Pipeline Safety Improvement Act of 1991

Pipeline Safety Improvement Act of 1991 - Amends the Natural Gas Pipeline Safety Act of 1968 and the Hazardous Liquid Pipeline Safety Act of 1979 (HLPSA) to direct the Secretary of Transportation to define environmentally sensitive areas to include: (1) earthquake zones and areas subject to substantial ground movements like landslides; (2) areas where groundwater contamination would be likely in the event of a pipeline rupture; (3) freshwater lakes, rivers, and waterways; and (4) river deltas and other areas subject to soil erosion or subsidence from flooding or other water action where pipeline facilities are likely to become exposed or undermined.

Makes the Secretary responsible for protection of the environment in issuing Federal pipeline safety standards. Requires pipeline operators to report to the Secretary any conditions that could have a significant adverse impact on the natural environment.

Revises the required pipeline inventory to include: (1) all pipeline facilities in environmentally sensitive areas, or in or immediately adjacent to cities, towns, or villages; and (2) all pipelines built before 1971.

Amends the HLPSA to direct the Secretary to survey and assess the effectiveness of procedures, systems, and equipment used to detect and locate hazardous liquid pipeline ruptures and minimize product releases from pipeline facilities. Requires the Secretary, within 12 months after completing the survey, to issue regulations to establish standards and require procedures and equipment for the rapid detection and location of pipeline ruptures and shut down of pipeline facilities located in or adjacent to environmentally sensitive areas and cities, towns, and villages.

Amends the HLPSA to direct the Secretary to issue regulations to require hydrostatic testing of all hazardous liquid pipelines regardless of date of construction.

Amends the Natural Gas Pipeline Safety Act of 1968 to direct the Secretary to: (1) issue regulations requiring operators of natural gas distribution systems to install excess flow valves in new or renewed gas service lines; and (2) publish guidelines for assessing the safety of existing cast iron gas pipes and for determining the conditions under which pipeline facilities using such pipes shall be replaced in order to ensure continuing pipeline safety.