1 Overview

Basic knowledge of design:

· Design requirements

· Design document

· Design conditions

Pressure vessel design:

According to the given process design conditions, following the current specifications and standards, under the premise of ensuring safety, the material is economically and correctly selected, and the structure, strength (stiffness), and sealing design are carried out.

Structural design - Determine reasonable and economical structural forms to meet the requirements of manufacturing, inspection, assembly, transportation and maintenance.

Strong (rigid) design - Determine the structural dimensions to meet the strength or stiffness and stability requirements to ensure safe and reliable operation of the container.

Sealing design - Choose the right sealing structure and material to ensure good sealing performance.

1.1 Design Requirements

The unification of security and economy

· Security is the premise, economy is the goal, and economy should be achieved as far as possible under the premise of fully guaranteeing security. Safety refers primarily to structural integrity and tightness. Economy includes high efficiency, saving of raw materials, economical manufacturing methods, low operating and maintenance costs, etc.

1.2 Design Documents

Design documents include: design drawings, technical conditions, design calculation, if necessary, should also include design or installation, use of the story. A stress analysis report should also be provided if designed according to the analytical design criteria.

The expression form of design is the embodiment of the designer's labor

Design calculation:

· Including design conditions, specifications and standards used, materials, corrosion margin, calculated thickness, nominal thickness, calculation results, etc.

· For a pressure vessel equipped with a safety discharge device, the safe discharge amount of the pressure vessel, the displacement of the safety valve and the discharge area of the burst disc shall also be calculated.

· When computer software is used for calculation, the software must be reviewed and evaluated by the "National Technical Committee for Standardization of Boiler and Pressure Vessels", and certified and filed with the Special Equipment Bureau of the General Administration of Quality Supervision, Inspection and Quarantine. The printed results should contain the software program number, input data and calculation results.

Design drawings - including general drawings and parts drawings

General drawing

· Including name, category and design conditions of pressure vessel;

· If necessary, the service life of the pressure vessel should be indicated;

· Material, grade and material requirements of main pressure components;

· Main characteristic parameters (such as volume, heat exchange area and number of passes of heat exchanger, etc.);

· Manufacturing requirements, heat treatment requirements, corrosion prevention requirements, non-destructive testing requirements;

· Requirements for pressure test and air tightness test, specifications of safety accessories;

· Location of pressure vessel nameplates, packaging, transportation, on-site welding and installation requirements, and other special requirements.

1.3 Design Conditions

Process design conditions - original data, process requirements (commonly used design conditions diagram).

The design condition diagram includes schematic diagram, user requirement, takeover table, etc

Schematic drawing - Schematic drawing of the container body, the structure size of the main internals, the position of the nozzle, the form of the support and other content to be expressed.

User requirement

(1) Working medium: medium scientific name or molecular formula, main components, specific gravity and harm;

(2) Pressure and temperature: working pressure, working temperature, ambient temperature, etc.;

(3) Operation mode and requirements: indicate continuous operation or gap operation, and whether the pressure and temperature are stable; When the pressure and temperature fluctuate, the frequency of change should be indicated

Rate and range of variation; Containers with open and frequent parking should indicate the number of driving and parking times per year;

(4) Others: It should also indicate the volume, material, corrosion rate, design life, whether to bring safety devices, whether to keep heat, etc.

The design condition diagram mainly has the following types:

General container condition diagram

Heat exchanger condition diagram: should indicate the heat exchange tube specifications, tube length and number of roots, arrangement form, heat exchange area and number of processes;

Tower condition diagram: should indicate the tower type (floating valve tower, sieve plate tower or packed tower), the number and spacing of tray, basic wind pressure and seismic design intensity and the type of site soil;

Stirring vessel conditions diagram: should indicate the type of agitator, speed and steering, shaft power, etc.

2 Design Criteria

From the failure form -- > select the failure criterion -- > obtain the corresponding design criteria -- > Judge whether the design is reasonable.

2.1 Pressure vessel failure

Definition: Pressure vessel in the specified use environment and time, due to changes in size, shape or material properties that endanger safety or loss of normal function.

Failure manifestations: leakage, excessive deformation, fracture.

1. Failure form of pressure vessel

(1) Strength failure - pressure vessel failure caused by material yield or fracture, called strength failure, including ductile fracture, brittle fracture, fatigue fracture, creep fracture, corrosion fracture, etc.

Ductile fracture is the fracture that occurs when the stress of the pressure vessel under the action of load reaches or approaches the strength limit of the material used.

Features: After fracture, there is macroscopic deformation visible to the naked eye, such as overall swelling, the circumference elongation can reach 10 ~ 20%, and the thickness at the fracture is significantly reduced; No debris, or occasional debris; The bursting pressure calculated according to the measured thickness is quite close to the actual bursting pressure.

Reasons: the wall thickness is too thin (the wall thickness is not designed and the wall thickness is reduced due to corrosion) and the internal pressure is too high (operation error, liquid expansion by heat, chemical reaction is out of control, etc.).

In strict accordance with the code design, material selection, equipped with the corresponding safety accessories, and transportation, installation, use, maintenance follow the relevant regulations, toughness fracture can be avoided.

Brittle fracture refers to the fracture that occurs when the deformation is small and the stress value in the shell wall is much lower than the strength limit of the material. This kind of fracture occurs in a low stress state, so it is also called low stress brittle fracture.

Features: There is no expansion of the container during fracture, that is, there is no obvious plastic deformation; The fracture is flush and perpendicular to the maximum stress direction. The rate of rupture is extremely rapid, often causing the container to break into pieces. Because the actual stress value of the container is often very low during brittle fracture, the safety accessories such as bursting disc and safety valve will not act, and the consequences are much more serious than the ductile fracture.

Reason: brittle material and defects. Improper material selection, welding and heat treatment make material embrittlement; Low temperature, long-term operation at high temperature, strain aging, etc., will also cause material embrittlement; Pressure vessel steel generally has good toughness, but if there are serious original defects (such as slag inclusion of raw materials, delamination, folding, etc.), manufacturing defects (such as non-penetration caused by welding, cracks, etc.) or defects generated in use, it will also lead to brittle fracture.

Fatigue fracture refers to the fracture that occurs after a certain period under the action of alternating load.

Alternating load - a load whose size and/or direction both change periodically (or irregularly) with time. Including pressure fluctuations during operation, driving and stopping, thermal stress changes caused by temperature changes during heating or cooling, stress changes caused by vibration, and alternating loads caused by additional loads caused by the container nozzle. It should be pointed out that cracks generated in the raw material or manufacturing process will also expand under the repeated action of alternating loads, resulting in pressure vessel fatigue.

Fatigue failure includes three stages: crack initiation, crack propagation and crack failure. The fatigue fracture is composed of crack source, crack propagation zone and final fracture zone.

Crack source - often located in the nozzle root, welded joints and other high-stress areas or defective parts.

· Crack propagation zone -- it is the most important characteristic area of fatigue fracture. It often shows the shape of shell, which is the trace left by the fatigue crack propagation process.

· The final fracture zone - the rapid fracture zone when the crack extends to a certain extent. The failure form caused by the remaining section can no longer withstand the applied load - "leakage before explosion", and the destruction needs a certain time.

Creep fracture means that the pressure vessel is loaded at high temperature for a long time, and the material continues to creep deformation with the increase of time, resulting in significant wall thickness thinning and bulging deformation, and ultimately leading to the fracture of the pressure vessel. It is characterized by ductile fracture from deformation and brittle fracture from stress.

Corrosion fracture has the characteristics of ductile fracture (thinning of uniform corrosion and pitting of local corrosion)/brittle fracture (fracture caused by intergranular corrosion and stress corrosion)

) Features.

(2) Stiffness failure - the failure caused by the deformation of the pressure vessel is large enough to affect its normal operation, such as the tower is subjected to wind.

(3) Instability failure - under the action of compressive stress, the pressure vessel suddenly loses its original regular geometry caused by failure.

(4) Leakage failure - failure caused by leakage. Its harm may cause poisoning, burning and explosion accidents, resulting in environmental pollution.

(5) Interactive failure - in practice, multiple forms of failure may occur simultaneously.

2. Failure criteria and design criteria

Design idea: Obtain the mechanical response (such as stress, strain, natural frequency, etc.) of the pressure vessel under steady or transient conditions, and determine the limit value of the mechanical response according to the most likely failure form of the pressure vessel to judge whether the pressure vessel can be used safely or whether it can obtain satisfactory results.

Failure criterion - Compare the results of mechanical analysis with the results of simple experimental measurements to determine whether the pressure vessel will fail. This criterion is called the failure criterion.

Design criteria -- According to the failure criteria, then consider various uncertainties, introduce the safety factor, and get the design criteria corresponding to the failure criteria.

Category: Strength failure design criteria

Stiffness failure design criteria

Design criteria for instability failure

Leakage failure design criteria

When designing the pressure vessel, first determine the most possible failure form, then select the appropriate failure criteria and design criteria, determine the design standard to use, and then design and check according to the standard requirements.

2.2 Strength failure design criteria

There are two main forms of strength failure: yield and fracture.

Common strength failure design criteria:

Elastic failure design criteria (ductile materials) - the initial yield of the overall part of the container is regarded as a failure.

1, one-way tensile - maximum tensile stress criteria

image.png

2, arbitrary stress state

(1) Maximum shear stress criterion

Tresca yield failure criterion - maximum shear stress yield failure criterion - third strength theory

image.png       image.png

(2) Specific energy criterion of shape change

-- Shape change specific energy failure treason -- fourth strength theory

image.png

(4-5)

3, stress intensity or equivalent stress

Uniform elastic failure design criteria: image.png

image.png

2. Plastic failure design criteria

1. Comparison of elastic failure criteria and plastic failure criteria:

Elastic failure design criteria Plastic failure design criteria according to the stress strength of the danger point to reach the allowable stress throughout the danger surface yield Application of ductile materials throughout the stress distribution is uniform, such as thin-walled containers ductile materials stress distribution is not uniform, such as thick-walled cylinders

2, plastic failure design criteria

-- Ideal elastic-plastic material, internal pressure thick wall cylinder

By image.png -- > image.png,P -- design pressure,Pso -- full yield pressure,image.png -- full yield safety factor

3. Design criteria for blasting failure

-- Vessel bursting as proof of failure -- Pressure vessels generally have strain hardening phenomenon. Bursting pressure is greater than full yield pressure

Blasting failure design criteria: image.png

4. Elastic-plastic failure design criteria

Also known as the stability criterion, it is believed that when the load variation range reaches the stability load, the container will fail.

Application: Suitable for a variety of loads do not increase in the same proportion, the size of the load changes repeatedly.

Initial yield load - the load corresponding to the maximum stress point entering the plasticity.

Stable state -- > The container bears a load slightly larger than the initial yield load -- > a small amount of local plastic deformation -- > residual stress field -- > If the container is subjected to a small load -- > less than the yield point after stress superposition -- > maintain elastic behavior -- > no new plastic deformation -- > "stable" state; -- > The load continues to increase -- > reverse yield, or accumulation of plastic deformation -- > loss of stability -- > gradual plastic deformation.

Stable load - the range of load variation corresponding to stable and unstable critical states.

Engineering: Because the container is not immediately destroyed after exceeding the stable load, the safety factor of the stable load is = 1.0, and the maximum load variation range is < the stable load.

5. Fatigue failure design criteria

Low cycle fatigue - the material will produce a certain plastic strain in each cycle, and the number of cycles when fatigue failure is low, generally less than 105 times.

Low cycle fatigue design curve - the curve of the relationship between the virtual stress amplitude and the number of allowable cycles obtained by experiment and theory.

Fatigue failure design criteria -- maximum virtual stress amplitude The allowable number of cycles determined by the low-cycle fatigue design curve is greater than the number of cycles required by the container, and the container will not suffer fatigue failure.

The theory of fracture mechanics -- the fatigue design criteria of pressure vessels with cracks, that is, the safety assessment of vessels under cyclic load is made according to the law of fatigue crack propagation and fracture.

6. Design criteria for creep failure

The stress is limited to the allowable stress determined by the creep limit and the lasting strength.

7. Brittle fracture failure design criteria

Brittle fracture - belongs to the research scope of fracture mechanics, which considers that there are defects in the material, and studies the failure law of the defects under the action of load and environment.

Application of fracture mechanics -- (1) Guiding material selection and design of pressure vessels (2) Safety assessment of in-service pressure vessels

Brittle fracture failure design criteria:

(1) Damage safety design - assuming that cracks exist, the structure can withstand the working load - container crack tolerance problem.

(2) Leakage after explosion design - the material has enough toughness, before rapid fracture, the crack has penetrated the wall thickness, resulting in leakage, which can avoid sudden rapid fracture and reduce losses

Description: Assumed crack, real crack (missed or in use)

Prevent brittle damage to containers:

(1) Material - According to the thickness of the compression element, stress level, the lowest metal temperature, load properties, the influence of the medium on the toughness of the material, put forward the material Charpy V notch impact work or fracture toughness acceptance indicators.

(2) Defects - minimize welded joints; Improve nondestructive testing technology.

(3) Design - from the level of nondestructive testing - assuming the presence of cracks in the high stress zone - using fracture methods - crack safety assessment - to ensure that the vessel does not suffer from low stress brittle failure.

2.3 Stiffness failure design criteria

Under the action of load, the elastic displacement and (or) Angle of the component are required to not exceed the specified value.

image.png(4-9)

2.4 Design criteria for stable failure

-- Prevent instability (circumferential, axial, local)

2.5 Leakage failure design criteria

Tightness, a concept used to compare or evaluate tightness's effectiveness for leaks, is standard. Tightness is expressed by the volume or mass of the sealed fluid passing through the leakage channel in unit time, that is, the leakage rate. Leakage and non-leakage (or zero leakage)