Level sensing and drain control are the parts of a vapor recovery unit that get the least attention and cause a surprising share of operating problems. The duty is straightforward in principle: detect liquid that has condensed inside the unit, and remove it before it interferes with the process. In practice the liquid arrives intermittently, in small quantities, often as a mixture, and the sensing point sits in a cold, wet, hydrocarbon laden environment.
Why drain control matters
Liquid held inside the separator or condenser affects performance directly. It reduces the effective volume available for separation, can be carried over into downstream stages, and in cold weather can freeze at the sensing point or in the drain line. A drain that works reliably protects the whole unit.
Sensor types and their fit
| Type | How it works | Practical considerations |
|---|---|---|
| Float operated | A float moves with the liquid level | Simple and visible; moving parts can foul or stick |
| Capacitance | Detects the change in dielectric conditions | No moving parts; reading can shift with product composition |
| Conductivity | Detects contact with conductive liquid | Widely used for point detection; depends on the liquid being conductive |
| Ultrasonic or similar | Measures distance to the liquid surface | Useful for a continuous reading; needs a clear path to the surface |
Design points that improve reliability
- Place the sensing point where liquid actually collects, not where it seemed convenient on the drawing;
- Size the drain line so it can clear a slug of liquid rather than only a trickle;
- Provide a means of checking and clearing the drain without opening the unit to atmosphere;
- Consider what happens if the drain line freezes, and provide protection where that is a real risk;
- Make the level indication visible during a routine walk round, so a fault is noticed early.
Problems seen in service
Sticking floats and fouled sensing surfaces are common where the liquid carries heavier fractions. Misreading caused by a change in product composition is a more subtle issue: a sensor that worked well on one fuel blend can behave differently on another. Drain lines that partially block are easy to miss, because the unit appears to be working while liquid slowly accumulates.
Frequently asked questions
How often should the drain system be checked?
As part of the normal walk round, with a more thorough check at each scheduled service. The most useful check is simply confirming that liquid leaves the unit when it should and that the indication matches what is happening.
Is a continuous level reading better than a point switch?
It depends on the purpose. A point switch is enough to trigger a drain action. A continuous reading is more informative when the trend in liquid accumulation is itself being monitored.
What causes a drain to work intermittently?
Partial blockage, a valve that does not fully open, a sensing point that only reacts to a particular liquid composition, or a drain line that freezes in cold weather. Checking these in order usually identifies the cause quickly.
Luoyang Wohong Petrochemical Equipment Co., Ltd. designs and builds vapor recovery units, membrane separation skids and condenser based recovery packages for fuel terminals, depots and service stations. Contact us to discuss a system selection or upgrade study.