Process engineer requested to design a system to load 50% sodium hydroxide into a carbon steel tank in his plant. Sodium hydroxide is considered a hazardous material, hence not recommended to move 50% sodium hydroxide at velocities over 6 ft/sec in carbon steel piping. The supply pump at the plant can generate a flow of up to 250 gpm. The recommended maximum pressure drop across the valves in the system is 3 psi. Specific gravity of the sodium hydroxide solution is 1.52. Please specify a control valve for this service.
Step 1: Find Cv
Fmax =250 GPM
ΔP =3 psi
Cv value is 178
Step 2: Find literature of Cv value (at various opening of ball valve) versus Valve sizes.
|Size (NPT)||Cv @10% Valve Opening||Cv @25%
At 50% or more opening, in order to meet Cv 178, then valve sizes should be 5″ or 6″ ball valve.
Step 3: Check piping minimum diameter.
What diameter pipe would correspond to a flow of 6 ft/sec?
A = πr² = π(d/2)²
v = 6 ft/sec
Fmax = 250 gpm = 0.557 ft³/sec
Since Fmax = Av = π(d/2)² · v
A pipe with diameter larger than 4.1 inches shall not exceed the 6 ft/sec requirement.
Answer: Combination between step 3 and 2 is valve and piping size should be 5″ inch.