2000 Hook-up Book

Clean Steam

Steam Quality vs. Steam Purity

Table 14: Differences in Steam Characteristics Quality Purity Particles

SYSTEM DESIGN

It is important to define the differ ence between steam quality and purity. Steam Quality — “The ratio of the weight of dry steam to the weight of dry saturated steam and entrained water. For example, if the quality of the steam has been determined to be 95%, the wet steam mixture delivered from the boiler is composed of 5 parts by weight of water, usually in the form of a fine mist, and 95 parts by weight of dry saturated steam. Likewise, if the quality of the steam has been determined to be 100%, there is no wet steam delivered from the boiler, 100% of the steam delivered from the boil er is dry saturated steam.” Steam Purity — “A quantitative measure of contamination of steam caused by dissolved solids, volatiles, or particles in vapour or by tiny droplets that may remain in the steam following primary sepa ration in the boiler”.

Boiler Additives Normally present Limited to process

Filtered

High

Typically 2.8 microns

Clean

Varies on System Design Varies on System Design

Varies Varies

Pure

None

Thus, the three different types of “clean” steam (filtered, clean and pure) can, and will, have different characteristics, summarized in Table 14. Note in particular that: 1. The quality of filtered steam will normally be high because water droplets larger than the filter element rating will be removed. Clean and pure steam systems will have a quality related to the design and operating characteristics of the generator, length and installation details of distribu tion system, insulation of system, number and effec tiveness of mains, drainage points, etc.

2. Boiler additives may well be present in filtered steam and also possibly in clean steam, but often this will be limited by process requirements. For example, the FDA restricts the use of certain additives, including amines, in any steam which comes into direct contact with foods or dairy products. 3. Assuming the generating and distribution system have been designed and installed correctly, the particles pre sent in a pure steam system will be water only. Dependent on feed water type, the same may also apply to clean steam systems.

Regulators for accurate pressure control

Clean Steam Distribution Main

Separators and filters for efficient conditioning of steam.

Process Vessel

Main drip traps for distribution system condensate removal

Pure/ Clean Steam Generator

Plant Steam

Hygienic ball valves for isolation on distribution systems.

Condensate

Diaphragm Valves

Condensate

Process steam traps for effective contaminated condensate drainage.

Condensate

Pure Water

Condensate

Figure 56 Clean/Pure Steam Generator and Distribution System

51