XYLENE POWER LTD.

NATURAL GAS PIPELINE SAFETY SETBACK - CALCULATION OF SAFETY SETBACKS FROM LARGE DIAMETER HIGH PRESSURE NATURAL GAS PIPELINES

By C. Rhodes, P. Eng., Ph.D.

INTRODUCTION:
An essential element of any electric power system is reliable load following generation that can be used to match the total electricity generation to the total electricity load. From a global warming perspective the ideal load following generator is a hydroelectric dam containing a large amount of storage. However, since about 1970 the Ontario government and other governments have failed to face both the political and practical issues involved in construction of major new hydroelectric dams and their associated electricity transmission lines.

Since about 2005 the Ontario Power Authority (OPA) has chosen to use natural gas fueled combustion turbines for supplying load following generation. In order to minimize electricity transmission line right-of-way requirements the OPA chose to locate the natural gas fueled power plants within urban areas. However, the OPA failed to adequately consider the public safety issues related to the large diameter high pressure natural gas pipelines that these combustion turbine power plants require. For public safety these pipelines should be installed in dedicated energy transmission corridors. The minimum width of these corridors is twice the minimum setback distance between the pipeline axis and the public. At present in Ontario, for natural gas pipes up to 20 inches in diameter, this minimum setback distance is municipally regulated. This web page focuses on determination of reasonable minimum setback distances, which distances are functions of both the pipeline diameter and the pipeline operating pressure.

DEDICATED ENERGY TRANSMISSION CORRIDORS:
Natural gas transmission pipelines in Canada have a relatively good safety record. There have been various explosive rupture failures with accompanying major fires, but the incidence of these failures and the related loss of life has been relatively small because most natural gas transmission lines are buried in dedicated energy transmission corridors in rural areas. The use of dedicated energy transmission corridors reduces the incidence of both accidental impact damage and long term corrosion damage and provides distance separation between the pipeline and the public.

A major error in the OPA's planning has been allowing routing of large diameter high pressure natural gas pipelines under public roads where these pipelines are subject to a high ongoing risk of damage by third parties engaged in drainage maintenance, installation and maintenance of other buried services and road construction. Furthermore, burial of large diameter high pressure natural gas pipelines beneath public roads eliminates much of the distance separation that is normally achieved by pipeline burial within dedicated energy transmission corridors in rural areas.

COST CONSTRAINTS:
To minimize capital cost natural gas is transported in steel pipes.

Major high pressure natural gas pipelines are generally designed for a maximum working pressure that causes an operating pipe hoop stress of about 30% of the pipes Specified Minimum Yield Stress (SMYS). A further margin of safety can be introduced by reducing the working pressure. However, practical material cost considerations usually prevent a major reduction in working pressure.

There are pipeline sections that operate at 50% of SMYS. However, such pipeline sections provide no safety margin against local earth movement (earth quakes) or local weld or corrosion problems. Pipelines routed through urban areas should be restricted to a maximum allowable operating pressure that causes a hoop stress of 33% of SMYS.

CORROSION PROTECTION:
1. To prevent external corrosion steel pipes conveying natural gas pipe are coated with a layer of electrically insulating plastic and/or are coated with electrically insulating material. The pipe steel is galvanically biased slightly negative with respect to the surrounding ground water. This bias is usually maintained by use of sacrificial magnesium electrodes that are electrically bonded to the steel pipe.
2.The galvanic bias attracts positive hydrogen ions in ground water toward any pipe steel that is exposed by imperfections in the pipe's external coating. The corresponding negative hydroxyl ions flow toward the positive sacrificial magnesium electrode.
3.The hydroxyl ions cause corrosion of the sacrificial magnesium electrode.
4.As long as corrosion is confined to the sacrificial magnesium electrode, corrosion of the pipe steel is prevented.
5.Eventually the sacrificial magnesium electrodes will corrode away or worse, they may be accidentally disconnected or stolen for their scrap metal value. Under these circumstances the galvanic corrosion protection mechanism is defeated and corrosion will occur anywhere that pipe steel is exposed to ground water, such as at a coating scratch that might have been inadvertently caused by a backhoe, trenching machine or utility pole auger used for work on an unrelated service.
6. A relatively new threat to buried steel pipelines is DC ground currents that can result from unbalanced power inverter equipment in neighboring buldings. Such DC ground currents can aggravate otherwise minor corrosion problems. This issue must be considered when a wind farm and a buried pipeline are in close proximity.

SAFETY:
Usually large diameter high pressure natural gas pipes are buried. The functions of the soil cover are to protect the pipe and its coating from damage due to UV radiation, external impact, thermal stress and frost heaving.

There are still real risks related to long term corrosion and to damage from mechanical equipment such as trenching machines, back hoes, utility pole augers and boom trucks. In the winter, when snow is piled high or during flood conditions the operators of back hoes, utility pole augers and boom trucks frequently scratch or damage other buried services, in spite of their best efforts to avoid such damage.

The risks of scratching or damage from mechanical equipment are greatly reduced if the large diameter high pressure natural gas pipeline is buried in a dedicated energy transmission corridor. Then almost all the risks due to installation and maintenance of electrical services, drainage culverts, fresh water pipes, storm sewer pipes, sanitary sewer pipes, low pressure natural gas pipes, telephone multipair cables, TV coaxial cables and fiber optic cables are eliminated.

Another means of improving safety is to ensure that buildings that routinely contain large numbers of people are not constructed within a specified setback distance from the axis of a large diameter high pressure natural gas pipeline. Similarly a setback distance should be maintained between the pipeline axis and outdoor locations where large groups of people routinely assemble.

RISKS:
The main risks to a large diameter high pressure natural gas pipeline are:
 
1.Improper engineering, fabrication or commissioning, including but not limited to inadequate:
a) Provision for hoop stress
b) Provision for thermal stress
c) Provision for sheer stress related to ground movement
d) Provision for pipe buoyancy
e) Mill testing of pipe steel
f) Weld inspection
g) Route choice (high and dry preferred to low and wet)
h) Burial depth
i) Pipe bedding and support
j) Corrosion protection
k) Hydraulic pressure testing
l) Drainage after hydraulic pressure test
m) Nitrogen pressure test
n) Documentation of magnesium electrode locations
o) Provision for insertion of pigs for automatic scanning of pipe wall thickness

2.Physical damage from external human activity. eg. The gas line is directly damaged by a trenching machine, backhoe, utility pole auger or boom truck leg.

3.Physical damage due to non-human activity. eg Earthquake, sinkhole, landslide or flood.

4.Minor outside surface damage in combination with loss of galvanic corrosion protection. eg Plastic coating is scratched by a trenching machine, backhoe or utility pole auger and the scratch is not promptly repaired. The magnesium electrode then rapidly corrodes away. Alternatively a magnesium electrode may be accidentally disconnected by a backhoe or utility pole auger or may be stolen for its scrap metal value.

5.Failure of the pipeline owner to periodically check that all the magnesium electrodes are still present and connected.

6.Failure of the pipeline owner to periodically fully check the actual pipe wall thickness using a pig type electronic inspection apparatus that scans the pipe wall from the inside and measures and records the pipe wall thickness as a function of linear and angular position.

Risks #2, #4 and #5 above are greatly magnified if the pipeline is installed in a road allowance instead of in a dedicated energy transmission corridor.

Risk #6 occurs if there are pipe joints, pipe elbows, pipe fittings, valves or compressor stations that are not designed to allow insertion and axial travel of the pig type electronic equipment for measuring the pipe wall thickness as a function of linear and angular position.

Risk #6 is greatly magnified if the pipeline maintenance personnel do not have adequate time to examine the pig data and to follow up risks identifyable via the pig data. It is essential that the pipeline owner employ staff whose first priority is pig data acquisiton, analysis and followup.

RUPTURE FAILURE MECHANISM:
If one makes a small hole with a diameter less than twice the pipe wall thickness in a high pressure natural gas pipeline the immediate result is a loud hissing noise as natural gas leaks out. The leaking high pressure natural gas will blow away soil in its path. The natural gas will mix with surrounding air and form a cloud with concentrated natural gas at its center and dilute natural gas at its edges. If the edge of this cloud with a natural gas concentration in the range 5% to 15% encounters a source of ignition such as a spark made by an electrical switch, there will be a delayed ignition explosion followed by a localized ongoing fire. However, the size of this fire will be limited by the size of the original small hole in the natural gas pipe.

However, if a hole in a high pressure natural gas pipe grows to an axial length that exceeds about four times the pipe wall thickness, a very different sequence of events takes place. At the axial ends of the hole the local hoop stress will exceed the material yield stress. The pipe will then immediately rip down its axis to form a rupture that has an open area several times the cross sectional open area of the pipe. This rupture discharges natural gas at the maximum possible flow rate from both open ends of the ruptured pipe.

PIPE RUPTURE SEQUENCE:
1.The pipe wall is thinned by corrosion, by cutting, by defective welding or by impact;
2.At the thin spot a hoop stress concentration develops that exceeds the yield stress of the pipe material;
3.The pipe wall deforms in a manner that magnifies the hoop stress concentration. This process can be observed in a stretched elastic band with a nick;
4.The pipe suddenly rips down its length causing a complete rupture. This process is similar to the sudden rupture of a fully inflated child's balloon that is hit by a dart.

DAMAGE SEQUENCE:
1.The escaping high pressure natural gas explosively blows away the soil over burden, forming a large crater in the ground;
2.The pipe rupture is fed with high pressure natural gas from both the upstream and downstream pipes.
3.The escaping gas makes a noise comparable to a large jet aircraft at takeoff;
4.The escaping gas mixes with the surrounding air. In regions where the volumetric natural gas concentration is in the range 5% to 15 % the mixture is highly flammable;
5.When the flammable gas mixture finds a source of ignition such as a flame, hot surface or electric spark there is an explosive delayed ignition pressure pulse. This pressure pulse is deafeningly loud and can break windows in buildings over a kilometre from the rupture location;
6.Then there is a steady state flame that is fed by high pressure gas flowing out of both open ends of the ruptured pipeline. This flame is impossible to extinguish and continues burning until it runs out of fuel. It typically takes the gas company one to two hours to close valves to isolate the ruptured section of gas pipe. The natural gas flame typically burns for several more hours.

SETBACK UNCERTAINTY:
Due to uncertainty regarding wind conditions and the position of the nearest point of ignition it is impossible to specify a practical safety setback distance that will ensure no damage or personnel injury from shrapnel related to the delayed ignition explosion. However, the subsequent fire emits a quantifiable amount of thermal radiation for which a reasonable safety setback distance can be calculated.

THERMAL RADIATION:
1.The thermal radiation intensity from the steady state natural gas flame is easy to calculate and is the basis of minimum setback calculations;
2.The radiation level may be substantially larger than calculated if black smoke from burning oil, wood or asphalt is conveyed by natural convection into the natural gas flame;
3.For a clean natural gas flame I have derived a formula for recommended safe setback distance as a function of pipe diameter and maximum operating pressure;
4.The distance Rs corresponds to a thermal radiation intensity from the natural gas flame equal to the solar irradiance (the maximum solar energy intensity incident on the Earth).
5. At distance Rs / 2 the thermal radiation intensity from the natural gas flame is four times as large as at distance Rs.
6. Natural gas pipeline rupture accident site photographs show that everything inside radius Rs / 2 burns to a crisp. Fire fighters cannot get closer than radius Rs / 2. At Rs / 2 the exposed surface temperature is about 200 °C.

PRESSURE PULSE:
The magnitude of the initial delayed ignition pressure pulse is unpredictable. The size of the delayed ignition explosion depends on the distance between the pipe rupture and the point of ignition. The larger this distance the larger the delayed ignition explosion. Depending on the location of the source of delayed ignition the pressure wave damage radius can exceed the radius of the thermal radiation damage by several fold. In extreme cases the delayed ignition explosion can be comparable to the blast wave from a small tactical nuclear weapon. For this reason it is important to limit the sizes of high pressure natural gas lines in urban areas. In the Middletown, Connecticut accident the delayed ignition blast wave shattered windows over 1.6 km away from the location of the natural gas release.

FORMULA FOR SAFE DISTANCE Rs:
In this document a formula is developed for the safe setback distance Rs from a natural gas pipe line required for personnel to avoid radiation skin damage from the steady state fire that follows a high pressure natural gas pipeline rupture.

It must be emphasized that the calculated safety setback applies to thermal radiation from combustion of clean natural gas. A delayed ignition explosion can cause blast damage beyond the calculated radiation safety radius. Toxic gases such as H2S can cause loss of life beyond the calculated radiation safety radius. If the natural gas burns in combination with other substances such as oil, coal, asphalt, wood, plastic resins, etc. soot forms. That soot increases thermal radiation and hence increases the required radiation safety radius.

The results of the formula developed herein are compared to the actual fire damage radius that occurred at Appomattox, Virginia where a 30 inch diameter buried high pressure natural gas pipeline ruptured and burned on September 14, 2008. Since then there have been major natural gas delayed ignition explosions and fires in urban areas such as at Middletown (suburb of Hartford), Connecticut on February 7, 2010 and at San Bruno (a suburb of San Francisco), California on September 9, 2010.

FORMULA DEVELOPMENT:
Consider a long straight natural gas pipeline that is subject to a sudden rupture that opens the full cross section of the pipe. To calculate the radiant heating consequences if there is a fire it is necessary to first find the natural gas mass flow rate out of the rupture. In reality there are two flows, because the pipes on both sides of the rupture discharge natural gas into the rupture. We will calculate one of these gas flows and then double the result to obtain the total mass flow rate out of the rupture.

Pa = the pressure in the pipeline distant from the rupture

Pb = the pressure at the point of rupture after the rupture. Normally Pb is atmospheric pressure.

Let Dp = pipeline inside diameter

Let Pi = 3.14159

Let Rma = gas density at pressure Pa

Let Rmb = gas density at pressure Pb

Let Rm(X) = gas density at linear position X.

The pipe cross-sectional area Ac is:
Ac = Pi (Dp / 2)^2

The mass of gas contained between X and X + dX is:
dM = Rm(X) Ac dX

Within the pipe but near the point of rupture the gas pressure drops and the gas expands. The linear velocity along the pipe increases. The energy contained in the compressed gas becomes linear kinetic energy. The linear velocity at the point of rupture is Vb. The mass flow rate from one pipe at the point of rupture is:
Rmb Ac Vb

Let X indicate linear position along the pipe.

Let T = time

Then:
V(X) = (dX / dT) = gas linear velocity

The gas kinetic energy between X and X + dX is:
(dM / 2) (dX / dT)^2

Let P = pressure at X

Let P + dP = pressure at X + dX

Conversion of pressure energy into kinetic energy as gas flows along the pipe gives:
-dP Ac dX = d[(Rm(X) Ac dX) (dX / dT)^ 2 / 2]
or
-dP = d[(Rm(X) (dX / dT)^ 2 / 2]

Integrating from Pa to Pb gives:
-(Pb - Pa) = [Rm (dX / dT)^ 2 / 2]b - [Rm (dX / dT)^ 2 / 2]a
or
(Pa - Pb) = [Rm (dX / dT)^ 2 / 2]b - [Rm (dX / dT)^ 2 / 2]a

Assume that as a result of the pipe rupture the natural gas supervisory control system closes valves distantly upstream and downstream from the pipe rupture. Then the initial condition at the location of each of these valves is no flow, or expressed mathematically in terms of the gas stream:
[dX / dT]a = 0

Hence:
(Pa - Pb) = [Rm (dX / dT)^ 2 / 2]b
or
[dX / dT ]b = [2 (Pa - Pb) / Rmb]^0.5

Fm = exiting gas mass flow rate from one pipe
= Rmb Ac [dX / dT]b
= Rmb Ac [2 (Pa - Pb) / Rmb]^0.5
= Ac [2 (Pa - Pb) Rmb]^0.5

Let Ec be the combustion heat release per unit mass of natural gas. Then the total combustion heat release per unit time H is given by:
H = 2 Fm Ec
where the 2 reflects the fact that the rupture is fed by two pipes.

Let Fr be the fraction of this heat that is emitted via radiation.

Let Rz = radius from the center of the flame to a surface subject to radiation damage.

Assume that the radiation is evenly distributed over a sphere with radius Rz and surface area 4 Pi Rz^2. Then at radius Rz the radiation intensity / unit area is:
Rz = (H Fr) / (4 Pi Rz^2)

Assume that to avoid skin damage the radiation intensity should be less than the most intense possible solar radiation incident on the Earth's surface ( 1365 W / m^2). This parameter is known as the Solar Irradiance. Hence, in terms of radiant energy, the safe distance Rs from the center of the flame is defined by:
(H Fr) / (4 Pi Rs^2) = 1365 watts / m^2
or
Rs = [(H Fr) / (4 Pi X 1365 watts / m^2)]^0.5
= [(2 Fm Ec Fr) / (4 Pi X 1365 watts / m^2)]^0.5
where Fm is given by:
Fm = Ac [2 (Pa - Pb) Rmb]^0.5
= Pi (Dp / 2)^2 [2 (Pa - Pb) Rmb]^0.5

Combining these two formulas gives:
Rs = [(2 Fm Ec Fr) / (4 Pi X 1365 watts / m^2)]^0.5
= [(2 Pi (Dp / 2)^2 [2 (Pa - Pb) Rmb]^0.5 Ec Fr) / (4 Pi X 1365 watts / m^2)]^0.5
= Dp (Pa - Pb)^0.25 [( [2 Rmb]^0.5 Ec Fr) / (8 X 1365 watts / m^2)]^0.5

The value of Fr can be found from a paper by J. P. Gore et al titled Structure and Radiation Properties of Large-scale Natural Gas/Air Diffusion Flames, published in Fire and Materials, Vol. 10, 161-169 (1986). These authors found that the radiation emission from a 207 MW natural gas flame measured at ground level about 11.9 m from the flame center was 6.37 kW / m^2.

The surface area of that sphere was:
4 Pi (11.9 m)^2 = 1778.62 m^2
Hence the emitted radiation was:
6.37 kW / m^2 X 1778.62 m^2 = 11330 kW = 11.330 MW
Hence:
Fr = 11.330 MW / 207 MW = .0547

This Fr value is in good agreement with other Fr data provided to this author by the Canadian Gas Research Institute.

NUMERICAL SIMPLIFICATION:
Pi = 3.1415928

Rmb = 16 gm / 22.4 lit
= 16 X 10^-3 kg / 22.4 X 10^-3 m^3
= .714 kg / m^3 = density of natural gas at standard temperature-pressure

Ec = (10.4 kWh / m^3) X (1 m^3 / .714 kg) X 3600 s / h = 52437 kJ / kg

Hence:
Rs = Dp (Pa - Pb)^0.25 [( [2 Rmb]^0.5 Ec Fr) / (8 X 1365 watts / m^2)]^0.5
= Dp (Pa - Pb)^0.25 [( [2 X .714 kg / m^3]^0.5 X 52437 kJ / kg X .0547) / (8 X 1365 watts / m^2)]^0.5
= Dp (Pa - Pb)^0.25 [[1.428 kg / m^3]^0.5 X .26266 kJ m^2 / kg-watts X 1000 J / kJ]^0.5
= Dp (Pa - Pb)^0.25 [1.195 kg^0.5 m^-1.5 X 262.66 J m^2 / kg-watts]^0.5
= Dp (Pa - Pb)^0.25 X 17.71 kg^0.25 m^-.75 m (J / kg-watts)^0.5
= 17.71 Dp (Pa - Pb)^0.25 kg^0.25 m^-.75 m (watt s / kg-watts)^0.5
= 17.71 Dp (Pa - Pb)^0.25 kg^-0.25 m^.25 s ^0.5

Units Check:
(newtons/ m^2)^0.25 = (kg m s^-2 m^-2)^0.25 = kg^0.25 m^-.25 s^-0.5

EXAMPLES:
YORK ENERGY CENTRE PIPELINE, KING TOWNSHIP, ONTARIO:
The York Energy Centre is a natural gas fueled air cooled combustion turbine based 400 MW electricity generation station located in YOrk Region, north of Toronto. This facility is served by a dedicated 16 inch diameter 600 psi natural gas pipeline running through a mostly rural area.

Dp = 16 inches = .406 m

Pb = 14.7 psia = 1 bar = 101 kPa = 1.01 X 10^5 newtons / m^2

Pa = 600 psia = 40.81 bar = 4122.4 kPa = 41.22 X 10^5 newtons / m^2

Hence:
Rs = 17.71 X .406 m X (40.21 X 10^5 newtons / m^2)^0.25 kg^-0.25 m^.25 s ^0.5
= 17.71 X .406 m X 402.1^0.25 X 10 (kg m s^-2 m^-2)^0.25 kg^-0.25 m^.25 s ^0.5
= 321.97 m

Application of this formula to the York Energy Centre pipeline gives a radiation safety distance of about:
Rs = 322 metres.
At Rz = 161 metres the radiation level will be four times as high as at 322 m and will cause spontaneous combustion of farm crops.

I strongly urge that in the case of the York Energy Centre pipeline a minimum 160 metre setback should be maintained from the pipeline center line to all human occupied structures and to all places of routine outdoor human assembly. This is an ongoing setback requirement that should be actively enforced by municipal authorities for the life of the pipeline. All parties should clearly understand that the radiation emitted by a pipeline rupture/fire is so intense that the only practical strategy for a rural fire department is to let the fire burn itself out. It is also unrealistic to expect persons within radius (Rs / 2) of a pipeline rupture/fire to be rescued by volunteer fire department personnel. If possible the municipality should attempt to enforce a 320 m setback instead of a 160 m setback. There could easily be litigation related to injury and property damage in the 160 m to 320 m setback zone.

APPOMATTOX, VIRGINIA:
On September 14, 2008 a 30 inch diameter buried natural gas pipeline that normally operates at a pressure of 800 psi ruptured and burned in a farmer's field near the intersection of Highway 26 and State Route 677 just north of Appomattox, Virginia. There was a modest delayed ignition explosion. Overhead news photographs showing the area where the crop burned were compared to distance calibrated overhead photographs from Google maps. It was found that with reference to the pipe rupture crater the area burned extended 311 m to the south-west and 275 m to the north-east.

Application of the formula for the radiation safety distance Rs gives:
Dp = 30 inch X .0254 m / inch = 0.762 m
Pa = 800 psi X 101 X 10^3 Pa / 14.7 psi = 549.66 X 10^4 Pa
Pb = 101 X 10^3 Pa = 10.1 X 10^4 Pa
Rs = 17.71 Dp (Pa - Pb)^0.25 kg^-0.25 m^.25 s ^0.5
= 17.71 X 0.763 m X (539.56 X 10^4 Pa)^0.25 kg^-0.25 m^.25 s ^0.5
= 651.25 m
Thus the area in which the crop spontaneously burned was the area where Rz < (Rs / 2).

SAN BRUNO, CALIFORNIA:
On September 9, 2010 at 6:11 PM a 30 inch diameter buried natural gas pipeline operating at a pressure of 400 psia ruptured and burned in a single family home residential area in San Bruno, California. San Bruno is a southern suburb of San Francisco, close to the San Francisco airport. The homes near the rupture location each had lot sizes in excess of one acre.

There was a modest delayed ignition explosion followed by a large natural gas fire that persisted for more than two hours. Uncontrolled secondary fires continued for more than eight further hours. The fire scene was attended by 67 fire trucks, 4 fixed wing aerial water bombers and 1 fire fighting helicopter.

Aerial photographs showing the area that burned were compared to distance calibrated Google maps. In spite of the large amount of immediately available fire fighting equipment almost all the homes (38) within a 150 m radius damage circle were completely destroyed. A further 17 homes were severely damaged and a further 53 homes sustained lesser damage. The center of the damage circle is displaced from the pipe rupture location by about 100 m. The cause of this displacement was a combination of local factors including natural gas exit velocity, wind, steep local terrain, local tree concentrations and asymetrical application of fire fighting resources.

Application of the formula for the radiation safety distance Rs gives:
Dp = 30 inch X .0254 m / inch = 0.762 m
Pa = 400 psia X 101 X 10^3 Pa / 14.7 psia = 274.8 X 10^4 Pa
Pb = 101 X 10^3 Pa = 10.1 X 10^4 Pa
Rs = 17.71 Dp (Pa - Pb)^0.25 kg^-0.25 m^.25 s ^0.5
= 17.71 X 0.762 m X (264.7 X 10^4 Pa)^0.25 kg^-0.25 m^.25 s ^0.5
= 544.3 m
Thus the calculated area of spontaneous combustion of homes was the area where Rz < (Rs / 2), which is a circle of radius 272 m.

It is clear from subsequent incident reports that absent the massive fire fighting resources that were immediately available close to the San Francisco Airport, the actual area of total destruction would have closely conformed to the calculated destruction radius Rz = 272 m.

The practical experience at San Bruno was that there is a limit to the capabilites of Urban Fire Departments. Even when there is an army of immediately available emergency personnel and almost unlimited fire fighting equipment, the municipal water mains and their pumping systems limit the fire fighting capacity. Water bombers designed for fighting large forest fires are of considerable help. However, the standby costs of maintaining a dedicated fleet of large water bombers that are available and ready to fly at a moments notice are prohibitive in most jurisdictions. In this respect the residents of San Bruno were particularly fortunate that there were four suitable water bombers immediately available.

An important conclusion from the San Bruno NTSB accident investigation report was that the pipe section that ruptured was defective at the time of original installation and had never been subject to an as-built hydraulic pressure test.

JERSEY CITY, NEW JERSEY:
In March 2012 this author became aware of a plan to build a 42 inch diameter 1200 psi natural gas pipeline through a densely populated area of Jersey City, New Jersey. This author's immediate response was that this plan is stupid because that pipeline would be a long term magnet for every anti-USA terrorist in the world. This author strongly recommends that this pipeline be rerouted outside the urban area, regardless of the extra cost.

If construction of this pipeline proceeds as planned, the consequences of a rupture failure, perhaps intentionally caused, would be comparable to the air burst of a tactical nuclear warhead. The safety radius Rs and the radius of probable total destruction Rz can be calculated as follows:
Dp = 42 inch X .0254 m / inch = 1.0668 m
Pa = 1200 psia X 101 X 10^3 Pa / 14.7 psia = 824.5 X 10^4 Pa
Pb = 101 X 10^3 Pa = 10.1 X 10^4 Pa
Rs = 17.71 Dp (Pa - Pb)^0.25 kg^-0.25 m^.25 s ^0.5
= 17.71 X 1.0668 m X (814.4 X 10^4 Pa)^0.25 kg^-0.25 m^.25 s ^0.5
= 1009.2 m
Thus the calculated area of spontaneous combustion is an area where Rz < (Rs / 2), which is a circle of radius 504.6 m. The perimeter length of that circle is 2 Pi (504.6 m) = 3170 m.

The only way to stop a fire of that size is to make back fires to create a fire break 3 km long and a block wide through the center of the city. The direct and consequential damages from the natural gas fire and the back fire would be unprecedented in United States history. The fire storm and consequent loss of life and property would be comparable to the WWII fire storms in Dresden, Hamburg, Tokyo and Hiroshima.

It is the hope of this author that common sense will prevail and that senior members of the United States government will do all necessary to force rerouting of this large diameter high pressure natural gas pipeline to a longer but much safer rural route.

One practical way to force rerouting of this pipeline is to immediately enact strong legislation to require the pipeline owners to continuously carry credible third party liability insurance and reinsurance sufficient to replace everything and everyone within 500 m of any potential pipe rupture location.

It must be emphasized that no amount of hydraulic pressure testing or pig testing will protect the public from intentional sabotage of such a pipeline passing through an urban area. The stress in the pipe walls is sufficient that even a relatively small suitably shaped explosive charge will cause a rupture failure.

CONCLUSIONS:
When a large diameter high pressure natural gas pipeline operating at its rated working pressure develops a crack or hole more than four pipe wall thicknesses in axial length the result is a sudden full cross section pipe rupture. The escaping high pressure gas blows away the soil overburden, forming a crater. Some time after the pipe rupture there is a large delayed ignition explosion followed by a steady state fire. This fire emits so much thermal radiation that it it is impossible to approach or extinguish the fire with conventional fire fighting equipment.

One can define a radiation safety distance Rs from the fire at which distance the radiation level is similar to the radiation level in the middle of the Sahara desert at noon on a very hot cloudless day. The formula is:
Rs = 17.71 Dp (Pa - Pb)^0.25 kg^-0.25 m^.25 s ^0.5
where:
Rs = radiation safety distance in metres
Dp = pipeline diameter in metres
Pa = pipeline working pressure in Pascals
Pb = atmospheric pressure in Pascals

At radii from the rupture less than (Rs / 2) combustable materials spontaneously burn.

There is additional danger if the natural gas fire triggers combustion of materials that form soot. If large amounts of soot mix with the natural gas combustion air the soot could increase the radiant heat output four fold which would double the safety radius Rs.

It should be emphasized that the above calculation applies to thermal radiation from steady state combustion of clean natural gas. The damage radius from the initial delayed ignition explosion could easily be larger. Based on eyewitness reports from Appomattox the sequence of events at that pipeline rupture/fire was a delayed ignition explosion followed by steady state combustion. The same sequence of events has occurred elsewhere.

The above calculation shows that even if someone is fortunate enough to survive the initial delayed ignition explosion, the temperature within the radiation safety radius Rs of the flame will rapidly rise past the point of human tolerance.

For large diameter high pressure natural gas pipelines passing through urban areas this author strongly recommends an initial as-built hydraulic pressure test to 100% of pipe SMYS (Specified Minimum Yield Strength) and a maximum operating pressure producing no more than 33% of SMYS. Furthermore, as long as the pipe remains in service in an urban area the pipe should be retested at least every five years with a non-combustable fluid to the larger of 50% of pipe SMYS or 150% of the maximum allowable operating pressure. These safety margins have been proven through many years of pressure vessel design, construction and use and are the basis of almost all modern pressure vessel safety codes.

In theory if the pipe could be assembled in the rigerously controlled conditions of a certified pressure vessel fabrication facility with complete material control and ideal welding, initial as-built hydraulic pressure testing to 50% of SMYS might be adequate. However, under the practical conditions that natural gas pipelines are assembled and welded in the field that degree of material and fabrication control is impossible. Hence the only solution is an initial as-built hydraulic pressure test to 100% of pipe SMYS. There is no x-ray, pig test, spectrograph, sampling test or inspection procedure that can replace a hydraulic pressure test to 100% of SMYS.

Given the limited resources of rural fire departments it is reasonable to assume that in the event of a high pressure natural gas pipeline rupture/fire they will simply ensure that the pipe is valved off on both sides of the rupture and then let the fire burn itself out. It is also reasonable to conclude that crops, buildings and other combustibles within a distance (Rs / 2) of the pipeline rupture/fire will spontaneously ignite and will be totally destroyed.

The principal objective of emergency services must be to immediately evacuate humans from inside the radiation safety radius Rs. It can safely be assumed that inside the radiation safety radius Rs damage to property will be extensive and inside Rz = (Rs / 2) almost everything will be destroyed. Most municipal fire departments are not equipped to function within the high thermal radiation levels that will occur inside Rz = (Rs / 2). Life and property insurance coverages should reflect this reality.

With reference to Ontario Fuels Safety Division publication PI-98/01 “Guidelines For Locating New Oil and Gas Pipeline Facilities” all the specified setbacks except the 200 m setback are totally inadequate for major high pressure natural gas pipelines. With respect to large diameter pipelines even the specified 200 metre setback is inadequate.

The Fuels Safety Division of TSSA should review document PI-98/01 with a view to making minimum setbacks from natural gas pipelines a function of pipeline diameter and operating pressure as set out herein. Code enforcement authorities should be realistic with respect to their expectation of emergency personnel working within high thermal radiation zones. In the event of a major natural gas pipeline rupture/fire the available emergency personnel will likely attempt to save human lives but in so doing they will likely sustain extensive personal skin damage. They will then be unable to fight or extinguish fires.

This web page last updated March 22, 2012.