Rice Lake Z6 Single-Ended Beam, SS Welded-seal, IP67, OIML C3 - Manual (Page 10)

6 Load Cell and Weigh Module Handbook
4.0 Selecting the Number of Supports and Load Cell Capacity
4.1 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 Section 22.0 on
page 47.
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.
4.2 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:
•
Is your dead weight accurate?
• Will the load be evenly distributed on all cells?
• Is the vessel fitted with an agitator or subjected
to shock loading?
• Is it possible the vessel will be overfilled,
exceeding your live weight value?
• Will the vessel be subjected to wind or seismic
l o a d i n g ? F o r m o r e i n f o r m a t i o n , s e e
Section 22.0 on page 47.
A good rule of thumb is to select a load cell with a capacity
25-50% 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 7.0 on page 11 for information on how
to determine this 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
e n v i r o n m e n t , s t a i n l e s s s t e e l o u t p e r f o r m s
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.