Common causes of Piping System Vibration

Piping vibration problems are real at onshore or offshore production facilities, pipeline stations, refineries, and petrochemical plants. Facility owners are extremely sensitive to these integrity risks because of the significant consequences associated with product releases from ruptured piping.

Recent examples in the press highlight situations where piping failures caused explosions, loss of life, environmental damage due to onshore and offshore spills, lawsuits, and facility shutdowns.  

Vibration can cause reliability problems on equipment, fatigue failure on process piping, and small branch connections including relief lines, instrumentation ports, nozzles, drains, and valves. Vibration on compressor and pump packages is also a reliability issue but is addressed in separate scopes of work.

In previous article it is stated that vibration does have a role in creating flange leak. This article briefly discusses the most common causes of Piping System Vibration.

Acoustically Induced Vibration

Acoustically induced vibration is a potential problem in high capacity, gas flow, pressure reducing systems. The pressure reduction can occur at control valves, at restriction orifices, safety valves, or when sonic flow occurs at a branch connection to a header. Failures can occur in only a few hours since the higher structural and acoustical natural frequencies are excited, and the material endurance limit can be reached in a short time. Failures have occurred in steam desuperheater systems, compressor recycle letdown systems, and safety letdown systems. Severe vibration has also occurred in pipline pressure letdown systems.

The approach to designing such a system consists of:

• Screening a system to determine if it is prone to acoustically induced vibration.
• Calculating sound power levels throughout the piping system downstream of the pressure reducer.
• Comparing the calculated sound power level to a design limit that is typically based on pipe diameter and thickness.
• Treating (i.e., modifying) the system if the sound power levels are excessive. Treatment alternatives include reducing the sound power level at the source (e.g., using a low noise control valve), improving the structrual integrity of the system, or some combination of methods.

Surge Induced Vibration

When the steady-state velocity of a fluid is suddenly altered, a pressure change occurs in the piping. The transient pressure variation is called hydraulic surge or water hammer. The pressure surge moves through the pipe at the speed of sound. Potential consequences are excessive internal pressure, pipe collapse, flange leaks, and large pipe movements.

Common causes of surge include:

• Rapid valve closure, especially in firewater systems and loading lines. Pressure is positive upstream and negative downstream of the valve. This is classic water hammer.
• Vapor pocket collapse. A collapsing vapor pocket causes two columns of liquid to collide with each other. A positive pressure rise occurs in both directions. Steam hammer is an example.
• Safety valve blowdown into a line full of liquid. The pressure rises as the released fluid collides with the fluid already in the line.
• Pump startup and shutdown. A pressure surge develops when the discharge valve downstream of the pump is left partially or fully open. For pump startup, surge is positive downstream and negative upstream. For pump shutdown, surge is negative downstream and positive upstream.

Slug Flow

Slug flow can cause flow-induced vibration in two-phase fluid systems. In a horizontal line, the vapor above the liquid can travel much faster than the liquid. This creates waves at the liquid surface and entrains some of the liquid into the vapor stream. At high vapor rates, slugs of liquid form across the pipe cross-section and travel at speeds that approach the vapor velocity. When this occurs, a wide range of reaction forces can occur at pipe bends, depending on the size of the slugs that are formed. Reaction forces developed at pipe bends due to slug flow can cause excessive piping vibration and movement unless the piping system is adequately restrained.

Wind Induced Vibration

Wind can cause piping vibration by vortex shedding from the pipe surface. If wind strikes at a right angle to the axis of a cylinder, aerodynamic forces due to vortex shedding occur at the following frequency that is a function of wind velocity & cylinder diameter.

These forces act on the pipe at right angles to the wind direction. Although the forces are small, the amplitude of vibration may be large if the shedding frequency is close to the natural frequency of the piping.

If a problem exists, the stiffness and the natural frequency of the piping should be increased by adding bracing, consistent with still meeting piping flexibility and associated equipment requirements. Mechanical snubbers and shock absorbers may also be used to change the stiffness and add damping to the piping system while still permitting its thermal movement.

Earthquakes

Earthquakes can cause piping vibration either directly due to resonance or by the motion of pipe supports or equipment connections. Piping in areas known to experience earthquakes should be checked for forces due to earthquakes.

Reading Source : Screening Piping Vibration Problems, Journal DOI: 10.1115/PVP2014-28081. Vincent A. Carucci. 2014.