Overview
ISO 5621:1984 - "Shipbuilding - Bilge mud boxes for machinery spaces and tunnels - General design characteristics" defines the general design characteristics and testing/definition terminology for mud boxes used between bilge pumps and bilge suctions for coarse filtration of bilge water. It applies to both deep‑sea and inland navigation ships and was prepared by ISO/TC 8 (Shipbuilding and marine structures).
Key topics and technical requirements
- Scope and purpose: Specifies functional design characteristics (filtration, volumes, layout) for mud boxes; users must still meet any statutory or class rules applicable to the ship.
- Types:
- Straight‑through (ST): inlet and outlet on same axis.
- Right‑angled (RA): inlet vertical, outlet horizontal at 90°.
- Volume definitions:
- Total volume (VT), upstream volume (VU), downstream volume (VD), and useful upstream volume (U) are defined and illustrated.
- Volume requirement: Downstream volume VD must be at least 0.5 × upstream volume VU to minimize flow restriction.
- Useful upstream volume (U): Standard gives minimum U values related to nominal branch size (DN). Numeric thresholds are provided in the standard for ST and RA types for DN ranges (40, 50, 65 and >65).
- Perforated strainer plate: Hole area of each aperture is approximately 79 mm²; the standard defines the strainer plate open‑area coefficient (ratio of aperture area to plate area) and the mud box open‑area coefficient (ratio of plate aperture area to branch cross‑section).
- Minimum open‑area coefficients (cmin) by DN:
- DN 40: 4.0; DN 50: 3.5; DN 65: 3.2; DN 80: 2.7; DN 100: 2.7; DN 125: 2.7; DN 150: 2.5; DN 200: 2.4; DN 250: 2.4.
- Materials guidance: Mud boxes may be made from cast iron or carbon steel (minimum 3 mm thickness). Carbon steel units should be hot‑dip galvanized or otherwise protected; where galvanized, zinc deposition should be at least 600 g/m². Alternative materials may be used by agreement.
Applications and users
- Primary use: Coarse filtration of bilge water ahead of bilge pumps in machinery spaces and tunnels.
- Who uses it: Shipyards, marine equipment manufacturers, naval architects, ship operators, classification societies, purchasers, and regulatory bodies specifying or verifying bilge system components.
- Practical benefits: Ensures consistent filtration performance, reduced pump blockage risk, clearer component specifications for procurement, and demonstrable compliance with recognized international guidance.
Designation example
- Standardized product labelling example: Mud box ISO 5621‑RA‑100
Related standards
- Prepared by ISO technical committee ISO/TC 8 (Shipbuilding and marine structures); consult other ISO shipbuilding standards and relevant class society rules for complementary requirements and statutory compliance.
Keywords: ISO 5621, mud box, bilge mud box, shipbuilding standard, bilge water filtration, strainer plate, open area coefficient, nominal size DN.
