COMMON FAULTS AND PREVENTION OF VALVES

There are many types of industrial valves for fluid control, such as ball valves, butterfly valves, globe valves. The performance of a valve that continuously maintains and completes a given function requirement during a certain period of working time and keeps a given parameter value within a specified range is called fault-free. When this performance of the valve is destroyed, failure will occur.

 

Common Failures of General Valves

1. Stuffing Box Leakage

The reasons for the stuffing box leakage are as follows:

(1) The corrosiveness, temperature and pressure of material and working medium are not compatible.

(2) The filling method is wrong, especially when the whole material is put in, which is the most likely to cause leakage.

(3) Stem processing accuracy or surface finish is not enough, or ellipticity, or scratches.

(4) The valve stem has been pitted or rusted due to lack of protection in the open air.

(5) Bending of valve stem;

(6) The filler has been used for too long and it has aged.

(7) The operation is too fierce.

 

2. Leakage of Closed Parts

The stuffing box leakage is usually called an external leak, and the closing part is called an internal leak. The closing piece leaks inside the valve is not easy to find. Closed parts leaks can be divided into two categories: one is the leakage of the sealing surface, and the other is the leakage of sealing ring root. The causes of leakage are as follows:

(1) The sealing surface is not grinded well.

(2) The sealing ring is not tightly matched with the valve seat and the valve disc.

(3) The valve disc is not firmly connected to the valve stem.

(4) The valve stem is bent and twisted, so that the upper and lower closures are not in alignment.

(5) Close too fast, the sealing surface is not in good contact or has been damaged.

(6) The material selection is improper, it can not withstand the corrosion of the medium.

(7) Use the shut-off valve and gate valve as the regulating valve. The sealing surface cannot withstand the erosion of the high velocity flowing medium.

(8) After the valve is closed, some media gradually cool down, so that the sealing surface appears slit, and it will also produce erosion phenomenon.

(9) Threaded connections are used between some sealing surfaces and seats and discs, which is easy to produce oxygen concentration batteries that may lead to corrosion.

(10) Because the welding slag, rust, dust and other impurities are embedded, or the mechanical parts in the production system fall off and block the valve core, so that the valve can not be closed.

3. Valve Stem Lift Failure

The reasons for the failure of valve stem lifting are as follows:

(1) The peration is too strong to cause thread damage.

(2) Lack of lubricant or failure of lubricant.

(3) Valve stem bending.

(4) Surface finish is not enough.

(5) Inaccurate tolerance, too tight bite.

(6) Stem nut tilt.

(7) Improper material selection, such as the stem and stem nut are the same material.

(8) Thread is corroded by medium.

(9) Open-air valves lack protection. Stem threads are covered with dust, or they are corroded by rain, dew, frost and snow.

 

4. Others

(1) Gasket Leakage

The main reason is not corrosion resistance, not suitable for working temperature and working pressure, and the temperature change of high-temperature valve. Use gaskets suitable for working conditions, check the material suitability of gaskets for new valves, and replace them if not. For high temperature valves, the bolt should be tightened again in use.

(2) Valve Body Cracking

It's usually caused by freezing. When it's cold, the valve should have insulation and heat tracing measures. Or the water in the valve and connecting pipeline should be drained clean after stopping use.

(3) Handwheel Damage

It is caused by impact or heavy operation of long lever. As long as operators and other relevant personnel pay attention to it, it can be avoided.

(4) Packing Gland Break

When compacting fillers, the force is not uniform, or the cover (usually cast iron) is defective. To tighten the packing, the screw should be rotated symmetrically and not skewed. In manufacturing, attention should be paid not only to large and key parts, but also to sub-elements such as capping, otherwise it will affect the use.

(5) Valve Stem and Valve Plate Connection Failure

The gate valve adopts the form of the valve stem long square head and the gate T-shaped groove to be connected, so the valve stem long square head wears faster.

Common Failures of Automatic Valves

1. Spring-loaded Safety Valve

(1) The sealing surface leaks. Maybe there is debris between the sealing faces. Or the sealing surface is damaged. This kind of fault should be prevented by regular maintenance.

(2) The sensitivity is not high. If it is spring fatigue, there is no doubt that it should be replaced. It is also possible that the spring is not being used properly. Because the rated pressure spring safety valve has multiple pressure sections, each pressure section has a corresponding spring.

 

2. Check Valve

(1) The valve flap is broken. The precaution is to use a check valve made of tough material.

(2) Media backflow. Repairing the sealing surface and removing impurities can prevent backflow.

 

The above description of common faults and prevention methods can only serve as inspiration. In actual use, other faults will be encountered. To actively and flexibly prevent valve faults, the most fundamental one is to be familiar with its structure, material and operation principle.