Valve common faults and causes and installation knowledge
Valve inspection and installation
Inspection before valve installation
- ① Carefully check whether the valve model and specification meet the drawing requirements.
- ② Check whether the valve stem and disc open flexibly and whether there is jamming and deflection.
- ③ Check whether the valve is damaged and whether the thread of the threaded valve is correct and complete.
- ④ Check whether the combination of valve seat and valve body is firm, the connection between valve disc and valve seat, valve cover and valve body, and the connection between valve stem and valve disc.
- ⑤ Check whether the valve gasket, packing and fasteners (bolts) are suitable for the requirements of working medium properties.
- ⑥ Old or used for a long time should be removed, dust, sand and other debris must be cleaned with water.
- ⑦ Remove the cover and check the sealing degree. The valve disc must be closed tightly.
Pressure test of valves
Strength test and tightness test shall be conducted for low pressure, medium pressure and high pressure valves, and spectral analysis shall be conducted for shell of alloy steel valve one by one, and the material shall be rechecked.
Strength test of valve
The strength test of the valve is the test under the open state of the valve to check the leakage of the external surface of the valve. For the valve with PN ≤ 32Mpa, the test pressure is 1.5 times of the nominal pressure, the test time is not less than 5min, and there is no leakage at the shell and packing gland.
Valve tightness test
When the valve is fully closed, check whether there is leakage on the sealing surface of the valve. The test pressure of the valve, except butterfly valve, check valve, bottom valve and throttle valve, should be carried out at the nominal pressure. When the working pressure can be determined, the test can also be carried out at 1.25 times of the working pressure, and the valve disc sealing surface is qualified if there is no leakage.
General rules for valve installation
- 1. The installation position of the valve shall not hinder the operation, disassembly and repair of the equipment, pipeline and valve body, and the appearance of the assembly shall be considered.
- 2. The valve on the horizontal pipeline should be installed with the valve stem upward or inclined at a certain angle. The handwheel should not be installed downward. The valve can be opened and closed by high-altitude chain and low-distance valve.
- 3. The arrangement is symmetrical, neat and beautiful; on the premise that the process allows, the valve handwheel is the most suitable for operation, generally 1.0-1.2m above the ground, and the valve stem must be installed along the direction of the operator.
- 4. For the valves on the parallel riser, the elevation of the centerline should be the same, and the clear distance between handwheel should not be less than 100 mm; the valves on the parallel horizontal pipeline should be staggered to reduce the pipe spacing.
- 5. When heavy valves are installed on water pumps, heat exchangers and other equipment, valve supports shall be provided; when valves are frequently operated and installed more than 1.8m away from the operation surface, fixed operation platform shall be set.
- 6. If there is an arrow mark on the valve body, the direction of the arrow is the flow direction of the medium. When installing the valve, it should be noted that the arrow points to the same direction as the medium flow in the pipeline.
- 7. When installing the flange valve, ensure that the end faces of the two flanges are parallel and concentric with each other, and double gaskets are not allowed.
- 8. When installing the threaded valve, a threaded valve should be equipped with a union for easy disassembly. The setting of the union should consider the convenience of maintenance, usually the water flows through the valve first and then through the union.
Precautions for valve installation
- 1. The valve body material is made of cast iron, which is brittle, so it can not be impacted by heavy objects.
- 2. When handling the valve, it is not allowed to throw it at random; when lifting and hoisting the valve, the rope should be tied to the valve body, and it is strictly forbidden to bolt on the handwheel, valve stem and flange bolt hole.
- 3. The valve should be installed in the most convenient place for operation, maintenance and overhaul, and it is forbidden to bury it underground. Inspection chamber shall be set for valves on directly buried and underground pipelines to facilitate opening, closing and adjustment of valves.
- 4. Ensure that the thread is intact, and wrap hemp, lead oil or PTFE raw material belt on the thread. When screwing, use a wrench to clamp the hexagonal valve body screwed into one end of the pipe.
- 5. When installing the flange valve, pay attention to tighten the connecting bolts along the diagonal direction, and the force should be even when screwing, so as to prevent the gasket from running off or causing the deformation and damage of the valve body.
- 6. The valve shall be kept closed during installation. For the screw valve close to the wall, the valve stem, disc and hand wheel should be removed for screw installation. During disassembly, the valve should be disassembled after the hand wheel is turned to keep the valve open.
Installation of common valves
Installation of gate valve
Gate valve, also known as gate plate valve, is a valve that uses the gate to control the opening and closing of the valve. It can adjust the pipeline flow and open and close the pipeline by changing the cross section. Gate valve is mostly used to do full open or full close operation of the fluid medium pipeline. Generally, there is no directional requirement for gate valve installation, but it can not be inverted.
Installation of stop valve
The globe valve is a valve which uses the valve disc to control the opening and closing. By changing the gap between the valve disc and the valve seat, that is to change the size of the channel section to adjust the medium flow or cut off the medium path. Attention must be paid to the flow direction when installing the stop valve.
The principle that must be observed when installing the stop valve is that the fluid in the pipeline passes through the valve hole from bottom to top, commonly known as “low in and high out”, and reverse installation is not allowed.
Installation of check valve
Check valve, also known as check valve, one-way valve, is a valve that automatically opens and closes under the effect of pressure difference between the front and rear of the valve. Its function is to make the medium flow in one direction and prevent the medium from reverse flow. Check valve according to its different structure, there are lifting type, swing type and butterfly clamp type. Lifting check valve and horizontal and vertical points. When installing the check valve, pay attention to the flow direction of the medium, and do not install it in reverse.
Installation of pressure reducing valve
The pressure reducing valve is a kind of valve which can reduce the inlet pressure to a certain outlet pressure by regulating, and keep the outlet pressure stable automatically by relying on the energy of the medium itself.
From the point of view of fluid mechanics, the pressure reducing valve is a throttling element whose local resistance can be changed. That is to say, by changing the throttling area, the velocity and kinetic energy of fluid will be changed, resulting in different pressure losses, so as to achieve the purpose of reducing pressure. Then, depending on the regulation of the control and regulation system, the fluctuation of the pressure behind the valve is balanced with the spring force, and the pressure behind the valve is kept constant within a certain error range.
Installation of pressure reducing valve
- 1. The pressure relief valve group is installed on the general vertical wall and is suitable for installation on the ground.
- 2. Profile steel shall be loaded into the wall outside the two control valves (commonly used for stop valves) to form a bracket. The bypass pipe is also stuck on the bracket for leveling and alignment.
- 3. The pressure reducing valve should be installed vertically on the horizontal pipeline without tilting. The arrow on the valve body should point to the flow direction of medium and should not be installed in reverse.
- 4. Stop valves and high and low pressure gauges should be installed on both sides to observe the pressure change before and after the valve. The diameter of the pipeline behind the pressure reducing valve should be 2 × 3 × larger than the inlet diameter in front of the valve, and the bypass pipe should be installed for maintenance.
- 5. The equalizing pipe of diaphragm pressure reducing valve should be connected to the low-pressure pipeline. Low pressure pipeline should be set with safety valve to ensure the safe operation of the system.
- 6. When used for steam decompression, drain pipe should be set. For the pipeline system which requires higher purification degree, a filter should be set in front of the pressure reducing valve.
- 7. After the installation of the pressure reducing valve group, the pressure test, flushing and adjustment shall be carried out for the pressure reducing valve and safety valve according to the design requirements, and the adjusted marks shall be made.
- 8. When flushing the pressure reducing valve, close the inlet valve of the pressure reducer and open the flushing valve for flushing.
Installation of drain valve
The basic function of steam trap is to discharge condensate, air and carbon dioxide gas in the steam system as soon as possible; at the same time, it can automatically prevent steam leakage to the maximum extent. There are many kinds of traps with different performance.
According to the different working principle of the steam trap, it can be divided into the following three types:
Mechanical type: The operation depends on the change of condensate water level in steam trap, including:
- Floating ball type: The float is a closed hollow sphere.
- Open up float type: The float is bucket type with opening upward.
- Open downward float type: The float is bucket type with opening downward.
Thermal static type: It depends on the change of liquid temperature, including:
- Bimetallic sheet: The sensitive element is bimetallic sheet.
- Steam pressure type: The sensitive element is bellows or ink cartridge, which is filled with volatile liquid.
Thermal power type: It depends on the change of thermokinetic properties of liquid.
- Disc type: Due to the same pressure, the flow rate of liquid and gas is different, resulting in different dynamic and static pressure, which drives the disc valve to act.
- Pulse type: When condensate water with different temperatures passes through the throttle orifices in series, different pressures will be formed between the two orifices to drive the valve disc to move.
Installation of drain valve
- 1. Block valve (stop valve) should be set in front and back, and a filter should be set between drain valve and front block valve to prevent dirt in condensate from blocking the drain valve.
- 2. An inspection pipe should be set between the drain valve and the rear block valve to check whether the drain valve works normally. If the inspection pipe is opened to emit a lot of steam, it means that the drain valve is broken and needs to be repaired.
- 3. The bypass pipe is set to discharge a large amount of condensate during start-up and reduce the discharge load of drain valve.
- 4. When the steam trap is used to drain the condensate water from the thermal equipment, it should be installed at the lower part of the heating equipment, so that the condensate pipe can be vertically returned to the steam trap to prevent water storage in the thermal equipment.
- 5. The installation position should be close to the drainage point as far as possible. If the distance is too far, air or steam will accumulate in the long and thin pipe in front of the trap.
- 6. Drainage should be considered when the horizontal pipeline of steam main is too long.
Installation of safety valve
The safety valve is a special valve which is normally closed under the action of external force. When the medium pressure in the equipment or pipeline increases more than the specified value, the medium pressure in the pipeline or equipment can be prevented from exceeding the specified value by discharging the medium to the system.
Safety valve is an automatic valve, mainly used in boilers, pressure vessels and pipelines. The control pressure does not exceed the specified value, which plays an important role in protecting personal safety and equipment operation. The safety valve can only be used after pressure test.
- 1. Before installation, the product must be carefully checked to see if there is a certificate and product manual, so as to clarify the constant pressure when leaving the factory.
- 2. The safety valve shall be arranged near the platform as far as possible for inspection and maintenance.
- 3. The safety valve should be installed vertically, and the medium should flow out from the bottom to the top, and the verticality of the valve rod should be checked.
- 4. In general, block valve can not be set before and after the safety valve to ensure safety and reliability.
- 5. Safety valve pressure relief: when the medium is liquid, it is generally discharged into the pipeline or closed system; when the medium is gas, it is generally discharged to the outdoor atmosphere;
- 6. The oil and gas medium can be discharged into the atmosphere generally. The vent pipe outlet of the safety valve should be 3 m higher than the surrounding highest structure, but the following conditions should be discharged into the closed system to ensure safety.
- 7. The diameter of the population pipeline shall be the minimum equal to the inlet diameter of the valve; the diameter of the discharge pipe shall not be less than the outlet diameter of the valve, and the discharge pipe shall be led to the outdoor and installed with elbow to make the pipe outlet face the safety zone.
- 8. When the safety valve is installed, when the connection between the safety valve and equipment and pipeline is open welding, the opening diameter shall be the same as the nominal diameter of the safety valve.
Common faults and causes of valves
Packing box leakage causes and maintenance methods
|
Cause of failure |
Maintenance method |
|
Incorrect filling method (e.g. the whole piece is coiled) |
Correct packing |
|
The valve rod is deformed or corroded and rusted |
Repair or replace |
|
Packing aging |
Replacement of packing |
|
Improper or excessive operation force |
Slow opening and slow closing, stable operation |
Causes and maintenance methods of valve rod failure
|
Cause of failure |
Maintenance method |
|
Valve rod damage, corrosion and tripping |
Replace valve parts |
|
Bent stem |
When the valve is not easy to open, do not pry the handwheel with a long appliance, and the bent stem needs to be replaced |
|
Stem nut tilt |
Replace valve or valve |
|
Open air valve rusted |
Open air valves should be maintained and the hand wheel should be rotated regularly |
Leakage reason and maintenance method of sealing surface
|
Cause of failure |
Maintenance method |
|
The sealing surface is worn and slightly corroded |
Regular grinding |
|
Improper closing, poor contact of sealing surface |
Slowly and repeatedly open and close several times |
|
The valve rod is bent and the upper and lower sealing surfaces are not in the center line |
Repair or replace |
|
The valve hole is blocked by impurities |
Open, remove debris, then slowly close, add filter if necessary |
|
The sealing ring does not fit tightly with the valve seat and disc |
Repair |
|
The connection between valve disc and valve stem is not firm |
Repair or replace |
Other faults, causes and maintenance methods
|
fault |
Cause of failure |
Maintenance method |
|
Gasket leak |
The gasket material is not suitable or is affected by medium in daily use |
Use the gasket suitable for the working conditions or replace the gasket |
|
Cracked valve |
Freezing damage or excessive force during installation of threaded valve |
Heat preservation and antifreeze, even and proper force during installation |
|
Hand wheel damaged |
Heavy object impact, long pole pry open, inner square hole wear chamfering |
Avoid impact, even force when opening, correct direction, file square hole or replace hand wheel |
|
Broken gland |
Uneven force when tightening the gland |
Tighten the nuts symmetrically |
|
Ram failure |
The wedge was not closed tightly due to corrosion, and the top wedge of double ram was damaged |
Grind regularly and replace it with carbon steel |
Common faults and causes of automatic valves
Common faults, causes, prevention and maintenance of check valve
|
Fault |
Cause of failure |
Maintenance method |
|
Backflow of medium |
1. Sealing surface damage between valve core and valve seat 2. Dirt between valve core and valve seat |
1. Grind sealing surface 2. Remove dirt |
|
Valve core does not open |
1. The sealing surface is stuck by scale 2. Shaft rusted |
1. Remove scale 2. Polish rust to make it flexible |
|
Disc broken |
The medium pressure in front of and behind the valve is close to the balance of “sawing” state, which makes the valve disc made of brittle material beat frequently |
The disc is made of ductile material |
Common faults, causes, prevention and repair of steam trap
|
Fault |
Cause of failure |
Maintenance method |
|
No drainage |
1. Steam pressure too low 2. Steam and condensate do not enter the trap 3. The pontoon is too light 4. Float stem stuck with sleeve 5. Valve hole or passage blocked 6. The thermostatic valve core is broken and the valve hole is blocked |
1. Adjust steam pressure 2. Check whether the steam pipe valve is closed and blocked 3. Add or replace the buoy properly 4. Repair or replace to make it flexible 5. Remove the clogged debris and install a filter in front of the valve 6. Valve core replacement |
|
Exhaust steam |
1. Valve core and valve seat wear and steam leakage 2. The drain hole cannot be closed by itself 3. The pontoon is small in size and cannot float |
1. Grind sealing surface 2. Check for dirt blockage 3. Increase pontoon volume appropriately |
|
Continuous working temperature drop |
1. The water displacement is lower than that of condensate. 2 |
1. Replace the appropriate steam trap 2. Installation of steam trap |
Common faults, causes, prevention and maintenance of pressure reducing valve
|
Fault |
Cause of failure |
Maintenance method |
|
The pressure behind the valve is unstable |
1. Pulse type is improper selection of valve diameter, medium pressure difference at both ends 2. Improper selection of spring type adjusting spring |
1. Replace the pressure relief valve as appropriate 2. Replace the appropriate adjusting spring |
|
The valve is blocked |
1. Control channel blocked by debris 2. The rust in the piston is stuck and cannot move down at the highest position |
1. Remove sundries and install filter in front of valve 2. Repair the piston to make it flexible |
|
Valve straight through |
1. The piston is stuck in a certain position 2. Lower main disc spring valve 3. The handle of pulse valve is stuck at the close position 4. The valve seat is jammed or seriously corroded 5. Film failure |
1. Repair the piston to make it flexible 2. Replace the spring 3. Overhaul and make it flexible 4. Remove dirt and grind sealing surface regularly 5. Film replacement |
|
The pressure behind the valve cannot be adjusted |
1. Failure of regulating spring 2. There is leakage in the cap and the pressure cannot be maintained 3. Wear or corrosion of piston and cylinder 4. The valve body is filled with condensate |
1. Replace the adjusting spring 2. Timely maintenance and replacement of gasket 3. Repair the cylinder and replace the piston ring 4. Loosen the plug and drain the condensate |
Common faults, causes, prevention and repair of safety valve
|
Fault |
Cause of failure |
Maintenance method |
|
Leakage of sealing surface |
1. There is dirt or wear on the sealing surface of valve core and valve seat 2. The center line of valve rod is not correct |
1. Remove dirt or grind seal 2. Straighten the center line of the valve stem |
|
Do not open when exceeding working pressure |
1. Lever stuck or pin rusted 2. The lever weight is moved 3. The spring is deformed or invalid when heated 4. The valve core is stuck with the valve seat |
1. Overhaul the lever or pin 2. Adjust the position of heavy hammer 3. Replace the spring 4. Exhaust test shall be conducted regularly |
|
Open before working pressure |
1. The heavy hammer of the lever moves inward 2. There is not enough spring force in the spring type |
1. Adjust the position of heavy hammer 2. Tighten or replace spring |
|
The valve core does not close automatically after opening |
1. Lever deflection 2. Spring bending 3. The valve core or valve stem is not correct |
1. Overhaul lever 2. Adjusting spring 3. Adjust the spool or stem |
Source: China Valve Supplier: www.epowermetals.com
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, 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.)
If you want to have more information about the article or you want to share your opinion with us, contact us at [email protected]
