The Care and Feeding of your Weigh Cells

Load Cells Are Super Accurate, but Finicky. Here Are Some of the Biggest Causes of Problems and How to Avoid Them

By Nancy Bartels

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Figure 2b illustrates a good load cell's installation; the legs from the tank are shorter, minimizing separate leg movement, and the load cell has a top and bottom frame to help induce a more uniform loading pattern. These figures show a common error; each weighing application is different.

One of the most overlooked factors is the vessel's peripheral connections and accessories, such as pipes, mixers, ladders, walkways, etc.

"Each of these external fixtures influences the weighing system. Pipes entering and exiting a vessel are always exerting forces on it. Under stable, ambient conditions; e.g., constant temperature, pipes and nozzles don't create serious problems for weigh cells. But thermal expansion and movement will adversely affect weighing results," says Titmas.

Figure 3 illustrates the possible forces and displacements of piping: A rigid pipe can lift some weight off the vessel and cause a displacement (dP). A force Fp along the pipe can push the vessel to the side and exert forces downward or vise versa.

By carefully considering pipe supports and nozzle design, it may be possible to reduce pipe problems. A simple way to dampen the effects of pipes on weighing measurements is to have all pipe connections enter from the side, advises Titmas. He recommends that if possible, pipe hangers should be placed as far away from the tank as possible to allow for flexing as the tank moves up and down during loading. "Rollers may help, but they will eventually fail and cause problems," he adds.

Some movement can always be expected. Flexible pipe connections and bellows may be the best solution. These connectors should lessen some of the influence of pipe forces on the vessel and moderate pipe fatigue. However, ensure that the flexible connections are truly flexible.

Installation

Once you ensure that your tank is properly designed and all pipe connections are flexible, the next step is to select the correct weighing system. Although shear beams and bending beams can be used, Titmas recommends compression-type load cells. Unlike shear-beam or bending beam, compression load cells are designed to eliminate the introduction of non-vertical loads. This is accomplished by carefully engineering a curved top and bottom. These rounded components allow the cells to "right" themselves during side loading by a principle of restoring forces.

Figure 4
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Mounting kits
Figure 4. The proper installation for turnbuckles assures
equal weight distribution between weigh cells.

Compression cells are designed to shift during vessel expansion and are constrained mechanically to eliminate the inluences of other non-vertical forces.

"A compression load cell shouldn't be bolted to the floor or to the sub-framing like typical beam load cells," says Titmas. "All external forces, including from mixers and thermal expansion, can't 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.

Mounting kits only perform well if aligned correctly to absorb side forces. Figure 4 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, a necessary configuration to avoid unnecessary axial binding. In this way, each turnbuckle shares the load equally across the entire base of the vessel.

Figure 5
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Thermal expansion
Figure 5. A 230 °C temperature change causes a significant
change in the weigh cell reading.

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 5,  a stainless steel vessel of AISI304/1.4301 material. 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) = 1.6 mm/m × 230 °C/100 °K = 3.68 mm/m; and

Expansion of the radius = 1.25 meters × 3.68 mm/m = 4.6 mm.

Figure 6
Proper installation
Figure 6. This unit typifies the application
of a weigh cell

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, but with limited success. "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," says Titmas. "Installing load cells that can mechanically expand with the vessel and ameliorate the influence of external side forces are the best solutions" (Figure 6).

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 cost thousands of dollars in lost product and wasted time.

Titmas' last piece of advice: "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."  

[Editor's note: This article is adapted from "Waylay Weight Woes" by Ryan Titmas, Chemical Processing, Jan.'07]

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