What is Positive Isolation?
Positive isolation is a method to totally isolate equipment or a section of the plant from a live process where in no leakage is tolerated due to the personal safety risks involved. A risk assessment should be performed to develop an appropriate isolation scheme. This assessment should consider the task of performing the isolation together with what the isolation is required for.
To determine the inherent risks to people, plant and the environment when carrying out the risk assessment, the following considerations should be made in regard to any of the properties of the primary or associated systems:
- Toxicity and Flammability of the fluid involved
- Pressure and Temperature
- Size of potential leak of hazardous materials
- The natural ventilation of the surrounding area
- The likelihood of pressurization of the line
- The risks and hazards involved in providing the isolation
Where positive isolation should be applied?
Following are the typical scenarios that require a positive isolation:
- Confined space entry: Many workplaces contain areas that are considered "confined spaces" because while they are not necessarily designed for people, they are large enough for workers to enter and perform certain jobs. A confined space also has limited or restricted means for entry or exit and is not designed for continuous occupancy. Confined spaces include, but are not limited to, tanks, vessels, silos, storage bins, hoppers, vaults, pits, manholes, tunnels, equipment housings, duct-work, pipelines, etc. [Ref1]
- Requirement for entry cannot be eliminated by equipment design
- Opening of an equipment for maintenance while the rest of the plant is operational and there is a consequent high level of risk of uncontrolled energy release and/or hazardous materials release
- Long term isolation, such as isolation between water injection and gas injection systems for WAG wells
- Equipment testing
- Hot work on pressurized hydrocarbon systems
- To isolate utility services
What are the various methods of positive isolation?
For positive isolation between two systems, the most reliable method is to remove a pipe section or a spool piece and install blank flanges at the open ends with appropriate material, size and rating, which is properly installed with the correct gaskets and securing bolts so that it could withstand the system design pressure. A typical schematic example is shown below for such a system:
Installing a removable spool piece is not always practical due to the reasons below:
- High pressure rating pipe
- High test pressure
- Spool piece weight
- Material handling
In such situations other means of positive isolation such as spade, spectacle blind, double block and bleed valve, single block and bleed, etc. can be used.
Note: Simple valves are not considered for positive isolation as they are subject to leakage.