Discussion on selection and maintenance of valves in chemical system
In the production of various chemical industries, valves are indispensable and important equipment components and devices. The valve is a device that can cut off, adjust, check, divert, change the direction of the medium, control the medium pressure, separate the medium water and gas, and ensure the safety of the pipeline system. Therefore, the correct selection, operation and maintenance of the valve is very important. In addition, the valve is also a facility that controls the medium in the pipeline. It is treated as a pressure bearing element in the pipeline device, but it is also a valve that derives from different types of valves with different operating methods, media, temperature, pressure, pipeline structure and functional requirements. To do a good job of the valve, the structure is very important, but the selection of materials is more important. Understanding the hot working method of materials and selecting the hot working method correctly is the key, but the importance of cold working on the processing quality of valve parts and the influence of assembly on the performance of valves cannot be ignored.
Characteristics of the valve
Valve as a pipeline device, there are three characteristics: the use of characteristics, structural characteristics, drive characteristics.
Use characteristics: it is designed to determine the main performance and use of the scope, the use characteristics of the valve are: the type of valve, valve type, valve main parts, valve drive mode.
Structural characteristics: It determines the valve installation, maintenance, repair and other aspects of some structural characteristics, structural characteristics are: the length of the valve structure and overall height, and the form of connection to the pipeline, the form of sealing seat, stem processing form.
Driving characteristics: there are two types of operating characteristics of the valve, 1 external force drive, 2 the use of internal parts of the valve deformation generated by the force or medium force drive .
Select the important technical parameters and requirements of the valve
Clarify the use of the valve on the device, working conditions, media characteristics, operating pressure, operating temperature, opening and closing time and frequency. Clarify the nominal diameter of the valve and the connecting pipeline, the connection method. Clarify the valve drive mode. Clarify the category of the valve. Clarify the type of the valve.
Determine the flow resistance of the valve, flow characteristics of the sealing sub-leakage level, life, drive the valve drive mechanism of the safety level. Determine the movement of the valve opening and closing member, is 90 ° or 180 ° rotation, is sliding type. Is the pressure (clamping) type, is the automatic type.
Should understand the internal structure of the selected valve, (such as anti-static, anti overpressure, anti-misoperation, etc.), in order to better and exactly ensure the correct selection of the valve .
Inspection of valves
Various types of valves inspection and test methods (national standards, Ministry standards and international standards), I think we all understand, here to communicate with the following points (for reference).
After the valve arrives at the place of delivery, the demand side according to the order requirements of the valve open box (package) check, carefully check the number of valves, caliber, pressure level, connection form, valve body bonnet material, appearance quality, accessories (spare parts), protective measures, etc. are in line with the order and relevant provisions.
Valve acceptance in principle should be placed indoors (large-diameter valves placed outdoors should be protective measures) to wait for inspection and testing. Directly sent to the site of the valve, after passing the inspection and test, the valve should be placed in a clean place and covered to prevent dust and other foreign objects from entering.
Generally speaking, the valve should be done before the installation of the valve strength and sealing performance test, low-pressure valve sampling 20%, medium and high-pressure valves should be 100% test to ensure the safe use of the valve.
Welded connection of the valve should be well test pressure blind (or conical seal, or O-ring seal bore) for test use (this type of test tooling can also be ordered with the valve supplier).
When using liquid (usually water) as the test medium, the air in the valve should be eliminated as far as possible to prevent the pressure value from being unstable when the test pressure is displayed. Test pressure should be gradually and slowly increase the pressure, not sharply and suddenly increase the pressure to prevent safety accidents, the choice of pressure gauge should be within 25 to 75% of the pressure gauge range, too low and too high are not in line with the requirements. Test pressure time and requirements should be implemented in accordance with the standard or order, test pressure, such as doubt can extend the test time or re-test, it is recommended that the valve installed in important locations should be at least two test pressure, the second test pressure in line with the provisions to be considered qualified. Throttle valve generally only do the valve strength, and packing, in the flange gasket static sealing test, not dynamic sealing test.
Test pressure, the valve closing force, can only be carried out by a normal physical strength of the person, and not with the help of levers and other tools (except torque wrenches), but the handwheel diameter greater than 320mm can be closed by two people together. Valves with sealing structure (such as gate valves, globe valves, etc.) should be checked on the sealing performance, check the packing gland should be loosened, so that the packing is in a free state, observe whether there is leakage at the packing, if the use of low pressure gas, it should be put some water above the stuffing box, observe whether there is bubble leakage. Valves with drive mechanism, should be tested together with the drive mechanism, and check the drive mechanism to open and close the force to meet the needs of the valve. The valve sealing vice in the test pressure shall not be coated with any grease (except for grease sealed plug valve).
After the valve test is qualified, the valve cavity should be excluded from the test medium, and wipe clean, make test records, the channel plugged bored cover to prevent foreign objects from entering .
Valve in the demand side for inspection and testing after passing, you can generally enter the installation phase (demand-side inventory of valves in three months and more, should be re-tested before entering the installation), installation of.
- (1) A single replacement valve, before installation should measure the length of the valve structure with flange and bolt holes and their center distance, meet the requirements, the valve will be opened to a slightly closed state after entering the installation.
- (2) If the new design of the pipeline, before the installation of the valve, the pipeline should be cleared to remove all kinds of debris and oil in the pipeline before the installation of the valve. After repair of qualified valves, in the installation, the requirements and (2) the same.
The location of the installation of the valve should be convenient for operation, the installation of valves on side-by-side pipelines should be maintenance, repair, disassembly and installation of space, the distance between the second valve handwheel should be greater than 100mm, if the distance between the pipeline is too narrow, the valve should be staggered installation. The valve should be installed on the branch pipe leading from the main pipe as close as possible to the main pipe to prevent the pipeline vibration and damage to the pipeline. Gate valve structure is a double gate should generally be installed upright, the single gate valve can be installed at any angle, but not allowed to be installed upside down, if installed upside down may affect the service life of the valve. For ease of operation, hand operated, electric, pneumatic, hydraulic driven gate valve should generally be installed upright. Block valve, throttle valve can be installed in any position in the pipeline, with the drive device should be installed according to the instructions, throttle valve because often to adjust the flow, should be installed in a more spacious location. Lift check valve can only be installed in the horizontal pipeline, spring vertical and swing check valve can be installed in the horizontal or vertical pipeline, but the swing check valve rocker pin should try to maintain the horizontal position. Ball valves, butterfly valves, diaphragm valves can be installed in any position on the pipeline, but with the drive mechanism should be installed horizontally as far as possible, three-way, four-way ball valve installation, the stem should be in a vertical position. Plug valve can be installed in any position, but should be conducive to the observation of the groove to facilitate operation, three-way, four-way plug valve should be tree straight or less than 90 degrees installed. Safety valves, whether spring-loaded or lever-loaded, in-line, pulse-loaded, or hammer-loaded, should be installed in an upright position with the stem vertical to the horizontal. The outlet of the safety valve (depending on the application) should generally avoid the phenomenon of back pressure, and if the outlet has a lead pipe, its diameter should not be smaller than the outlet diameter of the valve. Steam traps are generally installed on horizontal pipelines, while disc traps can be installed in other locations (steam traps are usually installed at the end of the saturated steam pipe or at the lowest point of the end of the steam tracing pipe, at the dead end of the lowest point where steam does not flow frequently, in front of the pressure reducing valve of the steam system, in front of the regulating valve, in the lower part of equipment such as steam separators and steam heaters, and in places where frequent draining is required).
Generally installed at room temperature valves, after installation is complete, the valve slightly open some, and then the valve in the flange bolts and then evenly tighten once to prevent leakage after the start of the pipeline. Due to the heavy self-weight of large-diameter valves, the installation may affect the force problem of the pipeline, so the installation of such large-diameter valves should consider setting up a bracket to ensure the safety of the pipeline.
Operation of the valve and precautions when operating
Valve operation, manual (without drive mechanism) valve, generally specified clockwise rotation for closing, counterclockwise rotation for opening, operation to see the logo to avoid misoperation.
As already mentioned, the handwheel, plate hand less than or equal to 320mm can only be operated by one person, the above can be operated by two people or one person with a force bar of no more than 0.5m operation. But diaphragm valves, non-metallic sealing surface valves are strictly prohibited to use the force bar operation to prevent damage to the valve. Some people are used to using the force stick, that the greater the closing force the better, in fact, is not right. If the closing force is too large, the greater the force on the seal and stem threads, and if it exceeds the sealing pressure and yield limit of the threads, the seal and stem threads will fail, and the valve will fail. I saw some operators using a live wrench to open and close the valve, I asked him why, he said the hand wheel or handle could not be found, I told him that it is dangerous for you to operate like this, in case the wrench slipped, you may be hurt. Therefore, you cannot use live wrenches to open and close the valve, and the handwheel and wrench that are broken or lost at the scene should be replaced in time.
When the steam valve is in high temperature work is closed, but for various reasons the pipeline stopped running, then the closed valve (especially the wedge gate valve) should be immediately opened slightly to prevent the sealing vice bite, damage to the valve. Installed in the high temperature pipeline valve, when the temperature rises above 200 ℃, the flange bolts will be heat elongation, easy to make the valve body cavity static seal leakage, then the bolts should be hot tight, in the hot tight before the sealing vice should be opened a little, to prevent the stem top dead, resulting in difficult to open the valve later. When the valve is closed, due to the temperature drop, the valve parts shrink, it will cause the sealing vice seal is not tight leak, you can re-open some, and then close again, generally will be sealed. When the ambient temperature drops to 0 ℃ and below, the shutdown, water shutdown of the valve should pay attention to open the bottom of the valve silk plug or bottom valve, to exclude condensation or water, so as not to freeze the valve, the valve can not exclude water should take insulation measures to prevent the valve from freezing.
Packing gland should not be pressed too tightly, it will accelerate the wear of the stem, and increase the opening and closing torque of the stem, the packing should be closed when replacing the upper seal, if there is no upper seal structure of the valve, generally can not just replace the packing with pressure, if it must be replaced. Packing should be installed according to the required number of turns, the long-term use, aging packing should be replaced in a timely manner, packing gland (gland) should be symmetrically pressed, and maintain a preload gap of more than 5 mm .
Operation of the valve, should be heard, smelled, seen, touched, than to find the abnormalities of the pipeline and the valve, belonging to their own responsibilities should be resolved in a timely manner, can not be resolved or the problem is relatively large should be reflected in a timely manner, please repairman to solve, absolutely can not delay, to prevent a major accident.
Valve maintenance in the process of operation
A pipeline system in the running time, the pipeline should be kept tidy, smooth and safe channel, various parts well lubricated, complete valve fittings, normal operation state, maintenance is the key.
Maintain the neatness of the valves on the pipeline, the valve paint is intact, often blowing with compressed air to prevent the valve surface positive ash, dirt, should be checked 1 to 2 times per shift, regularly open the flushing valve and the valve bottom of the plug wire, clean the valve bottom cavity of sewage, dirt.
Keep the valve moving parts of the lubrication parts should be often oil lubrication, high temperature valves should be added once a week to January molybdenum disulfide or graphite powder, can not add oil and butter, not often open the valve, lubrication cycle can be internal regulations of the enterprise. For should be greased should be regularly filled with grease, otherwise easy to wear and leakage. Keep all kinds of parts of the valve complete, due to pressure fluctuations, temperature fluctuations caused by loose flange nuts should be tightened in a timely manner, packing gland bolts and hand wheel on the loose nuts should also be tightened in a timely manner, long-term non-tightening will cause flange seal leakage and affect the accuracy of the fit, resulting in opening, closing is not in place.
As the valve often open, close, as the valve stem parts of the packing static seal is easier to wear, and cause leakage, is often seen in the pipeline system, the general is to use the packing method to solve the packing, packing should be prepared in advance to apply the packing, loosen the packing gland immediately filler to join, and then plate tight gland nut. If it is high temperature, ultra-high pressure, highly toxic media should take the appropriate protective measures.
Various types of driving devices supporting the valve should be extra careful maintenance, at least once a month maintenance content: the appearance should be clean, the device is not stained by steam, water, oil, electrical cabinets, hydraulic, pneumatic joints complete, good, the indicator is expressed correctly, so that the driving device to maintain good working condition, to meet the valve opening and closing requirements.
Valve maintenance is an effective way to extend the service life of the valve. He is divided into online maintenance and offline maintenance.
Online maintenance is also divided into shutdown (or not with pressure) maintenance and maintenance with pressure two.
Shutdown (without pressure refers to the pipeline upstream of the isolation valve isolation pressure) maintenance is generally speaking valve body is good, it can be divided into a, the seal vice failure, closed not dead (such as top-loading ball valves) need to be repaired in the machine, b valve suddenly can not open and close, it is estimated that in addition to the seal vice other parts of the damage (such as the gate, stem, etc.), these two types of maintenance if the enterprise maintenance force and experience is strong, anxious to resume production for a short period of time, and have parts (or to the original supply enterprise emergency order, but to find out the valve model, parts size, parts list, parts material), repair is no problem, because the valve has not been a separate test pressure, so in the start-up to carefully observe the valve’s working condition, timely measures.
Repair with pressure, as the valve appears a, valve body, valve cover leakage, in the flange seal leakage (external leakage), b, the drive device suddenly failed, can not open and close the valve, resulting in the normal operation of the pipeline device can not be. Pressure maintenance is a more dangerous work, must develop a maintenance program (including notification of the original supply enterprises), take security measures, if the medium is harmful, toxic should also do a good job of maintenance protection, if you want to weld should do a good job of fire prevention measures, in short, with pressure maintenance is a very difficult maintenance work, should pay great attention.
Offline maintenance is also known as the removal of the valve repair (repair after replacement of the valve). Can be returned by the enterprise itself and the supply unit repair or entrusted to other qualified valve repair units, no matter what unit maintenance is taken, on why to repair these valves (is urgent to use, is to repair the old reuse, is unable to find a replacement valve, or accountability rework), get please after these issues, also distinguish the purpose of repairing the valve, but also to put forward a reasonable maintenance program to ensure that the valve can be as scheduled repaired to meet the use of the unit .
In many conditions of use in chemical production, if there are several valves with the very same function, the valve can be selected according to the cost, use value, service life, maintenance, etc.. In particular, it is necessary to consider the material of the valve shell / cover and its internal structure situation with the materials used (mainly to consider the medium of high temperature – high temperature creep / low temperature – low temperature brittle / corrosive – reduce the strength / permeability – make the material failure / coking -affects the valve use function).
I based on years of field practice, in the selection and design of the valve should consider the following points.
- (1) Select the corresponding material according to the media characteristics (parameters).
- (2) pressure, temperature curve is the basis for the selection of materials and design.
- (3) Equipped with the material of the parts according to the expansion coefficient of the material.
- (4) Select the type and specification of the valve according to the use of the valve.
- (5) Select the internal structure of the valve according to the flow resistance loss (pressure drop).
- (6) Select the structure form and material of the opening and closing parts according to the high and low requirements of sealing.
- (7) Select the driving method according to the setting and operation requirements of the pipeline.
- (8) Special design according to the media and pipeline specificity.
- (9) The valve with frequent opening and closing should consider the life of its sealing vice and the pipeline to match the cleanliness of the medium.
- (10) The new pipeline cleaning, cleaning of the valve’s service life is a big impact.
- (11) The correct daily maintenance and operation of the valve is also an effective way to extend the service life of the valve.
Author: Kong Ji
Source: China Valves Manufacturer – Yaang Pipe Industry Co., Limited (www.sfutube.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Stainless Steel Valves, 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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