1. In winter, a water pressure test is conducted at a negative temperature.
Consequence: Because the pipe freezes quickly during the water pressure test, the pipe is frozen. Measures: Try to carry out a water pressure test before winter application, and blow the water clean after the test. Especially the water in the valve must be cleaned. Otherwise, the valve will rust when it is light, and freeze crack when it is heavy. When the water pressure test is taken in winter, the project should be carried out at a positive indoor temperature. And the water should be blown clean after the test.
2. Before the completion of the pipeline system, the flushing was not serious. And the flow rate and speed could not meet the requirements of pipeline flushing. Even the water pressure strength test is used to drain water instead of flushing.
Consequence: The water quality can’t meet the operation requirements of the pipeline system. This often leads to the reduction or blockage of the pipeline section. Measures: Rinse with the maximum juice flow rate in the system. Or the water flow rate that should not be less than 3m/s. The watercolor and transparency of the discharged water should be consistent with that of the inlet water visually.
3. Sewage, rainwater and condensate pipes will be concealed without a closed water test.
Consequence: It may cause water leakage and loss to users. Measures: The closed water test should be checked and accepted strictly according to the specifications. Buried underground, in the ceiling, between pipes and other concealed sewage, rainwater, condensate pipes should ensure no leakage.
4. It is in the process of the water pressure strength test and leakage test of the pipeline system. Usually only the changes in pressure value and water level are observed. And the leakage inspection is not enough.
Consequence: The pipeline system leaks after the operation, which affects its normal use. Measures: When the pipeline system is tested according to the design requirements and construction specifications, in addition to recording the pressure value or water level change within the specified time, it is especially necessary to carefully check whether there is leakage.
5. Using common valve flanges for butterfly valves
Consequence: The size of the butterfly valve flange is different from that of the ordinary valve flange. Some flanges have a small inner diameter. And the butterfly valve has a large flap. This causes the valve to be damaged due to its inability to open or hard opening. Measures: The flange of butterfly valves should be processed according to their actual size.
6. There are no reserved holes and embedded parts in the structure construction. Or the reserved holes are small in size and the embedded parts are not marked.
Consequence: During the construction of the heating and sanitation project, the building structure is chiseled, and even the stressed steel bar is cut off, which affects the safety performance of the building. Measures: Be familiar with the construction drawings of heating and sanitation projects, actively and conscientiously cooperate with the reserved holes and embedded parts in the construction of building structures according to the installation requirements of pipelines, supports and hangers, and refer to the design requirements and construction specifications for details.
7. When welding the pipeline, the butt joint of the pipe is not on a centerline. Leave no gap between the butt joints. And the thick-walled pipe does not shovel the groove. The width and height of the welding seam do not meet the requirements of the construction specification.
Consequence: The misalignment of the pipe in the centerline directly affects the welding quality and visual quality. There is no gap left in the butt joint. The thick-walled pipe does not shovel the groove. And when the width and height of the welding seam do not meet the requirements, neither is the welding strength. Measures: After welding the pipes to each other, the pipes should be on the centerline without the wrong mouth. Clearance should be left for the counterpart. Thick-walled pipes should be grooved. In addition, the width and height of the weld shall be welded according to the specification requirements.
8. The pipeline is directly buried in frozen soil and untreated loose soil. And the spacing and position of pipeline piers are improper, even in the form of a dry brick.
Consequence: Due to unstable support, the pipeline was damaged during backfill compaction, resulting in rework and repair. Measures: The pipeline shall not be buried in frozen soil or untreated loose soil, the spacing between piers shall meet the requirements of construction specifications, and the supporting pads shall be firm, especially at the pipeline joints, and shall not bear shear force. Brick piers should be built with cement mortar to ensure integrity and firmness.
9. The material of expansion bolts for fixing pipe supports is inferior, and the hole diameter for installing expansion bolts is too large or the expansion bolts are installed on brick walls or even light walls.
Consequence: The pipe support is loose, and the pipe is deformed or even falls off. Measures: Qualified products must be selected for expansion bolts. If necessary, samples should be taken for test inspection. The hole diameter of installation expansion bolts should not be larger than 2mm outside diameter. Expansion bolts should be applied to concrete structures.
10. The flange and gasket of the pipeline are not strong enough, and the connecting bolts are short or thin in diameter. The rubber pad is used for heat pipe, a double pad or inclined pad is used for cold water pipe, and the flange pad protrudes into the pipe.
Consequence: The flange joint is not tight, even damaged, resulting in leakage. The flange protruding into the pipe will increase the resistance of water flow. Measures: Flanges and gaskets for pipelines must meet the requirements of pipeline design working pressure. Rubber asbestos pad should be used for flange gasket of heating and hot water supply pipeline; Rubber gasket should be used for flange gasket of water supply and drainage pipeline. The flange gasket shall not protrude into the pipe, and its excircle should reach the flange bolt hole. Inclined pads or several gaskets shall not be placed in the middle of the flange. The bolt diameter of the connecting flange should be less than 2mm than the flange aperture, and the length of the bolt rod protruding from the nut should be 1/2 of the nut thickness.
11. Wrong valve installation method. For example, the water (steam) flow direction of the cut-off valve or check valve is opposite to the sign, and the valve stem is installed downward. The check valve installed horizontally is installed vertically. There is no opening and closing space for the handle of the open stem gate valve or butterfly valve, and the valve stem of concealed valve does not face the inspection door.
Consequence: The valve fails and the switch is difficult to repair. The valve stem faces downwards, which often leads to water leakage. Measures: Install the valve in strict accordance with the valve installation instructions. The open height of the open stem gate valve shall be reserved, and the rotating space of the handle of the butterfly valve shall be fully considered. All kinds of valve stems shall not be lower than the horizontal position, let alone downward. When installing concealed valves, it is necessary not only to set the inspection door to meet the needs of valve opening and closing but also to have the valve stem facing the inspection door.
12. The specifications and models of 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 branch pipes when the pipe diameter is less than or equal to 50mm; Cut-off valves are used for the dry and vertical pipes of hot water heating; The fire pump suction pipe adopts butterfly valve.
Consequence: It will affect the normal opening and closing of the valve, and adjust the resistance, pressure and other functions. Even when the system is running, the valve is damaged and forced to be repaired. Measures: Be familiar with the application scope of various valves, and select the specifications and models of valves according to the design requirements. The nominal pressure of the valve shall meet the requirements of the system test pressure. According to the requirements of construction specifications: cut-off valves should be used if the diameter of the branch water supply pipe is less than or equal to 50mm; When the pipe diameter is greater than 50mm, the gate valve should be adopted. Gate valves should be used for hot water dry and vertical control valves, and butterfly valves should not be used for fire pump suction pipes.
13. Before the valve is installed, the necessary quality inspection is not carried out according to the regulations.
Consequence: During the operation of the system, the valves are not flexible. The valves are not closed tightly, and water leakage (steam) occurs, which causes rework and repair. It even affects the normal water supply (steam). Measures: Before the valve is installed, the pressure strength and tightness test should be done. 10% of the quantity of each batch (same brand, same specification and same model) shall be randomly selected for the test, and not less than one. For the closed-circuit valves installed on the main pipe to cut off, the strength and tightness tests should be made one by one. The valve strength and leak test pressure shall comply with the Code for Acceptance of Construction Quality of Building Water Supply and Drainage and Heating Engineering.
14. The main materials, equipment and products used in construction are short of technical quality appraisal documents or product certificates that meet the current national or ministerial standards.
Consequences: the project quality is unqualified, there are hidden dangers of accidents. It can’t be delivered on time, and it must be reworked and repaired; The construction period is delayed, and the input of labor and materials is increased. Measures: The main materials, equipment and products used in water supply and drainage and heating and sanitation projects should have technical quality appraisal documents or product certificates that meet the current standards issued by the state or the Ministry; The product name, model, specification, code number of national quality standard, date of manufacture, name and location of the manufacturer, the inspection certificate or code number of ex-factory products shall be indicated.
15. Flip valves
Consequences: check valves, throttle valves, pressure reducing valves and other valves are directional. If they are flipped upside down, the throttle valve will affect the service effect and service life; The pressure relief valve doesn’t work at all, and the check valve may even cause danger. Measures: General valves, with direction signs on the valve body; If not, it should be correctly identified according to the working principle of the valve. The valve cavity of the stop valve is asymmetric from left to right, and the fluid should pass through the valve port from bottom to top so that the fluid resistance is small (determined by the shape). The opening is labor-saving (due to the upward pressure of the medium), and the medium does not press the packing after closing, which is convenient for maintenance.
This is the reason why the stop valve cannot be reversed. Part 2 Do not flip the gate valve upside down (that is, the handwheel is down), otherwise, the medium will remain in the bonnet space for a long time, which will easily corrode the valve stem, and it is forbidden by some technological requirements. At the same time, it is extremely inconvenient to change the packing. Open stem gate valves should not be installed underground, otherwise, they will corrode the exposed stem due to humidity. The lift check valve, when installed, ensures that its disc is vertical, so as to lift flexibly. The swing check valve, when installed, should ensure its pin shaft level, so as to swing flexibly. The pressure-reducing valve should be installed upright on the horizontal pipeline, and it should not be tilted in all directions.
16. Manual valve opening and closing with too-hard strength
Consequence: The valve is damaged at least. It will cause safety accidents at most. Measures: The manual valve, its handwheel or handle is designed according to ordinary manpower, taking into account the strength of the sealing surface and the necessary closing force. Therefore, you can’t use a long lever or a longboard hand to move the board. Some people are used to using wrenches, so they should pay strict attention to it.
Don’t use too much force, or the sealing surface will be easily damaged, or the handwheel and handle will be broken. Open and close the valve, the force should be smooth and not impacted. Some parts of high-pressure valves with impact opening and closing have considered this impact force, which is not equal to that of general valves. Before opening the steam valve, it should be heated in advance and the condensed water should be removed. When opening, it should be as slow as possible to avoid a water hammer.
When the valve is fully opened, the handwheel should be reversed slightly to tighten the threads to avoid loosening and damage. For open stem valves, remember the valve stem position when fully open and fully closed, and avoid hitting the top dead center when fully open. And it is convenient to check whether it is normal when fully closed. If the valve handle falls off, or there is large debris embedded between the valve core seals, the position of the valve stem will change when it is fully closed.
When the pipeline is put into use at the beginning, there is much dirt inside, so the valve can be slightly opened, and the high-speed flow of the medium can be used to flush it away. Then, it can be gently closed (it can’t be quickly closed or slammed to prevent the residual impurities from hurting the sealing surface), and then it can be opened again. Repeat this many times to flush the dirt, and then put it into normal work.
When the valve is normally opened, dirt may be stuck on the sealing surface. When it is closed, it should be washed clean by the above method, and then it should be officially locked. If the handwheel and handle are damaged or lost, they should be immediately assembled, and cannot be replaced by an adjustable wrench, so as to avoid damage to the four sides of the valve stem and ineffective opening and closing, resulting in accidents in production. Some media, after the valve is closed, will cool down, causing the valve to contract, so that the operator should close it again at an appropriate time, and so that the sealing surface will not have a fine seam; otherwise, the media will flow through the fine seam at a high speed, which will easily erode the sealing surface.
When operating, if you find that the operation is too laborious, you should analyze the reasons. If the packing is too tight, it can be relaxed appropriately. If the valve stem is skewed, the personnel should be informed to repair it. For some valves, in the closed state, the closing piece is heated and expanded, which makes it difficult to open; If it must be opened at this time, loosen the bonnet thread by half a turn to one turn to relieve the stress of the valve stem, and then turn the handwheel.
17. Improper installation of valves in a high-temperature environment
Consequence: causing leakage accidents. Measures: The high-temperature valve above 200℃ is installed at normal temperature. But after normal use, the temperature rises and the bolt expands due to heating. The gap increases, so it must be tightened again, which is called “hot tightening”. Operators should pay attention to this work, otherwise, it is easy to leak.
18. Not timely drainage in cold weather
Measures: When the water valve is closed for a long time in cold weather, the accumulated water behind the valve should be drained. After the valve stops, the condensed water should also be removed. The bottom of the valve is like a plug, which can be opened to drain water.
19. Non-metallic valves with too strong opening and closing force
Measures: Some non-metallic valves are hard and brittle, while others are low in strength. When operating, the opening and closing force should not be too strong, especially hard. Also, pay attention to prevent objects from bumping.
20. Too tight packing pressure of new valves
Measures: When new valves are used, the packing should not be pressed too tightly, so as not to leak, and to avoid too much pressure on the valve stem, which accelerates wear and tear, and make it difficult to open and close. The quality of valve installation directly affects the use, so we must pay attention to the direction and position of the valve, valve construction, valve protection facilities, bypass and instrument, and valve packing replacement.
21. No operating space in the installation position
Measures: The valve installation position must be convenient for operation; Even if the installation is temporarily difficult, the long-term work of the operators should be considered. It’s best to align the handwheel of the valve with the chest (generally 1.2m away from the operation floor), so it’s easier to open and close the valve. The landing valve handwheel should face upwards and do not tilt, so as to avoid awkward operation. The valve of the machine against the wall should also leave room for the operator to stand. Avoid overhead operation, especially acid and alkali, toxic media, or it will be unsafe.
22. Striking valves made of brittle materials
Measures: Be careful in installation and construction, and avoid striking valves made of brittle materials. Before installation, check the valve, and check the specification and model. Identify whether there is any damage, especially to the valve stem. Turn it a few times to see if it is skewed because the valve stem is most likely to be tilted during transportation. Also, remove the sundries in the valve. When lifting the valve, the rope should be tied to the flange instead of the handwheel or the valve stem to avoid damaging these parts. The pipeline connected to the valve must be cleaned. Compressed air can be used to blow away oxidized iron filings, mud sand, welding slag and other sundries. These sundries can not only easily scratch the sealing surface of the valve, but also block the small valve and make it invalid.
When installing the screw valve, the sealing filler (thread hemp plus lead oil or PTFE raw material belt) should be wrapped on the pipe thread, and it should not get into the valve, so as to avoid the accumulation in the valve and affecting the medium circulation. When installing flange valves, pay attention to tightening bolts symmetrically and evenly. The flange of the valve and the flange of the pipe must be parallel, with reasonable clearance, so as to avoid excessive pressure or even cracking of the valve. Pay special attention to valves with brittle materials and low strength. For valves that need to be welded with pipes, spot welding should be performed first, then the closing parts should be fully opened, and then welded to death.
23. The valve has not been kept warm and cold.
Measures: Some valves must also have external protection facilities, which are heat preservation and cold preservation. A heat tracing steam pipeline is sometimes added to the insulation layer. What kind of valve should be kept warm or cold depends on the production requirements. In principle, if the temperature of the medium in the valve is lowered too much, which will affect the production efficiency or freeze the valve, it is necessary to keep warm and even trace heat. When the valve is exposed, which is unfavorable to production or causes bad phenomena such as frosting, it needs to be kept cold. Thermal insulation materials include asbestos, slag wool, glass wool, perlite, diatomite, vermiculite, etc. Cold insulation materials include cork, perlite, foam, plastic, etc.
24. The trap is not equipped with a bypass.
Measures: Besides the necessary protection facilities, some valves should have bypass and instruments. A bypass is installed to facilitate the maintenance of steam traps. Other valves are also equipped with bypass. Whether to install the bypass depends on the condition, importance and production requirements of the valve.
25. Not replacing packing regularly
Measures: For inventory valves, some packing is not working well, and some packing is inconsistent with the used medium, so it is necessary to replace the packing. When the valve is faced with various kinds of different media, the stuffing box is always filled with ordinary packing, but when in use, the packing must be adapted to the media. When replacing the packing, press it in a round by round.
The seam of each circle should be 45 degrees, and the seam of each circle should be staggered 180 degrees. The height of packing should consider the room for the gland to continue to be pressed. At present, the stuffing chamber at the lower part of the gland should have an appropriate depth, which is generally 10-20% of the total depth of the stuffing chamber. For valves with high requirements, the joint angle is 30 degrees. It should be staggered by 120 degrees from the seam between the rings.
In addition to the above fillers, rubber O-ring (natural rubber resistant to weak alkali below 60 degrees Celsius, nitrile rubber resistant to oil below 80 degrees Celsius, fluororubber resistant to various corrosive media below 150 degrees Celsius), three-piece stacked PTFE ring (resistant to strong corrosive media below 200 degrees Celsius), nylon bowl ring (resistant to ammonia and alkali below 120 degrees Celsius) and other forming fillers can also be used according to specific conditions. A layer of PTFE tape is wrapped around the common asbestos packing, which can improve the sealing effect and reduce the electrochemical corrosion of the valve stem. When pressing the packing, turn the valve stem at the same time, so as to keep the circumference uniform and prevent it from being too dead. Tighten the gland evenly and do not tilt.