Pumps, Valves and Piping Under 46 CFR Part 56

What the rule requires

Relief protection at the pump discharge

Every system that may be exposed to pressure higher than its own maximum allowable working pressure must be safeguarded by appropriate relief devices, and the regulation then names the pump discharge in particular — 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 46 CFR §56.07-10.

That exception has two conditions and both must hold. The pump is centrifugal, and the installation as designed and applied cannot develop more than the system MAWP. A centrifugal pump feeding a line whose MAWP sits below what that pump can produce is not exempt.

Three numbers govern the valve itself:

Setting — must not exceed the MAWP of the system.
Relieving capacity — sufficient to prevent the pressure from rising more than 20 percent above the system MAWP.
Certified capacity point — for valves protecting piping systems only, rated relieving capacity must be based on actual flow test data and certified by the manufacturer at 120 percent of the set pressure of the valve.

The 20 percent and the 120 percent are measured from different references. One is accumulation above system MAWP; the other is the pressure at which the manufacturer certifies flow through the valve. They coincide only when the valve happens to be set right at MAWP, which is the usual case but not a rule.

Relief valves are also the one appliance in the whole piping chapter that does not enjoy the general exemption from Commandant approval — pipe, tubing, fittings, flanges and valves need no specific approval, except safety relief valves covered in part 162 46 CFR §56.01-10. Everything else must still be certified under part 50 and must still meet the applicable requirements for materials, construction, markings and testing.

The system is rated at its weakest component

MAWP must not be greater than the internal design pressure defined in 104.1.2 of ASME B31.1, and where a component such as a valve or a fitting has an 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 buys nothing if a 150-pound flanged fitting is spooled into the run.

The discharge pipe computes to 250 psig. A fitting in the line is rated 175 psig. A relief valve on that line is set at 250 psig. Acceptable?

No. The system MAWP is 175 psig, because system pressure is limited to the lowest component MAWP. A relief setting must not exceed the MAWP of the system, so 250 psig is above the ceiling by 75 psig. Reset to 175 psig or lower, and the capacity must then hold the pressure below 210 psig, which is 20 percent above 175.

Which systems are vital

Vital systems are those vital to a vessel's survivability and safety, and the regulation names them 46 CFR §56.07-5: fuel oil fill, transfer and service; fire main; fixed gaseous fire-extinguishing; bilge; ballast; steering and steering control; propulsion with its necessary auxiliaries and control systems; ship's service and emergency electrical generation with its auxiliaries. Anything not on that list is non-vital.

The grouping most instructors use is three F's, two B's, and the three that keep her moving — Fuel oil, Fire main, Fixed gas; Bilge, Ballast; Steering, Propulsion, Electrical generation. Two corrections to that device before you rely on it. The electrical entry is qualified: the generation systems and their auxiliaries count as vital insofar as they are vital to the vessel's survivability and safety. And the list is not closed — any other marine-engineering system the cognizant OCMI identifies as crucial to survival of the vessel or protection of personnel aboard is vital too. On an exam asking "which of the following is not a vital system," check the OCMI catch-all before eliminating.

What the drawings must carry

Diagrams of the listed systems must be submitted for approval prior to installation aboard ship — among them fuel oil service, transfer and filling, fire main and sprinkler, bilge and ballast, vent, sound and overflow, compressed air, lubricating oil, and boiler feed and blowoff . Arrangement drawings go in as well for all Class I, I-L and II-L systems, and for Class II fire main, foam, sprinkler, bilge and ballast, and vent, sounding and overflow systems.

For pumps, two figures must appear on the piping diagram: rated capacity and shutoff head. Pump characteristic curves are required for all pumps in the fire main and foam systems, and the only way out of submitting the curve is to show three values on the drawing — rated capacity and head at rated capacity, shutoff head, and head at 150 percent of rated capacity. Shutoff head is the figure that speaks to whether a centrifugal pump can drive the line above MAWP, which is why it is on the drawing for every pump and not just the fire pumps.

Designing for a hull that moves

Piping design must account for ship motion and flexure, including weight, yaw, sway, roll, pitch, heave and vibration . Five motions are named there; surge is not one of them, and adding it is a common way to talk yourself out of the right answer.

Three more design points that turn up as single questions. Components without specific ratings must be furnished to the Marine Safety Center for approval. Allowable stress values that are restricted by footnote or printed in italics must not be used at all, and where a material lists multiple stresses, the lowest value governs unless the Commandant approves otherwise — with temperature always understood as the actual temperature of the component. A material not listed requires permission from the Commandant before use.

Telling it apart: pipe, valve, fitting

The criterion is function within the pressure boundary — what the component does, not what it is made of 46 CFR §56.07-5.

Pipe joining fitting — joins branches of the system. Tees, wyes, elbows, unions, bushings.
Fluid conditioner fitting — operates on the fluid carried. Traps, drains, strainers, separators, filters, meters. Candidates misfile these constantly, reasoning that a strainer is equipment rather than a fitting. It is a fitting.
Special purpose fitting — expansion joints, slip joints, rotary joints, quick disconnect couplings. These may be subject to special design and testing requirements prescribed by the Commandant.
Nonstandard fitting — any piping component not fabricated under an adopted industry standard. Welding details for nonstandard fittings must be submitted as standard fabrication drawings 46 CFR §56.01-10.
Pipe and valves — outside the term entirely. "Fitting" means pressure-containing components other than valves and pipe. A thermometer well forming part of the pressure barrier is included; a gate valve is not.

Two size terms are separated the same way, by what they measure :

Nominal pipe size, or pipe size, is the commercial diameter of the piping.
Schedule relates to wall thickness, with the specific values given in ASME B36.10M and B36.19M.

Working a question

The No. 2 fuel oil transfer pump is a rotary screw type. Its discharge line computes to 200 psig, and a flanged fitting in that line is rated by its manufacturer at 150 psig. Determine whether relief protection is required, and specify the valve.

Classify the system. Fuel oil transfer is a named vital system. That governs material selection and plan submission, and it means the diagram goes in for approval before installation.
Fix the MAWP. The pipe computes to 200 psig, but the fitting is rated 150 psig, and system pressure is limited to the lowest component MAWP. The system MAWP is 150 psig. Every number that follows keys off 150, not 200.
Apply the exception test. The exemption from a discharge relief valve reaches only centrifugal pumps. This pump is not centrifugal, so the test ends there and a relief valve is required. Had it been centrifugal, the question would become whether it is so designed and applied that it cannot develop more than 150 psig — read shutoff head off the diagram and compare.
Set the valve. The setting must not exceed system MAWP. Set at 150 psig; anything above that fails on its face.
Size the capacity. Relieving capacity must prevent the pressure rising more than 20 percent above 150 psig, so the line must not exceed 180 psig with the valve lifting. The manufacturer certifies the rated relieving capacity at 120 percent of the 150 psig set pressure — also 180 psig here — from actual flow test data.
Close out the paperwork. The relief valve falls under part 162 and is certified as required in part 50 for valves, and must also meet § 54.15-10. The fuel oil transfer diagram carries the pump's rated capacity and shutoff head.

Watch what the distractors will do with step 2. An option offering 200 psig as the relief setting, and one offering 240 psig as the maximum accumulated pressure, are both built from the pipe figure instead of the fitting figure.

Where candidates lose the point

Answers "no relief valve — it's a centrifugal pump." The exception is conditional, and the condition is that the pump as designed and applied cannot develop pressure above system MAWP. Centrifugal alone does not carry it.
Reads the 20 percent as a setting tolerance and sets the valve 20 percent above MAWP. The setting must not exceed MAWP; the 20 percent is the ceiling on pressure rise while the valve is relieving.
Sizes the certified capacity at 120 percent of MAWP. It is 120 percent of the set pressure of the valve, which is a different reference whenever the valve is set below MAWP.
Answers that pipe, flanges and valves must be specifically approved by the Commandant. They need not be — but they must be certified under part 50 and meet materials, construction, markings and testing requirements. Safety relief valves under part 162 are the exception that makes this question worth asking.
Rates the system off the pipe. The lowest-rated component in the run sets MAWP, whether that is a valve, a flange or a fitting.
Treats ballast as non-vital because nothing in it burns or turns. Bilge and ballast are both on the vital list, and both require diagrams and arrangement drawings before installation.
Calls a strainer, steam trap or fuel oil meter something other than a fitting. All are fluid conditioner fittings.

Check yourself

A centrifugal fire pump discharges into a main with an MAWP of 175 psig. The pump's shutoff head corresponds to 160 psig. Is a discharge relief valve required?

No, provided the pump as designed and applied cannot develop pressure in excess of the system MAWP — which is the case where the pump cannot exceed 175 psig on the discharge. That is the sole exception to the requirement for relief valves at pump discharges. Change the pump to a non-centrifugal type on the same line and the valve becomes mandatory regardless of what pressure it can develop.

A relief valve is set at 120 psig on a system whose MAWP is 140 psig. At what pressure must the manufacturer certify its rated relieving capacity, and what pressure must the valve keep the system below?

Certified at 120 percent of the set pressure — 144 psig — on actual flow test data. The valve must have capacity enough to prevent the pressure rising more than 20 percent above the system MAWP of 140 psig, so below 168 psig. The two references differ because the valve is set 20 psig below MAWP.

You are checking a submitted diagram for a foam system pump. The characteristic curve is not attached. What must appear on the drawing instead?

Three values: rated capacity and head at rated capacity, shutoff head, and head at 150 percent of rated capacity. Curves are required for all fire main and foam system pumps unless those figures are shown. Separately, every pump on a piping diagram must show rated capacity and shutoff head.

Which of these is not a vital system: the emergency fire pump's supply, the ballast main, the potable water system, the steering gear hydraulics?

The potable water system. Fire main, ballast, and steering and steering-control systems are all named as vital. Anything not identified in the list is non-vital — subject to the OCMI's authority to identify any other marine-engineering system as crucial to the vessel's survival or the protection of personnel.

An installed spool piece uses a quick disconnect coupling. How is that component classified, and what follows from it?

It is a special purpose fitting, along with expansion joints, slip joints and rotary joints. Special purpose fittings may be subject to such special design and testing requirements as the Commandant prescribes.

A shipyard proposes an alloy for a fuel oil transfer line that is not among the listed materials. What is required?

Permission from the Commandant before the material is used. Testing requirements in subpart 56.97 may affect the design and should be considered at that stage, and specific systems carry their own design limitations under subpart 56.50.

Piping for an industrial system aboard a mobile offshore drilling unit — does Part 56 apply in full?

No. Piping for industrial systems on MODUs need not fully comply with this part, but must meet subpart 58.60, and the plans go in under that subpart 46 CFR §56.01-1. The vessel's own marine systems are not covered by that relief.