Dangers and main causes of piping vibration

WHAT IS PIPING VIBRATION?

Piping vibration is simply the movement of pipe away from its static, at-rest position. Some vibration can be seen by the naked eye, some vibration can be felt or heard, and some vibration occurs only occasionally under certain operating conditions, and may not be recognized until a piping failure has occurred.

WHY SHOULD I CARE ABOUT PIPING VIBRATION?

21% of hydrocarbon releases are due to vibration-induced fatigue failures (UK Health & Safety Executive). These releases can have a significant impact on public and employee safety, the environment, production and financial performance.

The piping system is by far the leading source of failures in facilities (Marsh & McLennan). This is in part due to the sheer quantity of piping in facilities, but also due to inadequacies in integrity programs to consider vibration and fatigue. The situation is compounded by the fact that commonly used design codes do not consider vibration in detail.

Overall, only a small portion of piping is of high risk of failure, but identifying those high-risk locations is the challenge. Piping vibration risks can be identified at any stage of the asset lifecycle, but few companies have a systematic approach to evaluating these risks.

many causes that lead to vibration in a piping system. Here are the most common causes:

Most Common Causes of Vibration in Piping:

  1. Excessive Pulsation
  2. Mechanical Resonance
  3. Inadequate Supports and/or Support Structure

Factors that contribute towards the above causes are:

  • Flow induced: Turbulence of flowing liquid causes Flow-induced vibration.
  • Equipment mechanical forces: Rotating and reciprocating equipment like pumps, compressors etc produce excitation forces.
  • High frequency acoustic vibrations generated by relief valves, control valves or orifice plates.
  • Pressure Pulsations from reciprocating equipment.
  • Water Hammer (Surge) or Momentum changes due to sudden valve closure.
  • Cavitation/ flashing caused by vapor bubble collapse
  • Sudden flashing of fluid.

Since the main factors affecting vibration are the strength of excitation and the flexibility of the piping system, there are two methods to solve piping vibration issues – either by reducing the level of excitation or providing more support to withstand the applied dynamic forces.

Effects Of Piping Vibration And Associated Risks:

Vibration can result in equipment damage, fatigue failure on process piping, and also cause fatigue in small branch connections including relief lines, instrumentation ports, nozzles, drains, and valves. The result of vibration on compressor and pump packages also causes reliability issues.

Identifying Risks:

A high number of pipe failures due to vibration are related to small-bore connections (SBCs). This is because of several reasons:

  • Huge unsupported valves
  • Concentration of stress concentrations at the main pipe weld or other vessel
  • Lack of timely vibration audit and fatigue risk
  • Gaps between the SBC design intent and site fabrication especially local support/bracing

In order to create safe and secure piping systems, free from unwarranted vibration, the individual piping components must not be mechanically resonant to the system excitation forces. Also, ensure there are no unnecessary bends as they offer a strong coupling connection between the mechanical system and pulsation excitation forces.

Engineers can identify piping vibration risks at any phase in the asset life cycle, but companies need to take a systematic approach. Taking a preventive approach with piping vibration analysis such that its potential impact can be resolved at the design phase.

WHAT PIPING IS MOST AT RISK?
Small bore connections are high risk locations for piping vibration - Dangers and main causes of piping vibration
Small-bore connections are high-risk locations for piping vibration

Around 80% of vibration-induced failures are associated with small-bore connections (SBCs). This is due to several reasons:

  • Stress concentrations at the weld to the main pipe or vessel
  • Large unsupported valves
  • Lack of evaluation of vibration and fatigue risk, beyond a reactive approach after a failure
  • Disconnects between the SBC design intent and site fabrication, particularly regarding local support/bracing

The remaining 20% are generally associated with parent pipe girth weld failure.

WHERE DOES PIPING VIBRATION COME FROM?

Two main factors affect vibration: the strength of the excitation and the flexibility of the piping system. Therefore, there are always two methods to solve piping vibration problems: reduce the level of excitation to which the piping system is exposed or support the piping system so that it can withstand the applied dynamic forces.

The excitation mechanisms that cause vibration originate from three main sources:

  1. Acoustic/pulsation – time-varying changes in fluid pressure
  2. Momentum – time-varying changes in fluid density and velocity
  3. Machinery – unbalanced forces from operating machinery

Three main sources of piping vibration

ACOUSTIC/PULSATION MOMENTUM MACHINERY
  • Dead leg pulsations (FIV)
  • Acoustic-induced (AIV)
  • Pulsations from rotating machinery
  • Rotating stall
  • Flow-induced turbulence (FIT)
  • Water hammer
  • PSV relief load
  • Multiphase or slug flow
  • Cavitation and flashing
  • Mechanical excitation from vibration
    transmitted to attached piping
HOW CAN I ADDRESS PIPING VIBRATION?
Typical vibration risks in a piping system   small - Dangers and main causes of piping vibration
Figure 1: Typical vibration risks in a piping system

Risk-based assessments are the most cost-effective and reliable method to identify, quantify, inspect, measure and mitigate vibration and fatigue risks. These assessments look at the vibration integrity problems a facility faces, from start-up to shut-down.

The risk-based approach allows for a proactive management of vibration, only where there are risks present. Risk-based assessments integrate with and complement conventional integrity management programs that typically focus on corrosion/erosion – saving time and money for operators.

Design and in-service inspection standards typically highlight the risk posed by vibration but fail to provide appropriate management strategies, often leaving the identification of vibration issues to operators.

PIPING VIBRATION RISKS IN ONSHORE AND OFFSHORE FACILITIES

Gas Plants, Refineries, Pipelines, Pumping and Compressor Stations

There are a number of common piping risks to be evaluated in a piping assessment:

  • Small-bore connections (SBCs) and branch attachments connect to the main process piping. These 

    Piping Failure Example1 - Dangers and main causes of piping vibration

    Cracked Vessel - Dangers and main causes of piping vibration

    Figure 2: Piping failure examples
    Piping Layout on an FPSO - Dangers and main causes of piping vibration
    Figure 3: Example of compact piping layout on an FPSO

     small attachments, typically less than 8 cm (3 inches) in diameter, are the most common cause of integrity problems. Even if the main process piping has acceptable vibration, the vibration can be amplified on SBC causing failures. For large facilities, there can be thousands of SBCs that pose this integrity risk.

  • Process piping vibration can cause excessive vibratory stress on nozzles and tees leading to cracks.
  • Failure of bypass lines, PSV or relief lines.
  • Transient related events such as starting, stopping, emergency shutdown, or closing and opening valves can cause momentum changes in the gas or liquid (fluid hammer), resulting is excessive stress.
  • Fretting and damage to pipe supports.  
  • Pipe stress analysis can be in conflict with vibration design requirements. Unless the design resolves the conflict between Mechanical Vibration Design (adding stiffness to control vibration) and the Piping Stress Analysis (increasing flexibility for thermal analysis), there are risks that the piping system will experience stress failures.
  • Figure 3 example of compact piping layout on an FPSO
  • Space on an offshore production facility is limited, and the piping layout is often very compact. As shown in Figure 3, the tight layout creates unique challenges in controlling piping vibration.
  • The piping system is often elevated, connecting rotating machinery to overhead coolers, vessels, or headers. Elevated piping is typically much more flexible than rigidly connected or buried piping because of the difficulties in designing sufficiently stiff piping supports (that prevent vibration).
  • Safety requirements often require “double block and bleed” valves on many small-bore connections. The geometry of these valve configurations and the heavy overhung weight creates much higher chances of excessive stress and failure.

OVERVIEW

Below is an overview of the Piping Integrity Assessment, which includes an Energy Institute AVIFF assessment, static stress analysis, and field vibration audit:

piping inegrity assessment overview chart - Dangers and main causes of piping vibration

The table below summarizes when each study is required:

TYPE ANALYSIS SYSTEM WHEN REQUIRED
Energy Institute
AVIFF Assessment
Small-bore connection (SBC) All fluids All connections not reinforced or braced
Flow-Induced Turbulence (FIT) All fluids High flow systems with flexible and infrequent supports
Flow-Induced Vibration (FIV) Gas systems only High flow systems with dead legs
Acoustic-Induced Vibration (AIV) Gas systems only Pressure-reducing devices like valves and orifice plates
Water hammer Liquid systems only Fast acting valves and emergency shutdown events
Momentum change Gas systems only Blowdown or PSV releasing events
Cavitation/flashing Liquid systems only Pressure-reducing devices like valves and pumps
Static Stress Analysis Piping stress analysis All fluids High temperature/pressure variation
Piping External Loads All fluids Local specifications and regulations (e.g., seismic zone)
Restraint and Structure Design All fluids Flexible supports required, or high risk of piping vibration
Machinery and equipment nozzle load analysis All fluids Machinery and equipment with low allowable nozzle loads
Advanced Studies Compressor surge control Centrifugal compressor systems Low inertia, high-pressure ratio, complex systems
Shell Transverse Acoustical (STA) All fluids (but gas systems typically) Thin-walled pipe near compressors and pumps
Pulsation analysis Reciprocating compressor systems High pressure, high power systems
Piping Dynamic Stress Analysis Reciprocating machinery systems High pressure, high power systems
Transient pulsation Liquid systems only Many operating scenarios with multiple units on and offline
Field Evaluation Piping vibration (steady-state) All fluids Always
Mechanical Natural Frequency (MNF) Testing All fluids Areas of high vibration or poor support
Piping vibration (transient) All fluids When transient events cause high vibrations
Pipe strain All fluids When suspect due to high-frequency vibrations
Small-bore connection vibration All fluids All connections not reinforced or braced

Dangers of piping vibration

The rotating of the equipment and the medium flowing in the pipe will cause mechanical vibration with low-intensity. The dangers of piping vibration are mainly as the following.
Harm to the worker
Pipeline vibration will interfere with the vision of the worker and make the worker have poor efficiency. The worker will feel fired, resulting in accidents. Working in a vibrating environment with considerable strength for a long time may cause greater harm to the worker.
Harm to the building
The building structure of some buildings will be damaged due to the difference in vibration strength and frequency of the pipeline. Common damage such as cracks of foundation and walls, peeling of the wall, sliding of the stones and even deformation of the foundation of the building will occur.
Harm to the the precision instrument
Pipe vibration will affect the normal operation of the precision instruments, accuracy and reading speed of the scale reading of the instrument, and even causing the scale reading not to be read. If the vibration is great, the service life of the instrument will be directly affected and the instrument will even be damaged.
Harm to the system’s main equipment
Long-term pipeline vibration causes uneven output of the main equipment of the system, affecting the mechanical performance and normal operation of the main equipment.

Source: China Seamless Steel Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@steeljrv.com

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Reference:

  • http://www.betamachinery.com/services/piping-vibration-and-integrity-assessment

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