Flow measurement in corrosive environments

Corrosion can occurs in all plants which makes choosing the correct flow meter a vital decision.

In all plants, some type of corrosion will affect the life and durability of the equipment. For example, corrosion is a common problem facing industrial wastewater treatment systems in petrochemical plants and refineries, as well as food/beverage, pulp/paper and many other industrial operations.

Water is used as a coolant, a cleaning solution or a way to move product and is often mixed with chemicals, solvents, disinfectants or other corrosive substances. The measured flow in natural gas production can be any combination of gas, condensate and water, and may also contain the corrosive gases methane or hydrogen sulfide.

When it comes to corrosion, it is important to recognise the question is not whether it will occur, but how severe it will be and what can be done to delay it. Properly considering purchase, installation, maintenance and replacement cost trade-offs, while selecting and commissioning plant equipment, can add up to considerable savings.

Typical hazards

There are several types of corrosion commonly found in processing plants, such as oxidative, galvanic, microbiological, pitting and erosion. The effects of corrosion range from unsightly rust, at a minimum, to catastrophic process failure, resulting in significant cost outlays.

When installing or replacing flow meters, it is important to consider the plant environment, as well as the specific process conditions. Oxidative, or generalised corrosion, such as rust, occurs when metals encounter water.

The hydrogen in water, or other acid catalysts, exposes the base metal to oxygen starting a chemical reaction. If there is exposure to saltwater, the problems are even worse.

Certain forms of chlorine, cyanide, hydrogen sulfide and methane are a few more of the many catalysts that trigger frustrating corrosion problems in certain metals when the resulting reactions form an oxide with the base metal.

With iron, the reaction forms iron oxide or rust. Choosing the correct material for a flow meter can mean the difference between a productive day or one filled with time consuming maintenance issues and operations impacts.

Galvanic corrosion occurs when two dissimilar metals come into contact. Every metal has an inherent, but different, electrical potential. If the difference in potential of the two mating materials is too great, then they are considered incompatible.

The result of galvanic corrosion is the gradual transfer of material from the anodic to the cathodic metal. Sometimes an anodic metal can be used as a sacrificial material to intentionally prevent corrosion elsewhere.

Microbiological corrosion (MIC) results when microorganisms breakdown or expose the metal to other types of corrosion. Once microbiological corrosion begins, it can be difficult to stop without the use of strong disinfectants such as chlorine, chlorine dioxide and ozone.

Typical cleaning merely reduces contamination to an acceptable level to avoid clogging of equipment like flow meters.

MIC is an obvious issue in wastewater systems and can be very frustrating when metering steam, cooling water and make-up water. Pitting is the result of localised corrosion and can be caused by uneven coatings, cavitation and damage from particles in the fluid stream that leave metal exposed for corrosion.

The risk is that pitting corrosion is difficult to detect and can remain localised in small areas, corroding metals through or weakening entire structures without detection. High pressure, high temperature and harsh environments pose serious concerns.

Selecting a flow meter with proven long-life and no moving parts is vital. While Orifice plates are commonly used in industrial applications, selecting a more robust Venturi or V-Cone® flow meter can save you both time and overall costs.

Erosion corrosion is the constant removal of material due to process media or abrasive substances slowly wearing away the surface of the metal. Erosion can occur throughout a system or at elbows, fittings and restrictions.

The result of corrosion is unnecessary maintenance and reduced life of flow meters, pumps, valves and piping.

Meter for corrosive conditions

Some flow meters are inherently more susceptible to corrosion, like those with moving parts that may rust or pit and those that may plug and foul. Consider the effects of pitting, cavitation, erosion, microbiological and oxidative corrosion for Orifice flow devices.

When selecting Venturi meters, the key factors to consider include conditioning plates and pitting, erosion and oxidative corrosion. McCrometer’s V-Cone Flow Meter redirects the flow to the outside of the pipe to reduce these effects.

Selecting materials according to your specific process conditions is also important. Appropriate coatings for harsh conditions avoid oxidative corrosion and pitting, and other similar materials to avoid galvanic corrosion.

A diagram showing how the redirected flow reduces corrosion effects

Materials should be compatible with the process media and able to withstand cleaning cycles and environmental conditions of your application. All major flow measurement technologies have corrosive attributes and drawbacks depending on the process, installation location and operating environment.

Choosing a flow meter that meets the process requirements and is versatile enough to be used throughout a plant is crucial. The V-Cone not only provides precision flow measurement in liquid, steam or gas, but requires virtually no maintenance in demanding processes where corrosive conditions exist.

The meter is ideal for use in a wide range of industries, from petrochemical and power generation to food/beverage, pharmaceutical, pulp/paper, water and wastewater. The device is based on advanced differential pressure technology, requiring no moving parts that may fail due to corrosion and has no spaces that may clog during use.

Built-in flow conditioning allows the V-Cone to achieve accuracy of ±0.5 per cent and repeatability of ±0.1 per cent with straight pipe runs of only 0–3 pipe diameters upstream and 0–1 pipe diameters downstream.

V-Cone’s unique design centrally locates the cone within the precision flow tube, which conditions the fluid flow by redirecting it to the larger surface area of the pipe wall and reshapes the velocity profile to create a low-pressure region immediately downstream. The fluid forms very short vortices as the flow passes the cone creating a highly stable low amplitude, high frequency flow profile and excellent signal stability.

The pressure difference measured between the static line pressure upstream of the cone and the low pressure at the downstream face of the cone is incorporated into a derivation of Bernoulli’s equation to determine the fluid flow rate.

The V-Cone operates over a wide flow range with an unprecedented 10:1 turn down, services line sizes from 0.5 to 120 inches (13 to 3,048 mm), and is available in standard stainless and carbon steel configurations or constructed to meet demanding requirements using hastelloy, duplex, chromoly, monel, inconel and more.

The result is V-Cone can be installed or retrofit virtually anywhere resulting in major cost-of-ownership savings.

AMS Instrumentation & Calibration distributes McCrometer products in Australia and New Zealand, in addition to a wide range of other leading manufacturers. With decades of experience, AMS provides industrial instrumentation and calibration equipment to industries, government departments and public utilities.

This article was featured in the July edition of The Australian Pipeliner. To view the magazine on your PC, Mac, tablet, or mobile device, click here.

For more information visit the AMS website.

If you have a project you would like featured in The Australian Pipeliner contact Managing Editor David Convery at dconvery@gs-press.com.au

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