Diagnosing Hydraulic System Pulsation and Noise

Pulsation, knocking or an unusual whine in a hydraulic system is one of the more common faults reported on board — and one of the more frequently misdiagnosed. It's often treated as a pump problem by default, when the pump is frequently just the component transmitting a problem that originates elsewhere in the circuit.

Start with what the symptom actually tells you

Pulsation and noise are two different symptoms that often get treated as one. It's worth separating them before doing anything else:

• Pulsation — a rhythmic pressure fluctuation, often felt through pipework or visible on a pressure gauge as needle flutter, usually tied to pump shaft speed or a multiple of it.
• Noise — audible knocking, whining or cavitation-type sound, which may or may not correlate with the pulsation frequency.

The two commonly occur together, but they don't always share the same root cause, and treating them as identical can send a diagnosis in the wrong direction from the start.

Common causes worth ruling in or out

Accumulator pre-charge

A hydraulic accumulator's job in a pulsation-damping role depends entirely on its pre-charge pressure being correct relative to system operating pressure. An accumulator with low or lost pre-charge (through a slow gas-side leak, or simply having been charged incorrectly at commissioning) loses most of its ability to absorb pressure spikes, and pulsation that the system was designed to tolerate becomes noticeable or damaging. This is worth checking directly — with the correct gauge, isolated from system pressure — rather than assumed from the accumulator's presence alone.

Air ingress

Air in a hydraulic circuit compresses under pressure in a way the oil doesn't, and that compressibility is a common source of both noise and erratic pulsation. Typical entry points include:

• A suction line fitting or seal that leaks air in under vacuum, even though it wouldn't leak oil out under pressure
• Reservoir level low enough to allow the return line or suction line to draw air, particularly under roll or pitch at sea
• A worn pump shaft seal, allowing air past the low-pressure side
• Air not properly purged after maintenance work on the circuit

Air ingress on the suction side is often intermittent and load- or motion-dependent, which is a useful diagnostic clue in itself — a fault that worsens at sea and settles alongside can point toward the suction side rather than the pump itself.

Pump condition and cavitation

Cavitation — vapour bubbles forming and collapsing within the pump due to insufficient suction pressure — produces a distinctive noise, often described as gravel or marbles in the pump. This can stem from a restricted suction line, a clogged suction strainer, oil that's too cold and viscous, or a suction lift that's simply too great for the pump's design. It's worth distinguishing genuine cavitation from air-ingress noise, since the corrective action is different in each case.

Pipework and mounting

Pulsation that's mechanically transmitted through rigid pipework, without adequate flexible sections or clamping, can resonate at points remote from the pump itself — sometimes making the noise appear to come from an entirely different part of the vessel. Pipe run length, support spacing and the presence (or absence) of pulsation dampers at the right points in the circuit all affect how much of the pump's inherent pulsation actually becomes audible or damaging elsewhere in the system.

A practical diagnostic sequence

Rather than replacing components on a guess, a structured approach generally holds up better:

1. Confirm the symptom with data — pressure readings, timing relative to pump speed, and whether it's constant or intermittent.
2. Check accumulator pre-charge directly, isolated from system pressure.
3. Inspect the suction side for air ingress — fittings, reservoir level, shaft seal condition.
4. Rule cavitation in or out based on suction pressure and noise character.
5. Only then consider the pump internals, which are usually the most expensive and time-consuming thing to investigate — and often not the actual cause.

General guidance versus vessel-specific diagnosis

The points above are common causes and a reasonable starting sequence, not a guarantee of what's wrong with any particular system. Hydraulic circuits vary enormously in design, and a fault that's typical on one system layout may be unusual on another. Where a fault has resisted a straightforward fix, or keeps recurring after repair, that's usually the point where an independent, on-site investigation is more useful than further remote guesswork.

This is the kind of problem Torven Marine is engaged for directly — see our troubleshooting & investigations service, or marine troubleshooting support in Vietnam if the vessel is operating in the region. If you're dealing with a hydraulic pulsation or noise issue that hasn't resolved with the usual checks, get in touch and tell us what you're seeing.