- 16 August 2007 -

Putting smart flow conditioning practices to work

Measuring and conditioning the flow of your product through your system's pipes can save money and enable you to free up valuable floor space. Installing a flow meter is not enough. Don Lundberg, senior engineer at Fluid Components International, explains why flow conditioning is also critical.

If you need to increase your plant's productivity and reduce its operational costs, focusing on flow measurement can result in significant monetary savings, potential product quality improvements and even a competitive advantage. The accurate measurement of gases, steam, liquids and slurries is critically important in many plants and processes that produce chemicals, energy and power, food, beverages, pharmaceuticals, pulp, paper, water and more.

Unfortunately, the measurement of flow is often an afterthought - especially in many plant expansions, equipment upgrades and retrofit projects. The selection and placement of flow switches and flow meters requires careful advance thought. Choosing the wrong flow sensing technology for your application or placing your flow instrument in the wrong location, too closely to pumps or valves, cannot only eliminate potential cost savings, but frequently results in unnecessary replacement or relocation projects.

Carefully planning for new or upgrade flow instrumentation in your plant, however, offers many advantages, including:

. lower initial instrument installation costs and faster start-up

. decreased energy costs to run burners, boilers, fans or ovens

. reduced consumption of process gases, such as chlorine or nitrogen

. lower consumption of process water and wastewater

. less frequent plant maintenance and equipment replacement.

The downside

In a perfect world, every plant would have an instrumentation engineer on staff who is familiar with all the different flow technologies on the market and has experience applying them in multiple processes or industries, enough money to buy the best equipment, and a great maintenance team to do the installation. You can stop reading if you are working for this
company, because this article is for the 95% of you who make up the real world.

Optimizing your flow instrumentat-ion can be simple if you follow these four broad guidelines:

. Choose the appropriate flow sensing technology for your process media.

. Consider the installation and maintenance requirements in advance.

. Be careful about installing other equipment around flow instrumentation.

. Consider adding a flow conditioner or flow straightener.

Flow sensing technology

When selecting a flow instrument, the first consideration should always be the process media to be measured: air, gas, steam, liquid or slurries. Many flow sensing technologies perform best in one or two media, such as
gas or liquids. Some will measure
slurries - and some won't at all. The industry's major flow
sensing technologies now available include:

. coriolis (mass)

. differential pressure

. electromagnetic

. positive displacement

. thermal (mass)

. turbine

. ultrasonic

. variable area

. vortex shedding.

All of these technologies have their advantages/ disadvantages, depending on your process and plant. Thinking about the process media, as well as your plant's equipment, environmental or other regulations, maintenance schedules, etc., will enable you to narrow the field to one or two best choices.

Installation issues

One of the most common problems when installing flow instruments, especially flow meters, is an inadequate straight run upstream and downstream from the instrument. Before specifying any flow meter, you must consult the manufacturer's specification to determine the necessary straight pipe run. Adding elbows, expansions or reductions, and/or or spiral piping in close proximity to instrumentation often further exacerbates the problem of inadequate straight runs. Having close coupled components and equipment, the lack of space, or a poor piping layout will all work together to alter the process media's tangential, radial and axial velocity vectors. The final result is flow disturbances, including swirl, jetting and velocity profile distortions.

Flow disturbances frequently have a significant effect on the performance of flow meters, pumps and other equipment. For example with flow meters, the irregular flow of process material adversely affects the accuracy and repeatability of many of the most popular flow sensing technologies: differential pressure, turbine, magnetic, thermal, ultrasonic and vortex shedding. Depending on the flow sensing technology, the straight pipe run requirements for flow meters varies-from 10 to 20 or more pipe diameters.

The solutions

One of the most practical ways to optimize flow meter accuracy and/or compensate for adverse installation conditions is the use of flow conditioners. With flow conditioners, it is often possible to increase the accuracy and/or repeatability of a flow instrument by 50% or more.

For example, thermal mass flow meters with the addition of a flow conditioner can be optimized to perform at +1% accuracy instead of their standard +2% accuracy. That increase in accurate measurement can result in huge savings in controlling expensive natural gas fed burners for plant boilers over a year's time.

There are many different types of flow conditioners, including:

. Honeycomb vane-type conditioners are, which are popular in HVAC or compressed air handling system applications where they provide airflow profile corrections. Many different designs and materials are available.

. Perforated plates are frequently selected for applications in natural gas pipelines or other clean
gases and liquid applications. They are simple to install and require no spool piece, but they can be prone to clogging in
dirty gas.

. Tab-type flow conditioners are
a good choice in clean or dirty gases and liquids because of the tapered design of their tabs. They provide excellent cross-mixing to remove swirl and correct velocity profiles with minimal pressure-drop.

. Tube bundles and vanes have been used for decades. Tube bundles are effective at
removing swirl, but they have the tendency to 'freeze' the velocity profile and therefore are not as efficient at isolating and correcting flow distortion anomalies.

Conditioner selection

To choose a flow conditioner, you will need to follow many
of the same guidelines required to specify a flow meter. The
process media is especially important. Be sure to consider the viscosity of the process media when selecting a flow conditioner. Some technologies are prone to clogging in slurries, for example, and/or contaminant build-up in dirty gas processes.

Don't forget that, in general, the more effective flow conditioners are at correcting flow profile distortions, the more pressure drop they may produce. If you are trying to reduce power consumption or pumping costs, or you need to speed product throughput, then you will want to minimise head loss or pressure drop.

For example, one flow conditioning technology that has proven itself effective is the tab-type flow conditioner. In its standard straight tube configuration, the Vortab flow conditioner consists of pipe fitted with a short section of swirl
reduction tabs combined with three arrays of profile conditioning tabs. Combining the anti-swirl and profile conditioning tabs creates a repeatable, flat velocity profile at the outlet of the pipe. An elbow flow conditioner also can be configured with the same tab-type flow conditioning technology.

Proven to work

Many different processes and different types of flow instruments can benefit from the installation of tab type flow conditioners. At an offshore oil pumping station, for example, the process engineering team needed to add a pump to increase capacity. Elbows feeding into the pump consisted of a 20 in inlet and reduced down to a 12 in section.

The engineer quickly determined there was no room for the pump's required straight run and no way to expand the platform to accommodate the pipe run. By inserting a tab-type elbow conditioner in the elbow itself, the engineer solved the space problem by ensuring that a properly conditioned flow entered the pump at a large cost savings. He also freed valuable real estate for other possible uses.

At an electric power facility, the plant team needed to add two identical 14 in centrifugal pumps to feed water into its main boilers. The pump configuration required that the line size drops from 16 in at the elbow to 14 in at the pump inlet. These pumps are powered by 350 hp electric motors.

When installing the pumps, the crew found that the indoor facility did not have adequate room for an upstream pipe run into the pumps. Using a tab-type flow conditioner placed in the elbow compensated for the lack of straight run and provided an equally distributed flow profile entering the pumps.

Conclusions

With thorough advance planning, the selection of the proper flow instrument, following best practice installation techniques, and the addition of flow conditioners where appropriate, you can optimise the performance of your flow instruments. In many cases, the simple reduction inflow meter straight pipe run with the use of flow conditioners can save plant floor space, and reduce material and installation costs to accommodate other equipment. Paying careful attention to
flow measurement can translate into big plant operational cost
savings.

Contact
Don Lundberg
www.fluidcomponents.com

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