78 Table 1. Steam Turbine Miınufacturer's Purity Requirements. ABB GE Westinghouse Cation Conductivity, < 0.5 < 0.2 < 0.3 mmhos/cm (mS/cm) Dissolved Oxygen, ppb < 10 (mg/L) Sodium, ppb (mg/L) 10 20 < 5 Chloride, ppb (mg/L) < 5 Silica, ppb (mg/L) 50 20 < 10 Copper, ppb (mg/L) <2 lron, ppb (mg/L) < 20 Na : PO4 Molar Ratio 2.3-2.7 Total Dissolved Solids, ppb 50 (mg/L) max. Aspects in the steam generation plant area that will benefit from betler input at the design stage of the projeci are discussed later in this paper. 2. Raw Water Evaluation - it AII Starts Here 1 was recently involved in a large lndependent Power projeci where the water is sourced from six different wells, and pumped in several rotational combinations. The ground is rich in clay, i.e. in aluminum silicate. There are also significant concentrations of iron. The EPC did a very poor job of evaluating the water source variability and proceeded to design the plant based on a single third-party composite sample of two of the wells. The aluminium was never reported and later it was determined to be a root cause of severe fouling of the reverse osmosis membranes, this taking place within the commissioning time period. Significant concentra�ions of barium also went undetected as testing thereof was never requested.Three of the untested wells later showed water characteristics greatly different from the two initially investigated. Of greater concern, however, was the EPC's poor decision to specify aluminium sulfate (alum) as cationic coagulant for this application and particular plant design, since (a) well waters are generally low in suspended and colloidal matter, and do not benefit from this step, and (b) alum use introduces a significant fouling risk in plants without conventional clarifiers. This was a classical case where the chemical water treatment supplier's expertise and knowledge of local water chemistries could have been invaluable early on in the process. They are involved with a very similar well water quality at two other client sites within a 5 km radius of the Power plant. The following is a list of recommendations regarding water source evaluation essential to sound water treatment plant design: water source(s) is/are properly sampled, not once but several ti mes at least. Grab samples are generally adequate although some applications absolutely cali for the use of composite samplers -for example, when wastewater or process water streams are involved. O Always allow water stream to run freely for a minimum of 20 minutes before sampling (preferably longer). ENERJi DÜNYASI EYLÜL 2006 O With surface waters in particular, it is i mportant to obtain dala that reflects seasonal changes. Total suspended solids, turbidity, colour and total organic carbon load are critical parameters. it is beneficial to have at maximum and minimum values. O Have samples analysed by a certified "independent" water laboratory but engage parallel testing through one of the major chemical water treatment companies, who have highly reputable labs, utilizing state-of-the-art technology, and personnel very experienced at picking up irregularities in results. Allow sufficient time for this process as lab turnaround times can be as long as two weeks and labs are often far from the plant site requiring longer shipping times. in many cases, lengthy custom clearing delays add to the frustration. O I get many calls from engineering firms requesting preliminary assistance with the evaluation of a certain water source. They will typically send me an analytical report from a "local" lab, usually the one closest to the construction site. it never ceases to amaze me how many such analytical reports come full of errors that include unbalanced ionic loads, with wrong or no units, and generally just totally inadequate for the establishment of acceptable water treatment design criteria. Far example, a lab resul! given as "X ppm silica" is generally worthless. You need both Total Silica and Reactive Silica concentrations for most applications. The above issues invariably result in delays as I request additional samples, particularly with remote planı sites. O The absence of any critical ionic species (e.g. barium, strontium, manganese) from a lab report will invariably lead to problems with the operation of the water treatment systems. Table 2 below shows the essential parameters required for an initial assessment of water treatment needs for most Power planı projects. O Labs will often report some parameters as "not available" or "undetermined". it could be a resul! of insufficient sample volume, or simply that the test was not requested. This is not generally unacceptable and should be queried. O it is alsa important for the lab to report an analysis showing a cation vs. anion balance (within a 10% reach is considered acceptable). Not only is this critical for sizing ion-exchange units, it is alsa important for the design of other unit operations and a reflection of a reliable analysis. Software that balances water analyses is freely available, easy to use and should be consulted. O Myth: The pH of a raw water sample is of critical importance. Though useful, it is not critical since the value will likely be inaccurate. This is because the pH of a given a es ii t f o t e time of sampling to the time of testing. O Understanding and quantifying the potential load of organic matter in a certain raw water is critical to good design. Far this purpose, a Total Organic Carbon (TOC) analysis is adequate tor most situations. The main strength of TOC analysis is that it is a "catch-all" value, as it measures virtually all organics present .in a water
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