TL;DR — Every piping system on a ship or barge must carry the pumps, valves, relief valves, flanges, fittings, pressure gauges, and liquid-level indicators needed for safe operation; relief valves at pump discharges must prevent pressure from rising more than 20 percent above the system's maximum allowable working pressure, and bilge and ballast systems are classified as vital systems requiring plan approval before installation. 46 CFR §56.01-1 46 CFR §56.07-10 46 CFR §56.07-5
What the Rule Says
Scope and General Equipment Requirements
Part 56 of Title 46 CFR governs piping systems and their appurtenances on ships and barges. 46 CFR §56.01-1 The regulation is explicit: every piping system installed on a ship or barge must be equipped with the necessary pumps, valves, regulation valves, safety valves, relief valves, flanges, fittings, pressure gauges, liquid level indicators, thermometers, and similar devices required for safe and efficient operation of the vessel.
One important carve-out: piping for industrial systems on mobile offshore drilling units (MODUs) need not fully comply with Part 56 but must instead meet subpart 58.60. This distinction appears on exams — do not apply Part 56 wholesale to MODU industrial piping.
Definitions That Drive Exam Questions
Several defined terms in §56.07-5 are tested directly.
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 pipe size) means the commercial diameter of the piping — i.e., pipe size — not the actual measured inside or outside diameter.
Schedule relates to wall thickness and refers to specific values given in ASME B36.10M (welded and seamless wrought steel pipe) and ASME B36.19M (stainless steel pipe), both incorporated by reference. 46 CFR §56.01-2
Fittings are pressure-containing piping system components other than valves and pipe. This category includes:
- Pipe joining fittings — tees, wyes, elbows, unions, bushings, and similar components that join branches of the system.
- Fluid conditioner fittings — traps, drains, strainers, separators, filters, meters, and similar components that operate on the fluid.
- Special purpose fittings — expansion joints, slip joints, rotary joints, quick-disconnect couplings, and similar items subject to special design and testing requirements as prescribed by the Commandant.
- Thermometer wells and other fittings forming part of the pressure barrier are also included.
A nonstandard fitting is a piping system component not fabricated under an adopted industry standard.
Vital Systems
The regulation enumerates which systems are classified as vital — meaning vital to a vessel's survivability and safety. Vital systems include:
1. Systems for fill, transfer, and service of fuel oil 2. Fire-main systems 3. Fixed gaseous fire-extinguishing systems 4. Bilge systems 5. Ballast systems 6. Steering systems and steering-control systems 7. Propulsion systems and their necessary auxiliaries and control systems 8. Ship's service and emergency electrical-generation systems and their auxiliaries vital to survivability and safety 9. Any other marine-engineering system identified by the cognizant OCMI as crucial to vessel survival or personnel protection
Any system not on this list is a non-vital system.
Design Conditions — Maximum Allowable Working Pressure and Relief Valves
The maximum allowable working pressure (MAWP) of a piping system must not exceed the internal design pressure as defined in ASME B31.1. 46 CFR §56.07-10 Where a component such as a valve or fitting has a lower MAWP than the computed pipe or tubing value, the system pressure must be limited to the lowest component MAWP.
Relief valve requirements are a high-frequency exam topic:
- 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, except for centrifugal pumps so designed and applied that a pressure in excess of the system MAWP cannot be developed.
- The relief valve setting must not exceed the system MAWP.
- Relieving capacity must be sufficient to prevent pressure from rising more than 20 percent above the system MAWP.
- The rated relieving capacity of safety and relief valves protecting piping systems must be based on actual flow test data, certified by the manufacturer at 120 percent of the set pressure.
Ship motion and flexure must be accounted for in piping system design, including weight, yaw, sway, roll, pitch, heave, and vibration.
Plan Approval Requirements
Before installation aboard ship, piping system diagrams must be submitted for approval for a specific list of systems. 46 CFR §56.01-10 Systems requiring diagram submission include:
- Steam and exhaust piping
- Boiler feed and blowoff piping
- Fuel oil service, transfer, and filling piping (including boiler fuel and internal combustion engine fuel)
- Fire extinguishing systems including fire main, sprinkler, inert gas, and foam
- Bilge and ballast piping
- Compressed air piping
- Lubricating oil piping
- Fluid power and control systems (hydraulic, pneumatic)
- Cargo piping
- Vent, sound, and overflow piping
- Sanitary drains, soil drains, deck drains, and overboard discharge piping
- Hot water heating systems where temperature exceeds 121 °C (250 °F)
- Refrigeration and air conditioning piping
Arrangement drawings (in addition to diagrams) must be submitted for all Classes I, I-L, and II-L systems, and for all Class II firemain, foam, sprinkler, bilge and ballast, and vent sounding and overflow systems.
Piping diagrams must include pipe diameters, wall thicknesses, design pressure, fluid temperature, applicable material specifications, and type, size, design standard, and rating of valves, flanges, and fittings. For firemain and foam system pumps, characteristic curves must be submitted, or alternatively the rated capacity and head at rated capacity, shutoff head, and head at 150 percent rated capacity must be shown on the drawing.
Where piping passes through watertight bulkheads or fire boundaries, plans of typical details of piping penetrations must be submitted.
Component Selection
Pipe, tubing, pipe joining fittings, and piping system components must meet the material and standard requirements of subpart 56.60 and the certification requirements of Part 50. 46 CFR §56.10-1 The requirements of subparts 56.15 through 56.25 govern component selection in place of ASME B31.1 sections 105 through 108.
Why It Matters on the Exam
QMED Oiler exam questions on this topic cluster around three areas:
1. Relief valve numbers. The 20-percent overpressure limit and the 120-percent certification pressure are precise, testable figures. Expect a question such as: "A relief valve protecting a piping system with a MAWP of 100 psig must prevent pressure from rising above ___." The answer is 120 psig (20 percent above MAWP). 46 CFR §56.07-10 A separate question may ask at what pressure the manufacturer must certify the rated relieving capacity — the answer is 120 percent of the set pressure.
2. Vital vs. non-vital systems. Examiners test whether candidates know that bilge systems, ballast systems, and fuel oil fill/transfer/service systems are all classified as vital. 46 CFR §56.07-5 Refrigeration piping, for example, does not appear on the vital systems list and would be non-vital unless the OCMI designates otherwise.
3. Plan approval triggers. Candidates must know which systems require diagram submission before installation. Bilge and ballast piping, compressed air piping, and fuel oil piping all require pre-installation diagram approval. 46 CFR §56.01-10 The exception for centrifugal pumps (no relief valve required if the pump design prevents overpressure) is also tested.
Common Pitfalls
Confusing the two 120-percent figures. The relief valve setting must not exceed the system MAWP — it is the relieving capacity that is certified at 120 percent of set pressure. The maximum pressure the system may reach is MAWP plus 20 percent. These are related but distinct requirements. 46 CFR §56.07-10
Assuming all pump discharges need relief valves. The rule requires relief valves at pump discharges, but centrifugal pumps that are designed and applied so that system MAWP cannot be exceeded are exempt. Positive-displacement pumps do not enjoy this exemption.
Misidentifying MODU industrial piping. Industrial piping on MODUs is not fully subject to Part 56 — it falls under subpart 58.60. Applying Part 56 requirements wholesale to MODU industrial systems is incorrect. 46 CFR §56.01-1
Treating nominal pipe size as an actual dimension. Nominal size is the commercial designation, not the measured inside or outside diameter. 46 CFR §56.07-5
Overlooking the lowest-component MAWP rule. When a valve or fitting in a system has a lower MAWP than the pipe itself, the entire system pressure must be limited to that lower value — not the pipe's computed MAWP.
Quick Check
Q1 — A piping system has a MAWP of 150 psig. What is the maximum pressure to which the system may rise before the relief valve must have discharged it?
180 psig. The relieving capacity must be sufficient to prevent pressure from rising more than 20 percent above the system MAWP (150 × 1.20 = 180 psig). 46 CFR §56.07-10
Q2 — At what percentage of the set pressure must a manufacturer certify the rated relieving capacity of a relief valve protecting a piping system?
120 percent of the set pressure.