Transcript Slide 1
Developed by Niels Bogh March 2006 Objective • At the end of the class you will be able to explain the differences of the three main types of flame safety devises used in the plants. – Flame rods – UV Mini peeper – UV self checker Flame Supervision • The flame supervisory system consist of a single or multi channel controller in connection with individual flame sensors that ensures the flame is present during operation. • If flame failure is detected the systems shuts down in a pre-determined safe manner. Flame safeties • Flame controllers • Flame Sensors – Flame rods – UV sensors (Purple peepers) – UV Self checker Flame rod. The flame rod is the safest flame supervision available since it inherently can not fail in the ON position like some of the UV detection systems. A flame must be present for the system to work. However with the micro amps generated in an industrial environment there are possibilities for failures where the system will not stay on. The following reviews the flame rod theory. Flame rod • FLAME CONTROLLER • . The controller applies alternating voltage between the flame sensing rod and the base of the flame (ground) • HOW DOES IT WORK? • The flame rod is positioned such that the high temperature sensing rod would be in contact with the flame. • The flame creates an ionized "path" between the center electrode/rod and the electrical ground. • The excitation voltage induces a current flow across this "path". This results in a very small DC offset current. DC offset is small, only micro amps. • This current is a stable, measurable signal which can then be used in the appropriate control scheme. If there is a flame present, the DC offset is detected by the controller, which tells the gas valve to remain open. Flame rod basic components Flame rod Furnace/burner 110V -360V AC depending on manufacturer. Insulator Ground. Flame rod • The figure show a flame present but no path for the DC current since the flame do not ionize so a ground is established. Flame but no path. Flame rod • A signal is established and the controller can amplify this signal so the flame safety will keep the gas valve open Flame with a ionized path for the rectified signal Micro Amp DC generated. Flame rod • Below are areas to keep in mind when using flame rods. The current detected is micro Amperes, wiring and other issues affect the reliability of the system. – – – – – – – – Moisture (Resistive leakage) Ignition interference (induced) High resistance connections (series resistance) Lead capacitance to ground (shunt) Line voltage (sensor output variations) Line transient or floating ground Low rod ground ratio (flame rod) Sot on flame rod • In short: The flame must hit the flame rod and create a path to the ground for the system to work. This can be done directly at the flame rod or between the flame rod and the burner body if the flame hits the body. Attachment of Flame rod and Igniter connectors UV Flame safeties Typical Honeywell Self Checker UV Scanner UV “Mini peeper’s” UV Flame Detection • An UV flame detector is a device that looks for a specific ultraviolet light emitted by the flame. • The light gets amplified by an optical lens and is projected to a lead sulfide coating on the back of the sensor. • The generated current is detected by the controller and is recognized as a flame. Mini Peepers • Will fail in the on position. • Can be used on equipment which gets shut down once every 24 hours and has automatic startup check. • Flame controller will fail flame if sensor is ON when gas is OFF during start check. • Manual check is still allowed. Self Checker • A self checker will interrupt the light signal from the flame a few times per minute and self check to ensure the flame sensor is still working. • A small shutter is activated and blocks the light from hitting the sensor surface. • The electronics keeps the flame signal on. • Used on continuous running equipment Flame controllers Flame safeties from GN Electronic and Honeywell