Rice Lake Z6 Single-Ended Beam, SS Welded-seal, IP67, OIML C3 - Manual (Page 41)

Attaching Piping to Weigh Vessels 37
20.0 Attaching Piping to Weigh Vessels
20.1 Attaching Piping to Weigh Vessels
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 20-1 shows a vessel mounted on load cells and
supported on an I-beam structure. An attached horizontal
pipe is rigidly supported a distance “l” from the vessel.
Figure 20-1.
When the vessel is loaded, it moves downward as shown in
Figure 20-2 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.
Figure 20-2.
The attached pipe also deflects downward by the same
amount Δh and it applies an upward force 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. Calibration using a load cell simulator will
not produce accurate results, since there is no way to
simulate the effects of attached piping.
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
r e q u i r e m e n t s o f t h e s y s t e m i n a d d i t i o n t o
recommendations of this section.
For a more rigorous treatment of the subject, the force exerted
on the vessel may be calculated using the following equation:
where:
D=outside diameter of pipe
d=inside diameter of pipe
Δh=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
next example illustrates the use of this formula.
Example I
A steel tank is supported on load cells and a steel structure with
deflections of .008" and .250" respectively under load. A 4"
schedule 40 pipe is attached horizontally with 36" free span
between the vessel and the first support point. What force F1 is
exerted upward on the vessel?
l