New designs or modifications of existing pump or gathering facilities are required frequently in many industries due to new operating conditions, increase in product demand, pump-pipeline interaction issues, pulsations problems, Net Positive Suction Head (NPSH) requirements, and so forth. In many cases, harsh transient events are experienced due to the lack of proper design, change in operating conditions, or unexpected process conditions. Piping systems are frequently subject to transient events such as slugs, water hammer, and cavitation that can create high amplitude forces and pressure spikes. Liquid systems are more susceptible to damaging forces during transients than gas systems due to the high density and incompressibility of the operating fluid. These events often result in high impact forces and vibrations that sometimes cause failures. This paper presents a general approach for modeling critical transient events in liquid and multiphase piping systems. In addition, case studies will be presented to identify the critical areas of the modeling, show model-field data comparison, results interpretation, and present some possible mitigation actions.
Transient events can introduce large pressure forces and rapid fluid accelerations into large pipeline systems, small manifolds, or distribution systems. These disturbances may result in different types of failures in pump, pipes, and devices such as pipe rupture or components reduced life due to cycle fatigue. Many transient events can lead to column separation or water hammer, which can result in catastrophic pipeline failures. Thus, transient flow simulation has become an essential requirement for increasing reliability and ensuring the safe operation of different pipeline systems and processes. Liquid systems are sometimes difficult to analyze for transient events as these can be dependent on factors that are not always known or finalized in the design stage or change during the lifetime of the system. Dependent variables include the characteristics of the fluid, the piping geometry, pressure reducing valves, operating conditions and nature of the events, scenarios and flow rates. Various modeling tools exist to provide reliable predictions and cost-effective solutions for hydraulic transient events, vibration or pulsation problems, but often multiple types of analyses must be used in combination to evaluate the system as a whole or solve complex problems.
The traditional method of analyzing transient events in liquid systems is to only use commercially available software to predict the extreme pressure conditions; however, often these transient fluid models need to be combined with mechanical or thermal stress analyses; dynamic pressure versus time inputs, and sensitivity studies or field data evaluations to be effective in resolving problems and ensuring pipeline integrity.