Heavy blow to overflow
Vibronic point level measurement for overfill protection.
You’ll find liquids handling in all process facilities, including tank farms, food plants, chemical or pharmaceutical production sites, and water and wastewater industry facilities. Some of these liquids are toxic, flammable, and reactive, or they cause explosive gases and pose a risk to personnel and the environment. A facility operator has to assure these dangerous liquids stay in the appropriate pipes, tanks, and vessels. In particular, they need to avoid over-spilling a tank during filling processes.
Local laws, government regulations, pollution control agencies, and insurance companies require preventive measures to inhibit tank overruns (see Fig.1), especially during unattended automated filling processes. Irrespective of the federal and state regulations of a certain country, automated filling processes always require a high-level alarm that causes an automatic flow shutoff to prevent an overfill. The reliability and degree of functional safety of this overfill protection system are related to the potential danger of the liquid and the surrounding plant or facility.
Overfill protection systems
An overfill protection system should stop product flow during delivery before the tank becomes full and begins releasing liquid into the environment. As a general rule, such a system consists of a high-level sensor, a logic solver, and a final element that shuts off the flow into the tank. The system in Fig. IB consists of a vibronic point level measurement device, an appropriate power supply, and a switch amplifier unit in the control room. A PLC- or DCS-based logic solver and a supply pipe shut-off valveThe combination of these components has to fulfill the high functional safety demands defined in ANSI/ISA-84.00.01-2004, Functional Safety: Safety Instrumented Systems for the Process Industry sector, or IEC 61508 and IEC 61511. High functional safety means these components either have to work reliably or give an alarm whenever maintaining the protection system. Thus, you must define different types of failures when discussing a safety system.
Failures
A system’s mean-time-between-failures (MTBF) generally characterizes that system or device. That number represents an average lifetime value for the system or device and includes all failures. With regard to a functional safety classification, not all failures are relevant. Failures the system detects and the alarm announces do not lead to critical situations. But failures leading to a malfunction and not detected and announced automatically are dangerous.
In the case of an overfill protection system, an operator would still rely on the system. In case of a demand, the system would fail. To track down these dangerous failures and to quantify the likelihood of a dangerous failure on demand, do a failure mode, effect, and diagnostics analysis (FMEDA). The guidelines for this appear in the standards IEC 61508/IEC 61511 and ANSI/ISA-84.00.01-2004. Using these guidelines, you can calculate a probability of a dangerous failure on demand (PFD). According to the different safety integrity levels (SIL1-SIL4), the probability of a dangerous failure cannot exceed given values.
To make the determination of a SIL classification manageable, divide the safety considerations into different components. Split the PFD values into the different components. A general recommendation is to weight the PFD value 35% on the sensor system, 15% on the logic solver, and 50% on the final element (see Fig.2). Finally, review the whole system. Account for statistical failures, and avoid systematical dangerous failures.
Point level measurement
Point level measuring devices for liquids see use in all process facilities. Numerous measurement technologies are available for these kinds of applications, such as float switches, vibration limit switches, ultrasonic gap switches, and capacitive or conductive limit switches. When considering high functional safety, choose a measurement method with low PFD values and no systematical failures during operation. Also, do a thorough investigation of the measurement sensor systematical failures, and consider the know-how gained from a large number of applications. Also, do a proven-in-use evaluation.
Vibronic point level
Vibronic measurement devices or tuning-fork systems fulfill the highest demands with regard to safety and reliability. The main advantage of vibronic point-level measurement devices over float switches with regard to functional safety is they use an active measurement principle. The device stays in vibration continuously, and an evaluation electronics always monitors it. It immediately detects sensor failure in almost all cases and avoids dangerous failures. Vibronic measurement devices meet the requirements of almost all point-level applications for liquids. A vibronic device is independent of the installation position. You can mount the same device from the top, the side, or the bottom. State-of-the-art devices have a broad application band-width where no calibration of the sensor is necessary. This sensor is independent of process influences, such as pressure and temperature. It’s independent of substance characteristics, such as conductivity, dielectric constant, and viscosity, and is independent of gas bubbles, foam, and solids in the process liquid.