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 Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by...
The System Integrator Giants program lists the top 100 system integrators among companies listed in CFE Media's Global System Integrator Database.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
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.
Mobile HMI; PID tuning tips; Mechatronics; Intelligent project management; Cybersecurity in Russia; Engineering education; Road to IANA
Save energy with automation; Process control system upgrades; Dispelling controll myths; Time-sensitive networking; Control system integration; Road to IANA
Additive manufacturing advancements; Machine vision enhances robotics; Fieldbus evolution; Process safety; Advice from System Integrators of the Year; Road to IANA
This article collection contains several articles on the Industrial Internet of Things (IIoT) and how it is transforming manufacturing.

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

Flexible offshore fire protection; Big Data's impact on operations; Bridging the skills gap; Identifying security risks
The digital oilfield: Utilizing Big Data can yield big savings; Virtualization a real solution; Tracking SIS performance
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

(copy 5)

click me