API RP 581 - 3rd Ed.2016 - Add.2-2020 - Risk-Based Inspection Methodology

RISK-BASED INSPECTION METHODOLOGY, PART 5—SPECIAL EQUIPMENT 5-53
F = 0.75 for conventional valves discharging to closed system or flare
c
F = 1.0 for all other cases
c
h) Adjustment for Environmental Factors
There are several environmental and installation factors that can affect the reliability of PRDs. These
include the existence of vibration in the installed piping, a history of chatter, and whether or not the
device is located in pulsing flow or cyclical service, such as when the device is installed downstream of
reciprocating rotating equipment. Other environmental factors that can significantly affect leakage
probability are operating temperature and operating ratio.
The operating ratio of a PRD is the ratio of maximum system operating pressure to the set pressure.
When the operating ratio is greater than 90 % for spring-loaded PRVs, the system pressure is close to
overcoming the closing force provided by the spring on the seating surface and the PRV will be more
likely to leak (simmer). This increased potential for leakage is taken into account by applying an
environmental factor to the default leakage curve. Similarly, an environmental factor is applied when the
operating margin is greater than 95 % for pilot-operated PRVs. Note that some pilot-operated PRVs can
function at operating ratios up to 98 % (see API 520 for guidance on operation margin).
An analysis of the industry failure rate data shows that PRDs installed in vibratory or cyclical service tend
to have higher leakage rates. The analysis showed, however, that the fail to open failure rates remain
about the same when a PRD is installed in these services.
If a PRV has a history of chattering, the installation should be modified or redesigned as soon as
possible to eliminate the chatter, since the effects of chatter may be very detrimental to the protection
provided by the PRD. An assumed adjustment factor of 0.5 is applied to the Weibull η parameters for the
POFOD and probability of leakage (POL) curves of a PRD that has a history of chattering in service.
Table 6.7 provides the environmental adjustment factors applied to the default POFOD and POL Weibull
curves.
The environmental factor, F env , is used to increase the POFOD or leakage by reducing the curve’s
characteristic life (Weibull η parameter). As shown in Figure 6.5, the modifier effectively shifts the
probability curves to the left.
i) Updating Failure on Demand Based on PRD-specific Testing Data
1) Tracking Historical Inspection and Testing Data
An inspection program should track each PRD’s testing and inspection history from its initial
installation. From this history, adjustments can be made to each device’s P fod and P l curves to
take advantage of the knowledge gained by the testing of a particular relief device in a specific
service.
After actual testing, data are obtained for a PRD, the probability functions of that device are
adjusted up or down (modifying the Weibull parameters) depending upon the results
(pass/fail/leak) of the device’s specific inspection tests and the length of service since the last
inspection. In this way, an increase or reduction in the recommended interval is obtained based on
historical test data.