In the United States a small one sentence in a regulation is now causing pipeline companies to spend hundreds of thousands of dollars on "surge studies" covering every mile of mainline transmission liquid line. Every consultant contacted proposes a different surge study. These study differences may include all or some equipment; review the entire pipeline or just parts of the system; and be viable for only the current operations. Will a new surge study be required when operations change, a new piece of equipment is added, or a new product is transported? This paper addresses surges in liquid pipelines; what causes them and the physics associated with the surge. The authors review various surge study techniques and recommend a surge study methodology for general pipeline operations that can help make surge protection proactive instead of reactive for pipeline operators. This paper addresses the DOT requirements for surge analysis on pipelines: the regulatory requirements, the significance of surges in pipelines, the physics of surges, and problems associated with surge simulation.
The regulatory requirement is, as Napoleon said constitutions should be, short and vague. In the Code of Federal Regulations, Title 49 Transportation under Part 195 Transportation of Hazardous Liquids by Pipeline § 195.406 Maximum operating pressure (b) No operator may permit the pressure in a pipeline during surges or other variations from normal operations to exceed 110 percent of the operating pressure limit established under paragraph (a) of this section. Each operator must provide adequate controls and protective equipment to control the pressure within this limit. In other words the regulatory authority realizes that there will be instances outside of normal operations where the pipeline will experience surges. This is acceptable as long as the pipeline stays within 110 percent of the operating pressure limit previously established. The operator must have controls and equipment to control the pressure surges within this limit. Lately, the regulators aren't assuming control and equipment on the system is working within these limits and wants proof that the operator is in compliance. The accepted method of proof is to have on file a surge study. So before contracting or performing a surge study perhaps one should review what a surge is.
The physics of a surge is quite simple. In accordance with Newton's second law of motion, stopping the flow in a pipe requires a force. Friction will stop the flow slowly, if the driving pressure is released. To stop the flow quickly by, say, closing a valve, more force is required. This force is provided by a reverse pressure gradient. It's this reverse pressure gradient that is of concern. For fast valve closures, the pressure upstream of the valve can become vary large. There have been cases, notably one in Saudi Arabia in the 1970s, of pipes being ruptured by the pressure increase associated with rapid valve closure.
