August 2020

Special Focus: Valves, Pumps and Turbomachinery

Myth busting: Unlocking the hidden value of Coriolis flowmeters in refining

Process engineers must make many decisions to meet production targets, while maintaining the safe and efficient operation of a process unit.

Process engineers must make many decisions to meet production targets, while maintaining the safe and efficient operation of a process unit. Changing process conditions, maintaining equipment, selecting catalysts, implementing design changes and executing turnarounds are all actions that process engineers take to maintain or improve the performance of a unit. What process engineers often overlook is the impact that flow measurement instrumentation can have on improving the optimization and safety of a unit.

It is easy to underplay the impact that flow measurement technology can have on a unit’s performance because, when many refineries in the U.S. and Europe were first built, there were not as many flow measurement technologies available as there are today. Most flow measurement points in a refinery are measured by traditional differential pressure (dP) meters that have had a strong track record for decades in providing reliable measurements for process control in refineries. However, a flow measurement device can deliver much more than just a repeatable measurement. As flow measurement technology has advanced, additional benefits can be derived from newer technologies, such as more process variables, higher accuracy, reduced maintenance, increased reliability and advanced diagnostics. These added benefits can contribute to increased unit uptime, improved safety and process optimization of a unit.

Coriolis flowmeters are one of those relatively newer flow measurement technologies with unique benefits and applications that process engineers can use to achieve optimization benefits. Coriolis flowmeters are multivariable devices that provide a direct measurement of mass flow, density and temperature for liquids, slurries and gases. The technology is based on the Coriolis effect, in which the inertia created by the fluid flow causes the meter’s tubes to twist proportionally to the mass flowrate. TABLE 1 shows a comparison of Coriolis technologies to traditional dP meters used in refining.

Although Coriolis flowmeters have now been utilized in refining for a few decades, especially in custody transfer and product blending applications, this flow measurement technology is still relatively unfamiliar to many process engineers. As a result, there are lingering misconceptions about the technology and what benefits Coriolis meters can provide. Understanding the incremental value that Coriolis meters offer compared to traditional volumetric technologies will provide inherent benefits that can help unit engineers solve their optimization and operational problems.

Myth #1: Coriolis flowmeters are only used for custody transfer applications

One of the most common myths that process engineers believe about Coriolis flowmeters is that they are best used for, or only used for, custody transfer applications. Coriolis flowmeters have high flow measurement accuracies—as good as ± 0.05% in liquids, ± 0.25% in gas and ± 0.0002 g/cc density accuracy for liquids. This is not only beneficial for custody transfer applications, but also for process unit monitoring and mass balance, feed characterization, product quality control and concentration measurements. The direct mass measurement and online density measurement are useful for fluids with changing density, viscosity and compressibility.

The process unit mass balance is a monitoring activity done to calculate yields and conversions, evaluate catalyst activity and optimize fractionator cut points. Achieving accurate unit mass balances can be a challenge because dP orifice meters are often used to measure unit mass balance points. These volumetric meters are impacted by changes in fluid properties and process conditions. Typically, the accuracy of volumetric measurements ranges from 1%–5%, depending on compensation. In addition, a representative density sample of the fluid is required to convert the volumetric measurement into a mass measurement to calculate the mass balance.

Coriolis flowmeters are extremely beneficial in these mass balance applications, compared to volumetric technologies, because the accuracy of direct mass measurement is not impacted by changes in pressure, temperature, composition or fluid density. Therefore, Coriolis flowmeters can provide more accurate and reliable charge and yield measurements for closing the balance and making better optimization decisions. A significant number of refineries have begun replacing their orifice plate dP measurements with Coriolis flow measurements on key mass balance points. This allows refiners to obtain a sustainable daily mass balance, and to optimize yields and drive unit profitability. One refiner reported calculated savings of nearly $40,000/d by more accurately following planning models with accurately measured process flows.

The online density measurement in Coriolis flowmeters can also be used for improved operations and as part of advanced process control schemes. Online continuous density measurements on feed streams with variable compositions—such as crude units, cokers, fluidized catalytic cracking (FCC) units and hydrocrackers—can utilize the density measurement from the Coriolis meter to enable feed-forward control functionality to prevent upsets and instability.

Another application where direct mass measurement has optimization benefits is with combustion control in fired heaters and boilers. The control strategy of fired heaters and boilers impacts safety, environmental compliance and efficiency. Many refineries use fuel gas for combustion. Fuel gas is made up of refinery offgases from various units; therefore, the composition and energy content of the gas can vary frequently. As the composition of the gas changes, the stoichiometric air required for combustion also varies. Excess air is supplied to the heater to ensure that a safe amount of air is present. However, there have been many cases where a rich fuel gas will deplete the oxygen to below the safety limits of the heater, resulting in a trip or shutdown of the heater. These trips or shutdowns most often occur when control schemes based on volumetric flow or pressure are used to control the fuel gas. Utilizing Coriolis flowmeters for mass-based control of fuel gas has proven to reduce oxygen variability and to provide more stabilized control of the combustion in the heater or boiler, resulting in increased uptime and safer operations. This is because the energy content and stoichiometric air required for combustion of the various components in fuel gas are more proportional on a mass basis than on a volumetric basis, making a mass-based control scheme of the fuel gas more stable than a pressure- or volume-based scheme. Many refiners have avoided trips and unsafe conditions by changing the fuel-gas control scheme from volume flow control to mass flow control. Studies have shown a net present value of between $250,000 and $1 MM by changing the control scheme.

Some additional applications where measuring mass and/or online density can be utilized for optimization benefits include online measurements for acid strength and the acid-to-hydrocarbon ratio in the contactor of sulfuric acid alkylation units, as well as chemical injection metering, hydrogen measurement and crude or product blending (FIG. 1).

FIG. 1. Coriolis flowmeters are multivariable flow measurement devices that measure direct mass, density and temperature, and can be used in critical refining applications (such as mass balance, combustion control of fired equipment, and blending) for improved optimization.

Myth #2: Coriolis flowmeters cannot be used for dirty or viscous process fluids

Another common myth for Coriolis flowmeters is that the meters cannot be used for dirty or viscous process fluids because the meter tubes will plug. One common use of Coriolis flowmeters in refineries is to measure liquid asphalt or molten sulfur in custody transfer loading applications. The viscous natures of these materials, and their ability to solidify if not kept within a certain temperature range, create a challenging measurement. Often, positive displacement meters or other traditional volumetric meters are used, but the maintenance for these meters is significant, and the moving parts are at risk of plugging. In addition, the accuracy of these meters degrades over time due to wear and tear, as well as to changes in fluid properties, such as the quality of the product or the viscosity, which changes as the temperature changes. Both the type of meter and the electronics play a key role in these applications. Some Coriolis flowmeter manufacturers have an optional software feature, specifically for molten sulfur applications available in the transmitter, that stops the tubes from vibrating if the fluid solidifies, thereby preventing damage to the meter.

Another challenging application area where Coriolis flowmeters can be used are residual flows. These are typically the most inaccurate measurements in a refinery; they are difficult measurements due to viscous process fluids and high temperatures. Coriolis meters have been successfully used to measure these streams.

An example is measuring coker unit feed streams. Coker units are among the most difficult units to mass balance due to the challenges in accurately measuring coke production. Therefore, having accurate charge and yield flow measurements is especially important to monitor and evaluate unit performance. Since the coker feed is normally quite viscous, it is difficult to get a fully developed flow profile required for volumetric- and velocity-based flowmeters. Since Coriolis flowmeters are independent of flow profile, composition and viscosity, they can provide accurate measurements for this challenging application.

Coriolis flowmeters can also be used in abrasive applications. For example, FCC slurry oil is an important but difficult measurement due to the residual, abrasive catalytic fines in the stream. Traditionally, wedge meters are used in these applications. Reliability, low accuracy and high maintenance costs must be considered. Coriolis flowmeters can be sized to keep the velocity through the metering tubes low and to reduce the impact and wear to the meter, while continuing to provide an accurate measurement. Diagnostic tools from some manufacturers can also be used to ensure that the integrity of the tubes is sound, as discussed later in this article.

Myth #3: Coriolis flowmeters are expensive. Another common myth is that Coriolis flowmeters are expensive

Often, engineers will only factor in the capital cost of the meter, as opposed to the total cost of ownership (TCO). Coriolis flowmeters do have a higher capital cost, but the TCO is typically lower for several reasons. First, Coriolis flowmeters are multivariable devices offering density, direct mass and temperature in a single device. To derive mass measurements from volumetric technologies, additional instruments (such as temperature transmitters, pressure transmitters and densitometers) would have to be installed and maintained.

Second, Coriolis flowmeter accuracy is independent of the flow profile; therefore, there are no upstream or downstream straight-run piping requirements and flow-conditioning elements required, as there are with volumetric flowmeters, which are flow profile dependent.

Third, Coriolis flowmeters have no moving parts or impulse lines, thereby significantly reducing maintenance requirements. In addition, some Coriolis flowmeters have advanced diagnostics that can be run in real time, which further reduces maintenance efforts compared to other meters. Online meter verification is a diagnostic software that monitors the health of the meter—both the sensor and the transmitter—by measuring the tube stiffness, checking the electronics functionality, and comparing these to the factory baseline. This can be performed without requiring a process shutdown or disrupting flow into the meter. Online meter verification maintains measurement accuracy and meter integrity, thereby driving cost reduction through early detection of issues and/or through extension of calibration intervals.

The online meter verification also aids significantly in providing a historical log of data and reporting for compliance. Complying to the U.S. Environmental Protection Agency’s (EPA’s) Petroleum Refinery Sector Rule, refineries must typically report fuel gas consumption, charge rates for units that are significant greenhouse gas contributors, and non-vent gas flows in flare systems for purge or supplemental gas. These requirements usually require a frequent calibration of the meters used for reporting every few years, and/or a validation of the accuracy of the devices used in reporting, to avoid fines or permitting issues. Online meter verification of Coriolis flowmeters has been accepted by numerous agencies globally. Applying traditional dP meters for these applications sometimes requires taking these devices offline to inspect and/or calibrate them during the required intervals mandated in these specifications.

Myth #4: Coriolis flowmeters have high pressure drops

Another area of concern for process engineers is the potential for pressure drops through Coriolis flowmeters. A tradeoff exists between the accuracy of the meter and the pressure drop; however, because of the very wide turndown for a Coriolis flowmeter, there are usually several options for the meter size, each with a different pressure drop. In most cases, the accuracy can be maintained over most, if not all, desired flow ranges. If the pressure drop is too high with a given meter size, a larger meter can be chosen that will decrease the pressure drop.

If volumetric meters are used and additional instruments or flow-conditioning elements are required, this adds to the overall pressure drop. Coriolis flowmeters do not need additional instruments or flow-conditioning elements, so the pressure-drop contribution comes only from the meter.

Myth #5: Coriolis flowmeters must be installed and oriented a certain way

Coriolis flowmeters are independent of flow profile, and do not require special straight-run or flow-conditioning requirements; therefore, installation is not limited to any of these constraints. Furthermore, the meter will work in any orientation, if the tubes are always full of process fluid. While the preferred orientations are “tubes down” for liquids and “tubes up” for gases, the “flag” orientation is also popular.

The mass movement to Coriolis flowmeters

The availability of more advanced flow measurement technologies has created new opportunities for process engineers to solve operational and optimization challenges differently. Historic beliefs about Coriolis flowmeters—they are too expensive, require strict installation requirements, are unable to handle viscous fluids, and are basically used just for custody transfers—are not true. For decades, refinery engineers have unlocked the value that Coriolis flowmeters provide across the refinery, and not just at the refinery gates. Whether it is around costs, safety, reliability or efficiency, Coriolis flowmeters, in certain refining applications, can be a simple but powerful solution for generating value in the form of optimization. These multivariable capabilities, along with the multiple benefits of Coriolis technology, are fueling a mass movement to utilize this technology more than ever before within refining process units. HP

The Authors

From the Archive



{{ error }}
{{ comment.comment.Name }} • {{ comment.timeAgo }}
{{ comment.comment.Text }}