In part one of our series, Surge Relief for Oil and Gas Liquids Pipelines, we provided background on surge conditions in pipelines transporting liquids, their effects on pipelines and why these effects matter to pipeline operators.
We’ll conclude this two-part series with a closer look at ways to prevent surge, methods of protection, surge systems and the need for pipeline hydraulic analysis.
What can be done to prevent surge
Some design approach and considerations can help prevent surge pressure from happening:
- Complete computer modeling of pipeline profile during the design stage, to determine the effects of unsteady flow of liquids transmission in the pipelines
- Stage pumps shutdown sequence, in conjunction with the closing of main pipeline valves
- Linked ship/shore emergency shutdown (ESD—for loading and off-loading tankers): historically, if the tanker’s ESD valve shuts in, the pump continues to operate for a period of time after the valve closure, causing surge pressure conditions. A linked ESD system uses the initiation of ESD on board the tanker to trip the pumps. The ESD valve closure time allows the pipeline flow to decay. These systems must insure the transfer pump is shutdown first.
- Stage ESD or motor operated valve closure times; extending the valve closure time will help reducing the surge effect
Surge protection methods
Even if all the above actions can help, a proper surge system should always be in place.
Several forms of surge protection are commonly used, each one with its pros and cons. The most common are accumulators, rupture discs, conventional spring-loaded relief valves, rupture pins and modulating relief valve (surge relief valves).
The fundamental requirements of surge relief systems include the need for fast acting, high capacity valves which can open very quickly to remove surge pressures from the line and then return to the normal (closed) state quickly but without causing additional pressure surge during closure. These valves are often required to open fully in very short periods of time, so that they may pass the entire flowing stream if conditions dictate.
In normal conditions, surge relief valves are closed, when a surge event interrupts normal steady-state flow, the fluid pressure exceeds the valve set point and the valve automatically and immediately opens, relieving sufficient liquid volume through a relief line to a surge tank to eliminate the surge. As the surge is dissipated, the valve closes again without slamming shut. It is then ready to act again upon any subsequent surge. In this way, the line is constantly protected from the buildup of pressure surges.
There are several types of relief valves as described earlier, namely spring loaded, rupture pin, gas loaded and pilot operated.
Relief valves that have gained wide acceptance and have become the surge relief valves of choice are the pilot and gas-loaded balanced piston flow style valves. These valves are an excellent choice for surge relief service because they are direct acting.
Pilot operated flow relief valves are generally slower to respond and most often used in refined or clean media products pipelines. As foreign material and viscosity can have an effect on performance, caution should be used if considering them for crude oil pipelines.
With reference to gas-loaded flow relief valve, nitrogen is the gas commonly used to charge the relief valve at the recommended set pressure. The nitrogen comes from a tank and its pressure is reduced by pressure regulators to the valve set point value. The nitrogen tank should be buried underground or insulated to minimize any change in temperature. Thermal expansion, caused by the increase in temperature of the nitrogen gas, can change the relief valve set point.
Surge relief valves can be provided separately, or integrated in a complete surge system onto a skid.
When properly specified and installed, surge relief systems can prevent accidents, reduce maintenance, and extend equipment-operating life. The design of these systems can be quite complex as many factors must be taken into account and various industry standards must be met.
The consequences and risks of a surge event can perhaps be avoided with an increased level of engineering at the implementation stage. These systems can be designed for on-site installation or supplied on a skid ready for installation at the site.
A typical surge relief system has single or redundant parallel surge relief valves; inlet and outlet manifolds sized to minimize the pressure loss; and a nitrogen system. Piping runs comprise all the necessary devices and instrumentation, including manual valves, pressure and temperature indicators and transmitters, a nitrogen control system, and, in some cases, full bore in-line Ultrasonic flow meters to determine the relief rate and quantity. This is the best metering technology for the application, as Ultrasonic meters do not have internal parts or restrictions that are undesirable in a relief path.
An important tool to avoid pressure surge is in the liquid pipeline management design: the planning and hydraulic analysis through advanced state-of-the-art simulation techniques.
The combination of both steady-state and transient hydraulic simulation enables better understanding of even the most demanding problems by providing appropriate analysis of the pressure surges relative to different operating events.
It is possible to carry out a hydraulic steady-state simulation and surge analysis, calculating time-invariant pressure, temperature and flow profile throughout a pipeline network for specified boundary conditions and network element set points. Using the results obtained by steady state simulation as the starting point, the simulation is further carried out for transient conditions.
Various pressure trends obtained by exposing the pipeline to various surge occurrence scenarios are and then compared with the Maximum Allowable Operating Pressure (MAOP) in pipelines. The result successfully determines the conditions that cause the formation of pressure surge.
A complete surge analysis will allow the operator to design the appropriate surge system and install it in the most desirable location. It will also allow increasing project feasibility and acceptance by public and clearance by regulatory authorities.