The knockout pot on a Stage III vapor recovery line is a small vessel with one job: catch liquid before it reaches the vacuum producer. Vapor pulled back from the station piping during unloading can carry droplets of condensate, and if that liquid reaches the blower it can trip alarms, wear internals or stop the unit altogether. A pot gives the liquid somewhere to settle out, so the machine handles vapor and the pot handles the mess. Stations that treat the pot as part of routine care have fewer unexplained shutdowns during deliveries.
Why Liquid in the Vapor Line Is a Problem
Vapor moving through underground lines cools as it travels, and heavier hydrocarbon fractions condense on the way. Low points in the piping collect that liquid, and a change in flow during unloading can push it forward as a slug. A vacuum producer is designed for gas, not for liquid: slugs upset the balance of the machine, load bearings in ways they were never meant to handle, and in cold weather can sit in the line as a block. The knockout pot sits between the line and the unit precisely because the problem is normal, not exceptional.
Where the Pot Sits and What It Holds
The vessel is installed on the vapor inlet ahead of the blower package, close enough to the unit that drained liquid can be handled on site. Inside, reduced velocity lets droplets fall out of the stream; what collects below is a mix of condensate and whatever the line has washed forward, occasionally including water that entered elsewhere. A sight glass or level mark shows how much has gathered, and a drain valve allows the contents to be returned to the tank system or a dedicated container as the site rules require.
Signs the Pot Needs Attention
| Sign at the unit | What it can mean | What to check |
|---|---|---|
| Liquid alarm during deliveries | Pot level high or sensor fouled | Drain the pot, then confirm the alarm clears and does not return immediately |
| Level rises faster than before | More condensate arriving from the line | Review weather, fuel type and slope of recently changed piping |
| Water rather than fuel in the drain | Water entering the vapor path upstream | Inspect seals, sumps and drain points along the vapor line |
| Repeated trips after draining | Liquid bypassing or sensor issue | Have the service technician check the sensor and internal condition |
Making Draining Part of Routine Care
- Check the level before the first delivery of the day, and more often in cold weather or after long idle periods.
- Drain into the approved container or back to the tank system as the site procedure describes.
- Note what came out: clear fuel, dark condensate or water all point to different upstream conditions.
- Close and test the drain valve so it does not weep vapor between visits.
- Record the check in the station log so patterns over a season stay visible.
How the Pot Works With the Rest of the Package
The pot is one guard among several. Upstream slope and sump design decide how much liquid arrives; downstream, the blower and its control system assume the vapor arrives dry. When a station reports repeated liquid alarms, the pot is where the investigation starts, but it is rarely where the problem ends. Treating the vessel as a symptom reader, rather than a nuisance that gets drained and forgotten, turns it into an early warning for line slope issues, seal wear and water ingress elsewhere.
Seasonal Notes Worth Passing to Crews
Cold weather thickens condensate and slows draining, so winter checks take a little longer and the pot often collects more water after freeze and thaw cycles. Summer deliveries push more vapor and more swelling liquid through the line. Crews who adjust how often they look at the pot, instead of keeping one fixed interval all year, catch changes while they are still cheap to fix.
How often should a Stage III knockout pot be drained?
There is no fixed interval that suits every station. Check before the first delivery of the day and after any alarm, then let what you find set the rhythm. Sites with long vapor lines or frequent condensate usually check more often than small stations.
Where should the drained liquid go?
Follow the site procedure and local rules. Most arrangements return product to the tank system through the approved route, or hold it in a dedicated container for disposal. It should never be poured out or mixed with surface water.
The pot keeps filling with water instead of fuel. What does that mean?
Water in the drain points to a source upstream: a seal, a sump cover or a drain point letting surface water into the vapor path. Draining alone will not fix it, so the line should be inspected section by section until the entry point is found.
Luoyang Wohong Petrochemical Equipment Co., Ltd. builds Stage III vapor recovery packages and supports stations with practical operation and maintenance guidance.