Span of control

Question: Why do some covered tasks in the ASME B31Q standard list a span of control of 1:0? What does that mean?

Answer: Let me first define span of control. It is the maximum number of nonqualified individuals that a qualified individual can direct and observe performing a covered task. For example, locating underground pipelines is a covered task with a 1:1 span of control. This indicates that a qualified individual can direct, monitor, and observe one unqualified individual in performing all aspects of the pipe locating task.

A span of control 1:0 ratio indicates that the task may not be performed by any unqualified individual (even under the direct observation of a qualified individual). These are a few of the covered tasks: joining pipe by heat fusion, electrofusion, compression fittings, and welding. In other words, the individual performing any of these tasks must be operator qualified.

Tip: Operator qualification (OQ) covered task designations and related issues must be written into the company’s OQ program manual. Span of control ratio requirements may be established in accordance with approved methods described in the ASME B31Q standard (e.g., subject matter expert consensus, rational process to determine task importance).


Surface temperature checks for heater plates

Question: Why do heater plates for fusion require surface temperature checks with a probe when they already have a temperature gauge?

Answer: The dial thermometer on the heater indicates internal temperature which can be different from the actual surface temperature. The dial thermometer can be used as a reference once the surface temperature has been verified with a surface probe (pyrometer).

If the surface temperature is within the proper range, you can then utilize the temperature thermometer on the heater plate, as a reference check prior to inserting the heater plate for each fusion joint. If the reference thermometer temperature drops, the surface temperature should be re-checked. The generator may have run out of gas or a heating element/thermoswitch inside the heater plate may have been damaged (this requires heater repair).

In addition, remember to shield the heater from cold weather and wind by keeping it stored in the insulated sling or stand; when not being used to make a fusion joint.

Tip: Always follow company procedures and/or PE pipe manufacturer’s heating temperature requirements. It is very important that before each fusion joint is made, that the heater surfaces be wiped with a clean, non-synthetic cloth.

Altitude and psig

Follow-up Question on Pressure (psig)

Question: Does altitude make a difference with psig?

Answer: Yes it does. Let me use my tire pressure example (32 psig) again to answer your question. Pound per square inch gauge (psig) is a unit of pressure relative to atmospheric pressure at sea level, which is 14.7. Atmospheric pressure pushes against the earth at 14.7 pounds per square inch at sea level, yet drops to 10.1 pounds per square inch at 10,000 feet. The bottom line is as altitude increases, atmospheric pressure decreases (see table below).
At sea level (14.7), 32 psig in tire is 46.7 psi
At 10,000 feet up in the mountains (10.1), 32 psig in tire is 42.1 psi

Altitude (ft.): Air Pressure (psi)
Sea Level: 14.7
1,000: 14.2
5,000: 12.2
10,000: 10.1


psi vs. psig in Pipeline Pressure Testing

Question: Regarding pipeline pressure testing, what’s the difference between psi and psig?

Answer: Here’s my (relatively) simple answer: Although psi and psig are both units of measurement for describing the amount of pressure a gas or fluid is exerting, the difference is that psig specifies what the measurement is relative to, whereas psi does not. In both, the letters “psi” represent pounds per square inch (the pressure resulting from a one pound-force applied to an area of one square inch).

Pounds per square inch gauge (psig) is a unit of pressure relative to atmospheric pressure at sea level, which is 14.7 (the “gauge” designation indicates that the readings are already adjusted to ignore surrounding atmospheric pressure). When a pressure gauge gives a reading called psig, it’s the difference between absolute pressure of the connected pressure source and the local atmospheric pressure.

Example: When you check a car tire (valve stem) with a tire pressure gauge, the instrument is reading the pressure difference between the air inside the tire and the air outside the tire. That’s called gauge pressure. Suppose the air pressure in your car tire is 32 psig. Then the absolute pressure inside is 32 + 14.7 = 46.7 psi. Sometimes this is written as 46.7 psia (pounds per square inch, absolute) to remind us we’re talking about an absolute pressure. In either case, we could just use psi.

Tip: Temperature changes will affect pressure during a test. A temperature increase and the resulting increase in pressure can mask a small leak. A temperature decrease can indicate a leak when there isn’t one.


Earthquake Protection for Pipelines

Dear Joe:
Is any extra support or safety equipment used to protect the gas pipeline in places like California (where there are lots of earthquakes)?

Angelo E. Polanco,
Connecticut Natural Gas

Answer: Earthquakes don’t occur only in the West; they can strike any location (e.g., Virginia and Alabama). California has a state law that requires natural gas operators to prepare for – and minimize damage to -pipelines from earthquakes.

For example, Pacific Gas and Electric (PG&E) has an earthquake fault crossings program that compiles geological survey data and analyzes/identifies areas susceptible to seismic activity. These studies provide critical information on how best to implement mitigation measures to improve safety at each fault crossing.

Mitigation measures may include modifying trench designs and/or adjustments, stabilizing unstable ground, making pipes more resilient, using larger pipe supports, replacing cast iron and steel pipe with polyethylene pipe, using rupture control valves, installing earthquake (seismic) actuated gas shut-off valves, and using remote controlled valves. Also, seismic vulnerability may be offset with flexible couplings or expansion joints and above ground fault crossings.

Tip: Part 192 represents the Minimum Federal Safety Standards, but each U.S. state may have additional regulations above those of the Federal requirements.


Purging and Nitrogen Slugs

Question: How and when is a nitrogen slug used in purging?

Answer: A nitrogen slug is a quantity of inert gas that can be initially injected when purging the air or gas from a section of piping. The procedures work like this:

  • To purge air from piping, natural gas is used immediately following the nitrogen slug injection to push out the slug and air ahead of it.
  • To purge natural gas from piping, air is used immediately following the nitrogen slug injection to push out the slug and gas ahead of it.

Tip: For safety, the purpose of purging with an inert gas (nitrogen) is to eliminate the formation of a combustible gas-air mixture within the pipe. Ensure that all purging procedures are followed in accordance with the company O&M Manual.


Plastic, Polyethylene, or PE?

Question: Why do your answers to questions about plastic pipe always identify the pipe as “polyethylene pipe (PE)”?

Answer: Most people in the industry refer to polyethylene gas pipe as “plastic” pipe. This is fine, but I’ve always preferred to make specific reference to it as polyethylene (PE) pipe.

There are different types of plastic pipe. Polyvinyl Chloride (PVC) piping is usually used for sewers and water main/service lines. Cellulose Acetate Butyrate (CAB) piping was installed for gas distribution services from the 1950s through the early 1970s. This material was susceptible to brittle cracking, resulting in gas leakage. Most of it has been replaced with PE pipe.

PE piping is the most widely used plastic piping material for the distribution of natural gas (over 90%) because of its unique thermoplastic properties: high impact strength, heat fusibility, crack propagation resistance, and the ability to be squeezed safely.

Tip: PE pipe accepted for fuel gas piping is covered by ASTM D 2513 requirements.