As discussed in one of my previous posts which can be found if you click in here, analytical tests and chemical treatment must be carried out in line with the chemical manufacturer’s recommendations. To keep the chemical levels within an acceptable range, the treatment must be added, but caution must be exercised, as excessive treatment can frequently cause more severe harm than minimal treatment. The results of the chemical analysis on the boiler water are recorded, and the effects of the added treatment can be tracked over time.
Following the analysis of the boiler water, a decision must be made regarding the amount and type of chemicals to be added to the boiler feed water, if any.
The treatment is, usually, added to a chemical injection tank and from there, it is routed to the boiler feed water lines. Chemicals for direct injection into boilers are combined with water in chemical injection tank. The combination is injected into the boiler water feed line immediately after the feed water control valve by a pump unit and the auxiliary boiler and the exhaust gas boiler share the same feed tank and treated feed water. Chemicals can also be put to the chemical injection tank and pushed from the tank into the boiler feed water lines using pressurized feed water.
The addition of chemicals must be done in line with the manufacturer’s recommendations.
There are several possible dosing points for a boiler system, and the choices depend on several factors like system configuration, boiler pressure and product combinations. Figure below shows typical dosing points for a low pressure system. All chemicals should be dosed with a suitable metering pump.
This will allow continuous dosage of the products and will minimise the handling of chemicals. Batch or slug dosing is never recommended.
Dosing point 1 is the dosing into the hotwell which is common, and can be used for non-volatile chemicals. Sulphite, alkalinity and scale inhibitors can be dosed here. Neutralising amines and oxygen scavengers based on DEHA, hydrazine or carbohydrazide should not be dosed here as there will be some vaporization in the hotwell. The dosage point should be below the water level, preferably close to a water inlet that can provide some mixing.
Dosing point 2 is the preferred point for volatile oxygen scavengers and neutralising amines. This will provide protection from dissolved oxygen as early as possible in the system, avoiding excessive vaporization in the hotwell.
Dosing point 3 is the dosage point which is used where separate dosing to multiple boilers is necessary. Scale inhibitors and some combined treatment are sometimes dosed here. Keep in mind that when dosing on the pressure side of the feed water pump, the metering pump need to be designed for pumping against a higher pressure.
Because of the constantly changing load on a boiler, daily monitoring of the chemical levels is important to make sure that the system is in good condition. For a small system sampling is typically taken from the boilers and the condensate return. More complex system would require samples from the feed water as well.
All sampling points should have a sample cooler to ensure the sample is at a temperature of 20-25°C when sampled.
This is important because it will prevent flashing of the volatile components like the amines and DEHA, yielding lower results than actual when sampling. Additionally, this will prevent burn incidents of the personnel. The water sampling procedure and cooler use explanation can be found in here.
When sampling, there should always be used clean sampling bottles. A good practice could be to have pre-labelled bottles so that the same bottle is used for the condensate each time. Trace amounts of boiler water from last sample may very well ‘ruin’ a condensate sample if bottles are mixed. Ideally, the bottle should be flushed with the water to be sampled a few times before the samples are collected.
Cleanliness is important when analyzing the water. Dirty hands and working benches may contaminate the samples. As an example, human sweat contains app. 6000 ppm chlorides, so there is not much needed to contaminate a condensate sample with chloride.
After due consideration of the feed system the operation of the deaerator (if installed) it is still necessary to apply a chemical oxygen scavenger to eliminate oxygen residuals and assist in the passivation of metal surfaces. There are various types of oxygen scavenger available to carry out this task and selection of the best approach is a function of the amount of oxygen present, risk, feed system design, economics and any particular limitations required by the process using the steam.
For oxygen control the most known chemicals that can be used:
- Sulphite – Sodium sulphite (Na2SO3) is widely used for oxygen scavenging. Sodium sulphite has been found satisfactory at pressures up to about 62 bar. Above these pressures decomposition products such as H2S and SO2 can affect steam purity.
- Hydrazine – Hydrazine, unlike sodium sulphite, does not increase the dissolved solids content of the boiler water. Hydrazine is very volatile and should be injected at the earliest possible point in the feed system.
- DEHA – (DiEthylHydroxylAmine) is an organic oxygen scavenger and metal passivator, enhancing formation of a protective magnetite layer. It is significantly more volatile than hydrazine, resulting in increased protection in the steam and condensate system. Being an amine, it has also some neutralising properties. It is more thermally stable than hydrazine and can be used for all types of boilers from low to high pressures.
- Carbohydrazide – is a ‘combined form’ of hydrazine. It was designed to minimise exposure to hydrazine vapours during handling. Carbohydrazide and its reaction products will add no dissolved solids to the water. Carbohydrazide can be used as an oxygen scavenger and metal passivator at both high (230 °C) and low (65 °C) temperatures. Carbohydrazide can be applied to boilers up to 170 bar.
- Erythorbic acid – is another effective oxygen scavenger and metal passivator, it is the only non-volatile scavenger which can be used for spray attemperation. It does not add measurable solids to the boiler water, is non-volatile, and will not jeopardies steam purity. Erythorbic acid can be used in boilers up to 122 bar.
- In low and medium pressure boilers it is usual to maintain a level of free OH alkalinity to aid in the prevention of corrosion of steel. The recommended level of free OH alkalinity is dependent on boiler pressure and heat flux and can be found in manufacturer’s manual.
- In high pressure boilers where there is a risk of caustic concentration and subsequent caustic attack it is common to apply a coordinated or congruent phosphate control program.
- Hide-out – In high pressure water-tube boiler it is sometimes observed that the concentration of soluble salts, notably phosphate salts, do not rise in line with other salts and when the boiler load reduces their concentrations suddenly rise. This is phenomena termed ‘hide-out’ and is due to the reduced solubility of sodium phosphate at temperatures above 250 °C. As load and temperature at heat transfer surfaces increases then some of the sodium phosphate will precipitate and measured PO4 reserves will fall. When load and temperature reduce the PO4 salts resolubilize and the reserve is seen to increase. When phosphate hide-out occurs there is a risk of permanent scale deposition and/or localized evolution of free caustic which in turn could lead to severe corrosion.
So for pH control, if there is a history of boiler deposits or phosphate hide-out is a recognized problem, it may be prudent to consider an All Volatile Treatment approach (AVT). This approach uses entirely volatile solids free chemicals such as Hydrazine, Carbohydrazide, Erythorbic acid and neutralising amines (Ammonia, Morpholine, Cyclohexylamine) to maintain the boiler pH at a level to high enough to control corrosion and give good passivation of metal surfaces. All steel systems are normally controlled at a pH of 9.2 – 9.6 and those containing copper or its alloys at a pH of 8.8 – 9.2. A drawback is that the boiler water is relatively un-buffered and if contamination occurs the boiler pH can be reduced dramatically. Additionally it is important to be aware of the amount of silica in the system as there is no free OH alkalinity to handle it correctly. High levels of silica in the feed-water will preclude this treatment approach.
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Source and Bibliography:
- WSS Water Treatment