Stress-Strain Curve is a graphical plot of a material’s Stress and its Strain. Stress is plotted on the Y-Axis and Strain is plotted on the X-axis. This Stress and Strain curve provides the relation between stress and strain and the material’s stress behavior with an increase in strain. In material science and mechanical engineering, the stress-strain curve is widely used to understand the strength, deformation, and failure criteria of any material. In this article, we will explore details about the stress-strain curve.
Generation of Stress-Strain Curve
The Stress-Strain curve is plotted during the tensile test of a test specimen inside the Universal Testing Machine (UTM). In that instrument, the force on the standard specimen is increased till its failure and a plotter keeps recording the stress and strain.
Fig. 1 shows a typical stress-strain curve of Steel.
Fig. 1: Stress-Strain Curve of Steel
The stress & strain curve shown above describes various engineering parameters as listed below:
Yield Strength:
The Yield Strength of a material is the maximum stress after which the elongation becomes plastic and permanent deformation starts. Once the yield strength of a material is reached, large deformation occurs with very little increase in the applied load. The material will regain its shape once the stress is removed if the yield point is not reached.
In the stress-strain curve, yield strength is the point from where the stress deviates its proportionality to strain. For a few materials, the yield strength in the stress-strain curve is distinct but for a few others, it is not. Hence, a concept of proof stress is used to denote the yield strength in the stress & strain curve for those materials. Proof Stress is indicated by drawing a parallel line to the linear portion of the stress-strain curve at a strain value of 0.002 (or 2%).
Ultimate Tensile Strength:
The ultimate tensile strength or tensile strength of a material is the maximum stress value of the stress-strain curve. This is the maximum stress value for any material before final failure. For brittle materials, tensile strength is used as a stress basis in design calculations. In the stress-strain curve, the ultimate tensile strength can be decided accurately for all types of materials.
Young’s Modulus:
Young’s modulus is defined as the ratio of stress to strain. It is a measure of the stiffness of an elastic material. As mentioned in Fig. 1, it is the slope of the line in the straight part of the stress-strain curve.
Importance of Stress-Strain Curve
The stress-strain curve of material provides engineers with a long list of mechanical properties needed for engineering design. The capacity of a material to withstand loads prior to fracture is obtained from the stress-strain curve. The allowable material stress values are normally decided from the yield strength value for ductile materials and from the tensile strength value for brittle materials. The curve also provides a rough estimate of its deformation under loading conditions.
The stress-strain curve also helps in fabrication processes like extrusion, bending, rolling, etc. From the curve, the amount of force required for plastic deformation can be calculated.
Stress-Strain Curve of Aluminum
The stress-strain curve of the most widely used ductile material Steel is shown in Fig. 1 above. Fig. 2 below shows the typical stress-strain curve for Aluminum. For Aluminum the yield strength is not distinct; So the yield strength is decided using the proof stress method.
Fig. 2: Stress-Strain Curve for Aluminum
Stress-Strain Curve for Cast Iron
Cast Iron is a brittle material. For brittle materials, yield strength is not present as these materials fail all of a sudden. So, tensile strength is the main important parameter for brittle materials like Cast Iron, glass, and Concrete. Fig. 3 below provides a typical stress-strain curve for cast iron and concrete.
Fig. 3: Stress-Strain Curve for Cast Iron & Concrete
Stress-Strain Curve for Elastomers
Elastomers normally exhibit permanent plasticity. So, the stress-strain curve of elastomers is quite different from ductile and brittle materials. Fig. 4 below shows a typical example of the stress-strain curve of elastomers, plastic material, and brittle polymers.
Fig. 4: Stress-Strain Curve for Elastomers
Stress-Strain Curve for Perfectly Plastic Materials
Perfectly plastic or ideal plastic material will not show any work-hardening during plastic deformation. Fig. 5 shows the stress-strain curve for perfectly plastic, linear elastic, viscoelastic, and elastoplastic materials.
Fig. 5: Stress-Strain Curve for Perfectly Plastic Material
Please note that
the stress-strain curves in tension and compression for all materials are different. The stress-strain behavior (curve shape) may be similar but stress values differ considerably.
the stress-strain curve at room temperature is different from the same curve at other temperatures. Fig. 6 shows a typical example of a stress-strain curve for stainless steel and fiber-reinforced composite materials.
Fig. 6: Stress-Strain Curve at Various Temperatures
Fig. 7 below shows a comparative image showing stress-strain curves of various materials.
Fig. 7: Comparative Stress-Strain Curve for materials
Pascal’s law: Definition, Formula, Applications, and Examples
Pascal’s law, also known as the principle of transmission of fluid pressure or Pascal’s Principle is a very important theory of fluid mechanics. The law was first stated by the French Mathematician and Physicist Blaise Pascal in 1653 and as per his name, this principle is known as Pascal’s law or Pascal’s Principle. In this article, we will explore details about Pascal’s Law, its formula, equation, applications, and examples.
What is Pascal’s law?
Pascal’s law in fluid mechanics states that a change in pressure at any point in a confined incompressible static fluid is transmitted evenly throughout the liquid in all directions.
The pressure remains constant and is transmitted equally to all parts and acts at the right angle to the wall of the enclosure. As pressure remains constant and Force=Pressure X Area; So, the force applied is proportional to the surface area. So, with an increase in surface area, the force will increase as pressure is constant.
Fig. 1: Pressure is Constant throughout the container
As can be seen in Fig. 1, the pressure generated due to applied force 1 (1000 N) is constant throughout (100 N/mm2). Also, Force 2 is the same as force 1 as the area of the output piston is equal to the input piston. If the area of the 2nd piston is 5 times that of the 1st piston, the force at the 2nd piston will be 5 times of the 1st piston. Note that, the pressure transmitted does not depend on the shape of the container.
Pascal’s law is the origin of many inventions used in our day-to-day life such as hydraulic brakes and lifts.
Pascal’s Law Formula / Pascal’s Law Equation
Let’s understand the above concept of Pascal’s law using mathematical equations. Refer to the Fig. 2 given below:
Fig. 2: Pascal’s Law Explanation
The above image (Fig. 2) explains the principle of the hydraulic jack. You might have seen in car garages that to lift a heavy car, a small force is applied. This is one of the best examples of Pascal’s law.
In Fig. 2, there are two pistons; Piston 1 and Piston 2. Force F1 is applied at Piston 1. So generated Pressure in the fluid medium of the container is, P1=F1/A1 (A1=Area of Piston 1). As per Pascal’s law, the same pressure will be transmitted throughout the container and will exert at Piston 2.
So, Pressure at Piston 2, P2=P1. Or, F1/A1=F2/A2 (A2=Area of Piston 2) Or, F2=F1 X (A2/A1) Since A2>A1; F2>F1
This area ratio A2/A1 is called the ideal mechanical advantage of the hydraulic lift. So you can see that by using a hydraulic jack area ratio of 200 one can easily lift a weight of 2000 Kg by applying a force of only 10 Kg.
Examples of Pascal’s Law /Application of Pascal’s Law
Using Pascal’s Law various pieces of equipment are manufactured which are used in day-to-day life. A few examples of the application of Pascal’s Law are listed below:
Hydraulic jack and hydraulic press.
Hydraulic Brakes for increasing resisting force in the vehicle braking systems.
Artesian wells, water towers, and dams.
Aircraft Hydraulic System: Hydraulic power systems in Aircraft use Pascal’s law to slow down aeroplanes on the runway. Also, used in flight control mechanisms, landing gears, etc.
Hydraulic Pumps: Hydraulic Pumps used in the Automobile industry uses the philosophy of Pascal’s Law.
Wide applications of Pascal’s law are also seen in hydraulic testing of pressurized tanks, calibration of pressure gages, pressing of oils such as olive, hazelnut, and sunflower oils, compression of wood stocks, etc.
Various Pneumatic devices like Dentists’ drills, jackhammers, paint sprayers, and air brakes on trucks, etc work on the principle of Pascal’s Law.
A Separator is a pressurized vessel used in the exploration and production (E&P) of an oil and/or gas field to split a multi-phase well stream into a gas stream and one or more liquid streams. An example of the pressurized vertical and horizontal separators used in separation & processing include:
Mechanical Design of the separator is done following ASME BPVC Sec. VIII Division 1 or Division 2 (Class 1 or Class 2).
The selection of Division 1 or Division 2 shall be based on both design and economic considerations. The use of the Code shall be limited to the following pressure such as:
ASME BPVC Sec. VIII Div. 1 for 200 barg
ASME BPVC Sec. VIII Div. 2 for 689.5 barg
If any of the following conditions apply, the vessels should be constructed in accordance with Division 2, unless otherwise specified by the Company.
Pressure vessels where weight savings are a critical factor such as offshore or marine applications.
If either of the following conditions applies, the vessels shall be constructed in accordance with Division 2, unless otherwise approved by the Company.
Pressure vessels with a nominal wall thickness greater than 38 mm (1½ in)
Pressure vessels with a design pressure of 10 MPa (1500 psi) or higher.
This article aims to present an overview of a simple explanation of a 3-phase separator.
Factors for Separator Designs
Designing an “optimum” set of separators requires a balance between the desired size (volume caused by phases of liquid and gas), operating pressure, separation efficiency, space limitations (vertical/horizontal), the material of construction, fabrication method, and installation cost.
Let’s briefly look at the common design factors of a separator below.
1). Size
In general, the sizing of the separator will be carried out by the Process Engineer based on the target volume to be achieved, flow rate, density, operating temperature and pressure, and the need to precipitate droplets from gas, oil, and water. Size parameters also include process considerations such as fluid velocity, fluid hold, and storage capacity. The output size consists of diameter and length/height (seam-seam) depending on the gas capacity or liquid capacity calculation. The final option of the optimal size is the most economical.
2). Pressure
The operating pressure is usually fixed by process conditions. Therefore, in some kinds of literature, separators could be categorized according to their operating pressure.
Low-pressure units: 10 to 180 psi (0.7 to 12.41 barg)
Medium-pressure units: 230 to 700 psi (15.85 to 48.26 barg)
High-pressure units: 975 to 1500 psi (67.22 to 103.42 barg)
This grouping directly affects the selection of the L/D ratio in the early stage of the design separator. The L/D ratio of the separator could be selected based on operating pressure and vice versa.
Table 1. Pressure vs Separator L/D Ratio
The optimum L/D ratio may refer to the above breakdown and will vary following these parameters:
Allowable stress: affected by temperature, inside diameter, TL/TL, and material selection. It will affect the primary and secondary stresses of the entire life of the separator
Corrosion Allowance: affected the shell, head, nozzles, and pressure-part thicknesses calculation output. It will affect the MAWP and MDMT final results
Joint Efficiency: affected the thickness calculation, welding quality, NDE, and fabrication requirement.
In mechanical design & analysis, it is recommended to have a suitable margin between operating and design pressure. Determination of design pressure can follow the guideline below:
Table 2. Guidelines for determination of Design Pressure
3). Temperature
Operating temperature is determined based on service fluid, and ambient site temperature and considering the maximum and minimum factors, normal and the worst case caused by start-up, shutdown, operational upset, auto-refrigeration, and other sources of cooling.
In mechanical design & analysis, design temperature is used to select the proper material of construction (MOC), flanges pressure-temperature rating
Table 3. Summary of ASME B16.5 rating for material group 1.1
and for specific cases affects the impact test requirements (e.g per UG-20(f) and UCS-66 from ASME VIII Div.1). Determination of design temperature can follow the guideline below:
Table 4. Guidelines for determination of Design Temperature
4). Orientation: vertical vs horizontal
a). Vertical Separator:
Used for small liquid holdup or in other words; it is mainly applied when there is a large amount of vapor detected from a small amount of the light and heavy fluid (less than 10-20% by weight).
Used for easier cleanout because of its smaller bottom area
More easily used to handle liquid surges because of the depth of the space
A smaller area is required especially for offshore platform cases.
A higher climbing ladder and platform for access are required.
b). Horizontal Separator
Used for large surge volumes because of its most efficiency large amounts of dissolved gas are present with the liquid
The horizontal separator has a greater capacity
Can add an additional boot to achieve liquid/liquid or vapor/liquid separation efficiency
A less static head affects the supports geometry
A bigger area is required but with less climbing ladder & platform.
4). Separation Process
The separation process can be described as either a 2-phase or 3-phase vessel.
3-phase separators:separate the gas from the liquid phase, and water from oil. Since free water does not settle out in the time it takes for the oil and gas to separate, a three-phase separator requires a longer retention time than a two-phase separator.
5). Retention Time
Retention time is an input parameter in determining the oil and water capacity to stay inside the separator. For a given retention time of oil and water, the procedure for establishing the diameter and length of the separator become easier.
Table 5. Residence Time as per API 12J
Configuration of 3-Phase Separator (Horizontal)
3-Phase separator in horizontal orientation consists of a shell, dished ends, and mostly two saddles. From a mechanical design perspective, horizontal separators offer less wall thickness in either shell, head, and support compared to vertical separators in the same shape and size. The vertical separator will tend to be influenced by wind and seismic load resulting in greater bending stress, and deflection.
From a process design perspective, the 3-Phase separator is typically similar to the 2-phase separator, but with additional internals to handle two immiscible liquids (oil and water) rather than one liquid. In a 3-phase separator, the vessel itself should be designed to separate the gas that flashes from the liquid, as well as separate the oil and water. Therefore, in the 3-phase separator, we will find additional control devices for controlling the liquid level (LLC) and pressure (PCV).
Fig. 1: Liquid level settling rates in 3-phase Separator
The Simplicity of Separation steps:
Feed inlet–> hit the Inlet device –> Gravity settling and separation of bulk liquid with big droplet –> Gas exists through Mist Extractor/Mist Eliminator/Demister.
The most common configuration of a 3-phase separator used in the separation process is shown in the following sample from Kimray Inc. :
1). Horizontal 3-Phase Separator with a Weir Plate
Fig. 2: Horizontal 3-Phase Separator with a Weir Plate
2). Horizontal 3-Phase Separator with an Oil Bucket & Weir Plate
Fig. 3: Horizontal 3-Phase Separator with an Oil Bucket & Weir Plate
Internals of a 3-Phase Separator
Other information that shall be remembered while designing the separator includes separation efficiency, capacity, and pressure drop. The separation efficiency depends on the droplet size target (e.g 500-micronwater and 200-micron oil) distribution in each phase of separation, hence to achieve more efficient separation, internals must be installed inside the separators.
In general, the 3-phase horizontal separator settler consists of three-zone compartments that automatically determined the internals type selection and installation. (Figure 4).
This device is designed according to the process requirement, and attached inside the separator by either welding or bolting connections and usually will be finalized by an internal Vendor (e.g from Sulzer) and approved by their Principal.
As explained in an early paragraph, it can be concluded that the 3-phase horizontal separator main parts including the Head, Shell, Inlet Pipe, Inlet Device, Gas gravity separation section, Mist Extractor/Mist Eliminator/Demister, Liquid gravity separation section, Manway (for inspection and maintenance), nozzles and Saddle Supports. Fig. 5 below shows a typical general arrangement drawing of a 3-phase horizontal separator.
Fig. 5: GA Drawing of a Typical 3-phase Horizontal Separator
Fig. 6 below shows a typical 3-phase horizontal separator from Kaji Site – South Sumatera, Indonesia.
Fig. 6 A typical 3-phase Horizontal Separator
Conclusions
The L/D ratio is one of the factors that influence the optimal design of a separator, without neglecting other considerations, taking into account the following points:
Process design reason
Economical reason
Operational reason
Mechanical design reason
In most EPC companies, determination of the type and size of separator must be made on an individual basis, based on justification and experience in daily-routine work according to relevant guidance and specification such as GPSA, Clients Specifications, Separators Handbook, International codes/standard,etc.
Dye Penetration Test: Definition, Principles, Procedure, Standards, Advantages, and Disadvantages
The Dye Penetration Test (DPT) is one of the simplest and oldest Non-Destructive Inspection methods. Also, known as the Liquid penetration test, the Dye penetrant test is widely used to detect surface discontinuities like cracks, fractures, porosity, grinding defects, incomplete fusion, leaks, impact fractures, pinholes, laps, and flaws in joints. This test or inspection method serves as an aid in finding irregularities in aluminum, cast iron, brass, steel and stainless steel, copper, magnesium, carbides, stellite, ceramics, and even certain rubber and plastic materials. So, the dye penetration test is suitable for both ferrous and non-ferrous materials and is highly economical as compared to the other non-destructive inspection methods.
Dye Penetration Test is also known as Dye Penetration Inspection, Penetrant Test, Liquid Penetrate Inspection, etc.
What is the Dye Penetration Test?
Dye penetration testing is a method that involves applying a liquid dye to the surface of a material to reveal any surface defects. The process is relatively simple yet highly effective, making it a popular choice for identifying cracks, leaks, and other surface imperfections that could compromise the structural integrity of a component.
Principles of Dye Penetration Test
The Dye Penetration test works on the philosophy of capillary action. A liquid with low surface tension can penetrate into a clean and dry surface if the liquid is kept for a certain time called “Dwell Time”.
A Liquid Penetrant has to be applied over the test specimen/object by dipping, spraying, or brushing. The excess amount has to be removed after the dwell time is over. A developer is applied sometimes. The main function of the developer is to draw the penetrant out of the flaw making an invisible indication, visible to the inspector. Depending on the type of dye used, the dye penetration inspection is performed under white or ultraviolet light.
Dye Penetration Test Procedure Steps
Depending on the penetrant system, component size, and discontinuity type, the procedure of the dye penetration test may vary. But, their general steps will be similar and can be presented as follows:
Pre-cleaning and Surface Preparation:
This is the most important and basic step. The examining surface is cleaned from grease, oil, water, paint, or any other contaminants. The penetrant must be able to freely enter the discontinuities. Cleaning methods may include solvents, alkaline cleaning steps, media blasting, etc. Sometimes, the sample may even require etching to make the defects open to the surface, dry, and free of contamination.
Penetrant Application:
Next, The liquid penetrant is applied on the specimen surface and allowed to soak into any flaws for its dwell time (generally 10 to 60 minutes). The dwell time varies depending on the used penetrant (viscosity: longer duration for high viscosity), test material, and defect sizes (smaller flaw sizes require longer penetration time). Dwell time is normally provided by the penetrant manufacturers and depends on the following:
the surface tension, contact angle, dynamic viscosity, specific gravity, and microstructural properties of the penetrant.
the atmospheric and capillary pressure of the defect opening.
the pressure of the entrapped gas in the flaw by the penetrant.
the radius of the defect.
Excess Penetrant Removal:
The excess penetrant needs to be removed from the sample surface. Depending on the dye penetrant type, The removal method is selected from water-washable, solvent-removable, lipophilic post-emulsifiable, hydrophilic post-emulsifiable, etc. Emulsifiers are used for the highest sensitivity level, and it chemically react with the oily penetrant, thus making it easier to remove using water spray. The excess penetrant has to be removed thoroughly otherwise, on the application of the developer, it may leave a background in the developed area that can mask indications or defects. Also, while using solvent remover and lint-free cloth, care must be exercised not to spray the solvent on the test surface directly, because this can remove the penetrant from the flaws.
Fig. 1: Dye Penetration Test procedure
Developer Application:
A thin layer of white developer is applied after the excess penetrant has been removed. Various types of developers are available like a non-aqueous wet developer, dry powder, water suspendable, and water-soluble. Based on the compatibility of the penetrant, the developer is selected. The developer basically draws back the dye penetrant from the defects to the surface so that those can be visible. This process is known as bleeding out. Developers are applied by dusting, spraying, or dipping.
The developer is kept on the test surface for sufficient time to extract the trapped penetrant out for visible indication. The minimum development time is usually 10 minutes. The bleed-out easily indicates the location, type, and orientation of the flaw in the specimen.
Developers are available in the following six standard forms:
Form a-Dry Powder
Form b-Water Soluble
Form c-Water Suspendable
Form d-Non-aqueous Type 1: Fluorescent
Form d-Non-aqueous Type 2: Visible Dye and
Form f-Special Applications.
Inspection:
Inspection is performed in the next step using adequate light. Inspection is done using visible light for visible dye penetrant and ultraviolet (UV-A) radiation of adequate intensity for fluorescent penetrant examinations. To understand the proper characteristics of the defects the inspector must be experienced enough.
Post-Cleaning:
The final step is to thoroughly clean the surface after inspection and recording of defects. The applied developer is removed.
Dye Penetration Test Standard
The following dye penetrant test standards provide guidelines for the test:
ISO 3059, Non-destructive testing – Penetrant testing.
ISO 12706, Non-destructive testing – Penetrant testing – Vocabulary
ISO 23277, Non-destructive testing of welds – Penetrant testing of welds – Acceptance levels
Dye Penetration Inspection Standards by the European Committee for Standardization (CEN)
EN 1371-1 & 2 Founding – Liquid penetrant inspection
EN 10228-2, Non-destructive testing of steel forgings – Part 2: Penetrant testing
EN 10246-11, Non-destructive testing of steel tubes – Part 11: Liquid penetrant testing of seamless and welded steel tubes for the detection of surface imperfections
ASTM Standards for Penetrant Test
ASTM E 165, Standard Practice for Liquid Penetrant Examination for General Industry
ASTM E 1417, Standard Practice for Liquid Penetrant Testing
ASME Boiler and Pressure Vessel Code, Section V, Art. 24 Standard Test Method for Liquid Penetrant Examination SE-165 (identical with ASTM E-165)
Dye Penetration Test Kit
The dye penetration test kit contains all the required elements for performing a dye penetration test. These kits are easily available in the market and contain the Penetrant, Cleaner, and Developer as shown in Fig. 2 below.
Fig. 2: Typical Dye Penetrant Test Kit
Penetrant Characteristics in the Dye Penetrant Test
Depending on the sensitivity level required, the penetrants of the dye penetration test should possess various important properties like:
Should be highly visible for producing indications.
Must flow easily over the test specimen surface.
Should be drawn easily into the defects by capillary action.
Must not be harmful.
Should be easily drawn back using the developer.
Penetrant Types in Dye Penetration Test
Depending on the physical characteristics and performance of the penetrant materials, they are of two basic types:
Type 1-Fluorescent Penetrants containing dyes that fluoresce under ultraviolet radiation and
Type 2-Visible Penetrants containing red color dye.
Based on the penetrant removal methods, dye penetrants are classified as follows:
Method A-Water Washable.
Method B-Post Emulsifiable, Lipophilic.
Method C-Solvent removable, and
Method D-Post-Emulsifiable, Hydrophilic.
Again, depending on the detectability of defect indication, five types of dye penetrant is available:
Level 1/2-Ultra Low Sensitivity
Level 1-Low Sensitivity
Level 2-Medium Sensitivity
Level 3-High Sensitivity and
Level 4-Ultra-High Sensitivity
Acceptance Criteria for Dye Penetrant Test
Mandatory Appendix 8 of ASME BPVC Section VIII Div 1 provides guidance regarding the acceptance criteria for the Dye Penetrant test. As per the above standard, All surfaces to be examined shall be free from:
Any relevant linear indications.
Relevant round indications with dimensions > 3/16″ (4.8mm).
Four or more relevant round indications in a line separated by 1/16″ (1.6mm).
Here,
Relevant Indications: All Indications with major dimensions greater than 1.5 mm (1/16 in.) shall be considered relevant.
Linear Indication: Any indication with a length greater than three times the width.
Rounded Indication: Any indication with a length equal to or less than three times the width. A rounded indication may be of circular or elliptical shape.
Advantages of Dye Penetration Test
The main advantages of a dye penetration test are:
Small defects can be detected easily.
Suitable for a range of materials. The test can be applied to various materials, including metals, plastics, ceramics, and composites.
Inspection is quick and can easily cover large areas and volumes.
Inspection of complex shapes can be performed easily.
Indications on the surface constitute a visual representation of the flaw and so an idea about the actual defect is obtained.
Portable and easily available.
Materials and associated equipment for the dye penetrant test are relatively inexpensive.
It is generally less expensive than other NDT methods like radiography or ultrasonics.
Disadvantages of Dye Penetration Test
However, there are a few drawbacks of the dye penetration test as follows:
Limited to the only surface-breaking defects.
Materials with a non-porous surface are suitable
Direct access to the test surface is a prerequisite for the test.
Sensitivity can be affected by the surface finish and roughness of the body.
Applications of the Dye Penetration Test
The Dye Penetration Test is a versatile NDT method with applications in a variety of industries. The majority of applications are performed on welds, castings, plates, bars, pipes, and forgings. Some of the major uses of dye penetration tests are found in
Aerospace Industries: It is used to inspect critical components like aircraft engine parts, landing gear, and structural elements for hidden defects that could compromise safety.
Manufacturing Sectors: In manufacturing industries, it helps detect surface defects in materials such as welds, castings, and forgings.
Chemical, Petrochemical, Pharmaceutical, Refinery, and Other Process Industries: The test is crucial for identifying leaks and cracks in pipelines, tanks, and pressure vessels, ensuring the integrity of these critical components.
Automotive Industries: In the automotive sector, it is used to assess the quality of welds, ensuring the safety of vehicles.
Construction Engineering: In construction engineering, the Dye Penetration Test can identify cracks and defects in concrete structures, bridges, and buildings.
Fig. 3 below shows a typical sample report for the liquid penetrant test performed on the welding of a piping line stop member to assess the quality of the work.
A pressure vessel nozzle is an opening in the pressure vessel through which fluid enters or exits the pressure vessel. The Nozzle, in general, projects from the pressure vessel’s surface and ends with a means of joining (flanged or welded) piping or equipment. To carry the normal process or operation of the pressure vessel these nozzle connections are required. The main functions of a pressure vessel nozzle opening can be any of the following:
To allow the content to move into the vessel or away from the vessel to help further processing of the fluid.
To allow the insertion of instrument items.
To allow for inspection or access to internal parts (Manholes).
To connect the nozzle with the pressure vessel, an opening is made in the vessel which in turn results in penetration of the pressure retaining wall. So, it weakens the boundary creating a discontinuity in the pressure vessel wall. Nozzle openings can be made in the shell or head parts of the pressure vessel. In this article, we will study the main types of pressure vessel nozzles used in process plants, their allowable loads, and some nozzle design points.
Parts of a Pressure Vessel nozzle
A pressure vessel nozzle consists of three parts
A flange Connection (for flanged connection with pipe)
Nozzle Neck part and
Reinforcement (in case required)
Fig. 1: Elements of a pressure vessel Nozzle
Types of Pressure Vessel Nozzles / Pressure Vessel Nozzle Types
Depending on the location of the nozzles they are grouped as
Shell Nozzles (Fig. 2) and
Head Nozzles (Fig. 2)
Fig. 2: Pressure Vessel Nozzle Types
Again, depending on the welding and positioning of the nozzles two types of nozzles are widely known.
Set-in Nozzle (Fig. 3): Nozzle is projected inside the vessel surface. The pressure vessel opening diameter in the shell/head coincides with the outer diameter of the neck.
Set on Nozzle (Fig. 3): Nozzle is seated on the vessel. The diameter of the pressure vessel opening in the shell or head coincides with the ID (inner diameter) of the nozzle neck.
Nozzles with added reinforcement: Additional reinforcing plate is added to withstand external nozzle loading. Preferred for non-cyclic loads.
Self-reinforced nozzles: Nozzle thickness itself is sufficient to withstand external nozzle loads and so an additional RF pad is not provided. Preferred for fatigue or cyclic loading. They are of two types:
Nozzles with straight hub
Nozzles with variable hub
Fig. 4: Self-reinforcement nozzles with straight and variable hubs
Also, the nozzles in a pressure vessel can be placed perpendicular or angular position with respect to the shell axis. It can be intersecting the vessel axis or offset. Various nozzle positions are shown in Fig. 5.
Fig. 5: Pressure Vessel Nozzle Positions
Allowable Nozzle Loads
Allowable nozzle loads for pressure vessels are provided by manufacturers. Normally, engineering companies have their own specification to decide minimum nozzle loads with respect to connected pipe size and flange rating. A similar pressure vessel nozzle loading table for vessels made from ferrous material from shell DEP 31.22.00.31 (unfired pressure vessel) is produced below (Fig. 6) as a sample.
Fig. 6: Pressure Vessel Nozzle loading table from DEP 31.22.00.31
From the above table the following conclusions can be made:
With an increase in flange rating, the nozzle load-carrying capability increases.
With an increase in nozzle size the load-carrying capability increases.
The capability of a vessel to withstand the external nozzle loading is decided based on WRC or FEA calculations. WRC calculation can be done using Caesar II, PV-elite, Start-Prof, or Code-cal software. FEA calculation can be done using Nozzle-pro, Nozzle-FEM, or Ansys software.
Design of Pressure Vessel Nozzles
For the design of Pressure Vessel nozzles, various codes and standards are available. However, the most widely used internationally recognized ASME BPVC Section VIII by the American Society of Mechanical Engineers (ASME).
Other pressure vessel codes and standards that are occasionally used are:
Europe: EN-13445
United Kingdom: British Standards BS PD 5500
France: CODAP
China: GB-150
Germany: A. D. Merkblatt Code
Nozzle design basically means three parts:
Deciding the nozzle size or nozzle opening
Designing and Selecting Nozzle thickness and
Calculating the reinforcement requirement based on pressure and external loads.
The size of the nozzle opening is normally decided by the process team depending on the volume of fluid input and output in the pressure vessel. Once the nozzle opening size is fixed the nozzle thickness requirement is calculated based on the design pressure of the contained/flowing medium. The calculated thickness is normally converted into standard nozzle thickness as per standard pipe thickness available following ASME B36.10 or ASME B36.19 standards. In the next step, the requirement of nozzle reinforcement is checked.
The design of pressure vessel nozzles is done following equations mentioned in ASME Section VIII, Div. 1 UG 36 to UG 45.
Nozzle design Calculation for pressure vessels is normally performed following the area compensation method. The detailed nozzle design methodology is explained in the following article: “Nozzle Reinforcement Calculation for a Cylindrical Nozzle“
Online Course on Pressure Vessels
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An automatic recirculation valve or ARV is a multifunctional valve. ARV ensures that a pre-determined minimum flow is transferred through a centrifugal pump at all times. So an automatic recirculation valve (Fig. 1) is basically a pump protection device. To avoid permanent damage (destruction) from cavitation and overheating (thermal damage) these valves serve a very important function.
Fig. 1: Automatic Recirculation Valve
Working Principle of Automatic Recirculation Valve
The heart of the automatic recirculation valve (ARV) is a check valve disk that senses the flow rate of the fluid. The valve disk is flow-sensitive and not pressure-sensitive. It controls the fluid flow and ensures that a specified fluid volume passes through. The controlling characteristics result in a consistent and stable flow over a wide range of pressure.
At the full capacity of the valve disk, the bypass closes. Again, when the flow decreases, the action is reversed and there is an increase in the flow rate. The fluid in that situation gets into the bypass system which is controlled by the orifices and is found at the bottom of the disk.
The fluid then flows through the annulus directing it to the outlet. The disk is lifted with an increase in the fluid flow. As a result, the bypass element which is important for the functioning of the bypass closes to limit the recirculation. This guarantees that the recirculation flow is more than the lowest volume of the flowing fluid through the pump.
Fig. 2: Typical Automatic Recirculation Valve
So the automatic recirculation valve sets out four functions in one body. They are:
Flow perception: The valve disc of the ARV automatically perceives the main flow of the processing system. Then according to the flow, it determines the main valve and bypass valve disc position.
Recirculation control: Automatic recirculation valve automatically adjust pump H – Q characteristics to realize recycling.
Bypass multistage pressure reducing: The bypass control system of the Automatic recirculation valve reduces the backflow medium from the high-pressure pump outlet to appropriate backflow to the low-pressure storage device with low noise and small wear.
Check: The automatic recirculation valve also provides a check valve effect that prevents the liquid backflow to the pump body.
Applications of Automatic Recirculation Valve
A common application is to protect pumps that handle hot water for boiler feeding or cooling water plants, where partial evaporation of the water content might otherwise cause the pump to run dry. Even in situations, when the flow rate of the main valve to the boiler is completely shut off, a minimum flow is maintained.
Another application of an automatic recirculation valve is to protect high-performance pumps during a start-up phase. In this scenario, several pumps are used in parallel with one on standby. The pump’s automatic recirculation valve enables the change to occur without damaging the pumps. In the Power, Paper & Pulp, Maritime, Refining, Fire protection systems, and Chemical industries, an automatic recirculation valve is widely used.
The bypass fully opens when the disk closes and there is no fluid flow. It helps to protect the pump from damage that would result if the pump continued to operate with no fluid.
Advantages of Automatic Recirculation Valve
The main advantages of an Automatic Recirculation Valve are:
Combines both check and bypass features
No external actuation required
low maintenance and reliable
Proprietary Cv calculation system
Cavitation prevention
Unique and stable design
Minimum flow protection
Easy installation
3 Phase Slurry and High-pressure variants available
Eliminate external power source
Eliminates installation and maintenance of complex conventional flow control loops
Centrifugal Pump Protection Scenarios using Automatic Recirculation Valve (ARV)
Fig. 3: Centrifugal Pump Protection
In a Centrifugal pump, the mechanical energy is transformed into pressure energy by means of centrifugal force. The impeller rotation acting on the fluid within the pump generates the centrifugal force. To avoid overheating, The pump always needs a minimum liquid flow. If the predefined minimum flow is not maintained, permanent pump damage can occur. Four different pump protection scenarios are reviewed below.
1) Non-return scenario
The pump does not have backflow prevention and therefore, the product will flow back through it once stopped. Therefore, A non-return valve (NRV) is always placed after the pump outlet. A reservoir is used to take the pump output when there is no process demand.
Fig. 4: Non-Return Scenario of Centrifugal Pump
2) Continuous flow scenario
To allow the required minimum flow back to the pump inlet, A manual bypass or leakage path can be added. This is a simple and effective system, which is in constant operation and therefore is inefficient and costly (energy costs).
Fig. 5: Continuous Flow Scenario of Centrifugal Pump
3) Control valve scenario
This comprehensive control valve solution is highly effective. In this scenario, a flow control valve is connected to a flow meter and allows the mainline flow to be metered. The control valve opens when the mainline flow decreases which allows the correct minimum flow required. However, this is a highly capital-intensive solution. It requires flow metering equipment, control, and non-return valves. No reservoir is required.
Fig. 6: Control Valve scenario of Centrifugal Pump
4) Automatic Recirculation Valve solution
The previous approach is costly and stands and falls within the integrity of the control system. An alternative and safer system are to combine the non-return valve, the bypass valve, and the control valve into an interconnected unit, known as “the automatic recirculation valve”. This valve closes during the no-flow condition automatically opening the bypass line, which is sized for minimum flow. When the mainline takes flow but less than the minimum, the bypass line, and the mainline are both partially open.
Fig. 7: Automatic Recirculation Valve Scenario
About the Author: Part of this article is prepared by Ms. Namita Modak, a Chemical Engineer with 14+ years of experience. Click here to know more about her.
