Steam purity - Boiler

Purity of steam indicates the extent of carry over of salts from boiler water with the steam. Sodium salts in boiler water gets carried over due to various reasons with the steam and deposits in the superheater inner surfaces and turbine blades causing reduction of equipment life and availability of the boiler. Steam drums are provided with drier box arrangement to avoid carry over.

Constant maintenance and inspection of these internals in an essential activity of the boiler operation. The purity of steam should be monitored periodically to improve the availability of the boiler.

The method of obtaining the steam sample plays a vital role in the measurement of its purity. It is recommended that the sampling lines should be of stainless steel and the cooling coils in the sample coolers should be located close to the sampling point to minimise settling and contamination from the sample line.

Sodium tracer technique permits measuring dissolved solids in steam to as low as 0.001 ppm. The condensed steam sample is aspirated through a small tube in the burner into the oxygen-hydrogen flame.

The flame, at about 1700 deg C, vaporizes the water and excites the sodium atoms, which emits a characteristic yellow light having a definite wavelength. The intensity of the emitted yellow light is a measure of the sodium in the sample.

The intensity of the light is measured with a spectrophotometer equipped with a photomultiplier attachment. The light from the flame is focused by the condensing mirror and is directed to the diagonal entrance mirror. The entrance mirror deflects the light through the entrance slit and into the monochromator to the plane mirror.

Light striking the plane mirror is reflected to the fiery prism where it is dispersed into its component wavelengths. The desired light wavelength is obtained by rotating a wavelength selector which adjusts the position of the prism.

The selected wavelength is directed back to the plane mirror where it is reflected through the adjustable exit slit and lens. The light impinges on the photomultiplier tube, causing a current gain which registers on the meter.

The amount of sodium in the sample is obtained by comparing the emission from the water sample to emissions obtained from solutions of known concentration.