Relieving pressure

Published:  12 September, 2007

Featuring constantly in the headlines, environmental sustainability has never been so important for organisations of all types. Such pressure is leading many plant

engineers to explore ways to reduce boiler house energy consumption. A common idea is to reduce boiler pressure to save energy, but does it work? Rick Plummer, UK engineering manager, Spirax Sarco, explains how steam system pressure and fuel economy are interlinked.

The pressure for companies to reduce their environmental impact grows daily. Consumers, investors, governmental bodies, the media and many other groups are all increasing the focus on the need for lower emissions. This is leading many boiler operators to explore ways to reduce boiler house energy consumption as a way to reduce total carbon emissions and save energy costs.

One question often asked is whether it is a good idea to run a steam boiler at lower than its design pressure to save energy. The answer is not always a simple yes or no.

There may be bigger issues to consider than relatively minor adjustments to boiler pressure. In particular, it is vital to ensure that all condensate and flash steam is recovered and its heat is re-used efficiently.

Ultimately what really counts is ensuring that the steam distribution system is as energy efficient as possible.

 

Higher distribution pressure is more efficient

Generating steam at a higher pressure will require more fuel. However, at its simplest level, the same amount of energy is used whether the boiler raises steam at 4 bar g or 10 bar g. This is because it is the connected load and not the boiler output that determines the overall energy consumption of a building or industrial plant.

However, we have to also consider losses around the system. Within the boiler itself the losses depend on the boiler's combustion efficiency, the heat transfer efficiency of its combustion chamber and fire tubes, and flue losses, which are likely to be higher at higher boiler pressures. However, these rises are marginal when compared to the benefits of distributing steam at a higher pressure.

The most efficient way to run steam plant is to operate the boiler at higher pressures, with pressure reducing equipment to lower the system pressure at the point of use. Using higher pressures increases the boiler"s thermal storage capacity, helping it to cope more efficiently with fluctuating loads and minimising the risk of wet and dirty steam being carried over into the distribution system. It also cuts the cost of materials, insulation and labour, since smaller bore steam mains can be used.

 

Consider all losses

However, there will also be losses from the steam distribution system. These include heat loss from pipework and fittings to the surrounding atmosphere, as well as steam leaks. Again, all of these losses will be greater at higher distribution pressures.

Potentially though, the most significant losses occur after the steam-using process, whether space heating for a building, or process heating in an industrial plant. Once the steam gives up the amount of heat that the process demands, condensate is released. In most cases, flash steam is also produced.

Now it gets interesting. If the condensate system is effective in recovering all or most of the heat in the condensate, and the flash steam is used by another process or fed to a recovery system, then losses will be minimal. In this case, the boiler operating pressure will not have much impact on the overall losses, and any efficiency gains may be offset by other considerations, such as the risk of wet and dirty steam being carried into the process.

On the other hand, if the hot condensate is not recovered effectively, or the flash steam escapes, then the losses will be large. In this case a lower operating pressure will produce lower losses. However, this would be an inefficient steam system and rather than adjusting the boiler pressure, much greater savings can be made by improving the plantís energy performance.

 

Achieving optimum steam distribution pressure

The optimum pressure within a system varies from plant to plant and depends on the maximum safe working pressure of the boiler and the minimum pressure required by steam-using equipment around the site.

Deciding on the optimum initial distribution pressure can be complex, involving the application, equipment and safety issues. Allowances must be made for steam pressure loss as the steam passes through the pipework to make sure the minimum pressure is met at the point of use, so it is important that the pressure is not altered in a misguided attempt to reduce fuel consumption.

There are other considerations too, such as whether the existing control valves and heat exchangers are adequately sized if the pressure is dropped.

It can take a degree of expertise and experience to successfully balance conflicting factors and arrive at the optimum pressure for a steam system. Thatís why many boiler operators would be better off getting advice from a steam system specialist.

 

Other ways to reduce fuel consumption

There are many effective ways to save fuel. It is estimated that industry could save up to 30% of the fuel feeding its boilers by combining established good practice with improved technologies.

 

Recover all condensate

Condensate can contain 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.

The benefits of condensate recovery do not end with energy savings however. It also saves water and treatment chemical costs, and even effluent charges may be reduced because less water is discharged to drain.

 

Install automatic boiler blowdown

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 and dump water down the drain. 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 as required. It is often cost effective to recover the heat from the necessary boiler blowdown using a simple flash vessel and heat exchanger system.

 

Create the right boiler feed tank conditions

Less fuel is needed to produce steam from hot feedwater. For example, using returned condensate to raise the feedwater temperature by 6įC gives a fuel saving of 1%. Ideally, feedwater should be maintained at 90įC.

 

Taking the next step

These are just a few of the factors that can determine whether a steam system operator is paying over the odds by running their plant below its optimum efficiency. A full energy audit can identify where energy is being wasted and suggest ways to win significant savings. Many companies do not have the necessary steam system expertise in-house, so calling in an external provider is often the best option.

For further information please visit: www.spiraxsarco.com/uk

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