Steam Drum Calculations Steam Velocity in Drum

When sizing steam drum internals, the chevron steam separator plays a significant role in ensuring the final steam purity (Figures 6.9 and 6.10). The boiler designer must have some idea how the steam generation varies along the length of the drum. This depends on whether we have a longitudinal gas flow boiler or a cross-flow type of boiler. If the chevron separator is located such that the steam flow enters it uniformly from all sides, then the vanes in the chevron will not be overloaded. In a steam generator, based on furnace heat loading and gas temperature profile along the convection gas path, one can obtain a good estimate of the steam generated along every meter length of the drum and then locate the chevron separator in the region where the steam generation is nearly equal from each end. This location need not be at the middle of the drum length. Similarly, in a cross-flow boiler, a large percentage of steam will be generated in the first few rows of the evaporator (due to the higher gas temperature at the inlet section), and baffling of internals has to be done considering this.

There are two vapor velocities in the drum space, which should not be exceeded to ensure that water droplets are not carried along with steam ensuring the chevron driers or vanes can perform well. Adhering to these norms will help lower the burden on the chevron separator.

The horizontal steam velocity V h 02 . 

 (6.20)  ρ v 

Steam out

Steam drum showing the flow of steam to chevron separator.

334 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers

Chevron

FIGURE 6.10

Steam drum internals. Note: (1) steam–water mixture, (2) wet steam, (3) dry stem, (4) drain pipe, and (5) belly pan.

( ρρ l − v ) 

The vertical steam velocity V v

where ρ l ,ρ v are the densities of saturated vapor and liquid, kg/m 3 .

Example 6.14

A steam drum of diameter 1219 mm and length 8 m is generating 50 ton/h of steam at 40 barg. Determine the steam velocity in the vertical and horizontal directions assum- ing 100% of steam leaving the water line in the vertical direction and 50% enters the separator from each direction. Check if the drum cross section is reasonable.

The density of saturated water from steam tables is ρ l = 1/0.00125 = 800 kg/m 3 and

that of saturated steam ρ v = 1/0.04792 = 20.87 kg/m 3 .

Then, V h should be less than 0.2 × [(800 − 20.87)/20.87] 0.5 = 1.22 m/s. V v should be less than (0.0762/0.2) × 1.22 = 0.47 m/s. The horizontal cross section area of steam space = π × 1.219 2 /8 = 0.583 m 2 . Volume of steam = 50,000/20.87/3,600 = 0.665 m 3 /s. Hence, 50% of the horizontal flow

velocity = 0.5 × 0.665/.583 = 0.57 m/s, which is far below the allowable value of 1.22 m/s. Hence, it is satisfactory.

The cross section for steam flow in vertical direction = 8.5 × 1.219 = 10.36 m 2 (including

the dished end lengths). The average vertical velocity = 0.665/10.36 = 0.064 m/s, which is far below the allowable value of 0.47 m/s.

Hence, the drum sizing is fine. The chevron separator has to be properly selected based on allowable steam purity. One has to ensure if the holdup times specified, if any, have been met.

Miscellaneous Boiler Calculations 335

Steam Drum Calculations Steam Velocity in Drum