with the young engineer an anecdote.
While driving down a gravel road in
southern India, the author passed a
small plant boiling down sugar cane
juices to produce molasses. He noticed
that the 20-in. diameter of the plant’s
brick boiler stack narrowed to perhaps
6 in. near the stack’s top.
The author reasoned that this reduction in the cross-sectional area of
the stack would increase the velocity
of flue gases exiting the top of the boiler’s stack, and that such an increase in
flue-gas velocity would decrease pressure at the top of the stack. Theoretically, this should result in a slight draft
increase in the boiler, located at the
base of the stack, to help draw in more
combustion air.
To confirm the theory, the author
stopped to ask two older gentlemen
working around the juice boiler why
they thought the top of their stack was
narrower than the bottom. He suggest-
ed the narrowing was an application of
the Venturi effect
The gentlemen responded that the
narrowing was not done to honor this
Mr. Venturi, as they’d never heard of
any such person. Rather, it was done to
honor a tradition handed down from
many generations of ancestors, hold-
ing that narrowing the top section of
the stack helps the boiler work harder.
This was another basic engineering
principle to consider before arriving at
the plant to begin the real work of field
observations and data collection.
Solving the problem
On site at Cartagena, the crude heater
showed multiple signs that it was operating in a draft-limited situation (Fig.
3):
• The burner air registers were
mostly closed.
• Yellow flames were licking up
against the radiant-section wall tubes.
• Wisps of white (e.g., SO2) flue gas
were escaping from leaks just below
the convective section.
• Increasing the fuel flow caused
the heater outlet temperature to drop—
a sure sign of lack of combustion air.
Alternatively, if you’re running a
heater at 2% excess O2 and the 30-mph
wind stops, the heater will be running
in an air-deficient mode, causing possible accumulation of explosive gases
in the convective section. If this type of
situation occurred with fuel gas on automatic temperature control, the consequence could be a heater explosion.
Venturi effect
During the 30-min. trek to the refin-
ery, past banana groves and hillsides
covered by withered corn stalks, the
author was reminded of and shared
In situations where wind is a factor,
however, it can be a significant prob-
lem, especially when we try to save
on energy costs by minimizing excess
combustion air in a fired heater.
The author observed in a Wyoming
refinery that a 30-mph gust of wind
will cause a sudden draft increase
of slightly more than 0.1 in. H2O, or
about 20%. This will increase the combustion air flow into a fired heater by
around 10%, which in the event of a
heater running with 2% excess O2 in
the stack means the excess O2 will
suddenly increase to about 4%.
NELSON-FARRAR COST INDEXES1
Refnery construction (1946 basis)
Explained in OGJ, Dec. 30, 1985, p. 145.
Dec. Nov. Dec.
1962 1980 2014 2015 2016 2015 2016 2016
Pumps, compressors, etc.
222.5 777.3 2,271.9 2,313.6 2,336.3 2,323.7 2,330.3 2,341.7
Electrical machinery
189.5 394.7 515.8 516.5 513.0 515.0 511.9 511.9
Internal-comb. engines
183.4 512.6 1,052.9 1,062.3 1,035.6 1,064.7 1,035.7 1,034.4
Instruments
214.8 587.3 1,533.6 1,554.4 1,597.5 1,569.2 1,595.7 1,604.1
Heat exchangers
183.6 618.7 1,305.0 1,305.0 1,221.2 1,305.0 1,221.2 1,221.2
Misc. equip. average
198.8 578.1 1,335.8 1,350.3 1,340.7 1,355.5 1,338.9 1,342.6
Materials component
205.9 629.2 1,571.8 1,434.9 1,403.1 1,339.8 1,414.9 1,441.2
Labor component
258.8 951.9 3,210.7 3,293.8 3,395.8 3,346.8 3,424.2 3,455.8
Refnery (infation) index
237.6 822.8 2,555.2 2,550.2 2,598.7 2,544.0 2,620.5 2,649.9
Refnery operating (1956 basis)
Explained in OGJ, Dec. 30, 1985, p. 145.
Dec. Nov. Dec.
1962 1980 2014 2015 2016 2015 2016 2016
Fuel cost
100.9 810.5 1,264.8 915.9 869.1 847.1 939.8 1,056.0
Labor cost
93.9 200.5 312.8 319.2 339.6 329.5 327.7 306.7
Wages
123.9 439.9 1,541.3 1,584.4 1,624.5 1,585.4 1,607.1 1,584.8
Productivity
131.8 226.3 493.1 497.1 479.1 481.2 490.4 516.8
Invest., maint., etc.
121.7 324.8 939.4 948.0 938.2 945.7 946.0 956.7
Chemical costs
96.7 229.2 472.3 434.6 409.9 421.8 427.0 425.5
Operating indexes2
Refnery
103.7 312.7 688.5 660.0 657.0 655.5 663.8 670.8
Process units
103.6 457.5 865.3 748.1 734.4 726.3 758.3 796.4
1These indexes are published in the frst of each month and are compiled by Gary Farrar, OGJ Contributing Editor.
2Add separate index(es) for chemicals, if any are used. Indexes of selected individual items of equipment and materials are
also published on the Quarterly Costimating page in frst issues for January, April, July, and October.
