Gas flow measurement: what you don't know can be very expensive

Inaccurate or inconsistent measurement of air and gases can result in serious accidents, emergency shutdowns, unplanned maintenance or production slow-downs.

There are six to eight viable air/gas flow measurement technologies on the market today, but only about half of them are suitable for the heavy-duty metering applications found in the most challenging chemical plant processes. Each of them has its own strengths and weaknesses, depending on exactly what needs to be measured, the required accuracy, where it needs to be measured, etc.

The truism “knowledge is power” definitely applies when it comes to choosing an air/gas flow meter for measurement tasks in chemical plants. The same flow sensing technology that you choose for one application in your plant is quite possibly the wrong choice in a different application that can even be in close proximity.

The cost of choosing the wrong flow meter in terms of extra maintenance, repairs and spares in large chemical plants can add up quickly to tens of thousands of dollars alone. If safety events or poor product quality or a production slow-down or environmental compliance issues occur, then the cost of failing to recognise the subtle differences in air/gas flow measurement technologies can be punitive.

Common measurement applications

Flow meters are used to measure air/gas flow rate and totalised flow. Due to the hazardous operating environment of chemical plants, air/gas flow meters generally require hazardous area approvals and often must be SIL compliant as part of a safety instrumented system (SIS) in many applications. Four of the most common and the most demanding air/gas flow measurement applications in chemical plants are gas distribution metering, flaring systems, tank blanketing and flue gas monitoring.

Gas distribution metering

Many chemical processes require large varying volumes of specific gases, such as nitrogen, argon and oxygen for inert ion or purging or blanketing; hydrogen is required as a catalyst and other specific gases are used as well. The accurate measurement of these gases is necessary for process control, gas inventory control and cost management.

Flaring systems

In petrochemical production, refining and storage, flare gas systems are used to burn off and dispose of waste, excess or off-gases and as a safety system (Figure 1). The accurate, responsive and reliable measurement of flare gas is essential in order to assure proper operation of the flare gas system, which protects people and equipment from hazardous combustible gas to maintain a safe working environment and to avoid environmental contamination.

Figure 1: Flaring system.

Tank blanketing

Nitrogen blanketing is used in the chemical and petroleum refining industries to reduce the hazards associated with flammable liquids, which supports plant safety and can help increase productivity. Blanketing or padding is a process of applying inert nitrogen gas to the vapour space of a tank or vessel (Figure 2), which minimises the possibility of an explosion or fire by reducing the oxygen content or the concentration of flammable or explosive vapours.

Figure 2: Tank blanketing.

Flue gas monitoring

Measuring the output of chemical plant waste gases through large stacks or flues with scrubber systems for environmental compliance requires multiple air/gas flow sensors, placed in strategic locations (Figure 3). Stack continuous emission monitoring systems (CEMS) must meet various international and national emission standards.

Figure 3: Stack gas monitoring system.

Air/gas measurement challenges

Accurate, dependable gas flow measurement applications present challenges to chemical industry plant, process and instrument engineers. The following issues require careful attention when choosing a flow meter sensing technology:

• Low and high flows: Sensitivity to low flow conditions is required to identify and measure leaking valves and the normal low flow associated in day-to-day operations. The capability to measure very high flows is needed during system upset conditions requiring a meter that needs to measure flow accurately over an extremely wide turndown range.
• Meter calibration: The calibration of flow meters specifically for hydrocarbon composition gases and matching to actual process conditions is essential.
• Large line sizes: As pipe sizes increase, the number of effective and suitable flow meter sensing technologies decreases.
• Available straight-run: All velocity-based flowmeter technologies have pipe straight-run requirements upstream and downstream from the meter in order to achieve accurate flow measurement. These straight-run requirements may not be available in crowded production sites and process plants.
• Limited access: Access to piping for installation, maintenance or servicing is frequently difficult. For example, spool-piece flowmeters can require prolonged process shutdowns and extensive on-site labour costs to install and continuously maintain the system, as opposed to insertion-style meters that can be easily inserted into or retracted out of the process through a ball valve.
• Agency approvals: When installing meters in hazardous (Ex) locations, the entire flow metering instrument should carry agency approval credentials for installation in environments with potential hazardous gases; enclosure-only ratings are inadequate.

Major gas measurement technologies

There are two basic types of flowmeters: liquid and air/gas. Liquid is primarily measured in terms of volumetric flow, while air/gas is a mass flow measurement because of the unique properties of gases (versus liquids). While some volumetric technologies can measure air/gas flow rates, there can be problems with totalised flow. Generally, the best choice is mass flow sensing technology when measuring air/gases — especially in critical applications.

Coriolis

The principle of operation for Coriolis flowmeters relies on a vibrating tube where the flow of a fluid causes changes in frequency, phase shift or amplitude, which is proportional to the mass flow rate. Coriolis meters are highly accurate and frequently used in custody transfer applications, but they are on the expensive side and require labour-intensive inline installation.

Differential pressure

DP meter sensors come in several designs, including orifice plates, pitot tubes and Venturis. The typical DP meter designs requires the fluid to move through or past two points of reference, creating a differential pressure rate that is equivalent to the rate of flow using the Bernoulli equation with some modifications. If the air/gas or fluid is dirty, there can be orifice clogging issues that require frequent maintenance in order to maintain accuracy.

Ultrasonic

Flowmeters designed with ultrasonic flow sensing technology rely on ultrasound and the Doppler Effect to measure volumetric flow rate. In ultrasonic flowmeters, a transducer emits a beam of ultrasound to a receiving transducer. The transmitted frequency of the beam is altered linearly by particles or bubbles in the fluid stream. The shift in frequencies between the transmitter and receiver can be used to generate a signal proportional to the flow rate.

Optical

Flow meters designed with optical sensing rely on laser technology and photo-detectors. This technology requires the presence particles in the gas stream. These particles scatter the light beam and the time it takes for these particles to travel from one laser beam to the other laser beam can be used to calculate the gas velocity and volumetric flow rate. These meters have good accuracy and wide turndown, but are traditionally expensive.

Thermal dispersion

Flowmeters with thermal dispersion sensors provide direct mass flow measurement. Two thermowell-protected platinum RTD temperature sensors are placed in the process stream. One RTD is heated while the other senses the actual process temperature. The temperature difference between these sensors generates a voltage output that is proportional to the media cooling effect, and can be used to measure the gas mass flow rate without the need for additional pressure or temperature transmitters.

Flowmeter calibration

In measuring flow accurately, second only to selecting the proper flow sensor is the method of calibration. There are two methods used in calibrating air/gas flow meters:

1. The Direct Method, where the meter is calibrated to a specific pure process gas or to the actual components of a mixed gas in use.
2. The Air Equivalency Method, where the meter is calibrated using air and then the calibration is adjusted with a predefined correction factor.

It is important to ask your supplier about the method of flowmeter calibration. You should know if manufacturers contract out and with whom, or if they operate their own calibration laboratory with direct method calibration test stands and equipment traceable to NIST and ISO/IEC 17025.

Installation considerations

When choosing an air/gas flowmeter technology, two of the most important criteria to consider are the location and the manufacturer’s installation requirements. Most flowmeter technologies require a stable fluid flow profile upstream and downstream from the point of meter installation; a specific number of pipe diameters in each direction. Flow sensors are potentially sensitive to swirling air/gas conditions in the pipe, or pressure drops (turndowns) or flow surges.

In many cases, irregular flow issues can be solved with flow conditioners. There are various types of flow conditioners that can be inserted strategically in the pipe to ‘straighten’ the flow before it reaches the flow sensor. They consist of tabs or honeycombs or vanes or other designs, which all straighten the flow.

There are two ways to install a flowmeter: inline or insertion. Inline flowmeters must be installed horizontally inside a section of the pipe. Insertion flowmeters are top mounted through a tap point.

Some flowmeters can only be installed using one method. Venturi meters, for example, must be installed inline (inside the pipe). In comparison, thermal meters, some DP meters (orifice plates) and others can be installed in either inline or insertion configurations.

Lastly, when considering installation requirements, some flowmeter technologies rely on direct mass flow sensors. Other flowmeters infer mass flow and require pressure or temperature sensors to be installed nearby along with transmitters, which can add to their cost and installation complexity.

Maintenance requirements

All flowmeters require maintenance. Some meters, however, require more maintenance. The type of air/gas fluid to be measured can have a major impact. Pure process gases in a benign plant environment are generally going to have less impact on a flowmeter than dirty waste gases.

Some meter designs require less cleaning or are easier to clean than others. For example, top-mount insertion-style meters with packing glands can be quickly pulled out of the pipe without shutting down the process and cleaned in place with compressed air and then returned to service.

Conclusions

There are many factors to consider when choosing a flowmeter for application within a chemical plant. A good checklist of considerations would include:

• Accuracy
• Repeatability
• Flow sensor technology
• Calibration type
• Installation requirements
• Maintenance
• Cost
   

In considering the cost of a flowmeter, there are three crucial factors to think about:

1. The purchase price of the meter
2. The installed cost
3. The lifecycle cost

Stopping your analysis at the purchase price is misleading when it comes to reviewing the true cost of instrumentation — especially flowmeters.

We’ve discussed the two types of flow meter installation. Insertion configuration flowmeters are simpler to install, which is going to result generally in a lower installed cost versus a flowmeter that is less expensive to purchase though it requires an inline installation.

The last factor to consider is the lifecycle cost. How long does the manufacturer expect the flowmeter to remain in service? Is its life span 5, 10 or 20 years? Over that lifetime, what kind of maintenance will be required? Some meters have movable parts that can break and require repair. Some meters depend on small orifices that tend to narrow or clog in dirty environments, requiring cleaning. These expenses can add up over time, which increases the cost of ownership.

Knowledge and experience with flowmeters is power. The more you know about flowmeter technologies, the better the decisions that you’ll make.

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