Specifications
METTLER TOLEDO Weigh Module Systems Handbook
(12/99)
5-16
Piping can have a significant effect on weighing accuracy, especially when many
pipes are connected to a tank with a relatively low capacity. By designing the
piping properly, you can reduce unwanted forces to a fraction of the tank’s live
load. Then you can compensate for the remaining forces when you calibrate the
scale. Since load cell simulators cannot simulate the forces produced by attached
piping, calibration must be performed on the installed tank scale.
You can use the following equation to calculate the force exerted by an attached
pipe:
0.59 × (D
4
- d
4
) × ∆h ×
E
L
3
where:
F
P
= Force exerted by pipe
D = Outside diameter of pipe
d = Inside diameter of pipe
∆h = Total deflection of pipe at the vessel relative to the fixed point.
Total deflection equals the load cell deflection plus support
deflection (see Appendix 7 for load cell deflection data).
E = Young’s modulus
L = Length of pipe from the vessel to the first support point
The value of E (Young’s modulus) varies for different types of material. Three
common values are listed below:
• Carbon Steel = 29,000,000 pounds/inch
2
(29 × 10
6
)
• Stainless Steel = 28,000,000 pounds/inch
2
(28 × 10
6
)
• Aluminum = 10,000,000 pounds/inch
2
(10 × 10
6
)
The equation assumes a rigid connection at both ends of the piping, which is
generally conservative. Use it to calculate the force exerted by each attached pipe.
Then add those forces to determine the total resultant force (F ) exerted by all the
piping.
Once you have calculated the resultant force, compare it to the following
relationship:
F ≤ 0.1 × System Accuracy (in %) × Live Load (pounds)
where:
For 0.1% System Accuracy, F ≤ 1% of Live Load
For 0.25% System Accuracy, F ≤ 2.5% of Live Load
For 0.50% System Accuracy, F ≤ 5% of Live Load
For 1.0% System Accuracy, F ≤ 10% of Live Load
If the resultant force satisfies this relationship, then the force exerted by the piping
is small enough that you can compensate for it during calibration.
F
P
=