What 178.337-9 means in plain language
FMCSR 178.337-9 governs the pressure relief devices on MC331 cargo tanks—the cylindrical steel tankers you see hauling liquids and gases on highways. These tanks operate under pressure, and pressure relief devices are safety valves that protect the tank and cargo by venting excess pressure buildup before it becomes dangerous.
In practice, this regulation requires that pressure relief devices on MC331 tanks meet specific design, installation, and operational standards set by DOT. The devices must function correctly, be properly sized for the tank and commodity, and be maintained so they work when needed. If an inspector finds that your tank's relief valve is missing, damaged, improperly installed, or cannot relieve pressure as designed, you can be cited for violating 178.337-9.
This code applies to carriers and drivers transporting hazardous materials in pressurized MC331 tanks. It is not an out-of-service eligible violation, meaning inspectors cannot immediately remove your vehicle from service based on this citation alone, though other related defects might trigger a separate OOS order.
What our enforcement data actually shows
Across our 13 million+ roadside inspection records, 178.337-9 is exceptionally rare. Our database shows exactly 1 citation on record, all-time. In the last 12 months, we recorded 0 citations. In the last 90 days, we recorded 0 citations. This code ranks #2796 of 3,036 FMCSR codes by citation volume—putting it in the bottom tier of enforcement activity.
The single citation in our records was not placed out of service, yielding a 0.0% out-of-service rate for this code. For context, the all-FMCSR average OOS rate is 31.4%, which means 178.337-9 is cited in a way that almost never results in an immediate vehicle removal. This suggests that when pressure relief device issues are found, they are often corrected on-site or the inspector does not view them as immediately safety-critical, or the defect is instead coded under a related hazmat regulation.
Who gets cited most
Our records contain only one citation for 178.337-9, issued to THOMPSONGAS LLC (USDOT 2812287). With only a single data point, state-level or carrier-level trends cannot be reliably established. This extreme rarity means that if you have received a 178.337-9 citation, you are in a very small group, and the defect found likely represents an unusual or severe gap in tank maintenance or inspection.
How severe is this compared to similar codes
Pressure relief sits within the broader hazardous materials category. Other common hazmat violations show dramatically different enforcement patterns. For example, our data shows general loading and unloading violations under codes 177.834A-HMC and 177.834(a) with 3,954 and 3,839 citations respectively—thousands of times more frequent than 178.337-9. Both of those codes carry OOS rates above 97%, indicating they are treated as severe roadside safety issues.
Placarding violations, such as 177.817(a), appear 2,274 times in our records with a 75.1% OOS rate. Even emergency response information maintenance violations (172.602(c)(1)) show 1,464 citations. By contrast, the single 178.337-9 citation reflects either excellent compliance in the field or possible underdetection due to inspector focus on more visible hazmat defects like placarding and loading procedures.
How to avoid it
Because 178.337-9 is so rarely cited, prevention advice must be grounded in tank maintenance fundamentals:
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Inspect pressure relief devices before every trip. Walk around your MC331 tank and verify that any visible relief valve or discharge outlet is not damaged, dented, or obstructed. Look for signs of corrosion, cracks, or missing bolts.
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Never modify or tamper with relief devices. If a valve appears stuck, leaking, or undersized, report it to your fleet maintenance team immediately. Do not attempt to adjust or bypass the device yourself.
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Verify tank certification and maintenance records. Before accepting a tank assignment, confirm that the tank has current DOT certification and that the relief device has been tested and certified to DOT specifications. Request maintenance logs showing when the relief valve was last inspected or replaced.
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Know your commodity. Different hazardous materials require different relief device settings. Confirm with your dispatcher that the tank is equipped with the correct relief device for the commodity you are hauling. Mismatched tanks and cargo can create pressure conditions the device was not designed to handle.
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Report defects during pre-trip. If you notice during your pre-trip inspection that a relief valve is corroded, leaking, or visibly malfunctioning, flag it in your vehicle condition report and do not depart. A failed relief device creates genuine safety risk for you, other motorists, and the environment.
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Coordinate with maintenance on tank condition. If your fleet operates MC331 tanks long-term, work with your maintenance team to establish a regular relief device inspection schedule (annually or per manufacturer specs). Preventive maintenance catches small corrosion or wear before it becomes a roadside citation issue.