Weighing precision is paramount when it comes to laboratory scales or adding micro ingredients to reaction vessels, but what about at the other end of the scale? Is weighing the best option for monitoring the content of silos and oversized vessels, where stock control often takes priority?
“Weighing is probably the most accurate way of measuring anything,” says Rod Morgan of Applied Weighing. “When you’re talking about the contents of a vessel, weighing is always accurate, even if the level is uneven [when measuring solids], or if the density of the contents varies. Weighing is always correct, provided it’s set up correctly.”
“Consideration has to be given to the fundamental difference between accuracy, resolution and repeatability. Customers often misunderstand these parameters and how they may relate to their specific application,” agrees Nick Burley, UK sales manager with Weightron Bilanciai, who also believes that weighing has some basic advantages over rival techniques based around level measurement.
“Techniques for measuring level are quite sophisticated these days and level control is pretty accurate for liquids, although there are a number of things that can affect it like temperature and pressure. But the thing with weighing is that it doesn’t matter what’s happening within the vessel. It will always give you an accurate result if installed correctly.”
“The fact that weighing is non-invasive is also very important,” adds Morgan. “Level systems can be affected by what’s going on in the vessel, by dust in the case of radar, for example. Even with level systems that should be non-contact like radar, a piece of metal could drop into the product and contaminate it, whereas weighing is all outside the silo and there’s no danger of a foreign body contaminating the product.”
When you’re talking about the contents of a vessel, weighing is always accurate, even if the level is uneven [when measuring solids], or if the density of the contents varies. Weighing is always correct, provided it’s set up correctly
Rod Morgan, Applied Weighing
On the other hand, the biggest drawback of weighing is that systems can be difficult to fit unless they are installed at the time the silo is first built.
“The main disadvantage with weighing is where clients come to us as an afterthought – many years after the silo was installed perhaps – and then it’s quite a significant job to incorporate weighing into the surrounding structures,” says Morgan.
“Some people have the foresight to put blocks where they might insert load cells in the future, but others don’t and they’re left with less accurate alternatives if they decide later on that they have to measure the contents of the vessel.”
Burley confirms that retrofitting weighing systems can be expensive and can even lead to structural problems if not carried out with care: “Weighing large vessels is fine if it’s designed in from the beginning, but if you’ve got existing vessels they may be supported or linked on a structure that may not be strong enough to fit load cells to it (you have to cut into the structure somewhere to fit them). Maintaining structural integrity is absolutely vital.”
He also cautions that existing structures may include pipework or other elements that can act as ‘force shunts’ or springs that impact on the apparent weight registered by the load cells: “Load cells are good at measuring load changes but they’re very good at measuring the things you don’t want to measure, as well as the things you do. If it’s designed right from the outset then you can consider all the things that might affect the results.”
According to Avery Weigh-Tronix senior technical manager Phil Tomlinson, even projects to install new silos can sometimes make the mistake of leaving weighing out of the loop until relatively late in the day: “Too often when a customer is installing a new vessel, they don’t involve anybody who knows about weighing until it’s on site. But one good thing to do is to involve the weighing company from the outset.”
So, for example, the designer may specify a large vessel with four legs. While this is certainly doable for weighing systems, it does raise the possibility of poor mounting leading to the vessel not resting properly on all four of the load cells.
In contrast, a three-legged vessel will naturally distribute the load effectively among the load cells and improves the stability of the system. “A silo with four legs may in fact only be sitting on three load cells. Three load points is a far better solution,” says Tomlinson.
Provided a system is designed, installed and commissioned correctly, weighing is a reliable, accurate and repeatable solution for monitoring the content of a silo. However, there are several factors to bear in mind in order to maintain good results in the longer term.
Take calibration for instance. It’s relatively straightforward to verify whether a system is measuring what goes in or out correctly.
Adding a 20-tonne tanker or lorry-load should register a 20-tonne change in the vessel and that can be verified by checking against a separate system based on flow metering or on a separate weigh bridge.
Too often when a customer is installing a new vessel, they don’t involve anybody who knows about weighing until it’s on site. But one good thing to do is to involve the weighing company from the outset
Phil Tomlinson, senior technical manager, Avery Weigh-Tronix
In addition, modern direct force calibration systems using hydraulic jacks to lift the weight of the vessel or apply a known load to the load cells can make calibration even quicker and easier.
“You can jack the vessel up or pull the vessel down by a known amount,” says Tomlinson. “The vessel doesn’t have to be empty.” Having jacking points designed in also makes it easier to swap out a defective load cell if necessary.
What’s trickier is to check whether the load cells’ zero point has drifted over time, because many of these massive vessels go for years without ever being emptied. However, that’s not a problem in most applications, where users are rarely looking to run stocks right down.
As Morgan explains: “For stockholding, they’re not usually interested in whether there’s 100 or 200 kg left at the bottom of the silo. They’re more interested in whether the silo is reasonably full or nearly empty. If you get down to the last 100 kg in the silo they’ve really left it to the last minute – they’d probably fill it long before then.”
“It may not be a big problem with many silos because they may just want to know what goes in and out over a relatively short period,” agrees Burley. “Even if the zero moves slightly, the measurement of difference will still be accurate.”
Housekeeping is vital
Good housekeeping is another vital component in maintaining long-term performance in these weighing systems, especially since they are often operating in outdoor, relatively uncontrolled environments.
“People tend to think they can fit and forget load cells. That might be OK in the pristine, regulated environment of the pharmaceutical industry, but if you’ve got a big, outdoor silo, regular visual inspection and good housekeeping is vital,” says Burley.
For example, Morgan suggests that particular attention should be paid in dusty environments where load cell assemblies can become caked with product: “Some people think that load cells are completely static, but actually they work by flexing a tiny amount. If they are caked with hardened deposits they may not be able to move. Regular cleaning and good housekeeping is absolutely critical.”
Other challenges of outdoor operations, such as installation on a windy site, can be overcome through the use of electronic filters that clean up the signal and the incorporation of anti-liftoff devices to prevent silos from blowing away during storms.
In summary, weighing is a good option for monitoring the contents of silos and large vessels, especially since it’s impervious to many of the challenges affecting competing systems based on level measurement.
However, if the user wants to ensure they have the most cost-effective and reliable solution, they should design the weighing system into the overall structure from the start. They also need to be wary of making any subsequent changes – adding pipework or insulation, for example – without fully appreciating the possible impact.