AQUAnet™ Improving Feedwater Treatment by Control
The chemistry of feedwater control is well understood and there is a raft of legislation and guidance notes spelling out minimum requirements and testing regimes but they all fall short in specifying suitable methods of controlling the application of those chemicals. The requirement to test water quality daily and adjust the chemical dosing rate based on the results may have sufficed when boiler plant was manned 24 hours a day but with the increase in boiler efficiency and 72 hour unmanned operation manual testing once or twice a week does not meets the requirements of current legislation.
To fully appreciate the need for improved feedwater treatment control we need a basic understanding of ‘Control Engineering Theory’. The fundamental laws of which apply to all control circuits irrespective of the different forms of equipment and instruments involved.
In reality, the application of controls is very simple, consisting of single or multiple control circuits of which there are only two types ‘Open Circuit’ and ‘Closed Circuit’ normally referred to as ‘Open Loop’ and Closed Loop’ control. In fact ‘Open Loop Control’ is a misnomer, simply because if it is ‘Open Loop’ it is classified as a ‘Process’, to be ‘In Control’ requires the intelligence of feedback.
Open loop control is when there is no direct relationship between the process or system and the controller. As the term implies, with ‘Closed Loop’ control each step in the loop responds to the step before and affects the step following.
An everyday example would be the switching ‘On’ of a boiler with a time clock to provide heat into a building: without a room sensor to feed information back, the control ‘Circuit or Loop’ is ‘Open’, to ‘Close’ the ‘Circuit or Loop’ a room thermostat is added to override the time clock.
Open loop control systems, those without the intelligence of feedback, tend to be very inefficient and wasteful.
With boiler feedwater treatment control, the vast majority of chemical dosing systems currently in use rely on the water-conditioning chemicals being added in proportion to the feedwater flowrate: ‘Open Loop’.
Boiler feedwater chemicals need to be applied and maintained at specified concentrations for their desired effects to be achieved. The object is to optimise the chemical characteristics in order to reduce the risk of corrosion, sludge precipitation or formation of deposits which may lead to tube or boiler failure.
In a boiler feedtank both the water quality and temperature vary throughout the course of each day. With an ‘Open Loop’ control system there is no intelligence that allows the dosing pump(s) to change the injection rate in response to changing needs or conditions.
The present method of manually setting the stroke length and pulse rate of dosing pumps enables chemicals to be added in a simple ‘Open Loop’ process which is based on the assumption that the feedwater quality is constant and the temperature is fixed. It is then left to the ‘Boiler Operator’ to manually take and analyse boiler water samples and adjust the pumps dosing rate accordingly.
When boiler houses were manned 24 hours a day the justification was that the boiler operator took regular daily samples and adjusted the dosing according to need: effectively closing the control loop.
With the increase in unattended boiler operation to 72 hours, that justification is no longer relevant.
Any risk assessment, a legal requirement under both the Management of Health & Safety at Work Regulations 1992 and The Provision and Use of Work Equipment Regulations 1998 must highlight the shortcomings of the existing ‘Custom and Practice’ ‘Open loop’ control regime especially when applied to boilers that have been repaired or operate unattended.
Following the Risk assessment, employers must then take any additional steps necessary to reduce risks identified either by elimination of the risk or by reducing it to as low as is reasonable practical (ALARP).
Automating feedwater chemical dosing and adding remote monitoring on unmanned sites closes the ‘Loop’ providing a safe, efficient and economical system to protect boilers from scale or corrosion damage and in so doing reducing risk and waste: saving energy and money.
Actual savings on fuel and reduction in blowdown vary with installation but typical savings are in the order of 1.7% to 3% in fuel and 30% in blowdown. With additional savings achieved through reduced maintenance and labour costs pay back for a complete system varies with steam demand but falls well within a 12 month period with the benefits throughout the lifetime of the boiler.