Improving Condensate Quality

Historically, steam condensate has been viewed as the best quality water in the energy center. Pure condensed steam has all the characteristics of distilled water, i.e. low concentrations of dissolved and suspended solids as reflected in very low specific conductance and filterable residue.

By comparison, raw boiler makeup contains hardness, alkalinity, chlorides, sulfates and metals that produce undesirable dissolved and suspended solids in the boiler.

Because engineers and scientists recognize the benefits of removing impurities from the feedwater before they enter the boiler, considerable attention has been given to designing makeup water pretreatment systems that effectively remove these contaminates from the raw water before they can cause problems such as scale deposition and corrosion.

Sodium ion exchange systems (water softeners) are capable of removing hardness before it reacts within the boiler to produce mineral scale deposits that adversely affect heat transfer. More sophisticated ion exchange demineralizers and membrane separation arrays (such as reverse osmosis) are capable of complete deionization of boiler makeup. These systems produce makeup water that is of such exceptional purity that it often exceeds the quality of steam condensate.

As a result of these advances, steam condensate is losing its status as “the best quality water in the energy center.” In fact, dissolved and suspended impurities in steam condensate can be several orders of magnitude greater than that in the makeup. In some cases, contamination of plant condensate is so severe that it must be discharged to drain rather than recycled for use as boiler feedwater.

However, because steam condensate is hot, it remains a very valuable resource from an energy conservation standpoint. Dumping the condensate to drain in order to protect the boiler is wasteful and unnecessary. Fortunately, several treatment options are available to optimize condensate quality for recycle to the boiler feedwater system.

FEEDWATER QUALITY

The ASME report, “Consensus on the Operating Practices for the Control of Feedwater/Boiler Water Chemistry in Modern Industrial Boilers,” provides water chemistry guidelines that promote the maintenance of steam purity and quality. This document provides recommendations across the entire spectrum of boiler types and pressure ranges. For the purposes of this discussion, however, we will focus our attention on typical industrial watertube boilers with turbine drives or superheat that operate at less than 750 psig. Additional information is available by reference to the ASME report.

ASME Boiler Feedwater Quality Guidelines

Parameter

Pressure Range

< 300

301 – 450

451 – 600

601 – 750

Hardness

0.3

0.3

0.2

0.2

Iron

0.1

0.05

0.03

0.03

Copper

0.05

0.03

0.02

0.02

pH

7.5 to 10

7.5 to 10

7.5 to 10

7.5 to 10

TOC

1

1

0.5

0.5

Oil

1

1

0.5

0.5

Note: Table indicates not-to-exceed values in mg/l (ppm). pH values are standard units

Boiler feedwater is a blend of makeup and returned condensate. In many low pressure plants that operate at less than 150 psig, the load is generally space heating and the percentage of condensate in the blend is high; typically 80% or more. Plants that operate at over 600 psig are power generating stations where the condensate return rate is also very high with low makeup requirements. In the mid-range, in addition to power generation, some of the steam is used in process applications where it may be lost, consumed or contaminated. In this case, the makeup requirements are much higher as the contaminated condensate must be discharged to drain to insure that the feedwater quality is maintained within acceptable limits.

Steam that is dry and pure as it leaves the boiler becomes contaminated as it gives up its heat, condenses and flows through the condensate return system. The primary contaminates are iron, copper, calcium and magnesium hardness, oil/organics and various process impurities.

Iron is commonly found in many condensate systems as a by-product of corrosion of the piping system. Likewise, copper may be detectable as a result of the general corrosion of copper alloy heat exchangers. Notwithstanding the application of condensate corrosion inhibitors such as neutralizing amines that keep the pH above 7.0, iron and copper corrosion is still a concern especially if oxygen is present. These metals are present either as impurities that are dissolved in the condensate or as insoluble by-products of the corrosion reaction. Insoluble materials are commonly called “crud” or “sludge.”

Hard water contamination is also a common problem. Steam that is used to supply coils in hot water tanks and to heat process baths and washing/rinsing operations often becomes contaminated with calcium and magnesium as it condenses and pulls hard water into the coil at a leak or crack. Locating the source of the hardness is challenging especially within central or district steam plants that provide steam to numerous buildings or processes. For plants that operate a generator, cooling water that leaks into the turbine condenser is a common source of hard water contamination.

Oil or organic contamination is a concern in that it tends to form adherent tar-like deposits in the boiler that are difficult to remove by routine blowdown. Oil combines with corrosion by-products such as iron and copper oxides to produce gummy, heavy deposits in condensate receiver tanks and the storage section of feedwater heaters.

In order to maintain the feedwater quality within ASME guidelines, it is often necessary to dump the condensate to drain to prevent dissolved impurities and crud from entering the feedwater system. If this is not done, the offending contaminates will enter the boiler where they must then be controlled by surface and bottom blowdown to prevent their accumulation as boiler scale. In either case, condensate dumping or aggressive blowdown wastes valuable heat energy and results in a reduction in overall boiler efficiency.

Considerable experience with reviewing boiler scale deposits and tube failures over the years suggests that a majority of material removed from a modern industrial boiler consists primarily of iron and copper oxides with lesser amounts of calcium and magnesium. This is in sharp contrast to the general belief that boiler scales arise due to the deposition of calcium and magnesium hardness; i.e. the classical “lime” scale so common years ago when the use of unsoftened makeup was more common. This shift is due to the universal use of ion exchange softening systems that have effectively eliminated hardness from the boiler makeup. As a result, metal oxides, hardness and process contaminates in the returned condensate have replaced makeup water hardness as the greatest threat to feedwater quality and boiler cleanliness.

IMPROVING CONDENSATE QUALITY

Treating condensate to remove undesirable impurities is not a common practice in boiler plants that operate below 600 psig. The general belief is that internal chemical treatment of the boiler water with specialty chemicals like precipitating agents, chelants and dispersing polymers is sufficient to keep feedwater contaminates fluid and easily removed by routine blowdown. Further, the use of neutralizing amines is a common practice to minimize corrosion in the condensate system. Iron and copper levels in the condensate are minimized by this practice assuming that pH is the only variable in the corrosion reaction. However, dissolved oxygen, if present, will cause accelerated oxidation (corrosion) of iron and copper even at pH values above 7.

In cases where chemical treatment is insufficient or ineffective in controlling condensate contamination, three (3) methods may be used to improve condensate quality;

1. Mechanical filtration

2. Deep bed ion exchange polishing

3. Powder resin polishing

Filtration: If the primary condensate contaminate is insoluble “crud,” then mechanical filtration using cylindrical string wound or metal filters offers the ability to filter out iron and copper oxides and reduce dissolved impurities. These filter are not as effective in removing dissolved solids that pass through the filter.

Deep bed ion exchange polishing systems utilize beds of ion exchange resin to filter out “crud” by in-depth filtration. This means that the iron and copper oxides along with other impurities penetrate deep into the bed rather than being trapped on the surface as with conventional filtration. The ion exchange resin also offers the advantage of removing unwanted dissolved solids that may have entered the condensate such as calcium and magnesium hardness.

Cation exchange condensate polishers are used in low and medium pressure systems to remove by-produces of iron and copper corrosion. This consists of a deep bed of macroporous strong acid resin that is operated in the sodium or amine form. The insoluble material is removed by depth filtration within the resin bed while the exchange capacity of the resin removes hardness.

A mixed bed of cation and anion exchange resin is a very common condensate polishing system especially in higher pressure applications. Again, crud removal is accomplished by in-depth penetration into the bed, which keeps filter cakes from forming on the surface. Dissolved cations and anions are exchanged for hydrogen and hydroxide which produces an effluent that is essentially of demineralized quality.

The endpoint of the service cycle must be carefully monitored to insure that the effluent meets the required quality limits. The point at which the service run will be terminated is dependent on type of polishing system (cation, mixed bed, or a combination) along with specific requirements of the steam plant.

Powder resin polishers are an alternative to deep bed condensate polishing. In this design, steel mesh or fiber-wound tubes are coated with a 1/8 inch layer of powdered resin that is applied from a slurry prior to each service run. The powdered resin functions as an effective filter to remove insolubles, but the ion exchange capacity is limited as compared to deep bed polishers. The powdered resin polisher is allowed to run until the pressure drop reaches 20 to 25 psig. However, these filters must be carefully monitored to insure that the exchange capacity is not exceeded prior to reaching the pressure differential cutoff. The ratio of cation to anion resin can be varied to adapt to the specific needs of the plant. The ratio of cation to anion resin ranges from 2:1 to 0.8:1 depending on the type of condensate contamination and plant operating conditions.

CONCLUSION

Improving condensate quality is a cost effective way to maintain clean boiler heat transfer surfaces, save energy and control operating costs. Several condensate polishing systems are available to achieve these goals. Each has its specific benefits and, therefore, should be evaluated based on site-specific water quality requirements and plant operating conditions. When done properly, the status of the plant condensate will once again be returned to “the best quality water in the energy center.”

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