Applying Flow Conditioning

Process industries are required to measure flow accurately to meet both plant operation and cost accounting objectives. However, these objectives can be difficult to achieve because the accuracy of many flow meter technologies can be adversely affected by flow disturbances. These are often caused by inadequate straight runs of pipe or other disturbances upstream of the flowmeter's location.

02/01/1999


KEY WORDS

 

  • Process control & instrumentation

  • Flow control/sensing

  • Flowmeters

Sidebars:
A 'look' at flow visualization

Process industries are required to measure flow accurately to meet both plant operation and cost accounting objectives. However, these objectives can be difficult to achieve because the accuracy of many flow meter technologies can be adversely affected by flow disturbances. These are often caused by inadequate straight runs of pipe or other disturbances upstream of the flowmeter's location.

Although many flowmeter manufacturers recommend a minimum of 20 dia. of straight run piping (in the absence of other disturbances), testing laboratories have shown flowmeter errors can exist even with flow disturbances up to 40 pipe dia. upstream. However, contrary to this recommendation, process plants are often designed (in the name of cost control and overall plant efficiency) to minimize plant floor space. Abbreviated floor space can result in inadequately short runs of straight pipe for flowmeter installations. The opposing concerns of improving flowmeter accuracy while controlling floor-space requirements can be resolved by using flow conditioners in "nonideal" piping installations.

In addition to short pipe runs, other conditions also interfere with accurate flow metering. These include upstream valves, fittings (elbows, tees, reducers, etc.), blowers, pumps, and compressors. These conditions generate flow disturbances which include swirl, jetting, temperature stratification, and flow profile distortion.

Flow disturbances adversely affect differential pressure, turbine, thermal, vortex shedding, ultrasonic, and magnetic flowmeter technologies. Poor piping practices increase flowmeter error, often outside of the specified performance limits of the flowmeter manufacturer. These errors often go undetected, as it is very difficult to make a comparative measurement of the actual flow and the flowmeter's indicated flow in the field. This creates a challenge for the user to determine the "real" flow in the system. Flow conditioners can solve this dilemma. A properly functioning flow conditioner isolates flow disturbance from the flowmeter, while minimizing pressure drop across it.

Common types of flow conditioners include:

  • Screen/vane-type—These flow conditioners were developed during early wind-tunnel test research and carried over to fluid testing;

  • Honeycomb-type/perforated plate/tube bundles—These ruggedized versions of screen-type conditioners are deemed suitable for industrial applications. The devices are relatively low cost; and

  • Tab-type.

Evaluating flow

When selecting a flow conditioner, first identify the disturbance problems that are likely to be present. For example, in long lengths of straight pipe, swirl reduction and velocity profile correction occur naturally due to diffusion and turbulent mixing.

Vane, honeycomb, and tube-bundle conditioners effectively eliminate swirl. However, the smaller the openings, the more vulnerable these conditioners are to fouling or clogging. Small diameters also are susceptible to buildup from viscous or sticky fluids.

Screens and perforated plates are good at removing flow profile distortions but also are susceptible to fouling or clogging. The more effective flow conditioners are at correcting flow profile distortions, the more pressure drop they produce.

Both antiswirl and flow profile conditioning are required to maximize downstream meter accuracy. In many plant situations, the burden on piping requirements and maintenance often leads to equipment compromise and lower flowmeter accuracy.

Tab-type flow conditioners use antiswirl tabs to remove fluid rotation, along with vortex tabs to eliminate irregular flow profiles from the flow stream. These tabs are welded to the inside of the flow conditioner body to create a durable device, see diagram. The result is a flat, nonswirling flow profile at the measurement location. Tab-type conditioners are highly immune to fouling and produce little pressure drop.

Mountings for this type of flow conditioner include an insertion sleeve design or "replacement" pipe sections equipped with either flanged, threaded, or butt-welded process connections that match existing piping. For large pipe diameters (up to 360-in. dia.), field mounting kits provide both loose tabs and instructions for welding or bolting into existing ducts or pipes.

Documenting improvement

Flow visualization techniques (see sidebar), along with standard empirical testing demonstrate conditioner effectiveness. During testing a known flow is measured by a meter downstream of "standard" disturbances. The test is conducted with and without a flow conditioner in place.

Visual comparison of flow patterns are used to the determine effectiveness of the flow conditioners used. Comparing actual meter reading vs. known flow determines reading errors for meter type, with and without flow conditioning. The accompanying table shows typical calibration test results for vortex shedding, thermal-mass, and turbine flowmeters. In all cases, flowmeter performance improved significantly by isolating it from the flow disturbance. Flow disturbance errors of up to 3% for vortex shedding; 8% for thermal-mass, and 15% for turbine flowmeters are regularly encountered in field installations.

Many flowmeter manufacturers recognize the need for flow conditioning to optimize performance. And, although operation manuals recommend ideal installations for their flowmeters, they often recommend flow conditioning in situations where downstream flow conditions are less than desirable.

For more information on Vortab, visit www.controleng.com/info

Typical Flow Meter Errors with/without Tab-Type Flow Conditioner

Flowmeter type

Downstream disturbance / Single elbow

Downstream disturbance / Double elbow

Vortex shedding

with

0.3%

0.5%

without

0.75%

3.0%

Thermal mass

with

1.0%

1.0%

without

8.0%

8.0%

Turbine

with

0.7%

0.7%

without

3.0%

3.0%


Author Information

David M. Feener is general manager for Vortab Co. located in San Marcos, Calif. Mr. Feener, a graduate of the University of California, Los Angeles, is a 25-year veteran of the flow control industry.


A 'look' at flow visualization

A number of techniques help fluid flow scientists visualize pipe flow. "Seeing" the effects of downstream disturbances confirms improvements resulting from use of flow conditioners. Typically, visualization lab tests use water flow through 4-cm. dia. clear acrylic pipe with a known upstream disturbance. During test runs—done with and without flow conditioners in place—the flow pattern in the pipe is illuminated and recorded on a videocassette recorder. Still frames from the recording can be reviewed individually if necessary.

A number of techniques help fluid flow scientists visualize pipe flow. "Seeing" the effects of downstream disturbances confirms improvements resulting from use of flow conditioners. Typically, visualization lab tests use water flow through 4-cm. dia. clear acrylic pipe with a known upstream disturbance. During test runs—done with and without flow conditioners in place—the flow pattern in the pipe is illuminated and recorded on a videocassette recorder. Still frames from the recording can be reviewed individually if necessary.

Visualization can be done several ways. The first technique mixes reflective particles with water upstream of a flow disturbance. The particles are illuminated by passing a laser sheet light through the acrylic pipe at the location where a flowmeter would ordinarily be mounted. The video records movement of the laser-illuminated particles, both with and without a flow conditioner in place.

Another technique uses a wire that passed horizontally through a vertical acrylic pipe connected to the test configuration. A short duration electrical pulse (150 V dc) is passed through the wire, releasing a small stream of hydrogen bubbles along its length. The bubbles then follow the swirl and velocity profile of the flow stream. Illuminated in conventional light, the flow pattern can be filmed with a video recorder.

Release of a red and green dye stream from opposite sides of a pipe downstream of disturbance configuration can also be used. The dye streams make the swirl pattern easy to see and videotape. Unlike the bubble technique, dye-stream injection provides visualization of the swirl only.



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