How to achieve leakage monitoring of double-layer composite pipes?

How to achieve leakage monitoring of double-layer composite pipes? Direct answer: By relying on the annular monitoring layer between the inner and outer layers, the oil leaked from the inner layer enters the gap and is captured by monitoring methods. One method is to fill the gap with dry nitrogen or vacuum to maintain pressure, and if the pressure or vacuum degree continues to decrease, it is judged as leakage; Another method is to install liquid sensors in the gap, which will sound an alarm when the oil is immersed in the probe. The monitoring layer transforms the "invisible underground leakage" into a "visible curve and alarm", which is the greatest value of the double-layer structure compared to the single-layer structure. This article explains the principles, applicability, and installation precautions of the two methods clearly.

Monitoring layer: gap between inner and outer layers

The double-layer composite pipe is a "pipe in pipe" structure: the inner layer transports oil, the outer layer provides protection, and there is a circular gap between the two layers, which is the monitoring layer. The leakage of a single-layer pipe can only be discovered through ground patrols or oil account discrepancies, and by the time it is discovered, the leakage often has been ongoing for a long time; The monitoring layer of the double-layer tube exposes the leakage signal in the first time, and the response is reduced from "month" to "hour". The monitoring layer also has a secondary function: even if the inner layer leaks, the outer layer can temporarily trap the oil and buy time for disposal.

Method 1: Pressure and Vacuum Monitoring

The method of dry monitoring is to evacuate or fill the gap with micro positive pressure inert gas and maintain the pressure. The monitoring end reads the pressure value in real time: if the value continues to decrease, it indicates that the gap has been invaded by oil or external moisture, and the system will sound an alarm. This method covers the entire pipeline, and any leakage at any location will be reflected on the pressure curve, and the leakage amount can be roughly estimated by the pressure drop rate. The key to installation lies in sealing: the gap sealing between the two end heads, tees, and flanges must be reliable, otherwise the pressure cannot be maintained and the monitoring system will report false alarms for a long time. The vacuum pressure holding method has stricter sealing requirements and is often used for stations with high-grade requirements.

Method 2: Liquid Sensor

The sensing method involves placing liquid sensor probes at intervals in the gap, and triggering an alarm when the oil flows along the gap to the probe position. The probe spacing is arranged according to the risk points: the joints, tees, elbows, and valve sections are encrypted, and the straight sections are regularly spaced. The advantage of this method is that it does not rely on pressure holding and is more tolerant of sealing defects; The limitation is that small leaks between probes may delay detection and require regular readings of probe status. Two methods can also be combined: main line pressure monitoring and installation of sensing probes at high-risk points.

Key points for installation and maintenance

Whether the monitoring system is installed well or not, half of the installation is done: the sealing of the head is solid, and a pressure holding test is conducted before the pressure monitoring is put into use, recording the initial pressure value as the baseline; Sensors should be constructed according to the diagram, and cables should be routed through dedicated slots to avoid mixing with power cables. During the operation period, the monitoring readings should be included in the inspection table, and slow pressure drops should be taken seriously - a continuous drop is a leakage signal, don't wait until it reaches the alarm limit to handle it. The disposal sequence after the alarm: first locate the leakage section, then arrange excavation and repair, and after repair, retest the pressure and restore the baseline.

Comparison of two monitoring methods

Alarm handling steps

1. Confirm the alarm signal, eliminate false alarms and communication failures;
2. Check the pressure gradient along the line or isolate and locate in sections;
3. Notify the station to stop using relevant pipelines and alert them;
4. Excavate to confirm the leakage points and degree;
5. After repair, retest the pressure to restore the baseline value;
6. Record and archive events, analyze root causes.

Frequently Asked Questions

Can single-layer pipes be equipped with leakage monitoring?

It can be installed later, and the mainstream approach is to lay sensing cables on the outer wall of the pipeline or set observation wells according to intervals, but it is a remedial plan after the fact; Priority should be given to building new stations with dual layers and monitoring layers in one step.

Is it important that the monitoring pressure slowly decreases but there is no alarm?

It's important. Continuous decline indicates that the gap state is changing, which may be due to seal aging or minor leakage. Follow the leakage investigation process once and do not wait until it falls to the alarm limit.

Do monitoring systems need to be maintained separately?

To be included in monthly maintenance: check the stability of readings, probe status, and cable integrity. Conduct a pressure test once a year to verify the baseline. Monitoring system failure is equivalent to half the value of the double-layer tube being lost.

This article is compiled by Luoyang Wohong Petrochemical Equipment Co., Ltd., which provides design, manufacturing, and operation and maintenance services for condensation recovery, membrane separation recovery, and complete solvent recovery equipment to chemical and pharmaceutical enterprises. If you need process evaluation or scheme configuration, please feel free to contact us for discussion.