The most common pressure class for pipeline transmission lines is class 600, which has a working pressure of 1440 psi. The valve ratings are in accordance with The American Society of Mechanical Engineers (ASME) standard B16.34 and API 6D.
Although a number of valves are in operation at each pumping station (for liquids) or compressor station (for gas transmission), the critical valves in a pipeline are spaced along its route. They serve as blocking or isolation valves to segregate pipeline sections for required maintenance or to help in cases of an accident. The minimum required spacing of these valves is prescribed in ASME B31.4, “Gas Transmission & Distribution Piping Systems” and ASME B31.8, “Pipeline Transportation Systems for Liquid Hydrocarbons & Other Liquids.”
Several factors influence valve spacing, including: 1) the amount of potential fluid leakage, 2) the impact of a release, 3) future development in the pipeline area, and 4) the time required to blow down (empty) an isolated section. Other criteria include how close the line is to occupied buildings and houses. According to B31.4, the distance between block valves could be as little as four miles apart for a gas pipeline.
Liquid pipelines have their own criteria for valve placement. They are placed: 1) at the suction end and discharge ends of a pump station, 2) on each line entering or leaving a storage tank area, 3) on each mainline at locations along the pipeline that will limit damage or pollution from accidental hazardous liquid discharge, 4) on each lateral take-off from the trunk line, 5) on each side of a water crossing that is more than a 100 feet wide, and 6) on each side of a reservoir holding water for human consumption.
Additionally, check valves may be installed on grades and the downstream side of rivers and streams for more protection from backflow conditions in case of a line breach.
Many block valve installations are outfitted with automatic shutdown controls. These controls are set to close the valve if pressure or flow rates change, indicating a possible breach in the line. By having these valves spaced throughout the line, the amount of potential fluid leakage that might occur during a line break is limited. Additionally, many pipeline valves are designated as emergency shutdown valves (ESD), which are remotely operated from the pipeline control center.
These block valve location requirements account for the numerous small, fenced-in valve installations visible when driving around areas with many pipelines—numerous pipeline block valves are located above the ground for easy maintenance. However, some are buried, with only the operating mechanism and auxiliary lubrication and bleed lines showing. These installation areas used to be the exclusive domain of gate valves. However, today welded body trunnion-mounted ball valves are very popular, especially for clean natural gas transmission lines. The unique welded body construction eliminates the potential body-bonnet leak path, while the only remaining leak path is up through the packing area.
Though fugitive emissions (FE) leakage has been a focal point in the refining industry for over 20 years, the upstream and midstream markets have been fairly immune from FE scrutiny. However, that situation is changing. According to MPL’s Daigle, “LDAR [leak detection and repair] for pipelines is becoming popular and required, especially since packing leaks are the most common leaks we deal with.”
One place where emissions of any type are unacceptable to almost everyone is in undersea pipelines. Because they are surrounded by water and vibrant marine life, undersea pipelines certainly have their own set of challenges. However, there are other key differences from on-land pipelines that affect design, including the design of the valves attached to the pipelines.
For example, undersea pipelines that connect wellheads to gathering points often operate at much higher pressures than their onshore counterparts. It is not uncommon for these lines to see 10,000 psi. Valves designed for this submerged service are critical, purpose-built flow control devices that absolutely must work properly when called upon to operate. Because of the unique undersea environment, standard API 6D requirements are not deemed tough enough, so a special underwater valve specification was written to cover these products: API 6DSS “Specification for Subsea Pipeline Valves.”