Types of Steam-Generating Equipment

Ordinary level control does not work in steam drums or kettle boilers. The problem is the lower density of the boiling water as compared to the greater density of the non-boiling hot water in the external level sensing chamber. I ve addressed this problem in my book, Troubleshooting Process Plant Control (Wiley, 2009). 

The boiling water will be at a 10 to 50 percent higher level inside the steam drum than the water in the level-sensing chamber or gauge glass. There are two ways of dealing with this problem  

The conventional method is to get a sample of steam condensate and measure it for TDS. If the blowdown water has a TDS of 1000 ppm and the sample has a TDS of 30 ppm, then the moisture content of the steam is 3 weight percent. The assumption is that the TDS is totally due to entrainment. 

My method (which hopefully you will agree is better) is explained in Fig. 22.4  

Three weight percent water in steam is a lot, and would be considered to be poor-quality steam. 

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