Piping Systems: Relief Protection, Fittings, and What the Drawings Must Show

180 psig is wrong on both counts. The capacity must prevent the pressure rising more than 20 percent above system MAWP, so 100 × 1.20 = 120 psig is the ceiling. Because the set pressure is also 100 psig, the manufacturer's certified capacity point — 120 percent of set pressure — happens to fall at the same figure, 120 psig. Drop the setting to 90 psig and the certification point moves to 108 psig while the 120 psig accumulation ceiling stays put.</details>

The system is rated at its lowest-rated component

The maximum allowable working pressure of a piping system must not be greater than the internal design pressure defined in ASME B31.1, and where a component such as a valve or a fitting has a MAWP lower than that computed for the pipe or tubing, the system pressure is limited to the lowest of the component maximum allowable working pressures .

Heavy-wall pipe does not rate the line. A 150 psig strainer in a run of pipe good for 300 psig gives you a 150 psig system, and the relief valve is set and sized against 150.

Vital systems

A system is vital if it is vital to the vessel's survivability and safety, and the regulation names them 46 CFR §56.07-5:

• Fuel oil — fill, transfer, and service
• Fire — fire-main systems and fixed gaseous fire-extinguishing systems
• Water control — bilge systems and ballast systems
• Manoeuvring and power — steering and steering-control systems; propulsion systems with their necessary auxiliaries and control systems; ship's service and emergency electrical-generation systems and their auxiliaries vital to survivability and safety

The list does not close there. Any other marine-engineering system the cognizant OCMI identifies as crucial to the survival of the vessel or the protection of personnel aboard is vital as well. Everything not caught by the list or by the OCMI is non-vital.

An oiler is asked about bilge and ballast more than any other pair on that list, and both are named outright.

What the piping drawings must show

Diagrams of the listed systems must be submitted for approval before installation aboard ship — bilge and ballast piping, compressed air piping, fuel oil service, transfer and filling, firemain and sprinkler, vent, sound and overflow, and lubricating oil piping are all on that list, along with steam and exhaust, boiler feed and blowoff, safety valve escape, tank cleaning, condenser circulating water, drains and overboard discharge, cargo, engine exhaust, fluid power and control, and refrigeration 46 CFR §56.01-10.

Two pump requirements sit inside that section and they are separate:

• Pump rated capacity and pump shutoff head must appear on piping diagrams. All pumps, all diagrams.
• Pump characteristic curves must be submitted for firemain and foam pumps only, and even then the curves may be omitted if the drawing shows rated capacity and head at rated capacity, shutoff head, and head at 150 percent of rated capacity.

Arrangement drawings are a further submission, required for all Class I, I-L and II-L systems and for Class II firemain, foam, sprinkler, bilge and ballast, and vent sounding and overflow systems. Where piping passes through watertight bulkheads or fire boundaries, typical details of the penetrations go in too.

Materials, certification, and the ship's motion

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. Piping materials and appliances — pipe, tubing, fittings, flanges, valves — are not required to be specifically approved by the Commandant, provided they comply with the applicable requirements for materials, construction, markings and testing. Safety relief valves are the carve-out: those are covered under part 162 .

Piping design has to account for the effects of ship motion and flexure: weight, yaw, sway, roll, pitch, heave, and vibration . That is the paragraph that separates a marine piping system from a shoreside one, and it is why supports and expansion arrangements get the scrutiny they do.

Two terms of art are worth fixing before anything else. Nominal size means the commercial diameter of the piping — the pipe size — not a measured bore. Weight, as used in this part, relates to wall thickness and refers to the specific values in ASME B36.10M and B36.19M . Piping itself means fabricated pipes or tubes with flanges and fittings attached for conveying vapors, gases or liquids, regardless of whether the diameter is measured inside or outside.

Telling it apart: the three families of fitting

A fitting is a pressure-containing piping system component other than a valve or pipe 46 CFR §56.07-5. The criterion that sorts the families is what the component does — join the system, act on the fluid, or absorb movement.

• Pipe joining fittings — they join branches of the system. Tees, wyes, elbows, unions, bushings. Most often misfiled: a flange, which the regulation lists among piping materials and appliances alongside pipe, tubing, fittings and valves.
• Fluid conditioner fittings — they operate on the fluid contained in the system. Traps, drains, strainers, separators, filters, meters. Most often misfiled: a strainer called a valve because it sits in the line with a handle on the cover.
• Special purpose fittings — they accommodate movement or disconnection. Expansion joints, slip joints, rotary joints, quick disconnect couplings. These may be subject to special design and testing requirements prescribed by the Commandant.
• Not fittings at all — valves and pipe, by the definition's own words. Thermometer wells and similar fittings forming part of the pressure barrier are included.

A nonstandard fitting is a component not fabricated under an adopted industry standard. Whether a fitting is standard or nonstandard is a question about the standard it was built to, not about its shape or its service.

Working a question

A gear-type lubricating oil transfer pump discharges into a line built of pipe computed good for 300 psig. A basket strainer in the discharge is rated 150 psig; every other component is rated 300. What relief protection is required, at what setting, and to what capacity?

1. Ask first whether the pump is exempt. The exemption at a pump discharge reaches only centrifugal pumps so designed and applied that pressure above system MAWP cannot be developed. A gear pump is positive displacement and will keep building against a closed discharge. Relief valve required.
2. Establish the system MAWP. The strainer's 150 psig is the lowest component rating in the line, so system pressure is limited to 150 psig, not the pipe's 300.
3. Set the valve. The setting must not exceed system MAWP: 150 psig or lower. Take 150.
4. Size the capacity. Capacity must be sufficient to prevent the pressure rising more than 20 percent above 150 psig — a ceiling of 180 psig with the pump running at full discharge into the relief.
5. Check the certification. Rated relieving capacity must come from actual flow test data, certified by the manufacturer at 120 percent of the set pressure — here 180 psig, because the setting was taken at MAWP.
6. Check the valve's pedigree. It must be certified under part 50 for valves and meet § 54.15-10; unlike the pipe, flanges and stop valves in the same line, a safety relief valve does not escape specific approval, being covered by part 162.
7. Check the paperwork. Lubricating oil piping is on the list of diagrams submitted for approval before installation, and the pump's rated capacity and shutoff head must appear on that diagram. No characteristic curve is required — that obligation reaches firemain and foam pumps.

Change one fact and watch the answer move: make the pump centrifugal with a shutoff head of 120 psig against a 150 psig system, and it satisfies the exemption on its own characteristics.

Where candidates lose the point

"Centrifugal pumps don't need relief valves." Half-learned and it costs the question. The exemption is conditional on the pump being so designed and applied that pressure above system MAWP cannot be developed. A centrifugal pump with a shutoff head above the system MAWP gets a relief valve like anything else.

Setting the valve at 120 percent of MAWP. The 120 percent figure belongs to the manufacturer's capacity certification and is referenced to set pressure. The setting itself must not exceed MAWP. A distractor offering "set at 120 percent of maximum allowable working pressure" is built for this error.

Reading the 20 percent as an operating allowance. It is the accumulation the relieving capacity must hold the system inside of while the valve is doing its work, not headroom you may run the system into.

Rating the system on the pipe. The pipe is usually the strongest thing in the line and the easiest number to find on the drawing. The rule takes the lowest component MAWP — the valve, the fitting, the strainer.

Calling a valve a fitting. The definition of fitting excludes valves and pipe by name. A question listing four components and asking which is not a fitting is answered by that clause, not by appearance.

Assuming everything in the line needs Commandant approval. Pipe, tubing, fittings, flanges and valves need certification under part 50 and compliance with materials, construction, markings and testing requirements — not specific approval. Safety relief valves are the exception.

Answering "pump curves" for every pump. Characteristic curves attach to firemain and foam pumps, and are excused when three specified points appear on the drawing. Rated capacity and shutoff head, by contrast, appear on the diagram for every pump.

Check yourself