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I recently helped upgrade an old (±20years) 4ton boiler (heavy fuel oil) with some monitoring instrumentation. I decided to have a look at the data and I'm not sure if these kind of oscillations are normal for an older boiler? I haven't really worked with boilers before.

Ignore the sudden drop (a topic for a different question), I'm referring to the oscillating behavior in the steady region. It seems to be rather...excessive? but follows a relatively consistent profile which leads me to believe its just a really barebones control system.

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SimpleJack
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    From the graph the cycle time is about 10 minutes. Probably nothing more complicated than "5 minutes on, 5 minutes off" heating cycle. – alephzero Oct 23 '20 at 20:27
  • @alephzero is this on-off cycle the normal control scheme for boilers? or this indicating that the boiler is oversized for the steam demand – SimpleJack Oct 23 '20 at 20:32
  • Does the burner offer proportional control or is it either fully on or fully off (i.e bang-bang)? – Graham Nye Oct 23 '20 at 21:26
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    Most every boiler I know of is on or off. – Eric S Oct 23 '20 at 23:27
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    This is called a jackshaft combustion control system. There is one on/off signal from the boiler management system, and it turns on both the fuel feed and the combustion air feed. When you start looking at variable flame controls, things get quite a lot more complicated in a hurry. [burner combustion control systems](https://www.controleng.com/articles/how-do-burner-combustion-control-systems-work/) – Phil Sweet Oct 24 '20 at 15:28

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Oscillations are normal. It's much harder to make a variable-output burner than a fixed-output burner, so the control systems for these are almost always "bang-bang": fully on, or fully off.

Are these oscillations normal for your boiler? It looks like the controller is doing a good job of keeping the pressure within a well-defined range, but you'll need to find someone who knows your particular system to know if it's an appropriate range.

Mark
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  • Understood. Such a guy used to exist but is likely sipping cocktails on a beach somewhere now which is why I am (rapidly) trying to fill that gap. The low end (~8bar) does seem to be a bit further below design pressure (~9.5bar) than I would like but the only way I see to fix that is reducing the cycle time – SimpleJack Oct 28 '20 at 08:09
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It is very difficult to cancel completely the oscillations in boiler pressure. I imagine that a boiler with an operating pressure between 8 ÷ 10 barg is small and most probably is heated with an on/off burner, in this case is impossible to eliminate the oscillations. But even when you can control the power of the burner and the feedwater flowrate you have to consider the shrink / swell effect that impacts the pressure oscillation.

Check https://controlguru.com/dynamic-shrinkswell-and-boiler-level-control/ for reference.

cactus-eng
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