Today’s guest post comes from Emerson’s Chris Womack.
It’s hard to overstate the importance of choosing the right surge-relief valve for the pipeline task—and installing it properly. After all, there’s a long list of spills and other accidents that relief valves could have prevented, including the 1979 nuclear near-disaster at Three Mile Island.
Emerson’s Trilochan Gupta lists a few more of these in his InTech article “Specifying Surge Relief Valves in Liquid Pipelines.” But he’s most concerned with sharing general tips about which relief valves to use, and what to look out for, in different pipeline and pump arrangements.
Here are a few choice tidbits from that article. But don’t stop here—read the whole thing. Trilochan offers a lot more.
First, isn’t it possible to use pipeline design to ameliorate surge problems? Here’s Trilochan:
Although many design approaches can help reduce surge pressure sin pipelines, a surge relief valve must be installed to protect the system. This is because the surge relief valve is often the last line of defense for a pipeline. If all else fails, a relief valve—as part of a complete surge relief system—can save the day.
So, where do you start in choosing the right valve size, location, and type?
The size of a surge relief valve is determined by two factors: location and setpoint pressure. Relief valves should be located nearest to the point where the increased pressure can occur. A simple conventional spring-loaded relief valve is unlikely to operate sufficiently fast enough to relieve a pressure wave as it passes the relief valve nozzle. By adopting the lowest set pressure allowed by a hydraulic transient surge study, the smallest surge relief system design can be used.
And a few specific elements you’ll have to take into account:
Factors that need to be considered include the modulating valve’s response time, valve flow coefficient (Cv), excess pressure above setpoint to reach required flow rate, and valve characteristic control curve.
Valve response time refers to the time it takes for a relief valve to open when the pressure setpoint is exceeded. The valve flow coefficient Cv is the quantity of fluid that will flow through a wide-open valve with a 1 psi pressure drop. Cv varies by valve size, type, and manufacturer.
Now basic facts about the two main types of relief valves you’ll choose from, pilot-operated and gas-loaded:
Both actuate when pipeline pressure exceeds the setpoint, but the gas-loaded valve responds faster.
A 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.
These valves protect the line against excessive pressure and surge or serve as a pump bypass to maintain a constant pump discharge pressure.
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.
These valves are capable of handling any hydrocarbon liquids, including dirty and viscous fluids, such as crude oil or any other heavy oils.
And of course, looking at the larger picture—properly designing a surge relief system:
The design of a complete surge relief system is dependent upon a complex range of factors, including the potential for pressure increases, the volumes that must be passed by the surge relief equipment in operation, and the capacity of the system to contain pressures. The surge relief system should also work in concert with other safety and control systems.
A correctly designed surge-relief system will include components to dampen or slow the relief valve on closing, and this often requires sophisticated reverse-flow plots. In Daniel surge relief valves, for instance, a check valve provides unrestricted flow when opening and a reduced orifice to limit closing speed. This allows a user to set different closing speeds as required by the system.
Once again, there’s a lot more in the article itself. Check it out here, and keep those pipelines protected!