Fluid_Flow_Rules_of_Thumb_for_Chemical

26

Rules of Thumb for Chemical Engineers

Table 2 Rel ief Valve Designat ions

Odf i ce Area (in. 2)

0.110 0.196 0.307 0.503 0.785 1.287 1.838 2.853

c D O E c F "~ G G) H | j O r9- K O L

3.60 4.34 6.38 11.05

N p Q R T

,9.,

16.0 26.0

8 x 1 0 ......

1.5 x 2.5

l x 2

1 . 5 x 3

2 x 3

2 . 5 x 4

3 •

4 x 6

6 x 8

6 x 1 0

1 . 5 x 2

Valve Body Size (Inlet Diameter x Outlet Diameter), in.

Plants, situations, and causes of overpressure tend to be dissimilar enough to discourage preparation of gener- alized calculation procedures for the rate of discharge. In lieu of a set procedure most of these problems can be solved satisfactorily by conservative simplification and analysis. It should be noted also that, by general assump- tion, two unrelated emergency conditions will not occur simultaneously. The first three causes of overpressure on our list are more amenable to generalization than the others and will be discussed.

This will also give conservative results. For heat input from fire to liquid containing vessels see "Determination of Rates of Discharge." The set pressure of a conventional valve is affected by back pressure. The spring setting can be adjusted to com- pensate for constant back pressure. For a variable back pressure of greater than 10% of the set pressure, it is cus- tomary to go to the balanced bellows type which can gen- erally tolerate variable back pressure of up to 40% of set pressure. Table 2 gives standard orifice sizes.

Determination of Rates of Discharge

Fire

The more common causes of overpressure are

The heat input from fire is discussed in API RP 520 (Reference 4). One form of their equation for liquid con- taining vessels is

1. External fire 2. Heat Exchanger Tube Failure 3. Liquid Expansion 4. Cooling Water Failure 5. Electricity Failure 6. Blocked Outlet 7. Failure of Automatic Controls 8. Loss of Reflux

Q - 21,000 FAw~

where

Q - Heat absorption, Btu/hr Aw - Total wetted surface, f t 2 F - Environment factor

9. Chemical Reaction (this heat can sometimes exceed the heat of an external fire). Consider bottom venting for reactive liquids. 5