WET News

AUGUST 2014 WET NEWS 17 The right feed water quality is crucial to optimise the operation of steam systems, says Mike Griffin, steam systems conditioning manager at Spirax Sarco. The challenge: Impurities can cause problems if le unchecked Whether the treatment regime is devised in-house or outsourced, raw water needs to be treated before being fed to the boiler M ost people think of wa- ter as plain old H2O and life for steam users would be far easier if things were really that simple. But the raw water supply that steam users rely on can oen have more to it, with a wide range of potential impurities that can cause a variety of problems if le unchecked in a boiler or steam distribution system. Common impurities include dissolved solids and gases, sus- pended solids and scum-form- ing substances, all of which may be problematic. Dissolved solids such as cal- cium and magnesium bicarbo- nates form an alkaline solution and salts that precipitate to cre- ate deposits. The alkalinity must be con- trolled to prevent such deposits damaging the performance of the boiler, for instance, by obstructing heat transfer or fluid flow. Silica and iron in the water supply can also result in unwanted deposits, although these are not as common. Gases such as oxygen and carbon dioxide dissolve easily in water. Oxygen is harmful in even small amounts because it causes pitting in metal sur- faces, while carbon dioxide dis- solved to form corrosive carbonic acid. In addition, deposits of carbonates and bicarbonates inside the boiler can release carbon dioxide when heated, which can then form more carbonic acid. Reverse osmosis Suspended solids are usually mineral or organic in origin and are not generally a big problem because they can be filtered out. However, they can cause real trouble in some equipment if they're not dealt with. For instance, they will cause severe blockages if they get through to the membranes of a reverse osmosis system. There are three main conse- quences of water impurities on a steam system – corrosion, deposits and foaming. Corrosion can damage the boiler surfaces, piping and other steam system equipment and it is likely to happen if there is dissolved oxygen or carbonic acid in the water. Carbonic acid can cause thinning of metal surfaces in contact with water, for example the bottom of pipework, while oxygen causes pitting of metal surfaces above the water, for example the top of pipework. If carbonic acid and oxygen are both present then the rate of corrosion rises by about 10%. The biggest problem with deposits is the disruption to heat transfer and reduced energy efficiency. A layer of deposits is a bar- rier against effective heat trans- fer and can reduce the overall energy efficiency of boilers dramatically. Essentially it forces the boiler to burn more fuel to raise the same amount of steam. A deposit layer just one mil- limetre thick can reduce boiler efficiency by 10%. In extreme cases, deposits can create local hot spots that can cause mechanical damage or even boiler failure. Once a problem with depos- its is identified, the resulting clean-up can also be problem- atic. The "quick fix" involves stripping down the system and cleaning it with acid, but this puts the boiler out of action and the aggressive acid can exacerbate any weaknesses in the equipment. Foaming Online cleaning is a more grad- ual process that dissolves the deposits over several months, or soens and loosens them so that they drop to the bottom of the boiler. Online cleaning involves THE CONCEPT • Faster steam system warm-up times • Reduced downtime • Fewer product rejects because steam reaches the point of use in the correct condition • Maximising heat transfer throughout the steam system • Minimising chemical, water and energy use • Reducing energy waste and carbon emissions • Lower fuel bills through improved energy efficiency NEED TO KNOW 1 The rate of corrosion can rise 10% if carbonic acid and oxygen are both present 2 Raw water quality can be variable and getting the wrong systems in place can mean paying more than necessary 3 Once a problem with deposits is identified, the resulting clean-up can also be problematic 3 Although dissolved solids levels will affect the performance of the boiler operation, other issues, such as total alkalinity or silica content can sometimes be more important THE VERDICT • The recommendation on most sites is to carry out testing several times a week, so any changes in the system chemistry can be identified and dealt with immediately • But in these days of rising energy bills and austerity, steam users simply cannot afford to have suboptimal systems eating into their utility bills and maintenance budgets. • INSIGHT WATER TREATMENT running the boiler at a higher alkalinity, with extra-low levels of dissolved solids and higher levels of chemical dispersants. The boiler can carry on gener- ating steam throughout the process, but the extra boiler blowdown needed to keep dis- solved solids at bay and to remove the extra sludge from the bottom of the boiler means losing energy, in addition to the extra money spent on treat- ment chemicals. This approach is also ineffective on silica deposits. Dissolved solids Foaming happens when the alkalinity of the water is too high, the boiler's levels of dis- solved solids are too high, there is excessive contamination in the condensate return or the dosage of certain polymers is too great in an inappropriate water treatment regime It can cause a variety of problems, from interfering with the readings on level instru- ments and pressure meters to the carryover of boiler water into the steam system. Carryover is especially trou- blesome elsewhere in the steam system, where it can contami- nate control valve surfaces, impair heat transfer and block steam traps.

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