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.


Pressure Regulation and Flow Control

Question: Regarding a regulation station, is pressure regulation the same as flow control?

Answer: They’re not the same.

The regulator valve opens and closes as necessary to allow the amount of gas to flow that’s required to restore and maintain downstream pressure. Pressure regulation only controls pressure to maintain a pressure setpoint.
The flow control valve opens and closes as necessary to allow the specified amount of gas to flow, regardless of downstream pressure. Flow control only controls flow to maintain a volume setpoint.


Tip: Control valves normally respond to signals generated by independent devices such as flow meters or temperature gauges.

Ball Valves vs. Lubricating Plug Valves

Question: Which type of valve is more suitable for downstream gas distribution companies–ball valves or lubricating plug valves?

Answer: First, valve selection should be based on technical application (sizing, flow capacity) and company standard requirements.

Ball valves and plug valves are generally quarter-turn valves (90o to open and close). Ball valves are generally less expensive, require no lubrication, can be pigged, and provide tight sealing with low torque.

Although ball valves are sometimes used for throttling (adjusting the restriction to control flow), they are most effective when fully open or closed because the seals are subject to damage due to impingement of high velocity flow. In comparison, lubricated plug valves must be resealed after only a few operations (high maintenance).

Failure to lubricate (grease) plug valves on a regular basis will result in valve operational problems. The cylindrical internal plug results in a reduced port, preventing pigging operations. Plug valves usually perform better with respect to throttling situations.

Tip: In earlier years, many companies installed lubricated plug valves in their systems. It is vital that these valves be maintained in accordance with manufacturer requirements and/or company O&M Manuals. In fact, the regulatory code requires that each valve necessary for distribution system safety must be checked and serviced at least once each calendar year (not to exceed 15 months).