Understanding Steam

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Understanding Steam

Steam

Steam is so familiar to all of us that we easily forget what a marvelous thing it really is. Steam will carry twenty times the BTUs per pound that Freon 12 will, nearly 15 times the BTUs per pound of F22, and over twice that of ammonia. Even when we use nuclear energy for power, it is only to heat water to make steam. For all of this, we tend to take steam for granted without much thought to how it really is produced or how it works best for us. So, let’s take a very basic, but serious look at steam.

Boilers make steam by boiling water. That’s about the most oversimplified statement we could make. Some very interesting things happen when water is boiled. At atmospheric pressure, water expands 1,600 times its original volume when turned into steam. This explains why a tea kettle may put off a plume of vapor half the morning before running dry.

Water at atmospheric pressure boils at 212 degrees F. The boiling water in an open vessel measures 212 degrees F.  Once 212 degrees F is reached, all heat  applied to the vessel does not increase the water temperature. The applied energy is going into changing the water from a liquid into steam, a vapor. The steam will give up this energy before turning back to a liquid. This energy is called Latent Heat and for most purposes this is considered the usable energy of steam.

Below the boiling point, the heat applied to the vessel goes into the water and raises the water’s temperature. We are able to sense this temperature change with a thermometer and we call this heat Sensible Heat.

For most purposes for which steam is used, it is the Latent Heat of steam that we utilize, for after the Latent Heat is given up, the steam re-condenses back into a liquid from a vapor. Wherever possible we return this condensed steam or water, commonly called condensate, back to the boiler for reuse. This conserves a large part of the Sensible Heat we had to apply to raise the temperature of the water up to 212 degrees F. Even if this condensate has come into contact with contaminating materials that make it unsuitable for return to the boiler, it can often be still be used to help heat new incoming water through a heat exchanger and reduce fuel requirements.

So far, we have only talked about an open vessel to the atmosphere. If we close this to atmosphere and as we continue to add heat above the boiling point, the expanding steam causes the pressure to raise. This increase in pressure however causes the temperature at which the water boils to rise as well. This calls for a further supply of Sensible Heat to get the water to its new, higher boiling point level. At the same time though, The Latent Heat required to convert the higher temperature water into steam is reduced. The next result is only a slight increase in the Total Heat required for each pound of steam. But, since that same pound (by weight) of steam will occupy 26.8 cubic feet of space at atmospheric pressure and only 2.15 cubic feet at 200 lbs per square inch pressure, it is easy to see why higher pressure is used in industry throughout the world! While occupying only 1/8 the volume at 200 psi, steam has only required 4 ½% more heat per pound. Smaller piping will serve the same system and will reduce piping radiant heat loses as well.

Steam can be in one of three forms, Wet, Dry or Superheated. Wet steam contains small water droplets entrained within it. These droplets contain no Latent Heat and therefore have nothing to contribute to the process before they are returned to condensate. Just 6% of water particles at 200 PSIG reduces the Total Heat in a pound of steam to less that the Total Heat in a pound of dry steam at atmospheric pressure.

Dry steam requires the elimination of water particles from the steam line. This can be done mechanically with baffles in the steam flow design so that water particles are deposited and left behind,  or by removing the wet steam from the water surface and applying additional heat to drive all the water particles into steam. Flue gas heat is often used for this additional heat.

Superheated Steam occurs when enough additional heat is applied to raise the steam temperature to any level above that of saturated dry steam. Some superheat is sometimes required to make sure that dry steam is available at the end of a long steam line. It would seem that superheat would be the course to take in all cases as a means to transfer more heat. In reality, the amount of additional heat transferred is not that great. One pound of steam at 200 PSIG superheated another 100 degrees F more has gone from 1200 BTUs Total Heat to 1260 BTUs, a gain of only 4.8%. While that small change was taking place, the saturated steam started acting like a perfect gas and expanded in volume, from our original 2.14 cubic feet to a volume change of 14.5%. The heat content per cubic foot of steam has actually decreased by superheating. Available latent Heat or usable heat in BTUs actually increases when you use lower pressure. This is why lower pressure is used for comfort heating in buildings in colder climates.

Reflecting then, we have eight times the volume of steam in the same space by raising the pressure from atmospheric to 200 PSIG but, we have a 14.5% decrease through superheating another 100 degrees at the same 200 PSIG. Superheat is therefore an advantage only when the heat requirement can be met by the smaller amount of energy required in the superheat and when the higher temperature gradient from the hotter steam is required by the process job.

For steam engines or turbines in simple cycle and combined cycle power plants (HRSG or, heat recovery steam generators), it is often part of the power unit’s design to use superheated steam. This is a specialized application of steam utilizing the entropy of steam, (which needs to be treated separately). Suffice it to say that after expansion through the engine, this steam has nearly used all of its heat units that were put into it and can often be put into additional use as in process applications, such as preheating supply water for makeup use.

We have not attempted to discuss all areas of steam use. For example, we have said nothing about steam trapping, a very necessary consideration in most steam systems. But we have zeroed in on steam and most of its basic simplicity, its great heat transfer capabilities, and the versatility of its applications. When these are taken together, it is obvious that steam will always continue to be a very important tool to industry. It will not become obsolete for there is nothing else that even comes close to having these properties.

Randall J. Harwood, circa 1980.

frick-steam-engine-illustration

 

 

Misc Readings & Research;

Click to view:     Pipe Fitters Handbook

Click to view:     3 Element Boiler Steam Drum Control

Click to view:     Saturated Steam Tables

Click to view:      understanding_steam_cycle_chemistry

Click to view:  Fluid Compatability Table:  FluidCompatTable

Click to View:     Steam Conditioning Valves: Case Study

 

Material Comparison Reference Chart

WROUGHT DESIGNATIONSALLOYSCAST EQUIVALENTS
304ASTM A-744CF 8
304 LASTM A-744CF 3
304 HASTM A-352CF 10
304 ModifiedASTM A-744CF 8 .4Min/0.8Max
Carbon
316ASTM A-351/A-744CF 8M
316 LASTM A-744CF 3M
317ASTM A-744CG 8M
317 LASTM A-744CG 3M
317 LMASTM A-744CG 3M Modified
Molybdenum 4.0-5.0
Carbon 0.3% Max
347ASTM A-744CF 8C
347 HASTM A-351CF 8C Modified
.4Min/0.8Max Carbon
405ASTM A-352CA 6NM
410ASTM A-487CA 15-C
Alloy 20 CastASTM A-744CN7M
Monel 400ASTM A-494M35-1
MonelASTM A-494M35-2
MonelASTM A-494M-30H
Duplex SS-BarASTM A-479Ferrallium 255
ASTM A-479S-31803
Duplex SS-CastASTM A-744CD-4MCU
Duplex SS-Cast 2205ASTM A-890CD-3MN
Nickel 200ASTM A-494CZ100
Inconel 600ASTM A-494CY40-CL1
Inconel 600ASTM A-494CY40-CL2
Inconel 625ASTM A-494CW6MC
Inconel X 750
Inconel 800ASTM A-351CT15C
WROUGHT DESIGNATIONSALLOYSCAST EQUIVALENTS
Inconel 800-HASTM A-351CT15C
Hast.ASTM A-494N7MCL 1
Hast. B-CastASTM A-494N-12MV
Hast. B-2 BarASME SB-335N10665
Hast. C-CastASTM A-494CW12MW
Hast. C-276 BarASME SB-574N10276
Hast. C4ASTM A-494CW2M
Hast. C4-Cast ModifiedASTM A-474CW2M Class 1 Modified
Si 0.40% Max
S 0.015% Max
Hast. C-22 BarASME SB 574NO 6022
Hast. C-22 CastASTM A-494CX2MW
Titanium-CastASTM B-367GR2
Titanium-CastASTM B-367GR3
Titanium-CastASTM B-367GRC5
TitaniumASTM B-367GR7B
Avesta 254 SMOASTM A-351/
NO 8926
CK3MCUN
Zirconium-CastASTM B-752GR 702 C
Zirconium-BarASTM B-550
Carbon & Low Alloy
Carbon-MolybdenumCastForged
1.25 CR – 0.5 MOWC6F11
2.25 CR – 1.0 MOWC9F22
5.0 CR – 0.5 MOC5F5
9.0 CR – 1.0 MOC12F9
9.0 CR – 1.0 MOC12AF91
API 600 Trim Number Chart
TrimMaterialSeatDiskBackseatStemNotes
1410410410410410
2304304304304304
3F310310310310310
4Hard 410Hard 410Hard 410410410seats 750BHN Min.
5HardfacedStelliteStellite410410
5AHardfacedNi-CrNi-Cr410410
6410 and Cu-NiCu-Ni410410410
7410 and Hard 410Hard 410Hard 410410410seats 750BHN Min.
8410 and HardfacedStellite410410410
8A410 and HardfacedNi-Cr410410410
9MonelMonelMonelMonelMonel
10316316316316316
11Monel and HardfacedStelliteMonelMonelMonel
12316 and HardfacedStellite316316316
13Alloy 20Alloy 20Alloy 20Alloy 20Alloy 20
14Alloy 20 and HardfacedStelliteAlloy 20Alloy 20Alloy 20
15304 and HardfacedStelliteStellite304304
16316 and HardfacedStelliteStellite316316
17347 and HardfacedStelliteStellite347347
18Alloy 20 and HardfacedStelliteStelliteAlloy 20Alloy 20

 Industry Associations

American National Standards Institute
www.ansi.org

ASTM International, formerly known as the American Society for Testing and Materials (ASTM)
www.astm.org

American Supply Association
www.asa.net

International Organization for Standardization
www.iso.org

Process Industry Practices
www.pip.org

Valve Manufacturers Association
www.vma.org

American Petroleum Institute
www.api.org

American Society of Mechanical Engineers
www.asme.org

E.I. Dupont Elastomer Chemical Resistance Guide
www.dupontelastomers.com

National Association of Purchasing Management
www.napmsyr.org

 

By | 2017-04-05T13:56:31+00:00 August 31st, 2010|Understanding Steam|Comments Off on Understanding Steam