If a trip to the doctor for a routine checkup resulted in a call for a drastic and costly medical procedure, there would be a legitimate reason to request a second opinion. If we apply the same concept to a major process unit in a refinery or chemical plant, an alarm triggered by a single field instrument could cause a complete shutdown with production grinding to a halt. This may be caused by an actual emergency and is entirely correct. On the other hand, a malfunction in that one instrument may create a false alarm with hugely expensive consequences.
The question for safety and process engineers, is how do we respond quickly to actual emergencies, while minimizing the possibility of false alarms causing production disruptions? Emerson has a very practical suggestion for flow measurements, and it’s the topic of my article at automation.com, Flow Meter Design Addresses SIL Reliability and Process Control Challenges. Many critical applications use two-out-of-three (2oo3) voting schemes where three safety instruments are measuring the same variable, and the safety function won’t trip without at least two of them sending the alarm.
The safety requirement calls for three separate devices so they can function independently, along with an additional device in the same area of basic process control measurement. This can become a complex arrangement, particularly where the variable is a flow measurement and the metering technology is a conventional differential pressure flow meter.
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.
Fortunately, there are more practical alternatives that solve all these and other problems at once: a vortex flow meter in a quad design, using Emerson’s Rosemount 8800 Series Vortex flow meter family. This is a very effective flow meter for liquids, gases, and steam across a wide temperature range. It is also possible to assemble into a compact quad configuration that is very efficient.
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 Rosemount 8800 Quad Vortex flow meter provides the ultimate redundant flow measurement solution to guard against spurious trips by using three safety-certified transmitters arranged for 2oo3 voting, plus a fourth integrated transmitter for process control. This assembly delivers exceptional SIL 3 reliability with a gasket-free, non-clog meter body that eliminates potential leak points, resulting in maximum process availability and fewer unscheduled shutdowns. Models for line sizes from 2 to 12 inches are available. The article includes a typical refinery application case study where a plant reduced maintenance costs substantially by switching to this approach.
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.
For more information, visit Emerson’s Flow Measurement Systems pages, and the Control & Safety Systems pages at Emerson.com. You can also connect and interact with other engineers at the Emerson Exchange 365 community.
