Steam Boiler Controls

Written by Jerry Ratzlaff. Posted in Instrumentation

These are a list of some of the controls that should be considered when putting together controls for a steam generator.  This list is meant to be a starting place for the process design and to offer topics for discussion.  

High Steam Pressure

The purpose of a high steam pressure shutdown is to prevent overpressure damage to generator tubes or related piping.  Generally speaking, the set point is 100 psi above the maximum normal operating pressure.  It is very important to note that this should always be below the set point of the PSV or the Maximum Allowable Working Pressure (MAWP).  

High Steam Temperature

The purpose of a high steam temperature is to detect and protect against a condition where superheated steam could cause tube failure.  This is because steam quality from a steam boiler is usually below 100%.  Superheat can cause to rapid and sudden tube failure.  This setpoint is commonly set 20 degrees above the maximum normal operating temperature.

Low Combustion Air

The purpose of this alarm is to detect a loss of blower pressure and protect against incomplete combustion.  This is usually detected with either a pressure switch or a differential pressure switch and is set just above the blower air pressure when the blower is in the low fire position.

Flame Failure

The function of a no flame shutdown is to detect loss of flame caused by burner malfunction or other abnormal condition. The purpose of this is to shut off fuel gas to the burner so uncombusted fuel doesn't go into the firebox and create an explosive atmosphere.  

The typical instrument is a flame scanner or an ultraviolet detector.  This is a discrete instrument and usually has no set point adjustment.

Low Steam Pressure

Low steam pressure instruments are to detect and protect against tube leaks or breaks in the radiant section of the steam generator.  The typical set point for this is 50 psi below the normal operating pressure.  If local, state or federal codes do not require the use of this device, it may be omitted or disabled if there are excessive “nuisance” shutdowns.

Low Steam Temperature

Low steam temperature is to detect leaks or breaks in the steam tubes.  This is a redundant shutdown device can be used in tandem with low steam pressure.  The typical set point is typically the corresponding temperature (based on steam tables) for the low pressure shutdown.  

Low Feedwater Flow

Feedwater must always flow through the tubes when the burner is activated.  The feedwater's purpose is two fold.  First, and most importantly, it is used for steam.  Secondly, the feedwater is used to keep the temperature of the tubes below their melting point.  If feedwater stops flowing through the tubes, the burner should be shutoff.  The set point for this depends on the type of generator/ boiler.  

High Steam Quality

This shutdown is prevent the steam from becoming superheated and causing fouling of the tubes.