What 178.337-9B-HMP means in plain language
FMCSR 178.337-9B-HMP governs the piping, valves, hoses, and fittings on MC 331 cargo tanks—the pressurized steel drums and cylindrical tanks you see hauling liquefied gases, anhydrous ammonia, and other hazardous materials requiring containment under pressure.
The regulation sets standards for how these components must be manufactured, installed, and maintained. Think of it as the plumbing code for hazmat tanks. Piping must be the right material and gauge. Valves must seal reliably and open/close predictably. Hoses must withstand the pressure and chemical environment. Fittings must be threaded or welded securely so nothing leaks during transport.
When an inspector cites you for this code, they found evidence that one or more of those components—a corroded valve, a pinhole in a hose, a loose fitting, or substandard materials—posed a risk of leakage or structural failure during transit.
What our enforcement data actually shows
Across our 13 million+ roadside inspection records, FMCSR 178.337-9B-HMP is cited infrequently: only 6 all-time citations, with 4 in the last 12 months and 2 in the last 90 days. The code ranks #2357 out of 3,036 FMCSR codes by citation volume.
The out-of-service rate for this violation is 0.0%—none of the 6 citations resulted in an out-of-service order. That stands in sharp contrast to the all-FMCSR average OOS rate of 31.4%, meaning inspectors who cite 178.337-9B-HMP typically allow the vehicle to continue operating, either because the defect was minor and immediately corrected, or because the tank and its systems remained functionally safe.
The monthly trend over the past 12 months shows sporadic enforcement: one citation each in April 2025, September 2025, February 2026, and March 2026. This pattern suggests the citation is situational—triggered when an inspector happens to conduct a detailed tank inspection and discovers a specific piping or valve defect—rather than endemic to any carrier or region.
Who gets cited most
Our data shows Ohio accounts for 2 citations over the last 180 days, the only state with multiple enforcement activity in that window. Both were cited but not placed out of service, indicating the defects found were correctable or not severe enough to ground the vehicle immediately.
Top carriers cited all-time include REIS TRUCKING INC (USDOT 228741), GRATE TRUCKING INC (USDOT 444396), I X L PROPANE INC (USDOT 461817), OVER XXI INC (USDOT 471779), NUCO2 SUPPLY LLC (USDOT 578807), and SALLEE INC (USDOT 668590)—each with one citation. This scattered distribution across six different carriers suggests no systemic compliance gap at any single fleet; rather, piping and valve defects are isolated incidents.
Vehicle makes cited include Freightliner and Peterbilt tractor units, along with tank manufacturers MTCI and others, reinforcing that the violation is not tied to a specific chassis model but to how the hazmat tank itself is equipped and maintained.
How severe is this compared to similar codes
Within the Hazardous Materials category, peer codes show a wide range of enforcement intensity and consequences.
177.834A-HMC (General loading/unloading hazmat) has 3,954 citations with a 99.2% OOS rate—meaning nearly every violation grounds the truck. 177.834(a) is equally serious, with 3,839 citations and 97.9% OOS rate. These codes reflect violations so dangerous (improper hazmat loading, uncontrolled discharge) that inspectors almost never allow the vehicle to move.
177.817(a) (Placarding violation) has 2,274 citations and a 75.1% OOS rate—still severe, but less absolute than general loading breaches.
By contrast, 172.602(c)(1) (Maintenance/accessibility of Emergency Response information) has 1,464 citations and a 0.0% OOS rate, matching 178.337-9B-HMP. That code governs whether the driver has access to hazmat reference materials; it's a paperwork-and-documentation issue, not a tank-integrity issue.
178.337-9B-HMP sits between these poles: it addresses physical tank infrastructure (piping and valves), which is critical to safe transport, but the rarity of citations and zero OOS rate suggest that defects, when found, are typically minor, field-correctable, or do not immediately jeopardize the load.
How to avoid it
Prevention starts with understanding what triggers the citation. Our co-occurring violation data shows that when 178.337-9B-HMP citations occur, they often appear alongside 173.24B1-HMBPMC (HM leaking from a bulk package/associated piping). This tells you the root cause is often visible leakage or corrosion.
Take these steps before every run and at rest stops:
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Walk the tank exterior before departure. Look for wet spots, stains, or corrosion streaks around all valve stems, coupling connections, and seams. Even a slow weep indicates a failing valve or hose and warrants a maintenance call.
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Inspect all visible piping and hose connections. Check that hoses are not cracked, pinched, or bulging. Verify that fittings are tight—no visible gaps or threads showing. Tug gently on each hose and line to confirm they don't flex excessively or separate.
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Check valve handles and caps. Make sure all pressure-relief and isolation valves move freely, are not corroded to the point of immobility, and have protective caps that fit snugly. A stuck valve or missing cap can mean the valve seat is damaged.
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Report defects immediately. If you spot even minor corrosion, a slow leak, or a loose fitting, alert your dispatcher and do not depart. These issues compound during transport due to vibration and pressure cycling.
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Use pre-trip checklists specific to your tank. Your carrier should provide a maintenance card that lists the inspection intervals for piping integrity. Follow it.
Our data also shows co-occurring defects in tires, cab parts, and exhaust systems in the same inspection events, suggesting that when detailed hazmat tank inspections occur, they happen alongside general vehicle safety checks. Stay ahead of all routine maintenance—brakes, tires, lights, windows—to reduce the likelihood of a roadside inspection that includes a close look at your tank's plumbing.