Do the boiler house walkabout
Regular maintenance of the boiler house is vital for ensuring a modern steam system is running smoothly. Rick Plummer, UK Engineering Manager of Spirax Sarco offers tips on how best to maintain the boiler house to ensure it is running to its full capability.
The boiler house is the engine room of the modern steam system and it pays to make sure it’s in good working order. In fact, the Government’s Carbon Trust estimates that poor operation and maintenance of industrial boilers wastes up to 10% of the energy input from fuel, while up to 25% of energy is lost through heat distribution systems and waste at the point of use.
So it’s worth taking time out to stroll around the boiler house and check that vital maintenance and testing is being carried out properly. Not only will it improve safety, but savings in energy, water and treatment chemicals may be there for the taking.
The first thing to do is check that all statutory daily and weekly tests of boiler water level alarms are being performed and recorded properly. Safety is the first priority here, but routine checks can often highlight inefficiencies worth tackling.
Water treatment chemicals are a big expense for boiler operators and it’s vital to make sure they’re being used at the right levels. So the next step is to check your water treatment records. Look particularly for hardness slippage through the water softener, contamination of the condensate return, and correct chemical dosing.
Another way of saving on chemicals is to check that the feedtank is being heated to the right temperature. Heating the tank to 85°C or 90°C will drive off most of the dissolved oxygen, reducing potential corrosion problems and giving scope for substantial savings.
In addition, less fuel is needed to produce steam from hot feedwater. For example, using returned condensate to raise the feedwater temperature by 6°C gives a fuel saving of 1%. Ideally, feedwater should be maintained at 90°C.
Adding hot condensate to the boiler feedtank can reduce the need for preheating, but simply feeding condensate into the top of the tank can be inefficient. As the condensate falls through the space above the water, vapour and energy can be lost and air will be admitted. A deaerator head can solve the problem by mixing returned condensate, flash steam and cold make up water as they enter the feedwater tank.
Check the feedtank is properly vented to prevent pressure build-up. It is best to fit a maintenance-free stainless steel vent head to safely vent excess flash steam. Although venting is an important safety measure, you should also watch out for any excessive venting of steam from the boiler feedtank or condensate return set because it could indicate leaking steam traps.
Return all condensate
Returning condensate to the boiler can save thousands of pounds per year. Condensate contains up to 20% of the energy in the steam from which it came. Returning water to the boiler feedtank typically recovers about half this energy, while the rest can be recovered by installing a flash steam vessel or pressurised condensate return system. As well as saving energy, condensate return saves water and treatment chemical costs. Effluent charges may also be reduced because less water is discharged to drain.
Using returned condensate as a higher proportion of the boiler feed means that the level of contamination in the boiler is generally lower. However, all boilers need to be periodically purged by blowdown. The key is to remove only enough water to maintain contamination at an acceptable level. Dumping any more than this is a waste of energy and treated water.
Many boiler houses use blowdown valves that are manually opened at regular intervals. The level of blowdown can be variable. An automatic TDS (Total Dissolved Solids) blowdown controller is a better option. By continuously monitoring TDS build-up in the boiler, the controller opens the blowdown valve only when required.
Improved efficiency can be achieved by recovering heat from the blowdown water as it flashes to steam. Flash steam can be separated in a flash vessel and injected back into the boiler feedtank.
When carrying out daily bottom boiler blowdown, which removes sludge from the bottom of the boiler, make sure you open the blowdown valve for no more than two or three seconds. Anything longer is just a waste. If bottom blowdown is automatic, check the settings.
Good insulation is another key measure. Check that all hot pipework is adequately insulated to prevent heat loss. This includes not only the pipes, but also flanged joints and the bodies of steam valves; all should be covered by insulated boxes or flexible covers. The feedtank must be properly lagged to reduce heat losses. A covering of 50mm will normally cut losses by about 90%.
Check the boiler start up routine and consider installing an automatic warm-up system. This comprises an electrically actuated valve controlled by a timer to open automatically at a pre-set time and at the correct rate to slowly heat up the steam system, eliminating the excessively long warm-up times that can occur with manual control.
If a boiler water high level alarm is not fitted, consider installing one. Overly high water levels in the boiler can cause wet and dirty steam to contaminate your process.
Finally, check the boiler gauge glass is not eroded or otherwise damaged. If it is, for safety’s sake replace it immediately.
Many of these measures won’t incur any cost, but even those that require some upfront investment should pay for themselves in a matter of months. After that, the ongoing benefits will be reflected directly in your company’s bottom line.
Note: These tips cover only the non-combustion side of the boiler house. For advice on improving boiler firing, contact your boiler supplier.
A major project to upgrade all boiler controls and install an advanced Spirax Sarco automation system is achieving up to 15% energy savings for one of the NHS’s largest boiler houses. The 1,000-bed St George’s hospital in South London now has a more reliable steam supply for its heating and hot water needs, and much improved on-site maintenance services as a result of the work.
The project upgraded the boiler house to comply with the Health and Safety Executive’s (HSE) Guidance Note PM5 on automatically controlled boilers and saw the installation of a complete boiler house automation system.
Several other energy saving measures were undertaken during the project. These included improving the site’s flowmetering capability, a detailed steam system survey and steam trap refurbishment programme, the installation of boiler blowdown heat recovery and installing a deaerator head on the boiler feedtank.
The results of the programme have been spectacular says Mr Shane King, Principal Engineer at St George’s hospital: “The aim was to improve energy efficiency and install systems necessary for de-manning the boiler house. The work has achieved between 12.5 and 15% energy savings, despite an increase in demand caused by adding several large buildings onto the site.”
As well as the energy savings, there have been significant reductions in carbon emissions. “The work carried out is having a very positive effect on our carbon emissions,” explains Mr King. “We are able to demonstrate that at St George’s, despite hospital capacity increasing year on year, we are making significant inroads into these targets.”