Common faults and preventive measures of valves

2020-11-06 9:55:56 No Comments Epower Metals News / Trade News

The performance that the valve continuously maintains and completes the given function requirements in a certain period of time, so that the given parameter value can be maintained within the specified range is called fault free. When the performance of the valve is damaged, it will cause failure.

Stuffing box leakage

This is the main aspect of running, emitting, dripping and leaking, which is often seen in factories.
The causes of stuffing box leakage are as follows:

① The corrosion, temperature and pressure of the material and working medium are not suitable;
② If the filling method is not correct, especially the whole packing is put in a spiral way, it is easy to produce leakage;
③ The machining accuracy or surface finish of the valve stem is not enough, or there is ovality or scoring;
④ The valve stem has pitted or rusted due to lack of protection in the open air;
⑤ The valve rod is bent;
⑥ The packing has been used for too long and has been aged;
⑦ Too much operation.

The methods to eliminate packing leakage are as follows:

① Proper selection of fillers;
② Load according to the correct one;
③ If the valve stem processing is unqualified, it should be repaired or replaced. The surface finish should be as low as ▽ 5, and more importantly, it should be more than ▽ 8 without other defects;
④ Take protective measures to prevent corrosion, and replace the rusted ones;
⑤ The bent valve rod should be straightened or renewed;
⑥ The packing should be replaced after a certain period of time;
⑦ Pay attention to stable operation, slow opening and slow closing, to prevent sudden temperature change or medium impact.

Closure leakage

Usually the stuffing box leakage is called external leakage, and the closure part is called internal leakage. The leakage of the shut-off part is not easy to find in the valve.
There are two kinds of leakage of the closing parts: one is the leakage of the sealing surface, the other is the leakage of the root of the sealing ring.
The causes of leakage are as follows:

① The sealing surface is not well ground;
② The sealing ring does not fit tightly with the valve seat and disc;
③ The connection between valve disc and valve stem is not firm;
④ The valve rod is bent and twisted to make the upper and lower closing parts out of alignment;
⑤ Closing too fast, sealing surface contact is not good or has been damaged for a long time;
⑥ Improper selection of materials can not withstand the corrosion of medium;
⑦ The globe valve and gate valve are used as control valves. The sealing surface cannot withstand the erosion of high-speed flow medium;
⑧ Some media will gradually cool down after the valve is closed, resulting in fine cracks on the sealing surface, which will also cause erosion;
⑨ Some sealing surfaces are connected with valve seat and valve disc by thread connection, which is easy to produce oxygen concentration cell and corrode and loosen;
⑩ The valve can not be closed tightly because of welding slag, rust, dust and other impurities embedded, or mechanical parts falling off in the production system.

Prevention measures include:

① Before use, pressure test and leakage test must be carried out carefully. If the leakage of sealing surface or root of sealing ring is found, it should be handled before use;
② Check whether all parts of the valve are in good condition in advance. Do not use the valve whose stem is twisted or the valve disc cannot be connected with the valve stem;
③ When the valve is closed tightly, it should be stable, not violent. If it is found that the contact between the sealing surfaces is not good or there is obstruction, it should be opened a little immediately to let the debris flow out, and then close it carefully;
④ When selecting valves, not only the corrosion resistance of valve body but also the corrosion resistance of closing parts should be considered;
⑤ In order to use the valve correctly according to the structure characteristics of the valve, the regulating valve should be used for the parts that need to adjust the flow rate;
⑥ If the medium is cooled and the temperature difference is large after closing the valve, the valve should be closed again after cooling;
⑦ When the valve seat, valve disc and sealing ring are connected by thread, polytetrafluoroethylene tape can be used as the filler between the threads so that there is no gap;
⑧ Filter should be added in front of the valve which may fall into impurities.

Valve rod lifting failure

The causes of valve rod lifting failure are as follows:

① Excessive operation will damage the thread;
② Lack of lubrication or lubricant failure;
③ The valve rod is bent and twisted;
④ The surface finish is not enough;
⑤ The fitting tolerance is not accurate and the bite is too tight;
⑥ The stem nut is inclined;
⑦ Improper material selection, for example, the valve stem and stem nut are of the same material, easy to bite;
⑧ The thread is corroded by medium (refers to the valve with Non rising stem or valve stem nut at the lower part);
⑨ The valve is lack of protection in the open air, and the valve stem thread is covered with dust and sand, or rusted by rain, dew, frost and snow.

Methods of prevention:

① Carefully operate, do not use strong force when closing, do not reach the top dead center when opening, turn the hand wheel back for one or two turns after opening enough to make the upper side of thread close, so as to avoid the medium pushing the valve rod upward to impact;
② Check the lubrication regularly and keep it in normal condition;
③ Do not use long lever to open and close the valve. Workers who are used to using short lever should strictly control the force to prevent the valve stem from twisting;
④ Improve the processing or repair quality to meet the specification requirements;
⑤ The material should be corrosion resistant and adapt to working temperature and other working conditions;
⑥ The valve stem nut should not be made of the same material as the valve stem;
⑦ When using plastic as stem nut, the strength should be checked, not only good corrosion resistance and low friction coefficient, but also strength should be considered. If the strength is not enough, do not use it;
⑧ The valve rod protection sleeve shall be added to the outdoor valve;
⑨ If the valve is normally open, the hand wheel should be turned regularly to prevent the valve stem from rusting.

Other

Gasket leakage:

The main reason is not resistant to corrosion, not adapt to the working temperature and pressure; there are high temperature valve temperature changes.
The gasket suitable for the working conditions shall be used. For the new valve, the gasket material shall be checked whether it is suitable, and if not, it shall be replaced. For high temperature valves, the bolts should be tightened again when using.

Valve body cracking:

It’s usually caused by freezing. In case of cold weather, the valve shall be provided with heat preservation and heat tracing measures, otherwise the water in the valve and connecting pipeline shall be drained after shutdown (if there is a plug at the bottom of the valve, the plug can be opened for drainage).

Hand wheel damage:

Impact or strong operation of long lever. It can be avoided by other operators.

Packing gland broken:

When the packing is pressed, the force is uneven, or the gland (generally cast iron) has defects. To compress the packing, the screw should be rotated symmetrically and not slanted. When manufacturing, we should not only pay attention to the large and key parts, but also pay attention to the secondary elements such as gland, otherwise it will affect the use.

Failure of connection between valve stem and valve plate:

There are many forms of connection between the rectangular head of the valve stem and the T-shaped groove of the gate valve. Sometimes the T-shaped groove is not processed, so the wear of the rectangular head of the valve rod is faster. Mainly from the manufacturing aspect to solve. However, the user can also make up the T-shaped groove, so that it has a certain degree of finish.

The ram of double ram valve can not press the sealing face:

The tension of the double gate is caused by the top wedge. Some gate valves have poor material (low grade cast iron) and wear or break soon after use. The top wedge is a small piece, and the materials used are not much. The user can make it by himself with carbon steel and replace the original cast iron.

Source: China Valve Supplier: www.epowermetals.com

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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