Understanding differential pressure flowmeters

Traditional flow measurement technique survives and thrives, often updated with current technology.

12/18/2013


Trying to force the fluid through an orifice creates a pressure drop in proportion to the flow rate.When a flow measurement technique that has been around since the automation dark ages continues to show among the top performing technologies, it indicates a high level of reliability, versatility, and adaptability. Such is the case with differential pressure flowmeters. As a flow metering technique, it consistently ranks among the top five.

This is really a very large family of designs that separates into two main categories: flow obstructions and Pitot tubes. For purposes of this discussion, we’ll consider only the former. When an obstruction is placed in a pipe, there will be a difference in fluid pressure upstream and downstream from the obstruction. This difference can be measured and characterized to provide the basis for calculating flow. (An electrical parallel is calculating amperage by measuring the voltage drop across a known resistance.) While it may not be the first flow measuring technique invented, it is certainly one of the earliest, and quickly followed the first practical pressure gages.

Understanding the concepts of a magnetic or Coriolis flowmeter may stretch your thinking but differential pressure flow measurement mechanics are easy to grasp (although the actual equations can get pretty complicated). This familiarity, combined with its simplicity, helps the technology survive. The most basic implementation involves placing a disk or other type of orifice in a pipe with an opening smaller than the pipe’s inner diameter, and then taking a differential pressure measurement comparing the up- and down-stream sides of the orifice. There are many practical advantages to this approach:

• The technology is simple and easy to understand;
• There are no moving parts;
• The sensor is very compact;
• Works equally well for gas or liquid;
• Can be configured to measure bi-directional flow;
• Usable over a very wide range of sizes;
• Devices can be mechanical or electronic;
• Relatively inexpensive; and,
• You can even build one yourself. (Watch a video demonstration.)

Since this approach uses a pressure sensor, suppliers have included all the technological advances of those product families as part of the package. There are some very sophisticated units available, and those improvements carry over to the flowmeter application. These can provide diagnostic information and multiple variables.

On the other hand, there are some drawbacks:

• It’s not the most accurate flow measurement technology. If high precision or wide turn-down ability is a key objective, there are better choices.
• Devices cause a pressure drop and reduce the pipe’s free passage. The extent of this depends on the design.
• Accuracy and turn-down are related to pressure drop. The more accuracy and range you need, the greater pressure drop you will have to tolerate.
• While there are no moving parts, the orifice itself is often a wear-prone point. If it gradually enlarges, the flow measurement will read proportionally lower. On the other hand, if it becomes partly obstructed with debris, the measurement will read higher.
• The same application and installation guidelines for pressure sensors apply here. Anything that interferes with the pressure readings (clogged impulse lines, etc.) will interfere with a true flow calculation.
• Changes in liquid characteristics (viscosity, multiphase flow, etc.) can affect readings. This varies by design.
• These are generalities, so discuss your specific application needs with any prospective suppliers.

Differential pressure flowmeters are available from a variety of suppliers. A search on “differential pressure flowmeter” at www.controleng.com turned up more than 350 results.

Peter Welander, pwelander@cfemedia.com



The Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by Control Engineering subscribers. Vote now (if qualified)!
The System Integrator Giants program lists the top 100 system integrators among companies listed in CFE Media's Global System Integrator Database.
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
This eGuide illustrates solutions, applications and benefits of machine vision systems.
Learn how to increase device reliability in harsh environments and decrease unplanned system downtime.
This eGuide contains a series of articles and videos that considers theoretical and practical; immediate needs and a look into the future.
HMI effectiveness; Distributed I/O; Engineers' Choice Award finalists; System Integrator advice; Inside Machines
Women in engineering; Engineering Leaders Under 40; PID benefits and drawbacks; Ladder logic; Cloud computing
Robotic integration and cloud connections; SCADA and cybersecurity; Motor efficiency standards; Open- and closed-loop control; Augmented reality
Programmable logic controllers (PLCs) represent the logic (decision) part of the control loop of sense, decide, and actuate. As we know, PLCs aren’t the only option for making decisions in a control loop, but they are likely why you’re here.
This digital report explains how motion control advances and solutions can help with machine control, automated control on assembly lines, integration of robotics and automation, and machine safety.
This article collection contains several articles on how advancements in vision system designs, computing power, algorithms, optics, and communications are making machine vision more cost effective than ever before.

Find and connect with the most suitable service provider for your unique application. Start searching the Global System Integrator Database Now!

Control room technology innovation; Practical approaches to corrosion protection; Pipeline regulator revises quality programs
Cloud, mobility, and remote operations; SCADA and contextual mobility; Custom UPS empowering a secure pipeline
Infrastructure for natural gas expansion; Artificial lift methods; Disruptive technology and fugitive gas emissions
Automation Engineer; Wood Group
System Integrator; Cross Integrated Systems Group
Jose S. Vasquez, Jr.
Fire & Life Safety Engineer; Technip USA Inc.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
The Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by Control Engineering subscribers. Vote now (if qualified)!
The System Integrator Giants program lists the top 100 system integrators among companies listed in CFE Media's Global System Integrator Database.
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
This eGuide illustrates solutions, applications and benefits of machine vision systems.
Learn how to increase device reliability in harsh environments and decrease unplanned system downtime.
This eGuide contains a series of articles and videos that considers theoretical and practical; immediate needs and a look into the future.
HMI effectiveness; Distributed I/O; Engineers' Choice Award finalists; System Integrator advice; Inside Machines
Women in engineering; Engineering Leaders Under 40; PID benefits and drawbacks; Ladder logic; Cloud computing
Robotic integration and cloud connections; SCADA and cybersecurity; Motor efficiency standards; Open- and closed-loop control; Augmented reality
Programmable logic controllers (PLCs) represent the logic (decision) part of the control loop of sense, decide, and actuate. As we know, PLCs aren’t the only option for making decisions in a control loop, but they are likely why you’re here.
This digital report explains how motion control advances and solutions can help with machine control, automated control on assembly lines, integration of robotics and automation, and machine safety.
This article collection contains several articles on how advancements in vision system designs, computing power, algorithms, optics, and communications are making machine vision more cost effective than ever before.

Find and connect with the most suitable service provider for your unique application. Start searching the Global System Integrator Database Now!

Control room technology innovation; Practical approaches to corrosion protection; Pipeline regulator revises quality programs
Cloud, mobility, and remote operations; SCADA and contextual mobility; Custom UPS empowering a secure pipeline
Infrastructure for natural gas expansion; Artificial lift methods; Disruptive technology and fugitive gas emissions
Automation Engineer; Wood Group
System Integrator; Cross Integrated Systems Group
Jose S. Vasquez, Jr.
Fire & Life Safety Engineer; Technip USA Inc.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
The Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by Control Engineering subscribers. Vote now (if qualified)!
The System Integrator Giants program lists the top 100 system integrators among companies listed in CFE Media's Global System Integrator Database.
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
This eGuide illustrates solutions, applications and benefits of machine vision systems.
Learn how to increase device reliability in harsh environments and decrease unplanned system downtime.
This eGuide contains a series of articles and videos that considers theoretical and practical; immediate needs and a look into the future.
HMI effectiveness; Distributed I/O; Engineers' Choice Award finalists; System Integrator advice; Inside Machines
Women in engineering; Engineering Leaders Under 40; PID benefits and drawbacks; Ladder logic; Cloud computing
Robotic integration and cloud connections; SCADA and cybersecurity; Motor efficiency standards; Open- and closed-loop control; Augmented reality
Programmable logic controllers (PLCs) represent the logic (decision) part of the control loop of sense, decide, and actuate. As we know, PLCs aren’t the only option for making decisions in a control loop, but they are likely why you’re here.
This digital report explains how motion control advances and solutions can help with machine control, automated control on assembly lines, integration of robotics and automation, and machine safety.
This article collection contains several articles on how advancements in vision system designs, computing power, algorithms, optics, and communications are making machine vision more cost effective than ever before.

Find and connect with the most suitable service provider for your unique application. Start searching the Global System Integrator Database Now!

Control room technology innovation; Practical approaches to corrosion protection; Pipeline regulator revises quality programs
Cloud, mobility, and remote operations; SCADA and contextual mobility; Custom UPS empowering a secure pipeline
Infrastructure for natural gas expansion; Artificial lift methods; Disruptive technology and fugitive gas emissions
Automation Engineer; Wood Group
System Integrator; Cross Integrated Systems Group
Jose S. Vasquez, Jr.
Fire & Life Safety Engineer; Technip USA Inc.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
click me