Petroleum Pipeline Case Studies
Petroleum vs Water Pipeline Transients – Are they the same thing?
Transient pressures in pipelines are caused by a change in the velocity of fluid in the pipeline, so it is reasonable to expect similar conditions in the measurement of pressures in petroleum and water pipelines. To an extent, this is true, however the petroleum pipeline industry differs from the water pipeline counterparts in several ways, including the safety aspect of operating pipelines filled with flammable liquids, the environmental consequences of product spills, and the monetary value of the product. The operating pressures of about 1500 psi are normal, and therefore a pressure transducer for line carrying a flammable liquid must be of the “double-membrane” type to eliminate the possibility of liquid spill. It must also be explosion-proof, and receive certification for that criterion. The transducer make and model will be scrutinized and analyzed by the pipeline operator prior to being permitted for use on that facility.
Transient pressures in petroleum pipeline stem from some of the same sources as to water pipelines, such as opening and closing valves, starting and stopping pumps. Yet in some respects the problems have different characteristics and problems.
- Gas vs Liquid – Many petroleum pipelines carry natural gas, which is compressible and not prone to the same pressure problems as liquid, or non-compressible liquids. So a distinction is used in design and regulation of liquids and gases. The discussion here will be limited to pipelines conveying liquids.
- Pumps – Petroleum products in some instances are pumped using positive displacement pumps, such as multiple piston pumps. These pumps will produce pulsations, often reaching several hundred psi in magnitude with a recurrence interval of several times per second. If the pressure is recorded digitally less than about 50 times per second, these pulsations will not be detected.
- Booster Stations – Petroleum pipelines are generally much longer than water pipelines, and the inherent friction losses necessitate the use of booster stations. The very nature of the frequent changes in pressure boost requirements results in strong pressure transients, and requires utmost care in the design and operation of the station pumps.
- Differing liquids – In those transmission pipelines which carry refined petroleum products, the operation includes switching from one product to another to meet demands. This in itself entails much switching of valves and pumps, again with the resultant pressure transients.
- Cavitation – The waterhammer event in water pipelines results from pipeline pressure dropping below the vaporization pressure of water. The comparable condition in petroleum pipelines can cause cavitation within the particular liquid being conveyed at the time, with very destructive consequences on seals, gaskets, and the pipeline itself.
- Maximum Operating Pressure – All pipelines have design pressure for both normal operating and surge pressures, and certainly in water pipelines the operator strives not to exceed these pressures. However there is no regulatory requirement that the pressure be monitored, nor is there any requirement that excessive pressure be reported. In petroleum pipelines, however, pressure testing is required by the US Department of Transportation regulations 45 CFR 195. It is a violation of these regulations to operate a pipeline even during surges if the pressure exceeds 110% of the established maximum operating pressure. The pipeline operator is required to measure pressure as necessary to control pressure within this limit, and to maintain records of pressure monitoring for as long as the facility is in use.
The regulatory requirements of petroleum pipelines put a somewhat different light on pressure monitoring as well. The Office of Pipeline Safety, US Department of Transportation, administers the pipeline safety program that is primarily contained within the 49 CFR 195. Among other requirements is this limitation that the pipeline may not be operated more than 10% above the operating pressure limit established for that pipeline. That is an upper limit not to be exceeded under any circumstances, even briefly during surge events. The TP-1 Transient Pressure Monitoring System is uniquely capable of monitoring pressures to meet the most rigorous regulatory requirement for pipeline operation.