Contact Section - Stacks - Spitting
May 22, 2003

Introduction

Stack spitting, fall-out and rain are terms that refer to the phenomenon of liquid or solid particles being emitted from a stack.

Condensation will occur in the stack as the hot, acid saturated gas leaving the absorber tower cools down as it passes along the duct going to the stack and then up the stack.  A liquid film forms on the inside walls of the stack and then drains down to the base.  As the liquid drains down the stack, the film becomes thicker and the liquid may begin to flow in streams.  If the upward velocity of gas is high enough, liquid droplets can be sheared from the liquid film and will be carried up the and out of the stack. 

Solids and iron sulphate can also be blown out of the stack, particularly during startups.  When a plant has been shutdown for a prolonged period of time iron sulphate will form on the inside of a carbon steel stack.  Overtime the sulphate will dry up causing it to loosen from the surface of the metal.  When the plant starts up, the sulphate can easily be stripped from the inside of the stack wall and emitted from the top of the stack.

The most obvious sign of stack rain is the discolouration of paint on buildings and cars.  Stack rain can be a costly problem for plant operators if they must repair the damage to employee cars and those in the surrounding neighbourhood.

The fall out from a stack can be monitored by placing a board on top of pole.   The board is painted white so any acid droplet landing on the horizontal board will discolour the paint.  This monitoring method will help to determine the degree of fallout and assist in settling or dismissing claims of damage to personal property due to stack fallout.  The board should be examined periodically to help pinpoint when incidences of stack rain occur.

Stack rain can be avoided by keeping the gas velocity in the stack low, reducing the vapour pressure of the acid in the absorber tower, ensuring stack drains and candle drains are open and ensuring candle drain cups are sealed.