The process pipe diameter in large-scale seawater desalination is generally DN300 to 1600, which is beyond the scope of use of general ball valves and stop valves. Compared with the gate valve of the same caliber (Z41H), the butterfly valve has the advantages of simple structure, easy corrosion resistance, short installation length, less steel consumption, and similar partial resistance coefficient of the valve. It is more economical and practical to choose a butterfly valve as a shut-off valve. According to the pressure level, butterfly valves can be divided into low-pressure butterfly valves and high-pressure butterfly valves.
Low pressure butterfly valve
The low-pressure butterfly valve can adopt the centerline non-pin-lined rubber butterfly valve. When the butterfly valve is less than or equal to DN500, the wafer connection is adopted. When butterfly valve ≥DN550, flange connection is adopted. When the diameter of the butterfly valve is less than 6in. (DN150) and the opening force is less than 400N, it is operated by the handle. When the diameter of the butterfly valve is greater than or equal to 8in. (DN200), it is operated with a gear box. Due to the low back pressure of the low pressure valve, the use of the centerline structure will not increase too much torque. This structure has two seals. The main seal is obtained by the pre-tightening force of the butterfly plate and the valve seat, and the second seal is obtained by the interference fit of the valve stem and the valve seat hole. Since the valve stem is completely isolated from the medium and does not penetrate into seawater, the valve stem can be made of 2Cr13 or equivalent materials. The valve body is made of ductile iron lining EPDM to increase the sealing performance. Since the valve body is not in contact with the medium, the material performance requirements of the valve body are reduced.
High pressure butterfly valve
When selecting the high pressure butterfly valve material, in addition to considering the corrosion factors of seawater, the pressure resistance of the material needs to be considered. When the working pressure is 69bar and the maximum pressure is ≥85bar (the closing pressure of the reverse osmosis high-pressure pump), due to the high back pressure, in order to reduce Torque, high pressure butterfly valve adopts double eccentric structure. When the nominal size of the butterfly valve is ≤DN500, the wafer connection is adopted. When the nominal size of the butterfly valve is ≥DN550, flange connection is adopted. The pressure grade is Cl600, and the valve body and disc material are dual-phase steel ASTMA995GR.4A. Because the valve stem is exposed to the medium, the valve stem is made of ASTMA276UNS31803, and the valve seat material is RPTFE. The double eccentric structure increases the local resistance coefficient. The butterfly plate and the valve stem need pins to be fixed, and the anti-corrosion requirements of the pins are the same as other flow-through components.
The check valve is usually installed at the outlet of the sea water pump to prevent the backflow of sea water and water hammer from damaging the equipment. At present, the check valves used in seawater desalination projects include slow-closing butterfly check valves, fully rubber-lined butterfly wafer check valves, single flap wafer check valves and single flap duplex steel wafer check valves.
Slow closing butterfly check valve
The main material of the slow-closing butterfly check valve is ductile iron. The mechanical or hydraulic slow-closing water hammer has good resistance to water hammer and is suitable for low-pressure use. The application of the product water section of the seawater desalination project.
Fully rubber-lined butterfly type wafer check valve
Fully rubber-lined butterfly-type wafer check valve is an improvement in anti-corrosion of slow-close butterfly-type check valve. The valve body and stem are fully rubber-lined, and the valve clack can be made of duplex stainless steel or nickel aluminum bronze. This type of valve is installed at the outlet of the low-pressure sea water pump and is suitable for installation on large-diameter pipelines. The nominal diameter of the valve ranges from DN200 to 1200. It is necessary to pay attention to the installation space requirements of the valve during the design. Improper installation of the valve will cause the valve disc and spring to act on the valve stem for a long time, destroy the seal at the contact between the valve body and the valve stem, infiltrate the medium, and cause the valve body to corrode.
Single flap wafer check valve
The single-leaf wafer check valve has simple structure and small installation space, and can be used under low pressure or high pressure conditions. The valve adopts duplex stainless steel as a whole, which has good seawater corrosion resistance, light weight, can be installed horizontally and vertically, and is easy to use. In the seawater desalination plant, a single-valve wafer check valve ≤DN250 is generally used. When the nominal size of the valve> DN250, the water hammer effect is obvious and the action noise is loud. Large-diameter single flap check valve is widely used in gas piping. The valve has a non-full bore, the maximum opening of the valve flap is 45°, the resistance coefficient increases, and the flow capacity decreases.