It is 8:30 in the morning: You have just gotten out of an early meeting, and you have a sinking feeling in your gut. You're the maintenance manager in a power unit, and you just learned Turbine 1 is going off line at midnight and you will be on steam bypass to export. Well, you know what that means, as it will be going through the letdown station "screaming." You also remember that when you worked in a large ethylene facility and the cracked gas compressor would go on recycle, the heavy system vibration led to structural vibrations and failure in a short time. You can only wonder if that will happen tonight as well.
The two situations discussed above involve high-energy systems. High-energy systems in these scenarios are piping systems that contain high steady-state and transient pressure, plus momentum effects that dissipate remarkable energy into the mechanical systems. Good examples of these systems are steam bypass and letdown stations, blowdown systems, vent relief systems and recycle systems.
Common problems
In the evolution of plant design, heavy attention is paid to the overall process, but things like vent relief, blowdown and letdown systems are not always well thought out in much detail. There may be a variety of reasons for this, but part of the problem lies in the fact that these systems are normally only temporarily in service. In these systems, fluids are typically going from a high pressure to a lower pressure. During this time, all sorts of interesting things can happen, such as:
- High noise.
- Vibration.
- Chatter.
- Failures.
- Performance shortfalls.
Troubleshooting methodology
In two-phase systems, we typically reach "choke" flow conditions within the letdown device. When this happens, shock waves are produced and can excite acoustic natural frequencies within the piping system. If these acoustic natural frequencies coincide with any mechanical natural frequencies, structural vibration may occur. A general methodology to troubleshoot and design these systems is as follows:
- Perform a process simulation of the system and evaluate fluid property conditions downstream and up.
- Determine if choke flow conditions will be achieved across the letdown device at any location.
- Depending on the complexity of the system, develop a computational fluid dynamic model of the letdown device.
- Perform a finite element acoustic analysis on the letdown device and inlet and outlet piping systems. Determine all acoustic frequencies.
- Perform a structural dynamics analysis of the system.
- Make sure shock waves and acoustic energy are contained within the letdown device.
- Decouple the mechanical from any acoustic natural frequencies in the piping systems. At this time, it is also a good idea to perform a code or fit-for-service analysis, since these systems are covered by ASME code and therefore fall under the Process Safety Management program.
In general, a good system will contain all the kinetic energy within the letdown device, and the Mach numbers on the outlet will be below 0.3. High Mach numbers in the piping system can lead to turbulence that can cause headaches from things like "side branch" excitation, among others.
Every situation is unique, but we have always found the methodology above to be very successful at isolating and solving even the toughest problems.
As I have said many times, make sure all work is reviewed by a professional engineer who is competent in the field. Nothing beats experience.
For more information, visit www.knighthawk.com or call (281) 282-9200.
