When vapor recovery performance drifts, attention goes naturally to the pump, the component with the nameplate and the service history. But the pump can only work with the vacuum that arrives at its inlet, and everything between the tank vent and that inlet decides how much of the work survives the journey. Suction lines are the quiet tax collectors of a vapor recovery system, and the tax is paid in pump life.
Where the losses come from
Suction-side losses accumulate from ordinary causes. Every bend and fitting adds resistance, and manifolds that grew bay by bay collect more of both than their drawings admit. Undersized runs force velocity up, and velocity multiplies the losses at each turn. Low points collect condensate that narrows the passage. And the oldest loss of all, leakage, hides in fittings and seals that look sound while admitting air that the pump must then move instead of vapor.
Reading the system before blaming the pump
The diagnostic order matters because it is cheap to expensive. The comparison below shows why experienced crews walk the line first.
| Suspect | How it shows up | Cost to verify |
|---|---|---|
| Leaking fittings | Vacuum weaker than pump data suggests | Walk the line, listen, soap test |
| Undersized diameter | Losses grow sharply with throughput | Compare performance across duty |
| Condensate pooling | Slow response, surging behavior | Check drains and low points |
| Excess bends and fittings | Chronic, uniform shortfall | Review against as-built |
| Pump wear | Confirmed only after the above | Measure at the inlet flange |
Only when the line has been cleared of these does the pump inherit the suspicion, and by then the measurement at the inlet flange will usually confirm or clear it quickly.
Layout principles that pay back for decades
Good suction design follows a few rules that never change: short runs and generous diameters ahead of the pump, slopes that carry condensate to drains rather than into the inlet, isolation valves positioned where maintenance can actually use them, and a filter or knockout stage placed where fouling is visible before it becomes a restriction. New builds get these from the drawing; retrofit sites get them by walking the line and fixing the worst offenders first, and even partial corrections usually move the numbers.
A field check any crew can run
- Measure vacuum at the pump inlet and again at the tank vent during unloading;
- The difference is the line tax; compare it against the design expectation;
- Walk the run with the difference in hand, checking fittings and low points;
- Soap test suspect joints and watch for fittings breathing air;
- Fix what is found, then repeat the two-point measurement and log the gain.
Frequently asked questions
How much suction loss is too much?
The honest answer is comparative: losses that were acceptable at commissioning become excessive when duty grows. Trending the two-point measurement over time turns a judgment call into a number, which is what the maintenance decision needs.
Do suction lines matter less on small units?
They matter proportionally more, because small pumps have less margin to spend fighting the line. Compact sites benefit from the same layout discipline even when the runs are short.
Can a leak on the suction side really go unnoticed for years?
It can, because the system still functions and the loss hides inside normal variation. It shows up as the pump working harder per liter recovered, which only becomes visible when someone trends the numbers.
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.