Maximum permissible Cycles of concentration (NC) From the relationship BD/S = [L/S] / [NC - 1] calculated at material balance in boiler section, as long as L/S is related to system parameters and it is independent to water analysis, the increase of NC lowers the BD/S ratio. So, our target is to increase NC to the maximum, for water and energy conservation. BD is necessary as to remove troublesome impurities from the boiler, and to keep material balance under control. In this section we have to examine the most important impurities and their effects to the boiler operation, as to define their maximum permissible limits. Undesirable impurities in boilers. The best for boilers is pure water. All other dissolved or suspended impurities are undesirable. In this section we are presenting the most troubleshooting and the problems related to them. Substance Problem Total Alkalinity a. Carry over b. Scale formation c. Caustic attack Suspended Solids a. Sludge deposition b. Carry over.
Total dissolved Solids a. Scale formation. b. Corrosion c. Carry over Silicate a. Silicate deposits. b. Deposition in steam and return lines The maximum permissible ΝC is limited by the maximum permissible concentration of above impurities. Alkalinity In most countries total alkalinity is the first factor that has to be examined. The presence of alkalinity is necessary as to control corrosion tendencies and to participate to scale inhibition programs. Magnesium requires alkalinity for its control, as to be precipitated into the boiler by the following reactions: Mg 2+ +OH - ----> Mg (OH) 2 (Broucite) 3Mg 2+ +2 SiO 3 +2 OH - -----> 2Mg SiO 3 *Mg (OH)*H 2 O ( Serpentine) Calcium is precipitated under phosphate treatment by the reaction: Ca 2+ +6CO 3 +2 OH - ------ 3 Ca 3 (PO 4 ) 2 * Ca(OH) 2 While in all organic: Ca 2+ +CO 3 -----> CaCO 3 Under insufficient alkalinity, in phosphate treatment the reactions will be Mg 2+ + SiO 3 ------> Mg SiO 3 Ca 2+ + SiO 3 ------> Ca SiO 3 3 Ca 2+ + 2 PO 3-4 ------> Ca 3 (PO 4 ) 2 3 Mg 2+ + 2 PO 3-4 ------> Mg 3 (PO 4 ) 2 which are hard and very difficult removable scales. In all organic treatment the reactions will be:
Mg 2+ + SiO 3 ------> Mg SiO 3 Ca 2+ + SiO 3 ------> Ca SiO 3 Ca 2+ +SO 4 -------> CaSO 4 *2 H 2 O Resulting also to hard and non-removable scale. While an alkalinity presence is necessary, the increase of its value over a certain limit will induce foaming and carry over problems and caustic attack of the boiler metals. Caustic in excess is reacting with the iron producing Metaferric Sodium and native Hydrogen: Fe + 2 NaOH Na2FeO2 +2[H] The [H] attacks sementite producing methane gas. Fe 3 C+4[H] -----> 3 Fe+CH 4 Sementite is a crystallographic substance, in very low quantity into the metal crystals and is responsible for the mechanical properties of the steel. Its deterioration from caustic attack is proportionally deteriorating the pressure resistance of the steel, while is not leading to significant metal mass loss. The danger of explosion is obvious, so even under low pressure conditions we have to observe low caustic alkalinity excess. According to the ASME STANDARDS for boiler pressure below 14 bar limit the total alkalinity to 700 ppm as CaCO 3. Normally there is not limit in total alkalinity only, but in the relationship of the different alkalinity measurements, related to the specific products in use. If the chemicals supplier has not defined another ratio, we suggest to keep the following relation: pa and ma in ppm as CaCO3. 200 < 2 * pa - ma < 400 Suspended Solids. Except of iron oxides, suspended solids are suddenly carried into the boiler from make-up or condensate. Mostly are created into the boiler as a result of the reactions of the hardness. Maximum permissible concentration is related to the efficiency of the sludge conditioner and the antifoam in use. If the chemicals supplier has not defined another ratio, we suggest to keep the following relation: NCmax = 300/TH MU TH MU is the total hardness in make-up water in ppm as CaCO3 Total dissolved Solids. The total Dissolved Solids (TDS) value will be responsible for corrosion, foaming and scale or sludge deposition. In low pressure boilers, while is in use soft water, we suggest the following rule:
NCmax = 3500/TDS MU TDS MU is the total dissolved Solids in Make-up in mg/l. Silica Silicates are forming silicate deposits, while the present a kind of volatility. So an amount of silica escapes to the steam, and when steam condenses, it is deposited as pure solid silica.. For low pressure boilers, we suggest you to use the relationship: NCmax=120/SiO 2 (MU) SiO 2 (MU) is the concentration of silicates in the make-up water (as ppm of equivalent SiO 2.). Maximum permissible Cycles of Concentration ( NC ) The overall maximum permissible NC is the minimum of the intermediate calculated NC, according to the relation: NCmax= min( maxncalk, maxncss, maxnctds, maxncsio2 ) Finally maximum permissible NC is a function of the operational parameters of the system, the nature of boiler water treatment in use and the make-up water quality. As to improve we have either: Change the operating conditions Change the boiler water treatment chemicals Change the make-up analysis. Improving cycles of concentration will provide conservation in water and energy.