Guidelines for Pressure Relief Design
5.0 OSHA Regulations
OSHA regulations for hazardous materials (29(CFR 1910.119) contains specific requirements for pressure relief system design and operation in addition to general requirements for storage and handling of selected materials. This is the controlling regulation in the USA. Designers and operators should possess knowledge of tis regulation to ensure compliance. Reviewing current editions of the NFPA, CGA, and API documents to check for current practices and utilizing them is a prudent practice.
This regulation deals with maintaining proper documentation showing that the design and design basis for pressure relief devices complies with Recognized and Generally Accepted Good Engineering Practices (RAGAGEP)
API Standard 521 defines the minimum documentation that must be developed and maintained. The documentation required includes:
1. Recommended minimum relief system design content
i. Relief system information
1. Relief Device Identification number
ii. Description of protected components
1. List of equipment and design conditions
2. P&ID’s, Mechanical drawings, etc.
iii. Design codes and Standards followed
1. Pressure design code
iv. Analysis of causes of system overpressure
1. Required rate/or area for each cause
2. Supporting calculations and assumptions
3. Consider all causes of overpressure
4. Credit for safety instrumented systems
v. System operating conditions
vi. System relieving conditions
vii. Relief device selection / configuration
viii. Pressure relief valve / rupture disc combination capacity factor (if applicable)
ix. Relief system required area
x. Relief system capacity if rupture disc only configuration
xi. Pressure relief valve rated capacity
xii. Spring loaded pressure relief valve cold differential test pressure
xiii. Pressure relief valve capacity correction for maximum back-pressure
1. Built-up back-pressure
2. Maximum and minimum superimposed back-pressure
3. Back-pressure capacity correction factor
xiv. Rupture disk specified burst pressure and manufacturing design range selection
xv. Rupture disc specified disk temperature
xvi. Relieving Fluid disposal requirements (closed or atmospheric)
2. Thermal radiation
3. Dispersion (toxic or flammable vapor)
4. Vapor cloud explosion
5. Environmental considerations
xvii. Relief device physical installation
1. Pipe stress analysis
2. Heat tracing
3. Pressure relief valve inlet / outlet line pressure drop
4. Relief device specification sheets
5. Criteria for vacuum protection
In general, the broader design methods for non-reactive systems are contained in publications of the American Petroleum Institute (API 520/521/20000)
Caution!!!! Chemically reactive systems and multiphase venting are not adequately covered by the API recommended practices. The design of these systems requires the use of DIERS methodology.
6.0 General Provisions ASME Code
The ASME BPV Code requires that all pressure vessels be provided with certified overpressure protection systems. The National Board of Pressure Vessel Inspectors has been designated by the ASME as an organization that can test and certify pressure relief devices. They publish a list of pressure relief devices that have been certified. National Board NB-18 (www.nationalboard.org) can be downloaded at no charge.
1) For definitions and terminology used such as set pressure, over-pressure, blowdown, operating pressure, accumulation pressure, as it relates to the vessel maximum allowable working pressure (MAWP) see Section 2 of ASME PTC 25.
2) UG-125 General
a. Other than unfired steam boilers [see UG-125(b)] all pressure vessels should be provided with the overpressure protection in accordance with the requirements of UG-125 through UG-138 overprotection by system design per UG-140.
i. It is the user or his / her designated agent’s responsibility to identify all potential overpressure scenarios and the method of overpressure protection used to mitigate each scenario.
ii. It is the responsibility of the user to ensure that the required overpressure protection system is properly installed prior to the initial operation.
iii. If a pressure relief device(s) is to be installed, it is the responsibility of the user or his / her agent to size and select the pressure relief device(s) based on its intended service. Intended service considerations should include, but not necessarily be limited to the following:
1. Normal operating and upset conditions
3. Fluid phases
iv. The overpressure protection system need not be supplied by the vessel manufacturer.
3) For ASME Section 1 valves used for fired equipment (boilers) the maximum accumulation of pressure is 3% above MAWP for one valve and 6% when using multiple valves.
4) (c) For ASME Section VIII non-fire case relief valves the maximum accumulation of pressure above MAWP is 10% or 3 psi, whichever is greater except for:
a. When multiple pressure relief valves are provided and set in accordance with UG-134(a), they should prevent the pressure from rising more than16% above MAWP or 4 psi, whichever is greater.
b. When a pressure vessel can be exposed to fire or other unexpected sources of heat, the pressure relief device(s) should be capable of preventing the pressure from rising more than 21% above the MAWP. Supplemental pressure relief devices shall be installed to protect against this source of excessive pressure if the pressure relief devices used to satisfy the capacity requirements of (c) and (1) have insufficient capacity to provide the required protection.
c. Pressure relief devices intended primarily for protection against exposure of a pressure vessel to fire or other unexpected sources of external heat installed on vessels having no permanent supply connection and used for storage at ambient temperatures (100 oF max) of non-refrigerated liquified compressed gases are excluded from the requirements of (a) and (b) above, provided:
i. The pressure relief devices are capable of preventing the pressure from rising more than 20% above the MAWP of the vessel.
ii. The set pressure marked on these devices shall not exceed the MAWP of the vessel
iii. The vessel has sufficient ullage to avoid a liquid full condition
iv. The maximum allowable working pressure of the vessel on which these pressure relief devices are installed is greater than the vapor pressure of the stored liquified compressed gas at the maximum anticipated temperature (<115oF or 45 oC) that the gas will reach under conditions; and
v. Pressure relief valves used to satisfy these provisions also comply with the requirements of UG-129(a)(5), UG-131(c)(2), and UG-134(d)2)
5) Other ASME and Design Requirements
a. For ASME Section VIII relief valves sized for fire exposure the maximum accumulation above MAWP is 21% no matter how many valves are being used.
b. For ASME Section I valves when using multiple valves, the 2nd and all remaining valves can be set at 103% of set pressure.
c. For ASME Section VIII valves when using multiple valves, the 2nd and all remaining valves can be set at 105% of set pressure.
d. For initial API sizing use a discharge coefficient (Kd) of 0.95 for vapor or gas sizing and Kd of 0.65 for liquid sizing.
e. For API sizing for two-phase flow use a discharge coefficient (Kd) of 0.95 for choked flow and 0.65 for non- choked flow.
f. For initial ASME sizing use actual orifice size and 90% of API discharge coefficient(Kd)
g. For final sizing after selection check manufactures catalogues to determine actual Kd’s and check performance charts to see if there are any negative effects of performance due to back-pressure (Kb). (See Vendor Charts of Kb vs Valve Capacity Performance)
h. Use rupture disc instead of a relief device or in conjunction with a relief device to relieve possible runaway reaction scenarios because rupture disc responds more quickly than a relief valve.
i. When using a rupture disc in combination with a relief valve use a combination capacity factor (Kc) of 0.9 for initial sizing. After selection you may consult manufacture’s catalogue for the rupture disc resistance coefficient (KR) to use as a piping resistance.
j. Pay close attention to the rupture disc operating margin to prevent premature bursting due to operating close to disc operating margin. Within 5 psi for < 70 psi set pressure with set tolerance of +2 psi. Within 10% for 70 psi > set pressure < 1000 psi with set tolerance at +3 psi, within 7% for > 1000 psi set pressure with set tolerance + 3 psi.
k. For gas, vapor, and two-phase flow the fluid can choke if the relieving pressure is twice the discharge pressure. Use choke pressure and flowrate instead of back-pressure when sizing.
l. For Two-phase flow use Homogeneous Equilibrium Model (HEM) to determine Maximum fluid flux (Gmax) entering the relief nozzle and for the inlet and outlet pressure drop determination.
m. API recommends using the Omega method for calculating the maximum flux (Gmax) through relief nozzle however, it does not give a method for calculating the inlet and outlet pressure drop. Use homogeneous diret integration (HDI) method instead. Which can be used for flashing flow, hybrid, frozen, sub-cooled flow for nozzle and inlet /outlet piping.
n. Perform two-phase test to determine likelihood of two-phase flow. Design for two-phase venting unless vapor-liquid disengagement correlations support all vapor designs.
o. Problems involving two-phase relief and piping flow should be referred to a process safety expert that can apply DIERS methodology.
p. Problems involving runaway reactions should also b e referred to a process safety expert that can apply DIERS methodology when self-heat rate is known or will be determined calorimetry testing.