Rice Lake Weigh Modules/Mount Assemblies - Manual (Page 11)

1-9
VESSEL WEIGHING SYSTEMS Design Elements
Number of Supports
The number of supports to be recommended is dependent on the
geometry, gross weight, structural strength and stability of the
vessel. The number of supports chosen for a vessel obviously
influences the capacity of the load cells required. In general, no
more than eight supports should be used. It becomes more difficult
to get even weight distribution on all supports as the number
increases beyond three. Below is a look at a number of examples.
Suspended Vessels
These vessels are very often suspended from an existing structure
which will sometimes dictate how many supports will be used. In
general, one or more supports may be used. Using three supports
or fewer has the advantage of not requiring adjustment of
the length of the support linkages to distribute the load
equally between all supports (assuming the cells are arranged
symmetrically on the vessel).
Upright cylindrical vessels in compression
The most convenient method of mounting is with three supports
arranged at 120
°
degree intervals. Correct weight distribution is
inherent to 3-point support and is preferred whenever possible.
With tall slender vessels or vessels subject to fluid sloshing, wind
or seismic loads, stability against tipping becomes a consideration.
In these situations, four or more supports should be considered.
See Appendix section on wind and seismic effects.
Square, rectangular or horizontal cylindrical
vessels mounted in compression
Because of geometry, it is usually most convenient to mount these
vessels on four supports, close to each corner. Higher capacities
may, of course, require more than four.
Load Cell Capacity
It is vital to the performance of a weighing system to select load
cells of the correct capacity. Here are some guidelines:
■ All load cells selected must be of the same capacity.
■ Estimate the vessel dead weight, including all piping, pumps,
agitators, insulation and vessel heating fluids.
■ Add the maximum live weight of product to be weighed to the
dead weight. This is the gross weight of the vessel and
contents.
■ Divide the gross weight by the number of legs or support
points. This is the nominal weight which will be carried by
each load cell.
■ Select a load cell with a capacity somewhat greater than the
nominal weight. The following should be considered when
determining how much greater the load cell capacity should
be:
1. Is your dead weight accurate?
2. Will the load be evenly distributed on all cells?
3. Is the vessel fitted with an agitator or subjected to shock
loading?
4. Is it possible the vessel will be overfilled, exceeding your
live weight value?
5. Will the vessel be subjected to wind or seismic loading?
For more information, see Wind and Seismic Effects on
Vessel Stability.
A good rule of thumb is to select a load cell with a capacity 50 –
100% in excess of the calculated nominal load per cell. Once the
load cell capacity has been determined, check that the live weight
signal is adequate for the instrumentation selected; see Section 4
for information on how to determine the microvolt-per-gradua-
tion for your system. This is particularly important when the ratio
of dead weight to live weight is high.
■ Additional factors to consider:
Load Cell Construction Material — In a corrosive envi-
ronment, stainless steel outperforms nickel-plated alloy steel.
Load Cell Protection — The ultimate degree of protection
can be achieved with hermetically-sealed load cells which
ensure the integrity of the strain gauge section of the cell in
corrosive or washdown applications.
Cable Length — Check that the standard cable length will
be adequate for your installation. Longer cable lengths are
available on special order.
■ See page 2-22 for compatibility information on mounts and
load cells by capacity. Capacity requirements may limit
practical applications of many models.
Selecting Number of Supports & Load Cell Capacity