7 Intelligent Boiler Sequencing - IBS
Mk8 MM Manual Page | 144
7.4 Steam Sequencing
7.4.1 Warming Steam Boilers
The difference between steam pressure sequencing is the warming periods; the IBS settings explained in
section 5.3.1 are the same (see section 5.4.2 for options 40, 41 and 53). By keeping the lag boilers at low
fire when they are in standby/ warming, when they are requested by the lead MM to come online and
contribute to the load, they will not be started from cold.
A steam boiler is at risk of thermal shock if not warmed before running at high fire. If the lag boiler is required
to contribute to the steam load, then the boiler must be warm in order to contribute quickly in a safe
manner. If the boiler is started from cold and allowed to fire at a high firing rate straight away, then this may
cause damage to the boiler. The tubes will increase in temperature and if the boiler started from a cold
position then this will cause thermal shock to the boiler.
Additionally, not warming a steam boiler can result in a slow response to meet the system’s steam demand.
For process applications and critical sites such as hospitals, it is imperative that the steam is met efficiently
and quickly. If one of the boilers fails or locks out, then it is very important that the next boiler in the
sequencing loop gets up to pressure as quickly as possible. Therefore, if this starts up from a cold status
then this will take a long time to get up to pressure safely. By warming this lag boiler this means that the
boiler will maintain a pressure, offset from the required setpoint in order to ensure that when required this
gets up to pressure quickly.
7.4.2 Implementing Steam Sequencing
When the IBS software control package is applied to steam boilers, its operation is similar to hot water
sequencing but with additional features and enhancements as explained below. In the case of heating
boilers only two states in the control form exist, either on or off. When steam boiler variations of IBS are
optioned there are three states which are controlled sequentially.
Just like in hot water sequencing, steam sequencing is used to ensure that only the minimum boilers
required are contributing to meet the required setpoint, reducing fuel consumption.
The steam sequencing operation has 4 sequencing states:
• On – the burner fires and modulates freely to meet the required setpoint. The burner will start and
stop according to the above and below offset differentials (see options 9, 10 and11).
• Standby – the burner remains at the low fire position to meet the standby setpoint (set as an
absolute value in option 42). The burner will start and stop according to the above and below offset
differentials (see options 9, 10 and 11).
• Warming – the burner remains at the low fire position to meet the standby setpoint (see option 42)
and runs according to a timer of X minutes firing (see option 54) and Y minutes not firing (see option
53). If option 54 is set to 0, then the burner will continually fire at the low fire position to meet the
standby setpoint in the warming state.
The burner can also be controlled by a (warming) thermostat fitted in the boiler shell, wired to
terminal 82 (see option/ parameter 156).
• Off – the burner does not fire.
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