TL;DR — Hydraulic piping must be rated to burst at no less than four times maximum operating pressure, and every hydraulic system must have at least one pressure-relieving device; positive-displacement air compressors must never run against a closed discharge, so each stage carries its own relief valve.
What the Rule Says
Hydraulic Systems — Design and Pressure Requirements
A hydraulic system transmits power through pressurized liquid (oil), exploiting the fact that a confined liquid transmits pressure equally in all directions and is practically incompressible — the principle known as Pascal's law. NAVEDTRA 14075 §7-2 A basic shipboard system consists of a reservoir, a pump, a relief valve, directional-control valves, and actuators (rams or cylinders for linear motion, hydraulic motors for rotation), all connected by piping.
Federal regulations impose specific design minimums on these systems. Piping and piping components must be designed with a burst pressure of not less than four times the system's maximum operating pressure. 46 CFR §28.405 46 CFR §28.880 Every hydraulic system must be equipped with at least one pressure-relieving device set to relieve at the system's maximum operating pressure. The system must be designed and installed so that proper operation is not affected by back pressure, and a malfunction of any unit must not render a connected or emergency system inoperative through back pressure. 46 CFR §58.30-5
All materials in a hydraulic system must be suitable for use with the hydraulic fluid employed and must remain ductile at the lowest operating temperature the vessel is likely to encounter. Designers must also account for the additional pressure imposed by hydraulic shock.
Operator Controls and Safety Devices
Except for hydraulic steering equipment, controls for hydraulic machinery must be positioned where the operator has an unobstructed view of the equipment and the adjacent working area. Under the commercial fishing vessel rules, operators must also be protected against falling or swinging objects and cargo. In all cases, controls must be arranged so the operator can quickly disengage the equipment in an emergency.
Hydraulically operated machinery must be equipped with a holding device to prevent uncontrolled movement due to loss of hydraulic system pressure. Under the more detailed commercial fishing vessel standard, the machinery must be either fail-safe or fitted with such a holding device. A system qualifies as fail-safe if a component failure results in a slow and controlled release of the load so as not to endanger personnel.
Flexible Hose Requirements
Nonmetallic flexible hose may only be used between two points of relative motion — for example, between a pump and the piping system. Under §28.405, hose must meet SAE J 1942 and, in applications not subject to torsional loading, must not exceed 30 inches (0.76 meters) in length. Under §28.880, hose assemblies are limited to the least amount of length that will afford maximum multidirectional movement of the equipment served, and hose end fittings must comply with SAE J1475 (Hydraulic Hose Fittings For Marine Applications); field-attachable fittings must be installed per the manufacturer's recommended practice. Each nonmetallic flexible hose must be marked with the manufacturer's name or trademark, type or catalog number, and maximum allowable working pressure.
For fluid power systems covered under Subchapter F, hose assemblies may be installed between two points of relative motion but must not be subjected to torsional deflection under any operating condition, must be limited to reasonable lengths required for flexibility, and sharp bends must be avoided. 46 CFR §58.30-20
Existing hydraulic piping, hose assemblies, and components may remain in service as long as they are maintained in good condition to the satisfaction of the Coast Guard Representative; however, all new installations or replacements must meet current specifications.
Hydraulic Fluid Cleanliness and System Maintenance
Hydraulic fluid does more than transmit force — it lubricates components and carries away heat. Dirt is the leading cause of hydraulic trouble because it scores pumps and valves and jams close-fitting spools. Water or air contamination causes spongy, erratic operation. The system is kept clean with fine filters; reservoir level and oil condition must be maintained; and trapped air must be bled out because it makes the system spongy and causes jerky motion. Hydraulic fluid used in fluid power systems must comply with the applicable fluid specification. 46 CFR §58.30-50
Pneumatic Systems — Design Requirements
Pneumatic systems use compressed air to transmit power and carry control signals. Air is compressible, so pneumatic actuators are suited to control functions and light-duty work rather than developing large, steady forces. A key safety characteristic is that pneumatic actuators are inherently fail-safe: a spring returns the actuator to a known position on loss of air pressure.
Pneumatic systems with a maximum allowable working pressure in excess of 150 psi must be designed with a surge tank or other acceptable means of pulsation dampening. Every pneumatic system must minimize the entry of oil into the system and must drain the system of liquids. All pneumatic cylinders in miscellaneous fluid power systems must comply with the applicable cylinder specification.
Air Compressors — Construction and Operation
Compressed air aboard ship is used to start large diesel engines, operate pneumatic controls and automation, run air tools, sound the ship's whistle, and blow through and clear lines. NAVEDTRA 14075 §7-1 NAVEDTRA 14104 §10-3
Most shipboard compressors are positive-displacement reciprocating machines: a piston draws air in through a suction valve on the down-stroke and forces it out through a discharge valve on the up-stroke into an air receiver. High-pressure air for diesel starting is produced in two or more stages because compressing to high pressure in a single stage generates excessive heat. Between stages, an intercooler removes the heat of compression and condenses out moisture; after the final stage, an aftercooler cools the discharge. Rotary screw compressors are widely used for lower-pressure control and service air.
Because compressing air heats it and wrings out water vapor while carrying over oil mist, the system must manage heat, moisture, and oil. Coolers control temperature; moisture separators, drain traps, and receiver drains remove condensed water; air driers may be fitted where control air must be very dry. Draining condensate from receivers, intercoolers, and separators is a routine watch duty — water carried downstream fouls pneumatic controls and, in a starting-air system, defeats engine starting and causes corrosion.
A recognized hazard is that oil mist in hot compressed air can ignite or explode. Oil carryover is therefore minimized, discharge temperatures are held within limits, and air receivers and lines are fitted with relief valves. On high-pressure systems, fusible plugs or bursting discs are fitted and must be kept clear and tested. Because positive-displacement compressors must never run against a closed discharge, each stage has its own relief valve.
Why It Matters on the Exam
Exam questions on this topic cluster around four areas:
1. The 4× burst-pressure rule. Expect a question that gives you a system operating pressure and asks for the minimum required burst rating of the piping. Multiply by four. 46 CFR §28.405
2. Pressure-relieving devices. Know that every hydraulic system requires at least one relief device set at the system's maximum operating pressure — not above it, not below it. 46 CFR §28.880
3. Flexible hose restrictions. Hose is permitted only between two points of relative motion. Under §28.405, the 30-inch length limit applies in non-torsional applications. Torsional deflection is prohibited under §58.30-20. Sharp bends must be avoided. 46 CFR §58.30-20
4. Compressor relief valves and the closed-discharge prohibition. Because a reciprocating compressor is a positive-displacement machine, running it against a closed discharge will overpressurize the stage. Each stage therefore carries its own relief valve. NAVEDTRA 14075 §7-1 NAVEDTRA 14104 §10-3
5. Pulsation dampening threshold. Pneumatic systems exceeding 150 psi working pressure require a surge tank or equivalent pulsation dampening. 46 CFR §58.30-5
Common Pitfalls
Confusing the relief valve set-point. The pressure-relieving device is set to relieve at the system's maximum operating pressure — not at four times that pressure. The 4× factor applies only to the burst rating of piping and components. 46 CFR §28.405
Misapplying the hose length limit. The 30-inch (0.76 m) maximum under §28.405 applies in applications not subject to torsional loading. Do not apply this figure universally; §28.880 uses a different standard (least length for maximum multidirectional movement), and §58.30-20 focuses on prohibiting torsional deflection and sharp bends rather than specifying a fixed length. 46 CFR §28.880 46 CFR §58.30-20
Overlooking the steering-equipment exception. The operator-visibility requirement for hydraulic controls does not apply to hydraulic steering equipment. This exception appears in both §28.405 and §28.880.
Assuming pneumatics and hydraulics are interchangeable for heavy loads. Because air is compressible, pneumatic actuators are suited to control and light-duty applications, not to developing large, steady forces. Hydraulics are the correct choice for steering gear, windlasses, winches, cranes, and controllable-pitch propellers. NAVEDTRA 14075 §7-2
Neglecting condensate draining. Water downstream of a compressor fouls pneumatic controls and defeats diesel starting-air systems. Draining receivers, intercoolers, and separators is a routine watch task, not an occasional maintenance item. NAVEDTRA 14075 §7-1 NAVEDTRA 14104 §10-3
Ignoring the oil-mist fire/explosion hazard. Oil mist in hot compressed air is an ignition and explosion risk. This is why oil carryover is minimized, discharge temperatures are controlled, and high-pressure systems carry fusible plugs or bursting discs in addition to relief valves.
Quick Check
Q1 — A hydraulic system has a maximum operating pressure of 2,000 psi. What is the minimum required burst pressure for its piping?
8,000 psi. Piping and piping components must be designed with a burst pressure of not less than four times the system's maximum operating pressure. 46 CFR §28.405 46 CFR §28.880
Q2 — At what pressure must the pressure-relieving device on a hydraulic system be set to relieve?
At the system's maximum operating pressure.
Q3 — Under 46 CFR §28.405, what is the maximum permitted length of a nonmetallic flexible hose in an application not subject to torsional loading?
30 inches (0.76 meters).
Q4 — Why must a positive-displacement reciprocating air compressor never run against a closed discharge?
Because it will overpressurize the stage. Each stage is therefore protected by its own relief valve. [NAVEDTRA 14075 §7-1](cite://navedtra-14