An Overview of Flow Computers and Their Function
Flow computers are essential components of large scale systems with multiple flows that need to be continuously analyzed and managed. They automate the data collection and data management processes so that operators can get a snapshot of the system status and make decisions quickly.
Flow computers also provide a means to calculate corrected values and interpret data from compensated flow meters. Compensated flow meters are those that need a certain amount of pressure to accurately measure flow rate. If the flow meter does not receive the required pressure, the flow rate reading can vary significantly from the actual flow rate within the pipe.
Uncompensated flow meters, also known as non-compensated flow meters, do not rely on specific pressure values and usually work within a defined pressure range. For applications where pressure is consistent, compensated flow meters work fine. For all other applications where pressure varies within a range, uncompensated flow meters should be used. Regardless, flow computers can collect data from compensated or uncompensated instruments and identify the actual flow rate by performing calculations.
Data Collection, Calculation and Recording
Flow computers collect analog and digital data from one or more flow meters and other sensors in a system. Such data can include:
- Flow rate
This information is recorded in the computer so that data trends over time can be evaluated. Some of this information can also be derived from certain parameters and inherent properties of the flow material. For example, flow temperature is often indicative of the flow density, which is used to calculate the specific gravity of the flow. If the temperature of the flow is measured and the density of the fluid is known from a manufacturer’s data sheet, then the computer can calculate the flow’s specific gravity without additional instrumentation. Viscosity can also be calculated as a function of flow temperature.
Temperature data is used to derive the corrected volume flow, which accounts for thermal expansion within the pipe. Density readings can also be used to calculate the corrected volume flow if the flow density and the fluid’s reference density are known. The corrected volume flow is then used to calculate the mass flow.
In addition, flow computers approximate data from nonlinear flow meter signals to create linearity curves. Such curves show whether the flow meter is operating within its design limitations. Off-reading graphs can be created to identify flow rates at which the flow meter falls outside its defined limits.
Some flow computer models are programmed to totalize flow values and output the fluid volume sum. Corrected totalized values can be calculated to account for thermal expansion. The corrected sum volume flow is then used to calculate the sum mass flow.
Here is a general sequence of calculations that flow computers execute:
- Collect flow and other sensor data.
- Calculate required flow parameters such as density and viscosity.
- Linearize the data and calculate the volume flow, which is also the uncompensated flow.
- Calculate the corrected volume flow if required.
- Calculate the mass flow rate.
- Totalize the flow and volume.
The final values from each calculation are displayed on the flow computer screen and saved in the hard drive.
Data Interpretation and Reporting
When the calculations are complete and all corrected data and values are defined, flow computers can compare the data to operator-defined values and perform certain actions. For example, if the flow rate at any point within the system exceeds or fall below a defined threshold, the flow computer can signal an alarm to a local or remote control system using relay connections. Typical relays include:
- Low flow alarm
- High flow alarm
- Prewarn alarm
- Security alarm
If the alarm remains active for a certain number of seconds or if multiple alarms occur in succession within a defined time frame, the control system can be programmed to shut down system equipment or ramp up pumping speeds to balance the system flow.
Flow computers also provide organized data reports for operators to identify patterns and detect changes in the system. Daily, monthly and yearly trend reports allow operators to visually see fluctuations and proactively adjust the system to compensate for known tendencies. Also, batching reports can be used to make precise modifications and anticipate changes. All data logs, calculation results and reports are transferred through serial ports to printers, modems or computers.
Flow computers are applicable to a wide range of industries but are most commonly used for larger systems with high volume flows. However, smaller systems that utilize multiple types of flow meters with deliver different data outputs can also benefit. Examples of industries that can benefit from flow computers include:
- Water and wastewater treatment
- Natural gas distribution
- Beverage production
- Chemical manufacturing
Instrument Verification Sets
Instrument verification sets are another type of flow computer that measure flow, temperature and pressure to verify the data. These sets are portable and can be used to test individual flow meters throughout a system. They are a useful supplement to permanent flow computer installations to troubleshoot specific problems and act as a calibration tool for the flow computers themselves. Examples of specific benefits include:
- Identifying individual flow meters that are operating outside of their design limitations
- Identifying flow meters that require calibration
- Determining leaks in the system
- Explaining discrepancies in the data
- Uncovering potential errors in the flow computer analyses
- Confirming flow computer results
- Establishing a regular flow meter calibration schedule
- Diagnosing flow meter errors and initiating maintenance or replacement of damaged or worn components
Adhering to National Standards
The National Institute of Standards and Technology is an organization operated by the U. S. Department of Commerce that determines calibration standards for instruments such as flow meters. These standards are typically included in flow computer calculations to determine calibrated values and assess the accuracy of each connected flow meter.
To conduct additional calibration tests and to receive guidance on interpreting the results of the calibrations, consider hiring a third-party professional calibration service. Regular professional service in addition to proper system maintenance can protect your instrumentation investments and keep them functioning optimally for many years.