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Using proper installation practices and adhering to maintenance schedules helps ensure reliable operation of safety-relief valves
Safety-relief valves are critical components of any pressurized system. In fact, a functioning safety-relief valve could be the most important piece of equipment on a process pressure vessel or pipeline. These valves essentially represent the last line of defense if something goes wrong in the process. If safety-relief valves are not functioning properly and can no longer achieve what they are designed to do, catastrophic failures can ensue, potentially causing significant financial burden, as well as putting lives at risk. Because of the potentially catastrophic consequences of relief-valve failure, adhering to recommended or mandated maintenance schedules, along with ensuring that proper installation practices are followed, is of utmost importance.
Several factors must be considered during the installation of a safety-relief valve. First is the compatibility of the inlet and outlet connections of the safety-relief valve with the vessel or piping to which it is connected. There are instances where the valve is not compatible with the existing connections. Note that the inlet connection of a valve cannot be larger than the mating connection size that is available on a vessel or line. On the discharge side, the connection on the outlet of a valve cannot be mated to a connection that is smaller.
Another common issue arises as a result of a misunderstanding about vertically versus horizontally oriented valve installation. It is important to understand that vertical (referring to vertical valve stem) installation is critical on most safety-relief valves. Most manufacturers do not recommend a horizontal installation, but there are some that seem to be ambiguous regarding this issue. If available, refer to the installation manual for the valve, but in general, vertically oriented installation is preferred in most cases. Horizontal installation presents challenges because when the valve sits sideways, it can allow a place for dirt to gather at a critical location within the valve. This can ultimately prevent the valve from reclosing, even after the slightest overpressure event. When installed in a proper upright position, the valve has no place for sediment to collect and thus, it will be less likely for debris to get caught between the seating surfaces and cause leakage or damage to the sealing surface. Failures of safety-relief valves due to this type of improper installation continue to occur.
Another important consideration for safety-relief valve installation is the match between the category of application and the type of valve: the proper type of valve must be installed for the correct service application. There are three main categories of applications for safety-relief valves: air/gas, steam and liquid. All three applications have separate ASME (American Society of Mechanical Engineers; www.asme.org) code certifications, and while some valves are engineered for, and can be applied to, all three services, the valves can only be code-certified for one type of service at a time. For example, it is not recommended to use a valve that is code-certified and designed for liquid, in a steam application.
FIGURE 1. Drip-pan elbows, such as the one shown here, collect and drain condensate (Image courtesy of Kunkle Valve, Inc., a Division of Emerson)
Regarding safety valves specifically for steam service, a quick installation tip involves a special fitting called a drip-pan elbow (DPE), used to collect and properly drain the condensate created during an overpressure event (Figure 1). This fitting is mounted as closely as possible to the discharge of the valve. As the steam is escaping through the external piping, much of it will cool and turn to liquid, where it runs back down the pipe. The job of the DPE is to collect the liquid condensate and drain it away, so that it does not back up into the valve. It is recommended in most steam installations. The only situations where it may not be used is if the steam contains other chemical vapors that might not be allowed to vent directly to the atmosphere. DPEs are also not needed or recommended for air/gas or liquid applications.
Finally, it is important that the safety-relief-valve discharge piping be properly supported. If too much weight is placed on the valve after installation, not only will it not function properly, but depending on the valve materials of construction, the valve body may even distort. Operators may believe that there is a defect with the valve, when in fact, it is a problem with the installation. Depending on the setup, the discharge piping may need to be supported with an external support system to prevent this from happening. In steam applications, the above-mentioned DPE, when properly installed, will help to isolate discharge piping stress from affecting the safety-valve body.
There are several factors that can affect best practices in terms of maintenance of a safety-relief valve. The biggest question most people have is: “How often do I need to test my valves to make sure they are performing correctly?” Fortunately, many companies have a maintenance schedule in place that addresses this question. Typically, the nature of the process has the biggest impact on this schedule. The questions to be asked are: “What exactly is the application? And how severe are the service conditions?” The answer to these questions will determine the maintenance schedule and ultimately, the testing frequency for the valves.
The more critical the application and the higher the risk associated with the safety-relief valve being non-functional, the tighter the preventative maintenance schedule should be. In addition, testing and maintenance requirements from the owner’s insurance company, state and local inspections and other regulations, may play a substantial part in dictating a regular maintenance schedule on these valves. As a general guideline only, the National Board of Boiler and Pressure Vessel Inspectors (Columbus, Ohio; www.nbbi.org) publishes a suggested testing interval for safety-relief valves (Table 1). Again, these intervals should be adjusted based on each user’s particular process details, application and experience.
In the petrochemical industry, the valves inserted into most processes will see much more harsh service conditions than those found in many other industries, such as the food-and-beverage sector. There may be certain instances where plant-outage timeframes might be too tight to have the safety valves repaired or replaced. In those cases, it is particularly important to keep spare valves on site for critical applications. In these situations, it may be possible to service them on a more frequent basis. However, this may not always be feasible based on the application. Across the board, many companies are now carrying a smaller spare parts inventory than they once did. Unless the application is critical, they may not necessarily be stocking a spare valve.
In these situations, having a preventative maintenance plan and taking full advantage of scheduled shutdowns is critically important. The maintenance on valves during a scheduled outage is much less expensive than maintenance during an emergency outage. Not only will the expedited repair costs rise during emergency situations, but the downtime experienced by the operation — and the lost revenue associated with this — can be devastating.
During scheduled maintenance, valves are typically sent to an ASME- and National-Board-certified testing facility. When tested, if a valve falls within the recommended set pressure on the affixed data plate and is not leaking, it will be deemed properly tested on that date and approved for use. However, if the valve fails the set pressure, has issues with sticking open, or is leaking severely, the valve will have to be disassembled and repairs will need to be made. In these situations, the valve internals are inspected, checked and refurbished or replaced with only OEM (original equipment manufacturer) parts to return the valve back to precise working condition.
Some valves are more prone to failure due to their age. In fact, many safety-relief valves in operation have been in use for decades. As with any product, the older the valve, the more difficult it becomes to service, due to reduced general availability of OEM parts or to simple obsolescence.
There are circumstances, however, when a safety-relief valve will fail while in use, and the common indicator is leakage. When there is process media leaking from the valve on a constant basis, while under its normal operating pressure, a repair or replacement needs to happen quickly. Keep in mind that the price of a repair is usually a fraction of the cost of purchasing a new valve. If the valve is just beginning to weep or leak, a judgement call needs to be made: “Can we wait until the next scheduled shutdown to address the valve’s issues?” If the normal system-operating pressure is near the set pressure of the valve, and the leak prevents the system from maintaining the necessary operating pressure, then the valve needs to be repaired immediately.
When servicing an ASME-certified valve, any required replacement parts must be purchased either directly from the OEM or from an authorized distributor for the OEM. All valve manufacturers must supply information on tolerances and critical valve dimensions to National Board-certified repair shops in order for proper repair and testing to take place. It should be emphasized that a National Board “VR” certified facility is the preferred choice.
The National Board Inspection Code (NBIC) does allow for operators who hold a “VR” stamp to have their maintenance personnel repair and recertify their own valves. However, some operators many not be as well-versed in valve repair as a dedicated “VR” shop, because their plant may contain numerous valves from a variety of manufacturers. The biggest mistake a plant can make is to have its valves repaired by a non-certified facility. With this approach, an operator heightens the risk of catastrophic failure.
In summary, safety-relief valves are critical pieces of equipment that must be installed properly, and regularly maintained and tested by repair shops that are certified by the National Board of Boiler and Pressure Vessel Inspectors. Failure in any of these areas can result in catastrophic failures that will ultimately cost the owners and operators money and even worse, possibly risk the safety of employees.
Tim Turner is the vice president of Advance Valve Inc. (212 Fabricator Dr., Fenton, MO 63026; Phone: 636.343.8525; Email: sales@advancevalve.com; Website: www.advancevalve.com). Turner has over 25 years of experience in supplying and repairing safety-relief valves.
John Kirk is a technical sales specialist at Advance Valve (same address as above); Kirk has 20 years of experience with Advance Valve, which, for nearly 50 years, has provided exceptional customer service, ASME-Code-certified safety-relief valves spanning multiple manufacturers, and National Board “VR” valve repair services.
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