10 January 2015
A steam raising plant and a steam distribution system are integral features of most process and manufacturing facilities. The cost of raising this steam is a significant overhead to the business and doubly so considering the carbon tax impacted costs these companies have recently begun to pay.
If you haven’t paid much attention to the efficiency of your steam system recently then it could be as little as 30% efficient i.e. for every 500,000 dollars spent on fuel only 150,000 of these are actually converted into useful work.
Don’t believe it? Then consider these typical system losses for an average boiler house and steam distribution system.
An inefficient steam management operation is a costly double whammy; costing more money to run but also increasing the company’s carbon tax bill.
Attention to the following key areas will yield savings in both energy efficiency and money:
The water softener you have installed should have duplex capability i.e when one half is regenerating; the other half is still providing a softened water stream to the boilers. This will ensure that no hard untreated water will reach the boilers and create scale within the boilers.
Scale build up reduces efficiency. Typically a poor water treatment regime could be costing up to 2% in efficiency loss.
Maintain the hotwell at the highest possible temperature. This will remove the majority of the oxygen from the feedwater, reduce treatment costs, and also help reduce corrosion in the boiler. A corroded boiler is not only inefficient; the corrosion will also shorten the boiler’s operating life.
However, It is important that the high hotwell temperature set point does not result in cavitation in the boiler feed pumps. As a general rule, the greater the height of the hotwell level relative to the feed pumps, the greater the water temperature they can pump before cavitation occurs. The tolerable temperature can be checked from charts. Cavitation will significantly reduce the life and performance of the feed pumps.
A higher water temperature also helps reduce the thermal shock on the boiler when the level is topped up and provides a minor improvement in the boiler output when compared to cold water top up.
Consider installing a floating ball blanket in the hotwell. Floating ball blankets are available from companies such as Allplas or Euro-matic. This will reduce the heat and flash steam loss from the hotwell and save energy. A blanket will also reduce the possibility of oxygen reabsorption back into the hotwell water.
Also, consider installing a floating ball blanket for general hot water tanks or hot fluid tanks as it will markedly reduce heat loss from the contents.
It is important that all clean condensate is returned to the boiler house. The recovery of the hot and softened water will reduce the heating and treatment costs of the boiler feedwater.
The mains steam pressure may not be enough to pump the condensate back to the boiler house from remote areas of the plant. A condensate receiver, fitted with an electric or steam powered pump will give the necessary boost to return the condensate across these distances.
A heat exchanger can be installed in the vent line of the condensate receiver to recover the heat of the flash steam and also the condensate that would otherwise be lost to the atmosphere. This recovers energy and reduces the water treatment costs. The heat recovered can be utilised for hot water heating, space or process air heating / preheating via duct mounted coil.
In some cases it might be necessary to also install a small U tube style vent line with a water seal to prevent the risk of the receiver pressurising dependant on how the heat is to be recovered and then utilised.
The condensate main should slope towards the local condensate receiver. This receiver should be vented to reduce the back pressure on the steam traps. Not only does this enable the steam using process to function as per design; it also allows systems running on variable steam pressures to all use a common condensate main.
All condensate and steam mains need to be well insulated. For steam valves, these should be provided with an insulated box around them or a premade insulated bag which can be tied around the valve (these are generally preferred for maintenance purposes as they are easier to refit!). An unlagged steam valve or flange, for example, represents a significant surface area for heat loss.
An infra-red camera survey of the steam and condensate mains, together with the boilers and hot water tanks is a good starting point for determining where the weaknesses are.
A well attended and insulated system could yield up to 5% in savings.
Where possible, slope the steam mains towards the next steam trap. The steam trap position should incorporate a flanged or screwed dirt pocket (for cleaning purposes). The dirt pocket should be the same diameter as the steam main it serves, and deep enough to provide a sump to capture the condensate in the line.
The end of a steam line should feature an air eliminator to prevent air build up in the system particularly during a cold start up.
Steam take offs should always be from the top of the steam main to ensure the driest possible steam to the process.
The steam trap strainers need to be checked regularly for cleanliness. It is suggested a first check should be carried out within 3 months of a new system being commissioned, and then every 12 months thereafter. This will ensure the delivery of the driest steam to the process and reduce the possibility of boiler chemical carryover into direct steam injected processes. Boiler carryover into process could result in product rejection due to the contamination.
Regularly check that steam traps are not stuck open. Statistics show that 10% of standard steam traps used in a steam system will fail every year. This wastes steam through excessive flash at the condensate receiver. Consider the use of fixed orifice traps from a manufacturer such as GEM (Gardner Energy Management). These are highly reliable as they feature no moving parts.
If the boiler is fitted with a continuous blowdown then consider the installation of heat exchangers to recover the heat from the blowdown liquid and vapour stream. A good application for this recovery stream, due to its proximity, is for hotwell top up preheating. Such systems are available from Spirax Sarco for example.
Ensure that new boilers are specified with an economiser fitted in the flue gas stream. In a lot of cases, subject to space, these can be retrofitted to existing plant to provide a source of heating for hotwell top up or for factory hot water demands an economiser can improve efficiency by up to 5%.
An oxygen trim system should be considered to improve boiler efficiency further. These are claimed to add up to 5% to boiler efficiencies. They work by controlling the quantity of excess air into the boiler. A volume of air is necessary to ensure complete combustion of the fuel; the “excess air” provides a small safety margin above this.
If this excess air is too high then heat is wasted from the boiler and so efficiency suffers. However, if it is too low then incomplete combustion, and the generation of CO is a risk. An oxygen trim system will precisely control the excess air to optimise performance.
Hot Water Generation
For localised hot water generation there are possibilities that can provide a more efficient solution than a large centralized hot water tank heated by steam and a pumped ring main that has to run around the factory.
If there is already an extensive steam distribution system around the plant consider the use of a localised Angelery Water Wizard supplied by Northvale, or products by Spirax Sarco, which can provide rapid response supplies of large quantities of hot water without any storage requirements.
Flue gas temperatures
Regularly monitor the temperature of the exit flue gases from the boiler. The benchmark should be set after the last tube clean and then regularly monitored after this point to log the increase in the temperature. The temperature increase will indicate that the tubes are becoming fouled and so heat exchange efficiency will reduce. The cleaning interval required to optimise the efficiency is a plant decision based on boiler availability and economics.
Pressure control to the plant and reduction in carryover risk
When comparing boiler quotes for new plants consider the amount of steam space within the boiler. A small steam space will result in a greater risk of carryover as the boiler will have less stamina to cope with sudden changes in load and will provide a less stable steam pressure to the plant. The lower priced boiler may be cheaper because it has a small steam space when compared to another. If the plant has a variable steam load then install the maximum steam space that can be afforded.
Avoid the use of a steam generator unless the load is very constant. Steam generators have a very small steam space and lack the ability to cope with a variable load without significant loss of pressure and the risk of carryover.
An alternative solution where there are large fluctuations in steam load is the use of a steam accumulator to even out the peaks that the system may require. This will flatten the load profile on the boiler and reduce the problems associated with large demand peaks. These can be installed local to the high demand process where required for the best results.
These have been around for a number of years without having caught on, but are worthy of investigation to squeeze the last few Kilojoules out of the fuel.
A turbulator is a device that fits inside each fire tube on the boiler. Generally fitted into the tube ends at the start of the last pass, they induce a swirl into the hot flue gases, and by converting the laminar airflow into turbulent flow, improve the heat transfer efficiency across the tube wall. These can be easily removed for tube cleaning purposes.
Where do I start?
In terms of starting an efficiency survey the best place to start is a steam trap operation and insulation survey. The hire of an infra-red camera for a day to survey these items would be very revealing on most plants and highlight some easy wins.
In a recent survey in the UK of 100 industrial premises and 300 boilers it was found that on average a saving of 7% could be made on steam generation costs for the site by the application of simple efficiency improvement measures.
These simple efficiency measures are outlined here along with some more detailed opportunities that could yield further efficiencies with some capital investment, and total savings of 10 to 20% are quite possible.
A 10 to 20% % saving in any industrial overhead would be considered to be significant and worthy of mention. Why not survey your own steam system now and get your name mentioned, in the right places!
ABOUT THE AUTHOR
Martin Bevis is a Senior Process Engineer at Wiley and can be contacted on 1300 385 988 or email email@example.com
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