Video tutorial: Differential pressure flowmeters

Using differential pressure to calculate flow is a very basic and common method. Here's how it works.

02/16/2010


DP video

Measuring flow with differential pressure

 

One of the most basic ways to measure fluid flow is by using a differential pressure measurement of fluid pressure up- and down-stream of an obstruction that creates a pressure drop. The pressure drop will be proportional to the amount of fluid flowing through the pipe, so the greater the flow rate, the greater will be the pressure difference. (The concept works with liquid or gas, but having a compressible fluid makes it somewhat more complicated. For purposes of this discussion, we'll stick with liquids.)

Watch the video .

A simple way to introduce such an obstruction is a plain orifice disk, where a device is inserted in the pipe with an opening that is somewhat smaller than the normal cross sectional area of the pipe. Liquid flowing through the orifice loses some pressure. That loss can be measured using a differential pressure device comparing readings above and below the obstruction.

This approach is very common commercially, and companies that have refined the technology have found sophisticated ways to optimize orifice size and shape, select the most desirable pressure measurement points, and otherwise create the highest accuracy and turn-down ratio with the least actual pressure loss.

There are many advantages to this approach:

• Simple sensor that can be compact with no moving parts;
• Bi-directional flow is possible;
• Scalable over a very wide range of sizes and flow rates; and
• Relatively inexpensive.

Of course there are drawbacks and limitations too:

• Limited accuracy and turndown range-If high precision is your primary objective, there are probably better technologies;
• Causes a pressure drop-Compensation for this may require higher pumping energy;
• Reduces the pipe's free passage-As sensor technologies go, this is one of the more invasive, but the extent varies among manufacturers;
• Accuracy and turn-down are related to pressure drop-In other words, the more accuracy and range you need, the greater pressure drop you will have to tolerate; and
• Orifice is prone to wear-While there are no moving parts, the orifice itself is often a wear-prone point due to the high liquid velocity. If the orifice gradually enlarges with wear, the flow measurement will read low. On the other hand, if it becomes partly obstructed with debris, the measurement will read higher. The extent of this also depends on the configuration.

The same application and installation guidelines for good pressure sensor practice apply here. Anything that interferes with the pressure readings, such as clogged impulse lines, will interfere with a true flow calculation. Additionally, changes in fluid characteristics, including viscosity, density, multiphase flow, and even temperature, can affect readings. Of course these are generalities, so discuss your specific application needs with any prospective suppliers.


The video shows a demonstration of a home-made sensor that illustrates the basic concepts.

 

See the growing catalog of instrumentation videos at CEtv.

-Peter Welander, process industries editor, PWelander@cfemedia.com
Control Engineering Process Instrumentation & Sensors Monthly eNewsletter
Register here to select your choice of free eNewsletters .



No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2015 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
Your leaks start here: Take a disciplined approach with your hydraulic system; U.S. presence at Hannover Messe a rousing success
Hannover Messe 2016: Taking hold of the future - Partner Country status spotlights U.S. manufacturing; Honoring manufacturing excellence: The 2015 Product of the Year Winners
Inside IIoT: How technology, strategy can improve your operation; Dry media or web scrubber?; Six steps to design a PM program
Getting to the bottom of subsea repairs: Older pipelines need more attention, and operators need a repair strategy; OTC preview; Offshore production difficult - and crucial
Digital oilfields: Integrated HMI/SCADA systems enable smarter data acquisition; Real-world impact of simulation; Electric actuator technology prospers in production fields
Special report: U.S. natural gas; LNG transport technologies evolve to meet market demand; Understanding new methane regulations; Predictive maintenance for gas pipeline compressors
Warehouse winter comfort: The HTHV solution; Cooling with natural gas; Plastics industry booming
Managing automation upgrades, retrofits; Making technical, business sense; Ensuring network cyber security
Designing generator systems; Using online commissioning tools; Selective coordination best practices

Annual Salary Survey

Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.

There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.

But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.

Read more: 2015 Salary Survey

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
This article collection contains several articles on the vital role that compressed air plays in manufacturing plants.
This article collection contains several articles on the Industrial Internet of Things (IIoT) and how it is transforming manufacturing.
This article collection contains several articles on strategic maintenance and understanding all the parts of your plant.
click me