Rice Lake 920i Digital Scale Controller Custom Weighing Systems

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RICE LAKE 920i CASE STUDY

Rice Lake’s 920i Continues to Allow Companies to Operate at Peak Efficiency

123 Corp manufactures and installs custom liquid and industrial gas distribution systems for the specialty and industrial gas filling industries. Their customers include distributors of industrial gasses, such as welding gasses; and distributors of medical gasses, oxygen and nitrogen for hospitals and home health care agencies.

123 also designs, manufactures, services and repairs cryogenic pumps, vaporizers, manifolds and control systems. Distribution of atmospheric gasses as cryogenic liquids is the most cost efficient way to deliver the product. Cryogens are dense liquids that allow the most molecules to be delivered in a transport. Cryogenic liquidsare transported from air separation production plants to 123's onsite bulk tanks where they are transferred to smaller cryogenic vessels, or converted to gasses and compressed to a higher pressure, typically 2,400 pounds per square inch gauge (psig) in gas cylinders.

According to one of the sales engineers, 123 Corps liquid transfill systems transfer cryogenic liquid argon, nitrogen, oxygen, and some other rarer gases into 180 to 265 liter liquid cylinders. This is accomplished by two methods: gravity/pressure transfilling and transfi lling utilizing cryogenic centrifugal pumps. Liquid cylinders are rated and stamped by the Department of Transportation, and must be filled by weight to avoid overfilling the DOT rating for the cylinder. Each cryogenic liquid has a different molecular weight, as well as temperature at which it exists as a cryogenic liquid. The gross weight of a typical 160 liter cylinder is 517 pounds in nitrogen service, or 710 pounds in argon service.

Liquid cylinders are designed to hold the rated weight and allow for expansion of the liquid as it warms over time. When a liquid cylinder is overfilled, the cylinder vents prematurely as it warms. Typically, a correctly filled liquid cylinder has a net evaporation rate (NER) of about 2 percent per day. Liquid cylinders are most commonly filled by the gravity/pressure method. This method utilizes the weight of the product in the bulk container (typically 1,500 to 13,000 gallon) as well as the pressure differential between the two vessels to accomplish transfer. The pressure differential is created by allowing the liquid to enter the liquid cylinder at a lower pressure by venting to atmosphere the gas at the top. The greater the pressure differential between the two vessels, the faster fi lling occurs. Filling by this method causes losses from venting called flash losses. A pressure differential of 100 psig is not uncommon and results in losses as much as 20 percent.

How Rice Lake’s 920i automates Multi-Scale,
Multi-Gas Gravity/Pressure Filling: The company uses the Rice Lake 920i digital programmable scale controller to control several two to five scale platforms. Each platform is capable of filling several different liquids; oxygen, nitrogen and argon. Mixing is rarely done. To start the fill process on the 920i, the user selects the liquid to be filled on a particular scale, enters the gross weight the container is designed to hold, and presses a start button which opens a filling valve. When the setpoint is reached, the valve closes, and the 920i indicates that the cylinder is full.

Human errors are still possible since cylinders can be partially filled with residual product. As a result, each container has a different tare weight. If a weight is entered that is greater than what the container can hold, the cylinder keeps filling and a lot of product is wasted venting to the atmosphere. The 920i enabled 123 corp to add an extra safety precaution. The 920i is programmed to read a thermocouple that senses the vent temperature in the cylinder. If it reaches cryogenic temperature, the 920i closes the valve. And a lot sooner than it would take a human operator to notice a problem.

How the 920i automates
Centrifugal Pump filling: In applications that require large quantities of filling at a fast rate, a cryogenic centrifugal pump is used. Cryogenic pumps can fill liquid cylinders at a rate of 25 gallons per minute (GPM). Traditional centrifugal pump filling incurs losses because, again, the gas is vented on the liquid cylinder during the filling process to keep the pump on its performance curve and avoid cavitation.

The 920i HMI attends to several more tasks that were once left to human attendants alone. If the liquid cylinder is warm, a result of not being filled in a long time, the pressure in the cylinder will rise to the point that the filling process has to stop to prevent the pump from cavitating. The 920i automatically puts the pump in bypass mode while the liquid cylinder is vented to the proper pressure, then restarts filling. The 920i automates priming and cool-down, bypass recirculation and controls processes including thermocouple input, 4-20 milliamps (mA) pressuretransducer input, and flow meters.

123 corp executives consider cost a main reason for switching to the 920i, PLC, scale indicator/controller and operator interface are all included in one package with the 920i. In the past, companies did not have a visual operator interface. The ability of the 920i to visually show the process on a flow sheet, what valves are open, what temperatures are at various points in the system, is a huge benefit. And it is all done seamlessly with one component.

How the 920i automates Custom Data
Acquisition and User Control Filling: A request from a large university was also accomplished using the 920i. The university had two locations which had bulk storage tanks that were shared by different department laboratories. The university wanted an accurate account of how much each lab was using with a more traceable method than a piece of paper filled out and handed in by students. They wanted the ability to allow only defined users, with passwords assigned by departments, to fill the liquid containers. The 920i collected the fill data, converted from pounds to standard cubic feet (SCF) in one case. In another case, a scale was not practical, so a flow meter was used and the cylinder was determined to be filled when cryogenic temperatures were detected on the vent line. The 920i enabled the administration to assign password protected accounts and print user reports for each department as well as add or remove users.

Hopefully after reading all this information, you realize how powerful the Rice Lake 920i controller is. Please click below if you need additional info or need a custom programmed system.

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