Accurate boiler blowdown

Published:  01 April, 2014

Boiler blowdown is probably one of the most regularly completed, but least understood areas of boiler operation. Blowing down a boiler is necessary to ensure that the boiler can deliver clean, dry steam to the heating system, but getting it wrong can lead to preventable energy losses. Paul Mayoh, technical manager at Spirax Sarco, reports.

In many boilerhouses, the blowdown valve is manually opened at regular intervals by the boiler operator and the water removed is just dumped to drain. The operators of these sites fail to recognise the tremendous cost savings that could be achieved by investing a relatively small amount in the latest automatic blowdown control systems. Payback times of only a few months are possible.

Why is boiler blowdown needed?

Blowdown is essential to control boiler water contamination. As a boiler generates steam, any impurities that are in the boiler feedwater and which do not boil off with the steam will concentrate in the boiler water. As these dissolved and suspended solids become more concentrated, steam bubbles fail to burst as they reach the water surface. They accumulate in the void above the boiler water and ultimately get carried over into the steam system. Not only does this make the steam leaving the boiler excessively wet, but it contains contaminants that could clog and damage control valves, heat exchangers and other equipment.

Blowdown is the process of drawing off boiler water, which is then replaced with clean water from the feedtank to reduce the concentration of contaminants. But removing hot boiler water also removes energy from the steam system.

How much energy is lost during blowdown?

The amount of energy contained in water taken out of a boiler during blowdown can be considerable. Blowing down a 10,000 kg/hr boiler running at 10 bar g can produce an energy flowrate of up to 240 kW. To put this into context, this energy flowrate would heat 19 houses with an average domestic central heating system rated at about 13 kW.

With this kind of energy flowing, it is it important to ensure that a boiler is not subjected to an excessive blowdown rate, which is typical with manual blowdown routines.

How is blowdown done?

There are two types of boiler blowdown, each performing a different function. Side blowdown controls the Total Dissolved Solids (TDS) level by drawing off boiler water via a small bore valve fitted just below the minimum boiler water level. Water can be taken off continuously or in bursts at regular intervals. This water is then passed into a heat recovery system for highest energy efficiency.

The second type, bottom blowdown, uses a large bore valve fitted to the base of the boiler. This method involves fully opening the valve for a few seconds at regular intervals, which is the best way to remove the sludge lurking at the bottom of the boiler. If not removed, the sludge can build up and cause hotspots and boiler damage.

Bottom blowdown is too intermittent to make heat recovery practical, so the blowdown is typically vented into a steel blowdown pressure vessel that can safely cool the water and vent any flash steam that is created. Several boilers can use the same blowdown vessel.

Is it better to control boiler blowdown manually or automatically?

Manually controlled boiler blowdown entails the boiler attendant opening the side and bottom blowdown valves at set intervals during the day. The drawback of this method is that, almost invariably, operators will err on the side of caution and a much larger volume of water will be drawn off than is necessary, wasting energy and water treatment chemicals.

Much better is to install automatic blowdown control systems. These ensure the minimum blowdown rate is achieved. Not only does this minimise energy loss from the boiler, but it also eliminates the need for skilled boiler operators to be tied up performing this tedious task.

Automatic TDS control measures the actual TDS level and water is drawn off the boiler continuously, but at a varying rate to maintain a steady TDS level. This is the most efficient way to control your blowdown and will save much energy.

Bottom blowdown is also best controlled automatically. This entails the automatic opening of the bottom blowdown valve at timed intervals to remove sludge. Again this has the added benefit of freeing plant personnel for other tasks and minimises energy loss.

Why install heat recovery?

Without heat recovery, the energy in the blowdown water would wasted, but with proper heat recovery about 80% of this energy can be recaptured. The savings are considerable with the system paying for itself within months.

As blowdown water is drawn off it flashes to low pressure steam, which can be captured by a flash vessel. This separates the flash steam from the condensate. From here, the flash steam is fed to the boiler feedtank to heat the feedwater, while the remaining hot condensate is passed through a heat exchanger. The heat extracted from this is also used to pre-heat the make-up water. Finally, the cooled condensate can be discharged to drain.

What other considerations are there?

Both side and bottom blowdown systems for boiler houses with multiple boilers need special safety measures.

The installation of TDS controls on multi-boiler plants should include a non-return valve and an isolation valve to prevent pressure from one boiler being imposed on another. This is particularly important when one boiler is shut down, as the TDS control valve may not be designed to seal against pressure on the downstream side.

Automatic blowdown in action

Medway Maritime Hospital shaved 2 to 3% off its main boiler house energy bill with the installation of two automated systems for boiler blowdown. The automatic TDS (total dissolved solids) systems monitor the build-up of contamination in the boilers and initiate a boiler blowdown when the level of contaminants reaches a preset threshold. Previously, periodic blowdown was carried out on the two boilers by hand.

“With manual blowdown we didn’t know exactly how often to blow down so we had to err on the side of caution,” says the hospital’s technical engineer - environmental, Phil Belton. This meant that the hospital was discarding more hot water than necessary.

“We identified the blowdown operations as one of the areas in the boiler house that could play a part in our carbon reduction operations. After all, we were spending an average of around £40,000 every month on boiler fuel,” says Belton.

As well as saving energy, automating the blowdown process also saves significant amounts of water and treatment chemicals. Belton highlighted a further benefit: “One other main bonus is the freeing up of manpower, so instead of losing a guy twice a day doing manual blowdown he can be doing other jobs. It’s a way of semi-automating the boiler house.”

How much are these systems?

The cost of boiler blowdown and heat recovery systems are relatively low and are quickly recouped through energy and manpower savings. A typical payback on investment on an automatic system would be less than two years.

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