- By Ellen Degnan
- December 09, 2024
- Emerson
- Feature
Summary
A vortex flowmeter design provides three independent safety measurements and a process control variable measurement in a maintainable meter body.
Large, continuously operating chemical plants and refinery processes are often faced with the need to measure critical flows that feed both interlocks and process control systems. Since these interlocks may go years between test intervals and the price of inadvertent shutdown is so high, two out of three (2oo3) safety integrity level (SIL) trip voting schemes are common. This then requires four independent measurements of the same process flow.
The common solution to this application has historically been a single orifice with up to four sets of process taps feeding four separate differential pressure transmitters. Unfortunately, those static pressure process connections are expensive to install, difficult to maintain and are prone to plugging and freezing. This article describes a solution for critical flow applications that is less costly and significantly more reliable than the alternatives.
Critical flow measurement challenges
Many petrochemical applications involve critical flows that must be carefully controlled to provide safe and efficient operation. In many situations, measurements from these same flows are fed into safety interlocks to protect equipment and personnel. The process flows may involve liquids, steam or gases, and in all cases are carefully monitored to ensure the proper ratios and flow rates are maintained. If flow drifts too far from design, the plant is often tripped offline, which results in expensive downtime and significant loss of production.
To improve reliability and avoid unplanned downtime, many of these critical flow interlocks are set up in a 2oo3 trip voting arrangement. This ensures a safe shutdown when required, while avoiding unnecessary trips due to a single transmitter failure. In such cases, the plant then needs four separate measurements of the same flow—three to feed the safety interlock and a fourth to feed the control system proportional-integral-derivative (PID) loop as its process variable. Each reading must be independent to avoid common cause failures, yet all must measure the same flow. This is not a trivial challenge.
While it is possible to install four separate flowmeters on the same line, the length of pipe required for such an installation would be significant because each flowmeter usually requires straight runs of piping before and after the transmitter. A common solution is to install a single orifice with four separate differential pressure transmitters measuring the drop across the orifice, but this approach has several drawbacks.
The first problem is one of independence; ideally each meter should have nothing in common with the others. In the case of an orifice with four transmitters, the orifice itself is common to all four, so a damaged orifice, or one with water trapped upstream, would affect all four readings. Another problem is cost. Installing four separate sets of orifice taps and static tubing is expensive, particularly if the static lines must be heat traced.
The tubing is also prone to plugging and freezing, which makes the signals less reliable. Fortunately, an innovative process flowmeter design has entered the market, and it is specifically designed for these types of applications.
Quad vortex flowmeter
For many applications, vortex meters are often the best flow measurement technology. These meters have vertical shedder bar installed in the flow path of a gas, liquid or steam process line (Figure 1). As the fluid moves past the obstruction, it creates tiny eddies or vortices on alternate sides of the bar. This effect is similar to what causes a flag to wave in the wind, or whirlpools to form as a paddle moves through water.
The rate of vortex formation is directly proportional to flow, so the vortex flowmeter uses a sensitive sensor within the shedder bar to count the eddies as they are formed. This count can then be converted to a volumetric flow. Pressure drop through the meter is minimal, and turndown can be as high as 20:1. Vortex meters do exhibit a phenomenon called low flow cutoff where the meter reading drops to zero at very low flow rates or high fluid viscosities. Otherwise, vortex meters are accurate and quite repeatable, making them often the best option for process and safety interlock flow measurements.
While it is possible to put four individual vortex meters in series to measure a particular flow, it would be quite unwieldy as each meter would require as much as 35 pipe diameters upstream and 10 diameters downstream. Fortunately, an innovative design combines four meters into a single body (Figure 2). The quad vortex meter design provides four completely independent flowmeters, each using its own sensor, but the meter only requires the straight run piping of a single unit.
Internally, each pair of vortex sensors is tied to a single shedder bar, but each has its own flexure sensor so common cause failures are minimized. The two shedder bars are parallel to each other and situated such that the second shedder bar amplifies the signal of the first, which avoids crosstalk and interference between the transmitters.
The new design works well for critical liquid, vapor or steam flow measurements and it is offered in sizes of 2 inches to 12 inches with a wide variety of body materials and flange ratings up to 1,500 psig. The meter can handle process media temperatures from -330 degrees F to 842 degrees F, which makes it suitable for cryogenic and superheated steam applications. It also eliminates the static connections required for orifice plates. This greatly reduces installation costs and eliminates the ongoing reliability problems associated with static sensor line heat trace, plugging and freezing.
While vortex sensor failure is exceedingly rare, the design allows the sensors to be replaced without removing the meter from service (Figure 3). The sensors can be ordered with a small bleed valve, typically referred to as the critical process valves, that allows the technician to confirm there is no pressure in the sensor compartment. Once confirmed, the sensor can be removed and replaced while the other three meters remain in operation.
The ability to maintain and repair a single meter while keeping the other units in operation can save significant costs in downtime and lost production.
Case study
A common critical flow application can be found on coking furnaces where multiple streams of process products are fed through a furnace (Figure 4). During furnace operation, it is critical that the flows through each path remain somewhat balanced to avoid tube overtemperature and failure.
Superheated steam is also periodically fed into each pass to remove coke from the tube walls and introduce emergency steam into the tube passes on shutdown. These flow measurements are critical to continuous furnace operation, so 2oo3 safety interlocks and a process measurement are required.
One refinery had been using a single orifice plate with four transmitters for this application, but continuous problems with impulse line plugging and freezing created significant maintenance costs and downtime. The orifice plate arrangements were replaced with quad vortex meters. Ongoing maintenance costs dropped dramatically and inadvertent outages and trips caused by sensor line plugging were eliminated. The meters have since been operating continuously with no measurement issues.
Final thoughts
Large, continuous chemical and petrochemical plants often run for years between outages and require 2oo3 SIL 2 or SIL 3 rated flow interlocks to enable these extended runs. In these applications, a quad vortex meter is often the preferred solution. Its compact design requires the same upstream and downstream meter runs as a single meter, yet it provides four completely independent flow measurements. Each meter can also be maintained and repaired without removing the other meters from operation. The design eliminates the problematic static pressure sensing lines associated with the traditional multi transmitter orifice plate solution, which reduces installation and maintenance costs.
For critical flow measurement applications in safety-related applications, four separate flowmeters have been the traditional solution, but these types of installations have issues related to cost, required upstream and downstream piping runs and maintenance. Quad vortex flowmeters provide all the advantages of four separate meters, while addressing the issues associated with these traditional designs, making them the best choice for many of these types of applications.
All figures courtesy of Emerson
This feature originally appeared in the November 2024 issue of AUTOMATION 2024.
About The Author
Ellen Degnan is a global product manager at Emerson based in Eden Prairie, Minn. She is responsible for managing Emerson’s Vortex Flowmeter portfolio. She has a Bachelor of Science degree in bioproducts and biosystems engineering from the University of Minnesota.
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