Both ASME Sec VIII Div 1 and Div 2 are used for pressure vessel design. Both divisions contain mandatory requirements, specific prohibitions, and non-mandatory guidance for pressure vessel materials, design, fabrication, examination, inspection, testing, certification, and pressure relief. So in a broad sense, both may seem to be similar but there are few distinct differences between both Divisions. In this article, we will explore the major differences between ASME Sec VIII Div 1 and Div 2.
Typical Column (Pressure Vessel) during the erection stage
ASME Sec VIII Division 1 vs ASME Sec VIII Division 2
ASME Section VIII, Division 1 is a straightforward design-by-rule method used by engineers to design pressure vessels based on rules. It’s conservative and usually leads to a sturdier design.
ASME Section VIII, Division 2 requires more detailed calculations and allows vessels to handle higher stresses, making it suitable for vessels with specific purposes and fixed locations.
The key difference between Division 1 and Division 2 is in how they handle stress. Division 1 uses normal stress theory, while Division 2 uses maximum distortion energy theory (Von Mises). The major differences between the two divisions of ASME BPVC Sec VIII Div 1 and Div 2 are tabulated below:
Parameters
ASME Sec VIII-Division 1
ASME Sec VIII-Division 2
Design Approach
ASME Sec VIII Division 1 is focused on a design-by-rule approach
ASME Sec VIII Division 2 on the other hand, is based on a design-by-analysis approach
Design Factor
The design Factor used is 3.5 on tensile and other yields and temperature considerations.
Design Factor of 3 (3.0 for Division 2, Class 1 and 2.4 for Division 2, Class 2) on tensile and other yield and temperature considerations.
Pressure Limit
Pressure typically up to 3000 psig. ASME Sec VIII Div 1 is more suitable for low-pressure applications.
Pressure is usually 600 psig and larger (less than 10000 psi). ASME Sec VIII Div. 2 caters to high-pressure applications.
Design Rules
Membrane – Maximum stress Generally Elastic analysis. Very detailed design rules with Quality (joint efficiency) Factors. Little stress analysis required; pure membrane without consideration of discontinuities controlling stress concentration to a safety factor of 3.5 or higher
Maximum Shear stress theory is the basis for Shell of Revolution. Generally Elastic analysis Membrane + Bending. Fairly detailed design rules. In addition to the design rules, discontinuities, fatigue, and other stress analysis considerations may be required unless exempted and guidance provided for in Appendix 4, 5 and 6.
Design Calculations
Simple Calculations.
requires more detailed calculations than Division 1
Failure Theory of Design
ASEM Sec VIII Division 1 is based on the normal stress theory
ASME Sec VIII Division 2 is based on maximum distortion energy (Von Mises criteria)
Experimental Stress Analysis
Experimental methods of stress analysis are not required in normal cases.
Experimental stress analysis is introduced and may be required
Material and Impact Testing
Few restrictions on materials; Impact required unless exempted; UG-20, UCS 66/67 provides extensive exemptions.
More restrictions on materials; impact required in general with similar rules as Division 1.
NDE Requirements
In ASME Sec VIII Div. 1, the NDE requirements may be exempted through increased design factors.
Div. 2 has more stringent NDE requirements; extensive use of Radiographic tests, Ultrasonic Tests, Magnetic Particle Tests, and Penetration Tests.
Welding and fabrication
Different types with butt welds and others.
Extensive use/requirement of butt welds and full penetration welds including non-pressure attachment welds.
Fatigue Evaluation
Not mandatory.
AD 160 for fatigue evaluation
Manufacturer
Manufacturers are to declare compliance with the data report.
Manufacturer’s Design Report certifying design specification and code compliance in addition to a data report.
Professional Engineer Certification
Normally not required.
Professional Engineers’ Certification of User’s Design Specifications as well as Manufacturer’s Design Report Professional Engineers shall be experienced in pressure vessel design.
Code Stamp and Marking
U Stamp with Addition markings including W, B, P, RES; L, DF, UB, HT, and RT.
U2 Stamp with Additional marking including HT.
Hydrostatic Test Pressure
1.3 times design pressure.
1.25 times design pressure.
Allowable Stress Value at a specified design temperature
Magnetic Particle Inspection (MPI) is one of the most widely used non-destructive inspection methods for locating surface or near-surface defects or flaws in ferromagnetic materials. MPI is basically a combination of two NDT methods: Visual inspection and magnetic flux leakage testing. Developed in the USA, magnetic particle inspection is extensively used to detect defects in the casting, forging, and welding industries.
MPI is simple, easy, fast, and very effective. This is the reason the Magnetic particle test is used in a variety of industries like automotive, oil & gas construction, chemical, and petrochemical plant construction, structural steel, aerospace, offshore structures, power generation industries, and pipeline industries. This is also known as the magnetic particle test or magnetic particle examination in NDT.
Basic Principle of Magnetic Particle Inspection
MPI uses magnetic fields and magnetic particles for detecting defects in ferromagnetic components. The basic principle of this inspection method is that the component specimen is magnetized to generate magnetic flux in the material which travels from the north pole to the south pole (magnetic flux exits at the north pole and enters at the south pole). Now if there is any discontinuity or flaws in the component, secondary magnetic poles are produced in the cracked faces. In this location, the magnetic field spreads out due to the air gap in the defect causing a magnetic flux leakage field. Such regions can be detected easily by using magnetic particles (iron powder), or a liquid suspension on the surface. Due to the magnetic effect, such particles are attracted to the flux leakage and make a cluster around the flaw making it visible. Refer to Fig. 1 showing the basic principle of magnetic particle inspection.
Fig. 1: Principle of Magnetic Particle Inspection
The magnetic particles can be dry or wet. Normally, dry particles can be used up to a temperature of 316 Deg C wet particles can be used up to a temperature of 50 Deg C.
Steps for Magnetic Particle Inspection
The magnetic particle inspection (MPI) is performed in the below-mentioned six steps.
1. Surface Preparation:
All surfaces and adjacent areas (within 1 inch) that will be examined must be free from rust, scale, sand, grease, paint, slag, oily films, or other interfering conditions. Unusually rough or non-uniform surfaces may interfere with magnetic particle cluster formation making interpretations of the magnetic particle inspection method’s indications difficult.
2. Inducing a Magnetic Field:
This is the most important step in the magnetic particle inspection procedure. In this step, place the equipment on the area to be tested and induce a magnetic field. Various types of magnetic particle inspection equipment are available. Widely used industrial equipment are Permanent magnets, Electromagnetic Yokes, Current flow probes, Magnetic Flow, Flexible coils, Threading bars, Adjacent cables, etc. The magnetization technique can be Longitudinal, Circular, or Multidirectional Magnetization. Equipment spacing in the inspection area is normally kept in between 3 inches to 8 inches. An ASME Pie Gauge or Burmag Castrol strip can be used to verify adequate magnetization of the part.
3. Applying Magnetic Particles on the Test Surface:
Both dry and wet magnetic particles can be either fluorescent or non-fluorescent (visible, color contrast) and are available in a variety of colors to contrast with the tested material. So accordingly choose the required particles for the magnetic particle inspection and apply them on the surfaces when the specimen is in magnetized condition.
4. Examine the component surface for defects
Remove the excess particles using light airflow and inspect the component for defects as per acceptable standards.
5. Repeat the test by changing the magnetic field
Two separate examinations are carried out on each area to be tested. The second examination is performed with the lines of flux perpendicular to those used for the first examination in that area.
Refer to Fig. 2 which shows the above 5 steps. An electromagnetic Yoke is used in the test to inspect the welding of two plates.
Fig. 2: Steps for Magnetic particle inspection
6. Demagnetization and Cleaning:
The presence of Residual magnetism in the component may interfere with the subsequent usage. Hence, the demagnetization shall always be performed on the parts once the magnetic particle inspection is over. The presence of residual magnetism can be verified using a calibrated Gaussmeter, Magnetic Field Meter, or a hall Probe Gauss meter. Residual magnetism must not exceed (+/-) 2 gausses.
After that, the parts shall be cleaned to remove all residual magnetic particle materials. If wet fluorescent MPI was performed, the part shall be scanned with the backlight to assure that the cleaning is adequate.
Advantages of Magnetic Particle Inspection/Test
The main advantages of magnetic particle inspection/testing are
Find flaws on the surface and near surfaces
Fast examination method with an immediate result
This is an easy method as compared to other NDT methods
Portable and low-cost equipment.
Defects are visible directly on the surface.
Relatively safe method.
Hot testing can be performed using dry particles.
The shape and size of the cracks are indicated.
Less training requirements.
Disadvantages of Magnetic Particle Inspection
The major drawbacks of magnetic particle inspection/examination are
MPI is limited only to ferromagnetic materials like steel, cast irons, etc. Non-ferrous materials, cannot be inspected.
The inspection is limited to small sections only. The examination of large parts may require the use of special equipment.
Equipment must be calibrated, with no permanent record of the result.
Before inspection thick paints (>0.005″) shall be removed.
Post-cleaning and demagnetization are normally required.
Magnetic flux and indications must be aligned for proper results.
Access may be a problem for the magnetizing equipment.
Testing in two perpendicular directions is required.
Codes and Standards for Magnetic Particle Inspection
Normally ASMESection V: Nondestructive Examination governs the magnetic particle inspection/examination methods for most organizations. However, there are various other codes and standards that provide guidance rules for magnetic particle test procedures as listed below
ISO (International Organization for Standardization) Standards for MPI
The following ISO standards govern the inspection by MPI.
ISO 3059, Non-destructive testing – Penetrant testing and magnetic particle testing – Viewing conditions
ISO 9934, Part 1, Part 2, and Part 3– Non-destructive testing – Magnetic particle testing – Part 1: General principles, Part 2: Detection media, Part 3: Equipment
ISO 10893-5-Non-destructive testing of steel tubes. Magnetic particle inspection of seamless and welded ferromagnetic steel tubes for the detection of surface imperfections
ISO 17638, Non-destructive testing of welds – Magnetic particle testing
ISO 23278, Non-destructive testing of welds – Magnetic particle testing of welds – Acceptance levels
European Committee for Standardization (CEN) Standards for MPI
EN 1330-7, Non-destructive testing – Terminology – Part 7: Terms used in magnetic particle testing
EN 10246-12, Non-destructive testing of steel tubes – Part 12: Magnetic particle inspection of seamless and welded ferromagnetic steel tubes for the detection of surface imperfections
EN 1290, Surface Crack Testing
EN 1369, Founding – Magnetic particle inspection
EN 10228-1, Non-destructive testing of steel forgings – Part 1: Magnetic particle inspection
EN 10246-18, Non-destructive testing of steel tubes – Part 18: Magnetic particle inspection of the tube ends of seamless and welded ferromagnetic steel tubes for the detection of laminar imperfections
ASTM E1444/E1444M Standard Practice for Magnetic Particle Testing
ASTM E 1316 Terminology for Nondestructive Examinations
ASTM A 275/A 275M Test Method for Magnetic Particle Examination of Steel Forgings
ASTM E543 Practice Standard Specification for Evaluating Agencies that Performing Nondestructive Testing
ASTM E 709 Guide for Magnetic Particle Testing Examination
ASTM A456 Specification for Magnetic Particle Inspection of Large Crankshaft Forgings
ASTM E 2297 Standard Guide for Use of UV-A and Visible Light Sources and Meters used in the Liquid Penetrant and Magnetic Particle Methods
The United States Military Standard: A-A-59230 Fluid, Magnetic Particle Inspection, Suspension CSA (Canadian Standards Association) Standards: CSA W59 Welded steel construction
MPI Standards of Society of Automotive Engineers (SAE)
widely used MPI standards from the society of Automotive Engineers are listed below:
AMS 3040 Magnetic Particles, Nonfluorescent, Dry Method
AMS 2641 Magnetic Particle Inspection Vehicle
AMS 3045 Magnetic Particles, Fluorescent, Wet Method, Oil Vehicle, Ready-To-Use
AMS 5355 Investment Castings
AMS 3041 Magnetic Particles, Nonfluorescent, Wet Method, Oil Vehicle, Ready-To-Use
AMS 3042 Magnetic Particles, Nonfluorescent, Wet Method, Dry Powder
AMS 3044 Magnetic Particles, Fluorescent, Wet Method, Dry Powder
Magnetic Particle Inspection Equipment
During magnetic particle examination, various kinds of equipment are required as listed below:
Electromagnetic Yokes
Magnetic Benches
Power Packs & Mobile Test Units
Demagnetising Units
These magnetic particle inspection equipment should be designed to be durable, fast, and reliable. Refer to Fig. 3 which shows a few of the magnetic particle test equipment.
Fig. 3: Magnetic Particle Inspection Equipment
Code Acceptance Criteria for Magnetic Particle Inspection
As per ASME Sec VIII Div 1 and Div 2, all surfaces examined by magnetic particle inspection shall be free from
Linear indication
Rounded indication with size greater than 3/16 inches.
Four or more rounded indications in a line separated by 1/16 inch or less.
Crack-like indications, irrespective of surface condition.
As per ASME B31.3, any cracks or linear indications are unacceptable.
Magnetic Particle Inspection/Testing Questions
The following pdf link provides a few sample questions for examinations for level 1 and level 2. Go to page no 46 directly to prepare answers for magnetic particle testing questions. Click here to open the pdf and start preparing.
Creep Rupture Usage Factor for Allowable Variations in Elevated Temperature Service
Appendix V of ASME B31.3 code covers the application of the Linear Life Fraction Rule, which provides a method for evaluating variations at elevated temperatures above design conditions where material creep properties control the allowable stress at the temperature of the variation.
What is Creep-Rupture Usage Factor?
The calculated value of Creep-Rupture Usage Factor “u” indicates the nominal amount of creep-rupture life expended during the service life of the piping system. If u ≤ 1.0, the usage factor is acceptable. If u > 1.0, the designer shall either increase the design conditions (selecting a piping system components of a higher allowable working pressure if necessary) or reduce the number and/or severity of excursions until the usage factor is acceptable
i – as a subscript, 1 for the prevalent operating condition
ti – total duration, h, associated with any service condition, i, at pressure, Pi, and temperature, Ti
tri – allowable rupture life, h, associated with a given service condition i and stress, Si
Ti – temperature, °C (°F), of the component for the coincident operating pressure-temperature condition i under consideration
C – Larson-Miller constant. C = 30 for 9Cr–1Mo–V; C = 20 for carbon, low, and intermediate alloy steels, except 9Cr–1Mo–V; C = 15 for austenitic stainless steel and high nickel alloys
TE – effective temperature, °C (°F) from Table A-1 or Table A-1M, find the temperature corresponding to basic allowable stress equal to the equivalent stress, Si, using linear interpolation if necessary. This temperature, TE, is the effective temperature for service conditions i.
The equivalent stress, Si, is calculated as follows
SL – the maximum stress due to sustained loads, during service conditions i
Spi – pressure-based equivalent stress, MPa (ksi)
Pmax – maximum allowable gage pressure, kPa (psig), for continuous operation of pipe or component at design temperature, considering allowances, c, and mill tolerance, but without considering weld joint strength reduction factor, W; weld joint quality factors, Ej; or casting quality factor, Ec
Sd – allowable stress, MPa (ksi), at design temperature, °C (°F)
Pi – gage pressure, kPa (psig), during service condition i
Calculation of Creep-Rupture Usage Factor
The latest version of modern professional PASS/START-PROF software includes the ability to automatically calculate the Creep-Rupture Usage Factor for the piping system.
Firstly, the material database contains the Larson-Miller constant for every material as shown in Fig. 1 below.
Fig. 1: Larson-Miller Constant in Start-Prof
Secondly, the operation mode editor contains the time duration for each operation mode
Fig. 2: Time Duration in Operation mode editor
And thirdly, the code stress table contains the column, where the “u” factor is printed. If you move the mouse over the table cell, you will see the calculation details (As shown in Fig. 3 below)
Fig. 3: Creep-Rupture Usage Factor output in Start-Prof
Minimum Design Metal Temperature or MDMT is the lowest temperature that a piping system with specified material and thickness can withstand. While designing piping systems (equipment) in cold regions where the environment temperature falls drastically or piping systems carrying cryogenic temperature fluid, MDMT is a critical factor. Considering the metal’s resistance to brittle failure, MDMT is the lowest permissible metal temperature for that thickness.
The piping designer shall verify that materials are suitable for service throughout the operating temperature range (maximum and minimum possible temperatures). Table A-1 and Table A-1M of ASME B31.3 code contain the minimum design metal temperature for which the material is normally suitable without impact testing. Refer to Fig. 1 where minimum design temperatures for a few carbon steel pipe materials are highlighted.
Fig. 1: MDMT of Carbon Steel Pipe and Tube
The MDMT for Carbon Steel in -29°C. So what does it mean? Can we use it below that temperature?
Below -29°C, ductile Carbon steel starts converting into brittle material. So impact test requirements as per the code arise as brittle carbon steel can easily fail catastrophically. However, the code provides few rules to use such materials below its minimum design metal temperatures as provided below:
Rules for using materials below its MDMT without impact testing
The use of a material at a design minimum temperature colder than −29°C (−20°F) is established by para. 323.2.2 and other impact test requirements. For carbon steels with a letter designation in the Minimum Temperature column, the curve in Figure 323.2.2A of ASME B 31.3 (Reproduced in Fig. 2) is used. MDMT depends on the nominal thickness.
Fig. 2: MDMT vs Nominal Thickness
Impact testing of the base metal is not required if the design minimum temperature is warmer than or equal to the calculated value of MDMT.
However, for steels, impact testing is not required if the stress ratio “r” 323.2.2 (b) is 0.3 or less, and the design minimum temperature is warmer than or equal to −104°C (−155°F), and temperature reduction may be used if 323.2.2 (c) rules are satisfied:
(1) The piping is not in the Elevated Temperature Fluid Service. (2) Local stresses caused by shock loading, thermal bowing, and differential expansion between dissimilar metals (e.g., austenitic welded to ferritic) are less than 10% of the basic allowable stresses at the condition under consideration. (3) The piping is safeguarded from maintenance loads, e.g., using a valve wheel wrench on a small-bore valve.
Also, for carbon, low alloy, and intermediate alloy steel materials (including welds) that have not been qualified by impact testing, the minimum temperature from Table A-1, Table A-1M, or Figure 323.2.2A may be reduced to a temperature no colder than −48°C (−55°F) by the temperature reduction provided in Figure 323.2.2B if 323.2.2 (c) rules are satisfied.
Fig. 3: Temperature Reduction calculation as per stress ratio
What is the Stress Ratio, r?
The stress ratio “r” is calculated as the maximum value from the following:
From all operating modes and force-based loadings, we calculate the maximum rating value r
From all operating modes’ force-based loadings (force+displacement)-based loadings we calculate the maximum value of r
How does Start-Prof take care of MDMT?
The latest version of professional PASS/START-PROF software includes the ability to check automatically if the impact test is needed or not.
Firstly, the PASS/START-PROF has a material database that includes the minimum metal temperature for all materials. For carbon steels with a letter designation in the Minimum Temperature, PASS/START-PROF calculates the minimum metal temperature automatically, according to Figure 323.2.2A.
Fig. 4: MDMT Consideration in Start-Prof
Secondly, the software automatically calculates “r” values for all operating modes of the piping system. And has the special option “Use MDMT Allowable Reduction” in project settings to verify if the temperature reduction is allowed or not as shown in Fig. 5
Fig. 5: Using MDMT Allowable Reduction in Start-Prof
Thirdly, for each pipe element in the system, the software determines if the impact test is needed or not according to the previously described rules of 323.2.2 (a), (b), (d), (e), (f), (g), (h), (i), (j) ASME B31.3-2018. The result is shown in the special MDMT table as shown in Fig. 6 below:
Fig. 6: MDMT Results in Start-Prof
Fig. 7: Checking MDMT output
After analysis, if the minimum design or ambient temperature from all operating modes is lower than the calculated MDMT value, the “Impact Test” requirement note is printed. Otherwise, the result simply shows “OK” for proceeding further.
To avoid impact testing, the stress ratio “r” value should be reduced as low as possible for the critical piping system elements. To do this, you need to create the piping stress analysis model in PASS/START-PROF and reduce the sustained and operation stresses by adding more support or flexibility to the piping system.
More information about the new modern pipe stress analysis software PASS/START-PROF you may learn from the resources web page.
The tank nozzle can be modeled using the special “Tank Nozzle” object in START-PROF software.
Tank connection modeling has a special significant features in comparison with pressure vessel modeling.
Tank Nozzle Movement due to Tank Temperature Expansion
Due to the large tank diameter, the temperature expansion can cause significant nozzle movement along its axis. This movement can be calculated by the following formula:
Tank nozzle displacement due to tank temperature expansion is modeled automatically using the “Tank Nozzle” object.
Tank Nozzle Movement due to Tank Settlement
Tank diameter is very large, due to this tanks usually have no foundation that can distribute its weight over the big soil area. Due to this, the tank settlement happens. Settlement value depends on soil type, tank weight, and dimensions, and should be calculated based on the geo-technological investigation report. The greatest settlement value is at the center of the tank, the lowest value is at the edges.
Since the piping connected to the nozzle is connected to the tank shell, we need to consider it during stress analysis. The settlement value should be specified in the “Tank Nozzle” object properties. To reduce the effect of tank settlement on piping the first support shall be kept sufficiently away from the tank nozzle
Storage Tanks are used for liquid storage and hence, are filled with liquid. The height of the liquid level is varying therefore the pressure on the tank shell is varying. The greatest pressure is near the bottom. The tank shell tries to expand near the bottom, but the bottom holds it. Due to this the nozzle moves radially outward and rotates in a vertical plane. This effect is significant for tanks with a diameter greater than 36 m.
According to API 650 code Appendix P radial growth of shell due to hydrostatic pressure:
Rotation of shell due to hydrostatic pressure:
G is the design specific-gravity of the liquid;
H is the maximum allowable tank filling height, in mm (in.);
L is the vertical distance from the nozzle centerline to the tank bottom, in mm (in.);
R is the nominal tank radius, in mm (in.);
t is the shell thickness at the opening connection, in mm (in.);
β is the characteristic parameter, 1.285/(R*t)^0.5 (1/mm) (1/in.);
E is the modulus of elasticity, in MPa (lbf/in.2);
DT is the normal design temperature minus installation temperature, in °C (°F);
a is the thermal expansion coefficient of the shell material, in mm/[mm-°C] (in./[in.-°F])
To reduce the nozzle rotation effect, it is recommended to turn the pipe 90° very close to the tank nozzle.
To consider this effect you need to specify the filling height and product density in the “Tank Nozzle” object.
Storage Tank Nozzle Flexibility
Tank nozzle flexibility can be calculated using the API 650 code or Nozzle-FEM software.
Storage Tank Nozzle Allowable Loads
Allowable loads are calculated using two methods.
The first method is according to API 650. The allowable values envelopes for moments ML, MC, and axial force FR are shown in the pictures below
The second method is according to STO SA 93-002-2009 code (Russian Standard). The allowable values envelopes for moments ML, MC, and axial force FR are shown in the pictures below
The method can be used if D and DN values are inside the following envelope
A Separator is a type of pressure vessel that is used to separate the gas and liquid from a two-phase mixture. The two-phase separator separates the liquid and gas phases from the mixture.
Pressure vessels are widely used in the process plant industry. Pressure vessels serve various functions such as
short-term hold-up, i.e. day tanks, surge vessels,
pressurized storage storages, i.e. bullets, Horton spheres,
Special purpose vessels such as reactors, columns, and jacketed vessels.
The most common shape of pressure vessels is a cylindrical shell with dished ends. Other types of end closures such as conical, and hemispherical are also used when appropriate. For large pressurized storage, a spherical shape may be chosen. The Standard Engineering codes used for the design of 2-phase separators are GPSA guidelines, and API 12J.
Orientation of Separators
Pressure vessels/Separators can be installed in vertical or horizontal orientation.
Vertical Pressure Vessels
Vertical orientation is preferred to horizontal orientation owing to the following advantages:
Lower plot space required: In most cases length (or height) of pressure vessels is more than the diameter. Therefore, the layout space required is lower when a vessel is placed vertically.
The vertical installation provides better utilization of vessel volume as the working volume between the high and low operating levels. This is illustrated in Fig. 1 below:
Fig. 1: Vertical vs Horizontal Pressure Vessel
Horizontal Pressure Vessels
A vessel may be oriented horizontally when higher mechanical strength is needed to support the weight. This is especially important in the case of high-pressure vessels and very long vessels (high L/D ratio). Horizontal vessels can be provided with two or more saddles.
Two-Phase (V/L) Separator Design
Depending on fluid phases the separators can be classified into two groups.
Two-Phase Separator and
Three-Phase separator
Two-phase separators handle two-phase fluids. One is the gaseous phase and the other is the liquid phase. While a three-phase separator can separate out three phases; normally a gas, oil, and water (two liquid phases and one gas phase). In the following paragraphs, we will briefly explore the design basics of two-phase separators.
Selection of Separator: Horizontal or Vertical
As a rule, a vertical drum should be chosen when the ratio of vapor to liquid volume is large (750 or more). The vertical drum is often preferred since the separation efficiency does not vary with the liquid level in the drum. Also, the plot space required is lower for the vertical drum.
The figure given below (Fig. 2) is used as guidance for the selection of the orientation of separators.
Fig. 2: Separator Selection Guide Chart
Choice of Separator Internals
Separator Internals are provided to increase the efficiency of the separator and reduce entrainment. The Internals available commercially is Demister Pads, Vane packs, Multi cyclones, or swirl decks. The size of droplets present in the two-phase flow entering the drum decides the type of internals to be used. Droplet size depends on the flow regime of the inlet pipe. The diameter of the inlet pipe should be selected to avoid dispersed, annular, or mist flow. The approximate size of droplets present in the vapor phase is given by:
Symbol
Description
Units
d
Drop diameter
m
D
pipeline internal diameter
m
g
acceleration due to gravity
m/s2
k
Souder’s Brown proportionality constant
k/s
ρ
density
kg/m3
σ
surface tension
N/m
v
velocity
m/s
W
mass flowrate
kg/s
P
pressure
bar
ν
kinematic viscosity
m2/s
μ
dynamic viscosity
Ns/m2
Subscripts
v
gas/vapor phase
l
liquid phase
Determining Separator Diameter
The design methods are based on Souder’s-Brown equation
The maximum allowable velocity of the vapor phase is given by the value of vmax calculated by the Souder’s-Brown equation. The diameter of a vertical separator is calculated based on the value of vmax.
In the case of a horizontal vessel, the full cross-section area for the flow of vapor is calculated based on the value of vmax. This is in turn used to calculate vessel diameter.
Typical Values of Proportionality Constant, k
Different values of the proportionality constant k are applied for the internals and orientation of the separator.
