The fluid systems in chemical plants and refineries today are extensive and complex, comprising countless feet of small-bore tubing that convey fluids in multiple directions with the goal of accomplishing specific critical functions. The complexity of these systems can pose significant challenges for plant employees because the threat of potential safety hazards rises as they become more complicated. It can be a daunting task to monitor each part of the system individually, so it is incumbent on system designers to make wise choices upfront to keep the plant safe. The decision to include pressure relief valves at critical points is crucial to building a safe fluid system within a chemical plant or refinery. Using these valves to vent off excess pressure can help prevent blowouts, damaged equipment, or system failures.
By Shaji Arumpanayil
Why relief valves are essential for safe operation
Relief valves operate as fail-safe measures in a chemical plant fluid system (see Figure 1). Though systems have pressure regulators to maintain the proper set pressure, the complexity of the system means chemical plants and refineries frequently encounter high pressures and potentially dangerous fluids.
The placement of relief valves does not happen by accident; it requires strategic placement from the design phase on to ensure they are properly situated within the system and set to the maximum allowed pressure for each segment.
Relief valves are triggered if they sense a higher pressure than is safe, and they open to allow the excess pressure to escape. When this happens, it may be necessary to capture the escaping pressure if the system media is toxic. If the air pressure is safe to allow in the atmosphere, then capture is not as important.
How to choose relief valves
The fluid system design and operation will dictate the type of relief valve to use. Options include four main types of valves:
- Proportional relief valves. These valves open gradually as pressure increases above the set pressure and close when system pressure falls below the set pressure.
- Proportional safety-relief valves. These valves operate in the same way as proportional relief valves. In addition, they carry Pressure Equipment Directive (PED) certification for use as a safety valve to accommodate specifications.
- Bleed and purge valves. Bleed valves can be used on instrumentation devices like multivalve manifolds or gauge valves to vent signal line pressure to atmosphere before removal of an instrument or to assist in calibration of control devices. Purge valves are simply manual bleed, vent, or drain valves.
- Medium-pressure relief valves. Medium-pressure proportional relief valves provide simple, reliable, over-pressure protection for a variety of general applications.
Why relief valves may be activated
Large-scale fluid systems are subject to many different conditions that may lead to overpressurization, but in general the reasons can be broken down into a few broad categories.
- Procedural error. Operators are not infallible, which means that on occasion they make errors that lead to overpressurization. For example, technicians may accidentally open a wrong valve in the system and cause pressure to rise downstream. It may be as simple as mistakenly calibrating the system, or the wrong type of pressure regulator may have been installed during routine maintenance. While proper training can help reduce the possibility of these errors, relief valves are essential to correct the problems human errors can produce.
- Component wear. Over time, fluid system components will wear down, particularly if the chemical being moved is caustic. For example, if the chemicals are corrosive, they can prematurely degrade system components. Such component wear may reduce pressure controls over time, leaving the relief valves as the last line of defense.
- Contamination. While pressure regulators are the primary means of keeping pressure at the proper levels, dirty or contaminated fluids can prevent them from functioning properly. Particulates can compromise the regulator’s seal, which allows excess pressure to creep past it, leading to increased pressure downstream. In such situations, relief valves can release the unwanted pressure before it becomes a safety hazard.
- Loss of power. More complex systems are often operated by electronic controls, which can lead to potential hazards if the plant loses power unexpectedly. Such sudden loss of control may prevent certain components from operating correctly and under those circumstances, relief valves can prevent the overpressurization and keep the system safe.
Where relief valves should be installed
To maintain chemical plant safety, relief valves should typically be installed within the following applications:
- High-pressure systems. In fluid system applications that exceed a certain psi, relief valves are an essential safety element that can help prevent potential accidents. Choosing high-quality small-bore tube fittings for such applications is a good way to protect against the potential dangers of overpressurization, but relief valves are a good supplemental safety measure for high-pressure applications.
- Toxic fluids or chemical applications. In applications where escaping system pressure would cause an immediate plant safety threat, relief valves are an essential safety measure. In the event of overpressurization in these applications, relief valves can be set up to capture vented pressure and media instead of releasing it into the atmosphere. Escaped fluid can then be disposed of safely.
- Sensitive equipment. One of the most important roles pressure relief valves can play is helping to protect sensitive analytical equipment from overpressurization. Expensive systems including online analyzers can be easily damaged if they experience higher pressures than they are rated for. This is especially important since analyzers are deployed to sample fluids directly from your main processes. An optimal regulator should be paired with a properly rated and set relief valve to ensure that pressure is sufficiently reduced from the main process by the time fluids are delivered to the analyzer.
Placing pressure relief valves properly throughout the industrial fluid system will improve the overall safety of the plant and should be considered at the time of initial system design. Other ways to design safety into the system from the beginning include avoiding tube fitting intermixing, eliminating unnecessary complexities in the system, and taking operational factors like vibration into account. If system designers pay attention to these factors and use relief valves judiciously, the plant will be much easier to operate safely.
How to optimize system design for chemical plant safety
Choosing the proper supplier can often be essential to designing the safest system possible. Look for a partner who can offer fluid system design specialists who fully understand the chemical and refining industries and who have a track record of success. Whether it is advice on where to install relief valves or how to choose tube fitting components, the supplier should be able to provide the necessary expertise to ensure chemical plant and refinery safety at all stages of the fluid system.
About this featured story
This featured story is an article originally published in Valve World India & Middle East journal (September 2022 issue). Interested in reading more articles about technical developments, company presentations, case histories, etc? Then why not take a print subscription or a free digital subscription? DIGITAL MAGAZINE SUBSCRIPTIONS ARE NOW FREE.
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