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:
- Excessive Pulsation
- Mechanical Resonance
- 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.
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|
- 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.
- Acoustic/pulsation – time-varying changes in fluid pressure
- Momentum – time-varying changes in fluid density and velocity
- Machinery – unbalanced forces from operating machinery
Three main sources of piping vibration
HOW CAN I ADDRESS 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
Small-bore connections (SBCs) and branch attachments connect to the main process piping. These
Figure 2: Piping failure examples 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.
The table below summarizes when each study is required:
|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
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.)
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