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Gas Process Automation

Many relatively simple industrial or laboratory processes that require control of flow or pressure can be greatly improved with automation.

by Sierra Instruments
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Problem: Many relatively simple industrial or laboratory processes that require control of flow or pressure can be greatly improved with automation. Using a mass flow controller to control a certain gas flow rate, for instance, is far more precise and has a faster speed of response than manually adjusting a needle valve while consulting a pressure gage.

Such processes can be automated using devices called Programmed Logic Controllers (PLC’s). These are digital computers used for automation of electromechanical processes. Such devices typically have a series of digital and analog inputs and outputs, and can be used to control relatively complex processes depending upon the number of these I/O channels and the programmed logic. For simple gas processes, PLC’s are generally overkill—being too expensive due to extra capabilities that are just not required.

Sierra Instruments' Compod™ + Smart-Trak® 2

Solution: Sierra’s Compod™ bridges the gap between manual operation and more complex PLC systems to enable simple gas processes like gas mixing and blending, batch control, leak testing, and process monitoring at a fraction of the cost. A Compod, coupled with Sierra’s Smart-Trak® mass flow meters (MFM’s) and controllers (MFC’s), greatly simplifies basic flow control installations and permits networking of multiple instruments using opensource MODBUS RTU protocol. With MODBUS, multiple instruments can be daisy-chained over a single network. Two digital outputs and one analog input can be configured by the end user for a wide variety of process controls. Unlike dedicated PLC’s, a Compod is a small device that mounts locally to the face of the Smart-Trak MFC.

One example involves leak testing. In a leak testing process, a component must be checked for leakage. A regulator is set manually to the leak-test pressure. This is usually a pressure above operating pressure of the DUT (device under test). A Smart-Trak MFC is given a setpoint appropriate to fill the DUT at a moderate rate. The MFC’s Compod monitors the flow rate. When the flow rate drops to zero, the pressure in the DUT is equal the pressure set by the regulator. The internal software in Compod senses that the flow has stopped and the Compod closes the valve in the MFC and trips an alarm (light) for the operator, telling him the DUT is pressurized and the leak test is ready to begin. The operator sees the alarm light and opens a ball valve manually. The leak test begins and a Smart-Trak mass flow meter with a Compod measures the flow rate. As long as the flow rate is below a pre-determined level, the DUT passes the test. If the flow rate is above this defined level, the Compod sends an alarm indicating the DUT is leaking and the DUT is failed.

Another use is in gas mixing and blending. In a typical gas mixing and blending process, a specific gas blend must be produced. For example, a synthetic air composition of 78% Nitrogen, 21% Oxygen and 1% Argon is required. A miniature Human/ Machine Interface (HMI) with LCD touch screen monitors and controls the system automatically. The HMI communicates to a pressure transducer mounted inside the tank and three Smart-Trak mass flow controllers, each with a Compod. Each MFC regulates the flow of one gas into the tank. The HMI maintains the exact proportion defined above by constantly monitoring each MFC. When the pressure reaches the set point, the HMI commands the Compod to stop the flow controllers. Should the pressure drop below the set point, the HMI sends a flow command to each controller to resume gas flow into the tank.

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