Gate valve encyclopedia and common fault elimination

The gate valve is a relatively common general valve with a wide range of uses, mainly used in water conservancy, metallurgy and other industries. Its extensive performance has been recognized by the market. In the many years of quality supervision and inspection work, the author has not only made some research on the inspection of gate valves, but also a more serious and meticulous study on the use and fault elimination of gate valves. 

Below is a general discussion of the structure, use, fault elimination, and quality inspection of valves.

    1. Structure

    The structure of the gate valve: The gate valve is a valve that uses the gate plate and valve seat to control the opening closing. The gate valve mainly consists of a valve body, valve seat, gate plate, valve stem, valve cover, stuffing box, stuffing pressure cover, valve stem nut, hand, etc. By changing the relative position between the gate plate and the valve seat, the size of the channel can be changed and the channel can be cut off. In order to that the gate valve can be closed tightly, the mating surface of the gate plate and valve seat has been ground.

    According to the different shapes of the gate valve, the gate valve can be divided into two categories: wedge type and parallel type.

    The gate plate of the wedge-type gate valve is wedge-shaped, and sealing surface forms an inclined angle with the center line of the channel. The wedge-shaped plate can be a single gate plate or a double gate plate.

    The sealing of the parallel-type gate valve is parallel to each other and perpendicular to the center line of the channel, which can be divided into two types: with and without support mechanism. the double gate plate with a support mechanism, when the gate plate is lowered, the wedge block of the two parallel gate plates is pressed against the valve seat by the inclined surface, the two gate plates are opened in the valve seat to cut off the flow passage. When the gate plate is raised and opened, the wedge block is separated from the mating surface the gate plate, and the gate plate rises to a certain height, and the wedge block is lifted by the projection on the gate plate. For the double gate plate without support mechanism when the gate plate slides into the valve seat along the two parallel valve seat surfaces, the gate plate is pressed against the valve body on the export side of the valve by the pressure the fluid, and the fluid is sealed.

    According to the different movement conditions of the valve stem when the gate plate is opened and closed, the gate valve is divided two categories: open stem gate valve and closed stem gate valve. For the open stem gate valve, the valve stem and gate plate rise and fall simultaneously when opening or closing; the closed stem gate valve, the valve stem only makes a rotating motion when opening or closing, and the valve stem's rise and fall cannot be seen. The gate plate makes upward or downward motion. The advantage of the open stem gate valve is that it can determine the opening height of the channel by the height of the valve stem rising, but it can the occupied height. When facing the handwheel or handle, turn the handwheel or handle clockwise, and the valve is closed.

    2. The occasion of use the principle of selection of gate valves

    01. Flat gate valve

    The occasion of use of the flat gate valve:

    (1) Oil gas transmission pipelines, and the flat gate valve with a guide hole is also convenient for cleaning the pipeline.

    (2) Product oil transportation pipelines and storage equipment.

    (3) Oil and gas mining wellhead devices.

    (4) Pipelines with suspended particle medium.

    (5) City gas transmission pipeline

    (6) Municipal water supply projects.

    The selection principle of the flat gate valve:

    (1) Oil and gas transmission pipelines, choose single gate plate or double gate plate flat gate valve. If the pipeline needs to be cleaned, choose a single gate plate with a guide hole open stem flat gate valve.

    (2) Product oil transportation pipelines and storage equipment, choose a single gate plate or double gate plate flat gate valve without a guide hole.

    (3) Oil and mining wellhead devices, choose a closed stem floating valve seat with a guide hole single gate plate or double gate plate flat gate valve.

    (4) Pipelines suspended particle medium, choose a knife-shaped flat gate valve.

    (5) City gas transmission pipeline, choose a single gate plate or double gate plate soft seal open flat gate valve.

    (6) For the water supply project, choose a single or double gate plate without a discharge hole and an rod flat gate valve.

    02. Wedge-type gate valve

    The applicable occasions of wedge-type gate valves: Among various types of valves, valves are one of the most widely used. They are 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 for the shape and size of the valve and the service conditions are relatively harsh. For example, high-temperature and-pressure working media require that the closing part ensures long-term sealing.

    Usually, when the service conditions or sealing performance requirements are reliable, high pressure, pressure cut-off (large pressure difference), low pressure cut-off (small pressure difference), low noise, cavitation and vaporization phenomenon, high-temperature media, temperature (deep cold), it is recommended to use a wedge-type gate valve. Such as the power industry, petroleum refining, petrochemical, offshore oil, urban in the water supply project and sewage treatment project, chemical and other fields are more applied.

    Selection principle:

    (1) The requirements for the fluid of the valve. For the working conditions with small flow resistance, strong flow capacity, good flow characteristics, and strict sealing requirements, choose the gate valve.

    (2 High temperature and high pressure media. Such as high-pressure steam, high-temperature and high-pressure oil products.

    (3) Low temperature (deep cold). For media such as liquid ammonia, liquid hydrogen, and liquid oxygen.

    (4) Low pressure and large caliber. Such as water supply projects and sewage projects.

    (5) Installation location: When the installation height is limited, choose a hidden rod wedge-type gate valve; when the height is not limited, choose open rod wedge-type gate valve.

    (6) Only when it can only be used for full opening or full closing, not for regulation and throttling, can wedge-type gate valve be selected.

    III. Common faults and maintenance

    01. Common faults and causes of gate valves

    After the gate is used, due to the medium temperature, pressure, corrosion, and the relative motion of each contact part, the following problems often occur.

    (1) Leak: It is divided into two types, namely external leakage and internal leakage. Leakage to the outside of the valve is called external leakage, and external leakage is common in the box and flange connection.

    The causes of leakage in the stuffing box: the variety or quality of the packing does not meet the requirements; the packing is aged or valve stem is worn; the packing pressure cover is loose; the surface of the valve stem is scratched.

    The reasons for leakage at the flange connection: the or size of the gasket does not meet the requirements; the processing quality of the flange sealing surface is poor; the connecting bolts are not properly tightened; the pipeline configuration unreasonable, and excessive additional load is generated at the connection.

    The reasons for internal leakage of the valve: The leakage formed by the valve not closing tightly is internal leakage which is caused by damage to the valve sealing surface or the sealing ring root not being tight.

   (1) Corrosion often occurs on the valve body, valve cover, valve stem, and fl sealing surface. Corrosion is mainly due to the action of the medium, as well as the release of ions from the packing and gasket.

    (2)ratching: The local surface of the gate plate and valve seat is pulled or peeled off when they are in relative motion under a certain contact pressure.

    02 of gate valves

    (1) Repair of external leakage of valves

    When tightening the packing, the bolts of the pressure cap should be symmetrically tightened to tilting the pressure cap and leaving a gap for tightening. At the same time, the valve stem should be turned while tightening the packing to make the packing around the valve stem uniform prevent it from being pressed too tightly, which would affect the rotation of the valve stem, increase the wear on the packing, and shorten the service life. If the surface of the stem is scratched, which makes it easier for the medium to leak out, the surface damage of the valve stem should be removed before use.

    For external leakage at flange connection, if the gasket is damaged, it should be replaced; if the gasket material is not properly selected, a material that can meet the requirements of use should selected; if the machining quality of the flange sealing surface is poor, it must be disassembled and the flange sealing surface should be reworked until it is qualified

    In addition, properly tightening the flange bolts and correctly configuring the pipeline to avoid excessive additional loads at the flange connection are all conducive to preventing leakage the flange connection.

    (2) Repair of internal leakage of valves

    The repair of internal leakage is to eliminate damage to the sealing surface and loosen at the base of the sealing ring (when the sealing ring is fixed by pressing in or screwing on the valve plate or valve seat). If the sealing surface is directly mach on the valve body and valve plate, there is no problem of base looseness and leakage.

    When the sealing surface is seriously damaged and the sealing surface is formed the sealing ring, the old ring should be removed and a new sealing ring should be installed; if the sealing surface is directly machined on the valve body, the damaged sealing surface be removed first, and then the new sealing ring or the machined surface should be polished into a new sealing surface. When the defects such as scratches, dents, and depions on the sealing surface are less than 0.05 mm, they can be removed by grinding.

    If leakage occurs at the base of the sealing ring, the sealing ring is pressed in for fixing, a Teflon tape or white thick paint can be placed at the bottom of the valve seat or the groove of the sealing ring ring and then the sealing ring is pressed in, so that the base of the sealing ring is filled; when the sealing ring is fixed by threads, a Teflon tape or white paint should be placed between the threads to prevent the fluid from leaking from the threads.

    (3) Repair of valve corrosion

    In general, the valve body valve cover are uniformly corroded, while the valve stem is often pitted. During repair, the corrosion products should be removed first. For the valve stem with pits, it be processed on the lathe to eliminate the depressions, use packing containing slow-release agents, or wash the packing with distilled water to remove the ions in the packing are corrosive to the valve stem.

  (4) Repair of scratched sealing surface

    During the use of the valve, the sealing surface should be prevented from beingched as much as possible, and the torque should not be too large when closing the valve. If the sealing surface is scratched, it can be removed by grinding.

   IV. Inspection of gate valves

    In the current market environment and user needs, iron gate valves account for a large proportion. As a product quality inspection personnel, in to being relatively familiar with the product quality inspection, it is also necessary to have a good understanding of the product itself.

    01. Inspection basis for iron gate

    The inspection basis for iron gate valves is the national standard GB/T12232-2005 "General Valves with Flange Connection Iron Valves".

    02. Inspection items for iron gate valves

    It mainly includes: marks, * minimum wall thickness, pressure test, shell test etc., among which wall thickness, pressure, and shell test are mandatory inspection items and key items. If there is an unqualified item, it can be directly judged as anqualified product.

    In summary, product quality inspection is the most important part of the entire product inspection, and its importance is self-evident. As a front- inspection worker, it is necessary to continuously strengthen one's own quality, not only to do a good job in product inspection, but also to have an understanding of the inspected products so as to better do a good job in inspection.