In an early post this morning, I discussed the need for “rain caps” and “weep holes” on our Pressure Safety Relief systems. And a dear friend and great ChemE reminded me that “weep Holes” are not just for draining rainwater. For example, this week I was working with an NH3 client, and I must state that NH3 gas can absolutely condense into liquid in the vent discharge piping after a Pressure Safety Valve (PSV) closes.
Even after a PSV closes, the conditions within a vent piping often promote condensation:
- When pressurized ammonia is released through a PSV, the rapid expansion (pressure drop) causes the ammonia’s temperature to plummet to its boiling point of approximately -28°F. This creates extremely cold vapor in the discharge piping.
- If the ambient temperature exceeds that of the cold ammonia residue in the pipe, heat transfer will occur. However, if the piping remains cold (e.g., in a refrigerated system or during rapid cycling), or if the vapor is exposed to colder ambient temperatures, it can readily reach its dew point and condense.
- Even after the valve seats, the discharge line is filled with cold ammonia vapor. As this gas cools or interacts with moisture/surfaces, it can transition to a liquid state, pooling at low points in the piping.
As we discussed in the earlier post, accumulated liquid ammonia in the discharge piping is dangerous for several reasons:
- Any liquid trapped in the discharge line creates a column of fluid. This exerts “backpressure” against the PSV disk. For standard spring-loaded valves, this can prevent the valve from opening at the intended set pressure or cause the valve to “chatter” (rapidly opening and closing), which can damage the valve seat and trim.
- If the PSV opens again while liquid remains in the vent line, the high-velocity vapor can strike the liquid slug, potentially causing a “liquid hammer” effect. This can create massive, instantaneous pressure spikes that can rupture piping, blow off pipe caps, or destroy the valve itself.
- Standing liquid ammonia, especially if it reacts with any trace moisture in the system, can accelerate corrosion.
- Furthermore, if the liquid freezes or plugs the line, the PSV becomes effectively inoperable.
To address this risk, consider these industry-standard practices:
- Low-Point Drains: As discussed previously, installing a weep hole (where permissible) at the lowest point of the discharge elbow is the primary method to prevent liquid pooling.
- Pitching: Ensure that all discharge piping is sloped (pitched) away from the PSV so that any condensed liquid drains naturally toward the exit point rather than back toward the valve.
- Insulation/Tracing: In specific refrigerated services, some engineers specify insulation or heat tracing on the discharge piping to maintain the ammonia above its condensation temperature, though this must be carefully evaluated to ensure you do not exceed the temperature rating of the valve materials.
- Closed Header Systems: For highly hazardous services, the best practice is often to route the PSV discharge into a closed-header system that leads to a knockout drum or a dedicated flare/scrubber system, which manages liquid accumulation centrally rather than relying on atmospheric vents.