Using Flow Sensors as Diagnostic Tools

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Equipment and Processing Report

Equipment and Processing Report, Equipment and Processing Report-10-15-2014, Volume 0, Issue 0

Changes in compressed air consumption are an early indicator of problems that need correction.

Compressed air is used throughout the pharmaceutical manufacturing facility to actuate valves, manipulate products, or pressurize vessels in processes from API production to secondary packaging equipment and at every process in between. Proper monitoring of compressed airflow and consumption can tell operations personnel much about the health of the system and provide specific and critical diagnostic information before larger problems occur. Monitoring also provides necessary data to ensure that long-term energy costs in the plant do not mysteriously increase.

Most applications or skids used in pharmaceutical production and packaging have an air preparation assembly on the front end to regulate pressure, filter the air to cleanliness levels required by FDA, provide a manual on/off valve, and monitor the system pressure. The pressure sensor in this assembly will alert the control system if there is a pressure drop that will cause actuation components to not perform properly or a pressure spike that can result in component or instrument damage.

Flow is related to but independent of the principles of air pressure. Although air pressure determines the amount of force or torque at the field device, the proper volume of air must be supplied to reach and maintain the pressure. Volumetric flow, however, is seldom measured in pharmaceutical plants today.

A leaking seal inside an actuator or a broken tube might not be enough to cause a pressure drop and can easily go undetected. Without a flow sensor, the problem may not be realized until there is a further or complete failure of the component and a downstream process fails. A pharmaceutical plant can have as many as 5000 pneumatic solenoid valves controlling processes and more than 5 miles of tubing, so there is a lot of opportunity to improve this analysis. 

Air loss as failure indicator
In its simplest use, a flow sensor, similarly to a pressure sensor, will provide a digital signal to the controller when its limits are exceeded. The flow sensor complements the pressure sensor well in this basic capacity because it ensures the equipment has the required airflow to function as designed.

Analog data provides an important tool for improving the heath of a system. It is possible to monitor the flow rate of a given process or timeframe and do comparative analysis.  This information is available to the controller, where it can be locally analyzed, displayed, maintained in a data log, used to create an alert, or passed on through the supervisory-control and data-acquisition system for further analysis.  


An increase in compressed air consumption can be an early indication of:

  • Leaking seals at a quarter-turn actuator or pneumatic-linear actuator

  • Early-stage cracks in piping or welds

  • Failing flexible tubing or fittings

  • Solenoid valves not closing properly

  • Failing pneumatic pumps

  • Problems with a nitrogen air blanket

  • Faulty vessel seals

Investigating the source of this consumption increase early on and correcting the problem will improve the performance and uptime of all automated plant equipment.

For further information, read the full article, Using Flow Sensors to Monitor Process System Health, in the upcoming November issue of Pharmaceutical Technology.

Craig Correia is head of Process Automation at Festo USA,