What is valve assembly

What is valve assembly?

Valve assembly is the last stage in the manufacturing process. Valve assembly is the process of combining all parts and components of the valve together to make it a product according to the specified technical premise.

The whole machine is the most basic unit of valve assembly, and several parts make up the parts of the valve (such as valve cover, valve clack parts, etc.). The assembly process of several parts forming parts is called component assembly, and the assembly process of several parts and components forming a valve is called general assembly. Assembly work has a great impact on product quality. Even if the design is accurate and the parts are qualified, if the assembly is improper, the valve will not meet the specified requirements, and even seal leakage will occur. Therefore, special care should be taken to adopt a fair assembly method to ensure the quality of the final product of the valve. The assembly process defined in the form of documents in production is called the assembly process specification.

Preparation before assembly

Before assembling the valve parts, it is necessary to remove the burrs formed by machining and welding residues, clean and cut fillers and gaskets.

Cleaning of valve parts

As a valve installed by the fluid pipeline, the inner cavity must be clean. In particular, valves used in nuclear power, medicine, and food industries have stricter requirements on the cleanliness of the valve cavity in order to ensure the purity of the medium and avoid medium infection. Before assembling, the valve parts should be cleaned, and the chips, residual oil, coolant and burrs, welding slag and other dirt on the parts should be washed and cleaned. The valve is usually cleaned by spraying with alkaline water or hot water (kerosene can also be used for scrubbing) or cleaning in an ultrasonic cleaning machine. After the parts are ground and polished, the final cleaning is required. The final cleaning is usually to clean the sealing surface with gasoline, then dry it with compressed air and wipe it with a cloth.

Packing and gasket preparation

Graphite filler is widely used because of its advantages such as corrosion resistance, good sealing performance and low friction coefficient. Packing and gaskets are used to prevent the medium from leaking through the valve stem and bonnet and the joint surface of the flange. These accessories must be prepared for cutting and receiving before the valve assembly.

Inspection before valve installation

  • ① Check carefully whether the valve model and specifications meet the requirements of the drawings.
  • ② Check whether the valve stem and valve clapper open flexibly, and whether they are stuck or skewed.
  • ③ Check whether the valve is damaged, and whether the thread of the threaded valve is correct and intact.
  • ④ Check whether the combination of valve seat and valve body is firm, the connection of valve disc and valve seat, valve cover and valve body, valve stem and valve disc.
  • ⑤ Check whether the valve gasket, packing and fasteners (bolts) are suitable for the requirements of the nature of the working medium.
  • ⑥ The pressure reducing valve that is outdated or left for a long time should be disassembled, and dust, sand and other debris should be cleaned with water.
  • ⑦ Clear the port seal and check the degree of sealing. The valve disc must be closed tightly.

Valve pressure test

Low pressure, medium pressure and high pressure valves should be subjected to strength test and tightness test. Alloy steel valves should also be subjected to spectrum analysis on the shell one by one, and the material should be reviewed.

Strength test of valve

The strength test of the valve is to test when the valve is open to check the leakage on the outer surface of the valve. For valves with PN≤32MPa, the test pressure is 1.5 times the nominal pressure, the test time is not less than 5min, and there is no leakage at the shell and packing gland.

Valve tightness test

The test carried out with the valve in the fully closed state is to check whether there is leakage on the sealing surface of the valve. The test pressure, except for butterfly valves, check valves, bottom valves, and throttle valves, should generally be carried out at the nominal pressure. At working pressure, it can also be tested with 1.25 times the working pressure, and the sealing surface of the valve disc is qualified.

General regulations for valve installation

  • 1. The valve installation position should not hinder the operation, disassembly and overhaul of the equipment, pipeline and valve body itself, and the aesthetic appearance of the assembly should be considered.
  • 2. For valves on horizontal pipelines, the valve stem should be installed upwards, or installed at a certain angle, and the handwheel should not be installed downwards. The valve, valve stem and hand wheel on the high-altitude pipeline can be installed horizontally, and the opening and closing of the valve can be remotely controlled by a vertical low chain.
  • 3. The arrangement is symmetrical, neat and beautiful; the valve on the standpipe, if the process permits, the valve handwheel is most suitable for operation with chest height, generally 1.0-1.2m from the ground, and the valve stem must follow the operator Direction installation.
  • 4. Valves on side-by-side risers should have the same centerline elevation, and the clear distance between handwheels should not be less than 100mm; the valves on side-by-side horizontal pipes should be installed staggered to reduce the distance between the pipes.
  • 5. When installing heavier valves on water pumps, heat exchangers and other equipment, valve brackets should be installed; when the valves are frequently operated and installed more than 1.8m away from the operating surface, a fixed operating platform should be installed.
  • 6. If there is an arrow mark on the valve body, the direction of the arrow is the flow direction of the medium. When installing the valve, make sure that the arrow points in the same direction as the medium flow in the pipeline.
  • 7. When installing flanged valves, ensure that the end faces of the two flanges are parallel and concentric with each other. Double gaskets are not allowed.
  • 8. When installing a threaded valve, in order to facilitate disassembly, a threaded valve should be equipped with a union. The setting of the union should consider the convenience of maintenance, usually the water flows through the valve first and then the union.

Precautions for valve installation

  • 1. The valve body material is mostly made of cast iron, which is brittle, so it must not be hit by heavy objects.
  • 2. When moving the valve, throwing is not allowed; when lifting or hoisting the valve, the rope should be tied to the valve body, and it is strictly forbidden to tie it to the handwheel, valve stem and flange bolt holes.
  • 3. The valve should be installed in the most convenient place for operation, maintenance and overhaul, and it is strictly prohibited to be buried in the ground. The valves on the pipelines directly buried and in the trenches shall be equipped with inspection wells to facilitate the opening and closing and adjustment of the valves.
  • 4. Ensure that the threads are intact and undamaged. Wrap hemp, lead oil, or PTFE tape on the threads. When screwing, use a wrench to clamp the hexagonal valve body screwed into one end of the pipe.
  • 5. When installing the flange valve, pay attention to tighten the connecting bolts diagonally, and use even force when screwing to prevent the gasket from running off or causing the valve body to deform and damage.
  • 6. The valve should be kept closed during installation. For threaded valves close to the wall, it is often necessary to remove the valve stem, disc and handwheel during installation before they can be turned. When disassembling, you should turn the handwheel to keep the valve open before disassembling.

Commonly used assembly methods for valves

There are three commonly used assembly methods for valves, namely, complete replacement method, repair method and selection method.

Complete swap method

When the valve is assembled using the complete interchange method, each part of the valve can be assembled without any trimming and selection, and the assembled product can meet the specified technical requirements. At this time, the valve parts must be processed completely in accordance with the design requirements to satisfy the requirements of dimensional accuracy and geometric tolerances. The advantages of the complete interchange method are: assembly work is simple and economical, workers do not need a high degree of skill, the production efficiency of the assembly process is high, and it is easy to organize assembly lines and organize specialized production. However, in absolute terms, when adopting a complete replacement assembly, the machining accuracy of the parts is higher. It is suitable for valves with absolutely simple structures such as globe valves, check valves, ball valves, and valves with medium and small diameters.

Selection method

The valve is assembled by matching method, and the whole machine can be processed according to economic precision. When assembling, a certain size with adjustment and compensation function is selected to achieve the specified assembly accuracy. The principle of the matching method is the same as that of the repair method, except that the method of changing the size of the compensation ring is different. The former is to change the size of the compensation ring by selecting accessories, and the latter is to change the size of the compensation ring by trimming the accessories. For example: the top core and adjustment gasket of the control valve type double-disc wedge gate valve, the adjustment gasket between the two bodies of the split ball valve, etc., is to choose special parts as compensation parts in the dimensional chain related to assembly accuracy. Adjust the thickness of the gasket to achieve the required assembly accuracy. In order to ensure that the fixed compensation parts can be selected in different situations, it is necessary to pre-manufacture a set of gaskets and shaft sleeve compensation parts hydraulic control valve models of different thickness sizes for selection during assembly.

Repair method

The valve is assembled by the repairing method, the parts can be processed according to economic precision, and then a certain size with adjustment and compensation function is repaired during assembly to achieve the specified assembly goal. For example, the gate plate and valve body of the wedge gate valve, because the processing cost required to realize the interchange is too high, most manufacturers use the repair method. That is, in the final grinding of the gate sealing surface to control the opening size, the plate should be matched with the opening size of the valve body sealing surface to achieve the ultimate sealing requirement. Although this method adds the plate matching process, it greatly simplifies the dimensional accuracy requirements of the previous processing process. The special personnel of the plate matching process are skillful, and generally speaking, it will not affect the production efficiency. Valve assembly process: The valves are individually assembled on a fixed site. The valve parts and components assembly and general assembly are carried out in the assembly workshop, and all the required parts and components are transported to the assembly site. Usually component assembly and general assembly are distinguished by how many groups of workers are carried out at the same time, which not only shortens the assembly cycle, but also facilitates the application of special assembly tools, and the requirements for the skill level of workers are relatively low.
Some foreign manufacturers or high-tech valves also adopt the mode of assembling the suspension line or assembling the rotary table.

Valve assembly

The valve is usually assembled with the valve body as the reference part in the order and method specified by the process. The parts and components should be reviewed before assembling to avoid the parts that are not deburred and cleaned from entering the final assembly. During the assembly process, the parts should be handled with care to avoid bumping and scratching the processing personnel. The moving parts of the valve (such as valve stems, bearings, etc.) should be coated with industrial grease. The bonnet and the flange in the valve body are mostly connected by bolts. When tightening the bolts, respond to the scale, interweave, tighten repeatedly and evenly, otherwise the joint surface of the valve body and the bonnet will produce flow control valve due to uneven surrounding forces. leakage. The handle used for tightening should not be too long to prevent the pre-tightening force from being too large and affecting the bolt strength. For valves with strict requirements for pre-tightening force, torque should be used to move the hand, and the bolts should be tightened according to the specified torque requirements. After the final assembly is completed, the holding mechanism should be rotated to check whether the movement of the valve opening and closing parts is maneuverable and whether there is any jamming. Whether the installation direction of the valve cover, bracket and other parts of the pressure reducing valve meets the requirements of the drawing, the valve can be tested only after all the reviews have passed.

Installation of common valves

Installation of gate valve

Gate valve, also known as gate valve, is a valve that uses a gate to control the opening and closing, and adjusts the flow of the pipeline and opens and closes the pipeline by changing the cross section. Gate valves are mostly used for pipelines that fully open or fully close the fluid medium. Gate valve installation generally has no directional requirements, but it cannot be inverted.

Installation of stop valve

The stop valve is a valve that uses the disc to control the opening and closing. By changing the gap between the valve disc and the valve seat, that is, changing the size of the channel section to adjust the medium flow or cut off the medium passage. Pay attention to the direction of fluid flow when installing the shut-off valve.
The principle that the shut-off valve must be installed is that the fluid in the pipeline passes through the valve hole from bottom to top, commonly known as “low inlet and high outlet”, and no reverse installation is allowed.

Installation of check valve

Check valve, also known as one-way valve, is a valve that automatically opens and closes under the action of the pressure difference between the front and back of the valve. Its function is to make the medium flow in only one direction, and prevent the medium from flowing backwards. According to the different structure of the check valve, there are lift type, swing type and butterfly wafer type. Lift check valve is divided into horizontal and vertical. When installing the check valve, you should also pay attention to the flow direction of the medium.

Installation of pressure reducing valve

The pressure reducing valve is a valve that reduces the inlet pressure to a certain required outlet pressure through adjustment, and relies on the energy of the medium itself to automatically maintain a stable outlet pressure.
From the point of view of fluid mechanics, the pressure reducing valve is a throttling element whose local resistance can be changed, that is, by changing the throttling area, the flow velocity and the kinetic energy of the fluid are changed, resulting in different pressure losses, so as to achieve the purpose of decompression. Then rely on the adjustment of the control and regulation system to balance the fluctuation of the pressure behind the valve with the spring force, so that the pressure behind the valve remains constant within a certain error range.

Installation of pressure reducing valve

  • 1. Vertically installed pressure reducing valve sets are generally installed along the wall at a suitable height from the ground; horizontally installed pressure reducing valve sets are generally installed on a permanent operating platform.
  • 2. The applied section steel is loaded into the wall on the outside of the two control valves (usually used for shut-off valves) to form a bracket. The bypass pipe is also stuck on the bracket and leveled.
  • 3. The pressure reducing valve should be installed upright on a horizontal pipeline, and should not be inclined. The arrow on the valve body should point to the direction of the medium flow and should not be installed backwards.
  • 4. Stop valves and high and low pressure gauges should be installed on both sides to observe the pressure changes before and after the valve. The diameter of the pipe after the pressure reducing valve should be 2#-3# larger than the inlet pipe diameter before the valve, and a bypass pipe should be installed for maintenance.
  • 5. The pressure equalizing pipe of the membrane pressure reducing valve should be connected to the low pressure pipeline. Low-pressure pipelines should be equipped with safety valves to ensure the safe operation of the system.
  • 6. When used for steam decompression, a drain pipe should be installed. For piping systems that require a high degree of purification, a filter is installed before the pressure reducing valve.
  • 7. After the installation of the pressure reducing valve group is completed, the pressure reducing valve and safety valve should be tested, flushed and adjusted according to the design requirements, and the adjusted mark should be made.
  • 8. When flushing the pressure reducing valve, close the inlet valve of the pressure reducer and open the flush valve for flushing.

Installation of trap

The basic function of the steam trap is to discharge the condensed water, air and carbon dioxide gas in the steam system as soon as possible; at the same time, it can automatically prevent the leakage of steam to the maximum extent. There are many varieties of traps, each with different performance.
According to the different working principles of traps, it can be divided into the following three types:
Mechanical type: Actions depend on changes in the height of the condensate level in the steam trap, including:

  • Float type: The float is a closed hollow sphere.
  • Open upward float type: The float is a barrel type with upward opening.
  • Open downward float type: The float is a barrel type with downward opening.
  • Thermostatic type: Relying on changes in liquid temperature to act, including:
  • Bimetallic strip: The sensitive element is a bimetallic strip.
  • Vapor pressure type: The sensitive element is a bellows or an ink cartridge, which is filled with volatile liquid.
  • Thermodynamic type: Action depends on the change of the thermodynamic properties of the liquid.
  • Disc type: Due to the different flow rates of liquid and gas under the same pressure, the different dynamic and static pressures produced drive the disc valve to move.
  • Pulse type: As condensed water of different temperatures passes through the two-pole series-connected orifice plate, different pressures will be formed between the two-pole orifice plate to drive the valve flap.

Installation steps of trap

  • 1. The cut-off valve (cut-off valve) must be installed before and after, and a filter should be set between the trap and the front cut-off valve to prevent the dirt in the condensate from blocking the trap.
  • 2. There should be an inspection tube between the trap and the rear shutoff valve to check whether the trap is working properly. If a large amount of steam is emitted from the inspection tube, it means that the trap is broken and needs to be repaired.
  • 3. The bypass pipe is set to discharge a large amount of condensate when starting and reduce the discharge load of the trap.
  • 4. When the trap is used to drain the condensed water of the heating equipment, it should be installed in the lower part of the heating equipment, so that the condensate pipe is connected to the steam trap vertically to prevent the heating equipment from storing water.
  • 5. The installation location should be as close as possible to the drainage point. If the distance is too far, air or steam will accumulate in the slender pipe in front of the trap.
  • 6. The drainage problem should be considered when the horizontal pipeline of the steam main pipe is too long.

Installation of safety valve

A safety valve is a special valve whose opening and closing parts are normally closed under the action of external force. When the medium pressure in the equipment or pipeline rises above the specified value, the medium is discharged outside the system to prevent the medium pressure in the pipeline or equipment from exceeding the specified value. .
Safety valves belong to the category of automatic valves, which are mainly used in boilers, pressure vessels and pipelines. The control pressure does not exceed the specified value, which plays an important role in protecting personal safety and equipment operation. Note that the safety valve must be pressure tested before it can be used.

  • 1. Before installation, the product must be carefully inspected to verify whether there is a certificate of conformity and product instructions to clarify the constant pressure at the factory.
  • 2. The safety valve should be arranged as close to the platform as possible for inspection and maintenance.
  • 3. The safety valve should be installed vertically, the medium should flow from bottom to top, and the verticality of the valve stem should be checked.
  • 4. Under normal circumstances, cut-off valves cannot be set before and after the safety valve to ensure safety and reliability.
  • 5. Safety valve pressure relief: When the medium is liquid, it is generally discharged into the pipeline or closed system; when the medium is gas, it is generally discharged to the outdoor atmosphere;
  • 6. Oil and gas media can generally be discharged into the atmosphere, and the outlet of the safety valve vent pipe should be 3m higher than the surrounding tallest structure, but in the following cases, it should be discharged into a closed system to ensure safety.
  • 7. The diameter of the population pipeline should be at least equal to the inlet diameter of the valve; the diameter of the discharge pipe should not be less than the outlet diameter of the valve. The discharge pipe should be led to the outdoors and installed with a bent pipe so that the outlet of the pipe faces a safe zone.
  • 8. When the safety valve is installed, when the connection between the safety valve and the equipment and pipeline is open-hole welding, the diameter of the opening should be the same as the nominal diameter of the safety valve.

Source: China valves Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.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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