Vibration of Piping

1Q. How do you know when piping vibration is excessive?

1A. When the Operators or Maintenance personnel start getting nervous, when flange leaks or rub leaks or cracks at welds start to develop, when the pipes are jumping excessively, are usually good signs. I usually use vibration data to quantify the magnitude of the problem. See "Hydrocarbon Processing Magazine August 2002, Case 10: Piping Vibrating Failures". Charts similar to the one referenced and shown below can aid in making decisions based on the amplitude and frequency of the vibrating pipe. For unsupported socket type welds that are vibrating excessively, it's usually not if they will crack, it's when will they crack.

2Q. Before a new plant starts up what are good things to look for in the piping system.

2A. Usually it's a good idea to have the piping stress isometric drawings available. These are the drawings used in the flexibility analysis and shows the critical areas. Since there may be many miles of piping, critical areas can be highlighted and reviewed. Things usually aren't the same in the field as the designer intended. Pipes may be off the supports, the supports missing or in the wrong locations, bellows with the shipping stay bolts still in place, spring hangers set wrong, pipes run the wrong way because of equipment blocking the original path, hundreds of socket welds with lack of penetration. Just about anything that could go wrong probably will. Most of the time it's a matter of adding something such as gussets or supports or finding out if it will be OK as it is. This subject is a book in itself.

3Q. What is the best book on piping for a new engineer?

3A. This is a personal thing, but I prefer the original to those that have used information and updated it. So I will have to go with a book out of print and given to me many years ago. "Design of Piping Systems, The M.W.Kellogg Company, Second Edition,1956, Wiley". The calculation methods have now been computerized, new support methods are out , but there are a lot of nuggets of good information in this book. Maybe you can find one on the used book circuit.

4Q. What is Thermal Stratification failures in piping?

4A. When a pipe is half full of a very hot fluid and the other side is not the pipe will "bow" towards the hot side due to the thermal difference. Tensile stress will also develop and if the fluid cycles, between half full and full, or half full and empty, fatigue cracking can develop if severe enough. Since the temperature is separated in the pipe the term thermal stratification is sometimes used.

5Q. What is acoustical vibration of piping and pressure vessels?

5A. This is similar to what occurs in an organ pipe. The fluid causes pressure waves in the piping system that can reinforce themselves and develop significant pressure pulsations. This is a complex subject and we once had 20 compressors pumping into a common line, with different combinations and loadings. When things were just right the waves would add and the piping would jump around violently. The system was tuned by adding special vessels which solved the problem. The book on vibrations in Book Information, takes you through some simple examples.