This document suggests the minimum requirements for the selection and installation of mechanical isolation devices such as isolation valves, spades, spectacle blinds, removable spool pieces, drain lines, and vent lines for topsides piping and equipment. This philosophy is needed to ensure that safety, regulatory, environmental, operability, and maintenance requirements related to positive isolation are met in an economically optimized manner.
Sufficient isolation valves and blinds or spades should be provided to ensure that safe, satisfactory isolation of each piece of equipment and each system is possible; therefore, isolation devices serve three functions:
- To enable an entire system, comprising of a number of equipment items and associated pipe work and instruments, to be taken out of service safely while other systems continue to function normally.
- To provide local, physical proof that a single piece of equipment is safely isolated from all possible sources of inflow prior to allowing maintenance and permitting entry of personnel.
- To provide the means to reinstate the equipment or system safely, following successful maintenance or inspection-testing and repair operation. The philosophy described by this document should be reflected in the Piping and Instrument Diagram (P&IDs).
2. Isolation Methods
Spectacle blinds are defined as any two (2)-position flange plates fitted between two mating pipe flanges, which under normal conditions are in open position. When rotated to its closed position, the spectacle blind blocks flow. In general, spectacle blinds are installed on-line sizes up to, and including 12 inches. A spade and spacer set is generally used on larger line sizes because of weight constraints. Insertion of the spade element of the set provides positive isolation.
Single isolation should be implemented by means of a single block valve. The valve may be a gate valve or a ball valve. Butterfly valves should be only used for water service as trivial leakages can be tolerated.
Double isolation should be implemented by means of two ball valves in a series (both closed), or single block valve, and spectacle flange or blank flange installed upstream of the fluid segment to be isolated.
Double Block and Bleed (DBB)
Double-block-and-bleed isolation should be implemented by means of two block valves and a piping spool or bleed ring in between, fitted with a bleed connection safely vented to the atmosphere. The use of a dual seat isolation valve (double piston type), with an integral cavity bleed safely vented to atmospheric pressure, should be considered for specific applications where cost and weight considerations are of very high priority.
3. General Notes
- Threaded connections should be avoided in hydrocarbon service. Welded connection using a flange at the boundary line between piping and instrumentation.
- All piping connections should be 1 inch (160 sch.) minimum.
- Use of triple offset butterfly valves may be considered for non-hydrocarbon, low pressure service (less than or equal to ASME 300#) to conserve space.
- Double isolation requirement in bigger lines may be met by providing removable spool.
- Credit may be taken for using an SDV to meet the isolation requirements, so long as the valve type requirement (i.e. ball, gate, or butterfly per above) is satisfied.
4. Isolation Categories
One of the following types of isolation is appropriate:
- Double Valve Isolation with Intermediate Vent Valve
This category is used for fluid systems having the highest energy, or potential hazard.
Isolation is provided by double isolation.
Isolation is provided by a single isolation valve. The valve should be tested to be tight shutoff (zero bubbles).
Isolation is provided by a single isolation valve, which is not a tight shutoff valve, provided the consequences and risks resulting from the fluid leakage have been assessed.
The selection of one of the isolation categories and methods mentioned above should be based on a risk assessment that considers the following factors:
- Isolation duration – Long duration isolation refers to an isolation period greater than the length of time, equivalent to one operational shift (i.e. greater than 8 hours or 12 hours). Short duration isolation refers to an isolation period less than the length of time equivalent to one operational shift (i.e. less than 8 hours or 12 hours).
- Isolation frequency
- Fluid pressure/temperature
- Line size
- Hazardous characteristics of fluids released in terms of toxicity, volatility, flammability, and/or harmful properties such as corrosivity
- Consequence of a fluid release
- Potential for pollution
A qualitative assessment of these factors and use of the logic flow diagram in below section 6 (General Isolation Philosophy) should result in the selection of a conservative isolation category.
Alternatively, the Mechanical Isolation Guidelines listed in section 7 may be used to determine an equally acceptable isolation category.
When applying the isolation requirements of section 6 and section 7, the System Pressure should be considered as the source of pressure and not the rating of the valves that would be used. For example, when isolating an ASME 600# vessel from a flare system at ASME 150# (e.g. via a manual depressurization line), the valves en-route to the flare should be 600# up to the last isolation valve where the piping specification break is taken. The source of pressure is less than ASME 150#, and therefore only a single block valve is required for isolation. (If it were based on the valve rating, then double block and bleed isolation would have been required.)
6. General Isolation Philosophy
7. Mechanical Isolation Guidelines
8. Pressure Vessel Isolation
All sources of hydrocarbons and hazardous fluids should be positively isolated from the vessel by means of blinds or spade and spacer arrangement.
All vessel connections should be 2 inch minimum. The provision of isolating valves and blinds, drain and vent lines, and purge connection points should be provided in accordance with the guidelines set out in above sections.
Where inlet and outlet piping are manifolded, such as to be connected to more than one nozzle on the vessel, a single spectacle blind located at the manifold isolation valve is acceptable rather than a blind at each vessel nozzle, provided it is clearly visible from the vessel.
Vessels connected to the flare system should have a globe valve installed downstream of a tight shutoff ball valve, which enables controlled depressurization of the vessel.
Connections to closed drain system from equipment and piping should have two block valves and a spectacle blind located at the downstream of first block valve. Where more than one drain line from the same vessel joining in a manifold with a common isolation valve, the spectacle blind should be located upstream of common isolation valve.
9. Isolation Arrangement
Figure 1: Closed Drain Arrangement for a Single Connection from the Vessel
Figure 2: Closed Drain Arrangement for Multiple Connections from a Vessel