WET News

16 WET NEWS AUGUST 2015 The right feed water quality is crucial to optimise the operation of steam systems, says Mike Griffin. The challenge: Impurities can cause problems if le unchecked INSIGHT WATer TreATMenT If companies opt to bring in specialist help, the contract should specify no more than a month between visits 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 4 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 • 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 budget. M ost people think of water 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, suspended solids and scum- forming substances, all of which may be problematic. Dissolved solids such as calcium and magnesium bicarbonates form an alkaline solution and salts that precipitate to create deposits. The alkalinity must be controlled 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. Reverse osmosis Gases such as oxygen and carbon dioxide dissolve easily in water. Oxygen is harmful in even small amounts because it causes pitting in metal surfaces, while carbon dioxide dissolve 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. 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. 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 barrier against effective heat transfer 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 millimetre 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 deposits is identified, the resulting clean-up can also be problematic. 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. The challenge: It pays to have the right water treatment The quality of raw water is key when it comes to the treatment plant selection process. D eciding on the optimum water treatment system is not always straightforward. Raw water quality can be variable and getting the wrong systems in place can mean paying more than necessary for treatment chemicals or perhaps even failing to prevent the potential problems. Water supplies and steam systems are extremely dynamic and their chemistry can vary day by day. The quality of raw water is obviously an important factor when choosing a water treatment plant. 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 and then dominate the selection process for water treatment equipment. Expert input can be the best approach for many steam system users, perhaps even outsourcing the water treatment entirely to a third party. Whether the treatment regime is devised in-house or outsourced, raw water needs to be treated before being fed to the boiler. The major water treatment processes include: • Water so ening supple- mented by chemical treatment in which various chemicals are added to the boiler feed– water in order to reduce the effect of deposits, corrosion and chemical attack within the system. Chemicals should generally only be added to the boiler feedwater in accordance with accepted standards, such as BS 2486: 1997 or BS EN 12953 –10 2003. Following the guidance in the standards should help prevent excessive dosing and provide an effective programme of trouble-prevention. • Reverse osmosis – a process in which the water to be treated is forced through a semi-permeable membrane to strip out nearly all the contam- inants, leaving behind a concentrated solution of impu- rities, which is rejected to waste. The pure water, or "permeate", will have had 98-99% of its salts removed. • Carbon filtration – if the water is heavily chlorinated, carbon filtration may also be required either between the water sof- tener and the Reverse Osmosis plant or before the water sof- tener itself. This is necessary to remove chlorine from the water, which otherwise could be highly corrosive. There are also important aspects of the day-to-day operation of the boiler itself that can have a big impact on water quality. For instance, heating the water in the boiler feed tank will reduce the concentration of dissolved oxygen and other gases. This minimises the oxygen-scavenging chemicals required and fewer chemicals means that less bottom blow- down is needed, which saves energy. In-house checks Most boiler owners recognise that they do not possess the in-depth knowledge and skills in-house to optimise their water treatment and steam system conditioning regimes. If they opt to bring in specialist help, the contract should specify no more than a month between visits. Even so, it may be prudent to carry out in-house checks between visits from the experts, and the specialist company should provide whatever support the end user needs to make those checks possible. For example, Spirax Sarco not only provides testing kits and water treatment engineers, but also makes sure that steam plant owners have all the training they need to use them properly. 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. Implementing an effective water treatment regime may seem like a complex and daunting prospect. But in these days of rising energy bills, steam users simply cannot afford to have suboptimal systems eating into their utility bills and maintenance budgets. n Mike Griffin is technical manager at Spirax Sarco.

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