TL;DR — Every piping system on a ship or barge must include all necessary pumps, valves, relief valves, gauges, and fittings for safe operation; relief valves are required at all pump discharges except centrifugal pumps that cannot exceed system maximum allowable working pressure, and the relief valve setting must not exceed system MAWP while relieving capacity must prevent pressure from rising more than 20 percent above MAWP. 46 CFR §56.01-1 46 CFR §56.07-10
What the Rule Says
Scope and General Requirements
Part 56 of Title 46 CFR governs piping systems and appurtenances installed on ships and barges. The regulation is broad: it covers every system that moves vapors, gases, or liquids through the vessel. 46 CFR §56.01-1
Each piping system must be equipped with the full complement of components necessary for safe and efficient operation. The regulation specifically names pumps, valves, regulation valves, safety valves, relief valves, flanges, fittings, pressure gauges, liquid level indicators, and thermometers as required appurtenances.
One important carve-out: piping for industrial systems on mobile offshore drilling units (MODUs) need not fully comply with Part 56 but must meet subpart 58.60.
Definitions That Appear on the Exam
The regulations adopt specific technical definitions that differ from casual usage:
Piping means fabricated pipes or tubes with flanges and fittings attached, used to convey vapors, gases, or liquids — regardless of whether diameter is measured on the inside or outside. 46 CFR §56.07-5
Nominal size (or nominal diameter) means the commercial diameter of the piping — i.e., the pipe size designation, not a precise measured dimension.
Schedule relates to wall thickness and refers to specific values in ASME B36.10M (for wrought steel pipe) and B36.19M (for stainless steel pipe).
Fittings are pressure-containing piping system components other than valves and pipe. They fall into three categories:
- Pipe joining fittings: tees, wyes, elbows, unions, bushings — components that join branches of the system.
- Fluid conditioner fittings: traps, drains, strainers, separators, filters, meters — components that operate on the fluid.
- Special purpose fittings: expansion joints, slip joints, rotary joints, quick disconnect couplings — subject to special design and testing requirements as prescribed by the Commandant.
Thermometer wells and similar fittings that form part of the pressure barrier are also classified as fittings.
A nonstandard fitting is a piping system component not fabricated under an adopted industry standard.
Vital Systems
The regulations define which systems are "vital" to a vessel's survivability and safety. This classification matters because vital systems carry more stringent design, material, and plan-approval requirements. Vital systems include:
- Fuel oil fill, transfer, and service systems
- Fire-main systems
- Fixed gaseous fire-extinguishing systems
- Bilge systems
- Ballast systems
- Steering systems and steering-control systems
- Propulsion systems and their necessary auxiliaries and control systems
- Ship's service and emergency electrical-generation systems and their auxiliaries vital to survivability and safety
- Any other marine-engineering system identified by the cognizant OCMI as crucial to survival or protection of personnel aboard
Any system not on this list is classified as a non-vital system.
Design Conditions and Pressure Relief
Maximum Allowable Working Pressure (MAWP): The MAWP of a piping system must not exceed the internal design pressure as defined in ASME B31.1. Where any component — such as a valve or fitting — has a lower MAWP than the pipe itself, the system pressure must be limited to the lowest component MAWP. 46 CFR §56.07-10
Relief valve requirements: Every system that may be exposed to pressures higher than its MAWP must be safeguarded by appropriate relief devices. Relief valves are required at pump discharges — with one specific exception: centrifugal pumps that are so designed and applied that they cannot develop a pressure in excess of the system MAWP do not require a relief valve at their discharge.
Relief valve settings and capacity: The relief valve setting must not exceed the system MAWP. The relieving capacity must be sufficient to prevent pressure from rising more than 20 percent above the system MAWP. For safety and relief valves protecting piping systems only, the rated relieving capacity must be based on actual flow test data, and the manufacturer must certify capacity at 120 percent of the valve's set pressure.
Ship motion: Piping system designs must account for the effects of ship motion and flexure, including weight, yaw, sway, roll, pitch, heave, and vibration.
Material selection: Materials for piping must be selected per §56.60-1(a). Where multiple allowable stress values are listed for a material, the lowest value must be used unless otherwise approved by the Commandant. Allowable stress values restricted by footnote or italicized in the ASME BPVC must not be used.
Plan Approval Requirements
Before installation aboard ship, piping diagrams for a specific list of systems must be submitted for approval. That list includes: steam and exhaust piping; boiler feed and blowoff piping; safety valve escape piping; fuel oil service, transfer, and filling piping; fire extinguishing systems (fire main, sprinkler, inert gas, foam); bilge and ballast piping; tank cleaning piping; condenser circulating water piping; vent, sound, and overflow piping; sanitary, soil, deck, and overboard discharge piping; internal combustion engine exhaust piping; cargo piping; hot water heating systems above 121 °C (250 °F); compressed air piping; fluid power and control systems (hydraulic, pneumatic); lubricating oil piping; and refrigeration and air conditioning piping. 46 CFR §56.01-10
Arrangement drawings (in addition to diagrams) must be submitted for all Class I, I-L, and II-L systems, and for Class II firemain, foam, sprinkler, bilge and ballast, and vent/sounding/overflow systems.
Piping diagrams must include: pipe diameters, wall thicknesses, design pressure, fluid temperature, applicable ASTM material or ANSI component specification, and type, size, design standard, and rating of valves, flanges, and fittings.
For firemain and foam system pumps, pump characteristic curves must be submitted — or in lieu of curves, the diagram must show rated capacity and head at rated capacity, shutoff head, and head at 150 percent of rated capacity.
Where piping passes through watertight bulkheads or fire boundaries, plans of typical details of piping penetrations must be submitted.
Piping materials and appliances — pipe, tubing, fittings, flanges, valves — are not required to be specifically approved by the Commandant (safety relief valves covered under Part 162 are the exception), but they must comply with applicable requirements for materials, construction, markings, and testing, and must be certified as described in Part 50.
Why It Matters on the Exam
QMED Junior Engineer exam questions on this topic cluster around four areas:
1. Relief valve rules — The 20-percent overpressure limit and the centrifugal pump exception are classic exam targets. Expect a scenario question asking whether a relief valve is required at a specific pump discharge.
2. Vital systems identification — Candidates are asked to identify which systems are classified as vital. Bilge, ballast, fire main, fuel oil, and steering are all vital; air conditioning and sanitary systems are not on the vital list.
3. Fitting classification — The three-way split (pipe joining, fluid conditioner, special purpose) is tested. Know that a strainer is a fluid conditioner fitting, while an expansion joint is a special purpose fitting.
4. Plan approval triggers — Knowing which systems require diagram submission before installation, and what data must appear on those diagrams, is directly testable.
Common Pitfalls
Pitfall 1 — Misapplying the centrifugal pump exception. The relief valve exemption applies only to centrifugal pumps that are designed and applied such that they cannot develop pressure exceeding system MAWP. Positive displacement pumps (reciprocating, gear, screw) do not qualify for this exception and always require a relief valve at the discharge. 46 CFR §56.07-10
Pitfall 2 — Confusing the 20-percent and 120-percent figures. The relieving capacity must prevent pressure from rising more than 20 percent above MAWP. The manufacturer certifies capacity at 120 percent of the valve's set pressure. These are related but distinct requirements — do not conflate them.
Pitfall 3 — Treating nominal size as an actual measurement. Nominal size is the commercial pipe size designation, not a precise inside or outside diameter. Schedule refers to wall thickness per ASME B36.10M or B36.19M. 46 CFR §56.07-5
Pitfall 4 — Assuming all valves need Commandant approval. Piping materials and appliances, including valves, do not require specific Commandant approval — they must comply with applicable standards and be certified per Part 50. Safety relief valves under Part 162 are the exception. 46 CFR §56.01-10
Pitfall 5 — Forgetting the MODU carve-out. Industrial piping on mobile offshore drilling units is not fully subject to Part 56; it must meet subpart 58.60 instead. 46 CFR §56.01-1
Pitfall 6 — Using the highest allowable stress value. When multiple stress values are listed for a material, the lowest value must be used. Italicized or footnote-restricted values in the ASME BPVC are prohibited.
Quick Check
Q1 — Is a relief valve required at the discharge of a centrifugal pump?
Not necessarily. Relief valves are required at pump discharges, but centrifugal pumps that are so designed and applied that they cannot develop a pressure exceeding the system MAWP are exempt from this requirement. 46 CFR §56.07-10
Q2 — By how much may system pressure rise above MAWP before the relief valve capacity is considered inadequate?
The relieving capacity must be sufficient to prevent pressure from rising more than 20 percent above the system MAWP. The manufacturer certifies capacity at 120 percent of the valve's set pressure.
Q3 — Which of the following is a vital system: (a) air conditioning, (b) bilge system, (c) sanitary drain system?
The bilge system is a vital system. Air conditioning and sanitary drain systems are not listed as vital systems under the regulation. 46 CFR §56.07-5
Q4 — A strainer installed in a seawater cooling line is classified as what type of fitting?
A strainer is a fluid conditioner fitting — a pressure-containing component that operates on the fluid contained in the system, as distinguished from pipe joining fittings (tees, elbows, etc.) and special purpose fittings (expansion joints, etc.).
Q5 — Before installing a bilge and ballast piping system on a new vessel, what must be submitted to the Coast Guard?
Piping diagrams for bilge and ballast systems must be submitted for approval prior to installation. Additionally, arrangement drawings must be submitted for Class II bilge and ballast systems. 46 CFR §56.01-10
Q6 — What pump performance data must appear on a firemain piping diagram if pump characteristic curves are not submitted?
The diagram must show: (1) rated capacity and head at rated capacity, (2) shutoff head, and (3) head at 150 percent of rated capacity.
Q7 — What does "nominal size" mean in the context of Part 56 piping regulations?
Nominal size (or nominal diameter) means the commercial diameter of the piping — the pipe size designation — not a precise measured inside or outside diameter.
Q8 — When multiple allowable stress values are listed for a piping material in the ASME BPVC, which value must be used?
The lowest value of the listing must be used, unless otherwise approved by the Commandant. Stress values that are italicized or restricted by footnote in the ASME BPVC must not be used at all.