Zhao Meigang
Daqing Oilfield Engineering Construction Company Building Materials Company
Summary:This document outlines the quality issues encountered during the production of epoxy powder anti-corrosion pipes, and subsequently analyzes the causes from both the process and equipment aspects. The ultimate goal is to identify solutions to improve product quality.
Keywords:Epoxy powder quality control improvement measures
Discussing the quality control of epoxy powder inner anticorrosive pipes during production.
ZHAO Mei-gang
(Daqing Oilfield Engineering Construction Company, Building Materials Company, Daqing 163000, China)
Abstract: This article describes the quality issues encountered during the production of epoxy powder internal anticorrosion tubes, analyzes the causes from the process and equipment, and ultimately proposes solutions to improve product quality.
Key words: epoxy powder; quality control; corrective actions
0 Introduction
Epoxy powder anti-corrosion pipe, utilizing solid fused epoxy powder applied to the inner surface of a steel pipe in a single coating process. This product offers advantages such as easy application, zero pollution, and excellent impact and bending resistance. It is widely used in oilfield gathering and injection projects, as shown in Figure 1.
1. Production Process for Epoxy Powder Anti-Corrosion Pipes
1.1 Main Production Equipment
Air compressor, epoxy powder control cabinet, epoxy powder fluidized bed, Venturi pump, powder spray gun, medium-frequency heating coil.
1.2 Production Process Flow
By adjusting the air pressure for the epoxy powder delivery and air supply in the spray booth, epoxy powder is delivered from the fluidized bed through the spray gun and applied to the inner surface of the steel pipe that has been preheated by microwave heating. The epoxy powder melts upon heating and forms a uniform anti-corrosion coating on the inner surface of the steel pipe. The process flow diagram is shown in Figure 2.
2. Quality issues associated with epoxy powder anti-corrosion pipes
After internal technical personnel conducted a survey to determine the frequency of non-compliance issues with epoxy powder anti-corrosion pipes in the previous year, they created a table similar to Table 1.
As shown in Table 1, "severe orange peel" has the highest frequency of defects, accounting for 58.10% of the total defects. This defect is the primary quality issue that leads to a lower pass rate for the epoxy powder anti-corrosion pipe, as shown in Figure 3.
3. Cause Analysis
Internal technical staff analyzed the existing production process and identified the following three reasons that lead to "severe orange peel" quality issues in the epoxy powder anti-corrosion pipes:* Insufficient mixing of the epoxy powder* Improper application of the epoxy powder* Inadequate curing conditions
3.1 The quality of steel pipe rust removal is not up to standard.
Our company uses compressed air to propel abrasive sand for blasting rust off steel pipes at both ends. For small-diameter steel pipes, the blasting nozzle cannot reach deep inside the pipe to perform rust removal, resulting in insufficient rust removal depth and non-compliance with standard requirements. Additionally, the friction and abrasion of the abrasive sand against the inner wall of the pipe during high-speed impact generate a large amount of dust, which not only pollutes the environment but also increases the dust level on the inner wall of the pipe, leading to serious orange peel appearance and adhesion issues in the epoxy powder coating.[1].
3.2 Improper spray gun structure
The existing spray gun consists of a spray nozzle and a stainless steel tube. The spray gun head is a direct-through type. The epoxy powder is sprayed along the direction of the steel tube, and the powder is dispersed within the internal space of the steel tube, rather than directly applied to the inner wall of the steel tube. Due to the excessive spray distance and the small contact area with the inner wall of the steel tube, the coating thickness becomes uneven, resulting in severe orange peel defects.
3.3 Mids-frequency heating coil mismatch
The current steel pipe preheating process involves using a medium-frequency coil to heat the steel pipe. The working principle of the medium-frequency heating device is as follows: The medium-frequency equipment converts three-phase high-frequency AC power into DC power, and then converts the DC power into a controllable medium-frequency current, which is supplied to the medium-frequency alternating current flowing through the capacitor and the induction coil (i.e., the medium-frequency heating coil). This generates heat and a high-density magnetic flux in the induction coil, and cuts the metal pipe material within the coil. The resulting eddy currents in the metal pipe material cause the atoms to move rapidly and randomly, resulting in heat generation through atomic collisions and friction, thereby achieving the purpose of heating the steel pipe.[2]As shown in Figure 4.
According to Faraday's law of electromagnetic induction, the number of turns in the induction coil directly affects the magnitude of the induced electromotive force. For example, when a steel pipe with a specific specification and induction coil is heated at a constant speed, the cross-section of the steel pipe can be considered as a closed coil. The rate of change of magnetic flux is determined by the frequency of the medium-frequency power supply and the magnetic induction strength in the closed circuit. If the size of the induction heating coil is too large, the magnetic induction strength on the inner wall of the steel pipe becomes weaker, resulting in a smaller induced electromotive force. The rate at which the inner wall of the steel pipe heats up is slow, leading to significant heat loss, which prevents the powder from reaching the required temperature for solidification. If the powder does not fully solidify, it can cause severe "orange peel" defects.
4. Remediation Measures
Through an analysis of quality issues in epoxy powder anti-corrosion pipes, three aspects of the equipment were identified as the root causes. To improve the quality of epoxy powder anti-corrosion pipes, the following three corrective measures were implemented by technical personnel:
4.1 Shot Blasting for Steel Pipe Cleaning Replaces Sandblasting
We procured blast cleaning machines for interior steel pipe surfaces through our in-house equipment department. These machines utilize abrasive blasting with steel shot to remove rust, replacing the previous process that used abrasive blasting with quartz sand.
This equipment can insert a spray gun and a blasting gun deep into the inner wall of the steel pipe. It uses compressed air to blast steel shot onto the inner wall of the pipe, and through blasting cleaning, it can remove oxidation, rust, and other foreign matter from the inner surface of the steel pipe, making the inner surface of the steel pipe smooth and achieving the cleaning purpose. Since the hardness of the steel shot is much greater than that of quartz sand, it is not easily pulverized during blasting, which reduces the generation of dust. The depth, cleanliness, and dust level of the anchor patterns on the inner wall of the steel pipe can also be well controlled, thereby ensuring the rust removal quality of the inner wall of the steel pipe.
4.2 Adopt a fan nozzle instead of a direct-spray nozzle.
To increase the contact area between the epoxy powder and the inner wall of the steel pipe, and to ensure that the powder is sprayed onto the inner wall of the steel pipe at a specific angle, the technical staff modified the existing direct-spray gun and added a trumpet-shaped diffusion head. The inner wall of the spray gun head should use a three-point support to ensure stability while also allowing the epoxy powder to be smoothly ejected from the gun head. See Figure 5.
4.3 Replace with appropriate low-frequency heating coil
Currently, the induction heating coils used in the factory have uneven coil spacing and excessively large coil sizes. This results in low actual heating efficiency and difficulty in achieving the required temperature for the powder on the inner wall of the steel pipe. To address this, technical personnel calculated and statistically determined the optimal coil specifications, and the equipment department procured different specifications of induction heating coils matched to the diameter of the heating steel pipe. This reduces heat loss and ensures that the steel pipe reaches the required preheating temperature, as shown in Figure 6.
5. Conclusion
Through the equipment modification and upgrade of our steel pipe rust removal system, powder coating system, and medium-frequency heating system, we have effectively reduced the occurrence of "orange peel" quality issues in epoxy powder-coated anti-corrosion pipes, thereby improving the pass rate of epoxy powder-coated anti-corrosion pipes. This ensures the quality of our epoxy powder-coated anti-corrosion pipe products, enhances the competitiveness of our company, and increases customer satisfaction.
References:
[1] SY/T 0442-2018, Technical Standard for Anti-Corrosion Layer Technology of Steel Pipe Welding Epoxy Powder.
[2] Yu Guoping, Xu Hao. Application of Mid-Frequency Induction Heating Energy-Saving Technology in Forging Production Lines [J]. Forging and Stamping, 2013, (17): 81-83.
The article was published in "Comprehensive Corrosion Control" Volume 34, Issue 2, 2020.