PRESSURE VESSEL
Learning Outcomes/Learning Objectives/Instructional Objectives
Describe what a pressure vessel is. Be familiar with the major types of pressure
vessels. Able to design pressure vessels.
After completing this chapter, you should be able to dothe following:
Pressure Vessel
Containers for fluids under pressure Used in variety of industries
Petroleum refining Chemical Power Pulp and paper Food
Main Pressure Vessel Components
Shells Manways Heads Nozzles
Tubesheets Flanges Flat cover plates Attachments parts
The basic data needed by the specialist designer will be:1. Vessel function.2. Process materials and services.3. Operating and design temperature and pressure.4. Materials of construction.5. Vessel dimensions and orientation.6. Type of vessel heads to be used.7. Openings and connections required.8. Specification of heating and cooling jackets or coils.9. Type of agitator.10. Specification of internal fittings.
Pressure Vessel/ Data Needed
Pressure Vessel For the purposes of design and analysis,
pressure vessels are subdivided into twoclasses depending on the ratio of the wallthickness to vessel diameter:
thin-walled vessels, with a thickness ratio ofless than 1:10; and thick-walled above thisratio.
General Design Considerations: Pressure Vessels
For vessels under internal pressure, the design pressureis normally taken as the pressure at which the reliefdevice is set.
This will normally be 5 to 10 per cent above the normalworking pressure.
Vessels subject to external pressure should be designedto resist the maximum differential pressure that is likelyto occur in service. Vessels likely to be subjected tovacuum should be designed for a full negative pressureof 1 bar, unless fitted with an effective, and reliable,vacuum breaker.
Design pressure
General Design Considerations: Pressure Vessels
The strength of metals decreases with increasingtemperature.
The maximum allowable design stress will depend onthe material temperature.
The design temperature at which the design stress isevaluated should be taken as the maximum workingtemperature of the material, with due allowance forany uncertainty involved in predicting vessel walltemperatures.
Design temperature
General Design Considerations: Pressure Vessels
Pressure vessels are constructed from plain carbon steels, lowand high alloy steels, other alloys, clad plate, and reinforcedplastics.
Selection of a suitable material must take into account thesuitability of the material for fabrication (particularly welding)as well as the compatibility of the material with the processenvironment.
The pressure vessel design codes and standards include lists ofacceptable materials; in accordance with the appropriatematerial standards.
In the United Kingdom, carbon and alloy steels for pressurevessels are covered by BS 1501 plates, BS 1502 section andbars. BS forgings, and BS 1504 castings.
Materials
General Design Considerations: Pressure Vessels
For design purposes it is necessary to decide a value for themaximum allowable stress (nominal design strength) that can beaccepted in the material of construction.
This is determined by applying a suitable "design stress factor"(factor of safety) to the maximum stress that the material could beexpected to withstand without failure under standard testconditions.
In the British Standard, BS 5500, the nominal design strengths(allowable design stresses), for use with the design methods given,are listed in the standard, for the range of materials covered by thestandard.
Typical design stress values for some common materials are shownin Table 1.
Design stress (nominal design strength)
General Design Considerations: Pressure Vessels
General Design Considerations: Pressure Vessels
The "corrosion allowance" is the additional thickness ofmetal added to allow for material lost by corrosion anderosion.
The allowance should be based on experience with thematerial of construction under similar service conditions tothose for the proposed design.
For carbon and low-alloy steels, where severe corrosion isnot expected, a minimum allowance of 2.0 mm should beused; where more severe conditions are anticipated thisshould be increased to 4.0 mm.
Most design codes and standards specify a minimumallowance of 1.0 mm.
Corrosion allowance
General Design Considerations: Pressure Vessels
Major loads1. Design pressure: including any significant static headof liquid.2. Maximum weight of the vessel and contents, under
operating conditions.3. Maximum weight of the vessel and contents under
the hydraulic test conditions.4. Wind loads.5. Earthquake (seismic) loads.6. Loads supported by, or reacting on, the vessel.
Design loads
General Design Considerations: Pressure Vessels
There will be a minimum wall thickness required to ensure thatany vessel is sufficiently rigid to withstand its own weight, andany incidental loads.
As a general guide the wall thickness of any vessel should not beless than the values given below; the values include a corrosionallowance of 2 mm:
Minimum practical wall thickness
Vessel Thickness and Diameter
The Design of Thin-Walled Vessels Under Internal Pressure
Minimum thickness required, e
1.0 Cylinders and spherical shells
Where Di is internal diameter f is the design stressand Pi, the internal pressure.
(1)
The Design of Thin-Walled Vessels Under Internal Pressure
The ends of a cylindrical vessel are closed by heads ofvarious shapes. The principal types used are: Flat plates and formed flat heads; Figure 13.9. Hemispherical heads; Figure 13.10a. Ellipsoidal heads; Figure 13.10b. Torispherical heads; Figure 13.10c.
2.0 Heads and closures
The Design of Thin-Walled Vessels Under Internal Pressure
Hemispherical, ellipsoidal and torisphericalheads are collectively referred to as domedheads.
They are formed by pressing or spinning; largediameters are fabricated from formed sections.
Torispherical heads are often referred to asdished ends.
2.0 Heads and closures
The Design of Thin-Walled Vessels Under Internal Pressure
Flat plates are used as covers for manways. Standard torispherical heads (dished ends) are the most
commonly used end closure for vessels up to operatingpressures of 15 bar.
Above 15 bar an ellipsoidal head will usually prove to be themost economical closure to use.
A hemispherical head is the strongest shape; capable ofresisting about twice the pressure of a torispherical head of thesame thickness.
The cost of forming a hemispherical head is higher than that fora shallow torispherical head.
Hemispherical heads are used for high pressures.
Choice of closure
The Design of Thin-Walled Vessels Under Internal Pressure
The thickness required will depend on thedegree of constraint at the plate periphery.
The minimum thickness required is given by:
3.0 Design of flat ends
(2)
The Design of Thin-Walled Vessels Under Internal Pressure
The values of the design constant and nominaldiameter for the typical designs shown in Figure13.9 are given below:
3.0 Design of flat ends
The Design of Thin-Walled Vessels Under Internal Pressure
The ratio of the hemispherical head thickness tocylinder thickness should be 7/17.
However, the stress in the head would then begreater than that in the cylindrical section; and theoptimum thickness ratio is normally taken as 0.6
4.0 Design of domed endsHemispherical heads
The Design of Thin-Walled Vessels Under Internal Pressure
Most standard ellipsoidal heads aremanufactured with a major and minor axis ratioof 2 : 1 .
For this ratio, the following equation can be usedto calculate the minimum thickness required:
4.0 Design of domed ends
Ellipsoidal heads
(3)
The Design of Thin-Walled Vessels Under Internal Pressure
The minimum thickness required:
4.0 Design of domed endsTorispherical heads
(4)
The Design of Thin-Walled Vessels Under Internal Pressure
4.0 Design of domed endsTorispherical heads
The ratio of the knuckle to crown radii should not be less than 0.06, to avoidbuckling; and the crown radius should not be greater than the diameter of thecylindrical section.For formed heads (no joints in the head) the joint factor J is taken as 1.0.
The Design of Thin-Walled Vessels Under Internal Pressure
Conical ends are used to facilitate the smoothflow and removal of solids from processequipment. Thickness required:
5.0 Conical sections and end closures
(5)
The Design of Thin-Walled Vessels Under Internal Pressure
For torispherical heads. Thickness required:
The design factor Cc is a function of the half apexangle a:
5.0 Conical sections and end closures
(6)
The length of the thickersection Lk depends on the coneangle and is given by:
(7)
Example 1Estimate the thickness required for the componentparts of the vessel shown in the diagram. The vessel isto operate at a pressure of 14 bar (absolute) andtemperature of 300C. The material of constructionwill be plain carbon steel. Welds will be fullyradiographed. A corrosion allowance of 2 mm shouldbe used.
Solution
Design of Vessels Subject to External Pressure
For an open-ended cylinder, the critical pressure tocause buckling Pc is given by:
1.0 Cylindrical shells
(8)
Design of Vessels Subject to External Pressure
For long tubes and cylindrical vessels this expression can besimplified by neglecting terms with the group (2L/pDo)2 in thedenominator; the equation then becomes:
For short closed vessels, and long vessels stiffening rings:
1.0 Cylindrical shells
where Kc is a function of the diameter and thickness of the vessel, and the effective length L' between the ends or stiffening rings; and is obtained from Figure 13.16. The effective length for some typical arrangements is shown in Figure 13.15.
(9)
(10)
Design of Vessels Subject to External Pressure
2.0 Design of stiffness rings
(11)
Design of Vessels Subject to External Pressure
Combining equations (11) and (12) will give an equation fromwhich the required dimensions of the ring can be determined:
2.0 Design of stiffness rings
(12)
(13)
Design of Vessels Subject to External Pressure
The critical buckling pressure for a sphere subject to externalpressure is given by:
Minimum thickness:
For a torispherical head the radius Rs is taken as equivalent tothe crown radius Rc.
For an ellipsoidal head the radius can be taken as themaximum radius of curvature; that at the top, given by Eq. 13.
3.0 Vessel heads
(14)
(15)
Design of Vessels Subject to External Pressure
3.0 Vessel heads
Example 2
A vacuum distillation column is to operate under atop pressure of 50 mmHg. The plates are supportedon rings 75 mm wide, 10 mm deep. The columndiameter is 1 m and the plate spacing 0.5 m. Check ifthe support rings will act as effective stiffening rings.The material of construction is carbon steel and themaximum operating temperature 50C, If the vesselthickness is 10 mm, check if this is sufficient.
Solution
References
R. K. Sinnott, Chemical Engineering Design, Coulson& Richardsons Chemical Engineering, Volume 6,Fourth edition, Elsevier, 2005.
Top Related