How to avoid vibrations in small-bore piping

Vibration of piping due to mechanical excitation is present in nearly any environment where small diameter branch connections (SBCs) of diameter size two inches and below are used. In initial piping designs, small diameter connections are often not considered or fully defined. While finite element techniques are often used to model the main piping system, it is time consuming and expensive to include all small bore piping. In addition, predictions of vibration and dynamic stress and strain amplitudes made using finite element models are subject to a range of uncertainty due to assumptions that must be made regarding the excitation sources, damping,

restraint stiffness values, end conditions, etc.

Below are some best practices prescribed in the paper, "Applying the Energy Institute and GMRC/PRCI guidelines for the avoidance or reduction of vibration problems in small diameter piping branch connections," presented at the 2016 Turbomachinery & Pump Symposia by Sarah Simons, Benjamin White and Francisco Fierro of Southwest Research Institute.

• Small bore piping wall thickness should be schedule 80 or higher

• Fittings with higher stress concentration factors (such as threaded connections) should be avoided in higher risk areas such as “near” the compression equipment. Therefore, weldolets are preferable over threadolets.

• Branch line lengths should be kept as short as possible with minimal attached weight to raise the structural natural

frequency

• If possible, locate the branch line away from valves, reducers, bends and tees in the main line where flow turbulence and acoustic induced vibration may cause problems

• Threaded fittings should be tight and back welded such that there are no exposed threads; threadolets are generally not recommended.

• Standard or higher schedules are recommended for main line piping to reduce the stress concentration at the connection to the SBC.

Restraints

The installation of an external restraint (support, bracing, etc.) will add stiffness to the branch line, raising its minimum mechanical natural frequency and reducing the risk of excessive vibration. Because there are an almost unlimited number of potential branch line configurations, it is not possible to recommend specific restraint configurations that will cover all feasible layouts. However, the following general guidelines are provided.

• The stiffer the restraint, the more effective it will be. A minimum restraint stiffness of 10,000 lbf/in in all three translational directions is recommended.

• A good restraint should be triangulated in multiple planes to provide stiffness in multiple directions. A restraint mounted to a tall, single slender support will typically be very flexible and ineffective in controlling vibration in directions other than the axial direction of the tall support.

• Simple weight supports and springs typically provide very little vibration control.

• It is typically preferred to brace the branch line back to the mainline piping. Bracing the branch line to a very stiff external support can increase bending stresses due to relative displacement from vibration and thermal expansion. A relatively stiff external support should not be installed too close to the branch connection.

• Strap type clamps are typically more effective than U-Bolt type restraints. If U-Bolts are used, they should be used in pairs to prevent rotation about the u-bolt.

• Gusset plates can add stiffness but also add high stress intensification factors and should be used with caution. If used, any gussets should attach to a reinforcing pad and not directly to the mainline.

• To prevent fretting, all clamps should be lined with a resilient liner material (Fabreeka or similar).