Steam quality and drum sizing
Steam quality and drum sizing
curve3104 (Mechanical) May 27, 2004
A certain area of water surface is required in a boiler from which to release the steam at an acceptable velocity with minimal carryover. I am looking for design standards or calculation methods for determining steam drum diameters and lenghts to ensure that the area of the normal water level would be acceptable for a given steam load.
Anyone know where I could find these answers or does anyone have experience with this topic?
Montemayor (Chemical) May 27, 2004
Proper disengagement, when considering 2-phase separation, is more related to the volume on top of a liquid surface - not to the area of the liquid surface. As an example, consider the difference in disengagement effect when dealing with a vertical cylindrical vessel as opposed to a horizontal cylindrical vessel. How much disengagement height is required (even prior to entering a demister pad) is a matter of the fluids in question, the pressures, and the physical configuration inside the vessel. This height value is normally an empirical one, dependent on the experience of the designer. There are no hard-and-fast rules here. Prior success and results deem what is more appropriate and recommended.
I believe that by basing yourself on a liquid surface area you are making a mistake. The answer is much more involved and complex than that. The Souders-Brown relationship is sometimes used for this application and in others, the settling velocity of droplets is used to size the separation vessel (particularly horizontal vessels). Most text books, Perry's Handbook, and the GPSA deal with this subject.
I hope this helps you out.
athomas236 (Mechanical) May 28, 2004
In the steam drums of water tube boilers, separation almost always takes place in two stages. The first stage is usually cyclone separators that use centrifugal force to separate the steam from the steam and water mixture entering the drum from the evaporator circuits.
This first stage is not 100% efficieny hence the need for a second stage. The second stage is usually demister pads that separate any remaining water from the steam.
With this approach the drum size is then determined to allow this separation equipment to be installed in the drum with access for maintenance. Some times there is a requirement for a minimum water storage quantity in the drum to cover the case when a feed pump is tripped and the standby pump started.
Each major boiler supplier has his own proprietary design of separation equipment which then leads to each supplier have different drum sizes for the same boiler operating conditions. I have kept records of drum sizes offered to us since about 1980 and there are very wide variations in steam drum loadings.
curve3104 (Mechanical) May 28, 2004
After giving this some additional thought, I am in agreement with you Montemayor. The height of the water level to the top of the drum or demister pad is the critical variable to determine steam quality. The overall length of the drum can be considered to be fixed based on other design criteria such as the required number of tubes in the boiler and heat release rates.
The question now becomes, is there a relationship between steaming capacity, the quality requirement of the steam, and the height to the water level that can be estimated to calculate the drum diameter?