Developments in pressure relief technologies respond to chemical processors’ need for improved reliability and safety
Pressure relief systems, whether rupture disks, pressure relief valves or other relief devices, are the last line of defense before a process enters a dangerous situation. “Processors never want pressure relief devices to activate, so it is important to take all possible steps to ensure that the process never gets to a state where this happens. However, if a pressure event does occur, you want to know that the device is reliably working and you want to be able to investigate the event effectively to ensure that it doesn’t happen again,” says Nick Petrosyan, customer success manager with TrendMiner (Houston; www.trendminer.com). This is especially true in the chemical process industries (CPI), where pressure relief events can have extremely hazardous and costly results. For this reason, pressure-relief-device providers are making improvements that increase reliability and safety of their wares, while new technologies are being introduced to help design, monitor and analyze pressure relief systems in an effort to investigate events and provide actionable information to avoid them.
The most commonly used pressure relief devices include pressure relief valves and rupture disks, which are designed to relieve pressure in a vessel or other process component when it reaches dangerous levels. Once activated, the devices release process fluids or gases to the environment or a closed recovery system. As the last line of defense, it is essential that they reliably operate in difficult chemical industry conditions.
While the basic operational design of pressure relief valves hasn’t seen drastic changes, providers of the components say they are making improvements that increase reliability, especially in difficult environments like those found in the CPI. GF Piping Systems (Irvine, Calif.; www.gfpiping.com), for instance, re-engineered its Type 586 pressure-retaining valve (Figure 1). The new valve is compact, and easy to install and operate thanks to a set screw that controls a non-rising spindle for even flow control in order to maintain pressure reliably. The valve includes a threaded bonnet, which eliminates the need for body bolts, so there is no need to re-torque. This makes it suitable for aggressive media.
“We use a threaded, plastic-on-plastic design, which is significant in the CPI,” says Jeff Sixsmith, product manager of valves and actuation with GF Piping. “Metals typically aren’t good in a corrosive atmosphere or one where there is a hot line running at high temperatures. In these cases, when the line is shut down, the bonnet connection will no longer be tight because plastic and metal expand at drastically different rates and the connection is no longer secure. However, because our connection is plastic on plastic, we don’t have that issue, so there’s no need to re-torque the valves because the connection is still secure. This ensures reliable operation.”
Farris Engineering (Brecksville, Ohio; www.cw-valvegroup.com/farris) recently began offering a restricted lift feature on its pressure relief valves, which allows users to customize the valve lift to meet specific capacity requirements. Restricted lift can improve a valve’s performance and stability, says Sean Croxford, business unit manager of engineering services with Farris Engineering. “Typically when you size a relief valve, the valve will always flow more than what the application requires, but restricted lift valves can help reduce inlet losses and built-up back pressures, ensuring better, safer performance,” he says.
Improvements have also been made on the rupture disc side. Oseco (Broken Arrow, Okla.; www.oseco.com) introduced its Safety Cartridge, which combines the traditional three components of a rupture disc system into one hermetically sealed component to provide simplified installation for critical applications and guaranteed leak protection by removing the need to seal the rupture disc within a holder (Figure 2).
“Rupture discs are designed to fail, but you don’t want them to fail too early due to mishandling or improper installation,” says Hunter Franks, chemical process segment engineer with Oseco. “Installation can be subjective. You have to clean and inspect the holder and then reinstall the disc. It is possible that not enough torque is applied or the holder is damaged or that it was installed incorrectly, which can lead to leakage. But this welded assembly is designed in such a way that it protects the disc, cannot leak and solves a lot of the installation issues that lead to premature failure.”
The importance of monitoring
“In the past, you had a relief device and it was there to protect the system from breaking or exploding, but what is happening now is that because of environmental and industry regulations, it’s becoming far more common to monitor pressure relief devices so that you know there was an event,” says Bob Klatt, manager, with Ameritrol (Vista, Calif.; www.ameritrol.com).
Andrew Cureton, wireless product manager with Emerson (Shakopee, Minn.; www.emerson.com) continues: “There’s currently a rule for the [petroleum] refining industry that mandates the monitoring of relief valves, and our expectation is that it will cascade to other industries, including the chemical industry or any industry that releases hazardous materials, such as methane or VOCs [volatile organic compounds], into the environment.”
But, there are reasons to monitor that go beyond compliance. “When a pressure relief valve activates, it’s an indication that the process is out of specification of normal operation because these devices are not meant to release under normal conditions. So, when you have a release, it’s an indication that something is wrong in the process,” says Cureton. “Often an operator will know that something is upset, but because the control system that manages the process is designed to compensate for the upset or the release, the release goes unnoticed.”
For example, if a boiler is feeding into a process and a downstream vessel releases, the control loop will kick up that boiler’s output to offset the difference and final product will continue to come out. The process will still yield as much as before, but there will likely be quality issues or more waste in the process; however, operators will not know why this is happening if they don’t know that they are releasing from a pressure relief device. Monitoring can help identify the cause of process upsets and process waste caused by pressure relief incidents that operators aren’t aware of.
One of the simplest and most common ways to monitor pressure relief devices is via thermal dispersion flow switches, such as the FX Series flow switch from Ameritrol (Figure 3). The thermal dispersion principle of operation features no moving parts for reliability. The sensor head employs two temperature sensors with a constant, very-low-power heating source physically attached to one of the temperature sensors. The second temperature sensor is isolated from the heating source and provides compensation for changing process temperatures. As flow changes, so does the temperature differential. This allows the device to be used as a flow/no flow sensor. A dual switch point option allows two separately adjustable switch points to be set using the same instrument.
At times, a pressure relief event is the only indication that a process upset has occurred and the sooner an event is detected, the sooner the root cause of the problem can be identified. Thanks to new sensing and software technologies, higher tech monitoring and root cause analysis is becoming easier.
Monitoring pressure relief devices provides real-time information, enabling users to pro-actively develop proper corrective action, optimize maintenance schedules and improve asset management while ensuring regulatory compliance.
For example, while rupture disks have been monitored for overpressure relief events for some time via the use of wireless sensors, pressure relief device providers, such as BS&B Safety Systems, LLC (Tulsa, Okla.; www.bsbsystems.com), realized that rupture disks often burst due to instantaneous and undetected pressure spikes in the process. “Seldom is the pressure and temperature of a process monitored at the point of installation of the rupture disk and the monitoring systems previously used were not designed to transmit process data anywhere close to the frequency at which a rupture disk responds to overpressure,” says David Garrison, business unit director for BS&B. “For this reason, many rupture disk activations are perceived to be caused by something other than a legitimate overpressure event. BS&B now offers equipment to monitor system pressure and temperature with a sampling resolution of up to 1,000 times per second, to which the rupture disk is exposed on the process side and the pressure on the downstream side. This system interprets and logs this information for subsequent use and analysis when a disk activates. The information can then be used by process engineers within the facility to help isolate the causes of the overpressue event.”
Additionally, BS&B has a new sensor design available to warn users if a rupture disk device is damaged, installed improperly or when process conditions have exceeded design limits for a rupture disk device before the disk bursts, says Garrison. “This sensor is used in a predictive role to anticipate condition and serviceability of the disk. It measures the strain applied to a rupture disk while in service and compares actual strain to a baseline determined during the manufacture of that specific rupture disk,” he says. “Strain values, outside of the known norm for that rupture disk device, indicate a risk of activation outside of tolerance, allowing it to be replaced before a nuisance activation occurs.”
The development of these rupture disk sensors has rapidly moved the “dumb” technology to contemporary “smart” technology with data gathered from advanced sensor signals capable of producing information useable for operators to plan on necessary rupture disk replacement at a convenient time, he says.
On the pressure-relief-valve side, Emerson offers the Rosemount 708 wireless acoustic transmitter (Figure 4). The device mounts externally and features ultrasonic acoustic event detection to allow visibility into steam traps and pressure relief valves by communicating acoustic level and temperature data, as well as device data, event status and leak detection via the WirelessHART network. To make the data provided by the sensor more actionable, it is tied into Emerson’s Plantweb Insight software platform. “This makes it a more complete solution and brings pressure-relief-valve monitoring into the world of pervasive sensing and the IIoT,” says Emerson’s Cureton. “The data coming from the sensor can be a little crude, but the software analytics of Plantweb Insight make sense of the data coming out of it. The software detects releases, alerts users on it and logs the duration in a way that can help create a report.”
The important value here, he says, is that when there are VOCs or other hazardous chemicals released that need to be reported, you can accurately report the releases rather than being dependent on the time of the last inspection. “Typically, if you have a release and it has been detected, the Environmental Protection Agency will fine the facility for the time between detection and last inspection, which, for pressure relief valves that are hard to reach, can be days or weeks and this can result in huge fines,” he says. “There can be a huge delta between a momentary release and something that’s been sustained.”
Further, monitoring and analyzing data can be helpful in other ways. “We are able to look for patterns of behavior and associate rises and falls with issues with the process. It is also able to identify which valve is releasing, helping to identify and correct the source of the release sooner,” explains Cureton.
Similarly, TrendMiner offers analytical software that indexes data from a historian and provides multiple ways to search for abnormal patterns, relevant events or time frames within historical process or asset data (Figure 5). It can be used to prevent repeated production issues by monitoring live processes and sending automatic notifications in the event of deviations, based on predefined settings to provide early warnings on process behavior.
Using time series data collected from temperature gages, pressure gages, flowmeters and other sensors, users can ask the software things like “How many times have I exceeded this pressure threshold while producing this product?”
“By examining these patterns and creating monitors based on patterns, you can find early indicators that a relief event may occur,” says TrendMiner’s Petrosyan. “If pressure swings or pressure or temperature oscillations usually precede a pressure event, you can capture those patterns inside TrendMiner and use them to generate new types of monitors that will alert you when something is about to happen.”
Further, he says, the software helps with incident investigation. “Processors try to design processes and plants to be safe and if you’re constantly exceeding key safety limits, then something is wrong,” he says. “Typically when there is a relief event, there is a release investigation that involves data analysis, looking through historical data and studying the sequence of events that led to the pressure excursion. This type of analytical software makes it easier to do a good job with the investigation because it looks at all the available data, which helps users determine the appropriate corrective actions necessary to make the plant safer and to prevent the event from happening again.”
And, to ensure that the pressure relief system is properly designed, to avoid potential incidents and comply with current regulations, Farris Engineering has created iPRSM, a solution for the design, audit and documentation of new and existing pressure relief systems. The web-based software provides a comprehensive approach to the management of pressure relief systems for safety compliance. It facilitates the evaluation of protected systems, systematically identifying problems and providing plant management with the critical tasks needed for overpressure protection compliance. The analysis capability acts as an effective management-of-change system and all reports and documentation necessary for regulatory purposes can be produced from iPRSM.
“This helps improve equipment and plant reliability, minimizing downtime and production losses related to overpressure events,” says Farris’ Croxford. “We are able to tie devices into this software and log events to provide actionable information back. Because it’s web enabled, one of our experts can look at any issues the clients may be having and assist them with taking the right course of action. It is like having an expert next to you while doing calculations. We put a lot of time and energy into providing subject matter expert advice to our clients and into helping them make their plants safer, more effective and within compliance.”
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