Isolation Philosophy
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 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.
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 Isolations alone (single
or double block) to Valve Isolations plus Positive Isolation (spade, removable
spool, etc).
Type of
Isolation Philosophy
1.Positive isolation by insertion of a spade, spectacle blind or removal
of a spool piece.
2.Valved isolation by closure of in-line
valves.
Positive
Isolation
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 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;
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:
o
Single
Block (SB)
o
Single
Block and Bleed (SBB)
o
Double
Block (DB)
o
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 flashpoints. 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.