Turning Pressure Safety Valves Into Smart Devices to Enhance Process Safety

Summary

Pressure safety valves (PSV) are designed to open at a preset pressure and release fluid until the pressure drops to an acceptable level. Nevertheless, numerous serious incidents have shown that operators often overlook critical performance indicators such as set-pressure drift, blowdown and chatter when PSVs are not actively monitored during operation.  

Let's examine how the incorporation of upstream and downstream pressure monitoring, rapid data collection and the inclusion of appropriate safety features in control system design can transform pressure safety valves from straightforward passive devices into intelligent, diagnosable assets.

Post-incident investigations around the world have examined several incidents that were caused by malfunctioning of the pressure safety valves. Standards exist that call for periodic inspection, maintenance and testing of PSVs; however, currently, no regulatory requirement mandates continuous online monitoring. In the absence of continuous monitoring of the PSVs, which are the last line of defense in a process, an increased dependency on the human factor may result in overlooked degradation, and the abnormal behavior may go unchecked. The absence of PSV diagnostics has frequently resulted in undetected malfunctions, expensive downtime and, in certain situations, fatalities across industries.

Shortcomings of in-service testing

Traditional, manual PSV onsite testing techniques (also called in-service testing) present several drawbacks. For example, the mechanical lift indicators in a PSV can only provide confirmation of stem movement. The bench testing can confirm calibration at a particular moment in time in controlled laboratory conditions, but it is unable to record in-service circumstances like fluid dynamics, process transients or back pressure effects. It is worth noting that although necessary, routine inspections frequently overlook the slow deterioration that takes place in between test intervals.

A drift in a PSV may occur much earlier than the next scheduled inspection. Accessing the PSVs installed at altitudes, removing them, transporting them to the laboratory, performing tests on them and installing them back is an expensive and labor-intensive process. It is also possible that PSV may be damaged during transportation and re-installation. Above all, these methods fail to cater to the dynamic behavior of the PSVs that occur under real process dynamics. Because of these reasons, operators are oblivious to problems that may occur during plant operations, such as chatter, abnormal blowdown, and set pressure drift.

Overcoming shortcomings of in-service testing

To overcome the shortcomings of In-Service Testing, different instrumentation technologies such as acoustic, pressure and temperature sensors can be used for continuous and uncrewed PSV monitoring. Here, however, we will cover only the pressure measurement technology to perform real-time PSV monitoring. The basic idea is to place the pressure transmitters on both sides of the PSV and examine the traces during events. A typical setup will be as follows:

• Upstream Pressure (P1): Determines the actual pressure in the pipeline or vessel.
• Downstream Pressure (P2): Measures backpressure and confirms flow by capturing flare header or discharge piping pressure.

The output from the upstream and downstream pressure sensors is transmitted to the control system comprising of a PLC, SCADA, DCS or a standalone system. The differential (ΔP = P1 – P2) can reveal partial lifts or abnormal reseat dynamics and highlights valve opening characteristics. from Engineers can obtain a real-time fingerprint of PSV performance by trending P1, P2 and ΔP together. Without the use of a lift-assistance device, the exact pressure at which the PSV lifts operate can be verified without accessing the pressure safety valve physically.

By comparing opening and closing points using upstream pressure transmitter, blowdown, which is the difference between actual set pressure and actual reseating pressure of a pressure safety valve, can be measured.  The pressure transmitter measures the set pressure at the exact movement when the pressure relief begins. It also measures the reset pressure when the pressure safety valve is reseated. By comparing both values, the blowdown is calculated which gives the real-world data about the actual performance of the valve under actual operating conditions.

Chatter is an abnormal and undesired phenomena in which the PSV rapidly opens and closes instead of the desired operation of opening fully, stabilizing the pressure and then closing once the process pressure is back to normal conditions. Chatter in a PSV can damage the seats and fracture the internal components of the valve and thus the primary function of the valve is compromised. In case of chatter, the upstream pressure (P1) oscillates widely because of the rapid opening and closing of the PSV while the downstream pressure (P2) records a spike each time the valve snaps open and a drop each time it closes. By trending P1 and P2 in the control system, the chatter can be detected, and preventive measures can be taken to prevent an incident.

The pressure at which PSV is required to open may drift away due to several reasons, such as improper valve sizing, corrosion and spring fatigue. Measuring the pressure across the PSV enables us to monitor the so-called set-pressure drift. If the operating pressure is getting closer to the set pressure of the PSV, it calls for a thorough investigation to ensure that the PSV has not drifted as well.      
  

The benefits of continuously monitoring PSVs

Continuous monitoring of the PSVs by identifying blowdown, chatter and set pressure drift earlier than catastrophic releases can prevent industrial incidents and hence save human lives, property, assets and the environment. Several standards and investigations have found that human factors and errors contribute significantly to most chemical accidents. Diagnostic through PSV monitoring through the control system such as PLC, SCADA and DCS provides an additional layer of protection to the process safety. It also saves a lot of hours spent on test lifting and speeds up the turnaround inspection times.

Considering design challenges

Design challenges must be carefully considered when implementing PSV diagnostics. Because transmitters must endure high pressure, high temperatures and transient loads during relief operations, sensor survivability is essential. PSV chatter measurement in particular needs highly accurate pressure transmitters with fast response time being a high-frequency transient phenomenon. It narrows down the options for pressure sensors that are generally available in the market.  It is thus widely accepted to measure the lift of the valve using differential pressure transmitter across a pressure relief device that provides a cost-effective method for PSV monitoring. It also allows the control system to measure the flow rate during overpressure events without needing expensive pressure sensors.

However, to perform the root cause analysis of valve failures due to vibration or critical infrastructure where the safety must be guaranteed under worst case scenarios, high speed pressure sensors are required that can pick up the chatter. It also mandates that the analog input modules in the control system are required to have high speed electronics that can sample the signal from the pressure sensors.

To maintain compliance with IEC 61511, it is equally essential that diagnostic monitoring stay distinct from safety-critical shutdown logic, making sure that the addition of diagnostics does not jeopardize the Safety Instrumented System's (SIS) integrity. The PSV diagnostic system is not a replacement for SIS; it can, instead, be considered as a supporting diagnostic/monitoring system to improve the reliability of External Risk Reduction Facilities (ERRF) which limit the consequences of an accident should it occur.

Final thoughts

A long list of incidents happened due to the poor performance of the PSVs across the industry show that PSVs cannot go unmonitored. A properly designed PSV monitoring system can play an instrumental role in environment protection by checking emissions. This additional diagnostic layer provides engineers with insight into PSV performance, predictive maintenance and confidence in overpressure protection of the critical infrastructure.