Steam quality is the percentage of entrained moisture present in steam. Pure steam (all water vapor molecules) with no moisture has a quality of 100%. Water in liquid form has a quality of 0%.

One pound of steam at 100 psig has a heat content for the vapor of 880 BTU's and the water of 309 BTU's for a total of 1189 BTU's. When one pound of steam at 100 psig is condensed, 880 BTU's is transferred to your process and the remaining water has a heat content of 309 BTU's. This change of state from vapor to liquid occurs with no loss in temperature. The remaining water (condensate) only provides 1 BTU of energy for each degree of temperature lost. That is why flooded steam systems provide very little thermal energy.

The figures sited above assume a quality of 100% which is virtually impossible in a real steam system. Steam at a properly operating boiler has a quality of 97-98%. As steam is transmitted through pipes; it gives up heat, condenses, and forms drops of water which carry through the lines. These water droplets are spent steam and reduce the quality. Typical delivered quality in most steam systems is ~90%. Using our 100 psig example, the BTU's of heat per pound of steam are reduced by 10% for a net of 792 BTU's.

When the steam quality is in the 85-98% range, water droplets are carried in the flow steam as a sort of fog. At about 85% quality or when the flow impinges on a surface, the moisture drops out and can be drained by a trap.

Steam with a quality less than 95% affects your process quality and output because of this spent steam. Wet steam is also very erosive which contributes to wear and leaks.

Steam separators are designed to remove this entrained moisture and return your steam quality to boiler quality.
Separators are made in two basic types; baffle style and cyclone. Although more costly due to design and manufacturing considerations, cyclone separators are your best choice to dry your steam.