What Is The Principle Of Valve Tightness?

- Jul 15, 2020-

In order to ensure that the valve can well cut off the flow of the medium so that it does not flow out, it must be ensured that the valve is intact and it is not damaged. There are many reasons for the damage of the valve box, including improper design, defective sealing contact surface, loose connection and poor cooperation between the valve body and the bonnet. All these problems can lead to poor valve orientation, leading to leakage problems. Yes, therefore, ventilation technology is an important technology for valve performance and quality, which requires systematic and in-depth research.

Since the birth of the valve, its sealing technology has also made great progress. So far, valve sealing technology mainly includes static sealing and dynamic sealing.

The so-called static seal usually refers to the seal between two static surfaces, and the most important sealing method of static seal is to use sealing.

The so-called dynamic seal mainly refers to the sealing of the valve stem, that is, the medium in the valve cannot expire with the movement of the valve stem. The density method of dynamic seal mainly uses stuffing box.

The valve type has a great influence on the choice of sealing fluid.

The on-off stroke of the lift gate valve is usually longer. If the operation is frequent, this long-distance linear motion will cause difficulties in sealing. In most cases, the operating frequency of such valves will not exceed once a week, or even only once a year. The gap between the packing gland, valve stem and stuffing box is very important. If the gap is large, linear movement will cause a part of the seal to be squeezed, or foreign particles will be dragged into the seal. Therefore, the cleaning ring can be installed at the bottom and sometimes at the top.

The globe valve usually adopts the lifting rod and rotary motion, and the sealing is the most difficult, because the valve rod will move in two directions at the same time, and the packing group will gradually contact the entire surface of the valve stem. Any eccentricity or non-roundness of the valve stem may cause damage and leakage of the packing element. Similar to gate valves, linear motion draws foreign objects into the process fluid through the sealing element.

Ball valves, butterfly valves and plug valves are commonly used quarter turn valves. When the valve stem rotates 90 degrees relative to the seal, the valve can complete the entire process from opening to closing. This movement means that it is the easiest to seal because it has a smaller stroke than other types of valves. Unlike linear motion, the rectangular rotary motion does not easily allow foreign particles to pass through the sealing element. The valve stem eccentricity problem is worth noting. Some sealing elements are very sensitive to the misalignment of the actuator, and even lead to a decline in the sealing performance of the valve stem.

The design of the stuffing box of the right-angle rotary valve often results in a limited selection of sealing elements. In many cases, the stuffing box is very shallow and it is difficult to achieve sealing under high pressure.

Control valve stem sealing is usually the most difficult. The main reason is that the operation is frequent and the stress of the stem sealing cannot be too high. If a control valve undergoes 100,000 valve stem cycles, other types of valves in the system usually only undergo 1500 cycles. High frequency cycling operation will cause the sealing element to wear out and reduce the sealing performance over time. In order to optimize the fluid control performance, the control valve stem cannot bear too much friction, so the sealing stress acting on the control valve is significantly lower than that of the manual valve. If the sealing element generates excessive friction on the valve stem, the valve action lags or there is a speed deviation, the valve stem action is too large, and the fluid control performance is reduced.

Linear control valves are more difficult to seal than rotary control valves. Similar to the right-angle rotary valve, the rotary control valve has only one circular motion mode, and the surface area of the valve stem to be sealed is significantly smaller than that of the linear control valve.

The stem material of metallurgical valves is usually relatively soft, and this should be taken into account when selecting seals. Ideally, the sealing element material is softer than the stem material, which minimizes stem wear. The gland bolt yield strength of some metallurgical special valves is low, and the load of the sealing element should be avoided to be close to the maximum stress that can be withstood.

Valve size is also the main factor affecting the choice of sealing elements.

For small valves, the annular cross-section between the stem and the inner wall of the stuffing box is small. In some cases, small is not necessarily a good thing, because it limits the choice of sealing elements. The annular cross-section of small valves is usually unique. 125", so it is difficult to install strong and novel sealing elements.

Large-sized valves are not without problems. Excessive size will cause excessive load on the stem and packing group. When the valve vibrates, the force generated may be too large for standard sealing elements. The temperature difference between different sections of large valves is also large, which may cause structural deformation.

For most types of valves, the ideal stuffing box size ratio is about 3-5 times the height of the cavity. If it is a right angle rotary valve with low sealing requirements, it can be effectively sealed even if the stuffing box is shallow. A stuffing box that is too deep means that the seal is easy to consolidate, resulting in loss of sealing stress and then leaking. Secondly, the high coefficient of friction of the valve stem can be an obstacle in certain applications.