By Vince Carucci
There's no question about it - excessive piping vibration can cause real problems. Threaded connections can loosen. Flanges can start leaking. Pipes can be knocked off their supports. And in extreme cases, a pipe fatigue failure can occur. But when is vibration excessive? Vibration is obviously too much if it causes a failure, but hindsight is 20/20. What about having to decide if vibration that is occurring is too much to tolerate? What about having to decide if vibration could be a problem while a piping system is being designed? The entire subject of piping vibration cannot be discussed in this article, but I will highlight several items to consider.
There are two major categories of piping vibration and several vibration types within each category. An engineer must know the type of piping vibration being dealt with before the vibration can be treated effectively. In other words, what is causing the vibration and what are its characteristics? The two major vibration categories are steady-state and transient.
Steady-state vibration is forced, repetitive, and occurs over a relatively long period of time. The force causing the vibration can be generated by rotating or reciprocating equipment, pressure pulsations, or fluid flow (liquid or gaseous). Excessive steady-state vibration can cause a fatigue failure in the pipe due to a large number of high stress cycles. This failure will probably occur at stress concentration point (e.g., branch connection, threaded connection, fillet weld, elbow, etc.). Steady-state vibration can also cause failures at small diameter connections and tubing (e.g., instrument lead lines) or cause flange leakage due to loosening of the studs. Steady-state vibration can be either low frequency (< 300 Hz) or high frequency (> to 300 Hz). Low frequency vibration will typically cause lateral displacement of the pipe, while high frequency vibration can cause flexural vibration of the pipe wall itself in addition to lateral pipe movement.
When compared to steady-state vibrations, transient vibrations occur for a relatively short period of time and are usually caused by larger exciting forces. Pressure pulses traveling through the fluid are the most common cause of transient vibration in piping systems. These pressure pulses exert unbalanced dynamic forces on the pipe that are proportional to the straight length of pipe between bends. A common transient piping vibration is water hammer that may be caused by rapid pump starts or stops or by quick valve closing or opening.
Once the type of vibration is determined, you can determine what (if anything) needs to be done about it.
Piping vibration can result in two types of problems - technical or psychological. A technical vibration problem is one that has the potential to cause a failure (or has already caused one) and must be dealt with.
In many situations, a pipe can vibrate forever and never cause a failure. However, it might still be a problem, a psychological one, to the personnel in an operating unit who see it every day and are concerned about it. If it is easy and inexpensive to fix a psychological vibration problem, it is usually better to do that rather than try to convince operating personnel that everything is really fine with that pipe shaking.
A pipe will not vibrate if it is prevented from moving. However, this does not necessarily help the piping system design from the standpoint of its ability to absorb differential thermal expansion. Therefore, when addressing a vibration problem, the flexibility design of the piping system must also be considered. Restraints that are added to reduce vibration must not increase the pipe thermal expansion stresses or end-point reaction loads to unacceptable levels. It may sometimes be necessary to use hydraulic snubbers to stop vibration rather than fixed restraints. Such snubbers permit pipe thermal movement while still dampening vibration.
Piping vibration, if severe enough, can cause failures. There are two main categories and several different types of piping vibration. The type and cause of pipe vibration must be determined before it can be dealt with effectively. Above all, any system changes made to alleviate piping vibration must be done in a manner that will not cause problems related to differential thermal expansion of the pipe.
Later articles in this series will discuss piping vibration further.