Isolation Philosophy describes methods for isolating sections of plant to permit safe operation, and provide access for maintenance or inspection. The methods described here are applicable for all hazardous and non-hazardous process and utility systems. Provision shall be made to isolate all equipment, either separately or commonly, within a process or non-hydrocarbon system.
Maintenance isolation is aimed at providing a safe level of isolation for the required activity. The level of required isolation will depend on the duration of the activity and the hazardous nature of the fluid and may range from Valve Isolation’s alone (single or double block) to Valve Isolation’s plus Positive Isolation (spade, removable spool, etc).
Type of Isolation Philosophy
- Positive isolation by insertion of a spade, spectacle blind or removal of a spool piece.
- Valved isolation by closure of in-line valves.
Isolation of plant for maintenance will be by positive isolation and/or valved isolation ie: Positive Isolation, where an absolute guarantee of segregation (i.e. no leakage) is required between the isolated section and the surrounding plant.
Positive Isolation is defined as a method of isolation that removes all practicable risk of accidental re-connection to source of the risk. Examples of Positive Isolation would include: Insertion of a spectacle blind or spade and spacer. Removal of a pipe spool with blind flanges fitted to flanges at risk of pressurization.
Spectacle blinds are normally preferred, except where the layout prevents the use of a spectacle blind or where the pipe diameter or pressure rating makes a spectacle blind impractical. In these cases positive isolation will be provided by a spade with removable spacer or removable spool depending on the specific nature of the requirement and the configuration.
Selection of Blanks, Spectacles and Spades (Blinds) And Spools wherever required shall be based on the Piping Material Specification according to equipment nozzle/line size. Spectacle blind shall be provided for all nozzles and lines less than 12”. Any size higher than 12” shall be provided with spacers/spades. Piping shall be designed to permit practical removal and re-installation via provision of sufficient pipe flexibility and appropriate mechanical handling procedures.
Where spools are used, the piping shall be designed to permit practical removal and re-installation via provision of sufficient pipe flexibility and appropriate mechanical handling procedures.
Positive isolation will be applied under the following circumstances:
- To permit isolation of major items of equipment or a section of plant which can be segregated for long term inspection or maintenance without requiring a complete process shutdown
- To permit confined space entry such as the isolation of vessels and tanks in preparation for entry or personnel.
- To prevent contamination, during normal operation, of utility supplies e.g. water, air and nitrogen where these are permanently connected to a process unit.
- To permit isolation of block valves provided on fill, vent and drain connections on process systems and equipment. These will be fitted with fully rated blind flanges;
- To segregate parts of the plant which may be subject to being over pressurized during alternative operating modes.
Valve Isolation Requirements
Valved isolation is used to maintain containment and is an acceptable option in low risk and non-entry situations e.g. control valve maintenance. Some situations will require both valved and positive isolation, whilst others will require only a valved arrangement. Valved isolation is also used to enable positive isolation to be installed or removed without the need for a complete plant shutdown.
The main classifications for valved isolation are:
- Single Block (SB)
- Single Block and Bleed (SBB)
- Double Block (DB)
- Double Block and Bleed (DBB)
Valved isolation shall be achieved using reliable manual operated valves and must provide a reliable positive seal. All isolations shall be tested and shown to be effective before containment is broken. The most secure valve isolation is provided by DBB, although unless the integrity of both valves is proven in the direction of the pressure differential at the highest pressure the system is expected to operate at for the duration of the isolation, the isolation cannot be considered as DBB.
Valve Isolation, where the isolation of the section is performed by one or more valves, but the guarantee of segregation is not required. The selection of single versus double block isolation shall be based on the fluid rank, as described herein.
This section presents the methodology to be used in determining the level of valve isolation required for the general applications. Required level of isolation shall selected based on the fluid handled in the system
Fluid Hazards and Isolation Requirements
The hazard posed by opening a line depends on the properties of the fluid, in particular: pressure, flammability, flash potential, toxicity, irritancy, and temperature. Where the hazard levels are determined to be high, the system is considered to be in severe service and a double block and bleed (DB&B) is required. Where the hazard levels are more moderate, a single block and bleed (SB&B) can be used. For low hazard levels, a single block only (SB) is required. The “moderate and low hazard levels are considered non-severe service.
Following fluid properties to be considered listed below for isolation.
Pressure: Pressure is the primary measure against which hazards are assessed. The nomination of system pressure should consider all conditions under which the isolation is required to seal. This may range from normal operating pressure to the process design pressure. Under certain maintenance applications consideration may be given to lowering the upstream pressure with the aim of reducing the hazard level and isolation requirement.
Flammability: In this philosophy, all hydrocarbon gases are considered flammable. Hydrocarbon liquids are considered flammable per the definition of the Institute of Petroleum’s Model Code of Safety Practice in the Petroleum Industry, Part 15 (IP 15) ie: Liquids with a flash point below 55°C
Liquids at a flash point below 100°C if handled at temperatures above their flash points. Hydrocarbons liquids such as diesel oil, hydraulic oil, and lubricating oil are considered combustible rather than flammable and can be referenced in the ranking table as Unclassified.
Flashing: In this philosophy, flashing refers to any liquid which is within 10°C of its boiling point at atmospheric pressure.
Toxic / Irritant: Toxicity and irritancy are per the appropriate Health and Safety Datasheets. With specific regard to hydrogen sulfide, toxic threshold levels in process hydrocarbons shall be established on a project basis but otherwise not below 500 ppm(mol).
Hot The philosophy threshold for hot fluids is 65°C consistent with the requirement of personnel protection on piping. However, consideration may be given to smaller inventories in which the fluid can be allowed to cool before any maintenance activities.