How to Maintenance and repair of the valve stem？ 

The valve stem is an important part of the valve, used for transmission, connected to the actuator or handle, the following directly drive the spool to move or rotate to achieve the valve switch or adjustment role.

 

Characteristics of the valve stem.

Stem in the valve opening and closing process is not only the moving parts, force parts, and seals. At the same time, it is subject to media impact and corrosion, but also friction with the packing. Therefore, in the selection of stem material, it must ensure that it has sufficient strength, good impact toughness, resistance to abrasion, corrosion resistance under the specified temperature. Stem is a wear parts, in the selection of materials should also pay attention to the mechanical processing properties and heat treatment properties.

 

Valve stem commonly used materials.

1、Copper alloy

The general choice of grades QA19-2, HPb59-1-1. Suitable for nominal pressure less than or equal to 1.6MPa, the temperature is less than or equal to 200 degrees of low pressure valves.

2、Carbon steel

A5, 35 steel is generally used, after nitriding treatment, suitable for nominal pressure less than or equal to 2.5MPa ammonia valve, water, steam and other media of low and medium pressure valves. A5 steel is suitable for valves whose temperature does not exceed 300 degrees; 35 steel is suitable for temperatures not exceeding 450 degrees . (Note: The actual route proved that the valve stem using carbon steel nitride manufacturing can not be a good solution to the problem of corrosion resistance, should be avoided.)

3、Alloy steel

Generally choose 40Cr, 38CrMoA1A, 20CrMo1V1A and other materials. 40Cr after chromium plating, suitable for nominal pressure less than or equal to 32MPa, temperature less than or equal to 450 degrees of water, steam, oil and other media. 38CrMoA1A after nitriding treatment, can withstand 10MPa pressure at a working temperature of 540 degrees, commonly used in power station valves. 20CrMo1V1A is nitrided to withstand 14MPa pressure at a working temperature of 570 degrees and is commonly used in power station valves.

 

Stem nut and stem with threaded fit, directly bear the axial force of the stem, and the card with the bracket and other valve friction. Therefore, the stem nut in addition to a certain strength, but also requires a small coefficient of friction, not rust, not with the stem bite and other properties.

 

The process characteristics of valve stem manufacturing.

1. The new stem threads are self-made extruded, with high thread strength and accuracy.

2. The concentricity of the threaded part and the light pole part is the same, which overcomes the long-standing problem of easy twisting and loss of the rolling die in the domestic rolling process.

3. The precision of the light bar part reaches mirror effect, easy to open and close, and good sealing.

4. The inverted shutter part is formed by CNC technology in one time, with high precision and good sealing performance.

5. The T-slot of the end part adopts forging and forming process, high efficiency and low cost.

 

The cause of valve stem deformation.

Stem mainly has more able to pressure, can be good insulation, there is less easy to decay and rust, it has the sealing function of the two sides, more resistant to wear and tear, and its service life is relatively long. Then the reason why the valve stem will produce fracture or deformation are so many points.

1. Area

The stem usually breaks at the bottom of the upper and lower threads, because the area is small and the force is concentrated and exceeds the standard.

2. Fracture during opening

Generally speaking, the most likely time for the stem to break is at the moment of opening. This is mainly because the gate has not yet been detached from the seat and the stem breaks at the root of the upper or lower threads, which is usually considered to be a stuck gate, but this is not the whole reason and is not the most important.

3. Valve body

The valve body of the middle cavity is closed after the abnormal heating and pressure drop, that is to say, after the valve is closed, closed in the upstream and downstream sides between the sealing surface of the middle cavity fluid pressure to be much higher than the upstream pressure situation. The reasons are as follows.

(1) expansion

(2) closed

 

Maintenance and repair of the valve stem：

Repair steps for valve stem strain

①Use a file to remove the burr from the valve stem. At the shallow part, use a flat shovel to a depth of about 1mm, and then use a sand cloth or an angle grinder to roughen it. A new metal surface will appear at this time. .
②Clean the surface with TL-700 metal cleaner to make the repaired surface free of oil, dust and dirt.
③Apply wear-resistant repair agent.
④Detail trimming.

Preparation and coating process of wear-resistant repair agent
①According to the volume ratio of 3.8: 1 to prepare repair agent;
②Scrape the adhesive on the scratched surface, the first pass should be applied as little as possible, from top to bottom, no bubbles are allowed;
③After 1h of the first pass of adhesive (that is, after the initial curing of the adhesive), reconstitute the repair agent as required and apply the second pass, which is 1 ~ 2mm higher than the original size;
④After natural curing for 1h, it can be heated with 80 ~ 100 ℃ iodine tungsten lamp for 3h.

Detail trimming process requirements
①Use a file, scraper, emery cloth and other tools to remove the adhesive that is higher than the original size. Measure at any time during operation. Do not make the adhesive layer lower than the original size, and reserve 0.5mm as the finishing amount;
②When the size reaches the finishing amount, use the pre-processed grinding tires for finishing (80-mesh emery cloth);
③When the size is higher than the original size 0.2mm, replace the emery and grind to the actual size accuracy.

Precautions:
Due to the on-site repair, in order to ensure the repair quality, the dust and oil stains around the repair (especially the upper part) must be cleaned up; after the repair, if there are defects (such as small pores, etc.), the glue should be repaired.

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