Gate valve is a relatively common general-purpose valve with a wide range of uses. It is mainly used in water conservancy, metallurgy and other industries. Its wide range of performance has been recognized by the market. In addition to the study of the gate valve, it also made a more serious and meticulous study on the use and troubleshooting of gate valves.
The following is a general discussion on the structure, use, troubleshooting, quality inspection and other aspects of gate valves.
1. Structure
The structure of the gate valve: the gate valve is a valve that uses a gate plate and a valve seat to control the opening and closing. Gate valve mainly consists of valve body, valve seat, gate plate, valve stem, bonnet, stuffing box, packing gland, stem nut, handwheel and so on. Depending on the change of the relative position between the gate and the valve seat, the channel size can be changed and the channel can be cut off. In order to make the gate valve close tightly, the mating surface of the gate plate and the valve seat is ground.
According to the different structural shapes of gate valves, gate valves can be divided into wedge type and parallel type.
The gate of the wedge gate valve is wedge-shaped, and the sealing surface forms an oblique angle with the center line of the channel, and the wedge between the gate and the valve seat is used to achieve sealing (closing). The wedge plate can be a single ram or a double ram.
The sealing surfaces of the parallel gate valve are parallel to each other and perpendicular to the center line of the channel, and there are two types: with expansion mechanism and without expansion mechanism. There are double rams with a spreading mechanism. When the rams descend, the wedges of the two parallel rams will spread the two rams on the valve seat against the inclined surface to block the flow channel. When the rams rise and open, the wedges and the gates will The matching surface of the plate is separated, the gate plate rises to a certain height, and the wedge is supported by the boss on the gate plate. The double gate without expansion mechanism, when the gate slides into the valve seat along the two parallel seat surfaces, the pressure of the fluid is used to press the gate against the valve body on the outlet side of the valve to seal the fluid.
According to the movement of the valve stem when the gate is opened and closed, the gate valve is divided into two types: rising stem gate valve and concealed stem gate valve. The valve stem and the gate plate of the rising stem gate valve rise and fall at the same time when it is opened or closed; when the concealed stem gate valve is opened or closed, the valve stem only rotates, and the lift of the valve stem cannot be seen, and the valve plate rises or falls sports. The advantage of the rising stem gate valve is that the opening height of the channel can be judged by the rising height of the valve stem, but the occupied height can be shortened. When facing the handwheel or handle, turn the handwheel or handle clockwise to close the valve.
2. The occasions and selection principles of gate valves
01. Flat gate valve
Application occasions of slab gate valve:
(1) For oil and natural gas pipelines, the flat gate valve with diversion holes is also easy to clean the pipeline.
(2) Pipelines and storage equipment for refined oil.
(3) Exploitation port devices for oil and natural gas.
(4) Pipelines with suspended particle media.
(5) City gas transmission pipeline.
(6) Waterworks.
The selection principle of slab gate valve:
(1) For oil and natural gas pipelines, use single or double slab gate valves. If it is necessary to clean the pipeline, use a single gate with a diversion hole open stem flat gate valve.
(2) For the transportation pipeline and storage equipment of refined oil, the flat gate valve with single ram or double ram without diversion holes is selected.
(3) For oil and natural gas extraction port installations, single gate or double gate slab gate valves with hidden rod floating seats and diversion holes are selected.
(4) For pipelines with suspended particle media, knife-shaped slab gate valves are selected.
(5) For urban gas transmission pipelines, use single gate or double gate soft-sealed rising rod flat gate valves.
(6) For tap water projects, single gate or double gate gate valves with open rods without diversion holes are selected.
02. Wedge gate valve
Applicable occasions of wedge gate valve: Among various types of valves, gate valve is the most widely used one. It is generally only suitable for full opening or full closing, and cannot be used for regulation and throttling.
Wedge gate valves are generally used in places where there are no strict requirements on the external dimensions of the valve, and the operating conditions are relatively harsh. For example, the working medium of high temperature and high pressure requires the closing parts to ensure long-term sealing, etc.
Generally, the service conditions or require reliable sealing performance, high pressure, high pressure cut-off (large pressure difference), low pressure cut-off (small pressure difference), low noise, cavitation and vaporization, high temperature medium, low temperature (cryogenic), it is recommended to use Wedge gate valve. Such as power industry, petroleum smelting, petrochemical industry, offshore oil, water supply engineering and sewage treatment engineering in urban construction, chemical industry and other fields are widely used.
Selection principle:
(1) Requirements for valve fluid characteristics. Gate valves are selected for working conditions with small flow resistance, strong flow capacity, good flow characteristics, and strict sealing requirements.
(2) High temperature and high pressure medium. Such as high pressure steam, high temperature and high pressure oil.
(3) Low temperature (cryogenic) medium. Such as liquid ammonia, liquid hydrogen, liquid oxygen and other media.
(4) Low pressure and large diameter. Such as water works, sewage treatment works.
(5) Installation location: When the installation height is limited, choose the concealed stem wedge gate valve; when the height is not restricted, choose the exposed stem wedge gate valve.
(6) Wedge gate valves can only be used when they can only be used for full opening or full closing, and cannot be used for adjustment and throttling.
3. Common faults and maintenance
01. Common faults and causes of gate valves
After the gate valve is used, due to the effects of medium temperature, pressure, corrosion and relative movement of various contact parts, the following problems often occur.
(1) Leakage: There are two types, namely external leakage and internal leakage. Leakage to the outside of the valve is called external leakage, and external leakage is commonly found in stuffing boxes and flange connections.
Reasons for the leakage of the stuffing box: the type or quality of the stuffing does not meet the requirements; the stuffing is aging or the valve stem is worn; the packing gland is loose; the surface of the valve stem is scratched.
Reasons for leakage at the flange connection: The material or size of the gasket does not meet the requirements; the processing quality of the flange sealing surface is poor; the connection bolts are not tightened properly; the pipeline configuration is unreasonable, and excessive additional load is generated at the connection.
Reasons for internal leakage of the valve: The leakage caused by the lax closure of the valve is internal leakage, which is caused by damage to the sealing surface of the valve or lax root of the sealing ring.
(1) Corrosion is often corrosion of the valve body, bonnet, valve stem, and flange sealing surface. Corrosion is mainly due to the action of the medium, as well as the release of ions from fillers and gaskets.
(2) Scratches: local roughening or peeling of the surface that occurs when the gate and the valve seat move relative to each other under a certain contact pressure.
02. Maintenance of gate valve
(1) Repair of valve external leakage
When compressing the packing, the gland bolts should be balanced to avoid the gland from tilting and leave a gap for compaction. While compressing the packing, the valve stem should be rotated to make the packing around the valve stem uniform, and prevent the pressure from being too tight, so as not to affect the rotation of the valve stem, increase the wear on the packing, and shorten the service life. The surface of the valve stem is scratched, which makes the medium easy to leak out. It should be processed to eliminate the scratches on the surface of the valve stem before use.
For the leakage at the flange connection, if the gasket is damaged, it should be replaced; if the material of the gasket is improperly selected, a material that can meet the requirements of use should be selected; if the processing quality of the flange sealing surface is poor, it must be removed and repaired. The flange sealing surface is reprocessed until it is qualified.
In addition, proper tightening of flange bolts, correct configuration of pipelines, and avoiding excessive additional load at flange connections are all conducive to preventing leakage at flange connections.
(2) Repair of valve internal leakage
The repair of internal leakage is to eliminate the damage of the sealing surface and the loose root of the sealing ring (when the sealing ring is fixed on the valve plate or seat by pressing or threading). If the sealing surface is directly processed on the valve body and the valve plate, there is no problem of loose root and leakage.
When the sealing surface is seriously damaged and the sealing surface is formed by a sealing ring, the old ring should be removed and a new sealing ring should be provided; if the sealing surface is directly processed on the valve body, the damaged sealing surface should be removed first. Remove, and then grind the new sealing ring or the processed surface into a new sealing surface. When the scratches, bumps, crushes, dents and other defects on the sealing surface are less than 0.05mm, they can be eliminated by grinding.
Leakage occurs at the root of the sealing ring. When the sealing ring is fixed by pressing, place tetrafluoroethylene tape or white thick paint on the valve seat or the bottom of the ring groove of the sealing ring, and then press the sealing ring to fill the root of the sealing ring; When the sealing ring is threaded, PTFE tape or white thick paint should be placed between the threads to prevent fluid from leaking between the threads.
(3) Repair of valve corrosion
Under normal circumstances, the valve body and bonnet are uniformly corroded, while the valve stem is often pitted. When repairing, the corrosion products should be removed first. For the valve stem with pitting pits, it should be processed on a lathe to eliminate the depression, and use a filler containing a slow-release agent, or clean the filler with distilled water to remove the filler that is harmful to the valve stem. corrosive ions.
(4) Repair of scratches on the sealing surface
During the use of the valve, try to prevent the sealing surface from being scratched, and the torque should not be too large when closing the valve. If the sealing surface is scratched, it can be removed by grinding.
4. Detection of gate valve
In the current market environment and user needs, iron gate valves account for a large proportion. As a product quality inspector, in addition to being familiar with product quality inspection, you must also have a good understanding of the product itself.
01. The detection basis of iron gate valve
Iron gate valves are tested based on the national standard GB/T12232-2005 “Flanged iron gate valves for general valves”.
02. Inspection items of iron gate valve
It mainly includes: signs, minimum wall thickness, pressure test, shell test, etc. Among them, wall thickness, pressure, and shell test are necessary inspection items and key items. If there are unqualified items, they can be directly judged as unqualified products .
In short, product quality inspection is the most important part of the entire product inspection, and its importance is self-evident. As a front-line inspection staff, we must constantly strengthen our own quality, not only to do a good job in product inspection, but also to Only by having an understanding of the inspected products can we do a better job of inspection.
Post time: Mar-31-2023