The Deck Seal That Looked Wet

A deck seal can look familiar, be entered confidently in the HVPQ, and still be misunderstood. On this vessel, the answer was not in the casing. It was in the way the water moved when inert gas started flowing.

Before a SIRE inspection, the HVPQ is checked line by line. During that review, the entry which stood out was that the deck seal had been recorded as wet type. Going by previous shipboard experience and the external appearance of the unit, wet type looked like a reasonable entry.
The unit had all the familiar external features like the inert gas line entering, line leaving towards the deck main, float switch, level indication, drains, and the usual fittings around the casing. Nothing visible from deck level forced another conclusion.

Yet this was almost a mirror image of a previous shipboard review. On that earlier vessel, the HVPQ correctly listed the deck seal as semi-dry, but the Chief Officer could not explain the operating principle behind it. On this vessel, the entry was incorrect, although the external appearance made the mistake logical. A closer look at the Kashiwa drawing and operating manual pointed towards the same semi-dry logic I had seen before, later confirmed by the manufacturer as a semi-dry water-displacement type seal.

One doubtful HVPQ entry can be corrected onboard but finding the same confusion across two vessels, from opposite directions deserved a closer look. A deck seal can be visually familiar, correctly or incorrectly recorded on paper, and still be operationally misunderstood. A correct answer in the HVPQ may satisfy the entry, but it is important that the crew understands the safety barrier protecting the vessel. The same difference between a checklist entry and the reason behind it was discussed in CO₂ on Oil Tanker Entry Permits.

The Barrier Behind the Entry

An incorrect HVPQ entry may appear to be a minor documentation error, but during a SIRE inspection it can become a formal observation. The deck water seal is part of the non-return protection preventing hydrocarbon vapour from returning towards the inert gas plant, boiler uptake, gas-safe areas and it is a primary safety barrier. The deeper concern is what a wrong entry reveals about an officer’s understanding of shipboard safety equipment.

The barrier function becomes critical when cargo tank pressure reverses. The seal-water column is what prevents vapour from returning towards the gas-safe side.

Under SIRE 2.0, the inspection is not limited to matching questionnaire entries against physical hardware. The inspection also tests whether the crew can explain the principle, basic handling and maintenance of critical tanker equipment fitted on their own vessel. IMO MSC/Circ.353, describes deck water seal types by their functional arrangements, including the familiar semi-dry arrangement where inert gas flow draws sealing water into a separate holding chamber by venturi action. If an officer states that the unit is semi-dry, but explains only a generic venturi mechanism when a water-displacement unit is fitted, the weakness is immediately visible.

Commercial vetting standards make this distinction even sharper. ExxonMobil MESQAC 2017, accepts wet and semi-dry deck seals, but does not accept dry deck seals. For vessels delivered after 30 November 2017, semi-dry seals relying on older venturi or equivalent arrangements are not acceptable. New semi-dry deck seals are required to be of the water-displacement type.

Selecting “semi-dry” on paper is therefore not enough. The vessel may be fitted with the correct water-displacement type and the equipment may be in good condition, but the inspection can still go wrong through the officer’s explanation. If the officer describes a venturi-type semi-dry seal, the inspector may reasonably conclude that the vessel is fitted with the older venturi arrangement. He may not open the maker’s drawing at that moment or recheck the internal details. The observation may simply record that the vessel appears to have a venturi-type semi-dry deck seal, which may not be acceptable under some oil-major criteria for newer vessels. This observation can affect vetting acceptance and may cost the vessel the next cargo.

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The Mechanics of Misidentification

The confusion surrounding deck water seals often comes from reliance on visual memory rather than operating physics. Officers build a mental catalogue of equipment throughout their careers. When a unit has a traditional casing, seal-water piping, external drains, level indication, and familiar deck fittings, recognition can easily default to wet type. Conversely, when many officers think of a semi-dry seal, they picture the classic IMO arrangement with a venturi nozzle and a separate holding chamber.

The familiar wet and semi-dry sketches explain why officers often carry two mental pictures: bubbling through water for wet type, and venturi action with a holding chamber for semi-dry type.

The fitted Kashiwa unit did not fit neatly into either shortcut. Resolving the HVPQ entry required moving beyond external appearance and looking at the physical movement of water across three distinct design principles:

Wet Type: Inert gas continuously bubbles through a standing water trap during operation. The principle is simple, but the gas remains in direct contact with the water seal throughout operation. The operational trap is assuming that water carryover or demister loading is normal simply because the seal has been identified as wet type.
Venturi Semi-Dry Type: Inert gas flow passes through a venturi nozzle, creating a low-pressure area that draws sealing water into a separate holding chamber. The gas passage does not rely on continuous bubbling through the seal water, but narrow openings can be vulnerable to soot or scale blockage. The operational trap is carrying that venturi-and-holding-chamber picture to a water-displacement unit and then looking for components or fault symptoms that do not apply.
Water-Displacement Semi-Dry Type (Kashiwa): When the system is stopped, water seals the inlet pipe. Once the inert gas fan starts, gas pressure pushes the water away in the guide pipe. Water carried with the gas strikes the baffle plate and drains, while the gas then passes through the demister pad. The operational trap is mistaking this for a simple wet seal and applying the wrong assumptions to water flow, drain condition, alarm response, or backflow protection.

The Kashiwa manual shows the real clue: gas flow pushes seal water away through the guide pipe, while carried water strikes the baffle plate and drains before gas passes through the demister pad.

DeepDraft made a similar design-basis point in MEG-4 Mooring Numbers: the numbers only matter when the crew understands what they are tied to. The unit looked wet from the deck and did not resemble the familiar venturi semi-dry sketch, but the operating sequence showed water displacement rather than continuous bubbling through a standing water trap.

The Kashiwa drawing did not clearly identify the seal type in the title block or main description, leaving the crew to classify the unit from the operating sequence. The main drawing or maker’s manual should state the deck seal category clearly: wet, semi-dry by venturi arrangement, or semi-dry by water-displacement principle. True operational competence depends on understanding what the liquid does when gas starts flowing, not on recognising the casing from another ship.

DeepDraft View

Familiar equipment creates confidence, and sometimes that confidence is inherited rather than earned. DeepDraft made the same point in Fire Wires on Tankers: a visible safety control still needs a valid operational basis. Onboard entries often pass from previous HVPQs, handover notes, office replies, or earlier inspection records. Over time, the entry remains while the physical reasoning behind it disappears.

This ambiguity highlights a documentation weakness in the maritime supply chain. Equipment manufacturers and shipyards should remove interpretation from safety-critical barriers wherever possible. General assembly drawings for deck water seals should clearly state the applicable SOLAS/IMO seal classification in the drawing title block or main technical description, instead of leaving the crew to extract the answer from operational clues inside the instruction manual. IMO guidance should also recognise water-displacement semi-dry seals more clearly as a distinct semi-dry arrangement, so modern designs are not judged only against the older venturi-based sketch.

IMO guidance must keep pace with modern deck seal arrangements.

Misidentifying a deck water seal carries direct operational risk. An officer who mistakes a water-displacement semi-dry seal for a simple wet type may apply the wrong assumptions to seal water supply, drain status, alarm response, or fault diagnosis during cargo operations. When cargo tank pressure reverses, the vessel is protected by the physical behaviour of the seal, not by the label entered in a questionnaire. The crew’s understanding has to match the equipment actually fitted on deck.

Sources Reviewed