Process Steam and its use in a Steel Plant
Process Steam and its use in a Steel Plant
Process steam is the general term used for steam which is used in process applications such as a source of energy for process heating, process cooling, pressure control and mechanical drives among others. Process steam is a popular mode of conveying energy and may come into contact with the final process or product.
Like compressed air, steam is often thought of as a utility which is often generated at a central location and then distributed to various points-of-use throughout the steel plant.
At atmospheric pressure the saturation temperature is 100 deg C. However, if the pressure is increased, this allows the addition of more heat and an increase in temperature without a change of phase. Therefore, increasing the pressure effectively increases both the enthalpy of water, and the saturation temperature.
Water and steam can coexist at any pressure, both being at the saturation temperature. Steam at a condition above the saturation condition is known as superheated steam. Temperature above saturation temperature is called the degree of superheat of the steam and water at a condition below the saturation condition is called sub-saturated water. Steam used as process steam is usually of the following types. (Fig 1)
- Saturated steam – Steam is said to be ‘saturated’ with energy at a given and constant pressure when the addition of more heat to the generation system results in more steam, but no rise in steam temperature. In this state, the steam cannot hold more heat energy in a given volume unless pressure is allowed to rise.
- Dry steam – It is the steam that contains 100 % of water vapour in the gas phase.
- Dry saturated steam – Achieving the above states of dry and saturated steam simultaneously is possible in theory. It is nearly impossible in practice when systems are optimized for generating saturated steam. The actual level achieved is measured as the ‘dryness fraction’. If the water content of the steam is 4 % by mass, then the steam is said to be 96 % dry and has a dryness fraction of 0.96.
- Superheated steam – When more heat energy is added to steam that has reached saturation, and no liquid water is present to consume that energy through evaporation, the temperature of the steam increases. In this condition, steam is said to be ‘superheated’.
Fig 1 Types of process steams
Depending on the intended use, and whether or not process steam comes into contact with the final product itself, it is to be filtered. When it comes to the transfer of energy, steam provides some unique characteristics which include, but are not limited to, the following.
- Steam has the ability to hold a great deal of energy, stored as heat, in a given volume.
- Steam gives up its heat energy at a constant temperature, eliminating heat gradients associated with other forms of energy transfer.
- Steam has a high rate of heat transfer, allowing for smaller heat transfer surface areas.
- Steam has the highest specific heat and latent heat
- Steam is easy to control and distribute
- Steam is cheap and inert
Process steam today is an integral and essential part of modern technology. Steam provides a means of transporting controllable amounts of energy from a central generating point, where it can be efficiently and economically generated, to the point of use. Therefore as process steam moves around a plant it can equally be considered to be the transport and provision of energy. Reasons for using process steam Include the following.
- Process steam is efficient and economical to generate
- Process steam can easily and cost effectively be distributed to the point of use
- Process steam is easy to control
- Energy is easily transferred to the process
- Process steam is flexible
Process steam is a principle energy source for some of the processes. It provides energy for process heating, process cooling, pressure control, mechanical drives, and component separation, and is also a source of water for many operations and chemical reactions. The popularity of process steam as an energy source stems from its many advantages, which include low toxicity, transportability, high efficiency, high heat capacity, and low production costs relative to other energy transport mediums.
The ability of steam to retain a significant amount of energy on a unit mass basis (normally within the range of 555 kcals per kg and 695 kcals per kg) makes it ideal for use as an energy transport medium. Since most of the heat contained in steam is in the form of latent heat, large quantities of energy can be transferred efficiently at a constant temperature, which is a useful attribute in many process applications. Steam is also used in contact applications such as the reforming of natural gas for direct reduced iron production. It is also used for stainless steel production in CLU process. In addition, process steam systems are used to control the pressures and temperatures of many processes, and in applications such as process heating and cooling, stripping of contaminants, facilitating fractionation of hydrocarbons, in certain drying operations, creation of vacuum (using steam ejector), operation of mechanical drive, quenching (regulation of the reaction temperature), dilution, injection, and source of process water.
The working pressure
The distribution pressure of steam is influenced by a number of factors, but is limited by the following.
- The maximum safe working pressure of the steam generating unit
- The minimum pressure needed at the consumer end
As steam passes through the distribution pipework, it inevitably loses pressure due to the following.
- Frictional resistance within the pipework
- Condensation within the pipework as heat is transferred to the environment.
Hence, allowance is required to be given for this pressure loss when deciding upon the initial distribution pressure.
Process steam must satisfy the following criteria be available at the point of use.
- In the correct quantity to ensure that a sufficient heat flow is provided for heat transfer
- At the correct temperature and pressure, or performance will be affected
- Free from air and incondensable gases which act as a barrier to heat transfer
- Clean, as scale (e.g. rust or carbonate deposit) or dirt have the effect of increasing the rate of erosion in pipe bends and the small orifices of steam traps and valves
- Dry, as the presence of water droplets in steam reduces the actual enthalpy of evaporation, and also leads to the formation of scale on the pipe walls and heat transfer surface.
The steam distribution system is the essential link between the steam generator and the steam user. Whatever the source, an efficient steam distribution system is essential if steam of the right quality and pressure is to be supplied, in the right quantity, to the steam using equipment or process. Installation and maintenance of the steam system are important issues which are required to be considered at the design stage.
As steam condenses in a process, flow is induced in the supply pipe. Condensate has a very small volume compared to the steam, and this causes a pressure drop, which causes the steam to flow through the pipes. The steam generated in the steam generator must be conveyed through the pipe work to the point where its heat energy is needed. Initially there are one or more main pipes, or ‘steam mains’, which carry steam from the steam generator in the general direction of the steam using plant. Smaller branch pipes then carry the steam to the individual pieces of equipment.
When the steam generator main isolating valve (sometimes called as the ‘crown’ valve) is opened, steam immediately passes from the steam generator into and along the steam mains to the points at lower pressure. The pipe work is initially cooler than the steam, so heat is transferred from the steam to the pipe. The air surrounding the pipes is also cooler than the steam, so the pipe work also begins to transfer heat to the air.
General layout and location of steam consuming equipment is of great importance in efficient distribution of steam. Technically steam pipes are to be laid by the shortest possible distance. However in reality a compromise is necessary while laying steam carrying pipes to suit the overall layout.
Apart from proper sizing of pipe lines, provision is also required for proper draining of the condensate which is bound to form as steam travels along the pipe.
Steam on contact with the cooler pipes will begin to condense immediately. On start-up of the system, the condensing rate will be at its maximum, as this is the time where there is maximum temperature difference between the steam and the pipework. This condensing rate is commonly called the ‘starting load’. Once the pipework has warmed up, the temperature difference between the steam and pipework is minimal, but some condensation still occurs as the pipe work continues to transfer heat to the surrounding air. This condensing rate is normally called the ‘running load’.
The resulting condensation (condensate) falls to the bottom of the pipe and is carried along by the steam flow and assisted by gravity, due to the gradient in the steam main that must be arranged to fall in the direction of steam flow. The condensate is usually drained from various strategic points in the steam main.
When the valve on the steam pipe serving an item of steam using shop is opened, steam flowing from the distribution system enters the shop and again comes in contact with cooler surfaces. The steam then transfers its energy in warming up an equipment and product (starting load), and, when up to temperature, continues to transfer heat to the process (running load).
There is now a continuous supply of steam from the steam generator to satisfy the connected load and to maintain this supply more steam must be generated. The condensate formed in both the steam distribution pipe work and in the process equipment is a convenient supply of useable hot boiler feed water. Although it is important to remove this condensate from the steam space, it is a valuable commodity and should not be allowed to run to waste. Returning all condensate to the boiler feed tank closes the steam energy loop, and is needed to be practiced wherever practical.
The objective of the steam distribution system is to supply steam at the correct pressure to the point of use. Hence the pressure drop through the distribution system is an important feature.
Proper sizing of steam pipelines help in minimizing pressure drop. The velocities for various types of steam are to be in the following range.
- Superheated steam – 50–70 m/sec
- Saturated steam – 30–40 m/sec
- Wet steam – 20–30 m/sec
Generating and distributing steam at higher pressure offers three important advantages as given below.
- The thermal storage capacity of the steam generator is increased, helping it to cope more efficiently with fluctuating loads, minimizing the risk of producing wet and dirty steam.
- Smaller bore steam mains are required, resulting in lower capital cost.
- Smaller bore steam mains cost less to insulate.
Having distributed at a high pressure, it is necessary to reduce the steam pressure to each zone or point of use in the system in order to correspond with the maximum pressure required by the application. Local pressure reduction to suit individual plant also results into dried steam at the point of use.
The most important components of a steam distribution system are (i) pipes, (ii) drain points, (iii) branch lines, (iv) strainers, (v) filters, (vi) separators, (vii) steam traps, (viii) air vents, and (ix) insulation. Other important aspects of process steam distribution system are as follows.
- The steam mains should be run with a falling slope of not less than 125 mm for every 30 metres length in the direction of the steam flow.
- Drain points are to be provided at intervals of 30 to45 m along the main.
- Drain points should also be provided at low points in the mains and where the steam main rises. Ideal locations are the bottom of expansion joints and before reduction and stop valves.
- Drain points in the main lines are to be through an equal tee connection only.
- It is preferable to choose open bucket or TD traps on account of their resilience.
- The branch lines from the mains are always to be connected at the top. Otherwise, the branch line itself will act as a drain for the condensate.
- Insecure supports as well as an alteration in level can lead to formation of water pockets in steam, leading to wet steam delivery. Providing proper vertical and support hangers helps overcome such eventualities.
- Expansion loops are required to accommodate the expansion of steam lines while starting from cold.
- To ensure dry steam in the process equipment and in branch lines, steam separators can be installed as required.
In practice, however, for steam pipes, a balance is drawn between pipe size and the pressure loss. The steam piping is to be sized, based on permissible velocity and the available pressure drop in the line. Selecting a higher pipe size reduces the pressure drop and thus the energy cost. However, higher pipe size increases the initial installation cost. By use of smaller pipe size, even though the installation cost can be reduced, the energy cost increases due to higher-pressure drop. It is to be noted that the pressure drop change is inversely proportional to the 5th power of diameter change. Hence, care is needed in the selection of the optimum pipe size.
Proper Selection, operation and maintenance of steam traps
The purpose of installing the steam traps is to obtain fast heating of the product and equipment by keeping the steam lines and equipment free of condensate, air and non-condensable gases. A steam trap is a valve device that discharges condensate and air from the line or piece of equipment without discharging the steam.
The following are the three important functions of steam traps.
- To discharge condensate as soon as it is formed.
- Not to allow steam to escape.
- To be capable of discharging air and other in condensable gases.
There are three basic types of steam trap into which all variations fall.
- Thermostatic (operated by changes in fluid temperature) – The temperature of saturated steam is determined by its pressure. In the steam space, steam gives up its enthalpy of evaporation (heat), producing condensate at steam temperature. As a result of any further heat loss, the temperature of the condensate will fall. A thermostatic trap passes condensate when this lower temperature is sensed. As steam reaches the trap, the temperature increases and the trap closes.
- Mechanical (operated by changes in fluid density) – This range of steam traps operates by sensing the difference in density between steam and condensate. These steam traps include ‘ball float traps’ and ‘inverted bucket traps’. In the ‘ball float trap’, the ball rises in the presence of condensate, opening a valve which passes the denser condensate. With the ‘inverted bucket trap’, the inverted bucket floats when steam reaches the trap and rises to shut the valve. Both are essentially ‘mechanical’ in their method of operation.
- Thermodynamic (operated by changes in fluid dynamics) – Thermodynamic steam traps rely partly on the formation of flash steam from condensate. This group includes ‘thermodynamic’, ‘disc’, ‘impulse’ and ‘labyrinth’ steam traps.
The alternatives to process steam system include water system and thermal fluids system such as high temperature oil. The advantages and disadvantages of process steam over other two systems are given below.
- High heat content. Latent heat of evaporation of water is 540 kcals per/kg
- Some water treatment costs
- Good heat transfer coefficients
- High pressure required for high temperatures
- No circulating pumps required, small pipes
- Easy to control with two way valves
- Temperature breakdown is easy through a reducing valve
- Steam traps are required
- Condensate is to be handled
- Flash steam available
- Blow down of steam generator is necessary
- Water treatment is required to prevent corrosion
- Reasonable pipework is required
- There is no fire risk
- Process steam system is very flexible