4.4.1 Isolating valves shall be installed for the purpose of isolating the pipeline for maintenance and for response to operating emergencies. 4.4.2 Valves shall be at locations accessible to authorized personnel and protected from damage or tampering, and shall be suitably supported to prevent differential settlement and movement of the attached piping. 4.4.3 Except for multiphase pipelines, oilfield water pipelines, and gas pipelines, isolating valves shall be installed on lateral lines connected to main lines. 4.4.4 Except as allowed by Clause 4.4.5, in determining the number and spacing of sectionalizing valves to be installed, if any, the company shall perform an engineering assessment that gives consideration to relevant factors, such as
a) the nature and amount of service fluid released due to repair and maintenance blowdowns, leaks, or ruptures;
b) the time to blowdown or drain down an isolated section;
c) the effect on inhabitants in the area of the blowdown gas release (e.g., nuisance and any hazard resulting from prolonged blowdowns);
d) continuity of service;
e) operating and maintenance flexibility of the system;
f) future development within the valve spacing section in the vicinity of the pipeline; and
g) significant conditions that could adversely affect the operation and security of the pipeline.
4.4.5 In absence of an engineering assessment conducted in accordance with Clause 4.4.4, the spacing of valves in the pipeline shall be as given in Table 4.7, but may be adjusted by up to 25% based upon factors such as operational, maintenance, access, and system design considerations.
4.4.6 For HVP and carbon dioxide pipelines, sectionalizing valves shall be located outside cities, towns, and villages, at the transition from Class 1 to a higher class location. 4.4.7 For HVP and carbon dioxide pipelines, in locations where a failure would constitute an extraordinary hazard, sectionalizing valves shall be equipped for remote operation and the maximum spacing between such valves shall not exceed 15 km. Note: Extraordinary hazards can exist in areas such as major industrial complexes, commercial navigable waters, and densely populated areas. 4.4.8 For HVP and carbon dioxide pipelines, emergency connections to facilitate depressuring or evacuating, or both, an isolated section of pipeline shall be provided on both sides of, and adjacent to, sectionalizing valves. The capacity of such connections shall be adequate for emergency conditions.
4.4.9 For HVP and LVP pipelines, valves shall be installed on both sides of major water crossings and at other locations appropriate for the terrain in order to limit damage from accidental discharge. Notes:
1) Consideration should be given to the installation of check valves to provide automatic blockage of the pipeline.
2) A major water crossing means a water crossing that in the event of an uncontrolled product release poses a significant risk to the public or the environment.
4.4.10 For gas pipelines, blowdown valves shall be located so that the sections of transmission lines between sectionalizing valves can be blown down. Sizes and capacities of the connections for blowing down transmission lines shall be such that under emergency conditions, the sections can be rapidly blown down. Locations of blowdown valves shall be such that the gas can be blown to the atmosphere without undue hazard.