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ASME STP-PT-046-2011 — Review of Safety Considerations for Natural Gas Pipeline Block Valve Spacing
🚧 ASME STP-PT-046-2011 is a natural gas pipeline safety research report prepared by Robert J Eiber Consultant Inc and Kiefner and Associates under the sponsorship of ASME Standards Technology, LLC (ST-LLC). It systematically reviews how the spacing and operator type of sectionalizing block valves on gas transmission pipelines affect public safety. The report examines prior studies on valve types and spacing from 1995 to 2006, analyzes the PHMSA incident database, and reviews NTSB serious gas transmission incident investigations, correlating the time required to make an area safe, fracture propagation lengths, and blowdown times with injuries, fatalities, and property damage. The findings were intended to help define spacing and operator requirements for block valves in gas pipelines, supporting possible revisions to ASME B31.8.
💡 Block valve spacing is a major factor in the construction cost of new pipelines, yet it is also the key to isolating a leaking segment after a rupture. Striking the right balance between cost and public safety is the heart of this report — and the reason ASME B31.8 is considering revised valve spacing rules.
1. Overview: Background of Valve Spacing Requirements 📌
Block valves have been used to sectionalize pipelines since the earliest pipelines were built. They have been required in pipeline codes — such as ASME B31.1.8, the predecessor of the ASME B31.8 committee — since 1952. The quantity and spacing of sectionalizing block valves has a significant impact on the construction cost of new pipelines. The report reviews several important prior studies:
- 1995: Remote and automatic main line valve technology assessment;
- 1997: Survey of design rationale for valve usage in various design codes;
- 2000: Development of a design basis for main line block valve spacing on gas transmission pipelines;
- 2005: White paper on equivalent safety for alternative valve spacing;
- 2006: Study on the safety impact of valve spacing in natural gas pipelines;
- Greenhouse gas considerations: volume limitations between valves driven by blowdown emissions.
2. PHMSA Incident Data Analysis 📊
Using the PHMSA incident database for 2002 through 2009, the report correlates injuries and fatalities with parameters such as the time to make the area safe and fracture propagation length. Key findings:
- In Class 1–3 locations, the time to make safe correlates strongly with injuries and fatalities — the slower the isolation and blowdown, the worse the consequences.
- Longer fracture propagation lengths enlarge the affected area and raise the probability of casualties.
- Total incident costs (from $2.5 million to $87.5 million for Class 1 incidents) rise with class location and hazard severity.
| Analysis Dimension | Data Source | Primary Observation |
|---|---|---|
| Proximity to the public | PHMSA database (2002–2009) | Injuries/fatalities in Class 1–3 strongly tied to time to make safe |
| Concentration of gas release | PHMSA database | Consequences more severe in higher-population-density areas |
| Time to make safe | Incident data regression | Longer time, higher risk of injury and death |
| Fracture propagation length | Incident data regression | Longer propagation, larger affected area and loss |
| Property damage / total cost | Class 1 incidents ($2.5M–$87.5M) | Cost rises with class location |
⚠️ From the 1969 Houston incident to the 2010 San Bruno rupture, the NTSB-investigated cases repeatedly demonstrate that excessive valve spacing or slow valve operation extends the duration of uncontrolled gas release and magnifies fire and explosion consequences. Any revision of valve spacing rules must be guided by these historical lessons.
3. Review of NTSB Serious Incident Investigations 🔍
The report reviews 13 serious gas transmission incidents investigated by the NTSB, including the 1969 Houston high-pressure gas incident, the 1974 Michigan-Wisconsin incident, the 1982 Hudson, Iowa, puncture/explosion/fire, the 1985/1986 Kentucky ruptures and fires, the 1994 Edison, New Jersey, explosion and fire, the 2000 Carlsbad, New Mexico, rupture and fire, the 2009 Florida Gas Transmission rupture alongside the turnpike, and the 2010 San Bruno, California, rupture and fire. The common lessons are:
- Continued gas release after rupture is the primary cause of large fires and secondary explosions;
- Valve spacing and operator type (manual, remote, or automatic) determine the speed of isolation;
- Higher automation and well-placed valves significantly shorten the window of hazard exposure.
4. Blowdown Time Related to Valve Spacing ⏱️
For a typical 36-inch (914 mm) diameter, 1000 psig gas transmission pipeline, the report estimates optimistic blowdown times with valves spaced from 1.4 to 20 miles (2.2 to 32 km) from the rupture. The conclusion: the larger the valve spacing, the slower the depressurization after a rupture and the higher the public-safety risk. In populated or high-consequence areas, shorter spacing or automatic/remote valves are the safer choice.
5. Engineering Practice Essentials 🛠️
- 🔍 Graded valve placement: add sectionalizing valves in high-consequence areas (dense population, near public facilities) and allow wider spacing in low-density areas to balance cost and safety.
- 🔧 Operator type selection: remote and automatic valves (R/A valves) sharply reduce response time and should be prioritized in critical segments.
- ⏱️ Blowdown check: during design, run blowdown-time calculations for the worst-case rupture location and ensure isolation/depressurization times meet emergency-response requirements.
- 📋 Emergency coordination: valve spacing design should be reviewed jointly with leak detection, emergency response, and public evacuation plans.
✅ The report gives ASME B31.8 a dual “data + case study” basis for revising valve spacing rules: by configuring spacing and operator type wisely, it is possible to optimize construction cost while maintaining equivalent safety, even as design stress levels increase.
6. Frequently Asked Questions ❓
❓ Q: How far apart are main line block valves on gas transmission pipelines?
A: Traditionally, U.S. main line block valve spacing is on the order of 10–20 miles, depending on class location and operator practice. This report recommends tighter spacing and more automation in high-consequence areas.
A: Traditionally, U.S. main line block valve spacing is on the order of 10–20 miles, depending on class location and operator practice. This report recommends tighter spacing and more automation in high-consequence areas.
❓ Q: What is the difference between manual, remote, and automatic valves?
A: Manual valves require personnel at the site and are slowest; remote valves can be operated from a control center; automatic valves close on their own when abnormal pressure is detected and react fastest. Construction and maintenance costs rise accordingly.
A: Manual valves require personnel at the site and are slowest; remote valves can be operated from a control center; automatic valves close on their own when abnormal pressure is detected and react fastest. Construction and maintenance costs rise accordingly.
❓ Q: Is shorter valve spacing always safer?
A: Not by itself — valve reliability and operating speed also matter. In general, however, smaller spacing and faster isolation shorten hazard exposure and improve public safety.
A: Not by itself — valve reliability and operating speed also matter. In general, however, smaller spacing and faster isolation shorten hazard exposure and improve public safety.
❓ Q: What is the direct impact of this report on ASME B31.8?
A: It provides the technical basis for the alternative valve-spacing design rules the B31.8 committee is considering, helping establish equivalent safety between higher design stress levels and different valve spacing allowances.
A: It provides the technical basis for the alternative valve-spacing design rules the B31.8 committee is considering, helping establish equivalent safety between higher design stress levels and different valve spacing allowances.
