Thread Content
This post was last edited by Mighty Ultraman. Edited on 2021-12-3 09:47. Valves are the most common types of equipment in chemical manufacturing plants. Installing valves may seem simple, but failing to follow the relevant technical procedures can lead to safety accidents... Prohibition 1: Conducting hydrostatic tests at negative temperatures during winter construction. Consequence: The pipes were damaged by freezing due to rapid ice formation inside them during the hydrostatic test. Measure: Try to conduct the pressure test before winter construction, and after the test, blow out all the water; in particular, the water inside the valves must be completely removed, otherwise the valves may rust in mild cases or crack due to freezing in severe cases. When hydraulic tests must be conducted during winter, they should be carried out while maintaining a positive indoor temperature. After the test, all water must be drained completely. Prohibition 2: Failure to carry out thorough pipe system flushing before completion, resulting in flow rates and speeds that do not meet the requirements for pipe flushing. Even replacing flushing with water pressure strength test drainage. Consequences: The water quality fails to meet the requirements for the operation of the pipeline system, and this often leads to a reduction in the pipe’s cross-sectional area or even blockages. Measure: Flush using the maximum designed flow rate within the system, or a water flow velocity of not less than 3 m/s. It is considered qualified if the color and transparency of the water at the outlet are visually consistent with those of the water at the inlet. Prohibition 3: Sewage, rainwater, and condensate pipes shall not be covered up without undergoing a water-tightness test. Consequences: It may cause leaks and result in losses for the user. Measure: The water tightness test should be strictly inspected and accepted in accordance with the specifications. Plumbed sewage, rainwater, condensate pipes, etc., that are installed concealed underground, within ceilings, or between pipes must be leak-proof. Prohibition 4: During the hydrostatic strength test and leak-tightness test of piping systems, only pressure values and water level changes are monitored, which is insufficient for detecting leaks. Consequence: Leaks occur in the piping system after it starts operating, affecting normal use. Measures: When testing the piping system in accordance with design requirements and construction specifications, apart from recording any changes in pressure or water level within the specified time frame, it is essential to carefully check for any signs of leakage. Prohibition 5: Do not use ordinary valve flanges for butterfly valve flanges. Consequence: The flanges of butterfly valves differ in size from those of ordinary valves; some flanges have a small inner diameter, while the valve disc of a butterfly valve is large, which can result in the valve not being able to open or in it being forced open and thus damaged. Measure: The flange should be manufactured according to the actual dimensions of the butterfly valve flange. Prohibition 6: No holes or embedded components are provided during building structure construction, or the holes are too small and the embedded components are not marked. Consequences: During the construction of heating and ventilation systems, the building structure is damaged, and in some cases the load-bearing rebar is severed, which affects the safety of the building. Measures: Thoroughly study the construction drawings for heating and plumbing projects, and proactively and carefully cooperate with the construction of building structures by providing the necessary holes and embedded components as required for the installation of pipes and supports, in accordance with the design specifications and construction standards. Prohibition 7: In building structure construction, there are no pre-installed holes or fittings, or the size of the pre-installed holes is too small and the fittings are not marked. Consequences: During the construction of heating and ventilation systems, the building structure is damaged, and in some cases the load-bearing rebar is severed, which affects the safety of the building. Measures: Thoroughly study the construction drawings for heating and plumbing projects, and proactively and carefully cooperate with the construction of building structures by providing the necessary holes and embedded components as required for the installation of pipes and supports, in accordance with the design specifications and construction standards. Prohibition 8: Pipes shall not be buried directly in permafrost or untreated loose soil; the spacing and location of pipe supports must be appropriate, and dry brick stacking should not be used. Consequences: Due to unstable support, the pipe was damaged during the compaction of backfill soil, resulting in rework and repairs. Measures: Pipes must not be buried in frozen soil or untreated loose soil. The spacing between supports must comply with construction specifications, and the supports must be secure. In particular, at pipe joints, no shear forces should be exerted. Brick piers should be built with cement mortar to ensure they are complete and sturdy. Prohibition 9: The expansion bolts used to fix pipe supports are of poor quality; the holes drilled for these bolts are too large; or the expansion bolts are installed in brick walls or even lightweight walls. Consequences: The pipe supports become loose, the pipes deform, and may even fall off. Measures: Only qualified expansion bolts must be selected; sampling and testing should be conducted when necessary. The diameter of the hole for installing an expansion bolt must not exceed the bolt’s outer diameter by more than 2 mm. Expansion bolts should be used on concrete structures. Prohibition 10: The flanges and gaskets used for pipeline connections are not strong enough, and the connection bolts are short or have a small diameter. Rubber gaskets are used for hot water pipes, while double-layer or beveled gaskets are employed for cold water pipes; the flange gasket extends into the pipe. Consequence: The flange connection is not tight, or may even be damaged, resulting in leaks. The flange gasket protruding into the pipe increases water flow resistance. Measures: Flanges and gaskets used for pipes must meet the requirements of the pipe’s designed working pressure. For the gasketing of heating and hot water supply pipes, rubber-asbestos gaskets are recommended ； Rubber gaskets are recommended for the flange gaskets of water supply and drainage pipes. The gasket of the flange must not protrude into the pipe; its outer edge should be at the level of the flange bolt holes. No inclined shims or multiple gaskets shall be placed in the middle of the flange. The diameter of the bolts used to connect the flanges should be at least 2 mm smaller than the diameter of the flange holes, and the length of the bolt shaft protruding from the nut should be half the thickness of the nut. Prohibition 11: Incorrect valve installation method. For example, in the case of globe valves or check valves, if the flow direction of water (steam) is opposite to what is indicated, the valve stem should be installed downward; horizontally mounted check valves should be installed vertically. Gate valves or butterfly valves with visible stems do not have any space for opening and closing, while the valve stems of concealed valves are not oriented toward the inspection door. Consequences: Valve failure, difficulty in maintaining the switch; a valve stem pointing downward often leads to leaks. Measures: Install the valves strictly in accordance with the installation instructions. For straight-stem gate valves, ensure sufficient clearance for the valve stem to extend during operation; for butterfly valves, take into account enough space for the handle to rotate. The stems of all types of valves must not be below the horizontal position, let alone point downward. For concealed valves, not only must inspection doors be provided to meet the requirements for opening and closing the valves, but the valve stem should also face toward those inspection doors. Taboo 12: The specifications and models of the installed valves do not meet the design requirements. For example, the nominal pressure of the valve is less than the system test pressure ； Gate valves are used for water supply branch pipes when the diameter is 50 mm or less ； Gate valves are used for the dry and vertical pipes in hot water heating systems ； Butterfly valves are used for the suction pipes of fire pump systems. Consequences: It affects the normal opening and closing of the valve, as well as functions such as resistance and pressure regulation. It even leads to valve damage during system operation, forcing repairs. Measures: Familiarize yourself with the application ranges of various valves, and select the specifications and models of valves according to design requirements. The nominal pressure of the valve must meet the requirements of the system’s test pressure. According to construction specifications: globe valves shall be used for water supply branch pipes with a diameter of 50 mm or less ； When the pipe diameter is greater than 50 mm, gate valves should be used. Gate valves should be used for the dry and vertical control valves in hot water heating systems, and butterfly valves should not be used for the suction pipes of fire pumps. Taboo 13: Failing to conduct the necessary quality inspections as required before valve installation. Measures: Before installation, valves shall undergo pressure strength and tightness tests. The test shall involve sampling 10% of the quantity in each batch (of the same grade, specification, and model), with a minimum of one sample. Prohibition 14: Poor installation quality. Consequences: The project quality is substandard, there are potential accident risks, it cannot be delivered for use on schedule, and it must be reworked and repaired ； This leads to delays in the project timeline and increased costs for labor and materials. Prohibition 15: Valves installed upside down. Valves such as stop valves, throttle valves, pressure reducing valves, and check valves all have a specific direction of flow. If installed incorrectly, the throttle valve may be affected in terms of its performance and service life ； The pressure relief valve doesn’t work at all, and the check valve can even be dangerous. Measure: For ordinary valves, there are direction indicators on the valve body ； In case there isn’t any, it should be correctly identified based on the valve’s operating principle. The valve chamber of the stop valve is asymmetrical on the left and right; the fluid must flow through the valve orifice from bottom to top, which results in low fluid resistance (due to the shape of the valve). It is easier to open the valve as the medium pressure acts upward, and once closed, the medium does not exert pressure on the packing, facilitating maintenance. This is why stop valves cannot be installed in reverse. Straight-stem gate valves should not be installed underground, as moisture can cause corrosion of the exposed valve stem. For swing check valves, it is necessary to ensure that their pin shafts are horizontal during installation, so as to allow for smooth swinging. Prohibition 16: Do not operate manual valves with excessive force. Consequence: Easy damage to the sealing surface ； Measure: Manual valve, whose handwheel or handle is designed for normal human effort, taking into account the strength of the sealing surface and the necessary closing force. Therefore, it cannot be moved using a long lever or long wrench. Some people* are used to using wrenches, and they must be careful not to apply too much force; otherwise, it is easy to damage the sealing surface, or the wrench may break the handwheel or handle. When opening and closing the valve, apply force smoothly without any impact. The various components of certain high-pressure valves that are operated by impact have taken into account this type of impact force, which is different from that encountered in ordinary valves. For steam valves, they should be preheated and of the condensed water removed before being opened. The opening process should be carried out as slowly as possible to avoid water hammer. After the valve is fully opened, turn the handwheel slightly in the opposite direction to ensure a tight fit between the threads, thereby preventing loosening and damage. For straight-stem valves, remember the position of the stem when the valve is fully open and fully closed, to avoid hitting the dead center when it is fully open. It also facilitates checking whether it is functioning properly when fully closed. If the valve seat comes loose, or large debris gets trapped between the valve core seals, the position of the valve stem when the valve is fully closed will change. When the pipeline is used for the first time, it contains a lot of dirt inside. The valve can be slightly opened to allow the high-speed flow of the medium to wash away this dirt, after which it should be closed gently (not quickly or forcefully, to prevent residual impurities from damaging the sealing surfaces). The valve should then be opened again, and this process repeated several times until all the dirt is removed, after which normal operation can resume. For normally open valves, dirt may adhere to the sealing surface; when closing them, it is necessary to clean them using the aforementioned method before closing them properly. If the handwheel or handle is damaged or lost, it must be replaced immediately; adjustable wrenches should not be used as a substitute, as this may damage the square shape of the valve stem, resulting in poor operation of the valve and potential accidents during production. In some media, cooling after the valve is closed causes the valve components to contract; therefore, the operator should close it again at an appropriate time to ensure that no gaps remain in the sealing surfaces. Otherwise, the medium will flow rapidly through these gaps, easily eroding the sealing surfaces. During operation, if it is found that the task is too difficult to carry out, the reasons should be analyzed. If the packing is too tight, it can be loosened slightly; if the valve stem is skewed, personnel should be notified for repair. In some valves, when they are closed, the closing element expands due to heat, making it difficult to open them. If it is necessary to open them at this time, the threads on the valve cover can be loosened by half a turn to one full turn to relieve stress on the valve stem, after which the handwheel can be turned. Prohibition 17: Improper installation of valves in high-temperature environments. Consequence: Causes a leakage accident. Measure: For valves that operate at temperatures above 200°C, since they are installed at room temperature, the temperature rises during normal use, causing the bolts to expand due to heat and increasing the gaps. Therefore, these bolts must be tightened again; this process is known as \"thermal tightening\". Operators need to pay attention to this step, as otherwise leaks can occur easily. Prohibition 18: Failing to drain water in cold weather. Consequence: Valves are prone to freezing damage ； Measure: When the weather is cold and the water valve remains closed for an extended period, the water accumulated behind the valve should be drained. After the steam valve stops supplying steam, the condensate must also be drained. There is a plug at the bottom of the valve, which can be opened to drain water. Prohibition 19: The opening and closing force of non-metallic valves is too high. Consequences: Valve damage ； Measure: Non-metallic valves are either brittle or have low strength; when operating them, the force used for opening and closing should not be too great, and forceful actions should especially be avoided. Be careful to prevent the items from being bumped. Prohibition 20: The new valve packing should not be tightened too much. Consequences: Difficulty in opening and closing, accelerated wear ； Measure: When using a new valve, do not compress the packing too tightly – just enough to prevent leakage. This avoids excessive pressure on the valve stem, which could speed up wear and make it difficult to open and close the valve. The quality of valve installation has a direct impact on its performance; therefore, great attention must be paid to the direction and position of the valves, the valve installation process, the protective devices for the valves, bypass systems and instruments, as well as the replacement of valve packing. Prohibition 21: No operating space at the installation location. Consequences: Difficult to operate, unsafe. Measure: The valve should be installed in a location that facilitates operation; even if installation is temporarily difficult, it is necessary to consider the operator’s long-term work needs. It is best for the valve handwheel to be at chest level (usually 1.2 meters above the operating floor), as this makes it easier to open and close the valve. The handwheel of the floor valve should face upward and not be tilted, to avoid awkward operation. For wall-mounted units, space must also be left next to the device’s valves for the operator to stand. It is necessary to avoid working with the head raised, especially when dealing with acids, bases, toxic substances, etc., as it is very unsafe. Taboo 22: Do not strike valves made of brittle materials. Consequence: The valve breaks. Measure: Care must be taken during installation to avoid striking valves made of brittle materials. Before installation, the valve should be inspected to verify its specifications and model, and to check for any damage, especially to the valve stem. It needs to be turned a few more times to check if it is skewed, as the valve stem is most likely to get damaged during transportation. The debris inside the valve also needs to be removed. When lifting the valve, do not tie the rope to the handwheel or valve stem, as this may damage those components; instead, it should be tied to the flange. The pipelines connected to the valve must be thoroughly cleaned. Compressed air can be used to blow away iron oxide filings, sediment, slag, and other debris. These debris not only easily scratch the sealing surfaces of the valves, but larger particles of debris (such as weld slag) can also block small valves, rendering them ineffective. When installing threaded valves, the sealing filler (flax mixed with lead grease or PTFE tape) should be wrapped around the pipe threads, avoiding contact with the inside of the valve, so as to prevent accumulation there and disrupt the flow of the medium. When installing flanged valves, be careful to tighten the bolts symmetrically and evenly. The valve flange and the pipe flange must be parallel, with an appropriate gap, to prevent excessive pressure on the valve or even its cracking. Special attention should be paid to brittle materials and valves with low strength. For valves that need to be welded to pipes, spot welding should be performed first, followed by fully opening the closing element, and then performing a full weld. Prohibition 23: Valves are not insulated or cooled. Consequence: Heat dissipation, valves being damaged by freezing ； Measures: Some valves also require external protective measures, namely insulation and heat retention. Heating steam pipelines are sometimes also added inside the insulation layer. It depends on the production requirements to determine which types of valves should be insulated or insulated from heat. In principle, whenever the temperature of the medium inside the valve drops too much, it can affect production efficiency or damage the valve; in such cases, insulation is necessary, and sometimes even heat tracing ； When valves are exposed, which is detrimental to production or causes undesirable phenomena such as frost formation, thermal insulation is required. Insulation materials include asbestos, slag wool, glass wool, perlite, diatomaceous earth, vermiculite, etc.; heat-insulating materials include cork, perlite, foam, plastic, etc. Prohibition 24: No bypass is installed for the steam trap. Consequences: Difficult to maintain/repair ； Measure: Add a bypass for the secondary line. Prohibition 25: Consequences of not replacing the packing regularly: Packing leakage. Measure: Since valves are used with a wide variety of different media, ordinary packing is always installed in the stuffing box; however, it is necessary to ensure that the packing is suitable for the specific medium being used. For high-demand valves, the joint angle is 30 degrees. The seams between the circles are offset by 120 degrees. When compressing the packing, turn the valve stem at the same time to ensure even compression throughout and to prevent it from being too tight; apply even force when tightening the gland, without tilting it.