Causes:
1. The poor operation makes the closing parts vulnerable to deadlocking or exceeding the top dead center and the joint is damaged and fractured.
2. The connection of the closing parts is too loose.
3. The material of the connector can not withstand the corrosion of the medium and the wear of machines because of the wrong choice.
Solutions:
1. Operate the valve correctly. Don’t use excessive force when closing the valve. And the valve cannot exceed the top dead center when being opened. After it is fully opened, the handwheel should be reserved a little.
2. The connection between the closing parts and the valve stem should be firm, and the non-return parts need to be installed on the threaded joint.
3. The fasteners for connection should be anti-corrosion and have certain mechanical strength and wear-resisting properties.
II. Leakage from the Packing
Causes:
1. The selection of the packing is not correct, with the packing being easy to corrode and not resistant to high pressure or vacuum, high and low temperatures.
2. The packing is installed in the wrong way with defects like replacing the big one with the small one, poor spiral wound joints and different degrees of tightness.
3. The packing has expired and lost its elasticity.
4. The accuracy of the valve stem is not high with defects like bending, corrosion and wear.
5. The number of packing rings is insufficient, and the gland is not pressed well.
6. The gland, bolts and other parts are damaged so that the gland can’t be compressed.
7. Improper operation with excessive force.
8. The gland is skewed, and the gap between the gland and the stem is too small or too big, causing the wear of the stem and the damage of the packing.
Solutions:
1. The choices of packing materials and types need to be in accordance with working conditions.
2. Install the packing correctly according to the relevant provisions. The packing ought to be placed turn-by-turn, and the joint should be 30° or 45°.
3. The packing which has been aging, damaged or used for a long time should be replaced in time.
4. The valve stem needs to be straightened and repaired after being warped and worn, and should be replaced if being destroyed seriously.
5. The packing should be installed according to the specified number of turns, with the gland being tightened symmetrically and evenly and the cover being kept a pre-tightening gap of more than 5mm.
6. Repair or Replace the damaged gland, bolt and other parts timely.
7. Maintain a uniform speed except for operating the percussive handwheel.
8. Tighten the gland and the bolt evenly and symmetrically. The gap should be properly increased if it is too small. And replace the parts if the gap is too large.
III. Leakage from the Sealing Surface
Causes:
1. The sealing surface is ground unevenly so that a tight line cannot be formed.
2. The joint center of the valve stem and closing part hangs in the air, or the joint is skewed or worn.
3. The assembly of the stem is incorrect.
4. Select the material of the sealing face improperly. And do not follow the working conditions when choosing valves.
Solutions:
1. Choose the right materials and types of gaskets according to the working condition.
2. Adjust carefully and operate smoothly.
3. Tighten the bolts with even force. Apply a torque wrench if necessary. The pre-tightening force needs to meet the requirements, neither too big nor too small. And the joint between the flange and the thread should have a pre-tightening gap.
4. The gasket should be adjusted in the middle with a uniform force. And it is not allowed to overlap or use the double gasket.
5. For the corrosion, damage and poor quality of static sealing surface, we should repair and grind it, and carry out the dye check to make it satisfy the requirements.
6. Pay attention to clean the gasket when installing. And the sealing face should be cleaned with kerosene, and the gasket shouldn’t be dropped.
IV. Leakage from the Valve Body and the Bonnet
Causes:
1. The casting quality of the iron castings is not high, and there are defects like blisters, loose textures and slags on the valve body and the bonnet.
2. Froze and burst in the cold.
3. Poor welding causes slags, stress cracking and other defects.
4. Cast iron valves are damaged after being hit by heavy objects.
Solutions:
1. Improve the quality of the casting. And conduct the strength test strictly according to rules before installing.
2. For valves with temperatures at 0℃ and below 0℃, methods of insulation or heat tracing need to be applied. And water in valves we stop using needs to be drained.
3. The operating instructions of welding need to be carried out accordingly. And the flaw detection and the strength test should be conducted for valves after welding.
4. It is forbidden to put heavy objects on the valves and impact cast iron and non-metal valves with hammers. The large-caliber valves need to be install
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