Evaluation of Water Flow in the Floor Tubes of Recovery Boilers with a Decanting Hearth, 2010 TAPPI/PAPTAC International Chemical Recovery Conference

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Water flow in the floor tubes of recovery boilers must be adequate to prevent DNB or internal deposits. Circulation calculations have served well in the design of boiler circuits, but trouble-shooting suspected circulation issues requires a more specific analysis. We have developed an ultrasonic flow monitoring (UFM) technique that has recently been applied to make wall/floor tube measurements in eight different recovery boilers with decanting hearths.

The nature of the design of these boilers is such that front and rear wall tubes that are closest to the side walls (sometimes called corner tubes) will generally receive less heat flux than other wall tubes. This results in lower circulating water flow velocities. Measurements showed that the typical front and rear tube (and by extension the floor tube) velocities away from the corners were from 3 to 5 ft/s depending on the boiler load. The corner front and rear wall tubes had velocities that were reduced by up to 60%. Tubes two or three away from the corner showed a velocity decrease of 30 to 40%.

Apparently the reduction in the flow in the floor tubes nearest the side walls still provides adequate flow to prevent DNB. In one recovery boiler, the UFM measurements confirmed that water flows were fine in the wall tubes and thinning in the tubes of concern was eventually pin-pointed to a water quality issue. In another boiler, UFM was used to check that circuit modifications had increased the water flow velocities in the corner wall (and floor) circuits after one of these tubes had overheated and failed.