Knowledge of Valve Installation for Pipeline (Part Two)

Installation of safety valves
Safety valves are special valves whose opening and closing parts are commonly closed under the action of external force. When the medium pressure in equipment or pipeline rises above the specified value, it can prevent the medium pressure in pipeline or equipment from exceeding the specified value by discharging the medium outside the system.

Safety valves belong to automatic valves, which are mainly used in boilers, pressure vessels and pipelines. The controlled pressure does not exceed the specified value, which plays an important role in protecting personal safety and equipment operation. Note that the safety valve must undergo a pressure test before it can be used.

1. Before installation, the product must be carefully inspected to verify whether there is a certificate of approval and product specification, so as to clarify the constant pressure situation at the factory.

2. Safety valves should be arranged around the platform as near as possible for inspection and maintenance.

3. The safety valve shall be installed vertically, and the medium shall flow out from bottom to top. The verticality of the valve stem shall be checked.

4. In general, block valves should not be set before and after the safety valve to ensure safety and reliability.

5. Relief by safety valve:
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. Oil and gas medium can be discharged into the atmosphere generally, and the outlet of vent pipe of the safety valve should be 3 meters 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 inlet pipeline shall be equal to the minimum inlet pipe diameter of the valve. The diameter of the discharge pipe shall not be less than the outlet diameter of the valve. The discharge pipe shall be led to the outdoor and installed with a pipe bend so that the outlet of the pipe faces the safety zone.

8. When the safety valve is installed, and the connection between the safety valve and equipment and pipeline is open-hole welding, the open-hole diameter shall be the same as the nominal diameter of the safety valve.
 
Common faults and causes of valves:
1. Causes of stuffing box leakage and its maintenance methods 

Cause of failure: The filling method is incorrect (e.g., the whole root is placed in a spiral way).
Maintenance method: Correct the packing.
 
Cause of failure: Deformation or corrosion of valve stems.
Maintenance method: Repair or replace them.
 
Cause of failure: The packing turns aging.
Maintenance method: Replace the packing.
 
Cause of failure: improper operation or excessive force
Maintenance method: slow opening and closing, stable operation
 
2. Causes of valve stem failure and maintenance methods

Cause of failure: valve stem damage, corrosion and tripping
Maintenance method: Replace valve parts.
 
Cause of failure: The valve stem is bent and twisted.
Maintenance method: When the valve is difficult to open, do not pry the handwheel with a long tool, and the bent valve stem needs to be replaced.
 
Cause of failure: Valve stem nut inclines.
Maintenance method: Replace valve parts or valves.
 
Cause of failure: The outdoor valve rusts out.
Maintenance method: The outdoor valve should get strengthened maintenance and the handwheel should be turned regularly.
 
3. Causes of sealing surface leakage and its maintenance methods
 
Cause of failure: The sealing surface is worn and slightly corroded.
Maintenance method: regular grinding
 
Cause of failure: improper closing and poor contact of sealing surface
Maintenance method: Slowly and repeatedly open and close it several times.
 
Cause of failure: The valve stem is bent, and the upper and lower sealing surfaces are not aligned with the centerline.
Maintenance method: repair or replacement
 
Cause of failure: Impurities block the valve hole.
Maintenance method: Turn it on; remove sundries, then turn it off slowly, and add filters if necessary.
 
Cause of failure: The sealing ring is not well-matched with the valve seat and the disc.
Maintenance method: Repair it.
 
Cause of failure: The connection between the valve disc and the valve stem is not firm.
Maintenance method: repair or replacement
 
4. Other faults, causes and maintenance methods

Trouble: gasket leakage
Cause of failure: The gasket material is not suitable or fails due to the influence of media in daily use.
Maintenance method: Use gaskets suitable for working conditions or replace gaskets.
 
Trouble: The valve is cracked.
Cause of failure: freezing or excessive force during installation of threaded valve
Maintenance method: Keep warm to prevent freezing, and apply even and proper force during installation.
 
Trouble: damaged handwheel
Cause of failure: heavy objects hit; the long lever pry is not opened, and the inner hole is worn and chamfered.
Maintenance method: Avoid impact; apply even force and correct direction when opening; file square hole or replace handwheel.
 
Trouble: The gland is broken.
Cause of failure: uneven force when pressing the cover tightly
Maintenance method: Tighten the nuts symmetrically.
 
Malfunction: failure of discs
Cause of failure: The wedge disc is not closed properly due to corrosion, and the top wedge of double discs is damaged.
Maintenance method: Grind it regularly and replace it with a top wedge made from carbon steel.
 
Common faults and causes of automatic valves
1. Common failures, causes, prevention and maintenance of check valves

Malfunction: media backflow
Causes of failure:
1. The sealing surface between the valve core and the valve seat is damaged.
2. There is dirt between the valve core and the valve seat.
Maintenance methods:
1. Grind the sealing surface.
2. Remove dirt.
 
Malfunction: The valve core does not open.
Causes of failure:
1. The sealing surface is stuck by scale.
2. The rotating shaft rusts.
Maintenance methods:
1. Remove the scale.
2. Grind rust to make it flexible.

Malfunction: The disc is broken.
Cause of failure: The medium pressure before and after the valve is in a state of dynamic equilibrium close to balance, which makes the valve disc made of brittle material flap frequently.
Maintenance method: Use flexible material discs.
 
2. Common faults, causes, prevention and repair of steam traps

Trouble: no drainage
Causes of failure:
1. The steam pressure is too low.
2. Steam and condensed water did not enter the steam trap.
3. The pontoon-type float is too light.
4. Float-type valve stems are stuck in the casing.
5. The valve hole or channel is blocked.
6. The thermostatic valve core breaks and then blocks the valve hole.
Maintenance methods:
1. Adjust the steam pressure.
2. Check whether the steam pipe valve is closed and blocked.
3. Properly add or replace the buoy.
4. Repair or replacement make it flexible.
5. Remove blocked sundries and install filters before the valve.
6. Replace the valve core.
 
Trouble: exhaust steam
Causes of failure:
1. Wear of the valve core and the valve seat lead to steam leakage.
2. The drain hole cannot be closed by itself.
3. The floating float is small in size and cannot float.
Maintenance methods:
1. Grind the sealing surface.
2. Check whether there is dirt blockage.
3. Appropriately increase the volume of pontoons.
 
Trouble: continuous working temperature drops
Causes of failure:
1. The displacement is lower than the condensate.
2. The amount of condensed water in the pipeline increases.
Maintenance methods:
1. Replace the appropriate steam trap.
2. Install steam traps.
 
3. Common faults, causes, prevention and maintenance of pressure reducing valves

Trouble: unstable pressure behind the valve
Causes of failure:
1. Improper selection of diameter for pulse-type valves
2. The medium pressure difference at both ends is large.2. Improper selection of the adjusting spring for spring-type valves.
Maintenance methods:
1. Replace the appropriate pressure-reducing valve.
2. Replace the appropriate adjusting spring.
 
Trouble: The valve is blocked.
Causes of failure:
1. The control channel is blocked by sundries.
2. Rust in the piston is stuck, so it cannot move down at the highest position.
Maintenance methods:
1. Remove sundries and install a filter before the valve.
2. Overhaul the piston to make it flexible.
 
Trouble: The valve is straight through.
Causes of failure:
1. The piston is stuck in a certain position.
2. The lower spring of the main valve disc is broken.
3. The valve handle of the pulse valve is stuck at the close position.
4. There is dirt stuck or serious corrosion between the main disc and the sealing surface of the valve seat.
5. Failure of the film sheet
Maintenance methods:
1. Overhaul the piston to make it flexible.
2. Replace the spring.
3. Regular maintenance makes it flexible.
4. Remove dirt and grind the sealing surface regularly.
5. Replace the film.
 
Trouble: The pressure behind the valve cannot be adjusted.
Causes of failure:
1. The adjusting spring fails.
2. The cap has leakage, so the pressure cannot be maintained.
3. Wear or corrosion of piston and cylinder
4. The valve body is filled with condensed water.
Maintenance methods:
1. Replace the adjusting spring.
2. Timely maintenance, replacement of gaskets
3. Overhaul the cylinder and replace the piston ring.
4. Loosen the plug and drain the condensed water.
 
4. Common faults, causes, prevention and repair of safety valves
Trouble: leakage of sealing surface
Causes of failure:
1. The sealing surfaces of the valve core and the valve seat are dirty or worn.
2. The centerline of the valve stem is not straight.
Maintenance methods:
1. Remove dirt or grinding the seal.
2. Correct and straighten the centerline of the valve stem.
 
Trouble: It does not open when the working pressure exceeds.
Causes of failure:
1. The lever is stuck or the pin rusts.
2. The lever-type heavy hammer is moved.
3. Spring-type springs are deformed or failed by heating.
4. The valve core is stuck to the valve seat.
Maintenance methods:
1. Overhaul the lever or the pin.
2. Adjust the position of the heavy hammer.
3. Replace the spring.
4. Do exhaust test regularly.
 
Trouble: It is turned on before working pressure.
Causes of failure:
1. The weight of the lever moves inward.
2. Spring-type elasticity is not enough.
Maintenance methods:
1. Adjust the position of the heavy hammer.
2. Tighten or replace the spring.
 
Trouble: The valve core does not close automatically after opening.
Causes of failure:
1. Deflection of the Lever-type lever
2. Bending of the spring-type spring
3. Deflective valve cores or valve stems
Maintenance methods:
1. Overhaul the lever.
2. Adjust the spring.
3. Adjust the valve core or the valve stem.
 
Maintenance of common valves
During the installation and use of the valve, due to reasons such as manufacturing quality and wear, the valve is prone to leak and loosen closure.

1. Glands leakage maintenance
(1) Small valve cover leakage maintenance
(2) Overhaul of large valve gland leakage
 
2. Can't open or open without circulating steam or water.
(1) Gate valves maintenance
(2) Globe valves maintenance
(3) The valve or pipeline is blocked.
 
3. It can't be closed or closed loosely.
(1) Loose closing: It may be that dirt is stuck between the valve seat and the valve core, or the valve seat and the valve core are scratched, so that the valve cannot be closed tightly.
(2) Can't shut it down: The force used is too large to open.
 
Valve grinding
When the depth is less than 0.05mm, all defects can be eliminated by grinding.
Using corundum (AI203), artificial corundum powder, carborundum (SiC), boron carbide, red iron powder, chromium oxide, diatomite, glass powder, diamond powder and grinding paste, etc. The most commonly used one is carborundum.

1. When grinding sealing surfaces made from cast iron, copper, bronze and brass: corundum (AI203), artificial corundum powder
2. When grinding the nitriding steel sealing surface: artificial corundum powder
3. When grinding the sealing surface made from cemented carbide: silicon carbide (SiC) and boron carbide
 
Grinding tools:
The best grinding tool material is pig iron, followed by soft steel and copper. When grinding globe valves, lifting check valves and safety valves, the sealing surface on the disc and the sealing ring on the valve seat can be directly ground against each other or separately.
 
Lubricant: Different lubricants are required for different grinding tools.
1. When using pig iron grinding tools, use kerosene as a lubricant.
2. When using mild steel grinding tools, use engine oil.
3. When using copper grinding tools, use engine oil, alcohol or alkaline water (alkaline water is natural alkali, and its main components are sodium carbonate and potassium carbonate).