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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Using load cells to accurately measure the content of tanks seems like a good idea. Yet at many facilities, they have been disconnected. They are left in place and the electrical cables connecting to the controls are cut because they never worked properly. Why? Several reasons. Ryan Titmas, product manager at Sartorius North America, Bohemia, N.Y., a manufacturer of weighing and liquid handling systems, outlines "the worst culprits."

Let's begin with a focus on the shear beam load cell. The alignment of the force vectors is important; these sensors can't discriminate against true vertical load and side/torque loading. You want to measure the true vertical load; i.e., the weight within the tank—not the shear loads. However, real-world conditions or improper installation often cause the load cells to see much more than vertical loading. These other factors cause the poor repeatability in weighing that ultimately leads to abandonment of load systems.

How a Shear Beam Works

A shear beam load cell is a precision-milled piece of metal that includes several strain gauges. These gauges are positioned at a 45° angle to measure the steel shear strain. They are "the heart of the weigh cell," says Titmas.
Strain gauges are zigzag-shaped strip conductors, etched from thin metal film. They operate on the principle that the electrical resistance of a conductor will increase when it is subjected to mechanical stress. This mechanical stress is caused by tiny changes in the film length and cross-section. The resulting voltage change is very slight; to measure it accurately, a Wheatstone bridge or some other familiar amplifying circuit is used.

The body of the load cell is usually constructed of mild steel, stainless steel alloy or aluminum. A typical load cell will undergo thousands of weighing cycles, so the composition of the metal is very critical to producing reproducible weighing results. A shear beam load cell is actually measuring the shear strain of steel, so if the steel itself isn't monolithic throughout, errors in weighing will occur. In general, it is critical that the load cell manufacturer obtain steel and aluminum that is high-grade to produce the most reliable installation.

Titmas points out that bending beam and shear beam load cells are the industry standard for bench and floor scales. Bending beam load cells are perfect for environments where static weighing takes place; they aren't intended for a process vessel. Weighing a vessel requires a load cell and mechanical support system with a high tolerance of non-vertical loading and the capability to make producible measurements.

Start with the Tank

A weighing application will never be successful without proper attention to the tank. The most important factor to consider when designing for a weigh cell is the construction of the weighing frame. The frame is where the load cells will be mounted. Whether the cell is a shear beam, bending beam or compression type does not matter. This structural steel is mechanically connected to all the load cells under the vessel to ensure that they can't move in different directions independently of each other. Because we're only interested in vertical compression forces, the design must resist external forces that could cause the load cells to tilt.

Figure 1 shows the best type of installation. Notice how the load cells are placed in the middle of the beams. Each cell is situated between two frames. This frame network ensures the load cells will move together, creating a more repeatable weighing environment. In Figure 1, the load cells are mounted on top of a bottom frame assembly that provides a rigid foundation. The weight of the vessel is being transferred to the load cells and then to the bottom frame. This transfer of force occurs with minimal deflection. Although the load cells can be mounted directly to the floor without the bottom frame, the floor must allow little deflection and settling.

"The load cells are measuring deflection down to thousands of a millimeter, so if your foundation deflects, you aren't going to achieve acceptable accuracy. Most architectural designs allow a floor deflection that would be unacceptable for weigh cell operation," says Titmas.

Figure 2a illustrates another typical design problem found in tank weighing. It shows load cells installed on the end of very long tank legs. In this example, errors are due to the legs moving in different directions. Measurements become unrepeatable as extraneous deflection causes variability in movement.

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
CLICK ON IMAGE TO ENLARGE IT
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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