Boiler Water Deposits – Part 3

August 3, 2020 - Nathan Olszak

Boiler Circulation and Deposits

boiler circulation, complete water solutions, industrial boiler

Boiler Circulation Process

The image to the left shows the process of boiler circulation. The left legs of the U-tubes represent downcomers and are filled with relatively cool water. The right legs represent generating tubes and are heated. The heat generates steam bubbles, and convection currents create circulation. As more heat is applied, more steam is generated and the circulation rate increases.

Boiler Circulation Process

If deposits form, like in the image to the right, the roughened surface and partially restricted opening resist flow, which reduces circulation. At a constant heat input the same amount of steam is generated, so the steam-water ratio in the generating tube is increased. The water in the tube becomes more concentrated, increasing the potential for deposition of boiler water salts.

In extreme cases, deposition becomes heavy enough to reduce circulation to a point at which premature steam-water separation occurs. When this happens in a furnace tube, failure due to overheating is rapid. When deposits are light they may not cause tube failures, but they reduce any safety margin in the boiler design.

Boiler Circulation Rate

Up to the point of premature steam-water separation, the circulation rate of a boiler is increased with increased heat input. Often, as illustrated in the image to the left, the inflection point (A) is above the nominal boiler rating. When the circuit is dirty, the inflection point of the circulation-to-heat input curve moves to the left, and the overall water circulation is reduced. This is represented by the lower broken line.

Circulation and Deposition

Circulation and deposition are closely related. The deposition of particles is a function of water sweep as well as surface charge. If the surface charge on a particle is relatively neutral in its tendency to cause the particle either to adhere to the tube wall or to remain suspended, an adequate water sweep will keep it off the tube. If the circulation through a circuit is not adequate to provide sufficient water sweep, the neutral particle may adhere to the tube. In cases of extremely low circulation, total evaporation can occur and normally soluble sodium salts deposit.

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