Water Pressure (Pump Curve) - NYC Energy Tools

Understand how to measure Water Pressure using the Pump Curve method. Learn which tools and specifications you need to capture valid water pressure readings and use them to infer water flow rate using pump curve and other data. Follow a clear, step-by-step procedure to take multiple spot measurements and record your data for analysis and use in calculations.

Used to measure:

Measurement Overview

What:Hourly water pressure measurements.

Why:Used to infer the water flow rate (in GPM) for a pump.

Duration:For water loop systems that operate year-round and are driven by outside air temperature (OAT), one full year of measurement (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.

When:Measurements should be taken when the system or component is operating under normal conditions.

Type of Measurement:Proxy.

expand_more

This technique uses water pressure measurements to infer the water flow rate (in GPM) for a pump. This is called the pump curve method because it uses the pump curve, pressure data, and technical specifications of a pump to determine the water flow rate, as illustrated in Figure 1. A water loop system can have multiple pumps, and the water flow rate for each pump must first be determined and then summed to obtain the total flow rate of the water loop. Measurement data is used to calculate how much heat is added or removed by the heating or cooling plant. Data is also used to calculate energy/heat recovered with a liquid-to-liquid heat recovery system.

Water pressure measurements are taken at the inlet (suction) and outlet (discharge) sides of a pump. To measure water pressure, use a programmable data logger with pressure transducers installed on the access ports of the piping network. This is preferred over a pressure gauge reading because the pressure gauge introduces greater measurement error.

To properly use this technique, you need the pump impeller size, model number, and pump curve, as well as the information on the pump motor nameplate, such as the horsepower and efficiency (for each pump in the water loop)–in addition to the pressure measurements. Without this information, this technique cannot be executed. Depending on how the piping network is designed in the facility, a combination of a direct measurement of water flow rate and this measurement technique can be used.

Measurements should be taken at one-hour intervals; do not use instantaneous values. For water loop systems that operate year-round and are driven by outside air temperature (OAT), 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.

Figure 1 illustrates a typical pump curve with the flow rate (in GPM) marked on the horizontal axis and total head (in meters) marked on the vertical axis. Flow rate was determined by applying the pump impeller size and the total feet of head to the pump curve.

Drawing of a pump curve with a picture of a pump nameplate overlaid on top — A typical pump curve marked with the determined flow rate on the bottom axis.

Total head is calculated by taking the difference between discharge and suction pressure and multiplying it by a constant. Design FT, design GPM, pump RPM, and pump horsepower are all obtained from the nameplate on the pump or from the technical specifications for that pump model.

\begin{equation*} \Delta P = Discharge \hspace{2mm} Pressure - Suction \hspace{2mm} Pressure\end{equation*}

\begin{equation*} Total \hspace{2mm} Head = \Delta P * 2.31 Pressure\end{equation*}

Where,

$\Delta P =$ differential pressure across the pump, psi

$Discharge \hspace{2mm} Pressure =$ water pressure leaving the pump, psi

$Suction \hspace{2mm} Pressure =$ water pressure entering the pump, psi

$Total \hspace{2mm} Head =$ operating pressure of the pump converted from psi, feet

$2.31 =$ constant equivalent to one pound of pressure per in $^2$ of water (at 60 degrees F), feet/psi

info Types of Measurements

Direct: Specifies a quantity of the exact item being measured.
Proxy: Provides an indication of the system operation but does not directly measure the quantity in question.

warning Measurement Feasibility

This technique should be used only if direct measurement of water flow rate with a flow meter is not possible.

Measurement Tools

The measurement tool needed for this procedure is a data logger with pressure transducers.

NYC agency personnel can borrow measurement data loggers, meters, or sensors from our tool library, by selecting tools based on the required measurement duration. See recommended tools found in our library:

HOBO Four Channel Analog Data Logger Onset UX120-006M

info

An analog logger that supports up to four external sensors allowing you to measure temperature, current, voltage, air flow, pressure and more in one single logger.

View Toolchevron_right

Gauge Pressure Sensor Ashcroft T-ASH-G2-500

info

Standalone data logger that monitors a motor’s on and off conditions using an internal AC magnetic field sensor.

View Toolchevron_right

info Recommended Tool

The Ashcroft Pressure Transducer measures voltage between 0V and 5V. The pressure transducer connects to the UX120-006M data logger which then converts the voltage signal to pressure in PSI units. Please refer to our video instructions for details on how to configure the UX120-006M to detect voltage and convert to pressure.

We recommend using this equipment, rather than the on-site pressure gauge, to determine the pump flow to reduce measurement error. One pressure transducer should be used for each side of the pump. Heating and cooling plants can have different pump configurations, such as primary-only and primary-secondary pumps. Additionally, pumps can operate at constant speed or variable speed.

warning Important

Make sure to identify the pump configuration and number of pumps for the plant so you borrow the right quantity of loggers and transducers.

info Logger Setup

Please note that loggers will require setup with a computer or phone using the manufacturer’s proprietary software prior to installation and measurement logging and that older loggers will require the use of a cable to connect to a computer. Refer to the measurement equipment’s user manual for a detailed description of setup requirements.

Measurement Steps

looks_one

Prepare Tools

Identify the temperature (hot or cold) of the pipes that will be measured. Use the manufacturer’s software to set up the logger. Refer to the user manual for detailed instructions on how to set up the logger.

Logging interval: 1-hour
Date and time to start logging
Date and time to stop logging
Values to measure: voltage (V) or pressure (psi)^[1]
Activate input channels on the logger
Type of sensor
Sampling interval: 1-second

How to setup 4-channel analog logger with CT to measure water pressure and pump curve (7 min).

looks_two

Install Tools

Confirm that the equipment is operational.
Locate the pressure gauges that are installed on the suction and discharge sides of the pump.
Connect the transducers to the data logger.
Place the data logger near the pipes, avoiding placing the logger on the pipe itself.
Confirm that the pressure gauge has a stop-valve to prevent water from flowing.
If there is no stop valve on the piping system, consider using another technique to quantify the water flow.
Remove the stop-valve and replace it with a T-shaped valve to stop the flow of water.
Connect the transducer to the T-shaped valve.
To avoid leaks, wrap Teflon tape around the installed equipment.

looks_3

Verify Data Acquisition

Wait 24-48 hours to verify data collection. Return to the location of the measured equipment and use a laptop or a phone with the manufacturer’s proprietary software installed to do the following:

warning Warning

Do not remove the logger or sensors. If you remove the logger, you may need to start over with the setup and installation process.

If necessary, connect the logger to a laptop or phone via USB cable. Otherwise, use the software to connect with the data logger via Bluetooth.
Analyze the data with a plot graph. This can be done with the proprietary software or Microsoft Excel after exporting the dataset as a .csv file.
Determine whether the results align with the expected operation of the system or component based on observed operational patterns or known equipment schedules.

How to check the data of loggers using HOBOware (10 min).

looks_4

Retrieve Tools and Data

After verifying that the logger is collecting data, do the following:

Allow the logger to collect data for the remainder of the measurement period.
After the measurement period has concluded, remove the logger from the panel.
If necessary, connect the logger to a laptop or phone via USB cable.
Use the proprietary software to stop the logger and end data collection.
Download all data from the logger and save the file in the .csv file format for analysis.

How to extract the data from the logger using HOBOware (4 min).

looks_5

Select Calculator(s)

These calculators will output estimated annual energy consumption or heat transfer for relevant components using measured data. Download the calculator for your specific component’s measurement strategy and add your measurement data.

Heat Exchangers

Liquid-to-Liquid Heat Transfer (Plate and Frame Heat Exchanger) Calculator

Estimates the annual heat transfer during the heating and cooling season of two fluids in a plate and frame heat exchanger.

Measurements Required:

Download Calculator download

Calculation Methodology

This methodology works with hourly data only and each measurement is taken at the start of the hour. If the system operates seasonally, measurements should be taken for the duration of the season at one-hour intervals. If the system is not weather dependent, then a minimum of six (6) weeks of hourly data is required for this methodology to work.

The following equations are used to calculate the sensible heat transferred by the heat exchanger for a full year. All data used in this section should be measured with data loggers per the measurements techniques above.

Table 2. Hourly measured data to calculate heat transfer.
Hourly Measured Values
$$T_1, T_2, T_3, T_4$$	Fluid Temperatures (F)
$$Q_1$$	Fluid 1: Water flow rate (GPM)
$$Q_2$$	Fluid 2: Water flow rate (GPM)

Calculate the mass flow rate of fluid 1 and fluid 2 leaving the heat exchanger for each hour interval.

\begin{equation} \dot{m_1} = Q_1 * 0.13368 * d \end{equation}

Where,

$\dot{m_1} =$ hourly mass flow rate of fluid 1 leaving the heat exchanger ($lb_m$/min)

$Q_1 =$ hourly flow rate of fluid 1 leaving the heat exchanger (GPM)

$d =$ density of water, 62 $(lb/ft^3)$

$0.13368 =$ unit conversion, GPM to $(ft^3/min)$

\begin{equation} \dot{m_2} = Q_2 * 0.13368 * d \end{equation}

Where,

$\dot{m_2} =$ hourly mass flow rate of fluid 2 leaving the heat exchanger ($lb_m$/min)

$Q_2 =$ hourly flow rate of fluid 2 leaving the heat exchanger (GPM)

$d =$ density of water, 62 $(lb/ft^3)$

$0.13368 =$ unit conversion, GPM to $(ft^3/min)$

Heat transfer should be calculated only when the heat exchanger is operating. This can be determined by checking the temperature difference across the recovery system in both the supply side and waste heat stream side. Another condition is that water flow should exist across the heat exchanger.

\begin{equation} \text{If} \hspace{2mm} \begin{cases} T_1 - T_2 > 1 & \text{if heat transfer occurs}\\ T_1 - T_2 < -1 & \text{if heat transfer occurs} \\ -1 < T_1 - T_2 < 1 & \text{if no heat transfer occurs} \end{cases} \end{equation}

Where,

$T_1 =$ meausured hourly temperature of fluid 1 entering the heat exchanger (F)

$T_2 =$ meausured hourly temperature of fluid 1 leaving the heat exchanger (F)

\begin{equation} \text{If} \hspace{2mm} \begin{cases} T_3 - T_4 > 1 & \text{if heat transfer occurs}\\ T_3 - T_4 < -1 & \text{if heat transfer occurs} \\ -1 < T_3 - T_4 < 1 & \text{if no heat transfer occurs} \end{cases} \end{equation}

Where,

$T_3 =$ meausured hourly temperature of fluid 1 entering the heat exchanger (F)

$T_4 =$ meausured hourly temperature of fluid 1 leaving the heat exchanger (F)

\begin{equation} \text{If} \hspace{2mm} \begin{cases} Q_1 > 0 & \text{if heat transfer occurs}\\ Q_1 < 0 & \text{if no heat transfer occurs} \end{cases} \end{equation}

Where,

$Q_1 =$ hourly flow rate of fluid 1 leaving the heat exchanger (GPM)

Calculate the sensible heat transferred by the heat exchanger for each hour interval.

\begin{equation} q_{s1} = \dot{m_1} * 60 * (T_1 - T_2) \end{equation}

Where,

$q_{s1} =$ sensible heat transferred by the heat exchanger every hour for fluid 1(Btu/h)

$60 =$ unit conversion, (min/hour)

$\dot{m_1} =$ hourly mass flow rate of fluid 1 leaving the heat exchanger ($lb_m$\min)

$T_1 =$ hourly fluid 1 entering the heat exchanger temperature (F)

$T_2 =$ hourly fluid 1 leaving the heat exchanger temperature (F)

\begin{equation} q_{s2} = \dot{m_2} * 60 * (T_3 - T_4) \end{equation}

Where,

$q_{s2} =$ sensible heat transferred by the heat exchanger every hour for fluid 2(Btu/h)

$60 =$ unit conversion, (min/hour)

$\dot{m_2} =$ hourly mass flow rate of fluid 2 leaving the heat exchanger ($lb_m$\min)

$T_3 =$ hourly fluid 2 entering the heat exchanger temperature (F)

$T_4 =$ hourly fluid 2 leaving the heat exchanger temperature (F)

Calculate the total heat transfer by the heat exchanger for a whole year.

\begin{equation} Q_s = \sum_{t=0}^{8760} q_s \end{equation}

Where,

$Q_s =$ total sensible heat transfer for a whole year (Btu/h)

$q_s =$ sensible heat transferred by the heat exchanger every hour (Btu/h)

$t =$ hours

Troubleshooting

This section provides some troubleshooting tips for the most common issues with tool installation.

Verify that the logger is calibrated. Re-calibrate the tool if necessary.

Footnotes

[1] This is dependent on the measurement capability of the chosen logger.