Section 3 — Process Safety & Loss Prevention

Register of Pressure Relieving Systems

IPE Engineering Practice IPE-EP-3-7-3

Document number: IPE-EP-3-7-3 · Section: 3 — Process Safety & Loss Prevention

SCOPE

• This Practice covers mandatory requirements for the setting up and maintenance of a Register of Pressure Relieving Systems and shall be used in conjunction with EP 3–7–1 , EP 3–7–2 and EP 3–7–4. The requirements of this Practice apply to refineries, tank farms and crude and product loading facilities.
• This Practice meets the requirements of the OSHA rule 29 CFR Part 1910.119 for process safety information with respect to pressure relief system design (see paragraph (d) 3 (D) of the OSHA rule).
• The design of a relief system is typically finalized only towards the end of a project, following considerable reworking of the design. As a result, the basis for the final design of the system is often not clearly recorded and difficulties are subsequently experienced when a unit modification is proposed and changes to the relief system are being considered. This situation inevitably leads to an unnecessary additional expenditure of engineering manhours and to some doubts regarding the integrity of the relief system. The Register of Safety Related Devices will help eliminate these problems.
• Any deviation from this Practice shall be in accordance with the procedure given in EP 1–1–3.
• An asterisk (*) indicates that a decision by the Owner or the Owner’s Engineer is required or that additional information is furnished by the Purchaser.
• A Revision Bar indicates all changes made to this Revision.

2.0 REFERENCES

The latest edition of the following standards and publications are referred to herein.

STANDARDS AND PUBLICATIONS

DEFINITIONS

• Accumulation – The pressure increase over the maximum allowable working pressure of a vessel during discharge through the pressure relief valve. Expressed as a percentage of the maximum allowable working pressure or in pounds per square inch.
• Back Pressure – The pressure that exists at the outlet of the pressure relieving device because of pressure in the discharge system (see also “Built–up back pressure” and “Superimposed back pressure).
• Built–up Back Pressure – The pressure in the discharge header that develops as a result of flow after the pressure relief valve opens.
• Burst Pressure – The static differential pressure at which a rupture disk device functions.
• Cause and Effect Diagram – A matrix showing the intended outcomes from the initiation of a shut down device, either manual or instrument, such as a high level, pressure or temperature switch. The outcomes are typically activation of designated equipment or closure of individual valves (e.g. fuel gas supplies to fired heaters).
• Contractor – Company or business that agrees to furnish materials or perform specified services at a specified price and/or rate to the Owner.
• Design Pressure – The pressure used in the design of a vessel to determine the minimum permissible thickness or other physical characteristics of the different parts of the vessel.
• Flare – A means of safe disposal of waste gases by combustion.
• Let–Down Station – A let–down station is a flow restriction where the upstream operating pressure is greater than the downstream design pressure. It normally consists of an arrangement of control valves, valves, and/or orifices plates. Less obvious situations include reverse flow through pumps or non–return valves, utility connections to processes, drains to closed drain systems, and heat exchanger tube failures.
• Operating Pressure – The pressure, measured in pounds per square inch gauge, to which the vessel is usually subjected in service.
• Overpressure – Measured in pounds per square inch gauge, is the pressure increase over the set pressure of the primary pressure relieving device. Overpressure is termed accumulation when the pressure relieving device is set at the vessel’s maximum allowable working pressure.

NOTE: When the set pressure of the first (primary) pressure relief valve to open is less than the vessel’s maximum allowable working pressure, the overpressure may be greater than the set pressure per the ASME Code, Section VIII, Division 1, paragraph UG–25.

• Owner – Inflection Point Engineering, LLC.
• Owner’s Engineer – A Inflection Point Engineering, LLC appointed engineer.
• Pressure Relief Valves – A generic term applied to relief valves, safety valves and safety relief valves.

• Pressure Relieving System – An arrangement of the pressure relieving device, piping, and a means of disposal intended for the safe relief, conveyance, and disposal of fluids in a vapor, liquid, or mixed phase.
• Protective Instrumentation – Instrumentation provided to prevent losses of all kinds, particularly in process upsets or emergencies, as distinct from instrumentation provided for normal control.
• Purchaser – The party placing a direct purchase order. The Purchaser is the Owner’s designated representative.
• Register of Pressure Relieving Systems – The Register outlines the design philosophy for the relief system and the pertinent data on which the design has been based. It is intended that the Register will be consulted during the process design of any future unit modification and updated as necessary following completion so as to maintain a comprehensive up–to–date record of the design basis for the relief system.
• Reliability Analysis – A mathematical technique for assessing in probability terms the performance of a component or system.
• Relief Valve – An automatic pressure relief device actuated by the static pressure upstream of the valve, which opens in proportion to the increase in pressure over the opening pressure. A relief valve is used primarily for liquid service.
• Rupture Disk Device – A device that is actuated by static differential pressure and is designed to function by the bursting of a pressure retaining disk.
• Safety Relief Valve – Normally used in gas and vapor service or in liquid service, is an automatic pressure relief device suitable for use as either a safety or a relief valve, depending on the application.
• Safety Valve – Normally used in gas and vapor service or in steam and air service, is an automatic pressure relieving device actuated by the static pressure upstream of the valve and characterized by rapid full opening or pop action.
• Set Pressure – Measured in pounds per square inch gauge, is the inlet pressure at which the pressure relief device is adjusted to open under service conditions. In a safety or safety relief valve in gas, vapor, or steam service, the set pressure is the inlet pressure at which the valve pops under service conditions. In a relief or safety relief valve in liquid service, the set pressure is the inlet pressure at which the valve starts to discharge under service conditions.
• Superimposed Back Pressure – The static pressure that exists at the outlet of a pressure relief device at the time the device is required to operate.

RESPONSIBILITIES

• The Register is a key safety document. The Plant Manager shall appoint a person or department to be responsible for compiling and updating this document.

• (*) For new unit construction, it will be the responsibility of the detailed engineering design contractor to produce the Register and ensure that it contains the necessary information in a comprehensive form. The Owner’s Engineer has the responsibility of ensuring that the information contained is complete and that it reflects the final engineering design. For subsequent unit modifications, the Owner’s Engineer must ensure that any changes which affect the relief system are recorded in the Register and that the design of the modifications is consistent with the design basis in the Register.

CONTENTS OF REGISTER

• General

For the Register to be of practical use, it is essential that the document be comprehensive to use and easy to update. It is therefore important that it includes only the data indicated in this Section (surplus information will tend to make it unnecessarily bulky).

• Design Philosophy

The Register shall include an outline summary of the philosophy adopted in the process design. This should address in particular (but not be limited to) the following:

• Types of utility failures considered, in other words, total, unit, or partial failures, etc.
• Whether multiple cases have been considered and if so, where and why.
• Protection downstream of let–down stations.
• Accommodation of fire relief for shell and tube exchangers where individual pressure relief valves have not been provided.
• Basis for the calculation of fire loads for air coolers and condensers.
• Instances where credit has been taken for operator intervention.
• Maximum fire areas considered and how they relate to the design of the surface water drainage system.
• Philosophy for sparing pressure relief valves.
• Basis for the sizing of discharge lines, including the maximum back pressure and velocity which have been considered.
• Values of pipe roughness used to size lines.
• The assumed position of bypass valves when control valves fail open.
• Unit capacities on which the design is based.
• The philosophy and design basis for the use of Protective Instrumentation in pressure relieving systems.
• List of Relieving Devices
• A list of every relieving device installed on the unit, shall be included in the Register containing the following information for each device:
• Tag No.
• Manufacturer
• Type
• Location
• Set Pressure
• Body and Orifice Size
• Manufacturer’s Capacity Factor (discharge coefficient)
• Discharge Location
• Sizing Calculations

• In addition to pressure relief valves, the list should include rupture discs and thermal relief valves. The list should also include pressure relief valves located on packaged equipment, such as lube oil skids or compressors, for example.
• Table 1 may be used for the list of relieving devices.
• Summary of Unit Relief Loads
• The Register shall include a tabulation of the relief loads generated for all identified causes of overpressure, indicating the case that sizes the pressure relief device. Table 2 and Table 3 may be used for the summary of relief loads.
• The summary should include, for each pressure relief device:
• Tag No.
• Location
• Set Pressure
• Discharge Location
• Relief Load for each cause of overpressure (flow, phase, molecular weight/specific gravity and temperature)
• The basis for each relief load should be clearly defined. Thus, when recording power failure it should be stated whether this is Plant wide, local (i.e. one unit or group of units), partial (affecting part of the supply distribution within a unit or group of units) or individual (single item of equipment).
• When specifying gas breakthrough through a let–down station, for example, the summary should state:
• A list of all the pressure relief valves with their size, type, set pressure and design capacity. In addition, the relief summary table should be completed for new plants and provided for existing plants on a selected basis.
• A list specifying the data pertinent to a let–down station. This should include the size, type, and fully open flow coefficient of the limiting valves or orifices in every route between the high and low pressure systems.
• Protective instrumentation system data. For each system it should include a schematic with every component specified together with the testing frequency and a reference to the study report which defines the system’s reliability.
• Where credit is to be taken for the pressure drop in interconnecting pipework to reduce the gas flow for the gas breakthrough situation, then the pipe lengths, diameters and fittings shall be included.
• Whether the control valve bypass has been assumed open or closed.
• The assumed liquid level in the low pressure vessel if this has any impact on the relief case.
• Where a pressure relief device is provided to protect more than one item of equipment, all the equipment protected should be clearly indicated. Where there is a fire relief case to be considered, a separate breakdown should be included showing the loads generated within each of the equipment items protected.
• Fire Areas and Fire Loads
• The Register shall include a list of the fire areas which have been considered in arriving at flare fire loads, indicating which pressure relief devices should be considered as relieving simultaneously. The tabulation should include:
• area considered

• Equipment item
• Tag No. of the pressure relief device through which the load is discharged (May not be located on equipment)
• Fire load (flow, molecular weight, temperature)
• Total load for area (flow, molecular weight, temperature)
• Table 4 and Table 5 may be used for the list of fire areas and fire loads.
• The section should include a plot plan of the unit marked up to show the fire areas considered.
• Principal Flare Loads
• The Register shall include a breakdown of the flare loads for each of the major utility failure cases, any HP/LP breakthrough (see EP 3–7–2), and the worst fire case. This will possibly include relief loads from other units not under consideration, which may be relieving at the same time.
• If pressure relief device discharges from the unit or units can be directed to more than one flare according to the flare sparing philosophy, then loads for each flare should be included.
• Table 6 may be used for the list of principal flare loads.
• Pressure Relief Device Process Specifications

This section contains a complete set of final process specifications (Data Sheets only) in accordance with EP 5–3–14 for pressure relief valves and rupture discs. In general, for each device these should include the limiting vapor and liquid sizing cases but may include data for additional cases where deemed appropriate (for example, for a high temperature relief case).

• Header Pressure Profiles

A series of layout drawings showing the back pressure at junctions and key points in the relief system pipe network for each of the major utility failure, HP/LP breakthrough, and fire cases shall be included in the Register.

• Pipeline Equivalent Lengths
• A tabulation of the piping equivalent lengths used for the purpose of estimating pressure relief device back pressures shall be included. Also included for each line and header shall be a breakdown of the number and type of fittings providing the basis for the equivalent length. This data should be for the final as–built design and should reference the number of the piping isometric or general arrangement drawing for the line.
• At Let–Down Stations, where credit is taken for the pressure drop in interconnecting pipework to reduce the gas flow for the gas breakthrough situation, then the pipe lengths, diameters and fitting shall be included in this section.
• Table 7 may be used to record the piping equivalent lengths.
• Control Valve/Restriction Orifice Data
• This section contains process Data Sheets for all control valves and restriction orifices which limit relief loads including control valves or orifices at Let–Down Stations. The Data Sheet should specify Manufacturer, type, size and rated conditions (including Cv and Cg) at normal and fully open positions. A brief process sketch should also be included, showing the location of the valve or orifice plate.

• Whenever a control valve or restriction orifice is replaced, this section of the Register must be reviewed to ensure that there has been no change that can give rise to an increased relief load. If an increased load can result, it is essential that the capacity of the associated pressure relief device and the header system be checked to ensure that the system is adequately sized. The Register must then be updated to reflect the change.
• Table 8 may be used for listing control valve data.
• Locked or Car–Sealed Block Valves
• A list of block valves that must be locked or car–sealed open, or closed during normal unit operation to safeguard the integrity of the relief system as designed shall be included in the Register. This may be required either to ensure a free vapor path from a relief valve to the flare, or to prevent the occurrence of an overpressure situation within the unit.
• Table 9 may be used for the list of block valves that must be locked or car–sealed.
• Protective Instrumentation Systems
• This Section of the Register contains details of any protective instrumentation systems per EP 3–7–4 which have been provided where credit has been taken in the sizing of closed relief systems (see Category 1 per EP 12–10–1). The information should include the following:
• P&ID showing the arrangement of sensors and final actuation devices. Cause and effect charts showing the relationship between inputs and outputs should also be included.
• A written description of the hazard being protected against together with a functional description of how the system is intended to work.
• Detailed testing procedures and frequencies shall be included.
• Details of operational requirements and any assumptions on how the system is to be operated.
• A section which lists all safety critical items which should not be modified without reviewing the safety implications.
• A record of the initial testing carried out at the manufacturer’s shop and on–site testing before commissioning.
• Detailed design drawings showing process, electrical, pneumatic, hydraulic, and power supply arrangements.
• Design specifications and purchase specification for all instruments.
• The Reliability Analysis, calculations and assumptions, if applicable.
• Fire–Resistant Insulation

A list of vessels equipped with fire–resistant insulation to limit the fire relief load shall be included in the Register. Credit may only be taken for insulation if it is specified ‘fire–resistant’ and is in accordance with the requirements of EP 11–2–1.

• Control Loop Segregation
• Where a control system is installed, in which failure of any part of the system gives rise to the governing relief load, the relief load and control system devices shall be recorded in the Register.
• Table 10 may be used to list of the relief loads that might arise from failure of a control system device.

6.0 TABLES

TABLE 1

LIST OF RELIEVING DEVICES

TABLE 2 SUMMARY OF RELIEF LOADS

TABLE 2

SUMMARY OF RELIEF LOADS (CONTINUED)

TABLE 3

NOTES TO ACCOMPANY SUMMARY TABLES

TABLE 4

FIRE AREAS AND FIRE LOADS WHEN UNIT IS SHUT IN

TABLE 5

FIRE AREAS AND FIRE LOADS WHEN UNIT IS CIRCULATING

TABLE 6

LIST OF PRINCIPAL FLARE LOADS

TABLE 7

PIPING EQUIVALENT LENGTHS

TABLE 8 CONTROL VAVLE DATA

TABLE 9

LIST OF LOCKED OR CAR-SEALED BLOCK VALVES

TABLE 10

LIST OF LOADS ARISING FROM A FAILURE IN A CONTROL SYSTEM

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