The gate valve is a relatively common general valve with a wide range of uses. It is mainly used in water conservancy, metallurgy and other industries. Its extensive use performance has been recognized by the market. In many years of quality and technical supervision and testing, the author has not only made certain research on the detection of the gate valve, but also the use of the gate valve, I have done more careful and meticulous research
The following is a general discussion on the structure, use, troubleshooting, quality inspection and other aspects of the gate valve.
A structure
The structure of the gate valve: The gate valve is a valve that uses the gate plate and the valve seat to control the opening and closing. The gate valve mainly consists of valve body, valve seat, gate plate, valve stem, valve cover, packing letter, packing pressure cover, valve stem nut, hand-wheel, etc. 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 close the gate valve tightly, the gate plate and the valve seat are ground.
According to the different shape of the gate valve structure, the gate valve can be divided into two categories: wedge and parallel.
The gate plate of the wedge gate valve is in shape, and the sealing surface is inclined to the centre line of the channel. The wedge between the gate plate and the valve seat is used to seal (close). The wedge plate can be a single gate or a double gate.
The sealing surface of the parallel gate valve is parallel to each other and perpendicular to the centre line of the channel. It is divided into two types: an open mechanism and a non-opening mechanism. There is a double gate with an open mechanism. When the gate lowers, the wedges of the two parallel gates support the two gates on the valve seat on the slope and cut off the flow channel. When the gate rises and opens, the shovel and the gate cooperate with the gate are separated, the gate rises to a certain height, and the wedge is held up by the con The double gate plate without an open mechanism. When the gate plate slides into the valve seat along the two parallel valve seat surfaces, the pressure of the fluid is used to press the gate plate on the valve body on the valve outlet side to seal the fluid.
According to the different movements of the valve stem when the gate is opened and closed, the gate valve is divided into two categories: open-bar gate valve and dark-bar gate valve. When the valve stem and gate plate of the open-bar gate valve are opened or closed, they rise and fall at the same time; when the dark-bar gate valve is opened or closed, the valve stem is only rotated, the rise and fall of the valve stem cannot be seen, and the valve plate is raised or lowered. The advantage of the open-bar gate valve is that it can judge the opening height of the channel through the rising height of the valve stem, but it can shorten the height of possession. When facing the hand-wheel or handle, turn the hand-wheel or handle clockwise, and the valve is closed.
The occasion and selection principle of the use of the second gate valve：
01 flat gate valve
Use of flat gate valve:
(1) Oil and natural gas transmission pipelines, flat gate valves with diversion holes are also convenient for cleaning pipelines.
(2) Transmission pipelines and storage equipment for refined oil.
(3) Oil and natural gas extraction port devices.
(4) Pipes with suspended particle media.
(5) Urban gas transmission pipeline.
(6) Water supply project.
Selection principle of flat gate valve:
(1) For oil and natural gas transmission pipelines, choose flat gate valves with single gates or double gates. If you need to clean the pipeline, choose a flat gate valve with a single gate with a diversion hole.
(2) For the transmission pipeline and storage equipment of refined oil, choose a single gate without a diversion hole or a flat gate valve with a double gate.
(3) For oil and natural gas extraction port devices, choose a single gate with a dark rod floating valve seat with a diversion hole or a flat gate valve with a double gate.
(4) For pipes with suspended particle media, choose a knife-shaped plate gate valve.
(5) For urban gas transmission pipelines, choose single gate or double gate soft seal open rod flat gate valve.
(6) For water supply engineering, choose single gate plate or double gate plate without diversion hole open rod flat gate valve.
02 wedge gate valve
Applicable occasions of wedge gate valve: Among various types of valves, gate valve is a widely used. It is generally only suitable for full opening or full closing, and cannot be used for regulation and throttling.
The wedge gate valve is generally used in places where there is no strict requirement for the external size of the valve, and the use conditions are relatively harsh. For example, for high-temperature and high-pressure working medium, it is required that the closed parts should be sealed for a long time.
Generally, when the use conditions or requirements of reliable sealing performance, high pressure, high pressure cut-off (large pressure difference), low pressure cut-off (small pressure difference), low noise, air hole and vaporisation phenomenon, high temperature medium, low temperature (deep cold), it is recommended to use a puncture gate valve. For example, there are many applications in the power industry, petroleum smelting, petrochemical, offshore oil, water engineering and sewage treatment engineering in urban construction, and chemical industry.
Selection principle:
(1) Requirements for the fluid characteristics of the valve. The gate valve is used for working conditions with small flow resistance, strong circulation ability, 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 products.
(3) Low temperature (deep cold) medium. Such as liquid ammonia, liquid hydrogen, liquid oxygen and other media.
(4) Low pressure and large size. Such as tap water projects and sewage treatment projects.
(5) Installation position: choose the dark rod gate valve when the installation height is limited; choose the open rod gate valve when the height is not limited.
(6) Only when it can be fully opened or fully closed, and cannot be used for adjustment and throttling, can the puncture gate valve be selected.
Three common faults and maintenance
01 Common faults and causes of gate valves
After the use of the gate valve, the following problems often occur due to the temperature, pressure, corrosion and relative movement of each contact.
(1) Leakage: There are two types, namely, external leakage and internal leakage. The leakage outside the valve is called the leakage, and the leakage is common in the packing box and flange connection.
Reasons for the leakage of the packing box: the variety or quality of the packing does not meet the requirements; the ageing of the packing or the wear of the valve stem; the loosening of the packing gland; the scratching on the surface of the valve stem.
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 improperly fastened; the pipeline configuration is unreasonable, resulting in excessive additional loads at the connection.
Reasons for the internal leakage of the valve: The leakage caused by the lax closure of the valve is an internal leakage, which is caused by the damage to the sealing surface of the valve or the lax root of the sealing ring.
(1) Corrosion is often the corrosion of the valve body, valve cover, valve stem and flange sealing surface. Corrosion is mainly due to the action of the medium, and also the release of ions in fillers and gaskets.
(2) Scratch: local surface pulling or peeling occurs when the gate plate and the valve seat are in relative motion under certain contact ratio pressure.
02 Maintenance of gate valve
(1) Repair of external leakage of valve
When pressing the filler, the upper gland bolt should be weighed to avoid the tilt of the gland, leaving a gap for compression. While pressing the filler, the valve stem should be turned to make the filler around the valve stem uniform and prevent the pressure from dying, so as not to affect the rotation of the valve stem, increase the wear of the filler, and shorten the service life. The surface of the valve stem is scratched, so that the medium is easy to leak out. It should be processed to eliminate the scratches on the surface of the valve stem before being used.
For the leakage of the flange connection, if the gasket is damaged, it should be replaced; if the gasket material is not selected properly, the material that can meet the requirements of use should be selected; if the flange sealing surface is of poor processing quality, the flange sealing surface must be removed and reprocessed until it is qualified.
In addition, properly tightening the flange bolts, correctly configuring the pipeline, and avoiding excessive additional loads at the flange connection are conducive to preventing leakage at the flange connection.
(2) Repair of internal leakage of valve
The repair of internal leakage is to eliminate the damage to the sealing surface and the laxity of the root of the sealing ring (when the sealing ring is fixed on the valve plate or valve seat by pressing or thread). If the sealing surface is directly processed on the valve body and valve plate, there is no problem of lax roots 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 equipped with a new sealing ring; if the sealing surface is directly processed on the valve body, the damaged sealing surface should be removed first, and then the new sealing ring or processed surface should be ground into a new sealing surface. When the scratches, bumps, crushes, dents and other defects of the sealing surface are less than 0.05mm, they can be eliminated by grinding.
The root of the sealing ring leaks. When the sealing ring is pressed into and fixed, the PTFE belt or white thick paint can be placed at the bottom of the valve seat or sealing ring groove, and then pressed into the sealing ring to fill the root of the sealing ring. When the sealing ring is fixed by a thread, the PTFE belt or white thick paint should be placed between the thread Leakage between the lines.
(3) Repair of valve corrosion
In general, the valve body and valve cover are evenly corroded, while the valve stem is often pitting. When repairing, the corrosive product should be removed first. For the valve stem with pitting pits, it should be processed on the lathe to eliminate the depression, change to a filler containing a sustained-release agent, or clean the filler with distilled water to remove the ions in the filler that have a corrosive effect on the valve stem.
(4) Repair of abrasions on the sealing surface
During the use of the valve, the abrasions on the sealing surface should be prevented as much as possible, and the torque should not be too large when closing the valve. If the abrasions on the sealing surface can be eliminated by grinding.
Detection of four gate valves
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 testing, you should also have a good understanding of the product itself.
01 Testing basis of iron gate valve
The detection of iron gate valve is based on the national standard GB/T12232-2005 ”General valve flange connection iron gate valve”.
02 Inspection items of iron gate valve
It mainly includes: logo, * small 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 a word, product quality inspection is an important part of the whole product inspection. Its importance is self-evident. As a front-line inspection staff, we must constantly strengthen our own quality. We should not only do a good job in product inspection, but also have an understanding of the inspected products, so that we can better do a good job in inspection.
Post time: Mar-23-2023