The
cast steel globe valve is force-sealed. When the valve is closed, pressure must be applied to the disc to force the sealing face not to leak. When the medium from below the valve disc into the valve, the resistance is required to overcome the operation force. It is the friction force of the stem, packing and the thrust produced by the pressure of the medium. The force of closing the valve is larger than the force of opening the valve, so the diameter of the stem is large. Otherwise, the stem top bending fault will occur. In recent years, since the emergence of the sealed valve, the medium flow of cast steel globe valve has changed from the top of the disc into the valve chamber. At this time, under the action of medium pressure, the force of closing valve is small. However, the force of opening valve is large, and the diameter of the valve stem can be correspondingly reduced. At the same time, under the action of the medium, this form of valve is also more rigorous. When the cast steel globe valve is opened, the opening height of the valve disc is 25% to 30% of the nominal diameter and the flow has reached the maximum. It indicates that the valve has reached a fully open position. Therefore, the fully open position of cast steel globe valves should be determined by the stroke of the valve disc.
The structure of cast steel globe valves is simpler than that of gate valves. The manufacturing and maintenance are more convenient. The sealing surface is not easy to wear and abrase. The sealing is good. There is no relative sliding between the disc and body sealing surface when we open and close it. When opening and closing, the valve stroke is small, and the height of cast steel globe valves is smaller than that of gate valves. However, the length of the cast steel gate valve is longer than that of a gate valve. Opening and closing torque are large, which is more laborious.
The fluid resistance is large, because the medium channel in the valve body is more tortuous. The fluid resistance is large and the power consumption is large. Then, PN≤16MPa (PN: the medium flow direction nominal pressure). The medium flowing direction is downstream, which is from the upper direction of the valve disc. When the nominal pressure (PN) is no less than 20MPa, the general medium flowing direction is countercurrent. The medium flow is from the direction of the disc to increase the sealing energy. When we are using it, the medium of cast steel globe valves can only flow in one direction. The direction of flow can not be changed. The disc is often eroded when it is fully open. The stem axis of cast steel globe valves is perpendicular to the sealing surface of the valve seat. Stem opening and closing stroke are relatively short. It has very reliable cutting action, making this valve suitable for medium cutting or regulating and throttling.
1. Disadvantages of cast steel globe valves
The fluid resistance is large, and the force required for opening and closing is large. It is not suitable for medium with particles, high viscosity and easy cooking. It has poor regulation performance.
2. Advantages of cast steel globe valves
The seal is designed by double seal (bellows + packing). If the bellows failure, stem packing will avoid external leakage. According to international standards, the seal has no fluid loss and can reduce energy losses. It improves the safety of plant equipment, the length of service life and reduces maintenance frequency and the operating cost. Durable bellows seal design guarantees the zero leakage of the stem and provides no maintenance conditions. Bellows sealing cast steel globe valves adopt bellows sealing design, which completely eliminates the defects of ordinary valve stem packing sealing aging fast and leaking. It not only improves the use of energy efficiency, increases the safety of production equipment and reduces maintenance costs and frequent maintenance, but also provides a clean and safe working environment.
