Basic knowledge of pressure piping inspection

Industrial pipeline on-line inspection, what parts should be focused on inspection?

  • (1) the outlet of the compressor and pump;
  • (2) compensator, tee, elbow (elbow), size head, branch pipe connection and dead angle of medium flow, etc.
  • (3) pipe components and welded joints near the damaged parts of the hanger.
  • (4) there have been problems affecting the safe operation of pipelines.
  • (5) the pipe section in the key part of the production process and the pipe section connected with the important device or equipment;
  • (6) pipe sections with harsh working conditions or subjected to alternating loads.

2 Why piping stress analysis? What does it contain? What are the purposes of various analyses?

  • Ensure the safety of the pipeline itself.
  • Ensure the safety of connected equipment;
  • Ensure the safety of civil structures.
  • It mainly includes static analysis and dynamic analysis.

Static analysis includes:

The first stress calculation of pipes under pressure, gravity and other loads – to prevent plastic deformation and failure;
Secondary stress calculation under thermal expansion, cold contraction and additional displacement at the end point to prevent fatigue failure.

  • The calculation of the force acting on the machine and equipment by the pipeline – preventing the excessive force and ensuring the normal operation of the machine and equipment;
  • The force calculation of pipe supports and hangers provides the basis for the design of supports and hangers.
  • The force calculation of flange on pipeline to prevent flange leakage;
  • Piping displacement calculation to prevent pipeline collision and excessive displacement of supporting points.

The dynamic analysis includes:

  • Natural frequency analysis of gas (liquid) column in reciprocating compressor (pump) pipeline – preventing gas (liquid) column resonance;
  • Pressure pulsation analysis of reciprocating compressor (pump) piping – control pressure fluctuation value;
  • Analysis of natural frequencies of pipelines to prevent piping system resonance;
  • Pipeline seismic analysis to prevent excessive seismic stress of pipelines.
  • Stress analysis of pipelines under impact loading – to prevent pipe vibration and stress.

3 What is a primary stress? The two stress? What loads are generated by them? What are the characteristics of these two stresses?

The primary stress is the stress generated by the interaction of pressure and gravity with other external loads. It is the stress required to balance external loads and increases with the increase of external loads.
Characteristics: there is no self limiting nature.
Secondary stress is caused by the constraints of pipeline deformation. It is caused by the thermal expansion, cold contraction and end displacement of pipeline. It is necessary to meet the continuous requirements of constraints or the deformation of the pipeline itself.
Characteristics: self limiting.

4 What is pipeline flexibility? How to design flexible pipes?

It is a concept that reflects the degree of difficulty of pipeline deformation, indicating the ability of pipeline to absorb heat account, cold shrinkage and other displacement deformation through its own deformation.
In design, it is necessary to ensure that the pipeline is flexible enough to absorb displacement and strain, so that the length of the pipeline is as short as possible or the investment is as small as possible.

Generally, the following methods are used to increase the flexibility of pipes:

  • Change the direction of the pipeline;
  • Waveform compensator;
  • Choose spring hangers.

5 What is the purpose of flexible piping design?

Ensure that the pipeline is flexible enough under the design conditions to prevent the following problems caused by cold shrinkage of heat account, additional displacement of end point, improper installation of pipeline support, etc.

  • Pipeline failure caused by excessive stress or metal fatigue.
  • Leakage occurs at piping connections.
  • Excessive pipe thrust or torque will cause excessive stress and deformation of the equipment connected with the pipeline and affect the normal operation of the equipment.
  • Piping failure is caused by excessive pipe thrust or moment.

6 Generally speaking, which point of stress on the pipe is larger? Why?

Generally speaking, the stress on the three way and elbow pipe is larger. Because, compared with straight pipe, the stress increasing coefficient of the three way and elbow pipe is larger.

7 What displacement can the fixed bracket, the guide bracket and the supporting bracket limit?

There are three functions of pipe hangers.
The weight load of the pipeline (including self weight, medium weight, etc.);
Limit the displacement of the pipeline to prevent unexpected displacement of the pipeline.
Control vibration to control swing, vibration or impact.
Fixed frame: limiting the linear displacement in three directions and the angular displacement in three directions;
Guide frame: the displacement in two directions is limited.
The bracket (or one-way thrust frame) restricts the linear displacement in one direction.

8 What problems should we pay attention to when designing vibrating pipe supports?

The bracket should use anti vibration pipe card, not simply supported.
The distance between supports should be determined by vibration analysis.
The structure of scaffolds and the rooting parts of scaffolds should have enough stiffness.
It is advisable to set up an independent foundation so as to avoid rooting in the beams and columns of the workshop.
When the temperature of the medium is high and the thermal expansion occurs, the flexibility requirement should be satisfied.
Support should be set along the ground.

Source: China Stainless Steel Pipelines Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)

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