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Interpretation Response #PI-14-0016

Below is the interpretation response detail and a list of regulations sections applicable to this response.

Interpretation Response Details

Response Publish Date: 04-21-2015
Company Name: Northern Utilities / Unitil Corporation    Individual Name: Mr. Christopher J. LeBlanc
Location state: NH    Country: US

View the Interpretation Document

Response text:

Mr. Christopher J. LeBlanc
Director, Gas Operations
Unitil Corporation
325 West Road
Portsmouth, NH 03801

Dear Mr. LeBlanc:

In a letter to the Pipeline and Hazardous Materials Safety Administration (PHMSA) dated September 5, 2014, on behalf of Northern Utilities, Inc. (Northern), Unitil requested an interpretation on the applicability of the Federal gas pipeline safety regulations at 49 CFR Part 192. Specifically, you asked about the requirements for maximum allowable operating pressure (MAOP) and system pressures during normal operation of a gas distribution system and during a system emergency caused by the failure of pressure regulating equipment.

You explained your system as follows: Northern operates a pressure regulating station at the point on its distribution system where it takes gas from Granite State Gas Interstate transmission pipeline.1 At the point of interconnection, the Granite State pipeline is operated at a MAOP of 492 psig. The MAOP of Northern's downstream distribution system is 56 psig. Northern's facilities at the station are configured as a dual-run. Run 1 is the primary run. Run 2 serves as a back-up if Run 1 were ever to fail in the closed position (diagram of the line was provided). Each Run is equipped with a "worker" regulator and a "monitor" regulator. On Run 1, the set point on the "worker" regulator is 53 psig. The set point on the "monitor" regulator is 55 psig.

A pressure gage is installed approximately six feet downstream of the pressure regulation equipment. The company's supervisory control and data acquisition (SCADA) system monitors a pressure sensor that is at the end of the distribution system (at the furthest point downstream of the regulator station). Recently, a failure of the worker regulator was simulated to assess the operation of Northern's over pressure protection. As the worker regulator failed the adjacent downstream pressure gauge indicated a gradual increase in pressure to 57.2 psig for approximately one minute, after which the pressure returned to and remained at the 55 psig set point of the monitor regulator. At no point during the simulation did the pressure rise above 57.2 psig. Northern believes that the observed pressure increase to 57.2 psig for approximately one minute resulted from the normal build-up pressure due to the mechanical operation of the monitor regulator. This assessment has been confirmed by the manufacturer of the regulator. During this simulated failure, the SCADA pressure sensor at the end of the Northern system did not register a pressure increase to 57.2 psig. The SCADA pressure sensor registered a pressure of 53 psig before the failure simulation, and a pressure of 55 psig after the failure simulation until the worker regulator was returned to service at 53 psig.

Based on the above information, Northern requests interpretation on the following two issues:

  1. During normal operation (i.e., no system emergency) of a high pressure distribution system with a properly established MAOP of 56 psig, does the operator violate § 192.621(a) if the system is operated above 56 psig?
  2. During a system emergency, such as a failed worker regulator, on a high pressure distribution system with a properly established MAOP of 56 psig, does the operator violate § 192.201(a) if the system pressure does not exceed 62 psig?

Section 192.621(a) states:

(a) No person may operate a segment of a high pressure distribution system at a pressure that exceeds the lowest of the following pressures, as applicable:

(1) The design pressure of the weakest element in the segment, determined in accordance with subparts C and D of this part.

(2) 60 psi (414 kPa) gage, for a segment of a distribution system otherwise designed to operate at over 60 psi (414 kPa) gage, unless the service lines in the segment are equipped with service regulators or other pressure limiting devices in series that meet the requirements of § 192.197(c).

(3) 25 psi (172 kPa) gage in segments of cast iron pipe in which there are unreinforced bell and spigot joints.

(4) The pressure limits to which a joint could be subjected without the possibility of its parting.

(5) The pressure determined by the operator to be the maximum safe pressure after considering the history of the segment, particularly known corrosion and the actual operating pressures.

Response 1 – Yes, the operator violates § 192.621(a) if the MAOP is exceeded during normal operating conditions. Under the regulation, operators must use pipeline pressure control equipment sized for pressure control with pressure sensors, actuators and control or relief valves that react in a timely manner and have pressure settings that do not exceed MAOP in accordance with Part 192.

Section 192.201(a) states:

(a) Each pressure relief station or pressure limiting station or group of those stations installed to protect a pipeline must have enough capacity, and must be set to operate, to insure the following:

(1) In a low pressure distribution system, the pressure may not cause the unsafe operation of any connected and properly adjusted gas utilization equipment.

(2) In pipelines other than a low pressure distribution system:

(i) If the maximum allowable operating pressure is 60 psi (414 kPa) gage or more, the pressure may not exceed the maximum allowable operating pressure plus 10 percent, or the pressure that produces a hoop stress of 75 percent of SMYS, whichever is lower;

(ii) If the maximum allowable operating pressure is 12 psi (83 kPa) gage or more, but less than 60 psi (414 kPa) gage, the pressure may not exceed the maximum allowable operating pressure plus 6 psi (41 kPa) gage; or

(iii) If the maximum allowable operating pressure is less than 12 psi (83 kPa) gage, the pressure may not exceed the maximum allowable operating pressure plus 50 percent.

Response 2 – No, the operator does not violate § 192.201(a) as long as the MAOP limits are met during a system emergency and the pipeline meets the Subpart D - Design of Pipeline Components requirements. In this case, the emergency operating limit is 62 psi (56 + 6 psi). Emergency operating overpressure conditions are only allowed for the time required to activate the overpressure protection device and are not meant for long term or frequently occurring normal operating or periodic maintenance conditions and, therefore, require immediate response by the operator either to shut down or reduce the operating pressure to the normal operating conditions.

Finally, we would note that based upon your actions described in your letter, there may be some confusion about appropriate testing and maintenance of a pressure limiting or regulator station for buildup and set point. Conducting a simulated test on a pressure limiting or regulator station that is not isolated from the system does not constitute a system emergency. It is a normal operation subject to the limitations described above. The pressure limiting or regulator station should be isolated from the system prior to any testing of buildup and set points.

If we can be of further assistance, please contact Tewabe Asebe of my staff at 202-366-5523.


John A. Gale
Director, Office of Standards
and Rulemaking

cc:PHP-30:OfficialFile:Sharepoint:Web site
T:/PHP-30:Interps:Open:Northern Utilities, Inc.-PI-14-0016-09-11-2014-Part 192

1 Northern and Granite State are wholly owned subsidiaries of Unitil Corporation.

Regulation Sections

Section Subject
§ 192.621 Maximum allowable operating pressure: High-pressure distribution systems