Gas plant is usually designed and built on the philosophy that methods are put in place to ensure that the plant can continue to operate whilst discrete sections of the plant are taken out of service for maintenance or inspection.
During maintenance and inspection work it must be possible to completely isolate equipment, or sections of equipment, to ensure a safe working environment for personnel.
In general it must be possible to positively isolate individual equipment, or groups of equipment within the processing system.
The exception is equipment in non-hazardous service where it is definitely established that, except for pressure and temperature, no hazardous condition can ever exist.
In determining the possibility of whether a hazardous condition can exist the following has been considered: the fluid toxicity, flammability, pressure and temperature and the possibility of personnel being exposed to the fluid.
Positive isolation of equipment is generally achieved by the closure of block valves plus the use of a spectacle blind or spade and spacer arrangement.
Removable spools are limited to:
- Pump and compressor suction and discharge lines.
- Shell and tube, plate type heat exchanger nozzle connections.
- Connection to relief systems on pressure vessels (only when there is no spare relief valve installed, in this instance an individual up- and /or downstream isolation valve will be installed).
- flare header train isolation valves.
- LP flare header train isolation valve.
The philosophy used for isolation with the gas plant is to provide valves at system boundaries. This allows complete trains of process or utility systems (rather than individual equipment items) to be positively isolated, thereby reducing the overall number of valves required.
An outline for the requirement for isolation of facilities is as follows:-
- tight shut off valves around the section, equipment or processing unit to be isolated.
- and drain provision in the section of plant between the respective isolation valves, and;
- pair of flanges at the point of isolation, along with either a spectacle blind, spade and spacer arrangement or removable spool piece.
General isolation shall be provided to separate one process system from another and from any utility systems.
When piping is ANSI class 900 or above, general isolation shall be achieved by double block and bleed isolation.
Single block isolation is acceptable for isolation of systems ANSI class 600 and lower.
Where isolation of systems containing condensate is required, double block and bleed isolation shall be provided for systems with a flange rating of ANSI class 600 and above. For systems containing condensate, single isolation is acceptable for ANSI class 300 and below.
Control valves, check valves and ESD valves shall not form part of isolation valves.
Positive isolation is being defined as being isolation achieved through the rotation of a spectacle blind in the closed position, or the insertion of the spade element of a spade and spacer set, or the removal of a removable spool piece and the subsequent blinding of the open process piping.
It should be noted that before positive isolation can be effected, general isolation of the system is required together with depressurization, draining and purging.
Positive isolation is required where an isolated system is required to be opened for maintenance, inspection or testing.
Where inlet or outlet piping is manifolded, such as to be connected to more than one nozzle on the vessel, e.g. multiple feeds to a column, a single positive isolation method located at the manifold isolation valve is acceptable rather than positively isolating each vessel nozzle, provided that the isolation is clearly visible from the vessel.
All vessels fitted with man-ways, hand-holes or inspection hatches are to be provided with spectacle blinds or spade/spacer arrangements on every process and utility nozzle except connection to the relief systems and connections fitted with appropriate valve(s) and a blind.
Lined vessels are provided with removable spools to allow for nozzle inspection.
Order of Effectiveness
The order of increasing safety effectiveness for mechanical isolation of piping and equipment is:-
1. closed valves (with securing devices)
2. closed and locked valves
3. double block & bleed (with securing devices)
4. double block & bleed locked valves
5. use of spade or blind (positive isolation)
6. physical disconnection (positive isolation)
A combination of the above isolation methods might also be a practical way of isolating equipment/components.
The isolation method to chose is also depending on medium and pressure rating of the piping/equipment.
The level of isolation also depends on what work needs to be carried out.
Reliance on Closed Valves
Securing of isolations by lock and key is only practicable if the isolation point is designed to have a lock fitted.
The gas plant has many valves which should be in the open or closed position. The position of these valves is indicated on the P&ID’s as LO (Locked Open) or LC (Locked Closed). These valves are normally equipped with a device to enable them to be locked in the desired position by padlock or CAR seal.
Valves which should be Locked Open can be found up- and/or downstream of Relief/ESD valves and in flare lines.
Valves which should be in the Locked Closed position are typical in compressor casing drains.
To enable a good and auditable control on these valve positions a “Locked open and closed valve” register is kept in which on a form & marked up P&ID each position of the valve is registered. The valves are either locked with a numbered padlock or CAR seal. The number of the padlock or CAR seal is entered on the registry form.
Besides the normally LO/LC valves also valves are registered which are used for system/equipment isolation.
The register is kept in the Shift Supervisor’s office and any changes in valve positions should be authorized by the Production General Manager.
If an isolation valve, however, can not be locked, the isolation must be properly secured and the isolation securing method in the PTW system must:
- Be highly visible.
- Make it impossible to move the valve without breaking the securing device and make it obvious that the securing device has been tampered with (e.g. a CAR seal; this is a seal with a unique number consisting of a steel wire with a metal seal secured with a break-off locking screw).
- Use of marked up P&ID’s showing the isolation requirement. Valve numbers and their position to be mentioned on Work Permit.
- Be simple to apply.
- The simpler it is the more chance is that it will be done correctly.
- Discuss the equipment isolation procedure and its importance during a tool box meeting with all personnel involved.
- Provide sufficient security of isolation to prevent inadvertent operation of a valve (e.g. presence of operator to monitor the maintenance activities).
- Although the preferred design practice is to use only one Relief Valve (RV) per vessel without any up- and downstream isolating valves several vessels are, for a combination of cost reasons and maximizing uptime of facilities, equipped with a double or triple relief valve system.
When this is the case this relief valve system is equipped with an interlocking system for the up- and downstream isolation valves. In case in a live system a relief valve needs to be isolated and removed for maintenance and the standby RV put on line, the interlock system opening and closing sequence will not allow an inadvertent isolation of the complete RV system.
In practice this means that each isolation valve is provided with a locking mechanism and key system with 2 unique keys. First the standby (isolated) RV has to be put online by opening the upstream isolation valve (the downstream valve is normally locked in the open position). When this valve is opened the 2nd key in the locking mechanism will be released. This key is used to isolate the online RV, starting with the upstream isolation valve and then the downstream valve. The key released when the downstream isolation valve is closed is kept in a key box in the control room and can be used at a later stage for the reversed sequence.
When the RV has been removed, blinds should be installed on the up- and downstream open flanges.
Mechanical Specification of Isolation Components
Ensure that spades, spectacle blinds, etc are of the same class as the systems in which they will be installed.
Spectacle blinds are already installed at many points in the installation and are of the same pressure rating as the associated piping.
Installing Isolation Components
The hazards associated with inserting a spade should be carefully considered before starting work and compared with other means of isolation which could be less hazardous to execute.
Isolation of Drain Systems
The need to isolate or de-isolate drain systems should be checked carefully as part of the overall isolation requirement. Especially in the closed drain system where de-isolated drains could cause a hazard (e.g. back flow of hydrocarbons from other drains).
The energy source (electrical, hydraulic, air or other) should be disconnected or the machinery isolated from the drive unit.
All electrical equipment should be regarded as live unless and until properly isolated and proven to be isolated.
Separation from Power Source
All electrical equipment should be separated from the source of supply before maintenance work is starting. Isolating or control circuit breakers should utilize padlocking facilities with a key control system.
Where such switches are not available, fuses should be removed and/or breakers or contactors racked out.
A warning label should be attached to each isolation point and identified on the Work Permit.
Test trials to start equipment items should be included as part of a final check to ensure effective isolation before maintenance work starts.