Why is flow metering important?
There are two fundamental reasons for the importance of flow metering and measurement. First, information, either for direct management needs - “You can't manage what you can't measure” - or indirectly for further analysis, including modeling. The second is its role in operational control. SCADA measurement for example is often directly linked to system controls, tracking trends and changes in system characteristics, just like a blood pressure check.
The hardware for flow measurement can be installed for either permanent or temporary use. An example of permanent flow metering is in the City of Austin on its Cross Town Tunnel, where permanent flow meters determine water velocity and ultrasonic sensors determine water depth and flow velocities within the pipeline.
The technology options of the different types of sensor are:
- Electromagnetic sensors – immersed in the fluid being measured, these use electromagnetic readings to determine any of the following:
- the velocity of flow in the pipe at the exact depth at which the sensor is placed, usually the lowest point
- the peak velocity of flow, typically at the surface of the fluid
- the average velocity averaged over all depths of the fluid in the pipe
- Ultrasonic sensors – positioned above the fluid, these devices send out an ultrasonic signal that hits the surface of the water at an acute angle. The speed of flow of the surface is calculated from the Doppler shift of the return signal
- Ultrasonic flow meters – using the same return beam and Doppler principles of the ultrasonic sensors, these flow meters are located within the fluid at the lowest point of the pipe.
The key questions to ask when considering which type and make of meter to use are:
- What is the precision of the data recorded?
- How is flow rate calculated?
- How is velocity corrected for average?
- Is there an indicator when the sensor fails?
Why is modeling important?
It is hard to see how the requirements of the capacity analysis and planning elements of CMOM can be met without modeling. Modeling is the best way to approach capacity issues, because it:
- Determines optimum size of a new pipe much more cheaply than building it and then finding out it is too small or large
- Analyzes the whole network, not just a few pipes – interpolating between meters and taking account of all interactions elsewhere in the network that impact on the area of interest
- Simulates hydraulics under a variety of conditions – wet, or dry and hot, or other conditions - even those planning levels that are so extreme they occur very rarely
- Forecasts into the future, against a number of different scenarios
- Tests assumptions and options in a very quick and safe environment
Hydraulic modeling check list
When selecting hydraulic modeling software, the following aspects should be considered:
Basis of Calculation - The model should be based on dynamic calculations and the St Venant equations – without these, the results are not accurate enough to be used as a basis for decision-making
Model Building – the modeling software should possess good links with GIS and other database and asset software, to facilitate the simple import of network data from existing asset information
Input data - Flow data should be based on two factors – base flows calculated from population and billing records, and wet weather flows from rainfall or radar data
Results – the modeling package should support easy sensitivity analysis, return period analysis, and volumetric analysis versus peak flows.
A CMOM System Evaluation and Capacity Assurance Plan
Under CMOM, a utility is required to prepare and implement a plan to evaluate the existing system, unless either the POTW (Publicly Owned Treatment Works) has already taken steps to correct structural and hydraulic deficiencies, or the discharge already meets the criteria of CMOM 122.42(g)(2).
If a plan is needed, it can be addressed in two parts:
The System Evaluation , covering that portion of the system over which the POTW has operational control, should include:
- Estimates of peak flow under target rain event, including return period and event duration, and spatial distribution of the impact of the event
- Estimates of capacity, including determining friction losses to prevent over-estimating capacity
- Identification of hydraulic deficiencies under specific storm events and specific spatial distribution of a storm
- Identification of major sources of extraneous flow contributing to overflows, using Sewer System Evaluation Surveys, Metering, and Modeling.
At the same time, capacity management and capacity assurance requires an action list to address short-term and long-term hydraulic deficiencies. Those deficiencies are revealed by metering and by modeling, as this presentation has sought to explain.