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Stationary Equipments 2014
Chapter No 2: Stationary Mechanical Equipments
Heat Exchanger Furnace Distillation Column Valve Boiler
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Stationary Equipments 2014
2.1 Heat Exchangers:
A heat exchanger is a device in which two fluid streams, one cold and another hot, are brought in thermal contact so that the heat is transferred from hot fluid to the cold fluid.
Heat exchangers are frequently used in chemical process industries, as well as in refrigeration, cogeneration and waste heat etc. In Oil Refining Complex 2 of Byco Petroleum, one purpose of heat exchanger is to preheat the crude.
The heat exchanger design code for mechanical design calculation is Tubular Exchanger Manufacturers Association (TEMA).
Classification of Heat exchangers:Heat exchangers are classified on basis of their construction and flow arrangement through them;
Shell and Tube Heat Exchanger:
Shell and tube heat exchanger are one of the most common heat exchangers found in all plants, they are most versatile heat exchanger and can provide large heat transfer area in small space. . The fluid flowing inside the tubes is called the tube side fluid and the fluid flowing on the outside of the tubes is the shell side fluid. Ease of maintenance is the reason that shell and tube type exchangers are preferred in every process industries as well as in other industries. U-type and I-type are the common types of shell and tube heat exchangers. In U-type heat exchanger there is no floating head.
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Heat Exchangers
Plate (plate and frame)
Multi-pass (parallel flow, counter flow, split flow, divided flow)
Tubular (shell and tube)
Single pass (parallel flow, counter flow, cross flow)
Construction
Flow arrangement
Stationary Equipments 2014
Components of Heat Exchanger:Main components of heat exchanger are:
Face Plate/Channel cover Channel head Shell Floating head Floating cover Tubes Baffle Tube sheet
The SHELL has the circular cross section and it provides a passage for the shell-side fluid. Selection of material for the shell is dependent upon corrosiveness of fluid and on working temperature and pressure. Most common material for shell is Carbon Steel.
The TUBES provide heat transfer area in shell and tube heat exchanger, tubes of 19mm and 25mm are commonly used. Common material for tube is carbon steel A-179.
BAFFLES are metallic plates usually in form of segments of circle having holes to accommodate the tubes. Functions of Baffles are- (i) to support the tubes, (ii) to maintain the tube spacing, (iii) to alter the flow direction which increases the heat transfer duration.
Tubesheets are plates having holes through which tubes are inserted. Tubes are appropriately secured to the tubesheet so that the fluid on the shell side is prevented from mixing with the fluid on the tube side.
Channels or heads are required for shell and tube heat exchangers to contain the tube side fluid and to provide the desired flow path.
Construction:
A set of tubes are contained in shell At the ends of the tubes, the tube side fluid is separated from the shell side fluid by the tube sheet(s).The tubes are rolled and press-fitted or welded into the tube sheet to provide a leak tight seal. Head(s) are bolted to the shell.
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Stationary Equipments 2014
KETTLE TYPE REBOILER:
Reboilers are heat exchangers typically used to provide heat to the bottom of distillation columns. They boil the liquid from the bottom of a distillation column to generate vapors which are returned to the column to drive the distillation separation. The heat supplied to the column by the reboiler at the bottom of the column is removed by the condenser at the top of the column.
Working:
Kettle reboilers are very simple and reliable. They may require pumping of the column bottoms liquid into the kettle, or there may be sufficient liquid head to deliver the liquid into the reboiler. In this reboiler type, steam flows through the tube bundle and exits as condensate. The liquid from the bottom of the tower, commonly called the bottoms, flows through the shell side. There is a retaining wall or overflow weir separating the tube bundle from the reboiler section where the residual reboiled liquid (called the bottoms product) is withdrawn, so that the tube bundle is kept covered with liquid and reduce the amount of low-boiling compounds in the bottoms product. A pressure safety valve is also installed to discharge steam when the internal pressure exceeds the preset value.
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Stationary Equipments 2014
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Sectioned view of U-type heat exchanger
Stationary Equipments 2014
2.2 Furnace:
A furnace is a device used for high temperature heating. It may be used as the heat source for fractional distillation columns.
Construction and Working:
Fuel flows into the burner and is burnt with air provided from an air blower. There can be more than one burner in a particular furnace which can be arranged in cells which heat a particular set of tubes. Burners can also be floor mounted, wall mounted or roof mounted depending on design. The flames heat up the tubes, which in turn heat the fluid inside in the first part of the furnace known as the radiant section. In this chamber where combustion takes place, the heat is transferred mainly by radiation to tubes around the fire in the chamber. The heating fluid passes through the tubes and is thus heated to the desired temperature. After the flue gas leaves the radiant section, most furnace designs include a convection section where more heat is recovered before venting flue gas to the atmosphere through the stack.
Common materials for the tubes in the furnace are P3, P5, P7, P11, and chemical requirement of these materials are given below in the table:
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Stationary Equipments 2014
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Crude Furnace
Stationary Equipments 2014
2.3 Column:
Distillation columns or towers are constructed to behave in the same way as a series of separate stills. Each 'still' section consists of a number of 'TRAYS' or contacting devices arranged vertically above one another in the column. The height of the tower and the number of trays or contacting devices it contains, depend upon the purity of the 'Fractions' required.
The Tray Type Column:
This is a tall, cylindrical column. Inside a series of trays are placed, one above the other. The trays are used to bring the rising vapour and falling liquid into intimate contact. They are very much more efficient in the separation process than other types of column and, they are also more costly.
Basic Operation in Distillation Column:
The mixture of two or more substances (liquids) with different boiling points are heated to a high temperature. The vapor enters the bottom of a long column (fractional distillation column), and rises in it. As the vapor rises through the trays in the column, it cools. When a substance in the vapor reaches a height where the temperature of the column is equal to that substance's boiling point, it will condense to form a liquid. The trays collect the various liquid fractions and finally the liquid is separated from the column.
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Stationary Equipments 2014
2.5 Valves:A valve is a device that regulates, restricts or controls the flow of liquids, gases and slurries through pipes and tubes.
Components of Valve:
Main components of any valve are body, bonnet, stem, handle.
Body: The valve's body is the outer casing of most or all of the valve that contains the internal parts.
Bonnet: A bonnet acts as a cover on the valve body. It is commonly semi-permanently screwed into the valve body or bolted onto it.
Stem: The stem transmits motion from the handle or controlling device to the disc. The stem typically passes through the bonnet when present.
Handle or actuator: A handle is used to manually control a valve from outside the valve body.
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Stationary Equipments 2014
Types of Valves:
Common types of valve include,
Gate valves: Gate valves are used to restrict the flow of fluids passing through a channel. They are operated by a circular wheel which is used to ascend or descend a rectangular gate in the passage of the fluid in order to restrict it. They have several advantages of less pressure drop and less/no leakage, they also provide good sealing when closed.Gate valves have several disadvantages as well, they produce noise and cavitation problem when opened partially.
Butterfly valves: A butterfly valve can be used to isolate and regulate the flow of a fluid. A butterfly valve functions like a disc mechanism. A disc is placed parallel or perpendicular to the flow direction as is connected to an actuator via connecting rod. When the actuator is rotated, the disc rotates and can be used to regulate and restrict the flow. These valves are good for large flow and low pressure. They can be operated easily and quickly and can be throttled.
Ball valves: Ball valves are most commonly used at any industry. It has a spherical ball with a central hole throughout so that when the hole is in line with the ends of the pipe, liquid starts to flow and when the hole is perpendicular to the ends of the pipe, the valve is closed. These valves are used for frequent operations as they can be opened or closed quickly. These valves have many advantages. They are less expansive, have low maintenance cost, have compact construction, no lubrication required, and provide tight sealing. But they are poor for throttling and throttling of pump can lead to the erosion of seat.
Globe valves: A globe valve is used for controlling and regulating the flow of liquids inside the channel. It consists of a disc type element with a stationary ring seat in a spherical body. This has an opening that forms a seat onto which a movable plug can be screwed in to close (or shut) the valve. In globe valves, the plug is connected to a stem which is operated by screw action using a hand-wheel in manual valves. These valves provide throttling and regulating of flow. But these valves can be very large and heavy for particular application. Other disadvantages include high head loss and noisy at high pressures.
Fire Hose: Main function of fire hose valve is to control the flow of fluid through various types of fire hoses for fire protection. Each hose valve is designated with a disc that is lifted or lowered into the seat
that opens or closes for fluid flow access to a hose.
Safety valve: It is a valve mechanism which automatically releases a substance from a boiler, pressure vessel, or other system, when the pressure or temperature exceeds preset limits. Two common types of safety valve are pressure safety valve (PSV) and pressure relief valve (PRV).
Pressure safety valves (PSV) are installed on gas pipe lines, when pressure exceeds a certain valve PSV operates and releases the excess pressure. PSV releases the pressure into atmosphere and these valves can only be calibrated by manufacturers.
Unlike pressure safety valve (PSV), pressure relief valves (PRV) are installed on liquid pipe lines, and they operate in proportional with the exceeding pressure. PRVs can also be calibrated by users.
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Stationary Equipments 2014
2.5 Boiler:
A boiler is a closed cylindrical vessel in which water is heated and converted into superheated steam and then this steam may be utilized for many purposes in any industry. In ORC-II, a water tube boiler is installed which has steam capacity of 50tons/hour, and this steam is being used for preheating the crude and to drive the steam turbine which in result drive the pump.
Types:
There are two major types of boiler, one is fire tube and other is water tube boiler.
In fire tube boilers water is kept inside the shell and the hot gases are passed through the tubes to convert water into steam. These types of boilers are not preferred in large industries because they have a tendency to burst when the steam pressure inside the boiler shell exceeds the set value and also the fire may cause damage to the tubes. Moreover in fire tube boiler only saturated steam can be produced that is why they have limited output. These boilers have a longer start-up time from a cold condition.
While in water tube boilers, water flows inside the tubes and the hot flue gases pass over the tubes. The heat transfer causes the water in tubes to convert into superheated steam. These types of boilers are widely used in large industries. There are further types of water tube boilers which include longitudinal drum boiler, cross drum boiler, and bent tube boiler. Bent tube boilers offer larger heating surface area in small space.
Main Components of Boiler:
At Byco, a water tube boiler is used to produce superheated steam, the basic parts of the boiler are listed below:
Mud chamber: It contains water which is then derived into the boiler shell in which subsequent heating will superheats it to steam
Steam chamber: The steam chamber collects the initial or what we may call the saturated steam which is further sent to the economizer for superheating.
Risers: Risers are the tubes through which saturated steam enters and gets collected in the steam chamber. The steam passes through the risers is saturated but not superheated.
Down tubes: These are the tubes which provide a returning path to the condensed steam to the mud chamber.
Fire head or fire section: Burners are placed inside the fire section which heats the water in the tubes. In order to avoid damage to the tubes such as due to thermal cracking, the tubes are sleeved. The internal temperature and pressure is controlled by a butterfly damper which is driven by a motor. The saturated steam is now taken to the economizer.
Economizer: The economizer which takes up the saturated steam now superheats the steam and the flue gases are discharged off to the fuel oil furnace.
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Stationary Equipments 2014
2.6 Pressure Vessels:
Pressure Vessels are used to store the fluids under pressure. Pressure vessels may be cylindrical or spherical.
Design Factor:
(i)Shape: Type (cylindrical or spherical) of vessel used depend upom application, location, nature of the fluid, and operating temperature and pressure.
(ii)Properties of Material used: appearance of unit, induced stresses, and cost and fabrication of material.
Classifications:
Pressure vessels are classified as
(i) Thin shells(ii) Thick shells
There are many criterion on which vessels are classified as thick or thin.
(a) Thin shells have t/ri ratio less than 1/20, otherwise shells are said to be thick.where ‘t’ is thickness and ‘ri’ is internal radius.
(b) According to ASME code, if P is greater than 0.385SE then vessel would be thick.where ‘P’ is internal pressure, ‘S’ is strength, and ‘E’ is joit efficiency.
Stresses:
Stresses induced in a vessel because of the fluid contained act in two directions, one is longitudinal and other is circumferential direction. Circumferential stress is also called Hoop stress and magnitude of Hoop stress is double as the longitudinal direction.
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