Liquid heaters and vaporizers – Safety devices – Alarms or indicators
Reexamination Certificate
2001-04-04
2003-02-18
Wilson, Gregory (Department: 3749)
Liquid heaters and vaporizers
Safety devices
Alarms or indicators
C122S448100, C122S504000
Reexamination Certificate
active
06520122
ABSTRACT:
TECHNICAL FIELD
The invention relates to pressurised steam boilers and their control, to a method and apparatus for detecting the level of water in a steam boiler and to a method and apparatus for assessing the mass flow of steam from a steam boiler.
BACKGROUND
In a known arrangement of a pressurised steam boiler, water is fed into the boiler at a controlled rate and is heated in the boiler to convert the water to steam. The heat required to convert the water to steam is provided by a burner whose hot products of combustion are passed through ducts in the boiler and then exhausted. The steam boiler is controlled by a boiler control system, which receives information from sensors indicating inter alia the level of water in the boiler and the presence of steam in the boiler, and which controls the flow rate of water into the boiler as well as sending a control signal to a burner control system that controls the burner The burner control system controls inter alia the flow of fuel and gas to the burner head in dependence upon a demand signal received from the boiler.
Pressurised steam boilers are potentially very hazardous because of the very high pressure that is maintained in the boiler and it is therefore essential for such boilers to have control systems that are extremely safe. One factor that is taken into account to ensure the safety of a system is the importance of maintaining the water level in the boiler within predetermined limits. The internationally recognised safety regime concerning adequate water level in pressurised steam boilers requires sensing arrangements to detect a first low water level (“first low”) below the normal operating range of the boiler and also to detect a second low water level that is even lower than the first low water level. When the first low water level is detected, the boiler control system sends a signal to the burner control system causing the burner to be switched off. Provided the water level then rises back above the first low water level the boiler control system sends a further signal to the burner control system allowing the burner to restart. If, however, the water level continues to fall and reaches the second low water level, the boiler control system sends a further signal to the burner control system preventing it from restarting without manual intervention. The requirement for manual intervention is inconvenient, but is regarded as a necessary safety requirement.
The false triggering of either the first low or second low is costly. The effect of a false triggering at the first low is to turn off the burner; at best that may simply lead to less efficiency because the burner is switched completely off rather than simply being turned down to a lower firing rate; in a worst case, however, as will be explained below, the false triggering bay lead to the burner being switched off at a time when the demand for heat in the boiler is especially high. False triggering at the second low is more damaging because it is likely to last longer given that the burner can be restarted only after manual intervention.
False triggering can occur without any fault in the equipment. In particular, it is not unusual for there to be a sudden demand for steam from a steam boiler; in that case there may be a significant drop in pressure within the boiler which can cause the water level in the boiler to rise (because of the small bubbles of compressed gas trapped within the water in the boiler). The reduction in pressure rightly leads to a signal passing from the boiler control system to the burner control system to increase the firing rate of the burner, while the increase in water level in the boiler causes the usual water flow into the boiler to be reduced or stopped. As the system then recovers and the pressure in the boiler rises, the water level in the boiler falls quickly and may well fall below the “first low” leading to the burner being turned off at a time when it should be operating, probably at full capacity. It is even possible that the fall in water level will reach the “second low” so that the burner remains off until an operator resets the system.
Safety considerations also have an impact on the techniques that are employed to measure the level of water in the boiler. Because of the importance of detecting the “first low” and the “second low”, separate probes are used to detect each of the levels; whilst one capacitative probe may sometimes be provided to sense water levels within the normal operating range, respective conductive probes, which sense whether or not they are in the water, but give no further indication of water level, are provided to detect the “first low” and the “second low”. Often other conductive probes are set at other levels so that those other levels can be detected in a similar way. Thus a large number of separate probes are provided. A capacitative probe is not regarded as sufficiently reliable for detecting the “first low” and the “second low” water levels. One particular concern is that the signals for such probes may be affected by stray electromagnetic radiation generated by devices in the vicinity of the probes.
Operators of pressurised steam boilers frequently purchase steam flow meters to measure the steam flows in the steam exit lines from each of the boilers. A frequent reason for installing such meters is for auditing purposes, to enable the amount of steam exported from the boiler to be compared to the amount of fuel used by the boiler. Such meters are, however, expensive.
SUMMARY
It is an object of the invention to provide an improved method and apparatus for controlling the operation of a steam boiler.
It is a further object of the invention to provide a method and apparatus for controlling the operation of a steam boiler in which the likelihood of a burner being shut down unnecessarily is reduced.
It is a further object of the invention to provide an improved method and apparatus for detecting the level of water in a pressurised steam boiler, and especially to provide a method and apparatus in which the number of probes that are required is reduced.
It is a still further object of the invention to provide a method and apparatus for assessing the mass flow of steam from a pressurised steam boiler without resorting to a steam flow meter.
According to the invention there is provided a method of controlling the operation of a steam boiler heated by a burner, the method including the following steps:
a) monitoring the level of water in the boiler,
b) monitoring the pressure of steam in the boiler,
c) monitoring the firing rate of the burner, and
d) controlling the flow rate of water into the boiler having regard to the signals resulting from a) and
b) and, at least for some signal conditions, also having regard to signals resulting from c).
By using the firing rate of the burner as one of the control inputs for determining the flow rate of water into the boiler and in that respect combining the burner control system and the boiler control system, it becomes possible to effect a more appropriate control of the water, reduce the number of times that the water level in the boiler falls below a first low water level at which the burner is switched off and thereby improve the efficiency of the boiler.
Whilst it is within the scope of the invention for the control of the flow rate of water into the boiler always to take account of signals resulting from monitoring the firing rate of the burner, it may be that the signals resulting from monitoring the firing rate of the burner are taken into account in a limited set of circumstances only. It is for example preferred that when
i) the monitoring of the level of water in the boiler shows a rate of increase above a predetermined level,
ii) the monitoring of the pressure of steam in the boiler shows a reduction in pressure at a rate above a predetermined level, and
iii) the monitoring of the firing rate of the burner shows that the firing rate is increasing at a rate above a predetermined level,
the controlling of the flow rate of water into the boiler is such that i
Kemp Brendan
Nichols Paul James
Autoflame Engineering Ltd.
Merchant & Gould P.C.
Wilson Gregory
LandOfFree
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