Guidelines for Pressure Relief Design
8.0 Set Pressure Guidenace
a. UG-134: Pressure Settings and Performance Requirements
i. (a) When a single pressure relieving device is used, the set pressure marked on the device shall not exceed the maximum allowable working pressure of the vessel. When the required capacity is provided in more than one pressure relief device, only one pressure relief device need to be set at or below the maximum allowable working pressure, and the additional pressure relief devices may be set to open at higher pressures but in no case at a pressure higher than 105% of the maximum allowable working pressure, except as provided in (b) below.
ii. (b) For pressure relief devices permitted in UG-125(c)(2) as protection against excessive pressure caused by exposure to fire or other sources of external heat, the device marked set pressure shall no exceed 110% of the maximum allowable working pressure of the vessel. If such a pressure relief device is used to meet the requirements of both UG-125(c) and UG-125(c)(2), the device marked set pressure shall not be over the maximum allowable working pressure.
iii. (c) The pressure relief device set pressure shall include the effects of static head and constant back-pressure.
1. The set pressure tolerance for pressure relief valves shall not exceed +/-2psi (15kPa) for pressure up to and including 70 psi (500 kPa) and +/- 3% for pressures above 70 psi (500 kPa), except as covered in (2) below.
2. The set pressure tolerance of pressure relief valves which comply with UG-125 ( c ) (3) shall be within - 0%, +10%.
iv. (d) The bust pressure tolerance for rupture disk devices at the specified disk temperature shall not exceed +/- 2 psi (15 kPa) of the marked busrt pressure above 40 psi (300 kPa)
v. (e) Pressure relief valves shall be designed and constructed such that when installed per UG- 135, the valves will operate without chattering and shall not flutter at the flow-rated pressure in a way that would interfere with the measurement of capacity or would result in damage.
vi. ASME BPV Code Section UG-127(b)(3)(b) requires that the pressure relief valve opens at the same time as the rupture disc when used in combination. This ensures that the rupture disc opens fully, and to prevent corrosion or fouling of the pressure relief valve if the pressure does not rise to the set pressure and the rupture disc is not replaced promptly. This practice also prevents leakage of possibly hazardous material from leaking valve.
b. Reactive Systems – These systems must be evaluated by an engineer with considerable experience in reactive systems identification, thermochemistry, adiabatic calorimetry, reaction kinetics and pressure relief design.
i. Reactive System Design steps
1. Identify all chemicals in the process
2. Determine which chemicals could foreseeably come in contact with one another, both intentionally or unintentionally.
3. Determine if a runaway reaction can occur. This requires a literature study and adiabatic calorimetry test to determine the onset temperature and reaction kinetics.
4. Determine the size of the required pressure relief device using DIERS methodology, which can involve scale-up experimental results and / or a software program that has been accepted by the AIChE and DIERS, such as Super Chems for DIERS and Mach II CDS Digital Engineer
5. Other than scenario identification, the guidance provided in API 520, Part 1 and Standard 521 are not applicable to reactive systems.
ii. Reactive Systems: Set Pressure- Another common practice is to specify the set point for devices well below the MAWP in order to prevent the temperature from reaching an uncontrollable level and allow the use of devices of practical size.
iii. Reactive Systems: References- Additional references for dealing with reactive systems can be found in the following references.
1. Guidelines for Chemical Reactivity Evaluation and Application to Process Design (CPS, 1995b)
2. Guidelines for Safe Storage and Handling of Reactive Materials (CCPS, 1995)
3. Essential Practices for Managing Chemical reactivity hazards (CCPS, 2003)
4. Guidelines for Process Safety in Batch Reaction systems (CCPS,1999)
5. Emergency Relief Systems Design Using DIERS Technology (Fisher, Forrest, Grossel, Huff, Noronha, Shaw, and Tilley: 1992)
6. Recommended Practices 101-Control of the Hazards of Reactive Chemicals (Howell, 2009)
c. Factors Affecting Opening Pressure – Factors that can affect the actual opening pressure of a relief device (depending on the type of device, relief system layout and the nature of the flowing fluid) are:
i. Device temperature at activation-
1. Temperature compensation- the spring constant for the spring in a pressure relief valve and the tensile strength of a rupture disk element are temperature dependent.
ii. Constant superimposed back pressure- must take into account for all but balanced devices
1. The actual set pressure is based on the difference between the desired opening pressure and the constant superimposed back pressure.
2. Device activation occurs in service at the sum of the differential set pressure and the superimposed back pressure.
iii. Variable superimposed back pressure
1. Must use balanced bellows valve or pilot operated valve
iv. Hydrostatic head –column of fluid force against spring or disc
1. Set pressure compensation is required for all types of relief devices (conventional or balanced) if hydrostatic head can develop in the inlet piping.
a. Conservative practice is to reduce the differential set pressure of the device by the amount of accumulated liquid hydrostatic head.
b. For non-reclosing devices, the head is typically of concern only for devices connected below the liquid level
c. Hydrostatic head development in the inlet piping to the device and/or submergence of the device
d. The differential set pressure of the device must be adjusted so that the device activates when the pressure in the vessel reaches the desired set point.
e. The compensation is the difference in liquid head level to the device inlet
2. Hydrostatic head development in the discharge piping from the device
a. Liquid accumulation can require set point compensation for either up flow of liquid or two- phase flow.
Note: The requirements for set pressure compensation are summarized in Table 2.3-3 Guidelines for Pressure Relief and Effluent Handling 2nd edition.
UG-140 OVERPRESSURE PROTECTION BY SYSTEM DESIGN:
(a) If the pressure is limiting- A pressure vessel does not require a pressure relief device if the pressure is self- limiting (e.g. the maximum discharge pressure of a pump or compressor) and this pressure is less than or equal to the MAWP of the vessel at the coincident temperature and the following conditions are met:
(1) The decision to limit the pressure by system design is the responsibility of the user. The user shall request that the manufacture’s data report state that overpressure protection is provided by system design per(a)
(2) The user shall conduct a detailed analysis to identify and examine all potential overpressure scenarios. The “cause of Overpressure” described in API Standard 521 shall be considered. Other standards or recommended practices that are more appropriate to the specific application must also be considered. A multidisciplinary team experienced in methods such as Hazards and Operability Analysis (HAZOP), failure modes, effects and criticality analysis (FMECA), “what-if” analysis, or other equivalent methodology shall establish that there are no sources of pressure that can exceed the MAWP at the coincident temperature.
(3) The results of the analysis shall be documented and signed by the individual in responsible charge of the management of the operation of the vessel.
This documentation shall include as a minimum:
i. Detailed process and instrument flow diagrams (P&ID’s), showing all pertinent elements of the system associated with the vessel.
ii. A description of all operating and upset scenarios, including scenarios involving fire and those that result from operator error, and equipment and/or instrumentation malfunctions.
iii. An analysis showing the maximum coincident pressure and temperature that can result from each of the scenarios listed in item (2) above does not exceed the MAWP at that temperature.
(b) If the pressure is not self-limiting – A pressure vessel may be protected from overpressure by system design or by a combination of overpressure by system design and pressure relief devices, if the following condition are met. The rules below are not intended to allow for the normal operation above the MAWP at the coincident temperature.
(1) The vessel is not exclusively in air, water, or steam service unless these services are critical to preventing the release of fluids that may results in safety or environmental concerns.
(2) The decision to limit the overpressure by system design is the responsibility of the user. The user shall request that the Manufacturer’s data report state that overpressure protection is provided by system design per (b) if no pressure relief device is to be installed, acceptance of the jurisdiction may be required.
(3) The user shall conduct a detailed analysis to identify and examine all potential scenarios that result in an overpressure condition and magnitude of the overpressure. The “Causes of Overpressure” described in API Standard 521 shall be considered Other standards or recommended practices that are more appropriate to the specific application mat also be considered. A multidisciplinary team experienced in methods such as hazards and operability analysis (HAZOP), failure modes, effects and criticality analysis (FMECA), “what-if” analysis, or other equivalent methodology shall conduct the analysis.
(4) The overpressure scenario shall be readily apparent so that operators or protective instrumentation will take corrective action to prevent operation above the MAWP at the coincident temperature.
(5) There shall be no credible overpressure scenario in which the pressure exceeds 116% of the MAWP times the ratio of the allowable stress value at the temperature of the overpressure scenario to the allowable stress value at the design temperature. The overpressure limit shall not exceed the test pressure. Credible events or scenario analysis shall be considered.
(6) The results of the analysis shall be documented and signed by the individual in responsible charge of the management of the operation of the vessel. This documentation shall include at a minimum:
i. Detailed process and instrument flow diagram (P&ID’s) showing all pertinent elements of the system associated with the vessel.
ii. A description of all operating and upset scenarios, including those involving fire, and those that results from the operator error, and equipment and/or instrumentation malfunctions.
iii. A detailed description of any safety critical instrumentation used to limit the system pressure, including the identification of all truly independent redundancies and a reliability evaluation (qualitative or quantitative) of the overall safety system.
iv. An analysis showing the maximum pressure that can result from each of these scenarios.
Warning Note: UG 140(b) (4) provides that operator actions may be considered as a part of the safety protection system. However, numerous texts (CCPS, 1994, 2007b and 2014) on recent incidents in the process industries show that operators alone are not reliable enough to provide the level of safety needed for protection by system design.
9.0 API Standard 521
HIPS- Annex E of API Standard 521 provides guidance on how to eliminate over-pressurization scenarios by means of high integrity protection systems (HIPS). The Standard discusses the use of instruments, final control elements (e.g. valves, switches, etc.) and logic solvers that are configured such that the probability of an over-pressurization scenario occurring is sufficiently low that it is no longer credible.
HIPS can be used to reduce a relief device size, but not necessarily eliminate it altogether. For complete information refer to API Standard 521.
Guidance for ensuring that the requirements are meet and the application properly performed can be found in:
1. Guidance for Safe and reliable instrumented Protective systems(CCPS, 2007a)
2. Layer of Protection Analysis-Simplified Process Risk Assessment (CCPS, 2007b)
3. Guidelines for Hazard Evaluations, 3rd Edition Procedures (CCPS, 2008)
4. Guidelines for Initiating Events and independence protection Layers in Layer of Protection Analysis (CPS, 2014)
There are three possible reasons that HIPS might be used to reduce the size of a mechanical relief device. A relief device may not be:
1. practical if a very large relief device area is required
2. reliable if there is a high risk of plugging
3. Economical if it requires an expensive effluent treatment device or system.