Steam Basics

Steam Basics - The Properties of Steam

 

The steam is a medium used to carry energy in the form of heat and pressure.

In industry, steam is used because it possesses many outstanding qualities, for example:

  1. It has a very high heat content
  2. It gives up its heat at a constant temperature.
  3. It is produced from water which is cheap and plentiful.
  4. It is clean, odorless and tasteless.
  5. Its heat can often be used over and over again.
  6. It can generate power and then be used for heating.
  7. It can be readily distributed and easily controlled.

Everything, or almost everything, that needs to be known about steam is in the steam tables.

For each pressure there is a corresponding boiling or saturated steam temperature, and these are tabulated for quick reference in the steam tables, or in a form of graph of the saturated steam curve.

As far as steam for heating and process is concerned, there are just two fundamental things that govern everything:

  1. The boiling point of water decreases with reduced pressure.
  2. The latent heat (the “heating” heat) of steam increases with reduced pressure.

As far as steam for Power is concerned, there are also two basic rules:

  1. Use the highest practical initial pressure and temperature.
  2. Use the lowest predictable exhaust or back pressure.

As far as steam for any purpose is concerned there is another rule that is of universal application:

NEVER PERMIT STEAM TO EXPAND FROM ONE PRESSURE TO A LOWER PRESSURE WITHOUT GETTING SOME USEFUL RESULT FROM THE EXPANSION.

The usual unit of heat is the BTU (British Thermal Unit).

A BTU is the quantity of heat required to raise the temperature of one pound of water through 1 deg F.  So to raise the temperature of one pound of water from 32 deg F to 212 deg F (the boiling point at atmospheric pressure at sea level) requires 180 BTU.  This heat is known as sensible heat.  To produce steam under these conditions it takes another 970 BTU’s per pound.  This is nearly 5 ½ times the heat needed to raise the water from 32 deg F to 212 deg F.  This heat is known as latent heat.

The heat which produces a rise temperature is sensible heat.  It is the practice to reckon that the sensible heat of water is nil at 32 deg F and the heat which produces a change of state without  a change in temperature is the latent heat.  The heat provided by dry saturated steam is latent heat.

The total heat of dry “saturated” steam at any pressure is the sum of sensible and latent heats.  At 212 deg F and atmospheric pressure the total heat is equal to 1150 BTU’s per pound.

The steam table shows the number of BTU’s present in each pound of steam, as total heat, as latent heat, as sensible heat at various steam pressures and as sensible heat at atmospheric pressure.  It shows the variation of the heat content as the steam pressure and steam temperature vary.

Note that if the pressure increases, the sensible heat increases, but the latent heat decreases.

Saturated Steam is steam generated in contact with water.

When steam is at a temperature corresponding to the liquid boiling point appropriate to its pressure, it is said to be “Saturated” – when no liquid is present at that temperature it is called “Dry Saturated Steam”.

So far we have considered only the heat content of dry saturated steam.  But steam is seldom dry and not always saturated.

When steam is generated in a boiler, the water surface is turbulent and droplets of water are thrown up into the steam.  Particularly when steam is being extracted from the boiler at a high rate, the movement of the steam towards the outlet will carry these droplets away and into the steam system.

Steam which contains these particles of water in a finely divided state is called wet steam.  If one pound of wet steam is made up of, say, 95% dry steam and  5% water particles, it is said to have a dryness fraction of .95.  The total heat of one pound of wet steam is less than the total heat of dry steam, because the water particles have escaped without receiving any latent heat.

The important effect of a small percentage of wetness in the steam will be realized when we consider at 200 PSIG if it contains 6% of water particles, has less total heat than dry steam at atmospheric pressure.

Steam Basics - How do you evaluate the condition of steam?

Wet steam has a white color and contains particles of water and the dry saturated steam is blue.

Two methods are adopted to ensure that steam leaving the boiler is dry.  The simplest is the provision of a drier in the steam space.  The steam passage out from the boiler is through  series of baffles so designed that the water particles in the steam shall be deposited and left behind.

The second method is to take the wet steam away from the water to another source of heat.

The extra heat first gives the missing latent heat to any water particles and turns the wet steam into dry saturated steam.  When there is no longer any water present, the steam itself absorbs the extra heat and becomes superheated steam.

Superheated steam at a given pressure can be at any temperature above that of saturated steam.  It expands when heated and contracts when cooled.  Saturated steam, by contrast, condenses when cooled.

Superheating increases the total heat of steam, but not by a very great amount.  For example, the total heat of one pound of steam at 200 PSIG which has been superheated by 100 deg F is 1260 BTU’s.  Although the temperature of the steam has been raised from 387 deg F to 487 def F the total heat has been raised only from 1200 to 1260.

The reason why only 60 BTU’s were added to one pound of steam on heating it 100 deg F at 200 PSIG is because the average specific heat of superheated steam at this pressure and temperature is .6.

The specific heat of any substance is the quantity of heat required to raise the temperature of one pound by 1 deg F.  The specific heat of water is therefore 1.  The specific heat of superheated steam varies according to pressure and temperature.  The higher the pressure, the higher the specific heat; the higher the temperature, the lower the specific heat.

Superheating provides a valuable safeguard against loss of heat content due to wet steam conditions, particularly when the steam is to be used for generating power.

The superheating of steam helps in preventing the formation of condensate in process plants and also increases the power output by delaying condensation during the expansion stage.  In turbines drier steam at the exhaust end will decrease the erosion of blades.

Steam Basics - What is Flash Steam?

Flash steam is vapor or secondary steam formed from hot condensate discharged into a lower pressure area.  It is caused by excessive boiling of the condensate which contains more heat than it can hold at the lower pressure.

Flash steam occupies many times the volume of water from which it forms.  For example, flash steam created by hot condensate flowing from 15 PSIG to an atmospheric pressure will have nearly 1,600 times the volume of the high pressure hot water.

If the pressure is increased both the boiling point and the heat content at boiling go up.  Conversely, if the pressure of the boiling water is reduced, the water must reduce its temperature and heat content to those corresponding to the lower pressure.  This means that a certain amount of heat must be released, and this excess heat will be absorbed in the form of latent heat, causing part of the water to flash into steam.

Flash steam can be valuable in some industrial processes, and some equipment can be operated by flash steam.  It may also be piped to the plants low pressure heating system or run through a heat exchanger to provide hot water or it may be piped into a lower pressure process.  The only requirement is that when flash steam is used in any one of these ways it must always be at a pressure lower than the original process, for example, condensate formed from an operation using 100 PSIG may be discharged or flashed into a 50 PSIG operation, then the 50 PSIG condensate flashed into a 25 PSIG system, etc.

This is commonly called the “cascading of pressures.”

When condensate is discharged to atmosphere it is usually quite easy to tell whether the steam formed is flashed steam or live steam.  If a strong jet or blast issues from the discharge line and is colorless at first, some live steam is present, a sign of a leaking trap, but if it is all white with no clear jet it is flash steam mixed with condensate, a normal phenomena.

Steam Basics - What causes condensate to form?

When steam contacts a cooler surface it gives up or loses some of its heat.  This loss of heat results in the formation of little droplets of water called condensate.  Even though a steam system is well insulated, there is always some loss of heat along the steam main or equipment, and condensate is formed.  At the lowest points in the system water is collected and drained away by a steam trap to keep the heating or process steam as dry as possible.

Steam Basics - Why should condensate be removed quickly?

The quick removal of condensate prevents the backing up of condensate and the blanketing of heating surfaces which would slow down the heating process and usually reduces efficiency.  In giving up heat to the process the steam turns back into condensate.  If we allow this condensate to collect it will eventually fill the steam space and keep the hot steam away from the heat exchanging surfaces.  This of course slows down the process until there is barely any heating done at all.  If this happens in heating coils and steam cannot reach the heating surface, it seriously impairs the heating ability of the coils.

Steam Basics - Steam Trapping - In the Beginning

The first method used to control condensate discharged was hand operated.  An operator would use a hand valve positioned on the discharge line reducing or increasing flow.  While this was considerably better than an open discharge line, it was still quite difficult to determine if the valve is open just the right amount.  The valve wide open would lose valuable live steam; if not open enough condensate will back up filling the steam space.  In addition, any change in the rate of condensate forming or change in steam pressure also causes the loss of live steam or the backing up of condensate, whichever may be the case.  In all cases, there is substantial efficiency loss.

Another early method of controlling condensate flow was to fit a plate or disc with a hole or orifice  in the center and simply insert this in the discharge line from the apparatus.  To correctly size the orifice you must first calculate the condensate flow, based on the condensing rate of the equipment.  Knowing the pressure at the orifice, the required size can be fitted from an orifice flow table.  The orifice plate is a very efficient device when correctly sized and condensate line operating conditions remain constant, but steady operating conditions happen very infrequently.  When condensate flow rate or pressure vary, the orifice selected is no longer the correct size and will either back up condensate or blow live steam.  An automatic device was necessary to let the condensate out, but hold back the steam, and this led to the development of today’s wide range of steam traps.

Steam Basics - What is a Steam Trap?

A steam trap is a device which distinguishes between water and steam and automatically opens a valve to allow water to pass out but which closes to steam and traps it.

Traps are of three broad kinds.  Those which distinguish water from steam owing to the difference in density of the two – these are mechanical types; those which distinguish by means of temperature – these are thermostatic types; and those which use velocity differences between steam and water – these are thermodynamic types.

The steam pressures at which steam traps must operate may be anywhere from vacuum to the highest in practical use.

The quantity of condensate which steam traps have to discharge on different jobs may vary from a trickle to a flood.  They may have to be suitable for saturated steam or for superheated steam.  They may have to discharge condensate at steam temperature, as soon as it forms in the steam space: or they may have to discharge it below steam temperature, after it has given up some of its sensible heat units.

There are about 40 steam trap manufacturers in the world today, but only few recognize the principles of operating, and out of the 40 manufacturers, probably ten share the greatest percentage of total sales volume.  It is, therefore, obvious that many steam trap manufacturers produce the same type of product with slight design variations that, they claim, have exclusive advantages over others.  The purpose of this website is not to identify particular manufacturers, but rather, identify the basic operating principles to help educate the marketplace.