Pressure Vessels

Pressure Vessels

A pressure vessel is defined as a container. Tanks, vessels, and pipelines which carry, store, or receive fluids are called pressure vessel. They are closed vessels which are capable of storing a pressurized fluid, and have a pressure differential between inside and outside, regardless of their shape and dimensions. Normally the pressure inside is more than outside of the pressure vessel excepting for few cases like submarines etc. Pressure vessels frequently have a combination of high pressure together with high temperature and in some cases flammable fluids or highly radioactive materials are stored I them. They are subjected to varied type of loads. They are most commonly used in different industries as heat exchangers, reactors, and storage vessels etc.

Pressure vessels are containers in which materials are processed, treated, or stored. They are vital part of operational units in the process industries. They are to perform required functions as needed by the process. They are used to carry out process operations such as distillation, drying, filtration, stripping, and reaction. These operations normally involve many different types of vessels, ranging from large towers / columns to small additive and waste collection drums. Pressure vessels are also used to provide intermediate storage between the processing steps. They can provide residence time for the reactions to complete or for contents to settle. These vessels are also being used for handling liquids and gases under pressure in the process units. The shape of the pressure vessels are to meet the requirements of the process.

Pressure vessels used in process industry are to be leak-tight pressure containers. The substance inside the pressure vessel can undergo change of phase such as water to vapour etc. In the reactor pressure vessels, the substance can mix with some other chemical substance for the required chemical reaction to take place. These types of conditions in the pressure vessel require the safety of the vessel for preventing the possible bursting or cracking and spoiling of the entire operation of the manufacturing process.

Due to the pressure acting on the pressure vessel, there is a chance for the contained fluid to leak out which can cause serious accident and loss of life. For this reason, the design, fabrication and testing techniques for the pressure vessels are controlled by the regulatory provisions. Further, all the pressure vessels used in the industries are to be certified by the regulatory agencies.

Pressure vessels can be classified according to their intended service such as temperature, pressure, materials, and geometry. Classification of the pressure vessels according to its use and shape is given in Fig 1. According to the intended use, the pressure vessels can be divided into storage containers and process vessels. Storage pressure vessels are used only for the storage of the fluids under pressure, and in accordance with the service they are known as storage tanks such as fuel oil storage tank or LPG (liquefied petroleum gas) tank etc. Process pressure vessels have multiple and varied uses, among them they can be heat exchangers, reactors (both horizontal and vertical furnaces come under this category), fractionating columns, and distillation columns etc. According to the shape, pressure vessel can be of cylindrical or spherical in shape, with different head configurations. The cylindrical vessels are normally preferred since they are simple, easier to manufacture, and make better use of the available space. The cylindrical vessels can be horizontal or vertical, and in some cases can have coils to increase or lower the temperature of the fluid. Boilers, heat exchangers, and chemical reactors etc. are normally cylindrical.

Fig 1 Classification of the pressure vessels

Spherical pressure vessels are normally used as storage tanks, and are recommended for storing large volumes. Since the spherical shape is the ‘natural’ form which the materials normally adopt when subjected to internal pressure, and hence spherical vessels are the most economical way for the storage of the pressurized fluids. However, the manufacture of these vessels is much more complicated and expensive compared the cylindrical pressure vessels. Spherical and the cylindrical pressure storage vessels are shown in Fig 2.

Fig 2 Types of pressure storage vessels

Pressure vessels are normally made from carbon steels or stainless steels and assembled by welding. In the earlier days, the operation of pressure vessels and boilers resulted in a number of explosions, causing loss of life and considerable property damage. Around 1920s, American Society of Mechanical Engineers (ASME) formed a committee for the purpose of establishing minimum safety standards for the construction for boilers. In 1925, the committee issued a set of rules for the design and construction of unfired pressure vessels. Presently all the countries have their national standards and Codes of practice for the pressure vessels. The Codes are living documents in that they are constantly being revised and updated by committees composed of individuals knowledgeable on the subject. Keeping current requires that the revised Codes be published after a certain period of time with addendas issued in between.

The design criteria in the Codes consist of basic rules specifying the design methods, design loads, allowable stresses, acceptable materials, and fabrication-inspection certification requirements for the manufacture of the pressure vessels. The Code guidelines cover several aspects of the designing of the pressure vessel, which include the material, allowable stress, welding, corrosion allowance, and sizing of the equipment.

Because of many hazards, it is imperative that the design of the pressure vessel is such that no leakage can occur. In fact, pressure vessels are designed to contain pressure and withstand the operating, mechanical, and thermal transients for a specified design life. In addition, they are designed to have safety to ‘leak before break’ (LBB). They are designed and manufactured following the practices mentioned in the standards and Codes, and hence do not require a detailed evaluation of all stresses. It is recognized that high localized and secondary bending stresses can exist but these are allowed for with a higher safety factor and design criteria for details of the pressure vessels. It is required, however, that all loadings (the forces applied to a vessel or its structural attachments) are to be considered while designing a pressure vessel..

Pressure vessels are designed to withstand the maximum pressure to which they are likely to be subjected in operation. For vessels under internal pressure, the design pressure (sometimes called maximum allowable working pressure) is taken as the pressure at which the relief device is to be set. This is normally 5 % to 10 % above the normal working pressure to take care of the possible operational disturbances during the minor process upsets. When the design pressure is decided, the hydrostatic pressure in the base of the column is to be added to the operating pressure, if it is significant. Pressure vessels subject to the external pressure are to be designed to resist the maximum differential pressure which is likely to occur in service. Pressure vessels which are likely to be subjected to vacuum are to be designed for a full negative pressure of 1 bar, unless fitted with an effective, and reliable, vacuum breaker.

In addition, the pressure vessels have to be designed carefully to cope with the operating temperatures. The strength of metals decreases with increasing temperature, so the maximum allowable stress depends on the temperature of the material. The maximum design temperature at which the maximum allowable stress is evaluated is to be taken as the maximum working temperature of the material, with due allowance for any uncertainty involved in predicting vessel wall temperature. The minimum design metal temperature is to be taken as the lowest temperature expected in service.

While the Code gives formulas for thickness and stress of basic components, it is upto the designer to select appropriate analytical procedures for determining stress due to the other loadings. The designer is also to select the most probable combination of the simultaneous loads acting on the vessel for an economical and safe design. Further, pressure vessels constructed in such a manner that the presence of a sudden change of section producing a notch effect is not recommended to be there especially for the low temperature range operations. Since the notches can create a state of stress which the material is incapable of relaxing high-localized stresses by plastic deformation, the materials used for low temperature operations are tested for notch ductility.

Steel is the material of choice for the pressure vessels because of its many advantages. As a construction material, steel is strong, affordable, reliable, and environmentally friendly. The unique combination of properties and characteristics of steel enable it to achieve performance levels required for the present day pressure vessels. The materials to be used in pressure vessels are to be selected from Code-approved material specifications. This requirement is normally not a problem since a large catalogue of tables listing acceptable materials is available. Factors which are needed to be considered in picking a suitable material are (i) cost, (ii) fabricability, (iii) service condition (wear, corrosion, operating temperature), (iv) availability, and (v) strength requirements.

Several of materials are used for the fabrication of the pressure vessels. The selection of material is based upon its appropriateness to meet the design requirements. All the materials used in the manufacture of the pressure vessels are to comply with the requirements of the relevant design Code, and to have traceability with the test certificate of the material. The selection of materials of the shell is to take into account the suitability of the material with the maximum working pressure as well as the requirements of the fabrication process.

Pressure vessels are made from plain carbon steels, low alloy and high alloy steels, other alloys, clad plate, and reinforced plastics. Selection of a suitable material is to take into account the suitability of the material for fabrication (particularly welding), as well as the compatibility of the material with the process environment. Carbon steels can be used down to 60 degree C. Notch ductility is controlled in such materials through proper composition, steel making practice, fabrication practice, and heat treatment. These materials have an increased manganese carbon ratio. Aluminum is normally added to promote fine grain size and to improve notch ductility. Embrittlement of carbon and alloy steel can occur due to service at elevated temperature. This is controlled by the addition of molybdenum which also improves tensile and creep properties. Two main criteria in selecting the steel for the elevated temperature service are metallurgical strength and stability. Carbon steels are reduced in their strength properties due to rise in temperature and are liable to creep. Hence, the use of carbon steel is normally limited to 500 deg C. Within the carbon steel, the quality and the composition of the steel decide the temperature and pressure upto which the pressure vessel can function safely.

The steel quality is to meet the guidelines of the Code. Plain carbon and low alloy steels plates are normally used where service conditions permit because of the lesser cost and greater availability of these steels. Pressure vessels from such steels can be fabricated by fusion welding and oxygen cutting if the carbon content does not exceed 0.35 %. The important materials normally used for the manufacture of the pressure vessels are generally divided into three groups as given below.

  • The first group of materials includes low cost materials such as cast iron, cast carbon and low alloy steel, and wrought carbon and low alloy steel. Steel under this group is to have fine grain, high toughness, and good surface quality. It is to have good weldability and good corrosion resistance for extreme weather conditions. Pressure vessels with formed heads are normally fabricated from low carbon steel wherever corrosion and temperature considerations permit its use because of the low cost, high strength, ease of fabrication and general availability of mild steel. Low alloy steels are used for special service.
  • The second group of materials includes medium cost materials such as high alloy steel (12 % chromium and above), aluminum, nickel, copper and their alloys, and lead. High alloy steels and non-ferrous metals are used for special service. Under this group comes pressure vessels made from stainless steels. Stainless steel pressure vessels are manufactured from 304, 304 L, 316 or 316 L grades of stainless steel for compatibility with many corrosive environments. Stainless steel provides a clean, contemporary appearance and it is corrosion-resistant to a large number of liquids and recommended when carbon steel or internal linings are not compatible with material being stored or processed.
  • The third group of materials includes high cost materials such as platinum, tantalum, zirconium, and titanium silver.

The allowable stress used to determine the minimum vessel wall thickness is based on the material tensile and yield strengths at room and at the design temperature. For design purposes, it is essential to decide a value for the maximum allowable stress (nominal design strength) which can be accepted for the material of construction. This is determined by applying a suitable safety factor to the maximum stress which the material can be expected to withstand without failure under standard test conditions. The safety factor allows for any uncertainty in the design methods, the loading, the quality of the materials, and the workmanship.

At temperatures where creep and stress rupture strengths do not govern the selection of stresses, the maximum allowable stress is the lowest of (i) the specified minimum tensile strength at room temperature divided by 3.5, (ii) the tensile strength at design temperature divided by 3.5, (iii) the specified minimum yield strength at room temperature divided by 1.5, and (iv) the yield strength at design temperature divided by 1.5.

At temperatures where creep and stress rupture strength govern, the maximum allowable stress which is considered is the lowest of (i) the average stress to produce a creep rate of 0.01 % / 1,000 hours, (ii) ‘X’ times the average stress to cause rupture at the end of 100,000 hours, where X = 0.67 for temperatures below 815 deg C, and (iii) 0.8 times the minimum stress to cause rupture after 100,000 hours.

The use of a welded joint can result in a reduction in strength of the part at or near the weld. This can be due to the metallurgical discontinuities and residual stresses. The efficiency of a welded joint depends only on the type of joint and on the extent of examination of the joint and it does not depend on the extent of examination of any other joint. The strength of a welded joint depends on the type of joint and the quality of the welding. There are four types of th welds namely (i) longitudinal or spiral welds in the main shell, necks or nozzles, or circumferential welds connecting hemispherical heads to the main shell, necks, or nozzles, (ii) circumferential welds in the main shell, necks, or nozzles or connecting a formed head other than hemispherical, (iii) welds connecting flanges, tube sheets, or flat heads to the main shell, formed head, neck, or nozzle, and (iv) welds connecting communicating chambers or nozzles to the main shell, to heads, or to necks.

The possible lower strength of a welded joint compared with the original plate is normally allowed for in design by multiplying the allowable design stress for the material by the weld joint efficiency. The value of the weld joint efficiency used in design depends on the type of joint and amount of radiography needed by the design code.

Corrosion occurring over the life of a pressure vessel is to be catered to by a corrosion allowance, the design value of which depends upon the vessel duty and the corrosiveness of its content. The corrosion allowance is the additional thickness of metal which is necessary to allow for the material loss due to the corrosion and erosion, or scaling. Corrosion is a complex phenomenon, and it is not possible to give specific norms for the estimation of the corrosion allowance required for all the situations. For carbon and low-alloy steels, where severe corrosion is not expected, a minimum allowance of 2 mm is normally used. Where more severe conditions are anticipated, this allowance is increased to 4 mm. Majority of the design codes and standards specify a minimum allowance of 1 mm.

A pressure vessel is a structure which is to be designed to resist gross plastic deformation and collapse under all the conditions of loading. The loads can be classified as (i) major loads, which are always to be considered in the vessel design, and (ii) subsidiary loads. Major loads are (i) design pressure which is to include any significant static head of liquid, (ii) maximum weight of the vessel and contents, under operating conditions, (iii) maximum weight of the vessel and contents under the hydraulic test conditions, (iv) wind loads, (v) seismic (earth quake) loads, and (vi) those loads which are supported by, or reacting on, the vessel. Subsidiary loads are (i) local stresses which are caused by supports, internal structures, and connecting pipes, (ii) shock loads which are caused by water hammer or by surging of the vessel contents, (iii) bending moments which are caused by eccentricity of the centre of the working pressure relative to the neutral axis of the pressure vessel, (iv) stresses which are due to the temperature differences and differences in the coefficient of expansion of materials, and (v) loads which are caused by fluctuations in temperature and pressure. The Code considers design pressure, design temperature, and, to some extent, the influence of other loads which impact the circumferential (or hoop) and longitudinal stresses in shells. It is left to the designer to account for the effect of the remaining loads on the vessel. Various national and local Codes are to be consulted for handling wind and earthquake loadings.

There is a minimum wall thickness of the pressure vessel is needed to ensure that the vessel is sufficiently rigid to withstand its own weight and any incidental loads. As a general rule the wall thickness of any vessel is not to be less than the value which includes a corrosion allowance of 2 mm.

Production process for pressure vessels

Pressure vessel fabrication is an extremely complicated process. It is a detailed and precise process. It involves the controlled cutting, forging or rolling, bending, welding and assembling processes of metals for vessel fabrication. Immense care needs to be taken to ensure that the quality of the vessel is excellent, and that the fabrication meets all the requirements of the Codes. Aside from that, there are several safety requirements which are to be adhered. Standards and Codes basically provide rules for design and fabrication of pressure vessels in order to ensure their safety, increase their longevity and functional value. A brief overview of pressure vessel fabrication process is described below. The entire production consists of several steps. Before the start of the fabrication process, there is a design step.

In the design step, first the design criteria are studied. The design criteria includes such aspects as the shape of the vessel, and details such as material of construction, length, diameter, internal pressure and  temperature etc. After this, with the help of the pressure vessel design software, the mechanical calculations are performed to determine the required material thicknesses and weld sizes. Then detailed fabrication drawings are made based on the design criteria and mechanical calculations for the purpose of the fabrication.

Fabrication of the pressure vessels is governed by the Codes. Under the Codes there are some requirements which are mandatory while there are others which are non-mandatory guidance for designing and fabrication and welding techniques along with choice of materials. A typical fabrication process for the pressure vessel is shown in Fig 3.

Fig 3 Typical fabrication process for the pressure vessel

In order to assemble or fabricate and tack the welded metal parts in place during the fabrication of the pressure vessels some or all of the following steps are needed.

First is the selection and collection of raw materials which are normally used materials. These materials can be plate, pipe, forgings, structural shapes, welding rod and wire, etc. The required number of plates is then cut into the required width and length for the vessel shells. A specialized cutting torch is used to cut the steel. A variety of torches can be used depending on material and required edge quality. Bevels are usually cut on edges at this time. After the cutting of the raw material as per the specified requirements, then the machining of certain parts is done if needed.

The cut plates are rolled into cylinders of required diameter. Normally the plate is rolled cold, but can be done hot in order to use plate rolls of a smaller capacity. Then the welding of the long seams of the cylinders is done. This activity is normally performed with submerged arc welding. All the welding operations are to be carried our only by the trained and certified welders.The next step consists of fitting and the welding of the pressure vessel cylinders which is normally performed with submerged arc welding. This is only needed if more than one shell is required to meet the required vessel length.

Next step consists of cutting and forming of the pressure vessel heads. Various cutting torches can be used to cut the plate for the pressure vessel head. Then the pressure vessel heads are formed using a variety of techniques, normally flanging and spinning or press forming is used. Assembly and welding of the pressure vessel heads to the shell is normally performed with submerged arc welding. After every welding, the quality of the weld is checked using non-destructive testing techniques for ensuring the integrity of the weld.

The next step is the cutting the holes for the nozzles. Holes are required to be cut into the steel plate for the required vessel nozzles. This is normally performed with a manual plasma-arc or oxy-acetylene cutting torch. Final edge preparation is performed by manual grinding.

Next the installation of the nozzles is done. Nozzles and reinforcement pads are welded by using a manual wire welding technique. Then the required structural supports and lifting devices are normally welded on using a manual wire welding technique. After this final quality checks are carried out. These checks are done by non destructive testing techniques. After the non destructive testing the pneumatic and /or hydrostatic pressure test is done for ensuring the integrity of design, material, and welding. After the pressure vessel has passed these tests, it is dispatched to the customer.

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