Valve maintenance basis

Planned maintenance of valves

Valve is the accessory of equipment and pipeline, its maintenance should be combined with the maintenance of equipment and pipeline.

According to the regulations for maintenance and repair of refinery equipment stipulated by the Ministry of chemical industry of raw materials and fuels, the repair contents of high, medium and low pressure valves for oil products, steam, water and various gases with nominal pressure lower than pnl6mpa and medium temperature lower than 550 ℃ can be divided into three categories: minor repair, medium repair and major repair.

The repair items included are as follows:

  • Minor repair: clean the oil nozzle and oil cup, replace the packing, clean the valve stem and its thread, remove the impurities in the valve, fasten and replace the bolts, complete the handwheel, etc.
  • Medium repair: including minor repair items, disassembly and cleaning parts, valve body repair, grinding seals, straightening valve stem, etc.
  • Overhaul: including medium repair items, replacement of valve stem, repair of bracket, replacement of spring and seal, etc.

In the indoor repair of the valve, in general, minor repair more common, but also should be disassembled to check and replace the gasket.

Valve maintenance under pressure

Valve maintenance under pressure is a complex, dangerous and technical work. In addition to being bold, careful and decisive, the maintenance of valves under pressure should also have a strict scientific attitude. Especially in explosive, highly toxic, high temperature, high pressure, radioactive and other media plugging work, it should be carried out in strict accordance with the pre-determined scheme and relevant safety regulations.
When maintaining the valve under pressure, pay attention to the following items:

  • 1. In the process of maintenance, it is necessary to strictly abide by the relevant safety operation procedures such as fire prevention, gas prevention and explosion prevention.
  • 2. Before dealing with the defects of the pressure valve, the treatment scheme shall be put forward, and the consent of the safety technical personnel and the cooperation of the operator shall be obtained. The treatment of important parts should go through joint research, and go through the procedures of fire ticket and poison ticket according to the regulations, then the defects of valves can be dealt with.
  • 3. The repair personnel with rich practical experience should be responsible for the defect treatment. There should not be too many on-site personnel and there should be one or two special guardians.
  • 4. There should be careful safety measures to be able to cope with the worst situation. Maintenance personnel and on-site personnel shall wear protective work clothes, shoes and hats, gloves and protective glasses according to the provisions of protective measures for flammable and explosive, high temperature and high pressure, toxic, highly toxic, strong corrosion and other media. Gas masks should also be worn when repairing toxic and highly toxic valves.
  • 5. In the process of dealing with valve defects, the safety baffle should be set, at the same time, it should be careful and careful, observe while doing, and carry out according to the specified scheme.
  • 6. Before tightening the thread, clean and penetrate the rusted thread with kerosene, and check whether the bolt and nut are complete; The thread should also be lubricated with rust remover or a layer of graphite powder. When tightening the nut, it is not allowed to tighten it with force to prevent the bolt and screw from breaking.
  • 7. When welding the valve, observe the welding operation procedures. The metal should be prevented from overheating and deformation, and new cracks should be produced. If there is a new leakage at the position where the seam has been twisted, it is not suitable to re stitch the seam to avoid expanding the leakage defect.

Assembly and disassembly of valve

Assembly and disassembly of threaded connection

(1) Anti loosing method of thread connection
Nuts, spring washers, stop washers, split pins and other connectors are commonly used on valves to prevent nuts from loosening. The connecting structure of the valve stem and the closing piece often adopts the locking method of the stop washer and the locking method of the winged washer, which has a good effect.

In addition, there are two methods to prevent the valve from loosing, one is the fixing method of saddle screw, the other is the point riveting method, both of which are used in the occasion of infrequent disassembly. Point riveting method is to rivet two or more points on the thread of the nut and bolt, so that the thread can be squeezed and deformed to achieve the purpose of anti loosing. If the nut is loose, it is difficult to solve with the above methods, the bonding method can be used.

(2) Identification of left-hand thread and right-hand thread
The most basic knowledge of valve assembly and disassembly is to correctly identify whether the thread is left-handed or right-handed. So, how can we find out whether the thread is left-handed or right-handed? It can be determined by the internal thread or external thread of the valve. In general, they form a positive and negative thread structure to prevent thread loosening. For example, the threaded connection on the stem and stem nut: the tightening ring on the stem nut is left-handed, the thread on the hand wheel is right-handed, and the trapezoidal thread of the stem nut is left-handed. In addition, the rising angle of the thread is to the left, and the thread rising to the left is called left rotation; On the contrary, it is dextral.
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(3) Method of assembling and disassembling bolts
The disassembly and assembly methods of bolts are usually related to the connection form, damage and rust degree. The following focuses on the disassembly and assembly of stud bolts; Removal of rusted bolts and nuts; Disassembly method of broken bolt.
① Disassembly method of stud bolt
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The method of disassembling and assembling the stud bolt by tightening the double nuts together. When the stud bolt is to be removed, the upper spanner will tighten the upper nut on the lower nut, and the lower spanner will turn the nut counterclockwise, and the bolt will be screwed out. If the stud bolt is reversed (left-handed), the upper wrench shall tighten the nut, and the lower wrench shall screw the nut out of the bolt clockwise; When the stud bolt is to be assembled on the valve piece, the lower wrench compresses the lower nut, and the upper wrench rotates the upper nut clockwise to tighten the stud bolt on the valve body. If the stud is anti thread, the lower wrench can compress the lower nut, and the upper wrench can rotate the upper nut counterclockwise to tighten the stud.
Another way to tighten the stud is to use a special nut with a screw on it. The internal thread of the nut is the same as that of the stud. When in use, screw the stud into the nut and tighten the screw, then screw the nut to turn the stud.
Removal method of rusted bolt and nut
Before removing the rusted and corroded bolts and nuts, soak them with kerosene and find out the screw direction, then slowly loosen about 1 / 4 and return to the original position; In this way, the bolt can be gradually screwed out by repeatedly advancing and retreating for several times. You can also use a hammer to knock around the bolt and nut to loosen the thread, and then loosen the bolt and nut (but in the process of knocking, do not knock the thread). Nuts that are difficult to disassemble by knocking can be heated by blowtorch or oxyacetylene flame to expand the nuts and screw them out quickly. For the stud bolts that are difficult to remove, after being soaked with kerosene, the pipe clamp can be used to screw them out at the position of the middle polished rod.
③ Disassembly method of broken bolt
The bolt is broken in the screw hole, which is a troublesome thing in disassembly.
Several methods of screwing out broken end bolts.

  • The method of file tenon and pipe wrench is suitable for the bolt with a height of more than 5mm outside the bolt hole;
  • The method of spot welding and screwing out is applicable to the condition that the bolt is outside the screw hole or the bolt is level with the screw hole. It is made of flat steel with a hole smaller than the screw hole, and the bolt is screwed out after spot welding and filling the hole;
  • The square hole wedge screwing out method is suitable for the bolt broken in the screw hole. The method is to drill a small hole in the middle of the bolt, knock it into the small hole with a square hole cone, and then screw it out;
  • The recovery method of drilling and tapping is suitable for the situation that the bolt can not be taken out. It is first to level the cross section of the bolt, after proofing and punching, drill with a bit smaller than the inner diameter of the thread, and finally tap according to the original thread.

Before using the above screwing out method, some conventional measures should be taken, such as soaking with kerosene and removing oil and rust on the surface (this is especially important for the screwing out method of spot welding, otherwise the welding will not be firm). If necessary, heat around the bolt and screw out the bolt under thermoplastic condition. Chemical corrosion method can also be used to screw out the broken bolts. Properly cleaning the rusted and corroded bolts and screw holes will speed up the screw out of the broken bolts (after screwing out the bolts, the screw holes shall be washed with water to avoid residual chemical corrosion of the valve).
(4) Tightening sequence of bolts
During valve assembly, the tightening degree and sequence of bolts have a direct impact on the assembly quality. For general bolt tightening is easier to do, the connector is not loose. However, the bolts of gasket and packing structure should be tightened first and then, symmetrically and evenly, and in turn. When each bolt is tightened properly, check whether the flange is skewed, and measure whether the gap between the flanges is consistent, so as to correct. Then tighten the bolts symmetrically and in turn, with a small amount of tightening, about 1 / 4 ~ 1 / 2 turn each time, until the required preload. Special attention should be paid not to screw too tightly, so as not to crush the gasket. Generally, it is subject to no leakage. Finally, check the flange clearance, which should be consistent and kept at more than 2mm.
The tightening sequence of bolts is shown in the figure.
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Assembly and disassembly method of key connection

Key connection forms include flat key, sliding key (guide key), inclined key (wedge key), semicircular key (crescent key) and spline. The figure shows the assembly form of key connection.
(1) Flat key
Flat key is widely used in valve, its cross-section shape is square and rectangular. Before assembling the flat key, the keyway should be cleaned, the edge of the key should be trimmed, the fit size and precision of the key should be repaired, and the round heads at both ends of the key should be filed. During assembly, the two sides of the key shall have slight interference, the top surface of the key shall have clearance with the hub, and the bottom surface of the key shall contact with the bottom of the shaft keyway. The assembly can be done by tapping with a hammer or clamping the flat key slowly with a vise to install it into the keyway of the shaft. During assembly, the copper sheet is used as a cushion to avoid damaging the shaft and key.
When the flat key is disassembled, the wheels should be removed first. The two ends of the flat key can be pulled up with tools such as a driver, or the two sides of the flat key can be wrapped with thin copper sheet, and the flat key can be clamped and pulled out with steel wire pliers or vice.
(2) Slide key
Sliding key is actually a special form of flat key. It can not only drive the hub to rotate, but also make the shaft and hub move relative. It is used in the clutch mechanism of transmission device and grinder. The sliding key can be assembled on the shaft or on the hub. The sliding key and its assembled keyway shall be closely matched without looseness, and fixed with countersunk screws. There should be a certain gap between the sliding key and its opposite sliding keyway on both sides and the top surface.
Except for the countersunk screw, the assembly and disassembly method of the sliding key is the same as that of the flat key. The countersunk screw shall be tightened and not loose, and shall be lower than the sliding key surface.
(3) diagonal key
The inclined key is similar to the flat key, but its top surface is inclined, with a slope of 1:100, and has a key head for disassembly. When assembling oblique key, the edge should be removed, and the fitting accuracy of key and groove should be repaired; Then, align the hub and shaft sleeve with the keyway, insert the inclined key with the inclined surface painted color into the groove, and check the contact surface. The contact surface shall not be less than 70%. If it is less than 70%, it shall be scraped and corrected. Finally, apply a layer of white lead oil on the inclined key and drive the inclined key into the groove. The tools used for the removal of the inclined key include the inclined key puller and the inclined key dial.
(4) semicircular key
The semicircular key is used on the shaft with smaller diameter and tapered shaft, which can automatically adjust the inclination in the keyway. Semicircle key assembly and disassembly method is similar to flat key.
(5) spline
Spline like a pair of internal and external meshing gear, high machining accuracy, large torque transmission, rarely used in the valve.

Source: China Valve Manufacturer – Yaang Pipe Industry Co., Limited (

(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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