P6 – Shell Type Exhaust Gas Heated Economizer

APPLICATION FIELD

IACS UR P6, “Shell Type Exhaust Gas Heated Economizers That May Be Isolated From The Steam Plant System,”

This set of rules applies to a specific category of waste-heat recovery equipment: shell-type economizers that raise steam or heat water using main engine or generator exhaust gas, arranged so that they can be valved off from the ship’s steam plant on the water/steam side while remaining physically in the exhaust gas flow path.

That last detail is the whole reason the UR exists as a standalone requirement.

it targets the specific configuration where the water circuit can be isolated but the heat source can’t be switched off the way a fired boiler’s burner can.

In practice, this covers the common arrangement on motor vessels where exhaust gas economizers reduce auxiliary boiler oil-firing by recovering heat that would otherwise go straight up the funnel.

PIPE CLASS / SYSTEM TYPE

This is a case where the “pipe class” question doesn’t apply. P6 isn’t a piping classification rule at all, an economizer is a heat exchanger, so its own pressure parts sit under general boiler and pressure vessel construction requirements rather than the P2.2 Class I/II/III system.

Where P2 classification does come into play is on the steam and feed piping connected to the economizer, which gets classified and constructed under P1/P2 in the ordinary way.

So the economizer shell itself and the piping running to and from it are governed by two different parts of the Rules, joined at the nozzle connections.

RELATED RULES

• Direct sibling: UR P1 and P2 govern the steam and feed water piping connected to the economizer, even though the economizer’s own pressure parts fall outside the piping classification system.
• Cross-reference: UI LL36, cited alongside P1’s own minimum wall thickness provisions, is mentioned in the IACS P-series contents alongside P6, reflecting the shared Load Line Convention backdrop for steam and related piping strength requirements.
• Cross-reference: IACS Recommendation Nos. 4 and 5 are referenced directly alongside P6 in the official IACS P-series listing, providing supplementary guidance connected to this requirement.
• Cross-reference: UI SC249 explicitly lists exhaust gas economizer components: casing door packing, manhole and hand-hole packing, gas shield packing for the soot blower, and flange packing and lagging for the associated steam, exhaust, fuel and drain line piping; 
• Statutory backdrop: General boiler and pressure vessel construction requirements (the equivalent of a Classification Society’s Boilers, Heat Exchangers and Pressure Vessels rule part) supply the baseline pressure-part design that P6 layers its isolation-specific safety requirements on top of.

RESTRICTION

The defining restriction in P6 is really the scope condition itself: the UR only apply for economizers so they can be isolated from the steam plant on the water side while exhaust gas keeps flowing through the shell.

An isolated-but-still-hot economizer is exposed to dry-firing and to the soot-fire risk that’s well documented in this equipment category, where accumulated unburnt carbon deposits on the gas side can ignite once conditions align, heat already present from the exhaust stream, air from scavenging, and a fuel source in the form of the deposits themselves.

DESCRIPTION

A conventional oil-fired boiler has a simple safety strategy: cut the fuel. An exhaust gas economizer offers no equivalent shutoff, because the main engine keeps running and keeps producing hot exhaust gas regardless of what’s happening on the water side of the economizer.

That difference is precisely why the UR’s deal in depth with economizers “that may be isolated from the steam plant system” as the trigger condition, the moment you give an operator the ability to valve the water side off for maintenance, standby, or load management, you’ve created a scenario where the shell keeps absorbing heat from the gas side with reduced or no water flow to carry that heat away. On top of the dry-firing concern sits the soot-fire risk, incomplete combustion upstream deposits unburnt carbon on the gas-side surfaces, and given enough heat, air and the right conditions that deposit can ignite, a failure mode that shows up regularly enough in service. (For this reason regular soot blowing and water washing are standard operational practice).

We’d read P6, in substance, as codifying the safety devices, protective arrangements and construction expectations that respond to this specific dry-firing and soot-fire exposure, layered on top of the general pressure-vessel construction rules that already govern the shell as a piece of pressure equipment.

IMPORTANT NOTE

• Soot fire history on exhaust gas economizers is well documented in service, and inadequate provision for cleaning access, water washing or soot blowing tends to surface as a recurring maintenance and safety finding rather than a one-off design flawn.

• Gasket and packing materials on the economizer casing, manholes, hand holes and associated soot blower and piping connections need to meet the non-asbestos requirements of UI SC249, a substitution using older asbestos-containing material during repair or overhaul is a straightforward compliance gap.