Gate valve is a more common general valve, widely used, mainly used in water conservancy, metallurgy and other industries, its wide range of performance has been recognized by the market, TWS in the quality and technical supervision and testing work for many years, in addition to the detection of gate valves have a certain research, but also on the use of gate valves, troubleshooting and other aspects of the more careful and meticulous research.
According to the different structural shapes of gate valves, gate valves can be divided into two categories: wedge type and parallel type.
The gate plate of the wedge gate valve is wedge-shaped, and the sealing surface is inclined to the center line of the channel, and the wedge between the gate plate and the valve seat is used to achieve sealing (closing). The wedge plate can be either a single gate or a double gate.
The sealing surfaces of the parallel gate valve are parallel to each other and perpendicular to the center line of the channel, which are divided into two types: with and without opening mechanism. There is a double ram with a bracing mechanism, when the ram goes down, the wedge of two parallel rams is propped up on the valve seat by the inclined plane, the flow channel is cut, when the ram rises and opens, the wedge is separated from the ram mating surface, the ram rises to a certain height, and the wedge is held up by the boss on the ram. When the ram slides into the valve seat along the two parallel valve seat surfaces, the pressure of the fluid is used to press the ram on the valve body on the outlet side of the valve to seal the fluid.
According to the different movement of the valve stem when the gate is opened and closed, gate valves are divided into two categories: rising stem gate valves and concealed stem gate valves. The valve stem and gate of the rising stem gate valve rise and fall at the same time when opening or closing; when the concealed stem gate valve is opened or closed, the valve stem only rotates, and the rise and fall of the valve stem cannot be seen, and the valve plate rises or falls. The advantage of the rising stem gate valve is that the channel opening height 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. Occasions and selection principles for gate valves
01. Flat gate valve
Occasions for flat gate valves:
(1) Oil and natural gas pipelines. Flat gate valves with diversion holes are also convenient for cleaning pipelines.
(2) Pipelines and storage equipment for refined oil.
(3) Oil and natural gas production outlets.
(4) Pipelines with suspended particle media.
(5) City gas transmission pipelines.
(6) Water supply projects.
Selection principle of flat gate valve:
(1) For oil and natural gas transmission pipelines, plate gate valves with single or double gates are selected. If you need to clean the pipeline, choose an open-rod flat gate valve with a single ram and a diversion hole.
(2) For the transportation pipeline and storage equipment of refined oil, the flat gate valve with single or double gate without diversion hole is selected.
(3) For the exploitation port device of oil and natural gas, the flat gate valve with single or double gate plate with dark rod floating valve seat and diversion hole is selected.
(4) For pipelines with suspended particle media, knife-shaped flat gate valves are selected.
(5) For urban gas transmission pipelines, single gate or double gate plate soft sealing open rod flat gate valve is selected.
(6) For water supply projects, single gate or double gate valve without diversion hole open rod flat gate valve is selected.
02. Wedge gate valve
Wedge gate valve application occasions: in various types of valves, gate valve is the most widely used one, it is generally only suitable for full open or full closed, can not be used for adjustment and throttling.
Wedge gate valves are generally used in places where there are no strict requirements for the external dimensions of the valve, and the use conditions are more harsh. For example, the working medium of high temperature and high pressure requires the closing parts to be sealed for a long time.
Generally, the use of 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 (cryogen), wedge gate valve is recommended. For example, it is widely used in electric power industry, petroleum smelting, petrochemical industry, offshore oil, tap water engineering and sewage treatment engineering in urban construction, and chemical industry.
Selection principle:
(1) Requirements for the fluid characteristics of the valve. Gate valves are selected for the 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 (cryogen) medium. Such as liquid ammonia, liquid hydrogen, liquid oxygen and other media.
(4) Low pressure and large diameter. Such as tap water projects, sewage treatment projects.
(5) City gas transmission pipelines.
(6) Water supply projects.
The selection principle of flat gate valve:
(1) For oil and natural gas transmission pipelines, plate gate valves with single or double gates are selected. If you need to clean the pipeline, choose an open-rod flat gate valve with a single ram and a diversion hole.
(2) For the transportation pipeline and storage equipment of refined oil, the flat gate valve with single or double gate without diversion hole is selected.
(3) For the exploitation port device of oil and natural gas, the flat gate valve with single or double gate plate with dark rod floating valve seat and diversion hole is selected.
(4) For pipelines with suspended particle media, knife-shaped flat gate valves are selected.
(5) For urban gas transmission pipelines, single gate or double gate plate soft sealing open rod flat gate valve is selected.
(6) For water supply projects, single gate or double gate valve without diversion hole open rod flat gate valve is selected.
02. Wedge gate valve
Wedge gate valve application occasions: in various types of valves, gate valve is the most widely used one, it is generally only suitable for full open or full closed, can not be used for adjustment and throttling.
Wedge gate valves are generally used in places where there are no strict requirements for the external dimensions of the valve, and the use conditions are more harsh. For example, the working medium of high temperature and high pressure requires the closing parts to be sealed for a long time.
Generally, the use of 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 (cryogen), wedge gate valve is recommended. For example, it is widely used in electric power industry, petroleum smelting, petrochemical industry, offshore oil, tap water engineering and sewage treatment engineering in urban construction, and chemical industry.
Selection principle:
(1) Requirements for the fluid characteristics of the valve. Gate valves are selected for the 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 (cryogen) medium. Such as liquid ammonia, liquid hydrogen, liquid oxygen and other media.
(4) Low pressure and large diameter. Such as tap water projects, sewage treatment projects.
(5) Installation location: when the installation height is limited, the dark rod wedge gate valve is selected; When the height is not limited, the open rod wedge gate valve is selected.
(6) Only when it can be fully opened or fully closed, and can not be used for adjustment and throttling, the wedge gate valve can be selected.
3. Common faults and maintenance
01. Common faults and causes of gate valves
After the gate valve is used, the following problems often occur due to the medium temperature, pressure, corrosion and the relative movement of each contact.
(1) Leakage: There are two types, namely external leakage and internal leakage. The leakage to the outside of the valve is called leakage, and leakage is common in stuffing boxes and flange connections.
The reason for the leakage of the stuffing box: the variety or quality of the packing does not meet the requirements; Aging packing or stem wear; Loose packing gland Abrasion on the stem surface.
The reason for the leakage of the flange connection: the material or size of the gasket does not meet the requirements; Poor processing quality of flange sealing surface; improper tightening of the connecting bolts; The piping is not properly configured, resulting in excessive additional loads at the joints.
Cause of internal leakage of the valve: The leakage caused by the loose closure of the valve is an internal leakage, which is caused by the damage to the sealing surface of the valve or the loose root of the sealing ring.
(1) Corrosion is often caused by corrosion of the sealing surface of the valve body, valve cover, valve stem and flange. Corrosion is mainly due to the action of the medium, but also the effect of releasing ions from the filler and gasket.
(2) Abrasion: local surface brushing or peeling phenomenon that occurs when the ram and the valve seat are in relative motion under a certain contact specific pressure.
02. Maintenance of gate valves
(1) Repair of external leakage of valve
When pressing the packing, the gland bolt should be symmetrically applied to avoid the gland tilting and leave a gap for compression. At the same time as pressing the packing, the valve stem should be rotated to make the packing around the valve stem uniform, and prevent the pressure from being too dead, so as not to affect the rotation of the valve stem, increase the wear of the packing, and shorten the service life. The surface of the valve stem is abrasive, so that the medium is easy to leak out, and it should be processed to eliminate the scars on the surface of the valve stem before use.
For the leakage of the flange connection, if the gasket is damaged, it should be replaced; If the material of the gasket is not selected properly, the material that can meet the requirements of use should be selected; If the processing quality of the flange sealing surface is poor, the flange sealing surface must be removed and reprocessed until it is qualified.
In addition, proper tightening of flange bolts, proper configuration of pipes, and avoidance of excessive additional loads at the flange connection are all conducive to preventing leakage at the flange joint.
(2) Repair of leakage inside the valve
The repair of internal leaks is to eliminate damage to the sealing surface and loose sealing at the root (when the sealing ring is pressed into the valve plate or seat with a thread). If the sealing surface is directly processed on the valve body and 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 the sealing ring, the old ring should be removed and a new sealing ring should be equipped; If the sealing surface is machined directly on the valve body, the damaged sealing surface should be removed first, and then the new sealing ring or machined 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.
When the seal ring is pressed-in and fixed, PTFE tape or white thick paint can be placed at the bottom of the valve seat or seal ring groove, and then pressed into the seal to make the root of the seal ring be filled; When the seal is threaded, PTFE tape or white paint should be placed between the threads to prevent fluid from leaking between the threads.
(3) Repair of valve corrosion
In general, the valve body and bonnet are uniformly corroded, while the valve stem is often pitted. When repairing, the corrosion products should be removed first, and the valve stem with pitting pits should be processed on the lathe to eliminate the depression, and the packing containing a slow-release agent should be used instead, or the packing should be cleaned with distilled water to remove the ions that have a corrosive effect on the valve stem in the packing.
(4) Repair of abrasion on the sealing surface
In the use of the valve, the sealing surface should be prevented from abrasion as much as possible, and the torque should not be too large when the valve is closed. If the sealing surface is abrasive, it can be eliminated by grinding.
Fourth, the detection of 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 the inspection of product quality, we must also have a good understanding of the product itself.
01. The testing basis of iron gate valves
The testing 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 valves
Mainly include: mark, small wall thickness, pressure test, shell test, etc., of which wall thickness, pressure, shell test is a necessary inspection item, but also a key item, if there are nonconforming items, can be directly judged as unqualified products.
In short, product quality inspection is an important part of the entire product inspection, its importance is self-evident, as a front-line inspection staff, we must continue to strengthen their own quality, not only to do a good job in product inspection, but also to have an understanding of the inspection of products, so as to better do a good job in inspection.
Tianjin Tanggu Water-Seal Valve Co.,ltd mainly produce resilient seated butterfly valve, gate valve, Y-strainer, balancing valve, check valve, balancing valve, back flow preventer etc.
Post time: Dec-26-2024
