Page 115 - Pressure Vessel Design Manual

General Design 95

PROCEDURE 2-20

ESTIMATING WEIGHTS OF VESSELS AND VESSEL COMPONENTS

Estimating of weights of vessels is an important aspect of possibility of being below thickness. The safety margin
vessel engineering. In the conceptual phase of projects, added by the mill is referred to as plate overage or over-
weights are estimated in order to determine costs and bud- weight percentage. The plate overage varies by the thickness
gets for equipment, foundations, erection, and trans- of the material.
portation. Estimated weights also help to get more In addition to the plate overage, the fabricator (or head
accurate bids from suppliers. Accurate weights are necessary manufacturer) also adds a thinning allowance to the head to
for the design of the vessel itself to determine forces and ensure that the head meets the minimum thickness in all
moments. areas. Depending on the type of head, the diameter, and
There are a number of different types of weights that are the thickness required, a thinning allowance can be deter-
calculated. Each weight is used for different purposes. mined. This can be as much as 1.5 in. for large-diameter
hemi-heads over 4 in. thick! The metal does not disappear
1. Fabricated weight: Total weight as fabricated in the during the forming process but may “flow” to the areas of
shop. most work.
2. Shipping weight: Fabrication weight plus any weight On a typical spun 2:l S.E. head, the straight flange will get
added for shipping purposes, such as shipping saddles. thicker and the knuckle will get thinner due to cold working.
3. Erection weight: Fabrication weight plus any weight The crown of the head should remain about the same.
installed for the erection of the equipment, such as any Therefore the completed head has a thickness averaging
insulation, fireproofing, piping, ladders, platforms. the initial thickness of the material being formed.
4. Empty weight: The overall weight of the vessel sitting After the weights of all the components are added for a
on the foundation, fully dressed, waiting for operating total weight, an additional percentage is typically added to
liquid. allow for other components and welding. The typical percen-
5. Operating weight: Empty weight plus any operating tages are as follows:
liquid weight.
6. Test weight: This weight can be either shop or field
test weight, that is, the vessel full of water. < 50,000 lb Add 10%
50,000-75,000 lb Add 8%
There are a number of ways to estimate the weights of 75,000-100,000 Ib Add 6%
vessels, depending on the accuracy required. Vessel weights > 100,000 lb Add 5%
can be estimated based on computer design programs. These
programs typically calculate the volume of metal for the The weight of any individual component can easily be
vessel shell and head and add weights for supports, nozzles, calculated based on the volume of the material times the
trays, and other components. Another fast and easy way to unit density weight given in Table 2-18. Any shape can be
get the volume of metal in the shell and heads is to use the determined by calculating the surface area times the thick-
surface area in square feet and multiply this by the unit ness times the density. The designer need only remember
weight for the required thickness in pounds per square foot. the density of steel for most vessels of 0.28331bh3 to de-
In addition to the base weight of metal in the shell and termine any weight. For vessels or components of other ma-
heads, the designer must include an allowance for plate terials, either the density of that material or the factor for
overages per Table 2-18. The mill never rolls the plates that material relative to carbon steel can be used. These
the exact specified thickness since there would be the values are also listed in the following tables.