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CHT - MSD TANKS

CHT / MSD Tanks

General Overview
Coatings described in this section are designed for application in Collection, Holding and Transfer (CHT) and Marine Sanitation Device (MSD) tanks. These tanks are used for the treatment of sewage onboard Navy ships.

In the last decade, re-preservation of shipboard tanks represented the #1 annual maintenance cost to the fleet. Re-preservation of CHT tanks alone costs over $25M annually. There are more than 600 of these types of tanks in the Navy's inventory. The Navy has seen a vast improvement in service life expectancy in the last few years with the installation of chemical-resistant high solids coating systems. However, coating systems designed for CHT and MSD tanks fail prematurely if not installed properly. These coating systems generally fail due to the lack of adequate surface preparation prior to coating installation (both new construction and repair).

The U.S. Navy has embarked on a major program to improve the reliability of tank coatings. This program involves new coatings, enhanced surface preparation requirements and strict process oversight/controls along with improved QA. The program's goal is to increase coating life expectancy for all tanks.

New Technology Areas

Chemical-Resistant, High-build, Solvent Free "Edge Wrap" Coatings
Tremendous success has been achieved in this area by shifting from solvent-based coating systems to new chemical-resistant, solvent-free, edge retentive coating systems. These new systems provide several important features. First, increased resistance to contaminants found in CHT/MSD systems is provided by coating chemistry. Second, due to their high-solids content and design, the coatings do not pull back from sharp edges during curing. These edge-retentive systems are capable of retaining at least 70% of the flat plate paint thickness on edges. This is an important feature since it eliminates many of the areas where coating failures begin. These coatings are also environmentally friendly. They are solvent-free or nearly so, which reduces volatile organic compound (VOC) emissions during painting evolutions.

Strict Process Oversight/Controls
Specific instructions have been developed for CHT/MSD tank preservation in the form of Preservation Process Instructions (PPIs). These requirements are discussed and are linked below.

Improved Surface Preparation
Enhanced surface preparation has been instituted, including strict adherence to surface preparation standards (SSPC/NACE/ASTM) and stricter soluble salts and relative humidity requirements.

Quality Assurance
QA oversight of the entire preservation process by inspectors trained in the new coating application and enhanced preservation protocols is vital to achieving the design life expectancy of these new systems. Minimum QA inspections include a check of the surface preparation and dry film thickness (DFT) of any remaining aged coating. DFT measurements after each coat are taken at a rate of at least five per 1,000 square feet of surface. Finally, the hardness of the coating is examined to ensure each coat is cured completely before the tank is filled.

Surface Preparation Requirements
Near-white blast cleaning (NACE 2/SSPC-SP 10)

Note
The relative humidity in the tank or void space must be maintained at a maximum of 50 percent from the start of abrasive blasting to cure of the topcoat.

Applicable Coatings Specifications and Approved Coatings

Product General Description
Material: Epoxy System

Note
Additional approved coatings may be listed in periodic NAVSEA Approved Coatings messages located on the Navy Community main page.

Note
For specific details on system application and design life expectancies, refer to Table 4 in NAVSEA STANDARD ITEM 009-32, "Cleaning and Painting Requirements."

U.S. Navy Guidelines and Instructions

Coating Application Process
Preservation Process Instructions (PPIs) were developed by NAVSEA 05P23 to document the latest requirements and procedures for coating applications. These documents contain information on cleaning, surface preparation, coating application, quality assurance requirements, checkpoints, and QA inspection forms. The applicable PPIs for CHT and MSD tank preservation are listed below.

Preservation Process Instructions

  • PPI 63101-000 Core
  • PPI 63001-001M-A  BALLAST TANKS, FLOODABLE VOIDS, COLLECTION, HOLDING, AND TRANSFER TANKS, AND FUEL/COMPENSATING FUEL TANKS (Abrasive Blasting) - Medium Durability
  • PPI 63101-001H BALLAST TANKS, FLOODABLE VOIDS, COLLECTION, HOLDING, AND TRANSFER TANKS, AND FUEL/COMPENSATING FUEL TANKS (Abrasive Blasting)

Key Application Requirements
Key application requirements to be used during preservation practices are referenced in a variety of document sources. These parameters should be considered and monitored during all preservation practices to assist in ensuring the longevity and service life expectancy of a coating. The requirements specified in the job order, ship specifications, 009-32, NSTMs, PPIs and additional repair documents have been tested and have proved the most effective measure to ensure coating service life. Some of the requirements referenced include environmental and quality assurance monitoring such as dehumidification, temperature, surface profile measurements and coating thickness measurements. For more information on these parameters, see Key Application Requirements.

Repair Requirements

Ship's Force
Ship's force personnel generally do not perform any CHT/MSD tank preservation work.

Repair Activities
The Standard Specification for Ship Repair and Alteration Committee (SSRAC) is responsible for providing technically and contractually sound standards for the Navy's ship repair and alteration community. The NAVSEA Standard Item applicable to surface ship preservation is Standard Item 009-32, "Cleaning and Painting Requirements." It contains cleanliness, surface preparation, and coating application requirements, along with complete system application instructions for each product (number of coats, coating thickness per coat, etc.) approved for CHT/MSD Tank preservation.

All current NAVSEA Standard Items may be found here.

New Construction Ships
New construction ship CHT tanks are painted in accordance with the applicable sections of the Ship Specification for that class of ship.