Nine Applications of Valves in Negative Pressure-Free Water Supply Equipment：
One, the cut-off valve should preferably be of a hard seal type. The cut-off valve requires the lowest possible leakage, as the leakage of soft seal valves is considerable. While the cut-off effect is good, they are not wear-resistant and have poor reliability. From the dual standards of both low leakage and reliable sealing, soft seal cut-off valves are not as good as hard seal cut-off valves. For example, the all-function ultra-lightweight regulating valve, which is sealed and equipped with wear-resistant alloy protection, has high reliability with a leakage rate of 10-7, and already meets the requirements for cut-off valves.
The double-sealed valve should not be used as a cut-off valve. The advantage of the double-saturation valve core lies in its force-balanced structure, which allows for a large pressure difference. However, its prominent drawback is that the two sealing surfaces cannot be in good contact simultaneously, leading to significant leakage. If it is artificially and forcibly used in cutting applications, the result is obviously unsatisfactory, even if many improvements have been made to it (such as double-sealed sleeve valves).
The spool of a straight travel valve is vertically throttled, while the medium flows horizontally in and out, causing the valve's flow path to become quite complex (resembling an inverted S-shape). This results in numerous dead zones, providing space for medium sedimentation, which over time can lead to blockages. In contrast, the throttling direction of an angular travel valve is horizontal, with the medium flowing in and out horizontally, easily carrying away impurities. Additionally, the flow path is simple, and there is little space for medium sedimentation, making the angular travel valve highly effective in preventing blockages.
Four, The stem of the straight travel regulating valve is thin, involving a simple mechanical principle: sliding friction is high, while rolling friction is low. The stem of the straight travel valve moves up and down, and slightly pressing the packing will tightly encase the stem, resulting in a large hysteresis. To address this, the stem is designed to be very thin, and packing is often made of Teflon, which has a low coefficient of friction, to reduce hysteresis. However, the issue that arises from this is that a thin stem is prone to bending, and the packing lifespan is also short. The solution to this problem is to use a angular travel valve stem, which is a type of control valve, where the stem is 2 to 3 times thicker than that of a straight travel valve. It also utilizes long-lasting graphite packing, offering good stem rigidity and extended packing lifespan. Surprisingly, this results in a smaller friction torque and reduced hysteresis.
The large cut-off pressure difference in angle seat valves is due to the very small torque generated by the resultant force of the medium acting on the rotating shaft, either at the valve core or on the valve plate, allowing it to withstand significant pressure differences.
Six, The desalinated water medium contains low concentrations of acids or bases, which have a significant corrosive effect on rubber. The corrosion of rubber manifests as expansion, aging, and low strength. The use of rubber-lined valves and diaphragm valves is ineffective, as the essence of the problem is the rubber's intolerance to corrosion. The rubber-lined diaphragm valves were later improved to be lined with fluorine-resistant materials, but the diaphragms of these valves are still prone to being folded and broken, causing mechanical damage and shortening the valve's lifespan. The current solution is to use water treatment ball valves, which can last for 5 to 8 years.
Seven, the double seat valve tends to oscillate at low opening. For a single-core valve, it exhibits good stability when the medium is a flow-open type; however, it has poor stability when the medium is a flow-closed type. A double seat valve has two valve cores, with the lower core in a flow-closed position and the upper core in a flow-open position. Consequently, at low opening, the flow-closed core is prone to cause valve vibration, which is why double seat valves are not suitable for operation at low openings.
The selection of valves is extremely important and complex. The calculation involves a simple formula, but its essence lies not in the complexity of the formula itself, but in the accuracy of the given process parameters. The selection process encompasses many factors, and a slight oversight can lead to improper selection. This not only results in a waste of human, material, and financial resources but also results in unsatisfactory performance, bringing about various usage issues such as reliability, lifespan, and operational quality.
Nine. The use of the pneumatic valve's piston actuator is common. For pneumatic valves, the piston actuator can fully utilize the air source pressure, making the actuator smaller in size than the diaphragm type and with greater thrust. The O-rings in the piston are also more reliable than those in the diaphragm type, thus its usage is expected to increase.
