This tutorial is intended to give an introduction to steam and to cover its basic characteristics. It gives an overview of the following areas with a test at the end:
Steam is formed from water which is relatively cheap and plentiful. When water is heated above a certain point it is converted into a vapour. This vapour is a white mist with minute droplets of water in the air. When water is boiling and continues to be heated in a boiler it will evaporate to form this vapour which is called steam. The amount of heat that is required to change water at its boiling point into steam is called the enthalpy of evaporation or latent heat. During the phase change time all the heat energy added is used to change the state of water to steam. Therefore there will be no increase in temperature of the steam/water mixture.
This phase is reversible. When a temperature gradient exists heat transfer will be the result. So when steam reverses this phase and condenses back into water it will give out exactly the same amount of heat that was added during its evaporation or latent heat stage.
Steam is used in so many of the goods and services that we take for granted. In fact modern life would grind to a stop without the use of steam.
Food and drink
Homes and buildings
Other uses, from semiconductors to tyres
Steam can be easily, efficiently and economically generated by modern boilers from a variety of fuels and sources.
The heat produced by steam can easily distributed to the required point of use. Steam flows due to a pressure drop in the system, therefore costly pumps for circulation are not needed. The pumps needed for other mediums normally result in a considerable electrical load, and cost. Steam has a high heat content meaning that in comparison to other systems relatively small diameter pipework is all that is required for distribution.
Steam can be accurately controlled. Only two port type valves are required reducing costs.
It is capable of transferring much energy with little effort. When steam condenses, this cycle efficiently transfers heat to the application being heated. The heat transfer properties of steam are high so the required heat transfer area for the application is relatively small.
Steam is not flammable and contains no toxins. This makes it ideal for many industrial purposes.
Steam has a unique and illustrious history being a vital part of the first ever industrial revolution, as well as many others after. It was quite literally the driving force behind the first industrial revolution, which took place in Great Britain. Steam drove engines which powered mills and factories, pumps and other equipment for mines, trains to transport goods and labour across the country, at ports these goods could be loaded onto steam powered ships which transported these goods across the seas, and ships returned loaded with raw materials.
The industrial revolution marked a major change to human life. It can be argued that the history of humans can be divided into pre and post industrial revolution. Prior to the industrial revolution; GDP per capita head, life expectancy, income, diet, in fact every experience of humanity had been largely unchanged for a millennia, from the times when humans learnt they could cultivate their own food from the land. The industrial revolution changed everything, the living standard of the masses of ordinary people underwent sustained growth, the like of which had never been seen before. Today we are still reaping the rewards of the industrial revolution. A middle class family living in a developed twenty first century country enjoys a life which a king or queen could probably only just afford prior to the industrial revolution. The industrial revolution created not just innovation, this has happened before (the wheel for example), but sustained perpetual innovation that continued at a pace that had never happened previously.
At the heart of this sustained perpetual innovation was steam. Steam was first used to drain the tin and coal mines. The steam engines were used to pump water from the mines enabling deeper mines than before to be worked, as well as bringing the coal and tin to the surface. The steam boiler was fired from burning coal from the coal mine itself. Iron works were created to produce iron in large quantities for steam engines and boilers. To clothe the factory workers the textile industry grew rapidly, using steam engines to power their spinning and weaving machines. These machines massively increased cotton spinning output per worker. To transport good around the country railways were built and the trains that ran on the tracks were steam powered using coal as the fuel. To transport goods and raw materials around the oceans of the world, steam ships were constructed. Steam power was used to industrialise the process of printing, and the massive expansion of newspaper and book printing resulted. This led to rising literacy and political awareness.
The industrial revolution spread across the globe and led to the industrialised world that we now live in and which steam still pays an important part in.
As stated before steam is formed from water that is heated till it boils and heat energy continues to be added.
So if we were to take a perfectly insulated glass of water at 0 °(water with no energy) the first step in the making this into steam is to heat it to 100 °C (we are assuming atmospheric pressure in this example, 0 barg). Therefore the first term that we must describe in our steam generation process example is the enthalpy of water, or otherwise known as the sensible heat of water (hf). In our example this would be the amount of energy required to heat the water from 0 °C to its current temperature of 100 °C. To take 1kg of water from 0 °C to 100 °C at atmospheric pressure takes an input of 4.19 kj of energy.
We are now at the stage where our hypothetical glass of water has been heated from 0 °C to 100 °C. To make steam we need to add further heat to convert the water into a vapour (steam). During this conversion the molecules attain enough kinetic energy to leave the liquid surface. This is called the Enthalpy of evaporation or latent heat (hfg). As the energy added is used to make steam no change in temperature will take place. We now have steam being produced from our glass.
Therefore the total energy of our steam (hg) will be the sum of the two enthalpy’s above:
hg = hf + hfg
The total heat or energy in the steam is defined as the total enthalpy of saturated steam (hg).
When water is heated and turned into steam different combinations of pressure and temperature cause the properties of steam to vary. The two different types of steam which can be produced are:
Saturated steam; is the type of steam that is most commonly used. Steam in this state has a temperature that corresponds to the boiling temperature of water at the existing pressure. The properties of saturated steam make it an excellent medium for heat transfer. Therefore it is widely used in the 130 - 200 °C temperature range.
Superheated steam; is formed by the further heating of saturated steam. Therefore superheated steam has a higher temperature than saturated steam at the same pressure. Superheated steam is not as commonly used as saturated steam. It is a very dry steam, these properties mean it is used mainly for physical drive or propulsion applications (e.g. steam turbines), and is not often used for heating or similar process purposes.
When steam is used by a process it gives up its heat when a temperature gradient exists. So when steam reverses its state it condenses back into water (called condensate). As this condensate is normally under pressure near the point where it has been used it will be water at above 100 °C. When this condensate is released to a lower pressure (say to atmospheric pressure in a vented condensate collection tank) its temperature must drop very quickly to the boiling point for this lower pressure (100 °C for example in an atmospheric condensate collection tank). The condensate must therefore “give up” the excess energy that it has to drop to this lower pressure. It does this by, some of the condensate re-evaporating into steam. This is called flash steam.
The quantity of flash steam generated can be calculated by knowing the flow rate of condensate, and the pressures before and after of the condensate. The higher the pressure before, and the lower the pressure after the more amount of (percentage) flash steam will be generated.
These are tables that allow steam designers and users to look up data on the properties of steam. This allows calculations to be made to ensure the best use of steam, and to help in identifying and diagnosing problems.
The tables themselves are a collection of data on temperature, pressure, volume, and the energy contained in water and steam. This data is traditionally expressed in the form of a table with columns for reading the data. For ease of use this website uses a calculator whereby the user inputs their known data and the software provides the output data without the need for scanning long lists of columns.
The data in steam tables has been measured in laboratories with precision instruments. This has been verified a great many times and therefore the data is very reliable.
Steam tables provide;
Steam tables consist of tables for five regions. This from sub-saturated water through to superheated steam.
The most commonly used steam table is the saturated steam one.
A steam and condensate system can be looked upon as a loop or cycle. There are several stages and pieces of equipment that make up this loop. Below is a short summary of this loop or cycle.
Water supply and treatment; this is essential to allowing the steam to be produced for the steam and condensate cycle. Raw water is treated using a water treatment plant and program.
Boiler hotwell feedtank; is the place where the treated water is sent to. If condensate is returned from the process it is mixed with this treated cold make up water in a deaerator.
The Boiler; is the heart of a steam system. Without it steam could not be generated. The boiler is heated in many ways, such as burning fuels like gas, oil or coal, from burning waste, or from waste heat recovery from a variety of sources. The boiler turns water into steam. There are many different types and designs of boilers available.
Steam distribution system; transports the steam to where it is required. It would typically consist of a steam header, steam distribution main, and steam off-takes from the main to the point of use.
Steam consuming plant; uses steam as part of its process. This could be to heat a medium by exchanging the heat in the steam by the condensing of the steam against a heat transfer surface, or to heat a medium by injecting the steam directly into it. It could be also be used to drive a mechanical piece of equipment like a steam turbine.
Condensate return system; collects the condensate formed from the steam consuming equipment (and from any distribution areas) and returns it back the boiler hotwell feedtank.
Steam is produced from water. It is therefore important for the water to be of the good enough quality so that; the steam produced is of the correct quality, and that the boiler, steam, and condensate systems functions correctly and do not break down or suffer harm.
All natural waters (including drinking water) contain different types and amounts of impurities. The natural water quality can vary from region to region, and country to country depending on its source and local minerals.
If we took a good quality drinking water and used it in a steam boiler it would cause problems and possible damage. Therefore it is important that water, we shall call it raw water is put through an appropriate treatment regime so that is becomes of a quality suitable for use in a steam boiler.
Raw water is normally supplied from a well or borehole, or from a towns water supply connection. The raw water is then treated using a water treatment plant and program.
This treatment will ensure that the boiler, steam and condensate system does not suffer from:
Scale formulation will result in the heat transfer surfaces becoming scaled up. This will reduce efficiency and drive up operation costs. If a boiler becomes scaled up it is a major job to clean the scale from it. Scale is caused by hardness being present in boiler feedwater. This is due to the water treatment plant and program not working correctly.
Corrosion and chemical attack will result in damage to boilers, steam and condensate pipes, and other equipment in the system. If the boiler feedwater has a PH that is too low then it has an acidic solution, which will result in the metal of the boiler and other parts of the system being attacked. This could mean serious damage over a period of time.
Raw water must be treated therefore to ensure that it becomes boiler feedwater quality. This is done by using a water treatment plant and a chemical program. For a simple shell boiler this may take the form of a base exchange softener plant. For a high pressure, high quality, power station water tube type boiler application, a demineralisation plant may be used.
Water treatment plants for steam boilers can generally be broken down into the following different types (depending on the application):
Base exchange softening plant. Consisting of a resin bed softener with a brine regeneration tank (regenerates the softener when required). This type of plant is simple and cheap to buy and operate.
Dealkalisation plant. This a base exchange softening plant but with a dealkaliser and de-gaser plant in front of it. It works by using a weak acid cation resin. This reduces the alkalinity and therefore the total dissolves solids (TDS) in the feedwater. This reduces TDS blowdown in the boiler and can reduce operating costs.
Reverse osmosis plant. The raw water is forced through a semi-permeable membrane providing good quality boiler feedwater. The impurities that are not desirable are rejected and either put to waste or re-used in other non-critical applications.
Demineralisation plant. A type of plant normally only used for applications where very high quality feedwater is required. Examples of these may be for applications where the steam generated is used to drive a steam turbine.
Boiler hotwell feedtank
The boiler hotwell feedtank is where water for the steam boiler is supplied from. The boiler hotwell feedtank is used to store this water ready for when the boiler requires it to produce steam. It is important that it is sized and designed correctly. The stored water in the feedtank is a mainly mixture of make-up water from the water treatment plant, condensate return coming back from the steam using process, and steam injected into the tank to keep the tank at its operating temperature.
The feedtank is kept hot (hence the name hotwell), but why is this required? It is kept hot to:
Minimise the oxygen and other gases contained in the feedtwater: If water in the feedtank is kept at a high enough temperature the amount of dissolved oxygen and other gases is reduced. This means that less chemicals (sodium sulphite) are required in the feedwater. This saves chemicals and cost. In addition it reduces blowdown required in the boiler.
To make sure that the boiler is not damaged: If cold water was sent to the boiler then thermal shock could be the result in the boiler. This cold water could introduce thermal shock to the hot surfaces in the boiler (boiler tubes and walls).Therefore hotter feedwater being sent to the boiler will reduce this risk.
To make sure the boiler performs as rated: If cold water is supplied to the boiler the boiler has to work harder to generate steam at the desired pressure and temperature. This results in the output of steam that the boiler can actually produce being reduced. A shell boiler’s rated output (F&A rating) will normally be based on feedwater being at 100 °C.
Due to these three reasons the water in the atmospheric boiler hotwell feedtank is typical kept at more than 80 °C but less than 100 °C (pressurised deaerator feedwater tanks are typically kept at 105 °C)
It will of course require energy to keep the feedtank hot (normally through steam injection into the feedtank). However the water would still need to be heated in the boiler so the energy used in heating the water in the tank is not additional energy. The tank will have some standing energy losses (heat loss). However provided the tank is properly insulated this will be negligible, certainly in comparison to the advantages to having the water in the tank hot.
Feedtanks types can normally be broken down into two main types. They differ due to the type of deaerator used on the tank.
Atmospheric boiler feedwater hotwell tank: This type of tank is vented to atmosphere so is not under any system pressure. It the most common type of design used in steam systems. The tank can be rectangular or cylindrical (either vertical or horizontal).
The tanks are constructed from a variety of different materials. These being; cast iron, carbon steel, or stainless steel. Stainless steel is normally judged to be the best material for this type of tank. This due to it being able to take the temperature for the application, it should not corrode like carbon steel, and it has a long life. Regardless of the type of material, the tank should be constructed to take the stresses that a boiler feedtank hotwell will have to endure. This will normally mean that stiffening and bracing will be a part of its design.
Atmospheric deaerator head. This device is where the condensate return and cold water make up is mixed. Cold water make up has a high oxygen content which is liberated when it is mixed with the hot condensate returned from the steam process. The gases which are released are removed via a vent on the top of the deaerator. The other advantage of the mixing is that flash steam in the condensate return can be condensed by the cold water make up water. This reduces any potential flash steam emissions venting from the tank
Steam injection. To keep the tank up to temperature a steam injection system is normally used. This injects low pressure steam into the tank to keep the desired tank temperature set point. The system would typically consists of a control valve system, control system, and steam injector.
Vent and overflow. To prevent any build-up of pressure the tank must be vented to an external area. This vent would come off the top of the tank. Off the side of the tank would be a “U” type syphon type tube filled with water to seal, acting as an overflow.
Feedwater off-take and drain. The purpose of the tank is to serve the steam boiler. To do this there will be a connection off the tank to serve the boiler feed pumps. These pumps maintain the water level in the steam boiler by forcing water into the boiler when required by the boiler level controls. The feedtank will normally also have a drain so that it can be drained down and inspected/cleaned when required.
Tank level controls. The tank will have some form of level control on it so that the water level is kept at the level that is desired. This can vary from float type controls to level probes and a solenoid or control valve.
Insulation. To make sure that standing energy losses are kept to a minimum the tank should be properly insulated.
Boiler make up feedwater supplied to the hotwell feedtank is the product of the water treatment plant and treatment program. However in addition to this some supplementary treatment may take place (dependant on a number of factors). This in the form of adding measured amounts of chemicals to either the feedtank itself or the feedwater pipes serving the steam boiler.
Pressurised boiler feedwater hotwell tank and pressurised deaerator: This type of feedtank and deaerator combination is generally used on large plant or where the quality of the feedwater needs to be very high. Live steam is used to elevate the temperature above 100 °C to drive off oxygen. Live steam is used in the deaerator head and is injected into the cold make up feedwater and the condensate return. A blanket of steam is also maintained over the top of the water in the tank to prevent any further gas being re-absorbed into the feedwater. Pressurised deaerators typically work at about 0.2 barg. As this type of tank is pressurised it must be fitted with appropriate safety devices and inspected periodically.
Different types of steam boilers
Steam is formed in a device generally called a boiler. Boilers can come in many different types and sizes. The shell type boiler is the most common type, and the type which we will use as our example throughout the rest of the boiler section of the tutorial.
Shell type boilers are so called due to the heat transfer surfaces being contained within a steel shell. The product of the combustion from the burner flows through the tubes in the boiler and then out through the flue/chimney/stack. The water which is to be heated to form steam is contained in the vessel. The heat from the tubes flows into the water.
The most common type of shell boiler is the two or three pass type. The term “pass” is used to define the number of times the product of the burner goes through the sections of tubes to heat the water in the boiler. A two pass boiler would typically have efficiency in the region of 78%. While a three pass boiler would typically have an of about 87%.
Steam boiler fittings and accessories
A boiler is more than just a shell and heat exchange surfaces. It needs many items on it to make sure that it functions safely in producing steam.
A fired boiler (as opposed to a waste heat boiler) requires a fuel to be combusted to make the boiler work. Therefore the boiler has a burner on it which the fuel is fed into. The fuel could be for example; oil, gas, or coal. It is the burner’s job to combust the fuel and adjust the amount of heat that it puts out. Therefore the burner will have a control system which adjusts it as required.
The product of the combustion will be exhausted from the boiler up the chimney, sometimes called a flue. The flue should be designed so that it can remove the gases from the boiler safely and handle the heat of these gases.
On the boiler shell itself there will be a boiler name plate. This contains details on the manufacturer and its design. It will also contain details on the output and pressure of the boiler.
As the boiler shell is a pressure vessel it needs to have protection from over pressure and risk of subsequent explosion. This is done with the use of a safety valve. There are international standards for this valve depending on the country of operation. In the US these standards are laid down by ASME. In Europe there are similar European standards (EN12953).
To open and close the outlet of the boiler a boiler crown (or stop) valve is used. This would normally be of rising globe type with an indicator on it. The valve only being used in an open or closed position. The body of the valve normally being made of steel or a material that does not have brittle fracture problems.
To generate steam there needs to be water in the boiler. With the boiler operating at pressure the water from the boiler feedwater hotwell is pumped into the boiler under pressure using a boiler feedwater pump. The pump must always be able to overcome the pressure in the boiler so that water can flow into the boiler. After the boiler feedwater pump and before the boiler there will be a check valve.
To protect the boiler, and produce the correct quality of steam the water quality in the boiler must be maintained. Even when a good and correct water treatment plant and regime is used there will still be a requirement to blow impurities out of the boiler. This is done in two ways; using TDS blowdown, and bottom blowdown.
The total dissolved solids level in the boiler must be kept to a set level or set point. This is normally done using a side connection on the boiler, a TDS conductivity probe, and a control valve. The probe measures the TDS level in the water in the boiler and opens and closes the valve as desired to remove the TDS laden water from the boiler.
The second blowdown from the boiler is normally located at the bottom and back of the boiler. It is used to remove the sludge or sediment that accumulates at the bottom of the boiler shell. Therefore it is only opened for a short period of time (a few seconds), and normally only a few times a day at most. The valve can be opened manually using a key to operate the valve (a key is used so only one boiler can be blowdown at any one time, on multi boiler installations). Alternatively it can be automated using an air operated valve on a timer.
During operation it is important that the pressure and water level in the boiler can be seen. Therefore the boiler should have at least one pressure indicator (pressure gauge of correct standard), and a water level indicator.
The water level indicator is normally called a gauge glass. For boilers less than 100 kW one gauge glass is deemed adequate. However above this two are required. The gauge glass itself should be blown down by opening the drain cock and discharging to the blowdown vessel on a regular basis. The frequency of this will be determined by legislation, guidance notes and standards in the country of operation. The reason for blowing down is to make sure that the water in the gauge matches the water quality in the boiler and so that the gauges are kept clean.
The correct level of water in the boiler is essential to the safe and proper operation of the boiler. This is done by the water level controls on the boiler. These level controls provide the signal to the boiler feedpump or feedwater valve to make sure that water is fed into the boiler when required and shut off when not required. The water level controls on the boiler also provide alarms in case of low water level (and in some cases high level alarms).
There are different types of level controls for boilers. These can range from probe type to float type. They can be mounted in the boiler shell using protection tubes, or in external chambers. The types of level controls and method of mounting used will be tailored around the manning requirements for the boiler house and local legislation (or guidance notes).
To make sure that air is removed from the boiler, and to make sure that vacuum conditions are not present, it would be normal to incorporate a method of venting air from the boiler and a vacuum breaker.
A steam distribution system transports the steam to where it is required. It would typically consist of a steam header, steam distribution main, and steam off-takes from the main to the point of use. At places on these would typically be; steam trapping, steam metering, air venting, isolation valves, and pressure reducing stations.
A steam distribution system starts in the boiler house and the off take from the boiler(s) are normally sent to a manifold or header. The steam manifold or header will then have numerous take off points to send the steam where it is required. To keep pipe size and therefore cost down steam is generally produced at a higher pressure and reduced down in pressure at point of use.
Steam pipework is normally constructed from carbon steel. For more exotic high temperature and pressure steam mains higher specification alloys may be used.
There are numerous standards throughout the world for steam pipework. The most common system globally is a standard from API (American Petroleum Institute). The pressure rating of the pipe is categorised by schedules (5-160). Other pipework standards can specify the pipe by different colour bands in respect to its pressure rating.
Steam mains and pipework can be sized by velocity or pressure drop. Allowances should be made for adequate pipework support and expansion.
Branch lines off a steam main or steam line should always come off the top. This to ensure that dry steam is sent down the branch line. If the branch line came off the bottom condensate would may be sent down the branch line in addition to steam.
When a steam system is started from cold it will be full of air that needs to be removed. If air is not removed properly the warm up period will be extended, and efficiency is compromised. In addition small amounts of air and other non-condensable gases will enter a steam system during normal operation. It is important that these be removed also.
The best way to remove air from a saturated steam system is to use an automatic air vent. Automatic air vents should be fitted at high points at the end of steam main for example, or at the high point on equipment where air could be trapped. The discharge from these automatic air vents should always be piped to a safe place.
Steam is a very valuable medium and it is common to meter its flow and usage. Steam metering can therefore be used to:
The types of steam meters used are many and varied. All have advantages and disadvantages. Types of flow meters can include:
A steam trap, and steam trapping is an essential part of a steam and condensate system.
From the steam boiler a steam distribution system transports the steam to where it is required. It would typically consist of a steam header, steam distribution main, and steam off-takes from the main to the point of use. At places on these would be steam trapping stations.
Once steam has passed from the steam distribution system it moves to the steam consuming piece of plant. This could be to heat a medium by exchanging the heat in the steam by the condensing of the steam against a heat transfer surface, or to heat a medium by injecting the steam directly into it. It could be also be used to drive a mechanical piece of equipment like a steam turbine. The piece of steam consuming plant will typically have a control valve or similar to control the amount of steam used in it. Depending on the piece of equipment it will probably have a steam trap or similar to remove condensate as it forms but close and trap steam when no condensate is present.
Therefore the purpose of a steam trap is to trap steam and stop live steam escaping, however when condensate (condensed steam), air and other non-condensable gases are present (or formed) the steam trap must discharge these.
Steam traps operated in many different applications, pressures, temperatures, and locations. There is no way that one size or type of trap can be suitable for all of these. Therefore steam traps come in different types and sizes.
Under ISO 6704:1982 there are three classifications of steam traps.
These can be defined below
Depending on the application it would be normal for the steam trap to be installed with other equipment around it to make a steam trapping station. A steam trapping would typically consist of upstream isolation, strainer, steam trap monitor (if fitted), steam trap itself, sight glass (if fitted), check valve, downstream isolation.
There are several ways of checking that a steam trap is working correctly. This can be incorporated as part of the steam trap station. This would be in the form of either a steam trap monitor before the trap (usually using a sensor wired back to a panel), or by a sight glass after the stream trap.
It is typical for steam to be generated at a higher pressure and for it to be used at a lower pressure. Therefore the pressure of the steam is reduced near the point of use by using a pressure reducing valve.
The reason for generating at high pressure and using at a lower pressure can be summarised below:
It is typical for pressure reducing valves to be accompanied by other equipment to form a pressure reducing station. This would typically include; a separator and steam trap, upstream isolation, strainer, upstream pressure gauge, pressure reducing valve, downstream pressure gauge, safety valve, downstream isolation.
The most common group of pressure reducing valves themselves can be divided into the following main groups:
Direct operating self-acting pressure reducing valves: A simple type of valve which functions by use of two springs, a bellows, hand wheel, and valve head/seat. The hand wheel is turned to adjust the pressure downstream of the valve (the set pressure). The pressure should be set with the valve in a dead end condition (with a stop valve closed downstream so that there is no flow).
When steam is turned on at start up the downstream pressure will be low so the valve will be open and will let steam through (due to the downward force of the adjustment spring). As the downstream pressure increases to its desired set point it acts on the bellows. This counteracts the force from the adjustment spring and closes the main valve when the set point downstream pressure is reached. After start up and during normal operation the valve will modulate in a similar way to attempt to keep the desired downstream steam pressure set point regardless of the steam flow rate.
The advantages of this type of valve is that; they are simple, cheap and reliable. The disadvantages are; that they suffer from proportional offset (called droop), certain designs can only cope with small loads, and any variation of inlet pressure can affect the resultant downstream pressure.
Proportional offset is commonly called droop. When the set pressure of the valve is set it is done on a dead end condition. This is normally down by closing a downstream isolation valve and adjusting the hand wheel on the pressure reducing valve until the pressure downstream of it reaches the desired set point. Therefore the pressure is set in a no flow mode (or near no flow). When the downstream isolation valve is opened steam can flow through the valve. When the flow of steam increases the actual downstream pressure that the valve provides will be different (lower) than the set point. This difference is called proportional offset or droop. Droop will increase as the flow of steam increases. Provided a direct acting pressure reducing valve is properly selected and applied this does not present a problem. However care should be taken with the sizing of the valve and thought should be given for what equipment the steam is being used to serve.
Pilot operated self-acting pressure reducing valves: A type of valve used where direct acting valves cannot be used. Direct acting valves can be used on many applications, however it does have some limitations which means that it is not suitable all. This is typically where a pilot operated pressure reducing valve is used.
The advantages of this type of valve are that; they suffer from very minimal proportional offset when compared to a direct acting valve. They will usually be smaller than a direct acting valve of the same capacity. They provide highly accurate downstream pressure control even when the upstream pressure can vary. The disadvantages are; that they are more complicated and therefore (normally) more expensive than direct acting valve, the more complicated design can be more prone to blockage, so it is important for a strainer and separator used. These are installed prior to a pilot operated pressure reduction valve to ensure that clean dry steam is supplied to it. On some applications where dead end conditions could result in condensate flowing back and flooding the valve a steam trap is positioned immediately downstream of the pressure reducing valve also.
On a pilot operated pressure reducing valve its main valve is either assisted or completely controlled by the operation of the pilot valve, which may itself be a small direct acting reducing valve. The pilot acts as to regulate the amount of opening of the main valve in a way that will maintain the flow at the desired level of pressure. The pilot valve may be integral with the main valve or may be a separate unit suitable for remote pressure sensing. It can also be used as part of a complex system governed by central control.
Actuated globe type control valves (electric or pneumatic actuated): Used where a direct acting or pilot operated pressure reducing valve would not be suitable or desirable. It uses a globe type control valve actuated by either an electrical or pneumatic actuator. The actuator opens or closes the valve on command from the pressure sensor downstream of the valve.
It has the advantage over self-acting pressure reducing valves of being very accurate under varying load conditions, the set point can easily be changed remotely, they can cope with a high turndown ratio, and they can be remotely monitored. The downside is that they are more expensive than their self-acting equivalents, and in the case of the electrically actuated version, to fail closed a large spring is required to close the valve.
A type of valve which is used to protect equipment, property, and life. It should be used as a backup in case of failure of normal plant. It does this by preventing overpressure of a piece of equipment or plant by opening at a pre-determined set pressure and releasing a volume of medium through it to make sure that excess pressure does not result. It may be the only device left to prevent a major and possibly fatal failure. It is therefore essential that a safety valve is selected, installed, and maintained in such a manner that it can operate when required at all times and under all conditions.
Typically safety valves are installed on; boilers and other pressure vessels, and after a pressure reducing valve (in case the pressure reducing valve were to fail). They should be sized and installed so that they ensure that the maximum allowable working pressure (MAWP) of a system or vessel is not exceeded, were the primary control systems to fail.
There are many safety valve standards across the world. The European EN ISO 4126-1 standard defines a safety valve thus:
Condensate recovery and flash steam
Steam is generated and used to heat a medium or to drive a mechanical piece of equipment like a steam turbine. To heat a medium the steam gives up its energy by exchanging the heat contained in it by condensing against a heat transfer surface. The steam therefore changes state back to water (called condensate). This condensate formed from the steam consuming equipment (and from any distribution areas) should be returned back the boiler hotwell feedtank. Condensate is a very valuable part of the steam and condensate cycle, as condensate contains valuable heat, and treated water which can be re-used in the steam and condensate cycle again. Therefore it is very important to wherever possible return as much condensate as possible to the boiler hotwell feedtank. A typical condensate return system might consist of a condensate return header to collect the condensate back to a certain point, a condensate return pump (could be electric or steam powered), and a condensate return main to return back to the boiler hotwell feedtank.
When steam is condensed it turns into water (condensate), which is removed via steam traps to the condensate return system. When the condensate is formed it is at a higher pressure than when it is discharged from the steam trap and into the condensate return system. A large number of condensate return systems will be atmospheric (i.e. vented to atmosphere and therefore at atmospheric pressure). When the condensate is formed it is under pressure and can stay as water at above 100 °C. However when it is expelled from the trap into the lower or atmospheric condensate recovery system it cannot exist as water and must give up some of its energy. It does this by re-evaporating some of its flow into what is known as flash steam. This can sometimes be called flashing. The percentage of flash steam generated can be calculated if the different pressures (steam and condensate system pressures) are known. As flash steam is a vapour and not a liquid it will require considerable more space than the condensate (water). Therefore care needs to be taken with the sizing of discharge pipework and condensate return lines so that the flash steam does not choke and impede the discharge and return of condensate back to the boiler hotwell feedtank.
For a steam system flash steam going to atmosphere can represent a loss of energy and treated water (could be up to 10 or 15% of the energy being used in the steam generation being wasted). Therefore flash steam emissions should be kept to a minimum. This can be done in a number of ways. The pressure of steam being used by the steam using equipment can be kept to the minimum possible so that the percentage of flash produced is kept to a minimum. The other way is to re-use the flash steam. This can be done by using a flash vessel, and other steam equipment to serve a low pressure piece of steam using equipment. The pressurised condensate flows through the flash vessel. This vessel separates the flash steam and the condensate. The condensate is removed via a steam trap to a condensate recovery vessel or direct back to the boiler hotwell feedtank. The flash steam is taken off the top of the flash vessel and is fed to the lower pressure steam using equipment. This steam equipment may also have a live steam top up which will be designed to allow live steam to flow when there is not enough flash steam available.
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