Convert pressure safety valves into smart devices to increase process safety

summary

The Pressure Safety Valve (PSV) is designed to open with a preset pressure and release fluid until the pressure drops to an acceptable level. Nevertheless, when the PSV is not actively monitored during operation, it is shown that operators often overlook important performance metrics such as set pressure drift, blowdown, and chatter.

Find out how and how converting pressure safety valves from simple passive devices to intelligent, diagnostic assets can be converted by including appropriate safety features in upstream and downstream pressure monitoring, rapid data collection, and control system design.

Post-Intext investigations around the world have investigated several incidents caused by malfunctioning pressure safety valves. There are standards that require regular inspection, maintenance and testing of PSVs. However, regulatory requirements currently do not require continuous online monitoring. Without continuous monitoring of the PSV, the final line of defense in the process, the increased dependence on human factors, can result in overlooked degradation, and abnormal behavior may not be checked. The absence of a PSV diagnosis resulted in frequent industry-wide deaths, undetected malfunctions, expensive downtime, and certain circumstances.

Disadvantages of in-service testing

Traditional manual PSV on-site testing technology (also known as in-service testing) has several drawbacks. For example, the mechanical lift indicator on a PSV can only provide a confirmation of the STEM movement. Bench tests can check calibration at a particular moment in controlled laboratory conditions, but cannot record situations within the service, such as fluid dynamics, process transients, or backpressure effects. It is important to note that routine testing frequently overlooks slow degradation that occurs between test intervals.

PSV drift can occur much faster than the next scheduled inspection. Accessing highly installed PSVs, removing them, transporting them to the lab, running tests, and retrieving them is an expensive and labor intensive process. It can also damage your PSV during transportation and reinstallation. Among other things, these methods cannot accommodate the dynamic behavior of the PSV that occurs under actual process dynamics. For these reasons, operators have forgotten about issues that can occur during plant manipulation, such as chatter, unusual blowdowns, and pressure drift settings.

Overcoming the shortcomings of in-service testing

To overcome the drawbacks of in-service testing, various instrumentation technologies such as acoustic, pressure, and temperature sensors can be used for continuous and undeniable PSV monitoring. However, here we will only cover pressure measurement techniques and perform real-time PSV monitoring. The basic idea is to place a pressure transmitter on either side of the PSV and examine traces during the event. A typical setup is:

Upstream Pressure (P1): Determines the actual pressure of the pipeline or container. Downstream Pressure (P2): Check the flow by measuring back pressure and capturing flare header or discharge pipe pressure.

Output from upstream and downstream pressure sensors is sent to a control system consisting of a PLC, SCADA, DCS, or a standalone system. Differentials (Δp= p1 – p2) can reveal partial lift or abnormal regenerative dynamics and emphasize the valve opening properties. You can obtain real-time fingerprints of PSV performance from the engineers by trending P1, P2 and ΔP together. Without the lift assistance device, the exact pressure at which the PSV lift operates can be verified without physical access to the pressure safety valve.

By comparing the opening and closing points using an upstream pressure transmitter, you can measure the blowdown, which is the difference between the actual set pressure and the actual recuperation pressure of the pressure safety valve. The pressure transmitter measures the set pressure with precise movement when pressure relaxation begins. It also measures the reset pressure when the pressure safety valve is reproduced. By comparing both values, a blowdown is calculated that provides real data on the actual performance of the valve under actual operating conditions.

Talking is an abnormal and undesirable phenomenon in which the PSV opens and closes rapidly, instead of the intended action of stabilizing the pressure and closing after the process pressure has returned to normal conditions. Talking on a PSV can damage the seat and destroy the internal components of the valve, thus impairing the valve’s main function. In the case of chatter, the upstream pressure (P1) vibrates widely due to the rapid opening and closing of the PSV, while the downstream pressure (P2) records the spike every time the valve snaps and drops with each stop. By making P1 and P2 popular in the control system, precautions can be taken to detect chatter and prevent incidents.

The pressure required for the PSV to open can be drifted for several reasons, including improper valve sizing, corrosion, and spring fatigue. By measuring the pressure across the PSV, you can monitor what is called set pressure drift. If the operating pressure is approaching the set pressure of the PSV, a thorough investigation is required to ensure that the PSV is not drifting as well.

Benefits of continuous monitoring of PSVs

By identifying blowdowns, chatting and set pressure drifts earlier than a catastrophic release, continuous surveillance of PSVS can prevent industrial incidents and thus save human lives, assets, assets and the environment. Several criteria and research have shown that human factors and errors contribute significantly to most chemical accidents. Diagnosis via PSV monitoring via control systems such as PLC, SCADA, DCS, and more provides an additional layer of protection for process safety. It also saves a lot of time spent on test lifting and speeds up turnaround inspection times.

Consider design challenges

Design challenges must be carefully considered when implementing PSV diagnostics. Sensor survivability is essential as transmitters need to withstand high pressures, high temperatures and transient loads during rescue operations. Especially for PSV chatter measurements, a very accurate pressure transmitter with a fast response time must be a temporary phenomenon of high frequency. Filter pressure sensor options that are generally available in the market. Therefore, it is widely accepted to measure valve lift using differential pressure transmitters across pressure relief devices that provide a cost-effective method for PSV monitoring. Control systems can also measure flow rates during overpressure events without the need for expensive pressure sensors.

However, to perform a root cause analysis of valve failure due to vibrations or critical infrastructure that must ensure safety under worst-case scenarios, a high-speed pressure sensor is required that can pick up chatter. It is also mandatory that the analog input modules of control systems require high-speed electronics that can sample signals from pressure sensors.

It is equally essential that diagnostic monitoring maintains a different stay than safety critical shutdown logic to maintain compliance with IEC 61511. Adding diagnostics does not endanger the integrity of the safety equipment (SIS). The PSV diagnostic system is not an alternative to SIS. Instead, it can be considered a support diagnostic/surveillance system to improve the reliability of external risk reduction facilities (ERRFs) that limit the outcome of an accident if it occurs.

Final Thoughts

The long list of incidents occurred due to poor PSV performance across the industry, indicating that the PSV is not being monitored. A well-designed PSV monitoring system can play an instrumental role in environmental protection by checking emissions. This additional diagnostic layer provides engineers with insight into PSV performance, predictive maintenance, and reliability for overpressure protection for critical infrastructure.