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Maintenance and overhaul of pressure vessels

Maintenance and overhaul

The pressure vessel is generally composed of six parts: cylinder, head, flange, sealing element, opening and connection pipe, and support. In addition, it is also equipped with safety devices, meters and internal parts for different production processes.

20201211002311 12275 - Maintenance and overhaul of pressure vessels

Pressure vessels are prone to explosions, combustion and fires due to sealing, pressure-bearing and medium reasons, which endanger the safety of personnel, equipment and property and pollute the environment.

Therefore, in order to ensure the safe production of chemical plants, the quality of daily maintenance and overhaul of pressure vessels is particularly important. At the same time, the quality of daily maintenance and overhaul of pressure vessels has laid a solid foundation for the stable, stable, long, full and optimal operation of the device.

Periodic inspection

The key points of inspection are as follows:

  • ① Cracks, overheating, deformation, leakage and damage of pressure vessel body, interfaces (valves, pipelines), welding joints, etc.;
  • ② Corrosion on the outer surface;
  • ③ The insulation layer is damaged, falling off, damp, cold running;
  • ④ Abnormal vibration, noise, and mutual friction between the pressure vessel and adjacent pipes or components;
  • ⑤ The damage of the support or the support, the sinking, tilting and cracking of the foundation, the integrity of the fastening bolts;
  • ⑥ Drainage (drainage, sewage) device;
  • ⑦ Stable operation; whether there are over-temperature, over-pressure and over-load operation;
  • ⑧ Whether the pressure vessel grounding facilities are intact;
  • ⑨ Pressure vessel monitoring measures and abnormal conditions with a safety status of level 4.

Anomalies

When one of the following abnormal phenomena occurs in the pressure vessel, the operator should take emergency measures immediately and report to the relevant department as required:

  • ① The working pressure, medium temperature or wall temperature, and a certain corrosive component in the medium composition exceed the allowable value, and the measures cannot be effectively controlled;
  • ② The main pressure components have cracks, bulges, deformations, leaks and other defects that endanger safety;
  • ③ Failure of safety accessories;
  • ④ The pipes and fasteners are damaged, making it difficult to ensure safe operation;
  • ⑤ The occurrence of fire directly threatens the safe operation of pressure vessels;
  • ⑥ Overfilling;
  • ⑦ The liquid level is out of control and cannot be effectively controlled even after taking measures;
  • ⑧ Serious vibration of pressure vessel and pipeline endangers safe operation;
  • ⑨ Other abnormal conditions.

Regular maintenance

  • 1. The user unit should combine equipment rectification and equipment repair to regularly solve and eliminate the phenomenon of running, emitting, dripping, leaking and poor sealing of the container. And according to the conditions of the container itself, the container should be regularly anti-corrosion and heat preservation work. The main bolts of the container should be greased regularly, and other bolts and fasteners should also be regularly anti-rust work.
  • 2. Vessels that are out of service and reserved for storage should be inspected, maintained, and “four-proof” (anti-freezing, dust-proof, moisture-proof, and anti-corrosion) work should be carried out regularly.

Emergency handling

When one of the following abnormal phenomena occurs, the operator has the right to take emergency measures and report it in time:

  • a. When the working pressure of the vessel, the temperature of the medium or the temperature of the vessel wall exceeds the specified value, and the vessel is overloaded, it cannot be effectively controlled after taking measures;
  • b. The main pressure components of the container or the container containing flammable, explosive, harmful, and moderately hazardous media are found to have cracks, bulges, deformations, leaks and other defects that endanger safety;
  • c. When a fire occurs at the position where the container is located or an accident of adjacent equipment has directly endangered the safe operation of the container;
  • d. When overfilling the container endangers safety;
  • e. When it is difficult to ensure the safe operation of the container due to the damage of the tube and fasteners of the container;
  • f. When the liquid level of the container is out of control and cannot be effectively controlled after taking measures;
  • g. When the container and adjacent pipelines vibrate and endanger the safe operation of the container;
  • h. The safety device fails and cannot be adjusted, endangering safe operation;
  • i. Occurrence of other circumstances that do not allow the container to continue to operate in the safety production technical regulations.

Pressure vessel failure and preventive measures

Pressure vessel failure refers to the phenomenon that the pressure vessel completely loses the original design function or fails to meet the original design requirements due to changes in the structural size, shape, and material properties within the specified use environment and life span, and cannot be used normally.
Common failure modes of pressure vessels can be roughly divided into four categories: strength failure, stiffness failure, instability failure and leakage failure.
Failure of pressure vessel strength:
The failure mode of a pressure vessel caused by material yielding or fracture under the action of pressure and other loads is called strength failure. It usually includes five situations: ductile fracture, brittle fracture, fatigue fracture, corrosion fracture, and creep fracture.

Ductile fracture

Ductile rupture is when the internal pressure of the container exceeds the safety limit, plastic deformation occurs first, and rupture occurs as the pressure continues to increase.
Features:

  • 1. The internal pressure is too high, exceeding the maximum working pressure of the vessel, and the design pressure has reached the burst pressure value of the vessel.
  • 2. Before the container ruptures, the container has obvious deformation, and the wall of the rupture is significantly thinned.
  • 3. Generally, no fragments fly out of the container with ductile rupture, only one opening.
  • 4. The fracture is torn.

The reason: 

  • 1. Violation of operating procedures, operating errors cause overpressure.
  • 2. The instrument control system is malfunctioning.
  • 3. The overpressure relief device fails.
  • 4. The storage of liquefied gas is seriously overfilled, resulting in too small gas phase space and overpressure when the temperature rises.
  • 5. The thickness of the container becomes thinner due to corrosion.

Precaution:

  • 1. Strictly abide by the safety operation regulations;
  • 2. Frequently check the flexibility and accuracy of instruments and safety devices;
  • 3. It is strictly forbidden to run over load or over temperature;
  • 4. Do the maintenance during operation.

Brittle fracture

Within the normal pressure range, the failure of the pressure vessel to rupture or explode without or without sufficient plastic deformation is called brittle fracture.
The reason:

  • 1. The brittle transition of materials;
  • 2. There are serious defects in the welding interface.

Features:

  • 1. The container has no macroscopic plastic deformation or small deformation;
  • 2. The container wall is not thinned, broken or occurs under low pressure.
  • 3. There may be fragments when broken;
  • 4. Brittle fracture mostly occurs when the temperature is low or the temperature changes suddenly.

Precaution:

  • 1. Choose materials with fewer defects and appropriate toughness;
  • 2. Design the structure to minimize stress concentration and take measures to eliminate residual stress;
  • 3. Before using the container, check it carefully according to the regulations.

Fatigue rupture

Due to the frequent pressurization and decompression of the container, the material is subjected to alternating stress, which causes the container to rupture after long-term use.
Three stages of fatigue fracture:

  • One is the fatigue crack forming stage;
  • The second is the crack fatigue growth stage;
  • The third is the stage of fatigue fracture.

Precaution:

  • 1. Minimize stress concentration in the design, adopt reasonable structure and manufacturing technology;
  • 2. Choose suitable anti-fatigue materials;
  • 3. Minimize unnecessary pressurization and depressurization times;
  • 4. Strictly control pressure and temperature fluctuations.

Corrosion cracking

General corrosion and local corrosion
General corrosion: Corrosion occurs evenly on the entire metal surface.
Localized corrosion: including regional corrosion, pitting corrosion, intergranular corrosion, stress corrosion and corrosion fatigue.
Preventive measures: Choose corrosion-resistant materials, try to reduce stress and stress concentration, and adopt various measures that can reduce the corrosion of the medium.

Creep failure

The parent material of the pressure vessel is subjected to tensile stress at high temperature for a long time, and the plastic deformation that occurs slowly is called creep. The rupture of the vessel caused by the creep of the material is called creep rupture. 【Creep rupture of containers is rare. 】
1. Pressure vessel stiffness failure
Caused by excessive elastic deformation of the pressure vessel.
2. Pressure vessel instability and failure
Under pressure, the container suddenly loses its original regular geometric shape and causes failure. The important feature of pressure vessel instability failure is that the elastic deflection is not proportional to the load, and the critical pressure has nothing to do with the strength of the material, but mainly depends on the size of the vessel and the elastic properties of the material.
3. Pressure vessel leakage failure
The failure of the sealing surface of various interfaces of the container or the leakage of penetrating cracks on the vessel wall. The leaked medium may cause combustion, explosion and poisoning accidents, and cause serious environmental pollution.
There are many reasons for pressure vessel leakage. Pressure components are subject to frequent vibrations to produce cracks, expansion nozzles loose, local corrosion and thinning of the wall perforation, local bulging deformation and sealing surface failure, etc., will cause the pressure vessel to leak due to leakage. Invalidate.

Common faults and measures for pressure vessels

Overpressure

Approach:

  • 1. The pressure vessel operator operates the corresponding valve and discharge device according to the specific operation plan to reduce the pressure to the allowable range;
  • 2. Immediately notify the process operation and equipment management department to find out the cause and eliminate hidden dangers;
  • 3. The overpressure situation may affect the safe use of related equipment, so you should continue to reduce the pressure immediately until it stops;
  • 4. Check whether the pressure components and safety accessories involved in the overpressure are normal;
  • 5. Repair or replace damaged parts;
  • 6. Detailed records of overpressure, repair and replacement of damaged parts.

Precaution:

  • 1. Observe the process discipline and operate in strict accordance with the process regulations of the pressure vessel system;
  • 2. Strengthen inspections, observe and record the display of related instruments;
  • 3. Strengthen the training of process operators, and be familiar with the process, operating procedures and emergency plans.

Over temperature

Approach:

  • 1. The pressure vessel operator immediately operates the corresponding valve according to the specific operation plan, and the spray device reduces the temperature to the allowable range;
  • 2. Immediately notify the process operation and equipment management department to find out the cause and eliminate hidden dangers;
  • 3. The over-temperature situation may affect the safe use of related equipment, and the temperature and pressure should be continued to drop immediately until it stops;
  • 4. Check the appearance, deformation and other safety conditions of pressure components and safety accessories involved in over-temperature;
  • 5. Repair or replace damaged parts;
  • 6. Record over-temperature conditions, repair and replacement of damaged parts in detail.

Precaution:

  • 1. Observe the process discipline and operate in strict accordance with the process regulations of the pressure vessel system;
  • 2. Strengthen inspections, observe and record the display of related instruments;
  • 3. Strengthen the training of process operators, and be familiar with the process, operating procedures and emergency plans.

Abnormal sound

Approach:

  • 1. The pressure vessel operator immediately observes whether the equipment pressure, temperature and other operating parameters are normal;
  • 2. Immediately notify the process operation and equipment management department to find out the cause;
  • 3. If the cause is unknown, immediately reduce the pressure until the vehicle stops;
  • 4. Check the appearance and deformation of pressure components and safety accessories involved in abnormal noise;
  • 5. Repair or replace damaged parts;
  • 6. Detailed records of over-temperature conditions, repair and replacement of damaged parts.

Precaution:

  • 1. Observe the process discipline and operate in strict accordance with the process regulations of the pressure vessel system;
  • 2. Strengthen inspections, observe and record the display of related instruments;
  • 3. Strengthen the training of process operators, and be familiar with the process, operating procedures and emergency plans.

Abnormal deformation

Approach:

  • 1. The pressure vessel operator operates the corresponding valve according to the specific emergency plan, and immediately reduces the pressure and stops;
  • 2. Notify the process operation and equipment management department to find out the cause;
  • 3. Check the deformed parts;
  • 4. Repair or replace deformed and damaged parts;
  • 5. Detailed records of overpressure, repair and replacement of damaged parts.

Abnormal vibration

Approach:

  • (1) The pressure vessel operator confirms the vibration source and eliminates it according to the specific emergency plan;
  • (2) If it may cause equipment damage, stop for testing.

Precaution:

  • 1. Strict process discipline to avoid abnormal fluctuations of operating parameters;
  • 2. Strengthen patrol inspection, check the integrity of piping system supports and hangers, find problems in time and eliminate hidden dangers.

Source: Network Arrangement – China Pipe Fitting Manufacturer – Yaang Pipe Industry (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 sales@epowermetals.com

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242424 - Maintenance and overhaul of pressure vessels
Article Name
Maintenance and overhaul of pressure vessels
Description
In order to ensure the safe production of chemical plants, the quality of daily maintenance and overhaul of pressure vessels is particularly important.
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www.epowermetals.com
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