Petroleum commodity releases can have serious consequences, so ensuring pipeline integrity is crucial for operators. Pipeline integrity is not just about preventing incidents, but is a holistic approach to the prevention, detection, and mitigation of these undesired commodity release such as surge. In this two-part series, we will provide background information on surge relief, surge effects, and steps to prevent surge conditions.
Pipeline Surge Definition
Surge is defined as a rapid change in pressure due to a change in flow rate inside a liquids pipeline. Pressure surges can be generated by anything that causes the liquid velocity in a line to change quickly, such as the starting or stopping of a pump, closure or opening of a manual or automatic valve or emergency shutdown (ESD) device.
All the events that cause rapid change in flow rate result in pressure waves, which travel upstream and downstream from the point of origin. For quick reference, for each foot/second fluid velocity there is approximately a 50-psi pressure rise. This increase has to be added to the normal operating pressure of the line, and the operator has to insure that this doesn’t exceed the Maximum Allowable Operating Pressure (MAOP) of the system.
Surge effects can be regulated and controlled by regulatory and industry standards. In the US, for example, the Department of Transportation (DOT) guidelines DOT Title 49 CFR Ch.1 Part 195.406 state:
No operator may permit the pressure in a pipeline during surges or other variations from normal operations to exceed 110% of the operating pressure limit…
Surge control systems are specifically designed to mitigate the increase in pressure; they consist of devices that are closed in normal conditions and open when a surge event occurs to discharge the excess pressure.
The proper sizing and placement of a surge system requires a surge analysis to accurately determine the pressure profile in the pipeline. As there are many devices in a pipeline, the surge analysis often involves detailed computer modeling to simulate the complex interactions of equipment, pipelines and fluid using normal, fault and emergency events together with the hazard and operability study.
Surge pressures are a very serious concern not only for onshore and offshore liquid pipelines, but also at truck loading terminals and tank farms.
Effects on Pipelines
The potential for damage due to surge events in a pipeline is illustrated by the following graph showing the effects of the closure of an ESD valve. Without surge relief, the pressure spikes to around 10 times the normal line pressure.As the pressure wave created propagates along the line, the cycling effect occurs. Some of possible events that can be caused include:
- Axial separation of flanges
- Pipe fatigue at welds
- Longitudinal splits of pipe
- Pumps knocked out of alignment
- Severe damage to piping and piping supports
- Damage to specialized components such as loading arms, hoses, filters, bellows, etc.
The graph also shows the effect on pressure if a surge pressure relief valve is used. The larger the capacity of the surge relief control valve, the lower the pressure spikes.
Why Does Surge Matter to Pipeline Operators
After surveying our customers worldwide in the petroleum transportation industry, we have found these to be the top challenges they face:
- Safety – avoiding pipelines from rupturing and potentially harming personnel, the environment and the community
- Pipeline integrity – customers want to lower their cost of ownership and extend their asset life. Surges can cause fatigue failure or catastrophic failure. Customers need their piping to be designed to withstand surges and avoid additional maintenance, inspection and repairs
- Operational flexibility – most customers design piping to meet current demand, but with growing energy demand they need the flexibility to operate at higher capacities and with varying demand
- Regulatory compliance – there are many design standards and codes in place that require surge analysis to be completed
- Business continuity – customers need to be a reliable transporter and ensure they are meeting their customers’ needs by delivering on time
- Brand reputation – customers want to maintain their brand integrity and value, and avoid public disasters and accidents
In part two, we’ll explore ways to prevent surge, surge protection methods, components in a surge system and planning & hydraulic analysis.