A baseline of every 5 years is recommended; however, there are some critical exceptions that would require more frequent inspections. And if the process is CLEAN (e.g., Non-Fouling), we could go 10 years WHEN the manufacturer permits. Here is a breakdown of these frequencies, using API 576 and NB Part 4.
9 Inspection Frequency
9.1 General
The inspection of pressure-relieving devices provides data that can be evaluated to determine a safe and economical frequency of scheduled inspections. This frequency varies widely with the various operating conditions and environments to which relief devices are subjected.
Inspections may be less frequent when operation is satisfactory and more frequent when corrosion, fouling, operational upsets, and leakage problems occur. Historical records, which reflect periodic test results and service experiences for each relief device, are valuable guides for establishing safe and economical inspection frequencies.
A definite time interval between inspections or tests should be established for every pressure-relieving device on operating equipment. Depending on operating experiences, this interval may vary from one installation to another. The time interval should be sufficiently firm to ensure that the inspection or test is made, but it should also be flexible enough to permit revision as justified by past test records.
In API 510, the subsection on pressure-relieving devices states the following for pressure-relieving device inspection intervals: “Unless documented experience and/or a RBI assessment indicates that a longer interval is acceptable, test and inspection intervals for pressure-relieving devices in typical process services should not exceed:
a) 5 years for typical process services, and
b) 10 years for clean (nonfouling) and noncorrosive services.
9.2 Frequency of Shop Inspection/Overhaul
9.2.1 General
The interval between shop inspection/overhaul of pressure-relieving devices should not exceed that necessary to maintain the device in satisfactory operating condition. The frequency of shop inspections/overhauls is normally determined by operating experience in the various services involved. Normally, the interval of a device in a corrosive and/or fouling service would be shorter than the interval required for the same device in a clean, nonfouling, noncorrosive service. Likewise, more frequent inspection and testing may be needed for pressure-relief valves subject to vibration, pulsating loads, low differential between set and operating pressures, and other circumstances leading to valve leakage and potentially poor performance.
Where an inspection or test history extending over a long period of time reflects consistent “as-received” test results that coincide with the CDTP (see 6.2.9.1), where no change in service is to be made, and where no conflict in jurisdictional requirements exists, an increase in the test interval may be considered.
Conversely, if the “as-received pop test” results are erratic or vary significantly from the CDTP, the inspection interval should be decreased or suitable modifications to improve the performance should be made. If a valve fails to activate on the test block at 150 % or more of CDTP, it can be assumed that it would have failed to activate on the unit during an overpressure event.
Where corrosion, fouling, and other service conditions are not known and cannot be predicted with any degree of accuracy (as in new processes), the initial inspection should be accomplished as soon as practical after operations begin to establish a safe and suitable testing interval.
9.2.2 Manufacturer’s Basis
Manufacturers of pressure-relieving devices are sometimes able to assist the user in establishing inspection and test intervals, especially if their designs contain features and components that require special consideration. For example, it may be necessary to inspect or replace certain parts, such as nonmetallic diaphragms in pilot-operated valves, at frequencies greater than those required for the parts of conventional pressure-relief valves. Rupture disks and bellows valves may also require special consideration. Manufacturers are familiar with the nature of the loading, stress levels, and operating limitations of their design and are able to suggest inspection intervals appropriate for their equipment.
9.2.3 Jurisdictional Basis
In some instances, the required frequency of inspection and testing of pressure-relieving devices is established by regulatory bodies.
7.1
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Rupture disk replacement should be done on a schedule based on the manufacturer’s recommendation, consequence of nuisance releases, past experience of the specific rupture disk installation, and the relative cost of an unplanned maintenance shutdown.
7.4
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If a disk’s manufacturer specifies a bolting torque procedure and the tightened bolts are loosened, the rupture disk should be replaced. Do not reinstall the disk once it has been removed from its holder, even though it has not been ruptured. When stresses are relieved by unbolting, the “set” taken by the disk during its original installation may prevent a tight seal and affect performance if reinstalled.
Rupture disks cannot be nondestructively tested and should be replaced on a regular schedule based on their application, the manufacturer’s recommendations, consequences of nuisance releases, and/or past experience. If a block valve is located ahead of the disk, the block valve should be locked or car sealed open during operation.