Slow Fill and Overheating Cylinder During Fast Fill
A fleet operator reports that a CNG van takes much longer than normal to fill at a public fast-fill station, and the station attendant notes the cylinder gets warm during fill. The van has a single Type 3 cylinder with an integral manual valve and a fill receptacle behind the fuel door. The receptacle has a check valve and a vent line. At the shop, a slow fill (overnight) reaches full pressure and the van runs normally. A fast fill at the shop's station stops short of full and the cylinder skin temperature rises noticeably. The receptacle's vent line is kinked where it passes through the fuel door opening. Static pressure after slow fill is 3,600 psi at 70°F. The fill receptacle's check valve was replaced last month with a generic part.
1. Explain how the kinked vent line and the generic check valve could each contribute to slow fill and heating.
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Kinked vent line: The receptacle vent line is the path for gas displaced during filling and for pressure relief/venting during the fill connection sequence. A kink restricts that path, which can:
- Slow the fill because the station's fill logic senses backpressure and derates flow.
- Cause trapped pressure in the receptacle/fill path that interferes with the check valve seating and the station's ability to complete the fill.
- Contribute to heating because restricted venting forces the station to throttle the fill rate or because gas is compressed against a restriction.
Generic check valve: A check valve that is not the specified part may have the wrong cracking pressure, flow coefficient, or seat design. If it is restrictive:
- Fill flow is reduced → longer fill time.
- The restriction causes pressure drop and turbulence, which adds heat.
- It may not seat/unseat correctly, causing the station to sense an incomplete connection and slow or stop the fill.
Heating during fast fill is normal to a degree (compression heat), but excessive heating with a short fill indicates restriction-driven pressure drop and reduced flow, not a cylinder fault - the slow fill reaching full pressure and normal operation supports that.
2. What is the diagnostic and repair plan, and what safety precautions apply?
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Diagnostic plan:
- Inspect the vent line - confirm the kink, measure bend radius, and check for crush damage. Replace with correct routing and secure it away from the door opening.
- Verify the check valve part number against the OEM specification. Replace the generic valve with the specified part.
- Flow-test the fill path - measure fill time and pressure rise against spec with a known-good station.
- Check the receptacle and fill hose for restrictions, debris, or damaged seals.
- Confirm cylinder temperature rise is within normal fast-fill limits after repairs.
Repairs: Replace kinked vent line with correct routing; install specified check valve; re-test fast fill.
Safety precautions:
- Defuel or isolate before opening the fill path; verify zero pressure.
- No ignition sources; ventilate the area.
- Wear PPE (gloves, eye protection) and use CNG-rated components only.
- Do not use a generic check valve - pressure-containing parts must meet the system specification.
- After repair, leak-test the fill path and confirm the receptacle closes and vents correctly.