If you’re in the business of transporting hydrocarbons through pipelines, you know that surge relief valves are often the last line of pipeline protection. Emerson’s Trilochan Gupta authored a recent Intech magazine article, Specifying surge relief valves in liquid pipelines, highlights the importance of system design and installation. You may recall Trilochan from last year’s Emerson Exchange in a post, Emerson Exchange 2011 – 7 Billion US Dollars per Year of Custody Transfer for the World Longest Heated and Insulated Pipeline.
Trilochan opens by citing some of the most notable industrial accidents such as the Three Mile Island nuclear reactor where relief valves were improperly installed. He describes what pipeline pressure surges are:
…pressure surges occur from sudden events, such as a valve closure or a pump trip, often triggered by an emergency shutdown (ESD). The moving fluid in the pipeline acts much like a train when it hits an obstacle; that is, each car slamming into the one ahead causes multiple surges.
The resulting pressure surge can be up to ten times the normal pipeline pressure—and can cause a pipeline rupture, blown valve or pump seals, spillage, and many other problems.
Trilochan explains the role of the surge relief valve:
The function of a surge relief valve is to quickly open when such a pressure surge occurs in order to relieve the high-pressure fluid to a holding tank or other safe outlet.
Normal occurrences in the process can induce surges. These include pump starts, pump power loss, valve openings and closings, and improper design and/or installation of the surge protection devices.
Regulations often govern the surge pressure limits. Trilochan cites the U.S. Department of Transportation (DOT) CFR Ch.1 Part 195.4064, which does not permit surges exceeding 110% of operating pressure limits.
The analysis and calculations of surge relief design is complex and pipeline engineers must have a solid understanding of pipeline hydraulics.
Two types of surge relief valves include pilot-operated and gas- (or nitrogen-) loaded. Trilochan notes:
…pilot-operated pressure relief valve is often used for pump protection duty and for applications where the relief valve is required to maintain pressure at a given setpoint. It can control pressure to within ±2 psi, regardless of upstream conditions.
Gas-loaded relief valves:
…are used for pipeline surge relief applications that require quick operating times and valves that can open fully. These valves are normally closed and open on increasing inlet pressure. Nitrogen gas is used to pressurize the valve piston in order to keep it in the closed position.
Emergency shutdown system (ESD) design should consider the pressure surge profiles:
For example, one of the most difficult surge problems occurs with tankers at loading terminals. If the tanker’s ESD valve shuts, the pump continues to operate for a period of time after the valve closes. A better system is for the ESD on the tanker to first shut down the pumps, and then close the ESD valves.
Skid mounted surge relief systems can incorporate the design and installation requirements with:
…properly sized surge relief valves, manifolds, and piping. These skids feature appropriate provisions for maintenance and a nitrogen charging control system.
A typical surge relief system skid has redundant, parallel surge relief valves; inlet and outlet manifolds sized to minimize the pressure loss; and a nitrogen system. All of these components are integrated on the skid, along with other required equipment, control system, and instrumentation.
Piping runs include the necessary instrumentation, including pressure and temperature indicators and transmitters, full bore in-line ultrasonic flowmeters, and a nitrogen control system.
See the excellent technical guide, An Introduction to Liquid Surge Relief, which can help you understand the concepts and considerations for a well-designed and implemented liquid surge relief system.