Steam drum stability

Ellison has published an extremely important factor for steam drum design called the Drum-Level-Stability Factor. As manufacturers have learned how to increase boiler design ratings, the criteria for steam drum design have lagged. The three historical steam drum design criteria have been  

The drum must be large enough to contain the baffling and separators required to maintain separation and steam purity. 

The drum must extend some minimum distance past the furnace to mechanically install the tubes. 

The drum must have a water holding capacity with enough reserve in the drum itself, such that all the steam in the risers, at full load, can be replaced by water from the drum without exposing critical tube areas. 

This fourth criterion can be met at a low steam drum cost. Only one percent of the cost of the boiler spent on the steam drum can provide it. The fourth criterion is met by requiring that the Drum-Level-Stability Factor (D.L.S.F.) be equal to 1.0 minimum. When this exists the steam drum level will be stable for wide and sudden operational changes. 

---

Ellison, G. L., Steam Drum Level Stability Factor, Hydrocarbon Processing, May 1971. 

Flare stack sizing and pressure drop is included with considerations of pressure drop through the safety valve headers, blowdown drums, flare headers, seal drum, etc. Elevated flare tips incorporating various steam injection nozzle configurations are normally sized for a velocity of 120 m/s at maximum flow, as limited by excessive noise and the ability of manufacturers to design tips which will insure flame stability. This velocity is based on the inclusion of steam flow if injected internally, but the steam is not included if added through jets external to the main tip. 

They should preferably not be used on steam service, steam boiler drums or superheaters, where an open bonnet-type SRV is generally preferred because of the temperature of the spring, which is required to be cooled off in order for it to keep its characteristics and maintain stability in the set pressure. 

The last batch of ONBALD super crude was produced eight months before the incident. It was stored in mild steel drums outside the plant. When it was decided to distil the ONBALD super cmde, samples were sent to the laboratory for thermal stability tests using a new thermal stability test apparatus. All seemed well, so permission was given for the distillation to begin. Finally the temperature was taken up slowly, by steam heating, to 405 K. The steam was then turned off, but the temperature continued to rise and the vacuum was... 

In this chapter, four basic distillation control schemes were introduced along with a number of variations on each scheme. The concept was introduced that there can be more than one way to successfully control a distillation column. In some schemes, the separation power base is controlled by the ratio of steam/feed and then the distillate flow rate or reflux flow rate can be manipulated to control an MRT point in the distillation column. In other schemes, the separation power base is controlled by the ratio of reflux/feed, and the steam to the reboiler or the bottoms flow rate can be manipulated to control an MRT point in the column. Control of reflux drum level and column base level was presented as basic to all control schemes. Control of column pressure was considered to have an overriding effect on the stability of distillation column control. 

---