Using Flow Sensors to Monitor Process System Health

Published on: 
Pharmaceutical Technology, Pharmaceutical Technology-11-02-2014, Volume 38, Issue 11

Using flow sensors, operators can monitor compressed-air use to help identify problems early and provides data for improving energy efficiency.

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 crucial 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 (see Figure 1) 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 valve controlling processes and more than five miles of tubing, so there is a lot of opportunity to improve this analysis.

Flow sensor function
Flow sensors are available as stand-alone products that can be mounted inline in an existing tube or pipe (see Figure 2a) or integrated into an air-preparation assembly with the filter and regulator (see Figure 2b). The flow sensor will send digital and analog signals back to the controller for analysis, data collection, or display on a touch screen. Most are freely programmable to set threshold values, window comparators, and hysteresis directly on the sensor. Flow sensors measure flow rates and/or volumetric air consumption. Flow sensors for compressed air are also suitable for use with nitrogen, thus allowing them to be used to monitor vessel sealing and blankets.

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.

Benchmarking efficiency
Industry is increasingly considering energy efficiency, and compressed air is recognized as a critical energy resource. Large companies are putting reduction targets into project objectives, which impacts equipment selection. Equipment manufacturers desire to quantify how new models have gained efficiency.

Flow sensors make air consumption transparent. Using the cumulative air consumption operating mode, the sensor works like a meter and it is possible to measure the volume of air consumption for a given process or over time. If, for example, a process is consuming 15% more compressed air compared to two years prior, the plant has gained a critical piece of diagnostic information that can help guide corrective and energy-savings actions.

Selecting a flow meter
When selecting and integrating a flow sensor, several points should be considered, as follows:

  • Determine where in the process you want to measure the airflow or consumption to help determine whether it should be integrated into an air preparation unit or mounted in-line downstream. To measure flow into equipment, a flow meter with high capacity, such as the flow meter shown in Figure 3 (Festo, SFAM), is typically installed inline.

  • Calculate the range of flow that will be measured. Flow sensors have different ranges, and selecting a sensor with wide band will impact data accuracy.

  • Decide if local display is required.

  • Define why and how the flow data will be used and select a sensor capable of providing flow rate, volumetric consumption, comparative flow analysis, or some combination.

  • Define pipe or tube size when the sensor will be mounted in-line.

  • Determine if electrical signals should use PNP or NPN-type configurations.

  • Calculate the number of analog and digital inputs required based on the number of flow meters and ensure these inputs are available.

In addition, users should create a functional description of how the sensor will be used and what type of analysis is expected so changes can be made at the control and programming level. Looking ahead, once this analysis is available, set up a plan on how it will be used. Data can be reviewed as part of preventative maintenance programs. Certain alerts can force operations personnel to investigate further.

Particularly with the energy conservation effort, once the information is available, the question shifts to “What do we do with this information?” and “How do we implement the solutions?” Until these steps are completed, the benefit of the flow sensor is not fully realized.

Costs of compressed air in North America are about $0.30 per 1000 scf.   This means a single leak can cost more than $3000 in annual electricity cost. A single flow sensor and a successful preventive maintenance program can have a payback period of a few weeks.

Whether designing a single piece of equipment or a complete facility, proper use of a flow sensor to monitor compressed air consumption can provide crucial data to increase uptime, improve troubleshooting of the process, and help manage long-term energy costs. Although it is easier to integrate flow sensors when equipment is initially designed, these components can be easily added to existing installations. To achieve the maximum benefit, a strong emphasis must be placed on follow through and preventative maintenance based on the data.

About the Author
Craig Correiais head of Process Automation at Festo USA,