In order to prevent overpressure in a liquid pipeline system, typical mitigation measures include increasing the valve closing time and/or installing a relief system. In this Case Study, a pump station is initially delivering crude oil to Terminal 1 by pipeline. A side-stream full delivery to Terminal 2 is then initiated from a mid-stream take off point on the pipeline. During the side stream operation, the main delivery operation to Terminal 1 is shut off.
A transient hydraulics analysis was carried out on closure of a valve at the side-stream delivery location (Terminal 2), which resulted in overpressure in the pipeline system when no relief system was operational. A relief system located at a relatively extended distance away was utilized for mitigating overpressures at varying valve closure times. Located at Terminal 1, this relief system was not along the crude oil flow path to Terminal 2. The impact of varying the location of the relief system was also examined.
This approach was successful in reducing the maximum transient pressures in the pipeline system. The maximum transient pressures reduced as the distance of the relief system decreased. The results also showed increasing transient pressures as the valve closing time increased instead of vice-versa as is typically expected. This unusual occurrence as well as the effect of further varying the location of the relief system is examined and explained in this study.
Steady state normal operations are typically employed as a basis for designing a pipeline system. During this process, the pipeline size, capacity and pumping or compression power requirements are established. However, ‘normal’ operation of pipelines also involves unsteady conditions that are initiated as a result of events such as the operation of automated valves or pumps (compressors for gas systems). Specifically, the sudden closure of an automated valve or stoppage of a running pump can create transient pressures within the system that exceed the steady state pressures. These transient pressures may exceed the ASME B31.4 (and Canadian CSA Z662) recommended maximum allowable surge pressure equivalent to 110% of the system maximum operating pressure (MOP). When this occurs, the pipeline system experiences overpressures that could lead to pipeline rupture.