Fire Detection and Suppression in Natural Gas Pipeline Compressor Stations

Transcription

1 THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 47 St., New York, N.Y GT-103 The Society shall not be responsible for statements or opinions advanced in papers or in discussion at meetings of the Society or of its Divisions or Sections, or printed in its publications. Discussion is printed only if the paper is published in an ASME Journal. Papers are available from ASME for fifteen months after the meeting. Printed in USA. Copyright 1987 by ASME Fire Detection and Suppression in Natural Gas Pipeline Compressor Stations COLTON W. MEYER, P.E. Pacific Gas Transmission Company Presented at the Gas Turbine Conference and Exhibition, Anaheim, California May 31-June 4, 1987

2 ABSTRACT The occurrence of fires in natural gas compressor stations is fortunately infrequent. The consequences, however, can be severe. This paper will discuss the design concepts and experience of Pacific Gas Transmission Company (PGT) with fire detection and suppression systems in its natural gas pipeline compressor stations. stations, most of these are small volunteer fire departments and are often not trained for or accustomed to fighting gas or oil fires. The company policy has been not to call in the fire department until company personnel are on the scene and have assured that the station has shut down properly and the gas piping has been depressurized. FIRES INTRODUCTION The occurrence of fires in natural gas compressor stations is fortunately infrequent. The consequences, however, could be severe, ranging from damage or destruction of equipment to loss of an entire station and even loss of lives. For this reason, the prevention, detection and suppression of fires are important considerations in the design and operation of compressor stations. This paper will focus on Pacific Gas Transmission Company's design concepts and experience with fire detection and suppression systems. The prevention of fires, through good design and operating practices, is of primary importance but is not the primary thrust of this paper. The PGT system includes 12 pipeline compressor stations with a total of 23 gas turbine powered compressor units, all located in the Pacific Northwest. Four stations have two units in a single building. The remaining stations have separate buildings for each unit. The stations are all remotely controlled via telemetry and operated from the Spokane, Washington Gas Control Center and are normally unattended. Some of them are located as much as two hours driving time from the nearest maintenance base. While there are local fire departments often within 10 to 15 minutes driving time of the The two major causes of fires in compressor stations are failures of the gas piping systems and failures of the lubricating and seal oil systems on the compressor units. Gas fires are potentially the most catastrophic and can normally be contained only by cutting off the supply of gas. Oil fires, on the other hand, are normally confined to a limited area and can often be extinguished by chemical, foam or inert gas fire suppression systems. PGT has experienced two major fires. Both were oil fires on gas turbine driven compressor units. On Sunday evening, October 3, 1976, a lube oil fire occurred at PGT's compressor station near Sandpoint, Idaho. At 8:47 p.m., station and unit emergency shutdown alarms were received at the Spokane Gas Control Center. The station was unattended at the time. Between 9:15 and 9:25 p.m. the first company employees arrived at the station to find the "B" unit engulfed in flames. The other unit at the station, housed in a separate building, was not affected. After the second employee arrived, the two men tried unsuccessfully to extinguish the fire using 150-pound dry chemical extinguishers. No fire suppression system was installed on this unit. The local volunteer fire department truck arrived at 9:40 p.m. and the fire was ultimately extinguished at 9:55 p.m. by a company employee using a water hose from the fire truck. 2

3 On the positive side, the station emergency shutdown system functioned properly. The unit was stopped and the station piping isolated from the main line and depressurized. A post-mortem investigation found that the fire, which caused extensive damage to the gas turbine, related equipment and building, was caused by an interaction of two separate problems. The outboard thrust bearing seal on the gas compressor failed, allowing excessive lubricating oil to flow into the drain cavity between the thrust bearing and the outboard journal bearing. This overloaded the drain line, causing a positive pressure of several pounds per square inch in the drain cavity, which normally operates at a slightly negative pressure. An improperly tightened 3/4 inch pipe plug had backed out, allowing a stream of oil to escape. This oil was ignited by contact with the exposed hot parts in the vicinity of the turbine bearing and on the exhaust duct, which operate at 800 to 1000 F. The flash point of the mineral oil was in the range of 600 to 750 F. A portion of the burning oil then flowed under the axial air compressor and combustion sections of the turbine. The fire in this area caused most of the damage. The turbine, a General Electric Frame 3, was removed and shipped to the G.E. repair facility in Salt Lake City on October 21, It was received back at the station on December 13, 1976 and returned to service on February 3, 1977, four months after the fire. The second major fire on the PGT system occurred on the morning of Tuesday, December 18, 1984, at a compressor station near Ione, Oregon. At about 5:15 a.m., a shutdown/ lockout alarm on the "B" unit was received by the Gas Control Center in Spokane. As no personnel were on site, a technician and a mechanic were called out. They reported to the maintenance base at Wallula, Washington and departed for the Ione Compressor Station (approximately 85 miles away) at approximately 6:45 a.m.. The first report by a neighbor of smoke coming from the compressor station was received at 8:15 a.m.. The crew arrived at the station at 9:05 a.m. to find a fire burning on the "B" unit. After verifying that the station emergency shutdown system had functioned properly and all gas was evacuated, they attempted to extinguish the fire using 150-pound dry chemical and 30- pound CO2 fire extinguishers. They were successful in extinguishing the flames but unable to prevent a flashback. The Ione Volunteer Fire Department was summoned and arrived at the station shortly after 10:00 a.m.. The fire was then extinguished by company personnel using the Fire Department's CO2 extinguishers. A water hose in the compressor building was used to cool the metal to prevent a flashback. The cause of the fire was determined to be a crack in the threads of a half inch pipe nipple in a gauge tap on the lube oil pump discharge line. The oil spraying from the crack was ignited by striking exposed hot surfaces on the load side of the turbine. The foam fire suppression system, which had been installed at Ione after the Sandpoint, Idaho, fire, had activated. It was the significant factor in confining the fire to the area between the power turbine and the gas compressor of the "B" unit. The "A" unit, which was in the same building, was not damaged. There was, however, major damage to the lighting and other systems in the building. The damaged unit, an Ingersoll-Rand GT51 power turbine with a G.E. LM 1500 gas generator driving an Ingersoll-Rand compressor, was installed in 1969 and had logged over 90,000 hours of operation. A preliminary assessment 3

4 indicated that replacement would be preferable to repair. PGT awarded a contract for a replacement unit based on competitive bids in early March The new unit was placed in service on November 26, 1985, eleven months after the fire. o Smoke Detectors, including Photoelectric Ionization o Flame Detectors, including FIRE DETECTION Fire detection can take a number of forms, starting of course with merely the visual observation of smoke and flame. Because this requires the presence of an observer, it won't work when a station is unattended. Most operators prefer something more sophisticated and utilize automatic systems at unattended locations. Typical functions of these fire detection systems in pipeline compressor stations include initiation of alarms, shutdown of equipment, blowdown of gas piping systems and operation of fire suppression systems. PGT also uses the fire detection system to shut down the building ventilation system to reduce the air available to the fire. Some of the common types of detectors are: o Heat Detectors, including High temperature, fusible link or bimetallic strip High temperature, pneumatic tube Rate of temperature rise Combination high temperature and rate of rise The lessons learned from these two fires have Infra-red influenced PGT's design concepts and policies Ultra-violet regarding fire detection and suppression. One obvious conclusion has been that there is Regardless of the type of system, reliability a significant risk of fire resulting from is of great importance. In PGT's experience, failures in the lube and seal oil systems on false operation has been a major concern. pipeline compressor units. Unnecessary shutdowns of equipment can increase operating costs and disrupt throughput. Station piping blowdowns are wasteful of gas. False operations of fire suppression systems are costly and in some cases can leave a mess to be cleaned up. In our search for the ideal system, PGT has tried different types of fire detection systems. We give them mixed reviews. Our original installations used high temperature fusible link or bimetallic detectors. While quite simple, they had the disadvantage that they would trip under high ambient temperature conditions, particularly if the unit was running. Turbine bleed valve operation would sometimes trip the detectors. To avoid false operation it was necessary to set the detectors to operate well above maximum normal temperatures. The frequency of false fire detection was at the annoyance level, but the costs were not major as we did not at that time have fire suppression systems installed on any of our units. After the fire at the Sandpoint Compressor Station in 1976, we installed fire suppression systems on certain units, specifically those where two units were in a single building. Due to our lack of confidence in the fire detectors, and in order to avoid the cleanup and recharging cost of false operations, we tied the operation of the fire

5 suppression systems to fire detection in conjunction with a low lube oil level alarm. We also changed the set points for the low lube oil level alarm and shutdown so that they would trip with less oil loss than was previously the case. We took this approach due to our belief that the most likely cause of fire was from oil system problems, and that in a major fire enough oil would be dumped (three to four barrels) to trip the low lube oil level alarm. We were not terribly comfortable with this solution; however, it was expedient and was likely to satisfy our primary concern of not losing both units at a station in a single fire. We also began looking at alternative fire detection systems. Our next move was to install ultra-violet detectors at those locations where we had installed fire suppression systems. These were found to be a substantial improvement. While ultra-violet detectors are susceptible to being set off by, among other things, welding and lightning, this has not been a serious problem if care is taken. We have found, however, as a plus, that they will detect a spark plug or cross fire tube blowout and will shutdown the unit. but had elected not to install them. After the fire, the subject was reopened. The potential benefits of installing fire suppression systems were thought to include: o Reduced risk to personnel (primarily from fighting fires) o Minimized fire damage and reduced downtime o Minimized loss of system throughput capacity o Reduced financial loss from fire damage and business interruption The potential costs or problems included: o Cost to install, maintain and operate the systems o Potential for false operation o Depending upon the type of system, risk to personnel or equipment Due to PGT's insurance coverage and its cost of service tariff, there was no significant economic benefit from installing fire suppression systems. For PGT, the primary consideration in doing so was to assure the ability to maintain service to our customers without interruption. We have recently installed both infra-red and pneumatic tube detectors at two stations and plan to do so this year at three more. Existing systems (high temperature and ultraviolet) will be left in operation. The operating performance of each of these fire detection systems will be evaluated for reliability and effectiveness for possible installation at other stations in addition to the high temperature and ultra-violet systems. FIRE SUPPRESSION The design of the PGT pipeline system is such that the loss of a single unit will have only a minor effect on system capacity as long as the other nearby units remain available for operation. Loss of both units at a two unit station, however, can be a serious concern. For this reason, PGT concluded that fire suppression systems were warranted at those stations where both units were contained in a single building such that a major fire could damage or destroy both units. The decision was made, therefore, to install fire suppression systems at the four stations with twounit buildings. PGT had considered the installation of fire suppression systems prior to the 1976 fire The types of fire suppression systems which were considered included CO2, Halon, dry 5

6 chemical, water and foam. CO2 and Halon were rejected due to the large volume required. As none of PGT's units have enclosures, the entire building would have to be filled. CO2 was also rejected due to personnel hazard. Dry chemical systems were rejected as they do not have a lasting effect. Re-ignition could occur requiring subsequent repeated operation. There is also a clean-up problem, as the residue can "cook on" to a hot unit. Water was considered undesirable due to the potential for damage to hot equipment by causing warpage. The system selected was an Aqueous Film Forming Foam (AFFF). A prototype system was installed in 1979 at the Rosalia, Washington Compressor Station. While this system met most design requirements, it was found that the premixed foam was unstable and did not have a reasonable "shelf life." The system was therefore modified to store water and the foam concentrate separately for mixing at time of use. AFFF systems were then installed in the remaining stations with two-unit buildings. The 1984 fire, defying all expectations, occurred at one of the four stations with a fire suppression system installed. The functioning of the system was a qualified success. There was no ignition of pooled oil under the unit, the major cause of damage in the 1976 fire. There was also no significant damage to the other unit in the building. The location of the fire was confined to the area between the power turbine and the compressor, above the level protected by the foam. Damage in this area, however, was major as was the damage to building systems. RECENT ACTIONS As a result of the most recent fire experience, we have made two changes in our control system with regard to fire detection. The first is to have a fire alarm telemetered to the Spokane Gas Control Center. Due to SCADA limitations which have since been eliminated, a fire alarm was previously lumped with other alarms requiring immediate dispatch of personnel to the station. The gas control operator in Spokane therefore only knew that there was a problem, not that there was a fire. Now, the operator can distinguish a fire from other alarms. The other change is in the activation of the fire suppression systems. Having installed multiple types of fire detectors at those stations with fire suppression systems, the fire suppression is now activated by the operation of any two different fire detection systems. The low lube oil level alarm has been eliminated from this logic. PGT considers multiple redundant fire detection systems to be cheap insurance in that they increase the chance of early detection of a fire. The voting system reduces the probability of false fire suppression system operation. Another action taken following the recent fire was to make a complete review of the fire suppression system design for compliance with the latest National Fire Protection Association codes and standards. FUTURE CONSIDERATIONS PGT does not have any definite program to install fire suppression systems on all our compressor units, although we have recently installed a system in conjunction with a turbine replacement at one station. We are continuing to consider other ways of reducing fire risk and improving our fire detection and suppression systems. Possible ideas include the following: o PGT has avoided spraying foam onto the hot parts of the turbine to avoid the risk of warpage damage. 6

7 o o o This is being reconsidered. Had the foam nozzles been directed at the ignition sources, the fire damage could have been minimized. While there may have been some warpage, it would probably have been repairable and would have been more than offset by the reduced fire damage. One way to reduce the risk of fire is to reduce or eliminate the sources of oil to fuel the fire. PGT plans to install dry seals in its compressors in conjunction with turbine replacements, thus eliminating the high pressure seal oil systems. The first dry seal was installed in We are also considering the use of magnetic bearings. PGT currently uses mineral oil for the compressor lube and seal oil systems. Fire resistant synthetic oils are available which have flash temperatures in the order of 1150 F, thus considerably reducing the chance of ignition. While these are used in the separate gas generator lube oil systems of aircraft derivitive units, PGT has not used these fire resistant oils for compressor lube and seal oil systems due to their potential deleterious affect on seals, gaskets and diaphragms in the downstream gas systems if oil is lost into the line. If the compressors are converted to dry seals, this would not be a problem. Fire resistant oils could then be used for the gas turbine lubricating systems. Even in the event of a unit shutdown due to a fire, we follow the manufacturer's recommendations for operation of the lube oil pumps during the cool-down period. While possibly protecting the shaft, this has the disadvantage of continuing to feed oil to the fire. We are considering shutting down the pumps to cut off the oil flow, even if it means sacrificing the shaft. This would also be tied into our voting logic with multiple fire detection systems to avoid losing a shaft on a false alarm. SUMMARY Based on PGT's experience, we believe that there is a significant risk of lube oil fires on compressor units; however, the extent of the damage from fires can be significantly reduced by properly designed fire suppression systems. Effective operation of the fire suppression systems is dependent in turn upon reliable fire detection systems which must be designed to minimize false operation. PGT's solution has been to install redundant fire detection systems and to require two independent systems to detect a fire before the suppression system is actuated. Fire suppression systems are not warranted for all applications. Factors to consider include whether the station is attended or not, economics, and the affect of the unit loss on the system operation. I hope, however, that PGT's experience may provide some food for thought. ACKNOWLEDGMENTS In writing this paper I have drawn greatly upon the knowledge and experience of various members of the PGT staff, including but not limited to Thomas J. Cusworth, John S. Dughman, John G. Lisk, Douglas E. Tuck, Alfredo W. Visitacion and Hiro Yamada. 7