seawater will cause some salt to accumulate on the surface of the butterfly valve, so we need to perform surface anti-corrosion treatment on this part of sanitary butterfly valves.

The caliber of sanitary butterfly valves is generally calculated according to the flow rate of pipeline media. If you want to correctly select the caliber of sanitary butterfly valves, you should first calculate the pipeline flow rate. In the general flow calculation process, the flow coefficient of the valve can be divided into the rated flow coefficient and the operating flow coefficient. The rated flow coefficient is an inherent characteristic of the valve. As long as the valve structure is determined, the rated flow coefficient will be determined accordingly, regardless of the temperature, pressure, density, etc. of the operating conditions. The operating flow coefficient is determined by the (maximum, normal, minimum) flow rate of the operating conditions and the corresponding operating temperature, valve front pressure, valve back pressure, medium density and other parameters, and has nothing to do with the valve structure.

In general, the selection principles of the caliber of sanitary butterfly valves are as follows.

① When the sanitary butterfly valve is used only as a switch valve, that is, the valve state is either fully open or fully closed, at this time, as long as the rated flow coefficient of the valve is greater than the maximum flow coefficient required for the working condition, the valve diameter is generally the same as the nominal size of the pipeline.

② When the sanitary butterfly valve is used as a regulating valve, in addition to the rated flow coefficient of the valve being greater than the maximum flow coefficient required for the above condition, it should also be considered that due to the limitation of the valve structure, the sanitary butterfly valve has poor regulating performance at a small opening. It is recommended that the valve opening be above 20° under the minimum operating condition and below 70° under the maximum operating condition. In order to achieve the throttling regulation effect, it is generally hoped that the regulation loss of the system should be mainly on the regulating valve, and the flow rate of the sanitary butterfly valve can reach about 5m/s.

When the sanitary butterfly valve meets the above requirements, the diameter often needs to be reduced (smaller than the nominal size of the pipeline), but it must not be less than half of the nominal size of the pipeline.

Common Faults of Sanitary Butterfly Valves

The rubber elastomer in the sanitary butterfly valve will tear, wear, age, perforate or even fall off during continuous use. The traditional hot vulcanization process is difficult to adapt to the needs of on-site repair. Special equipment is required for repair, which consumes a lot of heat and electricity, and is time-consuming and labor-intensive. Today, polymer composite materials are gradually being used to replace traditional methods, among which the most widely used is the Fushilan technology system. The superior adhesion and excellent wear and tear resistance of its products ensure that the service life of new parts is achieved or even exceeded after repair, greatly shortening downtime.

Selection of Sanitary Butterfly Valves

The butterfly plate of the sanitary butterfly valve is installed in the diameter direction of the pipeline. In the cylindrical channel of the sanitary butterfly valve body, the disc-shaped butterfly plate rotates around the axis, with a rotation angle of 0°-90°. When it rotates to 90°, the valve is in a fully open state.

The sanitary butterfly valve has a simple structure, small size, light weight, and consists of only a few parts. Moreover, it can be quickly opened and closed by rotating 90°, which is easy to operate. At the same time, the valve has good fluid control characteristics. When the sanitary butterfly valve is in the fully open position, the thickness of the butterfly plate is the only resistance when the medium flows through the valve body. Therefore, the pressure drop generated by the valve is very small, so it has good flow control characteristics. There are two types of sealing for sanitary butterfly valves: elastic seal and metal seal. For elastic seal valves, the sealing ring can be embedded in the valve body or attached to the periphery of the butterfly plate.

Valves with metal seals generally have a longer life than valves with elastic seals, but it is difficult to achieve complete sealing. Metal seals can adapt to higher operating temperatures, while elastic seals have the defect of being limited by temperature.

If the sanitary butterfly valve is required to be used as a flow control, the main thing is to correctly select the size and type of the valve. The structural principle of the sanitary butterfly valve is particularly suitable for making large-diameter valves. Sanitary butterfly valves are not only widely used in general industries such as petroleum, gas, chemical industry, and water treatment, but also in the cooling water system of thermal power plants.

Commonly used sanitary butterfly valves are two types: wafer-type sanitary butterfly valves and flange-type sanitary butterfly valves. The wafer-type sanitary butterfly valve is connected between two pipe flanges with stud bolts, and the flange-type sanitary butterfly valve is a valve with flanges, and the flanges at both ends of the valve are connected to the pipe flanges with bolts.

Use process of sanitary butterfly valve

At present, sanitary butterfly valves, as a component used to realize the on-off and flow control of pipeline systems, have been widely used in many fields such as petroleum, chemical industry, metallurgy, hydropower, etc. In the known sanitary butterfly valve technology, its sealing form mostly adopts a sealing structure, and the sealing material is rubber, polytetrafluoroethylene, etc. Due to the limitation of structural characteristics, it is not suitable for industries such as high temperature resistance, high pressure resistance, corrosion resistance, and wear resistance.

A relatively advanced sanitary butterfly valve is a three-eccentric metal hard seal sanitary butterfly valve, the valve body and the valve seat are connected components, and the valve seat sealing surface layer is welded with heat-resistant and corrosion-resistant alloy materials. The multi-layer soft stacked sealing ring is fixed on the valve plate. Compared with the traditional sanitary butterfly valve, this sanitary butterfly valve has the advantages of high temperature resistance, easy operation, no friction when opening and closing, and the sealing is compensated as the torque of the transmission mechanism increases when closing, which improves the sealing performance of the sanitary butterfly valve and extends its service life.

However, this sanitary butterfly valve still has the following problems during use:

1. Since the multi-layer soft and hard stacked sealing ring is fixed on the valve plate, when the valve plate is normally open, the medium forms a positive scouring on its sealing surface. After the soft sealing belt in the metal sheet interlayer is scour, it directly affects the sealing performance.

2. Due to the limitation of structural conditions, this structure is not suitable for valves with a diameter below DN200. The reason is that the overall structure of the valve plate is too thick and the flow resistance is large.

3. Due to the principle of the three-eccentric structure, the seal between the sealing surface of the valve plate and the valve seat is pressed against the valve seat by the torque of the transmission device. In the positive flow state, the higher the medium pressure, the tighter the seal extrusion. When the medium in the flow channel flows in reverse, as the medium pressure increases, the unit positive pressure between the valve plate and the valve seat is less than the medium pressure, and the seal begins to leak.

High-performance three-eccentric bidirectional hard-sealed sanitary butterfly valve, characterized in that: the valve seat sealing ring is composed of multiple layers of stainless steel sheets on both sides of the soft T-shaped sealing ring. The sealing surface of the valve plate and the valve seat is an oblique cone structure, and the heat-resistant and corrosion-resistant alloy material is welded on the oblique cone surface of the valve plate; the spring fixed between the adjusting ring pressure plate and the adjusting bolt on the pressure plate are assembled together.

This structure effectively compensates for the tolerance band between the sleeve and the valve body and the elastic deformation of the valve stem under the medium pressure, and solves the sealing problem of the valve in the process of bidirectional interchangeable medium transportation. The sealing ring is composed of multiple layers of stainless steel sheets on both sides of the soft T-shaped, which has the dual advantages of metal hard sealing and soft sealing, and has zero leakage sealing performance regardless of low temperature and high temperature.

Experiments have shown that in the positive flow state of the pool (the flow direction of the medium is the same as the rotation direction of the butterfly plate), the pressure on the sealing surface is generated by the torque of the transmission device and the effect of the medium pressure on the valve plate. When the positive medium pressure increases, the tighter the oblique conical surface of the valve plate and the sealing surface of the valve seat are squeezed, the better the sealing effect. When in the countercurrent state, the seal between the valve plate and the valve seat is pressed against the valve seat by the torque of the driving device. As the reverse medium pressure increases, when the unit positive pressure between the valve plate and the valve seat is less than the medium pressure, the deformation energy stored in the spring of the adjusting ring after being loaded compensates for the tight pressure between the valve plate and the valve seat sealing surface and plays an automatic compensation role. Therefore, unlike the existing technology, the utility model does not install a soft and hard multi-layer sealing ring on the valve plate, but is directly installed on the valve body. Adding an adjusting ring between the pressure plate and the valve seat is a very ideal two-way hard sealing method. It will replace gate valves, stop valves and ball valves.

Occasions where sanitary butterfly valves are applicable

Sanitary butterfly valves are suitable for flow regulation. Since the pressure loss of sanitary butterfly valves in the pipeline is relatively large, about three times that of gate valves, when selecting sanitary butterfly valves, the influence of pressure loss on the pipeline system should be fully considered, and the firmness of the butterfly plate to withstand the pipeline medium pressure when closed should also be considered. In addition, the limitation of the working temperature of the elastic valve seat material under high temperature must also be considered. The structural length and overall height of the sanitary butterfly valve are small, the opening and closing speed is fast, and it has good fluid control characteristics. The structural principle of the sanitary butterfly valve is most suitable for making large-diameter valves. When the sanitary butterfly valve is required to control the flow, the most important thing is to correctly select the size and type of the sanitary butterfly valve so that it can work properly and effectively.

Usually, in throttling, regulating control and mud media, the structural length is short and the opening and closing speed is fast (1/4 turn). For low-pressure cut-off (small pressure difference), it is recommended to use a sanitary butterfly valve.

In double-position adjustment, constricted channel, low noise, cavitation and gasification, a small amount of leakage to the atmosphere, and abrasive media, a sanitary butterfly valve can be used.

In addition to being used as a program control valve for large-scale pressure swing adsorption (PSA) gas separation devices, the metal-to-metal face-sealed triple-eccentric sanitary butterfly valve can also be widely used in the fields of petroleum, petrochemical, chemical, metallurgy, and electric power, and is a good alternative to gate valves and stop valves.

The center line sanitary butterfly valve is suitable for fresh water, sewage, seawater, salt water, steam, natural gas, food, medicine, oil and various acid and alkali and other pipelines that require complete sealing, zero gas test leakage, high life requirements, and working temperature between -10 degrees and 150 degrees.

Soft seal eccentric sanitary butterfly valve is suitable for two-way opening and closing and adjustment of ventilation and dust removal pipelines, and is widely used in gas pipelines and waterways in metallurgy, light industry, electric power, and petrochemical systems.

Metal-to-metal wire sealed double eccentric sanitary butterfly valve is suitable for urban heating, gas supply, water supply and other gas, oil, acid and alkali pipelines as a regulating and throttling device.

When sanitary butterfly valves are used for throttling adjustment under special working conditions, or when strict sealing is required, or when severe wear and tear, low temperature (deep cold) and other working conditions are used, it is necessary to use a specially designed metal seal with a three-eccentric or double-eccentric special sanitary butterfly valve with an adjustment device.

Stainless steel has strong corrosion resistance. Intergranular corrosion and stress corrosion of stainless steel are not common in seawater, but local corrosion such as pitting and crevice corrosion is prone to occur in seawater. In order to reasonably and effectively apply stainless steel in seawater, it is necessary to have a comprehensive and in-depth understanding of its corrosion behavior and laws.

In low-velocity seawater, 18-8 stainless steel is also prone to pitting damage. At this time, if cathodic protection measures are not taken, high Cr and Mo stainless steel should be used, which has excellent resistance to pitting and crevice corrosion. In seawater with a flow rate greater than 112m/s, the surface of stainless steel can get sufficient oxygen supply, it is difficult for marine organisms to attach, and it is difficult for sediments to accumulate. At this time, 18-8 stainless steel will not produce pitting.

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