Vapor recovery piping is often treated as a simple extension of the liquid piping, and on many units that judgement is close enough. Where the vapor stream is wet, warm and intermittent, however, the material and layout choices begin to matter more than they do on a dry, steady line. The service conditions that drive those choices are moisture, temperature cycling, and the presence of any aggressive component in the vapour.
The service conditions that matter most
Four conditions decide most of the material questions. Moisture content determines whether condensate will be present and where it will collect. Temperature range and cycling determine thermal movement and the loads on supports. Hydrocarbon composition determines compatibility with sealing materials and any elastomeric components. Where the stream includes an aggressive component, that governs the metal or lining choice regardless of the other factors.
Where moisture changes the decision
Wet vapour produces condensate in the low points of any line that is not adequately sloped or drained. The condensate is often more aggressive than the vapour itself, and it sits in exactly the places where inspection is most difficult. Sloping the line back to a collection point and providing proper drains addresses most of this, but the material at those locations should be selected in the knowledge that liquid will be present.
Selection factors and their influence
| Factor | Why it matters | What it influences |
|---|---|---|
| Moisture and condensate | Liquid collects at low points and drains | Drain provision, low point materials |
| Temperature range | Cycling imposes movement and load changes | Support arrangement and expansion handling |
| Vapour composition | Compatibility with seals and linings | Gasket and elastomer selection |
| Aggressive components | Drives corrosion allowance or lining choices | Base material or internal lining |
| Line gradient | Determines where liquid ends up | Layout and drain positions |
| Accessibility | Piping in trenches or congested areas | Joint type and inspection points |
Layout decisions that reduce material demands
- Slope all vapor lines continuously back towards a collection point;
- Place drains at every low point and at the end of each run;
- Minimise pockets where liquid can sit without draining;
- Support the line to allow the expected thermal movement without imposing stress;
- Locate joints where they can be inspected and maintained;
- Record the as-built gradient so future work does not accidentally create a pocket.
Joints, seals and the parts that age first
On most vapor lines the piping material outlasts the joints and seals around it. Gasket and elastomer selection should be made against the actual vapour composition and temperature rather than inherited from a standard specification, and the number of joints should be minimised on routes where access for maintenance is limited.
Frequently asked questions
Is stainless piping necessary for vapor recovery lines?
Not by default. It becomes justified where the vapour contains an aggressive component or where condensate chemistry demands it. For ordinary hydrocarbon vapour, the choice depends on the specific service conditions.
How important is the line gradient?
Very. A line that drains properly avoids standing condensate, which is the single biggest driver of localised corrosion and of liquid carryover into downstream equipment.
Which parts should be reviewed most often?
Drain points, low points and joints with elastomeric seals. These are the locations where condition changes first, and they are easy to inspect at defined intervals.
Luoyang Wohong Petrochemical Equipment Co., Ltd. designs and manufactures vapor recovery units, membrane separation skids and condensers for fuel depots, terminals and retail sites. Contact us for duty assessment and equipment selection support.