Water Flow RateMeasurement

Understanding Water Flow Rate Measurements

This technique uses a flow meter to measure the flow rate of a fluid within a pipe. The technique is typically used on water loop systems to measure the flow through heating and cooling plants, but it can also be used to measure the flow rate of other fluids (e.g., glycol). Measurement data is used to calculate how much heat is added or removed by the heating or cooling plant; energy/heat recovered by a liquid-to-liquid heat recovery system, and heating energy for domestic hot water.

This measurement technique is preferred over the pump curve method because flow rate is directly measured and removes the need for additional calculations. The pump curve method requires the differential pressure between the discharge and suction of all pumps for a water loop system and the technical specifications of all pumps.

However, a major challenge with this technique is that measurements must be taken on a straight pipe; the flow meter should not be installed near elbows or T-shaped pipes. Additionally, piping systems are typically covered with thick layers of insulation to minimize heat losses. To install the flow meter, the insulation must be cut and removed (Figure 2). CUNY BPL recommends conducting a site survey to examine the piping system in the facility and determine if taking a direct flow measurement is possible.

Measurements should be taken at one-hour intervals and not instantaneous values. When taking one-hour measurements, the flow meter will detect the flow several times every hour and store the average value. Sometimes flow can fluctuate over a one-hour period, but the instantaneous flow value will not indicate that change.

The duration of the long-term measurement and when to measure is dependent on the system and the operation of the building. For water loop systems that operate year-round and are driven by outside air temperature (F), one full year of measurement (12 consecutive months, 52 consecutive weeks, or 365 consecutive days) is required for the baseline and one full year for the reporting period. For water loop systems that operate during a particular season, the full season must be measured for the baseline and reporting periods. For water loop systems that are not driven by OAT it is recommended to measure flow rate for a minimum of six weeks. Measurements should be taken when the system or component is operating under normal conditions.

Type of Measurement

Direct

Measurement Equipment to Use

The measurement equipment needed for this procedure is an ultrasonic flow meter. Flow meters work by emitting ultrasonic sounds from the transducers to the pipe surface and into the fluid. Transducers operate in sets of two and both transducers act as transmitters and receivers of ultrasonic sound. The time it takes for sound to travel from one transducer to the other is called transit time. Flow rate is calculated by the meter using the transit time. This is the commonly used method to measure water or other liquids in a pipe. Refer to the NYC Energy Tools catalog to identify the correct flow meters for your project. The sensors of the ultrasonic flow meters available are clamp-on so there is no need to drill into the piping. By default, some flow meters are set to measure the properties of water. If glycol or other liquids will be measured refer to the user manual and change the settings of the meter to measure the fluid in the system. The contents of this guide are largely based off the DXN Portable Ultrasonic Measurement System, which can be found and borrowed through the NYC Energy Tools field equipment library.

Certain fluids contain moving particles such as bubbles or solid material. To accurately measure the flow rate of these fluids, some flow meters measure the transit time using the doppler effect. The moving particles in the fluid will cause the ultrasonic sound to shift in frequency (Hz) and the shift in frequency is directly related to the speed of the moving particles. Transducers used in this scenario are known as doppler transducers.

As mentioned, a major challenge is finding the right location to install the meter to avoid bad signals and no data collection. Manufacturers such as Instruments Direct Inc. recommend installing the flow meter on a straight pipe that is far enough away from an elbow. The distance from the elbow should be at least 10 to 15 times the external diameter of the pipe.

Please note that certain flow meters offered at the FELL will require setup with a computer or phone using the manufacturer’s proprietary software prior to installation. Refer to the measurement equipment’s user manual for a detailed description of setup requirements.

Measurement Steps

1. Preparation for Data Acquisition

Identify the location where the flow meter will be installed. Refer to the equipment’s user manual for detailed instructions on how to set up the meter(s).

• Logging interval: 1-hour (for variable speed pumps)
• Date and time to start logging
• Date and time to stop logging
• Value to measure: Water flow rate (GPM)
• Confirm if properties of fluid matches with the fluid to be measured

2. Installation of Equipment

1. Confirm that the equipment is operational.
2. Some piping systems have a layer of insulation to minimize heat loss. There are scenarios where the insulation must be cut to install the transducers of the flow meter. Consult with personnel who work in the facility to determine if this is appropriate. See , Figure 4 for reference.
3. If installing the flow meter is not possible then consider using the pump curve method to determine flow rate of the fluid.
4. Refer to the user manual of the measurement equipment to determine how to install the transducers.

3. Verify Data is Being Collected

Wait 24-48 hours to verify data collection. Return to the location of the measured equipment to do the following:

1. If necessary, connect the logger to the laptop or phone via USB cable. Otherwise, use the software to connect with the data logger via Bluetooth.
2. Analyze the data with a plot graph. This can be done with the software or Microsoft Excel after exporting the dataset as a .csv file. Please note that the format file may vary based on the measurement equipment.
3. Determine if measured results align with the expected operation of the system or component.

4. Retrieve Measurement Equipment and Download All Final Data

After verifying the meter is collecting data do the following:

1. Allow the meter to collect data for the remainder of the measurement period.
2. After the measurement period has concluded, remove the meter.
3. If necessary, connect the meter to the laptop or phone via USB cable. Otherwise, use the software to connect with the logger via Bluetooth.
4. If applicable, use the proprietary software to Stop the meter and end data collection.
5. Download all data from the meter and convert to csv file format for analysis.

Troubleshooting Our Measurement Equipment

Problem 1: Presence of Turbulence

Turbulence occurs when a fluid flows across an obstacle or a sharp corner. It can also occur if the fluid flows at a high speed due to drag forces between the fluid and the piping. The flow meter cannot accurately measure fluid flow if turbulence is present.

Problem 2: Data Accuracy and Measurement Interferences