Oil & Gas Journal - April 03, 2017

FIG. 2

28 in.

1 psi
Water

HEAD PRESSURE OF WATER AT 60° F.
DRAFT IN NECKED-DOWN FIRED HEATER STACK FIG. 1

Venturi effect

100 ft Draft = 0.45 in. H20

Stack
Point
A
Air temperature,

70° F.

∆P,
differential
pressure
Point B

300° F.

ft (Equation 11). Having already established a volume of 32,000 cu ft for the giant sphere, the balloon would be able to lift 740 lb (Equation 12).

Stack draft

A tall stack can develop a very strong sucking force. Let’s say the stack’s draft is 2 in. H2O (meaning pressure outside the stack is 2 in. H2O higher than pressure inside the stack). Let’s also say there is a metal plate measuring 2 ft × 2 ft to be pulled off the stack’s bottom. How much force, in pounds, would one have to use to pull off the plate?

First, we calculate the area of the plate as 4 sq ft (Equation 13), or—since we know there are 144 sq in. in 1 sq ft—576 sq in. (Equation 14).

Over the surface of this plate, there is a force of atmospheric pressure equal to 2 in. H2O pushing on the plate, which we can calculate as 1,152 cu in.

(Equation 15).

Since 1 cu in. H2O weighs 0.58 oz, we can determine the force required to pull off the plate would be 664 oz (Equation 16), or 42 lb (Equation 17).

The author encountered this problem in 1980 at Amoco Oil Co.’s Texas City, Tex., refinery where, while wearing a Scott Air-Pak, he had to pull off a manway cover on a reactor that was 150 ft high and full of hot flue gas from burning sulfur residue.

Stack height effect

Draft in a stack is not caused by combustion. Also, as the molecular weight of the flue gas is rather close to air, its effect is neglected. And while wind blowing across the top of a stack occasionally contributes to draft, its effect is generally ignored when calculating draft since it’s not always windy.

You may have seen on certain stacks that the top end of the stack is necked-down (Fig. 1). This is done to slightly increase the draft. It’s called the Venturi effect, which will be discussed later.