Optimising steam boilers efficiency
Published: 05 April, 2016
The UK’s high energy costs continue to present a major obstacle hindering the country’s manufacturing performance. A 2014 survey by the EEF highlighted the cost of energy as a major concern for UK manufacturers, with many identifying it a key factor impacting on their profitability and competitiveness. On sites where steam and hot water is used, significant cost savings can often be found in the boilerhouse and steam distribution system. Alan Hunt of ABB Measurement & Analytics offers advice on the best ways to optimise the efficiency of your steam boilers.
A survey by the Engineering Employers Federation (EEF) in 2014 highlighted the growing impact of energy on the competitiveness of UK manufacturing companies. Of those surveyed, 51 % expressed energy affordability as their greatest concern, whilst 53% saw rising energy prices as a potential threat to their competitiveness.
Whilst the recent dramatic fall in oil prices has provided some relief by helping to reduce production costs in some sectors using oil-fired production processes, the historic volatility of the oil markets means that permanent low prices should not be taken for granted. Rising climate change and energy taxes are also likely to further increase energy costs over the long term.
Although such rises cannot be avoided, their impact can nevertheless be mitigated by taking steps to maximise the efficiency of energy-intensive plant and processes. In the case of steam plant, various steps can be taken to improve both energy and environmental performance that can help achieve both short and long term cost savings.
Spotting areas for improvement
The Carbon Trust stresses that companies need to use a range of monitoring and targeting techniques to identify and implement energy saving measures. The Trust estimates that basic housekeeping enables many no- and low-cost savings, which can add up to 5% or more of an industrial energy bill, while more formal energy management schemes can achieve savings of 20 to 30% or more, depending on which industrial sector a company operates in.
However sophisticated a company’s planned energy management strategy might be, it starts with one essential realisation – you can’t manage what you can’t measure. For example, steam provides a popular and efficient form of heating throughout industry, as well as in building services, but how can boiler operators be certain that their steam systems are working as efficiently as possible?
Efforts must start in the boiler itself, where operators need to aim for the best possible combustion efficiency.
For the maximum amount of heat to be generated during combustion, there needs to be a perfect mix of fuel and air. Whilst this may seem straightforward, matching the correct quantity of fuel to the right amount of oxygen requires constant monitoring in order to compensate for the variety of factors that can affect combustion efficiency.
The optimum combustion process provides just enough excess air to completely burn the fuel, with the exact quantity depending on the fuel being used. Using a modern in-situ zirconia oxygen system and a temperature probe in the flue duct can ensure the plant is burning fuel optimally. If the oxygen level rises over time, it can also indicate the need for minor adjustments or repairs, while a rising duct temperature can indicate the need for tube cleaning, since fouling may be hampering heat transfer.
Rapid heat transfer
Heat transfer surfaces must be clean in order to conduct heat efficiently, but fouling can also be a problem on the “wet” side of the boiler too. Water quality is the key here, since any solid contaminants can cause a build-up of scale. Over time, this can accumulate, effectively acting as unwanted insulation.
Regular boiler blowdown is the obvious way to control contamination, although dosing the feed with chemicals such as ammonia or hydrazine also stops some chemicals getting that far. Careful, continuous monitoring of key parameters including conductivity, pH, dissolved oxygen, sodium, silica, hydrazine, phosphate, ammonia and chloride can also play a vital role in ensuring good long-term boiler chemistry.
Steam metering throughout the entire distribution system is crucial for good energy management. Proper metering allows operators to see exactly what’s going on. For example, meters can track the consumption of individual user processes across a site. This enables energy managers to encourage efficiency by introducing separate billing, or target energy saving measures where they will have the most effect. Trend information also enables operators to spot malfunctioning equipment or other problems as they develop.
Accurate metering is the key. Operators need to know the mass of steam moving around the plant, since this equates to the energy flow. Traditional differential pressure meters such as orifice plates require peripheral paraphernalia including differential pressure transmitters and a flow computer to produce mass readings for steam, all of which adds up to a high-maintenance headache.
In contrast, swirl meters have lower maintenance requirements and deliver greater accuracy – especially in applications where the steam flow varies over a significant range. Rather than an accuracy of two percent of the upper flow range, which is best an orifice plate can provide, swirl meters offer better than one percent accuracy over the entire flow range. Furthermore, the turndown is up to five times greater than that of an orifice plate.
Swirl meters rely on static veins at the entrance to the meter to force the fluid into rotation. The meter then measures the frequency of a helical secondary rotation that automatically develops within this pattern.
The frequency of the secondary rotation is directly proportional to the volumetric flowrate of the fluid, without any need to compensate for changes in pressure, temperature or density. The meters only need to know the temperature of the steam to calculate the mass flow.
Where companies are looking to retrofit meters on existing steam systems, swirl meters offer the added advantage of being able to fit almost anywhere. Most flow meters need to receive undisturbed flow to deliver accurate results. So they need to be positioned a good distance downstream from pipe bends, valves or other components that might interfere with the readings. Instead of requiring straight inlet and outlet runs of 15 pipe diameters and 5 pipe diameters respectively, which is typical of vortex meters, swirl meters need just three and two diameters in most applications.
With energy now featuring prominently on the agenda of many boardroom meetings, it is becoming increasingly important to have a detailed understanding both of current performance and how it could be improved.
Using instrumentation to assess the current efficiency of your boiler and steam systems can be a key tool in helping identify ways to improve combustion efficiency, reduce pollution, extend the life of equipment and reduce the frequency of unplanned stoppages.
For further information please visit: http://new.abb.com
Measure it to manage it
• Make sure you are only generating what you need. Measure the demand and compare it with what the boiler is generating. This will ensure you’re not wasting steam heating up your factory instead of your process, for example.
• Optimise the combustion process by monitoring the flue gases. Careful monitoring can help to strike a balance between supplying too much air, which carries heat away up the flue, and insufficient air, resulting in incomplete combustion.
• Make sure that boiler duty is at optimum efficiency. For example, don’t use two boilers at 30 percent output if you can run one at 60 to 70% output.
• Check your instrumentation is up to scratch. Modern instruments are typically more robust and more accurate. They are also easier to maintain, and are less prone to problems such as drift.
• If you measure steam or gas, measure the mass flow, not the volume flow. It takes ten times the energy to create 1 m3 of steam at 10 bar than at 1 bar, yet the volume is the same.