Petroleum refineries have traditionally employed differential pressure (dP) flow meters for most flow measurement points across the facility. These meters have historically been used because they are affordable upfront, and their repeatable measurements are well suited for process control in a variety of applications.
Although measurement repeatability is important for process control, there are additional benefits that a flow measurement technology can deliver, such as improved accuracy and reliability of a device. And, while dP flow meters have great application flexibility and repeatability, their accuracy can easily be impacted by changes in the composition or flow profile of the fluid flowing through them, or by changing process conditions such as pressure or temperature.
It is important to note that on the back end, dP flow meters require very regular maintenance as these technologies have impulse lines that can be prone to plugging depending on climate conditions and fluid properties. Like many mechanical meters, the accuracy of the device also degrades over time due to wear and tear on the orifice plate.
Differential pressure flow meters have often been viewed as a one-size-fits-all technology in refineries, but there are some areas in a refinery where using a higher accuracy and more robust flow measurement technology like Coriolis meters for process automation and control has generated added optimization benefits beyond measurement repeatability.
Coriolis meters are multi-variable 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 proportional to the mass flow rate. Coriolis meters, in comparison to dP flow meters, offer a more robust device without moving parts that can handle a variety of changing fluid properties and flow profiles while maintaining accuracy.
When looking to automate processes, evaluating flow meter technology attributes such as additional process variables, reliability and accuracy should be among the considerations. To that end, we’re seeing that the Coriolis meter is finding increasingly more uses across the refinery.
Below is an in-depth look at some of the applications where Coriolis meters are proving reliable and optimal for process control and automation.
In the offsite areas of a refinery, it is common to find Coriolis meters on hydrocarbon storage tank discharge lines. Over time, hydrocarbon storage tanks are prone to collecting water that needs to be discharged to water treatment units.
There are several different methods for draining tanks, both manually and automatically. Traditional level measurement systems and manual methods often result in hydrocarbon being released into the water streams, which can potentially result in non-compliance with environmental regulations, inaccuracies in the mass balance and loss of valuable hydrocarbon or the need for re-processing. A Coriolis meter can be employed in a control loop to automate the process of draining water from storage tanks to avoid such losses of hydrocarbon to the water treatment systems.
Because the Coriolis meter can measure density, the density measurement can be used as an interface detection to detect between water and hydrocarbon being drained. Once the change in density is detected, the flow meter transmitter can trigger an event to close the valve and significantly reduce the amount of hydrocarbon withdrawn into the water stream.
Furthermore, the mass measurement of Coriolis meters can be used for mass balance purposes for measuring the amount of water drawn off from tanks.
Volatility in the crude market is driving refiners to extend beyond traditional crudes and incorporate opportunity crudes into their processes. The benefits to refiners are that opportunity crudes are readily available and often at discounted prices. However, there are challenges and risks that come with that convenience. Among the challenges are that opportunity crudes can be sour, heavier, much lighter or more acidic. Any one of these can make the crude more difficult to process because it often doesn’t meet the original design and configuration conditions of the refinery.
To process these kinds of crudes, they need to be blended to bring them as close to matching the original crude properties the refinery was set up for in order to reduce the risk of corrosion, fouling, bottlenecks and process slowdowns.
Typically, crudes are blended using tank gauging or volumetric flow meters to meet certain properties. Tank gauging can be problematic because although the level measurement is accurate, other variables, including imperfections in the uniformity of the tank, temperature gradients, heels, live tanks and stratification can lead to measurement inaccuracies. The accuracy of the volumetric flow meters used to blend crudes can also be a source of measurement error because the accuracy of these technologies is impacted by changing properties of the crude.
Sophisticated crude blending systems involve advanced blending control software, a near-infrared analyzer system that can provide a full characterization of crude, and accurate metering.
However, even just using better metering such as Coriolis meters can result in significant savings. One refinery was using uncompensated dP flow meters to ratio the components of the crude blend. The refinery found that changes in the crude properties affected the accuracy of the measurement too significantly, so it switched to using tank level measurement. But this was an issue because the refinery would fill and draw from the same tanks. The refinery could not control to a precise ratio based on either method and also faced demurrage costs because it was not on schedule with emptying tanks due to poor measurements. The refinery installed four Coriolis meters to precisely control the blend ratios enabling it to take advantage of the price differential between various crude stocks and decrease demurrage costs. This resulted in $1.5 million in savings in five months, as well as operational benefits of improved flexibility for crude unit operation, leading to better cuts and improved optimization downstream.
Precise ratio blend control of components is also critical in fuel blending. The end goal is to create an optimal blend based on inventory or profit that meets target product specifications like Reid Vapor Pressure and Octane while minimizing giveaway.
Like crude blending systems, fuel blending systems can also be sophisticated and require blend control software, near-infrared analyzer and metering. Inaccurate flow measurement can have a huge impact, resulting in products that do not meet specifications or result in quality giveaway.
Traditional technologies like turbine flow meters used in these applications lose their accuracy due to two-phase flow or entrained vapor in the line or mechanical wear over time. In addition, because turbine flow meters have moving parts, the maintenance of these meters can be costly. Coriolis meters improve blending system flexibility to meet differing seasonal or geographic demands by measuring different fluids without needing to be recalibrated since the meter accuracy is not dependent on composition. Coriolis meters also have no moving parts and significantly reduce maintenance costs. They are also rich in diagnostic information, making it easy to verify the accuracy over time.
Acid control in alkylation units
Coriolis meters can be used to reduce acid consumption by online measurement and control of acid strength in sulfuric acid alkylation units. Density measurements from Coriolis meters can be measured and correlated to acid concentration at the interstage settlers, as well as the final spent acid settler. Monitoring the spent acid strength online allows for better optimization and control of fresh acid. Coriolis meters are also used to determine the acid-to-hydrocarbon ratio in the contactor, replacing high maintenance, low reliability sight gases.
The right mixture of air and fuel is critical for fired equipment control. Many refineries use fuel gas as the fuel source in boilers and fired heaters. Most combustion control schemes for fuel gas are based on volumetric or pressure-based control.
Normally, the largest contributor of instability in fired equipment control is due to changes in the composition in the fuel gas, and therefore the energy content. Excess air is typically added to provide a safety margin due to the periodic and sudden step changes in energy content. Traditional volumetric and pressure-based control schemes are not as responsive to these step changes and often result in trips of the heaters as oxygen levels run below safety limits during a rich fuel condition. A mass-based control scheme using Coriolis meters, however, can better stabilize control because the fuel gas component heating values and stoichiometric air requirements are much more proportional on a mass-basis than a volumetric basis.
Many refiners have avoided trips and unsafe conditions by changing the fuel gas control scheme from volume flow control to mass flow control. Studies show a Net Present Value of between $250,000 and $1 million by changing the control scheme. An added benefit for Coriolis technology in this application is automated environmental reporting of fuel gas consumption and verification of meter accuracy used for reporting purposes through online diagnostic software built into the transmitter, which enables verifying the meter accuracy and health of the meter without any process disruptions.
Coriolis meters are multi-variable devices that deliver multiple benefits in multiple applications. Based on the examples above, Coriolis meters can provide additional process variables, online meter verification and reporting, and greater measurement flexibility while maintaining high accuracies. There are a variety of applications where Coriolis meters can be used for automating or improving control in a refinery in simple to complex applications, from interface detection to stabilizing the control of fired equipment. Regardless of the complexity of the application, all the benefits available with Coriolis technology are designed to deliver beyond process control purposes and impact optimization, safety, reliability and efficiency.
Meha Jha is a refining industry marketing manager for flow solutions at Emerson. Since joining Emerson, she was selected for Emerson’s Engineers in Leadership program, where she worked on projects related to mass balance and using flow measurement solutions to optimize production. She earned a bachelor's degree in chemical engineering from Auburn University.
Julie Valentine is director of global refining flow solutions for Emerson. She first joined Emerson in 1993 and worked as the refining industry marketing manager for Micro Motion products for 14 years, until 2007. In 2015, she became responsible for all of Emerson’s flow technologies for the refining industry. Valentine has authored numerous technical papers on various applications of flow technology in the refining industry and is a co-inventor on two U.S. patents. She is a member of several working committees for the instrumentation and control group of API’s Refining and Equipment Standards program. She holds a bachelor’s degree in chemical and petroleum refining engineering from the Colorado School of Mines.