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|Typical boiler system problems involve corrosion of feedwater systems; scale, deposition, and corrosion in the boiler; corrosion of condensate systems; and problems due to impurities in produced steam. ProChemTech manufactures various chemical products and equipment, which when properly applied, can economically control all of these problems.|
Oxygen Corrosion Control
Oxygen which enters a boiler system will contribute to accelerated corrosion of the feedwater system, boiler, and condensate return system. The primary method to control oxygen entry into the boiler system is to deaerate added makeup. Once the makeup is deaerated, various organic and inorganic oxygen scavenger compounds are added to complete oxygen removal. An excess residual of oxygen scavenger in the boiler water, for instance 40 to 60 mg/l in the case of sulfite, is recommended to protect the boiler at all times. Oxygen scavengers should be added to the boiler makeup water stream immediately after deaeration so as to protect all downstream components of the system. Oxygen enters the condensate system via passage through the boiler and introduction via condensate tank vents. Oxygen based corrosion is controlled in the condensate system by a combination of good oxygen removal upstream, use of volatile oxygen scavengers, and/ or use of filming amines. Proper use of neutralizing amines, to maintain the pH above 8.0 and 8.5 su, also aids in oxygen corrosion control.
ProChemTech produces a wide variety of oxygen scavenger products for use in boiler systems, which permits our water treatment specialists to specify the exact chemistry needed for safe, economical operation of any boiler system. Products are available as concentrated powders, easy to use liquids, and as blended multi-component formulations.
Scale and deposition in a boiler system can be caused by precipitation of two classes of materials: scale forming minerals, introduced with the makeup water; and corrosion products, generated within the boiler system itself due to poor corrosion control. The primary means for control of scale caused by minerals in the makeup water is to treat the makeup water prior to use via cation exchange softening, reverse osmosis, or demineralization. This treatment should remove the majority of the scale causing minerals, usually salts of calcium and magnesium, so that boiler chemical use is minimized.
Even with the best pretreatment, some form of scale and deposition control chemistry is needed in the boiler water to control residual traces of scale forming minerals as well as any products of corrosion formed within the boiler system. Scale control products are basically classed as precipitating or non-precipitating based upon their action in preventing scale formation. Precipitating products, usually based on carbonate or phosphate chemistry, actually precipitate the hardness minerals within the bulk boiler water to form a fine "mud". This mud is maintained in suspension via use of polymer dispersants and removed in the routine boiler blowdown. Non-precipitating products, usually based upon sequestrant and chelant chemistries, function by chemically complexing the scale forming minerals to form a soluble compound, thus preventing any scale or mud formation.
As with oxygen scavengers, a residual of scale control chemistry is maintained in the boiler water to provide protection 100% of the time. For instance, phosphate chemistry is usually controlled to maintain a free phosphate residual of 20 to 40 mg/l.
ProChemTech produces products based on all four major scale control chemistries, as well as all commonly used polymer dispersants, for use in boiler systems. This range of products allows our water treatment specialists to specify the exact chemistry needed for safe, economical operation of any boiler system. Products are available as easy to use single, dual, and multi-component liquid formulations. In many cases, appropriate scale and deposition control chemistry can be obtained in one blended product by judicious selection of the various active components. This benefits the user by reducing the number of individual chemical products required for the water treatment program.
Boiler Corrosion Control
While often not discussed, boiler internals must also be protected against corrosion caused by contact with water. The best method of protection is to simply maintain the boiler water OH alkalinity above 250 mg/l, thus rendering it non-corrosive to steel boiler internals. Both the carbonate and phosphate scale control chemistries also require the presence of OH alkalinity to properly form the scale preventing precipitates.
This fact, that boiler water should be maintained above a specific OH alkalinity value for two different specific reasons, nicely demonstrates the multiple actions often encountered in water treatment chemistry. In addition, it should be noted that in many cases sufficient alkalinity is present in the makeup water so that no supplemental alkalinity needs to be added to the boiler system to maintain the cycled boiler water alkalinity at an appropriate level. We bring this point up as many "water treatment experts" routinely add alkalinity as part of their boiler water treatment programs when it is not needed. This addition, in many cases, actually results in increased blowdown where the boiler cycles are limited by alkalinity considerations. Such actions amount to nothing less than theft, as the customer is both purchasing chemicals that are not really needed and the use results in increased boiler blowdown, which is extremely costly.
ProChemTech has a complete selection of single and multi- component alkalinity adding products, as well as many products with no added alkalinity, so that a water treatment program can be tailored exactly to each customer's needs.
Condensate Corrosion Control
|Corrosion in a condensate return system can result from two
items, entry of oxygen or low pH in the condensate.
Boiler condensate has a naturally low pH due to the formation of carbon dioxide in the boiler, from breakdown of carbonates present in the makeup water, and its subsequent carryout and dissolution in the condensed steam. When carbon dioxide dissolves in water, it forms carbonic acid, which can easily corrode the materials from which most condensate systems are constructed. To maintain the condensate pH in a minimal corrosion range, usually between 8.0 and 8.5 su, various volatile amines are added to the boiler. These compounds carry out with the steam and condense, forming an alkaline solution which neutralizes the carbonic acid.
Another approach, which can be used in place of or in combination with neutralizing amines, is to feed a filming amine into the boiler. Filming amines, typically based on octadedylamine, also volatilize and carry out with the steam. When they condense, however, they preferentially form an organic film on exposed metal surfaces, the film protecting the metal from attack by acidic condensate and oxygen.
Selection of the proper condensate corrosion control program can be quite complex as the ability of the various amines to carry throughout a steam system changes with steam pressure and distance from the boiler. The specific amine(s) fed into each boiler should be based upon the steam pressures and geometry of the steam system so as to ensure that the pH of all return condensate is within the recommended range.
ProChemTech produces a wide variety of condensate products, based on all accepted amines, for use in boiler systems. This permits our water treatment specialists to specify the exact chemistry needed for safe, economical operation of any boiler system. Products are available as concentrated single component and blended multi-component formulations.
Poor steam quality can cause problems via deposition on heat exchangers, erosion of turbines, and product contamination. These problems can be avoided by close control of boiler water dissolved solids levels and use of antifoam compounds to control boiler foaming, thus giving improved steam quality. ProChemTech incorporates an extremely effective anti-foam in all our basic boiler scale and deposition control products, and also makes the material available as a concentrated single component product.
The various boiler chemistries developed by the company permit the maximum boiler cycles possible to be obtained via use of effective anti-foams, no alkalinity products, organic oxygen scavengers, and selection of minimal solids scale and deposition control products.
The company is also a major supplier of wastewater treatment technology and has integrated this knowledge with its boiler water technology to provide clients with a unique ability to reuse treated wastewaters as boiler makeup. Being a provider of complete wastewater systems and specific wastewater treatment chemistries, while knowing the various inherent limitations of boiler water treatment chemistry, has made the company the world leader in this new, rapidly expanding field.
To date, several projects in the railcar cleaning and repair industry have been successfully completed where wastewaters have been treated and then reused as boiler makeup water.
Chemistry Program Control
We have also found that lack of proper chemistry control is a major cause of boiler system water problems. To insure proper application of our chemical products, the company has designed proprietary control systems which utilize a blend of manufactured and purchased units to provide the most reliable standard, and customized, chemical feed and blowdown systems for control of boiler water chemistry available.
For more information, please contact:
Minimum Drum Diameter
Sizing of steam disengaging drums relys on experience as much as specifics, but the following guidelines should be considered.
Except for very special designs, the minimum inside diameter of steam drums for HRSG service should be a minimum of 48 inches.
Steam drum water surge volume should be determined for rate of change due to anticipated process operating conditions and for anticipated rate of change due to system pressure increase or decrease. If not known, a minimum rate of change to be considered should not be less than 20% per minute. The drum sizing must allow for the swell volume to be accomplished without actuating the high liquid level alarm or causing liquid carryover into the saturated steam piping. Likewise, the shrinkage should be accomodated without actuating the low level alarm.
The vapor velocities in the free space above the normal liquid level should not exceed the following velocities at design flow rate and normal operating pressure.
|Vh||=||Horizontal velocity, ft/s|
|Vv||=||Vertical velocity, ft/s|
|rl||=||Liquid density, lb/ft3|
|rv||=||Vapor density, lb/ft3|
Feedwater Holdup Time
In addition to accomodating the shrinkage volume discussed above, the drum size should allow for a minimum of two minutes holdup time at design flowrate in the advent of a loss of feedwater.
The steam drum must be sized to accomodate the internals necessary to meet the guaranteed steam purity requirements, as well as the riser and downcomer conections. The internals include the primary separators, baffles or centrifugal, the secondary separators or dry pipe, as well as the channels or plenums needed for collecting the steam and water mixture from the risers. All steam drum internals should be removable without cutting. All internals should be able to be removed through the drum manhole.
There are many types of drum internals available, so we are only suggesting what may be used. For low pressure drums, under 50 psia, simple primary baffles and secondary dry pipe should be sufficient for most services. For low pressure drums integrally connected to a deaerator, no dry pipe is required. For other pressures and services the use of centrifugal primary separators with chevron secondary scrubbers are recommended. We aslo recommend the use of a dry pie on internal connection of steam outlets even when chevrons are used.
The minumum area of chevrons required may be calculated using the following:
|Amin||=||Minimum area of chevrons, ft2|
|Wn||=||Net steam flow, lb/hr|
|Vv||=||Specific volume of drum vapor, ft3/lb|
|Vl||=||Specific volume of drum liquid, ft3/lb|
So, if you had a single row of chevrons 8" high, you would need 1.5*Amin in length or if you had a double row, i.e., one row on each side of dry pipe, you would need a length of 0.75*Amin. Of course, for larger diameter drums, you could use 12" high chevrons and reduce the length requirement accordingly. You should keep in mind, however, that increasing the height of chevrons reduces the working area in the drum which is needed for drum swell, etc.
The minumum number of centrifugal separators may be calculated using the following:
|Ncent||=||Number of centrifugals|
|Ws||=||Steam Make, lbs/hr|
|Vv||=||Specific volume of vapor, ft3/lb|
|Vl||=||Specific volume of liquid, ft3/lb|
|Cratio||=||Design Circulation ratio|
We are assuming 12" centrifugals for this case(remember, they must go through our 12"x16" manhole) so it is fairly easy to determine the length required for a single row. But to reduce the required length, you may stagger the cans and/or place them along both sides of the drum. Additionally, the formula is designed to provide a 1 psi pressure loss through the centrifugal separators at design load.
Feedwater Distribution Pipe
The boiler feedwater distribution pipe should have the following minimum pipe inside diameter.
|di||=||Inside pipe diameter, in|
|Wbfw||=||Feedwater flow, lb/hr|
|Volbfw||=||Specific volume of feedwater, ft3/lb|
Length of the pipe should be approximately the full length of the drum. Pipe should be secured and supported approximately every 35 diameters. Pipe should be perforated approximately 12 inches on center. Total flow area of perforations should not be less than area of the pipe. Feed pipe should have at least one breakaway joint at connection to external nozzle inside drum. Above 3 inch size should be flanged and below 3" should be threaded. End of pipe should be capped with a vent hole at top. The perforations should be orientated downward and toward the chemical feed pipe. The pipe should be below low liquid level and, if possible, in the stream of effluent coming from the primary separators and going to the downcomers. The entry into the drum must be fitted with a thermal sleeve if required by ASME code. Preferred location is horizonal, on centerline of drum head. If drum has thermal sleeve, it may not be located below horizontal, which would allow debris to collect in sleeve. If the economizer is designed such that steaming can occur at any operating condition, this design is not adequate. Special design for steaming economizer is not covered in this discussion.
Continuous Blowdown Pipe
The boiler continuous blowdown pipe should have the following minimum pipe inside diameter.
|di||=||Inside pipe diameter, in|
|Wbldn||=||Blowdown flow, lb/hr|
|Volsatliq||=||Specific volume of drum water, ft3/lb|
To calculate the blowdown flow, you must know the solids in the feedwater. If known, you can base the blowdown flow on the following ABMA standards for drum water conditions, unless the specifications require a more conservative level.
|American Boiler Manufacturers Association|
|Boiler Water Standards|
|Pressure at Drum Outlet, psig||Total Solids, ppm||Total Alkalinity, ppm||Suspended Solids, ppm|
|2001 & Higher||500||100||5|
So if you had a feedwater with a total solids of 20 ppm and you were operating at 650 psia, then the blowdown based on percent of feedwater flow would equal 20/2000*100 = 1%, or based on feedwater flow it would equal 20/(2000-20)*100 = 1.01%.
The minimum pipe diameter to be used for this service is 3/4 inch IPS. Length of pipe should be full length of drum. Pipe should be perforated on approximately 12 inch centers. Perforations should be orientated in an upward direction. Blowdown pipe should have a threaded breakaway joint at connection to internal nozzle. The blowdown pipe should be located below the water discharge of the primary separators. If the separators are located on both sides of the drum, the blowdown pipe should be divided and ran on each side.
Intermittent Blowdown Pipe
A rule of thumb to use is for steam flows up to 150,000 lbs/hr, use 1 1/2 inch; and for greater than 150,000 lbs/hr, use 2 inch size. Preferred location is dependent on type of HRSG. In double drum designs, the intermittent blowdown pipe should be located in the mud drum. For single drum designs, the blowdown pipe should be located in the bottom of the drum. The purpose of the intermittent blowdown is to make quick corrections to water level as well as sludge removal. It should be pointed out that there is not much "sludge" in modern boilers with sufficient water treatment. This blowdown pipe may also be used as part of the overall boiler draining procedure.
Chemical Feed Pipe
Minimum internal pipe size should not be less than 1/2 inch IPS. There is no specific sizing rules since normally at time of design, the flow is not known. A rule of thumb to use is for steam flows up to 50,000 lb/hr, use 1/2 inch; for 50,000 to 150,000 lbs/hr, use 3/4 inch; and for greater than 150,000 lbs/hr, use 1 inch size. This pipe should be 304 SS material including the nozzle entering the drum. Good practice is to use a ss sleeved entry nozzle designed such the the chemical feed pipe can be replaced easilly. Pipe should be supported every thirty-five diameters. Chemical feed pipe should be capped and vented at end. Preferred location is where the wash from the feedwater pipe will mix the chemicals well before they can enter the downcomers. Care should be taken so that chemicals cannot be drawn up by the contiuous blowdown.
Other Miscellaneous Nozzles
The minimum connection size for any connection should not be less than 3/4 inch. All nozzle connections up through 2 inch shall be minimum schedule 160.
Manholes should be 12 inch by 16 inch or larger and must be equiped with yoke and hinged cover. Sizing must be able to facilitate removal of all internals without cutting.
All drums should have a minimum of two steam outlets, manifolded together outside the drum. This reduces the demand on the dry pipe and chevrons, as well as, lowers the horizontal velocity in the vapor space. An exception to this rule may be made for units with a total steam flow of less than 50,000 lbs/hr and there is no superheater. The minimum piping area should not be less than:
|di||=||Inside pipe diameter, in|
|Wn||=||Net steam flow, lb/hr|
|Volsatvap||=||Specific volume of drum vapor, ft3/lb|
Normal Steam Drum Connections
|Connection||Design Pressure, psig||Type|
|Steam Outlets||All Pressures||Welded|
|Safety Valves||Under 650||Flanged|
|650 and Over||Welded|
|Chemical Feed w/Sleeve||All Pressures||Flanged|
|Feedwater Inlet w/Sleeve||Under 650||Flanged|
|650 and Over||Welded|
|Water Columns, Lower||Under 650||Flanged|
|Conns w/Sleeves||650 and Over||Welded|
|Test Connections||Under 650||Flanged|
|650 and Over||Welded|
|Pressure Gauges||Under 650||Flanged|
|650 and Over||Welded|
|650 and Over||Welded|
|Sampling Connections||Under 650||Flanged|
|650 and Over||Welded|
|Continuous Blowdown||Under 650||Flanged|
|650 and Over||Welded|
|Intermittent Blowdown||Under 650||Flanged|
|650 and Over||Welded|
|Risers||All Pressures||Rolled or Welded|
|Downcomers||All Pressures||Rolled or Welded|
For our sample HRSG that we have frequently referred to in this discussion, our steam drum might look like the one shown below. When we reviewed the natural circulation that would be the case for this HRSG, since it is an O-Frame design, we concluded that we needed 16 centrifugals. Since we have risers entering the drum from both sides, it makes sense to use two rows of centrifugals, so without staggering, we would need at least 8 feet of length. Since we had 28 tubes wide at 4" spacing, the inside width of the evaporator would be 28 * 4/12 = 9.33'. If we add 18" to allow for the casing and structure, then the seam length of the drum would be 10'-8" which is plenty of room for our centrifugals.
Our guide for the chevrons indicates we need a minimum of 12 ft2. So if we assume 8" high with two rows, we would need 9 feet of length. With our seam length, we have plenty of room for our chevrons.
Following our rule of two outlet steam connections, the maximum vapor flow at any point in the upper part of the drum would be 104396/4 = 26,099 lb/hr. Using our formula for maximum velocity, Vh, from above, we are okay up to 3.85 ft/s. if we assume the normal water level, NWL,is at centerline of drum, our net flow area is 6.28 ft2, and our velocity would be only 1.15 ft/s without discounting the area blocked by chevron hangers, so we are okay.
The first thing we notice after checking the times is that we don't meet our requirement for 2 minutes between NWL and LLSD. We can achieve this by raising the NWL to 26 inches and lowering the LLSD to 6 inches. But, this shows how tight the design of this drum is due to the short length. We could of course increase the diameter, but this would affect the cost, or we could increase the length, but then we couldn't ship because of the width of the module. Remember, when we change these levels, we must recheck the vapor velocities.
The insulation in an HRSG is extremely important for a number of reasons. The insulation provides a means of keeping the heat contained in the HRSG where it can be absorbed by the heat exchanger tubes, resulting in higher overall efficiencies. The insulation also keeps the external shell cooler making it safe for operating and maintenance personnel to safely work around the equipment. This cooler casing temperature also results in the structural stability of the overall structure.
Twenty years ago, much of the insulation used in HRSGs was the gunned or cast refractory. This material often, was mixed on site at the HRSG manufacturer's shop, and thus frequently varied in insulating properties. The more popularly used mixes like 1:2:4 LHV and others became standard and over time the insulating properties became very predictable. This was improved upon by the offering of proprietary mixes, by a number of companies, which were packaged in controlled environments and were thus more predictable in their application.
During the early eighties, ceramic fibers became accepted in the industry and since they are a much better insulator, they quickly caused a decline in the use of refractory. In general, 3 inches of ceramic fiber blanket could do a better job than 6 inches of refractory and weighed much less. As an example, if we have a hot face temperature of 1200 °F and an air temperature of 70 °F on a vertical wall with no wind blowing, 6" 1:2:4 LHV gunned has a cold face temperature of 205.5 °F where 3" 8# 2300°F ceramic fiber would have a cold face temperature of 161.2 °F. And this is with a weight less than 10% of the gunned refractory, which reduces freight cost. Furthermore, the ceramic fiber blanket does not require "drying" in the field as would be required with the refractory.
Refractory is still used in special cases and in areas where it is more durable or easier to install. The floor of a unit which must be walked on during maintenance and inspection, may use castable refractory or brick, or both because it is more durable. End tube sheets, when they have multiple tube penetrations such as in an end supported tube convection may utilize gunned refractory because it is easier(less costly) to apply between the openings for the tubes then ceramic fiber blanket.
The use of fiber insulation in the high velocity ducting, normal in HRSG designs, quickly brought out a problem that didn't occur in other equipment such as Direct Fired Heaters or Boilers. The damage to the fiber material due to the high velocities, over 50 ft/sec, encountered in the ducting requried the use of metal liners. These thin metal liners themslves also present a design problem which is discussed elsewhere.Heat Loss Through Insulation:
The heat loss due to radiation may be calculated using the Stefan-Boltzman formula.
|hr = Heat loss by radiation, Btu/hr-ft2|
|e = Emisivity of surface, assumed at 0.95|
|T1 = Temperature of surface, °R|
|T2 = Temperature of surroundings,°R|
The heat loss due to free convection may be calculated using the following method.
|hc = Heat loss by convection, Btu/hr-ft2|
|C = A constant, assumed at :|
|1.79 for an arch or roof|
|1.39 for a wall|
|0.92 for a floor|
|Tavg = Average temperature of wall and surroundings,°R|
The heat loss due to forced convection, where the air velocity is greater than zero, may be calculated using the following method.
|hfc = Heat loss by forced convection, Btu/hr-ft2|
|V = Velocity of air across surface, ft/sec|
To visualize the differences in the various materials, used for insulation, the following calculator can be used to run calculations for some of these materials under different conditions.