Online Feature: What you need to know about emergency isolation valves
AUTHOR: Peter Jessee, Valin Corporation, Portland, Oregon
Industrial process plants are complex facilities with thousands of components that play a role in the operation of the plant. Not all these components hold the same consequences if they are improperly selected or if they fail to perform their designed function.
Emergency isolation valves (EIVs) are critical to the safety of refinery and hydrocarbon processing plants. EIVs are typically controlled by a separate safety instrumented system (SIS) to isolate the flow of a flammable or dangerous fluid into a process. They are also known as emergency shutdown valves, safety shutoff valves or chopper valves.
Standard off-the-shelf EIVs adhere to specific industry standards for fuel shutdown service. However, an EIV can be designed to meet the unique needs of a specific customer and application.
Standard shutoff valves. Several manufacturers offer standard shutoff valves to meet the needs of their customers in their target market. There are two primary standards agencies in North America that regulate these types of valve assemblies. Most systems in the U.S. are designed to meet the requirements of Factory Mutual (FM) Global Class 7400 (fire safe class 7440). In Canada, the CSA International Standards Specification Z21.21/CSA 6.5 Automated Valves for Gas Appliances and CGA3.9-M94 Automated Safety Shut Off Gas Valves are most applicable. This type of valve is offered with some variety of body, trim and seat materials, solenoid coil voltages, and limit switch design to meet the needs of most customers.
Standard EIVs are designed to shut off liquid or gaseous fuels quickly and reliably to fuel burning equipment such as boilers, furnaces, process heaters and burners, among others. Customers can be confident that these EIVs will meet the regulatory requirements of their plant and those of their insurer, as they are designed for a wide range of processes and conditions. This type of valve can be used for most fuel safety applications in refineries and hydrocarbon processing plants.
There are a lot of reasons to use a pre-designed shutoff valve. Product bulletins include a clear selection guide to assure the buyer or specifier selects an appropriate valve. The manufacturer has had to submit their products to FM or CSA for testing and approval, and the standards agencies regularly inspect the manufacturer’s facilities and review their quality procedures. Most importantly, the manufacturer shoulders the liability if a properly selected valve fails in an emergency.
There are also limitations when trying to use a standard EIV. First, these valves are designed to shut off commonly used fuels and might not be a good choice for shutting off a flammable process ingredient in a refinery or chemical plant. Second, if the plant where it will be used has an unusual air supply pressure, control voltage requirement or electrical classification requirement, users may not be able to find a standard valve that is appropriate. Third, these valves must be repaired at a shop approved by the standards agency to retain approval rather than being maintained by plant personnel. Lastly, the components the manufacturer offers might not be on the plant’s approved manufacturer’s list (AML).
If there is any question of whether a standard model is right for a particular application, it is important for the buyer to consult with a process control expert to avoid costly catastrophic failures.
Custom-designed EIVs. The other type of EIV is a valve assembly that is custom designed for the specific process, regulatory, safety and environmental requirements of the customer’s refinery or process plant. Some choose this option from personal preference, but most users only pick this option when a standard valve cannot be found that meets their requirements.
To successfully design a custom EIV, two different sets of criteria must be met. First, the process requirements (i.e., pressure, temperature, flow rate and fluid properties) must be clearly understood and components capable of handling them need to be selected. Second, the safety and regulatory requirements of applicable standards agencies, plant insurers and local inspection authorities must be incorporated into the design. This can require an iterative process before all the various requirements are met.
Custom designing an EIV is obviously more complex than selecting a standard catalog product. It also puts more liability on the party responsible for designing the EIV. The plant owner should carefully select who they want to give this responsibility to so that the completed design will reliably perform the required shutdown function.
There are several advantages to this approach. First, you can accommodate process fluids that are hotter, have a higher pressure or are more corrosive than typical fuels. These include plant gas, partially-refined oil, petrochemicals, alcohols and other chemical feedstocks that must be safely isolated in a plant upset or emergency. As nearly all standard EIVs use “soft” seats limited to a maximum temperature of approximately 260°C (500°F), a custom EIV with a metal seat can be used when fluid temperatures exceed this level.
Second, users can select components that are on a plant’s standard/AML lists. This means that operations and maintenance personnel will be familiar with the new equipment being procured, and ready to use or repair them in an emergency.
Third, the custom EIVs can meet the specific electrical classifications or other regulatory requirements that may not be available from standard EIVs designed for the most common standards.
Lastly, a custom EIV can be designed to levels of safety and reliability not offered by standard EIVs.
Meeting safety integrated level (SIL) requirements. Plant owners may identify locations that require extremely high levels of reliability. When part of a SIS, an EIV may be specified to provide a SIL-2 or SIL-3 level of reliability. This will require SIL analysis to confirm that the EIV will operate reliably when called on. The SIL is determined by the expected reliability of each component of the EIV and how frequently the valve can be tested to verify that it is still performing as expected.
Many refineries and plants are being run for several years between shutdowns, so waiting for a maintenance shutdown to test the valve might prevent a valve from achieving the necessary SIL. To generate a higher SIL, a partial stroke test (PST) device can be specified to permit more frequent testing. A PST allows initiating the movement of the valve towards the safe position, confirming it has started to move and then returning to the operational position before the fuel or process flow can be interrupted.
Another way to improve reliability is the use of redundant solenoids that prevent a single point of failure from causing a nuisance trip that shuts down the process. Similarly, to assure the valves can cycle during an emergency that has damaged the air supply system, actuators can be sized to operate at pressures below the normal system pressure.
To add robustness to these critical devices, many refineries require EIVs to use ANSI 300 valves, even if process conditions would allow ANSI 150 valves. The added flange bolting and greater wall thicknesses provide additional safety.
The EIV supplier should be required to provide documentation to confirm they meet the necessary requirements. This should include actuator torque calculations to show the safety factor of the actuator, maximum allowable stem torque values for the valve to confirm the actuator will not damage a stuck valve and valve stroke time calculations. After assembly, the EIV should be tested to confirm it operates as expected. Many plants send a representative to witness the vendor’s testing to confirm that these critical safety devices provide the performance that they are expecting.
Whether they are off-the-shelf or custom designed, EIVs are one of the most critical pieces of equipment in a refinery. The proper operation of EIVs is crucial for the safety of the workers in the plant and plant equipment, and lack of performance can be extremely costly.
BIO
Peter Jessee is a Process Application Engineer for Valin Corporation, a leading technical solutions provider for the technology, energy, life sciences, natural resources and transportation industries. Valin offers personalized order management, onsite field support, comprehensive training and applied expert engineering services utilizing automation, fluid management, precision measurement, process heating and filtration products.
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