The Selection and Maintenance of Chemical System Valves

Characteristics of the valve
 
As a pipeline device, valves have three characteristics: service characteristics, structural characteristics and driving characteristics.
 
Usage characteristics: It determines the main usage performance and usage scope during design, and the usage characteristics of valves include: valve category, valve type, valve main parts and valve driving mode.
 
Structure features: It determines some structural features of the valve in terms of installation, maintenance and repair, etc., which include: the structural length and overall height of the valve, the connection form with the pipeline, the form of the sealing seat, and the processing form of the valve stem.
 
Driving characteristics: There are two kinds of working characteristics of the valve, one is driven by external force, the other is driven by force or medium force generated by deformation of internal parts of the valve.
 
Important technical parameters and requirements for valve selection
 
Define the purpose of the valve on the device, working conditions, media characteristics, working pressure, working temperature, opening and closing time and frequency. Define the nominal diameter and connection mode of valves and connecting pipes. Define the driving mode of the valve. Define the categories of valves. Identify the type of valve. Determine the flow resistance and flow characteristics of the valve, the leakage grade of the sealing pair, the service life and the safety grade of the driving mechanism driving the valve. Determine the movement mode of the valve opening and closing piece, whether it is 90 or 180 rotation or sliding. It's press (clamp) tight, and it's automatic. Know the internal structure of the selected valve (such as anti-static, anti-overpressure, anti-misoperation, etc.), so as to ensure the correct selection of the valve.
 
Check the valve
 
I think everyone knows about the inspection and test methods of various valves (national standards, ministerial standards and international standards). Here, I will share the following points with you (for reference). After the valve arrives at the delivery place, the buyer shall unpack (package) the valve according to the requirements of the order, and carefully check whether the quantity, caliber, pressure grade, connection form, valve body and bonnet material, appearance quality, accessories (spare parts), protective measures, etc. of the valve meet the order and relevant regulations. After acceptance, the valve should be placed indoors in principle (outdoor protection measures should be taken for large diameter valves) for inspection and test. The valves sent directly to the construction site shall be placed in a clean place and covered to prevent dust and other foreign bodies from entering after passing the inspection test.
 
Test of valves
 
Generally speaking, the strength and sealing performance of valves should be tested before installation, 20% of low-pressure valves should be randomly selected, and 100% of medium and high-pressure valves should be tested to ensure the safe use of valves. The welded valves shall be provided with pressure test blind plates (or cone seals or O-ring seals) for test use (such test tools can also be ordered with the valve supplier). When liquid (usually water) is used as the test medium, the air in the valve should be exhausted as far as possible to prevent the pressure value from showing instability during the pressure test. During the pressure test, the pressure should be gradually and slowly increased, and sudden pressurization is not allowed to prevent safety accidents. The pressure gauge should be selected within 25~75% of the range of the pressure gauge, and neither too low nor too high can meet the specified requirements. The pressure test time and requirements shall be implemented according to the standards or orders. If there is any doubt during the pressure test, the test time may be extended or re-tested. It is suggested that the valves installed in important positions should be tested at least twice, and the second test can be qualified only if it meets the requirements. Generally, throttle valves are only tested for static sealing of valve strength, packing and flange gasket, but not for dynamic sealing.
 
During the pressure test, the closing force of the valve can only be carried out by a person with normal physical strength, and tools such as levers (except torque wrench) are not allowed. However, when the diameter of the handwheel is larger than 320mm, it can be closed by two people together. The valve with upper sealing structure (such as gate valve, globe valve, etc.) should check its upper sealing performance. During the inspection, the packing gland should be loosened to make the packing in a free state. Observe whether there is leakage at the packing. If low-pressure gas is used, put some water above the stuffing box to observe whether there are bubbles leaking out. The valve with driving mechanism shall be tested together with the driving mechanism, and whether the opening and closing forces of the driving mechanism meet the requirements of the valve shall be checked. The sealing pair of the valve shall not be coated with any grease during the pressure test (except the grease-sealed plug valve). After the valve passes the test, the test medium in the valve cavity should be removed, wiped clean, the test record should be made, and the channel should be blocked with a stuffy cover to prevent foreign bodies from entering.
 
Pipeline installation of valves
 
Generally, the valve can enter the installation stage after the buyer has passed the inspection test (if the buyer has stored the valve for three months or more, the pressure test should be carried out again before installation). During installation:
 
(1) For individual valve replacement, the structural length of the valve, flange and bolt hole and their center distance should be measured before installation. After meeting the requirements, open the valve to the micro-closed state before installation.
 
(2) If it is a newly designed pipeline, before installing the valve, the pipeline should be pigged to remove all kinds of sundries and oil in the pipeline before installing the valve. When installing the repaired valves, the requirements are the same as those in Article.
 
The installation position of valves should be convenient for operation. The valves installed on side-by-side pipelines should have space for maintenance, repair and disassembly. The distance between the handwheels of two valves should be greater than 100mm. If the distance between pipelines is too narrow, the valves should be installed in a staggered way. Install valves on the branch pipes leading out of the main pipe as close as possible to the main pipe to prevent pipeline vibration and damage. Gate valves with double gates should generally be installed vertically. Gate valves with single gates can be installed at any angle, but they are not allowed to flip, which may affect the service life of the valves. For the convenience of operation, gate valves driven by hand, electric, pneumatic and hydraulic should generally be installed vertically. Cut-off valves and throttle valves can be installed at any position in the pipeline; those with driving devices should be installed according to the instructions; throttle valves should be installed in a spacious position because of the frequent flow adjustment. Lift check valves can only be installed on horizontal pipelines. Spring vertical and swing check valves can be installed on horizontal or vertical pipelines, but the rocker pin of swing check valves should be kept in a horizontal position as far as possible. Ball valves, butterfly valves and diaphragm valves can be installed at any position in the pipeline, but those with driving mechanism should be installed horizontally as far as possible. For the installation of three-way and four-way ball valves, their valve stems should be in an upright position. Plug valves can be installed at any position, but it should be easy to observe the groove and operate. Three-way and four-way plug valves should be erected straight or installed at less than 90 degrees. Safety valves, whether spring-type, lever-type, built-in, pulse-type, or heavy hammer-type, should be installed vertically, and the valve stem should be vertical to the horizontal plane. The outlet of the safety valve (depending on the application) should generally avoid back pressure. If there is an outlet pipe, its diameter should not be less than the outlet diameter of the valve. Steam traps are generally installed on horizontal pipelines, and disk traps can be installed in other positions (steam traps are usually installed at the end or lowest point of saturated steam pipes, the dead end of the lowest point of steam tracing pipes, the position in front of the pressure reducing valve and regulating valve of steam system, the lower part of steam-water separator and steam heater, and the places that need constant drainage).
 
Generally, for valves installed at room temperature, after the installation is completed, slightly open the valve, and then tighten the middle flange bolts of the valve symmetrically again to prevent the pipeline from leaking after startup. Due to the heavy weight of the large-diameter valve, the installation may affect the stress of the pipeline. Therefore, when installing the large-diameter valve, the bracket should be considered to ensure the safety of the pipeline.
 
Operation and matters needing attention during valve operation
 
When the valve is running, it is operated manually (without driving mechanism). Generally, it is stipulated that clockwise rotation is closed, and counterclockwise rotation is open. When operating, the logo should be clearly seen to avoid misoperation. As mentioned earlier, the handwheel and wrench with a diameter of less than or equal to 320mm can only be operated by one person, and those above can be operated by two people or by one person with a booster bar of no more than 0.5m However, diaphragm valves and nonmetallic sealing surface valves are strictly forbidden to operate with booster bars to prevent damage to the valves. Some people are used to using booster bars, thinking that the larger the closing force, the better, which is actually wrong. Because the closing force is too large, the sealing pair and the valve stem thread bear greater force. If the sealing pressure of the sealing pair and the yield limit of the thread are exceeded, the sealing pair and the valve stem thread will be deformed, and the valve function will also be invalid. When I saw some operators at the scene using adjustable wrenches to open and close valves, I asked him why. He said that the handwheel or handle could not be found. I told him that it was dangerous for you to do so, and you might be hurt if the wrench slipped. Therefore, the adjustable wrench cannot be used to open and close the valve, and the handwheel and wrench that are broken or lost on site should be filled in time.
 
When the steam valve is closed when working at high temperature, but the pipeline stops running due to various reasons, the closed valve (especially the wedge gate valve) should be slightly opened immediately to prevent the sealing pair from biting and damaging the valve. For valves installed on high-temperature pipelines, when the temperature rises above 200℃, the middle flange bolts will be heated and stretched, which will easily lead to leakage of the static seal in the middle cavity of the valve body. At this time, the bolts should be hot-tightened, and the sealing pair should be opened a little before hot-tightening, so as to prevent the valve stem from being blocked, which will make it difficult to open the valve in the future. When the valve is closed, due to the temperature drop, the parts of the valve contract, which will cause the sealing of the sealing pair to be lax and leak. You can re-open some, and then close it again, and it will generally be sealed. When the ambient temperature drops to 0℃ or below, attention should be paid to opening the plug or bottom valve at the bottom of the shut-down and water-cut valves, so as to eliminate condensed water or accumulated water, so as to avoid freezing the valves. For valves that cannot eliminate accumulated water, heat preservation measures should be taken to prevent the valves from freezing. Packing gland should not be pressed too tightly, which will speed up the wear of valve stem and increase the opening and closing torque of valve stem. When replacing packing, the upper seal should be closed. If there is no valve with upper seal structure, it is generally not allowed to change packing with pressure casually. If it must be replaced. Packing should be installed according to the specified number of turns, and the packing that has been used for a long time and aged should be replaced in time. Packing gland (gland) should be pressed symmetrically, and a pre-tightening clearance of more than 5mm should be kept.
 
During the operation of the valve, the abnormality of the pipeline and the valve should be found by listening, smelling, seeing, touching and comparing. If it falls within the scope of one's own duties, it should be solved in time. If it can't be solved or the problem is serious, it should be reflected in time, and the repairman should be asked to solve it. There must be no delay to prevent a major accident.
 
Maintenance of valves during operation
 
During the running time of a pipeline system, the pipeline should be kept clean and tidy, the passage should be unobstructed and safe, all parts should be well lubricated, the valve fittings should be complete, and the maintenance is the key. Keep the valves on the pipeline clean and tidy, and the valves are painted in good condition. They are often purged with compressed air to prevent positive ash and dirt on the valve surface. They should be checked 1-2 times per shift, and the flushing valve and the plug wire at the bottom of the valve should be opened regularly to clean up the sewage and dirt in the bottom cavity of the valve. Keep the lubricated parts of the moving parts of the valves lubricated frequently. For high-temperature valves, molybdenum disulfide or graphite powder should be added once a week to a month, and oil and butter should not be added. For valves that are not frequently opened, the lubrication cycle can be stipulated in the enterprise. For those that should be greased, grease should be greased regularly, otherwise it is easy to wear and leak. Keep all kinds of parts of the valve complete. If the hexagon nuts with flange is loose due to pressure fluctuation and temperature fluctuation, it should be tightened in time. If the packing gland bolt and the nut on the handwheel are loose, it will cause flange seal leakage and affect the fitting accuracy for a long time, resulting in inadequate opening and closing. Due to the frequent opening and closing of the valve, the static seal of the packing as the valve stem is easy to wear and cause leakage, which is a common problem in the pipeline system. Generally, it is solved by adding packing. When adding packing, the applicable packing should be prepared in advance. Loosen the packing gland and immediately add the packing, and then tighten the gland nut. If it is a high-temperature, ultra-high-pressure and highly toxic medium, corresponding protective measures should be taken. The driving devices matched with all kinds of valves should be carefully maintained at least once a month: the appearance should be clean, the devices should not be contaminated by steam, water and oil, the electrical cabinet, hydraulic and pneumatic connectors should be complete and in good condition, and the indicator lights should be clearly and correctly indicated, so that the driving devices can keep in good working condition and meet the requirements of valve opening and closing.
 
Maintenance of valves
 
Maintenance of valves is an effective way to prolong the service life of valves. It is divided into online maintenance and offline maintenance. On-line maintenance is divided into shutdown (or without pressure) maintenance and pressurized maintenance. Shutdown (without pressure finger, there is partition pressure of the partition valve upstream of the pipeline) Generally speaking, the valve body is good for maintenance, which can be divided into
 
A. If the sealing pair fails and can't be closed (such as top-mounted ball valve), it needs to be repaired on-machine; if the valve B suddenly fails to open and close, it is estimated that other parts except the sealing pair are damaged (such as gate, valve stem, etc.). For these two kinds of maintenance, if the enterprise has strong maintenance strength and experience, is eager to resume production in a short time, and has parts (or make an emergency order from the original supplier, but find out the valve model, size, parts list and parts materials). During maintenance under pressure, due to valve A, the valve body and bonnet leak, and the middle flange seal leaks (leakage).
 
B. The driving device suddenly fails to open and close the valve, which causes the pipeline device to fail to operate normally. Maintenance under pressure is a dangerous job, so it is necessary to make a maintenance plan (including notifying the original supplier) and take safety measures. If the medium is harmful or toxic, the maintenance and protection work should be done well, and if welding is required, the fire prevention measures should be done well. In short, maintenance under pressure is a very arduous maintenance work, which should be highly valued. Offline maintenance is also called valve removal maintenance (maintenance after valve replacement). It can be repaired by the enterprise itself and returned to the supplier or entrusted by other qualified valve repair units. No matter what unit is used for repair, if you ask these questions about why you want to repair these valves (whether they are in a hurry to use them, whether they are old for reuse, whether they can't find a replacement valve, or whether they are responsible for repairing them), you will know the purpose of repairing the valves, and then you will be able to put forward a reasonable maintenance plan to ensure that the valves can be repaired on schedule and meet the needs of the users.

Conclusion
 
In many working conditions of chemical production, if there are several valves with the same functions, the valves can be selected according to the cost, use value, service life and maintenance. In particular, the material of the valve shell/cover, its internal structure and the used materials should be considered (mainly considering the high temperature-high temperature creep/low temperature-low temperature brittleness/corrosiveness of the medium-reducing strength/permeability-making the material invalid/coking-affecting the function of the valve). Based on years of field practice, I should consider the following points when selecting and designing the valve:
 
(1) Select the corresponding materials according to the media characteristics (parameters).
(2) Pressure and temperature curves are the basis of material selection and design.
(3) According to the expansion coefficient of materials, materials of parts shall be equipped.
(4) Select the model and specification of the valve according to its purpose.
(5) Select the internal structure of the valve according to the flow resistance loss (pressure drop).
(6) According to the requirements of high and low sealing, choose the structure and material of the opening and closing parts.
(7) Select the driving mode according to the installation and operation requirements of the pipeline.
(8) Make special design according to the particularity of medium and pipeline.
(9) For valves with frequent opening and closing, the service life of sealing pair and the cleanliness of pipeline and medium should be considered.
(10) The cleaning of newly-built pipelines has great influence on the service life of valves.
(11) Proper routine maintenance and operation of the valve is also an effective way to prolong the service life of the valve.