Waylay weight woes

Production managers, quality control managers and maintenance engineers alike have encountered weigh cells that never worked properly. Why? Why have so many switched from a supposedly high accuracy load cell solution to old fashioned level probes or sight glasses? This article touches on the worst culprits for problems and how to avoid them.

By Ryan Titmas, product manager, Sartorius North America

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Figure 6. Compression weigh cells are generally the choice for resisting influence of non-vertical forces.

A compression load cell shouldn’t be bolted to the floor or to sub-framing like typical beam load cells. All external forces, including from mixers and thermal expansion, cannot be isolated from beam-type load cells. The interference from these forces can be effectively eliminated with the proper installation of compression cells.

One of the tools available with compression cells is the mechanical restraint. Mechanical restraints are simple devices though they are capable of taking care of most complex side loading influences when used properly. These devices are generally installed on the sides of the load-cell mounting kits. They are similar in many ways to classic turnbuckle assemblies. These turnbuckle assemblies absorb non-vertical forces from direct side loading, e.g., wind, and torque forces created from vertically-mounted mixers.

Figure 7 is a typical compression-cell mounting kit with an integrated turnbuckle.

Figure 7. The proper installation for turnbuckles assures equal weight distribution between weigh cells.

Figure 7. The proper installation for turnbuckles assures equal weight distribution between weigh cells.

Mounting kits only perform well if aligned correctly to absorb side forces. Figure 7 illustrates how to properly install mounting kits in both three-legged and four-legged vessels. As you can see, the turnbuckle assemblies are tangential to the tank in three-leg applications. The four-legged installation is usually more stable than the three-legged system. This type of turnbuckle arrangement is used to absorb torque forces from mixers and distribute side forces equally among the constrainers. Mounts in the four-leg tanks are installed so that no one turnbuckle points toward another. This configuration is necessary to avoid unnecessary axial binding. In this way, each turnbuckle shares the load equally across the entire base of the vessel.

Let’s consider one final issue: thermal expansion of the tank. Thermal expansion and contraction occurs for a variety of different reasons, the most common being: jacket heating or cooling, environmental or diurnal temperature changes, and the addition of hot or cold fluids to the vessel. Were a shear beam or bending beam to be used there would be no physical way to prevent vessel expansion or contraction from inducing a load on the cells.

For example, with this type of installation, heating would cause the vessel walls to press outward on the load cells. Stress would create a force vector in the downward direction, registering as a weight change on the cells. This expansion will cause drastic, unrepeatable, weight changes from a few pounds or kilograms up to hundreds of kilograms.

Consider Figure 8, a stainless steel vessel of AISI304/1.4301 material.

Figure 8. A 230°C temperature change causes a significant change in the weigh cell reading.

Figure 8. A 230°C temperature change causes a significant change in the weigh cell reading.

The weigh system is calibrated under ambient conditions (20°C) but operated at a temperature of 250°C when the jacket heating system is running. The engineering calculations are presented below:

Radius of the tank (R) = 1.25 m
Expansion coefficient (k) = 1.6 mm/m/100°K (different depending on material!)
Temperature Difference = 250°C – 20°C = 230°C
Strain (ΣT) = 1.6mm/m × 230°C/100°K = 3.68 mm/m
Expansion of the radius = 1.25 meters × 3.68 mm/m = 4.6 mm

The expansion of the tank 4.6 mm in a shear beam weighing applications will produce severe weighing errors. Compression load cells are not physically bolted to the floor, but rather are floating and allowed to shift slightly when the tank expands. This type of cell, if properly installed, avoids the inaccuracies of thermal expansion.

Some homemade remedies have been tried to compensate for thermal expansion. Blowing cool air on load cells, wrapping insulation around load cells, or dowsing cells with water are not adequate solutions for the effects of tank expansion on load cells. Installing load cells that can mechanically expand with the vessel and, secondly, ameliorating the influence of external side forces are the best solutions (Figure 9).

Figure 9. A typical application for a weigh cell.

Money talks

No matter the industry, weighing accuracy equals money, and in most cases it equals lots of money. Operating a plant with vessels that don’t weigh properly can cost time and plant efficiency, which equates to a cost. Inaccurate tank weighing can literally cost thousands of dollars in lost product and wasted time. It is critical to consult with an experienced tank weighing company, familiar with not just load cell specifications, but also the engineering behind the system.

Ryan Titmas is a product manager at Sartorius North America in Edgewood, N.Y.; e-mail him at ryan.titmas@sartorius.com.

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