Discussion on water quality parameters related to boiler feedwater and boiler water.
Thread Content
On June 9, 2018, I posted a thread titled: “How to control the rapid increase in chloride levels in boiler water?” ”In this previous post, a brief introduction to the boiler water treatment process in our plant was provided, as follows: municipal water supply — fire water tank (secondary water supply) — raw water pretreatment system (activated carbon filter — multi-media filter — regulating water tank — hydrogen ion exchanger — intermediate water tank — ammonia addition system not in use) — sodium ion exchanger — soft water tank — atmospheric thermal deaerator — 5 x 20T boilers — steam distribution cylinder. The raw water pretreatment system is designed to reduce alkalinity, while sodium ion exchangers are used to lower hardness. To meet the plant’s process standards, when the chlorine content in the boiler water exceeds 120 mg/l and the alkalinity exceeds 23 mmol/l, manual bottom drainage of the boiler is required. As long as the set conductivity threshold is not exceeded, automatic conductivity control is utilized to determine the timing for automatic surface and bottom drainage of the boiler. However, when the steam flow load on the boiler is high, or when it is necessary to operate two boilers simultaneously, especially during winter, the level of chlorides increases rapidly. I have checked every stage of the process: the chloride content in the water fed to the sodium ion exchanger is between 12 mg/l and 15 mg/l (this takes into account the possibility that the failure of the sodium ion exchanger could lead to an increase in saltwater levels as a result of the processes of bed loosening, replacement, regeneration, cleaning, and water production). A chemical dosing system is used in the deaerator, and the chloride level in the soft water at the detection point of the deaerator also remains between 12 mg/l and 15 mg/l. This indicates that the parameters of the soft water remain within acceptable limits before it enters the boiler. To ensure the accuracy of the tests, the boiler’s water is completely drained at the start of operation, and then soft water is added again. It can be observed that once the steam flow load on a single boiler increases but does not exceed the rated value, the chloride level rises quickly; similarly, when the steam flow requires the use of two boilers, the chloride level also increases rapidly, often reaching levels between 160 mg/l and 175 mg/l. The alkalinity increases slowly and remains within the specified range. To this day, it is still not clear what the cause of this issue is, nor how to control the chloride level Chemicals are also added to the boiler body, but they are not chemicals containing chloride ions; they are mainly used for corrosion prevention. In our most recent work in November 2019, we unexpectedly discovered some unclear matters; we invite everyone to discuss them. First, each time the hydrogen ion exchanger in the raw water pretreatment stage fails and is cleaned to a pH value of 3 to 4, it can be put into use in accordance with the manufacturer’s instructions. (The water in the intermediate tank comes from a mixture of water controlled by the manual alkalinity valve after the water exits the regulating tank, and water controlled by the electric alkalinity valve after the water exits the hydrogen ion exchanger.) Above the intermediate tank, there is a total alkalinity meter that automatically adjusts the opening of the electric alkalinity valve. ) However, it was recently found that after cleaning the hydrogen ion exchanger, the chlorine level in the intermediate water tank was more than 10 times above the normal level; this has not happened before. The alkalinity of the intermediate water tank was within acceptable limits after cleaning. This phenomenon occurs in both hydrogen ion exchangers. Secondly, the PH of the intermediate water tank
Before answering the question, determine two things first: 1. What is the rated pressure, and has there been any overheating? 2. What are the main uses of steam? Only then can a targeted response be given. The answer is as follows: The GB/T 1576-2018 Standard for Water Quality of Industrial Boilers does not set a limit on the chloride ion concentration in boiler water; however, this does not mean that such a limit is not required. From a corrosion perspective, the allowable CL- concentration in boiler water should be determined based on whether scaling occurs inside the boiler, as it relates to under-scale corrosion. It is recommended that this concentration not exceed 200 mg/l. So it’s fine that you have set the limit at 120mg/l, and the actual measured values are often between 160mg/l and 175mg/l – it’s not a serious issue ； However, deoxygenation and the addition of scale inhibitors are still necessary, as there is a relationship between them. The boiler is inherently a concentration process, and the fixed drainage system is used to control the appropriate concentration ratio. The alkalinity should also increase accordingly. Note: In the H—Na process, the main source of chloride ions is the raw water (which varies with chlorine addition, region, and season), as well as the regeneration chemicals (which is why GB/T 1576 stipulates that the chloride ion level after regeneration should not exceed 1.1 times the limit). The hydrogen ion exchanger is operating normally, so there is no change in chloride levels; therefore, the chloride concentration in the intermediate water tank is more than 10 times higher than normal. This is not due to the effect of the regenerant, but rather possibly influenced by the quality of water in the regulating tank. Furthermore, the weak hydrogen ion exchanger already has a slightly acidic nature after removing hardness and reducing alkalinity; therefore, the pH of the intermediate water tank
The rated steam pressure of the steam distribution cylinder is 0.95 to 1.05 MPa, while the set pressure and actual pressure for each boiler range from 1.00 to 10.5 MPa. The steam is used to meet the requirements of product manufacturing processes, as well as for deaeration, air conditioning, and heat and cold exchange applications. What do you mean by overheating?
The regulating water tank has been tested, and the chloride level there is also 15 mg/L. The level of chloride in the intermediate water tank is entirely determined by the opening degree of the electric valve for the hydrogen ion outlet; the pH of the hydrogen ion outlet is between 3 and 5, and the chloride level there is 6 to 10 times higher than that in the municipal water supply. The opening degree of the manual outlet valve for the water tank is fixed.
So how can more precise blowdown criteria be established for chloride ions in boiler water? Both our deaerators and boilers are equipped with chemical dosing. We have explicitly asked the manufacturer; our drugs do not contain chloride ions, chlorate ions, or chlorine compounds. We have conducted our own tests on the deaerator, and the chloride content in the water from that deaerator is also 15 mg/L, with no change before the water enters the boiler.
Your rated pressure and rated evaporation capacity are not high; it is recommended to use 1.0 as per Table 1 of GB/T 1576-2018 for the feed water and boiler water
Is sodium hydroxide added when adjusting the water tank? Pay attention to the control of sodium chloride in sodium hydroxide ； In the case of a weakly acidic hydrogen ion exchanger, the statement that \"the level of chlorides in the intermediate water tank is entirely determined by the opening degree of the valve controlling the flow of hydrogen ions, with the pH of the hydrogen ion output being between 3 and 5\" is incorrect. Weakly acidic resins remove carbonate (hydrogen) salt hardness but still leave behind a certain amount of alkalinity; therefore, the pH should not be this low. Check the system again to isolate the regenerant.
Adding ammonia to the feed water and deaerator, as well as adding chemicals inside the boiler, are all normal operations; therefore, it is unlikely that this will lead to an increase in chloride levels. If there is an increase in chloride levels and alkalinity beyond the concentration multiplier of the boiler, there is no need to investigate further for the cause.
No chemicals are added to the adjustment tank; the water comes directly from the municipal supply after passing through multi-media filters and activated carbon filters. The pH value of the hydrogen ion-containing water is indeed between 3 and 5, which is in line with the manufacturer’s specifications. The pH level in the adjustment tank is set at 7, and then the two streams of water are combined.
This post was last edited by zhanyl628 on 2019-11-15 at 10:11. Understood; it will be adjusted using raw water. One more thing to confirm: is the hydrogen bed made of weakly acidic resin? If so, the low pH value of the effluent should occur at the beginning of operation! If it is of the strong acid type, it is normal. But in any case, the cation exchange resin removes cations, while the amount of anions remains unchanged.
30% concentration HCL is used for hydrogen ion regeneration