Solutions to common problems with valves
Valve cleaning steps
The valve parts of the
valve company must go through the following processes before assembly:
1. According to the processing requirements, some parts need to be polished, and there should be no processing burrs on the surface;
2. All parts are degreased;
3. After degreasing is completed, pickle and passivate. The cleaning agent does not contain phosphorus;
4. After pickling and purification, rinse with pure water. There should be no chemical residue. This step is omitted for carbon steel parts;
5. Wipe the parts one by one with a non-woven cloth. No lint or other hair should remain on the surface of the parts, or blow dry with clean nitrogen;
6. Use non-woven fabric or precision filter paper dipped in analytical pure alcohol to wipe each component until there is no dirt left.
Daily maintenance of valves
1. Pay attention to the storage environment of the valve. It should be stored in a dry and ventilated room, and both ends of the passage should be blocked.
2. The valve should be inspected regularly, dirt on it should be removed, and anti-rust oil should be applied on its surface.
3. After installation and application, the valve should be regularly inspected to ensure its normal operation.
4. Check whether the valve sealing surface is worn, and repair or replace it according to the situation.
5. Check the trapezoidal thread wear of the valve stem and valve stem nut, whether the packing is out of date, etc., and make necessary replacements.
6. The sealing performance of the valve should be tested to ensure its performance.
7. The valve in operation should be in good condition, the bolts on the flange and bracket are complete, the threads are not damaged, and there is no looseness.
8. If the handwheel is lost, it should be replaced in time, and an adjustable wrench cannot be used to replace it.
9. The packing gland is not allowed to be skewed or have no preload gap.
10. If the valve is used in a harsh environment and is easily contaminated by rain, snow, dust, sand and other contaminants, a protective cover should be installed on the valve stem.
11. The scale on the valve should be kept complete, accurate and clear, and the lead seal and cap of the valve should be kept.
12. The insulation jacket should have no dents or cracks.
13. When the valve is in operation, avoid knocking it or supporting heavy objects.
FAQs about valves
1. Why can’t the double-seal valve be used as a shut-off valve?
The advantage of the double-seat valve core is that it has a force-balanced structure that allows a large pressure difference. However, its outstanding disadvantage is that the two sealing surfaces cannot make good contact at the same time, causing large leakage. If it is used artificially and compulsorily in cutting-off situations, the effect will obviously not be good. Even if many improvements are made for it (such as double sealing sleeve valve), it is not advisable.
2. Why does the regulating valve easily oscillate when the double-seat valve operates at a small opening?
For a single core, when the medium is flow-open, the valve has good stability; when the medium is flow-closed, the valve has poor stability. The double-seat valve has two valve cores. The lower valve core is closed and the upper valve core is open. In this way, when working at a small opening, the closed-flow valve core will easily cause vibration of the valve. This is a double-seat valve. The reason why it cannot be used for small opening work.
3. Which linear stroke control valve has poor anti-clogging performance, while the angle stroke valve has good anti-clogging performance?
The spool of a straight stroke valve is a vertical throttling device, while the medium flows in and out horizontally. The flow path in the valve cavity must turn and reverse, making the flow path of the valve quite complicated (the shape is like an inverted S-shape). In this way, there are many dead zones, which provide space for the precipitation of the medium, which in the long run will cause blockage. The throttling direction of the quarter-turn valve is the horizontal direction. The medium flows in horizontally and flows out horizontally, which can easily take away the dirty medium. At the same time, the flow path is simple and there is little space for the medium to settle, so the quarter-turn valve has good anti-blocking performance.
4. Why is the cut-off pressure difference of the quarter-turn valve larger?
The cut-off pressure difference of the quarter-turn valve is large because the resultant force generated by the medium on the valve core or valve plate exerts a very small torque on the rotating shaft. Therefore, it can withstand a large pressure difference.
5. Why is the stem of a straight stroke regulating valve thinner?
It involves a simple mechanical principle: large sliding friction and small rolling friction. The valve stem of a straight stroke valve moves up and down. If the packing is pressed a little tighter, it will wrap the valve stem very tightly, resulting in a large hysteresis. To this end, the valve stem is designed to be very small, and the packing often uses PTFE packing with a small friction coefficient in order to reduce the hysteresis. However, the problem derived from this is that a thin valve stem is easy to bend, and the packing life is also short. To solve this problem, the best way is to use a rotary valve stem, that is, a quarter-stroke regulating valve. Its valve stem is 2 to 3 times thicker than a straight-stroke valve stem, and it uses graphite filler with long life and high valve stem stiffness. Good, the filler has a long life.
6. What is a hard seal?
The leakage of the shut-off valve is required to be as low as possible. The leakage of the soft seal valve is the lowest. The shut-off effect is of course good, but it is not wear-resistant and has poor reliability. From the dual standards of small leakage and reliable sealing, soft seal cutting is not as good as hard seal cutting. For example, the full-function ultra-light control valve is sealed and protected by wear-resistant alloy. It has high reliability and a leakage rate of 10 to 7, which can already meet the requirements of a shut-off valve.