IBP1294_09

ASSESSING THE EFFICIENCY OF AUTOMATICALLY CONTROLLED VALVES (ACV) FOR PIPELINE SECTIONING Leandro S. da Veiga1, Marcos J. M. da Silva2, João Paulo de B. Leite3, Renata N. R. dos Santos4, Rodrigo B. O. Jardim5, Thiago C. do Quinto6

Copyright 2009, Brazilian Petroleum, Gas and Biofuels Institute - IBP This Technical Paper was prepared for presentation at the Rio Pipeline Conference and Exposition 2009, held between September, 22-24, 2009, in Rio de Janeiro. This Technical Paper was selected for presentation by the Technical Committee of the event according to the information contained in the abstract submitted by the author(s). The contents of the Technical Paper, as presented, were not reviewed by IBP. The organizers are not supposed to translate or correct the submitted papers. The material as it is presented, does not necessarily represent Brazilian Petroleum, Gas and Biofuels Institute’ opinion, or that of its Members or Representatives. Authors consent to the publication of this Technical Paper in the Rio Pipeline Conference Proceedings.

Abstract

In order to mitigate the effects caused by the rupture of a gas pipeline and following ASME B 31.8 recommendations, block valves are installed in these structures. However, many transportation companies also install devices capable of infer the occurrence of an accident in a gas pipeline. The most common devices are the ones that actuate when pressure in gas pipeline reaches a low value early established (PSL) and those which close valves due to high rate of pressure drop (line-break). Line-break has the function of identifying as fast as possible the occurrence of a rupture in a gas pipeline by high rate of pressure drop in that line. Although PSL presents a later actuation when compared to the line- break, it represents redundance to the line-break system, since it is able to isolate the segment where the accident happened even if other devices or the operator had not done it before. The growing of gas pipelines transport capacity has been generated transients capable of causing an erroneous shut down of the shut down valves (SDV). The aim of this paper, therefore, is to present how the operational limits of SDV can be overcome with remote operation using SCADA System.

1. Introduction

Pipelines are generally the most economical solution to transport large quantities of natural gas over land, and consequently, the most employed worldwide. The oldest long-distance interstate gas lines date to the late 1920s. Particularly, the advances in welding technology join steel pipes in the 1920s made it possible to construct leakproof, high-pressure, large-diameter pipelines (Kiefner, J. F, et al). Manufacturers began to form pipe with electric resistance- welded or flash-welded processes, a significant advance in the reliability of the longitudinal seam, and the electric arc girth weld was developed, a significant improvement over acetylene girth welds in use previously. Simultaneously, the industry began to develop material-quality standards and consensus standards for the safe design, construction, operation, and maintenance of pipelines (CASTANEDA, C. J, et al). Yet, the production of natural gas in Brazil only started around the early 60´s, in the northeast of the country, boosting the development of major industry projects of the region. Hence, in 1974 the first interstate gas pipeline was constructed, with 235 km connecting the states of Bahia and Sergipe. The Brazilian gas pipeline network started then to develop but restricted to the northeast coast. Later, between 1980 and 1995, with the discovery of reserves in the Campos Basin, the consumption of natural gas increased significantly, to about 2.7% of the domestic energy matrix, promoting the fast development of the southeast gas pipeline network. In 1999, with the conclusion of a 2539 km gas pipeline connecting Brazil to Bolivia, with capacity for transporting 30 million cubic meters per day, there was a sharp increase in the domestic supply of natural gas, raising its participation in the domestic energy matrix to nearly 10%. In addition, the discovery of new reserves over the last few years has opened new perspectives for rising domestic ______1 Control and Automation Engineer – TRANSPETRO 2 Master, Mechanical Engineer – TRANSPETRO 3 PhD, Civil Engineer – TRANSPETRO 4 Master, Civil Engineer – TRANSPETRO 5 Civil Engineer – TRANSPETRO 6 Chemical Engineer – TRANSPETRO

Rio Pipeline Conference and Exposition 2009 consumption. The country’s gas pipeline network reached currently nearly 7000 km and has some other 1000 km under construction, mainly to connect the northeast to southeast gas pipeline network. The design of the Brazilian gas pipelines has followed ASME B31.8 standard and complying with Brazilian NBR 12712 standard, being the latter strongly based on the American standard. Both standards encompass, in addition to gas pipeline construction aspects, others related to operation and maintenance. In particular, criteria for the spacing of blocking valves to enable pipeline sectioning. This allows, for instance, the isolation of certain portion of a pipeline ensuring that maintenance can be safely carried out and also limiting inventory losses in case of maintenance or accidents. The code recommendations are restricted to the spacing of such valves and not making any allusion to its specific type and operation features. These are usually shutoff valves manually operated. However, in order to add local intelligence units capable of providing more safety to the gas pipelines and further limiting the inventory losses, concepts of emergency shutdown systems (ESD) were embodied in the sectionalizing block valves of gas pipelines. A particular type of valve was designed to allow the quick full closure activated by preset pressure triggers. These automatically controlled valves (ACV) were mainly intended to replace the manually operated valves in strategic places, as for example, areas of difficult access and also to isolate an upstream or downstream gaseous inventory. Later, with technological advances in instrumentation, data gathering units and communication systems the operation and control of pipelines were gradually shifted to highly equipped remote operation control centers (OCC) and also the sectionalizing block valves were designed to operate remotely, known as remotely controlled valves (RCV). All the three types used for pipeline sectioning have their technical and/or operational up and down sides, so that the choice has been left to the judgment of the operators. Petrobras, the Brazilian pipeline operator in the 1970s, aware of its little experience and limited resources for monitoring the gas pipelines, decided to adopt mainly automatically controlled valves for gas pipeline sectioning. Initially, on/off valves with low pressure triggers, but currently about 97% of gas pipelines contain shutdown valves featuring actuators with double triggers: low pressure and sharp pressure drop. While the Brazilian gas pipelines network was operating with low and steady flows, substantially under its capacity, the ACVs rendered an elusive impression of safety, which served particularly to ease the worries of environmental regulators. However, the gas consumption is growing quickly and operation complexity is very different from that of the 1970s. The gas pipeline network has become an essential part of the Power system of the country. Since the Brazilian production is mainly offshore, the pipeline network is consisted mainly of trunk lines along the coast line, with no redundancy or storage facilities for contingency situations. As a consequence of such changes in operation scenario, apart from few particular cases, the ACVs are no longer suitable for pipeline sectioning, resulting in many undesired and spurious valve closure and supply failure. The arbitrary use of ACVs has become a threat to energy supply system and must be reviewed. In fact, it was observed that in the few incidents that the ACVs should close, they fail to actuate. Natural gas consumption has grown steeply in Brazil, encouraged by federal government programs, reaching the mark of 60 million m³/day in 2008, with a forecast for 2012 of 130 million m³/day. In order to meet this demand, besides investing in new forms of natural gas transportation such as LNG and CNG, Brazil has continuously invested in new gas pipelines as well as in retrofitting those already in place to the enhance transportation capacity. There are, currently, more efficient ways to ensure safety and prevent supply failures, and the local operators are moving towards shifts in operational policies to accommodate the higher demands in the pipeline network complexity. The new National Operations Control Center, provided with some of latest technologies for simulations and remote monitoring, is capable of more precise diagnosis and actions, allowing it to remotely oversee and operate gas pipelines and also compression stations as a unique system in a safety manner.

2. Pipeline Sectionalizing Block Valves

As mentioned before, the shutoff valves employed for pipeline sectioning may be classified in three types according to the way they are controlled and/or operated: manually operated valves, automatically controlled valves (ACV) and remotely controlled valves (RCV).

2.1. Manually Operated Valves The manually operated valves generally represent lower costs and are the most employed for intermediate sectioning valves. In the case of large diameters or buried underground valves, depending on the operational requirements, powerful actuators and gearbox control may be required, increasing the costs comparably to the automatically controlled valves.

2.2. Automatically Operated Valves The automatically operated valves (ACV) are actuated valves which borrowed some of the concepts of the ESD systems. An emergency valve is usually operated by means of a pressurized fluid. One method of operation

2 Rio Pipeline Conference and Exposition 2009 involves an actuator using hydraulic or gas pressure to retain the valve in its normal, for example, open, position. When the emergency valve is to be shut, the hydraulic or gas pressure is released and a metal spring or other mechanism closes the valve. The hydraulic or gas pressure is normally controlled by electrically controlled solenoid valves. An electrical signal is provided to the solenoid valve by an electrical control line. An interruption of the electrical signal will operate the solenoid valves to release or divert the hydraulic or gas pressure and hence closes the valve. Actuator triggers are usually defined as high and low pressure (Hi-Lo) or pressure change thresholds:

ƒ Activation by pressure drop rate– the valve is set to close when the rate of upstream or downstream pressure drop (∆pressure/∆rate) reaches a predetermined threshold.

ƒ Activation by high/low pressure – the valve closure occurs when the upstream or downstream pressure reaches preset high or low limits. Pipeline operational conditions are usually taken into account in order to establish these limits, such as minimum delivery pressure in accordance with contract obligations and minimum acceptable pressure for the compressor suction linked to the pipeline.

Technical advantage is the quick isolation of a section without dispatching personnel and requiring knowledge of valve status using SCADA. Economic advantages are minimizing company liability, and potential for minimizing gas customer outage by quickly isolating section. Main disadvantages are the higher costs and potential for inadvertent closure. There are a number of reports of malfunctions and cases where the valves fail to closed on demand. In order adjust the valve to each of the above mentioned situations, it is crucial to define the operational conditions of the pipeline and evaluate potential transients. Transients are usually related to consumer, compression station engaging or stopping, and variations in processing units. The lower limit pressure set will depend on the minimum pressure threshold allowed in the pipeline operations, with allowance for a safety margin. In case of pipeline rupture, the pressure drops until that threshold is met and the upstream and downstream valves, in relation to the rupture, isolates this section of the pipeline and prevent further inventory losses. However, it may take considerable time until the pressure drops sufficiently to meet the preset lower limit. Thus, in order to reduce inventory losses, a pressure drop rate trigger is usually in place, attempting to reduce the delay in actuation. The difference between the two systems lies in the monitoring principle for determining the drop rate, presented as follows:

2.2.1 Pneumatic Line-Break Actuators The pneumatic actuator, as shown in Figure 1, uses a reference vessel and a calibrated cylinder to measure a pressure drop rate. Thus, when the pipeline is being pressurized, the directional flow valve allows for pressure equalization between the vessel and the pipeline. When depressurized, however, the gas located in the reference vessel returns to the pipeline, not by the directional valve, but through the calibrated orifice where a pressure differential is produced during depressurization. This differential is generated through the restriction imposed by the orifice, which makes the pressure in the reference vessel fall in slower rate than the pressure in the pipeline. Therefore, each manufacturer defines a relation between the pressure value generated in the orifice and the pressure drop rate in the gas pipeline. Once the pressure value related to a set pressure drop rate is reached, a pilot valve is activated to close the ACV. The adjustment of the pressure drop rate in a pneumatic actuator is set through the curves provided by the manufacturer, which convert the target pressure drop rate into a pressure value to be adjusted in the differential pilot valve (as shown in Figure 1). The differential pressure generated in the orifice may also depend on the distance from an ACV to the event, making this detection process very empirical. Attempting to prevent inadvertent actuations during operational transients such as compression stations rapid rump-up or shutdown, and sudden rises in consumption from major consumers, delaying devices are sometimes installed in the pneumatic line-break circuit. Such device should only allow the valve actuation if the pressure drop rate is sustained above the preset value for a period of time. Nevertheless, in the case of the pipeline rupture, the pressure drop rate should remain above the permissible value for a longer period than the set delay a pressure, activating the valve in case of accidents.

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Figure 1. A line-break pneumatic actuator

2.2.2 Electric Line-Break Actuators Electric actuators, in turn, have simpler line-break logic. With the help from a pressure transducer, the pressure in the pipeline is measured through an adjustable sampling rate, usually every five seconds or longer, and the pressure drop rate is calculated at every reading. The calculated rate is compared to the adjusted value prompting the ACV to close in case the rate corresponds to or exceeds the adjusted value. Electric line-breaks allow adjustments be made so to reduce problems of inadvertent activation due to abrupt depressurization resulting from operational transients. A handy feature usually available in the electric system is the facility for variation of time on the calculation of pressure drop rates.

2.3. Remotely Operated Valves Known in the art is a system for controlling shutoff valves which comprises a self-contained power source in the form of an electrically driven blower with a check valve and an additional reservoir and further comprises distributing piping and electromagnetic remote control valves. This system suffers from the disadvantage that, owing to the sophisticated design and the need for a constant supply of substantial electric power, it does not provide sufficient control reliability during breakdown situations where failure of electric power feed may occur. This system comprises a main control apparatus which includes solenoid-operated remote control valves, a pneumatic distributor, a gas filter and dryer unit, and a reservoir containing silicone fluid, and further comprises a duplicating apparatus in the form of hand operated pumps with valves and flow restrictors, the duplicating apparatus being connected with the main control apparatus. In this system, the natural gas being transported is used as a power source. Therefore, with the gas insufficiently filtered and dried, formation of hydrates may occur in the system at freezing ambient temperatures. Furthermore, during breakdown situations where the pressure in the gas pipeline is not enough to operate the system, the main control apparatus is inoperative and the duplicating apparatus has to be operated manually. Thus, in this system the use of the duplicating apparatus fails to provide greater reliability of remote control. This remote control system is energized by a powerful electrical signal which has to be maintained throughout the actuation of the shutoff valve involved. Therefore, a source of electric power is required for the operation of the system and additional devices for amplification and storage of the control impulse are required for the operation of the telecontrol elements, which adversely affects the reliability of shutoff valve control, particularly in the event of a breakdown. During breakdown circumstances, when a drop of pressure and failure of electric power supply may occur, the dependability of control is reduced sharply.

3. ACV Operational Problems

One of the biggest problems found in ACVs with a pneumatic line-break is their vulnerability to operational variations. In other words, pressure drop rate adjustments are highly dependent on the pipeline’s operating pressure. 4 Rio Pipeline Conference and Exposition 2009 Figure 2 below shows the curve of a pneumatic line-break actuator. This figure shows that if the pressure drop rate is adjusted to a certain pressurization condition in the pipeline and that pressure happens to drop, the line-break becomes more sensitive, i.e. without any modification in its physical adjustment, rate detection will be reduced.

Figure 2: Line-break adjustment chart

Figure 2 above shows an example of the described situation. For a 0.9 bar/min drop rate adjustment, taking into account a pipeline operation pressure at 55 bar, a 1.5 bar pressure differential will be adjusted, as shown by the red arrow in the chart. If the operation pressure is lowered to 35 bar, which might occur at the end of a week of a gas pipeline’s operation, the pressure drop detection adjustment will have been reduced to 0.7 bar/min. Another great problem in this actuator is its inability to distinguish operational transients from possible transients indicating ruptures or leaks in the pipeline.

3.1. Increase of Noise Due to Compression Station When in service, these compression stations induce upstream pressure drop and downstream pressure rise. This upstream drop occurs in a short period of time (seconds), during which the pressure can fall by about 5 kgf/cm² in less than one minute. As for downstream pressure, the worst problem occurs at the time compression stations stop, when upstream and downstream pressures equalize, generating in the downstream area near the compression station a great drop in pressure in the pipeline, reaching around 15 kgf/cm² in a minute. To define the line-break adjustments of the ACVs as previously described, a number of possible operational scenarios are studied. These scenarios undergo simulations and thereafter the operational transients found in each of them are compared. Then, the highest pressure drop transient in those simulations is selected plus a safety margin of 10%. These simulation studies have revealed that due to difficulties with filtering operational transients in pneumatic line-break actuators, ACVs need extremely high adjustments in their pressure so as to avoid unwanted closures. For example, ACVs near compression stations (from 1 km to 5km) need to be adjusted to values that, depending on the system settings, may top 20kgf/cm² per minute. This situation is one of the limitations of pneumatic line-break systems: the short range of their actuators, i.e., the relation between the highest and lowest adjustment value. Indeed, this limitation is a result of another greater one, which is the fact that an ACV is a device with a local intelligence and as such it cannot identify what occurs in the entire network. The next item describes a new operational paradigm which provides new solutions to tackle those limitations.

3.2. Operational Limitations Brazil does not have an official database of accidents in relation to pipelines, not even of cargo-carrying pipelines specifically. Nonetheless, data from the end of 2008 and beginning of 2009 gathered by national companies handling cargo-carrying pipelines contain analyses showing that their ACVs were not able to respond, either upstream or downstream of the rupture. In only one of the accidents where there was a complete rupture of the pipeline was the 5 Rio Pipeline Conference and Exposition 2009 ACV line-break activation confirmed downstream of the accident. Still, it could not prevent cargo loss because the breakdown site continued to be fed with natural gas by the upstream section of the rupture. These analyses show that due to the distance between the ACVs and the events (maximum of 10 km), the line-break could not identify those accidents. Simulations of one of the accidents showed that the ACVs situated upstream and downstream of these events would not have been able to spot such occurrences in the gas pipelines due to the fact that such occurrences mimicked operational transients. Due to the vulnerability in gas pipeline accident detection, line-break adjustments are set in values very close to those of operational transients. In practice, these transients are very difficult to predict, so the adjustment values set turn out to be lower than those in operational reality. As a result, closures may take place prompted by normal operational transients with any rates higher than the estimated adjustment value set for a specific ACV location. Another factor contributing to the inefficiency of prompt ACV actuation in response to a line-break is the shock wave propagation mode when a rupture occurs in the pipeline (Mahgerefteh, H., et al). For example, the figure below presents the case of a pipeline located at the North Sea carrying natural gas. That pipeline has a 145- kilometer length and 34-inch diameter. The gas fluid velocity is about 10 m/s, the pipeline has a 133 bar operational pressure and a mean temperature of 283 K or 9.85 ºC. Even with the final quantitative result being directly linked to the operational condition of each particular pipeline, the qualitative information is similar to that of other pipelines. In order to evaluate rupture detection capacity in a pipeline, settings adjusted to the class Location 3, i.e. ACVs located a maximum of 16 kilometers away, are used here as an example. Considering that these two ACV’s will not respond, it is possible to study the pressure and velocity profile propagation. The following chart shows how pressure and velocity profiles act as they move along downstream of the event site. Upstream conditions are not taken into account here because the shock wave is damped by moving against gas flow direction, producing longer time results than those studied here. The propagation of this shock wave is crucial, for it is the force responsible for altering the velocity and pressure profiles across the pipeline after a rupture event.

Figure 4: Pressure and temperature profiles near a rupture plane. (Mahgerefteh, H., et al)

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Figure 5. Time location of the outflow of gas near (downstream of) the pipeline rupture. (Mahgerefteh, H., et al)

Figure 4 shows velocity and pressure profiles of gas flow in a number of time periods encompassing the first 50 seconds after a rupture event at the mark 0 km indicated in Figure 5. The principal characteristics analyzed in the above figures are summarized as follows. The rupture produces a shock wave propagation which results in a significant change in gas flow pressure and velocity near the rupture plane, but the conditions downstream of the rupture remain identical to those prior to the event during the periods of time immediately after its occurrence. This is illustrated in Figure 4 through the constant pressure and velocity curves. The position corresponding to the flow inversion downstream of the rupture is illustrated in Figure 5, in which the instants after the accident where there is flow inversion are identified. To make it clearer, the flow inversion location is also mapped in Figure 5, along with the time each site takes to notice such flow inversion. These figures reveal that the ACVs located beyond 6 to 8 kilometers from the event notice a pressure drop rate (less than 10 bar) very similar to an operational transient, as mentioned. Moreover, the time between the accident identification through the pressure drop rate or low pressure becomes insignificant in face of the accidents caused at the rupture site itself by thermal radiation from a possible fire. Another factor that has been proved by studies is its failure to minimize the damages caused to people near pipeline rupture. The thermal radiation produced in the event of fire and the noise provoked by the gas mass launched into the atmosphere cannot be minimized by shortening the distance between shutdown valves (Mahgerefteh, H., et al)

4. Centralized Remote Operation as Alternative Solution

Each figure shown in the text must bear a title and be numbered in Arabic numerals. Figures should be centered, and referred to in the text as follows: “Figure 1 indicates...”. The captions must be equally centered below each figure. A line must be skipped between the figure, the captions and the body of the text. With the advance of automation and data networks, centralized and remote operations of geographically- segmented systems are becoming a new operational paradigm in pipeline activity. The main factor responsible for making this type of operation possible is the SCADA system. SCADA stands for Supervisory Control and Data Acquisition and is used to describe the group of supervisory software that monitors industrial plants. Still, the definition of that acronym is much broader and refers to all the software, hardware and data network that allow for remote supervision, control and operation of plants as well as highly segmented systems like cargo-carrying pipeline networks. Transpetro, a subsidiary of Petrobras, uses the SCADA structure in its gas and oil pipeline operations 24 hours a day. The oil and gas pipeline network throughout the country can be remotely operated from the company’s National Operational Control Center, located at its headquarters in Rio de Janeiro. At this control center, with just one mouse click operation technicians are able to interact with pipelines and terminals, turning pumps on and off, opening and closing valves and altering operation sites throughout the pipeline network, in addition to detecting leaks and carrying out simulations of future operational conditions. Indeed, to make these remote operations possible, it takes more than automation infrastructure; it also takes skilled technicians. For this reason, prior to assuming their positions at the CNCO, every staff member undergoes rigorous practical and theoretical training, which includes the handling of abnormal situations, so as to evaluate their response in accordance with established procedures. The benefits this system provides are not only in relation to the centralization of all the operation at the company’s headquarters. As the gas pipeline network is highly interconnected in many regions in the country, the SCADA system allows for a much broader view of the pipeline system as a whole, facilitating the identification of operational anomalies and quicker mitigation of them. From the operating consoles, which display information such as 7 Rio Pipeline Conference and Exposition 2009 pressure, temperature and flow rate of natural gas in many sections, it is possible to identify footprints of certain events such as start and stop of compression station operations, customer consumption, ACV closures and pipeline rupture. In terms of ruptures, the SCADA system has a greater advantage over local ACV units, for these can only visualize local pressure transients, which often do not indicate a pipeline rupture. The SCADA system, in turn, by gathering data from other sections in the network, allows operators to identify and distinguish normal operational transients from abnormal ones. The following item describes the limitations found in ACVs that can cause unwanted activations or even failure to activate depending on their proximity to an event.

5. Conclusion

The operation and maintenance history from Transpetro’s gas pipeline network shows that arbitrary use of ACVs for pipeline sectioning introduce serious risks to market supply and does not imply in additional safety. In fact, it could be stated that the large number of ACV malfunctions and inadvertent closure introduces a potential risk in delaying or preventing fast emergency responses. This restates chapter IV, section 846(c) of ASME B31.8[8]: “This Code does not require the use of automatic valves nor does the Code imply that the use of automatic valves presently developed will provide full protection to a piping system. Their use and installation shall be at the discretion of the operating company.” It also supports the conclusions of studies (Sparks, C. R., et al) conducted by GRI, reviewing injuries and fatalities that occurred in U.S. DOT reportable incidents from 1970 through 1992, to assess whether rapid closing main line valves might reduce the number of injuries and fatalities that occurred in incidents on pipelines: “Data were obtained for 80 incidents over the time period from 1970 to 1992. There was only one incident in which the application of a quick closing valve could have prevented the injury that occurred. In the remainder of the incidents, immediate burns or impact from the gas released caused the injury or fatality, and quick closing valves would not have mitigated the consequences of the incidents to people.” The only incident in which a faster response could have possibly prevented an injury involved a burn to a contractor’s employee about 49 minutes after the incident occurred and took 85 minutes to close the valve. The operational results obtained by Transpetro through the centralized operation of its gas pipeline network reveal that the company is able to safely operate its pipeline in accordance with a new established paradigm. Actuators that were once necessary today can be replaced with more efficient diagnosis and actions in case of eventual operational anomalies. This means that the operational intelligence, which had been segmented and limited to locally ranging equipment, is now centralized, but with a much broader capacity due to the SCADA system. The potential of damage from a pipeline failure because of such factors as population density, pressure, and pipe diameter, and the probability of a pipeline failure due to such factors as subsidence, and proposed contiguous construction activity, should be used as criteria.

6. References

KIEFNER, J. F.; CHERYL J. TRENCH, C. J. Oil Pipeline Characteristics and Risk Factors: Illustrations from the Decade of Construction, API/Pipeline Performance Tracking System (PPTS) Report. Washington, D.C. NTSB. 2002. CASTANEDA, C. J. History Beneath the Surface: Natural Gas Pipelines and the National Historic Preservation Act," The Public Historian 26, no. 1 (Winter 2004), pp. 105-21. MAHGEREFTEH, H.; SAHA, P. AND ECONOMOUA, I. G. Study of the Dynamic Response of Emergency Shutdown Valves Following Full Bore Rupture of Gas Pipelines. Process Safety and Environmental Protection, 1997, vol.75, no. 4, pp.201-209. EIBER, R.; MCGEHEE, W. B.; HOPKINS, P.; SMITH, T.; DIGGORY, I.; GOODFELLOW, G.; BALDWIN, T.R. AND MCHUGH, R. Valve Spacing Basis For Gas Transmission Pipelines. Pipeline Research Council International, Inc. 01/01/2000. SPARKS, C.R.; BOWLES, E. B.; GERLACH, C. R.; HARRELL, J. P.; MCKEE, R. J.; MORROW, T. B. Remote and Automatic Main Line Valve Technology Assessment, Final Report, GRI Report No. GRI-95/0101, July 1995. ASME, American Society Of Mechanical Engineers. B31.8: Gas Transmission and Distribution Piping Systems, New York, NY, 2007 ABNT. NBR 12712: Projeto de sistemas de transmissão e distribuição de gás combustível, Rio de Janeiro, 2002.

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