Hydraulic and Pneumatic Systems: Set Pressures, Holding Devices and Fluid Condition

Relief device set to relieve at 2,500 psi — the system's maximum operating pressure. Piping and piping components designed for a burst pressure of not less than 10,000 psi, four times that figure. The four-times factor is a design property of the pipe, never a valve setting.</details>

Controls, holding devices and fail-safe

Controls for hydraulic equipment are located where the operator has an unobstructed view of the equipment and the adjacent working area, and hydraulic steering equipment is expressly excepted from that requirement . Controls are also arranged so the operator can quickly disengage the equipment in an emergency. Under the operator must additionally be protected against falling or swinging objects and cargo.

Loss of pressure is where the consequence lies. Hydraulically operated machinery must be equipped with a holding device to prevent uncontrolled movement due to loss of hydraulic system pressure. §28.880 allows the alternative of a fail-safe arrangement and defines it: a system is fail-safe if a component failure results in a slow and controlled release of the load so as not to endanger personnel. That definition is the operative test. A system that drops its load quickly on a hose failure is not fail-safe, however predictable the drop.

Hose and fittings

Nonmetallic flexible hose is used only between two points of relative motion, including between a pump and a piping system, and must meet SAE J 1942. Where the application is not subject to torsional loading, hose and hose assemblies are limited to a length of not more than 30 inches (0.76 metres) and are installed in accordance with the manufacturer's rating and guidelines .

§28.880 handles length differently: hose assemblies between two points of relative motion are limited to the least amount of length that will afford maximum multidirectional movement of the equipment served, with no inch figure given. That section also requires hose end fittings to comply with SAE J1475, field attachable fittings to be installed by the manufacturer's recommended method, and each nonmetallic hose to be marked with the manufacturer's name or trademark, type or catalog number, and maximum allowable working pressure. Existing piping, hose assemblies and components may stay in service so long as they are maintained in good condition to the satisfaction of the Coast Guard Representative, but every new installation or replacement meets the current requirements of the section.

For the systems reached by subpart 58.30, hose and fittings meet subpart 56.60, hose assemblies must not be subjected to torsional deflection under any condition of operation, lengths are held in general to what flexibility requires, and sharp bends are avoided 46 CFR §58.30-20. Longer lengths may receive special consideration where machinery cannot otherwise operate properly.

Which systems the detailed rules reach

The detailed requirements of subpart 58.30 attach to a named list of installations: main and auxiliary steering apparatus including bow thruster systems; cargo hatch operating systems, unless fitted with an alternate mechanical means and approved by the Commandant as fail-safe; watertight door operating systems; automatic propulsion boiler control systems; starting systems for internal combustion engines used for main propulsion, auxiliary power, the prime mover of required emergency apparatus or manoeuvring thruster propulsion; centralised control systems for main propulsion and auxiliary machinery; lifeboat handling equipment; controllable pitch propeller systems; remote control installations for the piping systems listed in §56.01-10(c)(1); all systems containing a pneumatic or hydropneumatic accumulator; cranes, hydraulic elevators and other material or personnel handling systems not approved as fail-safe; any system in the cargo area of pump rooms on a tank vessel or in cargo-handling spaces on a liquefied flammable gas carrier; all pneumatic power and control systems with a maximum allowable working pressure over 150 psig; and any other system the Commandant judges a hazard to seaworthiness or personnel 46 CFR §58.30-1.

Anything outside that list is not unregulated. It must still submit system diagrams for the record, use hydraulic fluid complying with §58.30-10, be tested per §58.30-35(c)(2), and have all pneumatic cylinders comply with §58.30-30 46 CFR §58.30-50.

Condition of the fluid and the air

Because continued service of existing hydraulic components turns on their being maintained in good condition, the maintenance content is examinable in its own right. Hydraulic fluid transmits force, lubricates the components and carries away heat, so its condition governs the health of the whole system. Dirt is the leading cause of hydraulic trouble: it scores pumps and valves and jams close-fitting spools. Water or air contamination produces spongy, erratic operation, which is why air is bled out and fine filters, reservoir level and oil condition are kept up NAVEDTRA 14075 §7-2. The recurring faults are leaks, contamination, and worn or sticking valves, and the relief valves that protect the system from overpressure are set and tested rather than assumed good.

On compressed air systems the principal source of trouble is the compressor. Maintenance procedures are essentially the same across compressor designs and capacities, with high pressure compressors requiring additional safety precautions and adherence to the manufacturer's procedures NAVEDTRA 14075 Ch. 7 — Compressed Air Systems. Gasketed joints, pipe joints and bolts are sized for the specified torque applied with the correct tool; if a joint cannot be brought up tight without an oversized wrench or a handle extension, something is wrong with the assembly, and the extension will distort the gasket surface or twist the bolt off without achieving a tight joint.

Telling it apart

The criterion is what the number does — whether it is a setting, a design margin, or a threshold that switches a rule on.

• Relief valve setting — the system's maximum operating pressure, no margin added; at least one such device per hydraulic system 46 CFR §28.405. Misfiled as "four times".
• Piping and piping components — burst pressure not less than four times the maximum operating pressure. A design property, never verified by lifting a valve.
• Fluid power cylinders — bursting pressure not less than 4 times the maximum allowable working pressure, proven by calculation or certified burst test report 46 CFR §58.30-30.
• 150 psi / 150 psig — the pneumatic threshold. Over 150 psi MAWP the system needs surge tank or pulsation dampening 46 CFR §58.30-5, and over 150 psig it falls inside the §58.30-1 list.
• 30 inches — the §28.405 hose length ceiling where the application is not subject to torsional loading. §28.880 substitutes least length affording maximum multidirectional movement.

Hydraulic against pneumatic separates on one property: compressibility. A confined liquid transmits pressure equally in all directions and is practically incompressible, so hydraulics develop the large controllable forces wanted for steering gear, windlasses, winches, cranes, watertight doors and hatches, and controllable pitch propellers. Air is compressible, which makes it a cushion rather than a stiff medium NAVEDTRA 14104 Ch. 2 — Principles of Pneumatics, so pneumatics are used chiefly for control signals and light duty, and are inherently fail-safe in the sense that a spring returns the actuator to a known position on loss of air NAVEDTRA 14075 §7-2. That everyday sense of fail-safe is not the regulatory definition in §28.880(g).

Working a question

A hydraulic deck crane runs at a maximum operating pressure of 1,500 psi. The lifting ram has an effective piston area of 12 square inches. A nonmetallic hose spans the pump and the rigid piping, and the system is not fitted with an accumulator.

1. Identify the governing principle for the force. Pressure exerted at any point upon an enclosed liquid is transmitted undiminished in all directions, and the force acting on a piston is directly proportional to its area NAVEDTRA 14104 Ch. 2 — Principles of Hydraulics. Force is pressure times area.
2. Compute the ram force. 1,500 psi × 12 in² = 18,000 lb.
3. Fix the relief setting. One pressure relieving device minimum, set to relieve at 1,500 psi 46 CFR §28.405. Nothing in the arithmetic above changes that setting.
4. Fix the piping design. Burst pressure not less than 4 × 1,500 = 6,000 psi, and the cylinder itself not less than 4 times its maximum allowable working pressure 46 CFR §58.30-30.
5. Deal with the suspended load. A holding device is required to prevent uncontrolled movement on loss of hydraulic pressure, unless the installation is fail-safe as §28.880(g) defines it.
6. Check the hose. It runs between two points of relative motion, pump to piping, which is the permitted use. If the application is free of torsional loading, its length must not exceed 30 inches.
7. Locate the crane in the regulatory scheme. Cranes and material or personnel handling systems not approved by the Commandant as fail-safe are inside the subpart 58.30 list 46 CFR §58.30-1, so the detailed requirements apply rather than the §58.30-50 minimum set.

Where a gauge reading downstream of a reduced-diameter section is at issue, apply Bernoulli's principle instead: at a constriction the velocity increases and the pressure decreases, and static pressure at any point is the original static pressure less the velocity head at that point and less the friction head consumed reaching it . Velocity head returns as pressure when the fluid slows; friction head does not come back.

Where candidates lose the point

Setting the relief device above maximum operating pressure to stop it lifting in service. It attracts because it is what a careless watchstander would do to a nuisance valve. The regulation names the setpoint as the system's maximum operating pressure, and no allowance above it is provided.

Reading "four times" as a test pressure or a relief setting. It is a burst pressure the piping, components and cylinders are designed to, established by drawings and calculations or a certified burst test report for cylinders.

Marking a hydraulic steering installation deficient because the operator cannot see the rams from the control. Hydraulic steering equipment is excepted by name from the unobstructed-view requirement. The quick-disengage requirement is not similarly excepted.

Calling an installation fail-safe because the load stops when pressure is lost. Under §28.880(g) fail-safe means a component failure produces a slow and controlled release of the load so as not to endanger personnel. The same definition governs the cargo hatch exemption in §58.30-1(a)(2).

Treating a small pneumatic control system as outside subpart 58.30 by size. Scope turns on function and pressure, not size: over 150 psig MAWP, or containing a pneumatic or hydropneumatic accumulator, puts it in the list.

Blaming the pump for spongy, jerky deck machinery. Air or water contamination causes spongy, erratic operation, and trapped air is bled out for exactly this reason. Dirt, not wear, is the leading cause of hydraulic trouble.

Applying the 30-inch hose limit everywhere. It is stated in §28.405(i) and qualified by "in an application not subject to torsional loading"; §28.880 and §58.30-20 govern length in words, not inches.

Check yourself