Boiler protection

June 2005 Maintenance, Test & Measurement, Calibration

One of Eskom's power stations uses non-contact infrared pyrometers from PREI Instrumentation to monitor furnace flame health on its coal-fired drum boilers. There are six boilers and each uses 10 pyrometers, four looking directly at the flame on the floor level, and the other six are mounted around the boiler between the pulverised fuel entry points.

The pyrometers are used in the boiler protection system as a protection against loss of furnace ignition or poor flame health. On boiler start up, all pyrometers monitor the flame health. Fuel oil is used as the basic support of the furnace fireball until such time as the pulverised fuel (PF) flow is great enough to maintain its own flame. When PF is introduced into the oil flame the temperature in the furnace increases greatly. At a temperature of 900°C rising, monitored by three of the four top pyrometers, the protection of circuitry latches and the system is now armed.

Once stable fires are achieved and the oil support is removed the pyrometers (which armed the protection system) now constantly monitor the furnace flame health.

The protection system operates on the detection of these scenarios:

1. Any three of the four top pyrometers detecting flame temperature of less than 600°C falling.

2. Any three of five pyrometers on the left side (two on the top, three of the lowers) detecting flame temperature of less than 600°C falling (monitors unstable fires).

On detection of any of these conditions the protection system initiates a master fuel trip, which does the following:

a) Trips out the primary air fans and mill motors (elimination of PF).

b) Trips out the fuel oil pumps and oil burners (elimination of fuel oil flame).

c) Trips out the propane gas supply (needed to ignite the oil fires).

The boiler is thus starved of fuel, and the operator can then initiate a furnace purge and clean out any unwanted fuel/gases.

For more information contact Peter Tait, PREI Instrumentation, 011 448 2172, prei@hixnet.co.za, www.eprei.co.za

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