Plant & Works Engineering October 2022

24 | Plant & Works Engineering www.pwemag.co.uk October 2022 Energy & Environmental Management Focus on: Boilers, Burners & Controls What are the implications of de-rating boiler pressure? In these days of pushing the boundaries on energy efficiency it’s very easy to connect a lower working temperature in a boiler to a lower flue gas temperature and therefore higher efficiency but is that really the case? The Combustion Engineering Association (CEA) explains further. W hile it’s true that a 15°C reduction in flue gas temperature will save most steam users 1% of the boiler running cost, it isn’t quite that simple when it comes to looking at the effect on the system as whole. Let’s look at the facts: One thing to remember is that you still need to move the same amount of energy so lower pressure steam will need to travel faster through the same pipes. Good design practice says that one should design for around 25 m/s in a pipeline with an upper limit of 40 m/s in exceptional circumstances. That could mean that some of the steam pipework will need to change to accommodate the lower pressure. If you stick with the existing pipework you will experience: Increased pressure losses; a 6” pipe exposed to 10 bar steam at 25 m/s will experience a pressure, drop of 21 mbar along a 10m straight length compared to the same mass of steam entering at 6 bar (40 m/s) experiencing 32mbar of loss. This may not sound much but this is a relatively short piece of straight pipe, once you factor in a longer pipe run, isolation valves, a strainer, and a control valve the pressure drop could be considerable. Increased noise; in the above example the 10-bar steam line would have a noise of 62 dB(A) compared to 72 dB(A) for the 6 bar line; add in some other noise or perhaps even a control valve or steam meter, and you could easily take the noise above 80 dB(A). Control valves and steam meters are usually sized with a much higher velocity so it is extremely important to check their suitability for the new operating conditions with a specialist; they simply may not be able to pass the same mass of steam at the lower pressure. High velocity is also problematic inside the boiler. As the pressure falls, the size of steam bubbles increases and the required volume flow to achieve a given mass flow, also increases; this leads to a much more violent