Rice Lake Weigh Modules/Mount Assemblies - Manual (Page 54)

VESSEL ATTACHMENTS
3-2
Piping
Attaching Piping to Weigh Vessels
Figure 3-2
The attached pipe also deflects downward by the same amount
∆
h
and it applies an upward force, F
1
, to the tank.
The effects of piping are particularly severe when several pipes are
attached to a low-capacity weigh vessel. Through proper design, the
upward forces exerted by the pipes can be reduced to a small
percentage of the vessel’s live load. Then, by calibrating the vessel
with weight, the remaining effects can be compensated for. Calibra-
tion using a load cell simulator will not produce accurate results,
since there is no way to simulate the effects of attached piping (see
Calibrating Vessel Weighing Systems in Section 4 for further
details).
Some common rules of thumb for piping design are as follows:
• reduce deflection of the vessel support structure to a minimum.
• use the smallest diameter, lightest wall pipe possible.
• all pipes must run horizontally away from the vessel.
• place the first pipe support 20 to 30 times the pipe diameter
away from the vessel (for example, for a 2" diameter pipe, the
first support would be placed at least 40", and preferably 60",
away from the vessel).
NOTE: Pipe diameters and wall thickness, pipe support intervals,
etc., must be chosen consistent with the functionality, structural,
and reliability requirements of the system in addition to recom-
mendations of this section.
For a more rigorous treatment of the subject, the force F
1
exerted
on the vessel may be calculated using the following equation:
where:
D = outside diameter of pipe
d = inside diameter of pipe
∆ h = total deflection of the pipe at the vessel relative to the
fixed point.
E = Young’s modulus
= 29,000,000 for mild steel
= 28,000,000 for stainless steel
= 10,000,000 for aluminum
l = length of pipe from the vessel to the first support point.
This yields conservative results, since it assumes that the pipe is
held rigidly at both ends. In practice there will be some give in both
the support point and its attachment to the vessel. The example on
the following page illustrates the use of this formula.
Without question, attached piping is by far the largest source of
error in vessel weighing. Hence the piping arrangement must be
carefully planned in the design of any weigh vessel.
Figure 3-1 shows a vessel mounted on load cells and supported on
an I-beam structure. An attached horizontal pipe is rigidly sup-
ported a distance “l” from the vessel.
l
Figure 3-1
When the vessel is loaded, it moves downward as shown in Figure
3-2 as a result of:
1) The deflection of the load cell (.005" to .015" at full load), and
2) The deflection of the support structure.
l
∆
h
l 3
F
1
=
.59(D 4 – d 4 ) x (Dh) x E