Understanding Pressure-based Flowmeters

A “plug” of fluid can be accelerated by applying a difference of pressure across its length. The amount of pressure applied will be in direct proportion to the density of the fluid and its rate of acceleration. Conversely, we may measure a fluid’s rate of acceleration by measuring the pressure developed across a distance over which it accelerates.

We may easily force a fluid to accelerate by altering its natural flow path. The difference of pressure generated by this acceleration will indirectly indicate the rate of acceleration. Since the acceleration we see from a change in flow path is a direct function of how fast the fluid was originally moving, the acceleration (and therefore the pressure drop) indirectly indicates fluid flow rate.

A very common way to cause linear acceleration in a moving fluid is to pass the fluid through a constriction in the pipe, thereby increasing its velocity (remember that the definition of acceleration is a change in velocity). The following illustrations show several devices used to linearly accelerate moving fluids when placed in pipes, with differential pressure transmitters connected to measure the pressure drop resulting from this acceleration:

Pressure-based Flowmeters

Another way we may accelerate a fluid is to force it to turn a corner through a pipefitting called an elbow. This will generate radial acceleration, causing a pressure difference between the outside and inside of the elbow which may be measured by a differential pressure transmitter:

Pressure-based Flowmeters

The pressure tap located on the outside of the elbow’s turn registers a greater pressure than the tap located on the inside of the elbow’s turn, due to the inertial force of the fluid’s mass being “flung” to the outside of the turn as it rounds the corner.

Yet another way to cause a change in fluid velocity is to force it to decelerate by bringing a portion of it to a full stop. The pressure generated by this deceleration (called the stagnation pressure) tells us how fast it was originally flowing. A few devices working on this principle are shown here:

Pressure-based Flowmeters

Reprinted from "Lessons In Industrial Instrumentation" by Tony R. Kuphaldt – under the terms and conditions of the Creative Commons Attribution 4.0 International Public License.

Reboiler Condensate Level Measurement Using a Non–intrusive Magnetostrictive Level Transmitter

Magnetostrictive Level TransmitterIntroduction

Reboilers are found throughout refineries and are critical to reliable plant operation. Reboilers are designed to operate with no liquid condensate level. Unintentional condensate flooding of reboilers results in a greater risk of corrosion since corrosion processes occur in the liquid phase. Uncontrolled corrosion can lead to reboiler failure and unplanned shutdown costing billions of dollars.

The Application

The customer is a fractionation plant which produces ethane and propane. The application, reboiler condensate level, is critical to the plant operations. Process conditions: The transmitter is mounted as a non–intrusive device, so pressures are of no consequence.
  • Ambient temperature: –2 to 43 deg. C (28.4 to 109.4deg. F)
  • Process temperature: 65 deg. C (149 deg. F) 
  • Process pressure: 22 barG
The Challenge

The condensate–pot level indicator was unreliable and insensitive to the variations in level–control– valve opening. Condensate–pot level control was poor, and the control valve had to be operated on manual. Finally this competitor transmitter failed and was replaced by the earlier generation magnetostrictive AT200 transmitter years ago. The customer is now looking to upgrade the measurement system.

The Solution

ABB offered the advanced next generation LMT series magnetostrictive transmitter for the level measurement. The LMT200 initially was added as a redundant measurement for ensuring the performance and reliability. The customer appreciated the advanced features including the Easy setup, built–in waveform and diagnostics capabilities of the LMT. This resulted in a higher confidence and switched the LMT measurement loop as the primary for the level control through the control system.

For more information contact Hile Controls of Alabama by visiting https://hilealabama.com or by calling 800-536-0269.

Reprinted with permission from ABB Measurement & Analytics.

Your Choice for Process Control Instrumentation - Hile Controls of Alabama

When you need pressure, temperature, level, flow, gas detection or analytical instrumentation, think Hile Controls of Alabama.  Hile provides process instrumentation for the oil and gas, chemical, power, plastics, mining, water and waste water, pharmaceutical and bio-pharmaceutical, food and beverage, pulp and paper, and government-related industries. Located in Pelham, Alabama, Hile Controls of Alabama proudly serves the states of Alabama and Mississippi, as well as Western Tennessee and the Florida Panhandle.