How to carry out quality control when installing process pipeline in chemical plant?

If the chemical plant is compared to a person, the equipment must be the “heart part”; the process pipeline for transporting raw materials, water, gas, finished products, etc. is the “artery”, which plays an important role in the production process. Therefore, at the beginning of project construction, it is necessary to strictly control the quality of process pipeline installation. If the quality is not good, it will greatly affect the normal production in the future. As a chemical construction unit, controlling the quality of process pipeline installation is the key to the survival of the installation enterprise, and also the guarantee that the production unit can “work safely” in the future.


To control the construction quality of process pipeline, we should control the whole process from the beginning of construction preparation to the end of purging and air tight test. We can divide the project into the following three stages:

Preparation stage

To understand the characteristics of the process construction of the project, and determine the construction difficulties and the main parts prone to quality problems, the following aspects shall be generally taken into consideration:

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1. Organization:
Establish the quality assurance system, organization and quality management system, define the quality responsibilities of each management department, and implement the responsible person; once the personnel of the project department are determined, they shall not be replaced before the completion of the project.
2. Technology:
Be familiar with the construction drawings carefully, understand the main functions of each equipment of the construction device from the flow chart, and be familiar with the medium, pressure, material, temperature, etc. of each process pipeline connected between the equipment, as well as the high temperature, high pressure, special material and special medium pipelines, so as to define the main contents of quality control and prepare the process pipeline construction of the project Main objectives of quality control.
When reviewing the construction scheme, clear and reasonable requirements shall be put forward for quality standards, construction methods of main projects and quality assurance measures.
3. Personnel training:
According to the pipeline design materials of the project, the welding procedure specification shall be prepared, the qualified welding procedure qualification meeting the specification requirements shall be made, the welder shall be trained and the welding qualification of the corresponding project shall be obtained.
4. Material procurement:
According to the material, specification, model and standard required by the material preparation plan, qualified material suppliers with processing and manufacturing capacity and manufacturing qualification shall be selected.
The material acceptance must meet the relevant requirements; the quality certificate shall be the original, the content shall be consistent with the physical object and the data shall be true to meet the standard requirements; the valve must also be subject to the hydrostatic test when arriving at the site, and all alloy steel pipes, pipe fittings, valves, etc. must be retested for chemical composition (spectral analysis); high pressure pipes, pipe fittings, valves must be 100% retested.
5. Acceptance of welding materials:

The appearance package shall not be damaged or damped, and the marks (model, brand, specification, production date, production batch number) on the package shall be consistent with the attached quality certification documents. If they are inconsistent, they must be rechecked to be qualified.

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Setup script

Before construction, engineering and technical personnel must conduct technical disclosure to construction personnel, mainly including construction methods, quality assurance measures, quality standards, etc.; in order to ensure the prefabrication quality, it can avoid the impact of weather and environment on construction, which is more conducive to the control of construction quality. During installation, the following main aspects shall be paid attention to:
1. Blanking and processing:
It mainly controls the length of pipe cutting, the straightness of incision, the cleaning of flash and burr, and the processing of groove; generally, the pipe cutting below DN100mm is carried out with grinding wheel cutting machine, the large caliber medium and low pressure pipe cutting is generally carried out with gas cutting or plasma cutting, and the groove can be polished by hand grinding wheel machine.
High pressure pipe cutting and beveling are generally completed by pipe bed or beveling machine; if the flatness of the incision exceeds the standard, it will affect the uniformity of the gap between the nozzle groups.
The groove processing should also be carried out according to the angle required by the regulations. If the groove is too large, it will cause waste of labor, materials and machinery, increase the installation cost, and the groove is too small, which is easy to cause welding defects such as incomplete penetration and slag inclusion. Therefore, the final purpose of the quality control of the blanking processing process is to ensure the welding quality and installation size.
2. Identification:
Each welded junction of prefabricated pipeline must be marked, especially alloy steel and special material pipeline.
Mark the welding date, welder number and crater number, and paint each pipe section with pipeline number and pipeline section number according to the single line diagram, so as not to cause confusion in installation, make the crater untraceable, cause the handover data inconsistent with the physical object, and make mistakes in the use position of special material pipeline, etc.; the prefabricated pipe section must be closed with the end nozzle.
3. Welding:
The welding quality control is the most important control link in the process pipeline construction, which requires the welder to strictly follow the welding operation instructions; there are many factors affecting the welding quality, in addition to the influence of nozzle assembly and groove, the welder’s quality, welding materials, weather and environment, etc;
The welding work is carried out in the workshop or on the ground, with little environmental impact and relatively good quality control, mainly controlling the baking quality of welding materials, strictly following the baking and distribution system, controlling the amount of each welding rod received by the welder, and supervising the normal use of the welding rod barrel;
If the process requires preheating and heat treatment of the welded junction, the preheating and heat treatment temperature of each junction must be strictly controlled, and the NDT of the backing quality shall be paid attention to for the welding of high-pressure pipe;
4. Installation:
The on-site installation of process pipeline is the difficulty of quality control, which is restricted by the on-site installation conditions and environmental factors. If necessary, appropriate quality assurance measures shall be taken;
The welding joints welded on site are generally fixed joints, and the quality is difficult to control, so it is necessary to control the following aspects:

  • In the prefabrication stage, each welder shall be observed and the qualification rate shall be counted, because human is the first element in the quality control link, and the quality of nozzle field assembly shall also be carried out according to the requirements;
  • In case of wind, snow, rain, high humidity and other weather, effective protective measures must be taken before welding is allowed;
  • It is very difficult for the alloy steel pipe to be charged with argon gas, so it is the weakest link of on-site quality control, and also the most important link of quality inspection. It can take flux skin or flux cored wire for backing, but this process must pass the welding process assessment and approval, and the welder has passed the examination and qualified.

Construction inspection

The quality of the project depends on inspection to a certain extent. As a necessary part of process pipeline installation, nondestructive testing, pressure test and purging are also a means of process completion, and also the inspection of construction quality. This requires that there must be a certain plan and arrangement, strictly in accordance with the requirements of the specification.
1. NDT
NDT comparison must be carried out in strict accordance with the requirements of the specification. The welded junction of each welder’s pipeline shall be inspected by sampling. If unqualified welded junction is found, it must be repaired and double sampling inspection until it is qualified. Otherwise, 100% of the welded junction of the welder shall be inspected. The hardness of the alloy steel pipeline shall be measured after heat treatment;
2. Pressure test.

  • A. Before pressure test: the pipeline installation works within the pressure test scope have been completed in accordance with the design drawings except for painting and heat insulation, and the installation quality meets the relevant regulations; the pressure gauge for test has been calibrated, and within the weekly inspection period, its accuracy shall not be less than level 1.5, the full scale value of the gauge shall be 1.5-2 times of the maximum pressure to be measured, and the pressure gauge shall not be less than 2 pieces; the pipeline to be tested has been separated by blind plates in unrelated systems, The safety valve, bursting plate and instrument elements on the pipeline have been disassembled; before pressure test, air shall be exhausted during liquid injection.
  • B. Pressure test can only be conducted after all pipes requiring NDT are qualified and approved by relevant departments and units.
  • C. During hydrostatic test, the ambient temperature should not be lower than 5 ℃, and anti freezing measures should be taken when the ambient temperature is lower than 5 ℃.
  • D. In the process of pressure test: the pressure shall be increased slowly in the hydraulic test. After the test pressure is reached, the pressure shall be stabilized for 10min, then the pressure shall be reduced to the design pressure, and the pressure shall be stopped for 30min. If the pressure does not drop and there is no leakage, it is qualified. For pipes with large alignment difference, the static pressure of the test medium shall be recorded in the test pressure.
  • E. After pressure test: remove the blind plate in time and drain the liquid. Negative pressure shall be prevented from forming during liquid discharge, and it shall not be discharged anywhere.

3. Blow wash.

  • A. The purging scheme shall be prepared before purging, and the specific purging method shall be determined according to the use requirements of the pipeline, the working medium and the degree of dirt on the inner surface of the pipeline. Steam pipes shall be purged with steam; non thermal pipes shall not be purged with steam.
  • B. For pipes with special requirements, corresponding purging methods must be adopted. The equipment and pipes that are not allowed to be purged shall be isolated from the purging system; the purging sequence shall be carried out in sequence according to the main pipe, branch pipe and drain pipe, and the purged dirt shall not enter into the qualified pipeline.
  • C. When flushing, the maximum flow rate should be adopted, and the flow rate should not be less than 1.5m/s; the water flushing should be carried out continuously, and the water color and transparency of the outlet should be consistent with the visual inspection of the permeate mouth.
  • D. Before purging the steam pipe, warm up the pipe, drain the water in time, and check the thermal displacement of the pipe; the purging sequence shall be carried out in the order of heating cooling reheating. At the same time, the flow rate shall not be less than 3m / s.
  • E. After the pipeline is purged to be qualified and reset, no other work affecting the pipe cleaning shall be carried out.


The quality of a project determines the service life of the project. A good owner evaluation may set up a good image for the construction company in the market, and a good image can bring a good development space.
In the increasingly fierce market competition environment, quality will be a very important part of many weights. Only by strictly controlling the quality can we have greater development.
Source: China Steel Pipeline Manufacturer – Yaang Pipe Industry Co., Limited (

(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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