Tank Truck Relief Valves: Accept Them As Layers of Protection?

“The ancient Greeks and Romans also held the view that acceptance is the beginning of wisdom.”  — Simon Van Booy

PHA teams often face the question of whether to address tank truck loading and unloading.

On the one hand, trucks in transit are under the jurisdiction of the U.S. Department of Transportation (DOT), not the Occupational Safety and Health Administration (OSHA). Because trucks in transit are not under the jurisdiction of OSHA, they are not covered under the Process Safety Management (PSM) standard.

On the other hand, if there is a catastrophic failure of a tank truck while it is loading and unloading, the facility must deal with the consequences.

Once a facility decides to address tank truck loading and unloading, there is a second question to address: in overpressure scenarios, should they take credit for the protection and risk reduction provided by the tanker’s pressure relief device?

Reasons To Give No Credit For A Tank Truck’s Relief Valve

There are four questions offered to justify not taking credit for a tank truck’s relief valve:

1. How do we know that every tank truck even has a relief valve?
2. If it does have a relief valve, how do we know that it works?
3. If it works, how do we know what the set point is?
4. Even if the set point is correct, how do we know if the relieving capacity is sufficient?

These are valid concerns, any one of which would disqualify a tank truck relief valve as an independent layer of protection (IPL). Before tackling these questions, however, let’s review the types of tank trailers, or “cargo tanks,” as the DOT refers to them.

Types Of Cargo Tanks

The DOT specifies several types of cargo tanks.

DOT 406:  These are non-pressurized tankers typically used to haul flammable liquids, such as gasoline, or non-flammable liquids, such as milk. They are permitted to have a maximum allowable working pressure (MAWP) in the range of 2.65 psig to 4 psig with a typical MAWP specification of 3 psig. Their typical capacity is in the range of 9,000 to 9,500 gallons.

DOT 407:  These are pressurized tankers for hauling hazardous liquids, such as chemicals or crude oil. They have a MAWP of at least 25 psig, but usually less than 40 psig. Trailers in chemical or solvent service have a typical capacity in the range of 5,000 to 7,000 gallons. Trailers in crude oil service typically have an MAWP of 25 psig with a much larger capacity, in the range of 8,400 to 13,000 gallons.

DOT 412:  These are pressurized tankers for hauling corrosive liquids, which is reflected in their materials of construction and use of liners. Their MAWP must be at least 5 psig. However, an MAWP in the range of 35 to 50 psig is not unusual, because of the high vapor pressure of some corrosive liquids. DOT 412 trailers typically have a capacity in the range of 3,500 to 5,500 gallons.

Although not really in the scope of this discussion, there are two other types of cargo tanks worth mentioning:

MC 331:  These tankers are primarily for hauling compressed gases that liquify at room temperature when under pressure, such as propane, anhydrous ammonia, and LPG. Their MAWP must be at least 100 psig and no more than 500 psig. More importantly, the design pressure must be at least as much as the vapor pressure of the commodity being hauled when it is at 115 psig. They typically have a capacity in the range of 5,200 to 19,600 gallons.

MC 338:  These are jacketed cargo tanks for hauling cryogenic liquids. They must have a design pressure, or MAWP, of at least 300 psig. The jacket is filled with insulation or under vacuum, making them much like giant Thermos^® bottles on wheels. They typically have a capacity in the range of 4,500 to 5,550 gallons.

Does Every Tank Truck Have A Relief Valve?

The DOT specifies the design and construction of tankers. DOT 406 cargo tanks, DOT 407 cargo tanks, and DOT 412 cargo tanks are each required to have a pressure relief system in accordance with general requirements in 49 CFR 178.345-10. How would someone know if a tanker was built to DOT specifications? Cargo tanks must be marked, per 49 CFR 178.345-14, with a corrosion-resistant metal nameplate and specification plate, which, among other things, must identify the applicable DOT specification. The plates must be on the left side of the trailer, near the front.

Cargo tanks built to MC 331 or MC 338 specification must also have pressure relief systems, designed, installed, and maintained in accordance with the Compressed Gas Association Bulletin S-1.2, Pressure Relief Device Standards, Part 2 – Portable Containers for Compressed Gas. As with other DOT specified cargo tanks, MC 331 and MC 338 must be marked with corrosion-resistant metal nameplate and specification plate which include the DOT specification number.

Does The Pressure Relief System Work?

The DOT requires that every cargo tank constructed in accordance with a DOT specification be inspected and tested. This includes

• External Visual Inspections
• Internal Visual Inspection (with exceptions)
• Lining Inspections (only for lined cargo tanks)
• Leakage tests (except for MC 338 cargo tanks)
• Pressure tests
• Thickness Tests (only for unlined cargo tanks, except for MC 338 cargo tanks)

This is relevant to pressure relief because external visual inspection includes inspection of pressure relief valves, and for those in corrosive service, the pressure relief valve must be removed and bench tested. The DOT regulation on inspection and testing of cargo tanks, 49 CFR 180.407, requires that external visual inspection (including bench testing the pressure relief valves, when called for) must be done at least once per year.

Annual inspection and bench testing is more often than many pressure relief valves in fixed service get.

What Is The Pressure Relief Set Point?

For cargo tanks designed and built to DOT 406, DOT 407, and DOT 412, the set pressure for any relief must be no less than 120% of the MAWP and the set pressure for primary relief must be no more than 132% of the MAWP. The pressure relief device must reclose at not less than 108% of the MAWP. All of this is according to 49 CFR 178.345-10.

If there is any uncertainty about the set pressure of the relief device, it will appear on the device itself.

What Is The Pressure Relieving Capacity?

For cargo tanks designed and built to DOT 406, DOT 407, and DOT 412, the minimum emergency vent capacity is also stipulated in 49 CFR 178.345-10, in Table I. The minimum is based on the fire case. The smallest exposed area shown in the table is 20 square feet, which requires a vent capacity of 15,800 scfh. The largest area shown in Table I is 1,000 square feet, which requires a vent capacity of 445,000. A typical chemical tanker, a DOT 407 trailer (42’ long, 5’ diameter) with a full volume of 6,169 gallons has an exposed surface area of 700 square feet. For a typical trailer, then, the standard requires an emergency venting capacity of at least 363,700 scfh.

As an alternative for DOT 412 cargo tanks, the DOT allows in 49 CFR 178.348-4(d)(3) for calculating a smaller venting capacity, but only for trailers in dedicated service hauling a material that is corrosive but has other hazards. The equation is

Q [scfh] = 37,980,000 A^0.82 (ZT)^0.5 / (ΔH_vapC)(MW^0.5)

and is a function of absolute temperature [°K], heat of vaporization [BTU/lb], and molecular weight, as well as the exposed surface area [square feet]. The equation is derived from the equation API 521 uses to calculate heat transfer in the fire case.

Again, if there is any uncertainty about the rated flow capacity of the relief device, it will appear on the device name plate, at the rating pressure.

A Reasonable Basis For Risk Assessment During A PHA

Cargo tanks designed, built, and maintained in accordance with DOT requirements must have pressure relief adequate to address the external fire case. The external fire case is usually the sizing case for pressure relief. For other relieving cases, the DOT requirements for relief capacity found in Table I of 49 CFR 178.345-10 will be the basis for determining whether the pressure relief is adequate. For most chemical shipments, an emergency venting capacity of at least 363,700 scfh is a reasonable basis for risk assessment during a PHA.

Accepting Tank Truck Relief As An IPL

It is reasonable to accept the relief devices on tank truck trailers as an IPL, as long as the trailer meets a DOT specification. There are some non-code trailers on the road, so it will be important to check. Even though responsible trucking companies will only use trailers meeting DOT specifications, it will not hurt to require that shippers only load material into DOT specified trailers.

Then, when the truck arrives on site for delivery or pick-up, personnel in shipping and receiving should make one last check of the nameplate on the trailer to confirm that it is, in fact, a DOT code trailer. For chemical shipments, that usually means a DOT 407 or DOT 412 trailer, although there may be the occasional shipment in a DOT 406 trailer. If the trailer is something else, or not marked, that should be handled by management of change.