 # What is the liquid height and equivalent diameter ?

1. Introduction
2. Pipe full or partially filled ?
3. Height of liquid in the pipe calculation
4. Equivalent diameter calculation

## 1. Introduction

It happens that pipes are not actually full, for example in some return pipes, condensate...etc... then the assumption that they are full can lead to some erroneous calculations. [Brannan] is reporting a method proposed by Durand in order to know if a pipe is totally full or not.

## 2. Pipe full or partially filled ?

To know if the pipe is full or not, it is necessary to calculate Q/d2.5
- If ≥ 10.2 it means the pipe is full
- If < 10.2 the pipe may be partially full only, further calculations are required to know the height of liquid in the pipe and the equivalent diameter

With :
Q = flow rate in gpm
d = pipe diameter in in

## 3. Height of liquid

The method is proposing to have x = ln (Q/d2.5)

Then calculate H/D = 0.446+0.272x+0.0397x2-0.0153x3-0.003575x4

With :
H = height of liquid in the pipe in ft

## 4. Equivalent diameter calculation

The equivalent diameter can be calculated from the following correlation :

De/D = -0.01130+3.040 (H/D) -3.461 (H/D)2 +4.108 (H/D)3 -2.638 (H/D)4

With :
D = pipe diameter in ft
De = equivalent diameter in ft

Source
Rules of thumb for Chemical Engineers, Brannan, Elsevier, page 5