Why Accurate Flow Measurement Is Critical

Accurate control of pumps is critical in potable water applications. The goal is to improve efficiency with precise tracking of water volume through distribution systems, while identifying potential performance problems. This requires effective flow measurement.

Background

Regulatory and legislative efforts to reduce the lead contained in all plumbing-related products that come in contact with potable water has increased. United States federal and state agencies stipulate that components of potable water systems comply with the U.S. Environmental Protection Agency’s Safe Drinking Water Act (SDWA) through standards established by NSF International, a not-for-profit, nongovernmental organization recognized for its expertise in health and environmental sciences.

Standards addressing potable water applications include NSF/ANSI Standard 61: Drinking Water System Components—Health Effects and NSF/ANSI Standard 372: Drinking Water System Components—Lead Content.

NSF Standard 61 is a drinking water approval, whereas Standard 372 is a method of measurement for determining “lead free” (weighted average of 0.25 percent or less) characteristics. Standards 61 and 372 are intended to ensure health protection for products in contact with potable water. The standards require that prefabricated domestic pump systems have certification as an entire system.

Understanding Pump Performance

Manufacturers involved with food and beverage production and other sanitary processes, as well as operators of municipal water systems, use pumps compliant with the SDWA. These pumps are made of stainless steel or other nonhazardous materials and must be easy to disassemble and clean.

Accurate control of pump systems is a critical concern in such applications, which are often characterized by intermittent low flows and reduced water consumption. Today, water delivery systems feature large pipes with low-flow velocities and low overall volume capture.

The flow generated by a pump is volumetric in nature. The precision of any measurement is heavily dependent on the fluid properties. Inaccuracies in flow metering can be caused by process conditions including temperature changes, pressure variations, air entrapment and wear of components.

Data indicates that pump flow output can vary significantly due to component degradation, environmental or system changes, or a combination of these.

Flow rates through pumps are metered for a number of reasons: to calculate distribution system losses by subtracting the total of readings from total supply; to optimize disinfection; to treat corrosion; to monitor overall pump performance; and to determine gross billings in custody transfer.

Most aspects of a pump’s performance can be associated with the flow of fluid. Understanding pump performance involves a basic knowledge of related specifications and performance curves.

Choosing a Metering Technology

Industrial, commercial and municipal organizations have a choice of flow measurement technologies for achieving accuracy and safe water contact in pump control applications. A variety of mechanical and electronic meters are available for liquid flow measurement.

The most commonly used options range from turbine meters with stainless steel wetted components, to electromagnetic meters and both inline and clamp-on ultrasonic meters—all of which can be safe for use in potable water.

Turbine meters are a proven technology and low-cost solution for smaller line sizes. They are known for accuracy, wide turndown and repeatable measurements. Turbine meters produce a high-resolution pulse rate output signal proportional to fluid velocity and, hence, to volumetric flow rate. These devices are limited to clean fluids and require periodic recalibration and service.

Some turbine-based flow meter designs are available in a lead-free bronze alloy. They comply with lead-free drinking water provisions and can be certified to the NSF/ANSI standards. All components of these lead-free alloy meters comprise the certified system.

Nutating disc flow meters are also commonly used in water metering—particularly with smaller lines. They have a reputation for high accuracy and repeatability, but viscosities below the threshold adversely affect performance.

Since most potable water processes involve liquids that are conductive, electromagnetic flow meters are suitable for many associated flow measurement tasks. These meters help meet measurement accuracy objectives, enabling more efficient pump operation at a lower cost. They provide diagnostics to deliver enhanced reliability, process insight and reduced costs.

Electromagnetic flow meters have become an affordable long-term solution for a wide range of applications. They are designed to achieve up to ± 0.2 percent accuracy and the nonintrusive, open-flow tube design virtually eliminates pressure loss. The meters have no moving parts or flow obstructions and are relatively unaffected by viscosity, temperature and pressure when correctly specified.

Electromagnetic meters have signals that are linear with flow. Turndown can approach 20-to-1 or better without losing accuracy. They offer encoded outputs.

Many are choosing noncontact ultrasonic flow meters, which are inherently compliant with NSF regulations and installation without disturbing or cutting the pipe. Providing a turndown ratio that is comparable to electromagnetic flow meters, modern ultrasonic meters have been enhanced with increased power and sampling rates.

Transit-time ultrasonic meters are designed to determine bidirectional flow rates. Ultrasonic waves transmit upstream and downstream through the pipe wall and liquid flowing in the pipe. By measuring the difference in the travel time and knowing the pipe size, the meter determines the velocity and flow.

Clamp-on transit-time ultrasonic meters have no moving or wetted parts, can suffer no pressure loss and can provide maintenance-free operation. By measuring the speed of sound of the fluid, they can report changes in fluid temperature and/or other properties.

Clamp-on ultrasonic flow meters can troubleshoot a wide range of flow issues, particularly in pumps. These ultrasonic meters reside outside the pipe and enable installation flexibility across a wide range of pipe sizes, no contact with internal liquid and no moving parts.

A clamp-on ultrasonic flow meter can be a valuable tool to manage pump and system performance. The meter can provide the baseline and load profile information needed to optimize system efficiency and reduce energy consumption and costs.

Using cloud-based data analytics, the latest flow meters enable greater visibility and control. The interval meter data provides actionable intelligence to optimize water management. Cellular endpoints broadcast meter reading and event data to system software. This helps identify leaks and other anomalies in performance.

Article originally appeared in Pumps & Systems May 2019 issue