At Holland, we’ve been helping our customer create successful batching and custody transfer applications for over 60 years. In this post, we’ll take a closer look at two different ways we can measure mass, Coriolis flow meters and load cells. We will look at the benefits and drawbacks to each technology.
Load Cells Mount Under Vessels and Measure Weight
Load cells sense force and are coupled with scale electronics that convert this signal to weight (pounds force). Load cells can be mounted to a platform in a truck or railcar for custody transfer. In batch or blend systems, they are mounted to a mix tank or vessel. The weight of the product is calculated by subtracting the empty weight of the mechanical system (tare) from the full weight (gross). The force measured by the load cell includes both vessel weight and raw material weight.
Load cells have been used for many years in the process industry. Most engineers and technicians are familiar with them and understand how to use and apply them. Load cells are an excellent choice when dry products are to be measured or where large quantities are being measured. Calibration is also well understood for load cells. Weights and Measures organizations can be relied upon to select reference standards and calibrate existing systems that fall out of balance.
That being said, there are certain challenges and drawbacks inherent to load cells. First and foremost, we need to make sure our load cells are isolated from the surrounding system. Rigidity of the support structure (load cells necessitate flexible hose connections), external vibration from other processing equipment, or even internal sloshing from mixers can affect process measurement.
Load cells also limit the number of ingredients that can be added to a batch process at a single time. Load cells are dumb. They can only measure total vessel weight and are generally only mounted on the receiving vessel, not the bags or totes feeding the process. This makes multiple product addition impossible and effectively limits our processing capacity by increasing batch time.
Micro Motion Sanitary Coriolis Flow Meter
Coriolis meters directly measure the mass flow rate of liquids, slurries, and gases. Mass flow rate is unaffected by changes in fluid composition or density. Mass meters also directly measure product density and temperature. The rate measured can be totalized to determine the specific amount of process fluid being added to a vessel during a batch. This allows us to add multiple products simultaneously to the receiving vessel.
They can handle a wide range of process fluids and viscosities and also have relatively few flow conditioning considerations. This means that simple good piping practices, such as adequate support, vertical mounting consideration, and limiting induced stress or torque, will result in stable, accurate system measurement.
They are excellent in dosing or metering applications where we need to account for every last drop. They have accuracies of +/-0.05% and can measure flows as low as 0.04 lb/min. Because they are taking a weight measurement, they are excellent for custody transfer. Milk is often paid for by pound and milk’s density can vary with temperature, which will skew traditional volumetric flow meters. When it comes to custody transfer, you want to make sure you only pay for what you get.
That being said, there are drawbacks to Coriolis meters as well. Meter performance can be impacted by two-phase or “slug flow”. They are also constructed of thin-walled metal tubing and material selection and process fluid compatibility must be taken into consideration when selecting a meter. There is a limited turndown with these meters and high upfront cost can make multiple meter purchase a significant investment.
It is a newer process technology, carries a high upfront cost, and is not as well understood by technicians. Calibration is often costly and involves removing the meter from the process. Most units do, however, have intrinsic mechanisms that let you know when a meter is not functioning properly. The high upfront cost is also mitigated by the ease of installation and reduced structural support necessary.
Tube design inherently creates a restriction to fluid flow and pressure loss which must be compensated for. Only select models are available with a straight through design which raises drainability and cleaning concerns.
Load Cells vs. Coriolis Meters- Which should I use?
In short, it depends. Let us explain. If you’re measuring bulk quantities of dry goods in an environmentally neutral condition and don’t need to be extremely precise (better than 0.1 lbs), then you’re fine with load cells. As mentioned previously, they are straight forward and well understood. Piping rigidity can be compensated for with the use of flexible hoses.
If this is a dosing or extremely precise metering application with low flow, we would guide you towards a Coriolis meter. As mentioned previously, Coriolis meters allow for multiple product addition which can decrease batch times and effectively increase system capacity without augmenting any auxiliary equipment. This presents ROI opportunities and helps justify the high upfront cost.
Ultimately, the best measurement technology should be selected on a case by case technology. At Holland, we’ve been helping our customers deal with problems like these on a day-in-day-out basis for over sixty years. Contact Holland today and we’ll get you pointed in the right direction.