- Published on Wednesday, 16 April 2008 22:08
- Written by Bart Collins
In today’s ever-changing, fast-paced world, uptime is at a premium, while remaining in compliance with governmental and plant regulations is a requirement. With increasing demands placed on plants to produce, it is necessary to look at different ways to perform inspections on pressure relief devices and minimize plant downtime. Many plants in the United States have opted to modify their plant PSM (process safety management) programs to allow a “test only” on each pressure relief device in lieu of a complete disassembly and “VR” overhaul. The intent of this article is to explain the pro’s and con’s of both methods, while balancing additional demands of productivity, cost, safety and environmental issues.
A “test only” can be performed by two different methods: (1) “In-situ” online testing while the valve is still in service; or (2) bench testing—removing the valve and sending it to a local repair/test center for verification of relief pressure and seat leakage testing.
PRO’S: In-situ testing gives the end user an economical advantage over other methods as it allows the plant to keep the process unit running while testing is being performed. While this type of testing is typically more expensive on a per-valve basis than bench testing, it is much more economical for the end user in the total overall cost of performing the test.
CON’S: In-situ testing produces very little results regarding the total functionality of the valve. While it does provide a start-to-discharge point (the pressure in which the valve actually starts to lift), it does not put the valve into lift, does not provide any type of external joint leakage testing (back pressure test), does not allow for a visual inspection of the inlet and outlet, and typically cannot accurately inspect for seat leakage. In addition, in-situ testing can become very time-consuming depending on the location of the valve and the process it is in. In some cases, the valves will be located in a “fresh air” application or in an area that requires the equipment to be intrinsically safe.
PRO’S: Bench-testing allows a full functional test to be applied to each valve by putting the valve into lift (lift may vary based on volume of test equipment). It also allows the valve to blow down and reseat and allows for a seat leakage test. In addition, the valve is back-pressure tested to inspect for any external leakage from bolted or threaded joints. While this is usually the most economical of all PSV inspections on a per-valve basis, the end user typically experiences internal costs such as loss of production, use of plant manpower to remove and install the valves, additional materials required for the installation, and any other peripheral safety concerns dictated by the location of the valve.
CON’S: Bench testing is typically known as the “norm” for test-only inspections; but it also has its own pitfalls. While it does provide both a full functional and seat leakage test, it does not provide the end user with a dimensional and visual examination of the PSV’s internal working components. All manufacturers of ASME-certified PSV’s provide dimensional tolerances, clearances and visual inspection points that are required to maintain the valve’s integrity and ensure that it is in “like new” condition. Additionally, a “test only” does not provide an internal visual inspection for corrosion and/or erosion of the body and bonnet cavities, internal parts, bearing points and spring—all of which can affect the valve’s performance and its total life in service.
Full Inspection and Overhaul
A process involving a pretest, full disassembly, internal inspection and functional test is the only true way to ensure that your PSV is in the best possible condition. The pretest is performed before the PSV is disassembled. The results from the pretest are used to help gauge whether a valve would have relieved in service and at what pressure, whether it would have reseated and whether it had seat leakage. By performing a pretest and then an entire overhaul and inspection, each part of the PSV is cleaned, inspected, reused/repaired/replaced, assembled, tested and returned to service.
A certified “VR” overhaul performed by a certified repair company authorized by the National Board of Boiler and Pressure Vessel Inspectors1 is the common industry standard followed and accepted by the majority of regulatory agencies and end users throughout the United States. In some cases, end users will accept a non-VR repair as long as it meets certain criteria spelled out within their own PSM program. While the “VR” is the preferred method, plant specific repair criteria may be acceptable with some end-users. A listing of all authorized “VR” stamp holders can be found in the National Board’s NB-182.
PROs: As stated above, a full inspection and overhaul is the only true method of determining the condition of a valve. When a valve is disassembled and inspected, it gives you a full view of the internal condition of the valve, a full visual and dimensional inspection of all internal parts, verification of the spring number and range, any potential visual corrosion or erosion, and a full functional test demonstrating consistent, repeatable performance. Coupled with the pretest results, this is the only accurate way to predict frequency of inspections required to comply with plant PSM requirements.
CONs: There are two main downsides to a full inspection and overhaul; additional time and cost of repair. To properly overhaul any PSV, there are certain requirements that must be followed in order to provide a “VR’d” valve. This method can take 3-4 times longer (time may vary based on the size and condition of the valve) than that of a test only. Additionally, there are additional labor costs for the repair, as well as costs associated with transportation, plant labor (for removal and installation) and unit downtime. These factors need to be taken into consideration along with production schedules to maintain a true and accurate PSM program.
PSV Management Software
Determining the methods and frequencies of inspection is not an easy task. As stated, there are many factors—not just the “pop-point” of a PSV—that affect whether inspection frequencies should be shortened, extended or maintained.
Managing all of this information can be a daunting task. Having an internal database or using a repair/testing facility that is equipped with the proper database is essential to a successful PSV program. The ability to manage this information, help predict frequencies of repair, and assist in tracking history and total life cycle cost for each PSV is a critical component of the PSM process.
There are many methods of PSV inspection that are used to meet end-user PSM requirements. Determining which method is the safest and most practical is not to be taken lightly. Although cost will always be a factor in the decision, safety, quality and efficiency considerations should play an equal (if not greater) role in determining PSM frequencies and methods.
A PSV is a plant’s last line of defense when it comes to protecting life and property. A properly maintained PSV will ensure that the end user, the community and the environment will not be exposed to equipment failure resulting in a catastrophic situation.
1 National Board of Boiler and Pressure Vessel Inspectors; “NBIC” www.nationalboard.org
2 National Board of Boiler and Pressure Vessel Inspectors; “NB-18” www.nationalboard.org