Steam Boiler Feedwater Treatment
Pressure Systems Safety Regulations 2000 state that:-
'The primary objective is to deal with the dangers of stored energy'. The regulations are concerned with all matters affecting the mechanical integrity of those parts of the plant which contain pressure.
Water treatment or the lack of it will have a significant impact on the safe operation of Steam systems: Steam generation and distribution.
An efficient water treatment regime can only be achieved by continual monitoring and control of the variable parameters.
When questioned, most Engineers and Boiler Operatives believed that either dosing chemicals directly into a Boiler Feedwater Tank at a preset rate or having the chemical dosing pumps linked to the operation of the boiler feed pump or modulating feedwater valve was the best means of protecting the boiler internals from scale or corrosion.
History paints a different picture:-
The majority of boiler repairs are as a result of 'Furnace' or 'Fire' tube failure.
Too little oxygen treatment causes tube failure due to 'Oxygen Pitting' whilst too much increases the solids content inside the boiler resulting in increased blowdown, wasting energy and increasing fuel costs.
Scale formation on boiler tubes can be the result of either over dosing the hardness control (with a phosphate based treatment) or underdosing the hardness control: Whatever the cause the result is an increase in metal temperatures due to inefficient heat transfer, reduced boiler efficiency and increased fuel costs.
The whole concept of chemical dosing is based on an 'Open-Loop Control' system with no feedback except for a manual test once a day or once a week. For an 'Open-Loop' control system to work effectively, the precise feedwater characteristics, temperature, hardness and flow rate relative to the feed pump characteristics have to be known with the dosing pumps arranged to provide an output proportional to those characteristics. Any change in feedwater temperature or quality or differential pressure across the pump will cause an imbalance.
'Closed-Loop' or 'Feedback' control is inherently stable and eliminates all the short comings of the Open-Loop system
Adopting a 'Dosing on Demand, Closed-Loop' control regime maintains boiler efficiency, reduces fuel costs, saves money and most importantly improves safety.
The three main criteria that require constant monitoring and accurate control are:-
- Boiler feedwater (temperature & flow rate)
- Chemical composition of the boiler water
- Dissolved and Suspended solids content