The pipelines of the three-stage oil and gas recovery equipment do not carry dry gas, but rather oil and gas mixed with condensate. When gasoline vapor flows along the pipeline, it will precipitate out liquid oil droplets when it encounters cold. If these condensate cannot be discharged, they will accumulate at the low points of the pipeline, blocking the air passage, rusting the pipe wall, and possibly freezing and cracking the joints in winter. The entire significance of pipeline slope design is to ensure that the condensate always has a path to follow and flows back to the storage tank smoothly, without settling halfway. I have seen many stations on site with inexplicably low recovery efficiency, and after investigation, the problem was found to lie in a section of horizontally laid pipeline without any slope. This article discusses how to determine the slope, how to handle low points, and how to conduct acceptance inspection during installation.
Three core principles of slope design
Firstly, the condensate must flow towards the storage tank: the recovery pipeline, from the fuel dispenser end to the tank area, should maintain a downward slope towards the storage tank throughout the entire route, allowing the condensate to flow back by gravity without relying on any power. Secondly, "pocket" formations are not allowed throughout the entire route: local low points formed when the pipeline climbs over obstacles are where condensate can accumulate, and these should be avoided during the design stage by adjusting the direction or burial depth. Thirdly, the slope gradient should be set with a margin: the value indicated on the design drawings is subject to construction setting-out and backfill settlement, so the gradient should be set slightly steeper than theoretically required to allow for construction errors.
Five steps for slope design and installation
- Survey and determine the direction: First, confirm the relative height and distance between the fuel dispenser end and the tank farm vent, and plan the shortest pipeline route with minimal climbing;
- Determine slope by section: Draw a slope map for each section, specifying the slope direction and slope value. The slope direction must uniformly point towards the tank side;
- Avoiding pocket-shaped low points: When obstacles must be crossed, take an arc-shaped route and ensure that the highest point does not cause suffocation, and the condensate at the lowest point can still return to the tank;
- Construction setting-out review: During laying, use a level gauge to review the actual slope section by section. Take photos before backfilling for preservation, and don't wait until it's buried to verify;
- Ventilation and flow test acceptance: After installation, conduct a ball passing or ventilation test to confirm that the pipeline is unobstructed and there are no liquid accumulation points, before handing over for operation.
| Control link | What to do | Frequently Asked Questions |
|---|---|---|
| Towards planning | Shortest path, minimal climbing, and uniform slope orientation refer to the tank | To save trouble, we detoured along the cable trench, crossing many hills and low-lying areas |
| Slope value | Mark the slope direction and numerical values segment by segment, leaving a margin | Only marked "slope making according to specifications", construction is at a loss |
| Pocket avoidance | Adjust the direction or burial depth to eliminate local low points | Overcoming obstacles forms a pocket shape, and the accumulated condensate increases |
| Setting out and review | Recheck the level meter section by section and take photos for preservation | Laying by hand, and unable to verify after backfilling |
| Flow passage acceptance | The ball passing or air passing test confirms that there is no trapped liquid | Only ensuring air tightness without ensuring flow passage, putting the equipment into operation with hidden dangers and defects |
| Completion archiving | Put the slope aspect map and test records into the data box | There will be no records to refer to for future investigations |
How to determine if there is a problem with the slope after operation
Slope defects may not manifest immediately but may reveal themselves after a period of operation: slow declines in recovery efficiency, abnormal fluctuations in pump loads, repeated leaks or corrosion at a fixed point in the pipeline, all warrant suspicion. The method of diagnosis is straightforward: open an inspection port at the suspected low point or feel the temperature difference with your hand. The temperature sensation at the condensate accumulation section and the normal section will be different; this is more pronounced in winter, when freezing occurs first at the low point. Once the problem is confirmed, the preferred solution is to "re-establish the slope". For sections with significant changes, a short condensate recovery pipe can be added to direct the accumulated fluid back into the system. For newly built or renovated stations, include the slope diagram, setting-out records, and ball passing test records in the completion documents. This will save a lot of detours during future after-sales troubleshooting. For stations where the situation is uncertain, provide the pipeline routing diagram to the after-sales personnel of Luoyang Wohong Petrochemical Equipment Co., Ltd. They can conduct a remote written consultation before deciding whether to visit the site.
Frequently Asked Questions
What is an appropriate slope gradient?
The specific value is related to the pipe diameter, condensate volume, and pipe section length, and it is not appropriate to provide a universal number without considering the site conditions. The principle is to prefer a slightly steep slope rather than a gentle one, and each section must have a clear slope direction. It is recommended that surveyors mark each section on the slope map according to the site conditions, and the construction team should set out and review according to the map.
What should we do if we suspect there are low points in the old pipelines that have already been buried underground?
First, confirm the symptoms: Is the decrease in recovery efficiency concentrated under certain refueling conditions, or is there leakage at a specific location? After confirmation, decide whether to perform local excavation to re-establish the slope or add a condensate recovery short pipe for drainage. It is not recommended to excavate extensively based on intuition. Diagnose first, then act.
Which is more important, slope or pipe diameter?
The two are two sides of the same coin: the pipe diameter determines the gas flow rate and resistance, while the slope determines the direction of condensate. If the pipe diameter is too large, the flow rate will be insufficient, and condensate is more likely to accumulate. Therefore, the pipe diameter and slope should be determined together during the design stage and cannot be decided separately.
This article is compiled by Luoyang Wohong Petrochemical Equipment Co., Ltd. The company provides design and manufacturing, skid-mounted customization, installation and commissioning, as well as after-sales operation and maintenance services for three-time oil and gas recovery equipment to gas stations and oil depots. If you need on-site inspection or scheme configuration, please feel free to contact us for discussion.