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Treated Water Tank Linings Repair

by Mar 4, 2016Uncategorized

Introduction

Nuclear Power Plants are going to be facing decisions on re-lining the aging fleet of their treated water vessels. The majority of these vessels were purchased with previously installed shop applied 3/16 inch butyl rubber liners, the duplication of these liners in place will pose a great challenge. I have chosen to present this paper to help plan for alternatives for these upcoming projects. wpfigure1

TRADITIONAL RUBBER LINERS FOR TREATED WATER

MATERIAL DESCRIPTION

A synthetic rubber, or elastomer, butyl rubber is impermeable to air and used in many applications requiring an airtight low permeation rubber liner. Polyisobutylene and butyl rubber are used in the manufacture of adhesives, agricultural chemicals, fiber optic compounds, ball bladders, caulks and sealants, cling film, electrical fluids, lubricants (2 cycle engine oil), paper and pulp, personal care products, pigment concentrates, for rubber and polymer modification, as a gasoline/diesel fuel additive, and even in chewing gum, for protecting and sealing certain equipment, tanks, vessels, piping systems, the first major application of butyl rubber was tire inner tubes. This remains an important segment of its market even today.

SURFACE PREPARATION (LINING REMOVAL)

MECHANICAL REMOVAL METHOD

The rubber liner has to be completely removed from the interior of the vessel as well as all penetrations. Cut lines are made in the tank with a straight edge razor followed by an air chisels to strip the rubber liner as shown in the below illustration #1. This process can be very labor intensive and the technicians must also be cautious not to gouge the substrate thus causing additional repairs once the liner is removed. Illustration #1 Rubber Liner Removal Mechanical Methods

UHP REMOVAL METHOD

The rubber liner has to be completely removed from the interior of the vessel as well as all penetrations. 40,000 psi water is used in conjunction with a air driven rotary nozzle with a minimum of 3 cutting jewels. Each of the cutting streams lacerates the rubber while the water mechanically removes the rubber from the surface leaving a white metal finish often exposing the original surface profile. Illustration #2 Rubber Liner Removal Ultra High Water Illustration #2 Surface Preparation After Lining Removal (Rubber Liner): The surface is inspected prior to sandblasting for contamination; grease, oil, dirt, rust or other foreign matter. All foreign matter will be removed by buffing, burning or washing with solvent and will conform to the SSPC-SP1 specification. All surfaces to be rubber lined are sandblasted to a clean gray white metal surface finish with a 2.0 mil minimum profile for proper metal to rubber bond. All sandblasted areas will be checked for the proper surface profile using our Surface Profile Comparator. The blasted surface will conform to SSPC-SP5 (or NACE 1) specification for a white metal blast. The surface is also inspected for pits, crevices and pin-holes. If any are found, the surface has to be re-surfaced. The entire area to be coated will be vacuumed to eliminate any dust being trapped between the coating and substrate. The vacuuming process is done in 2 stages, first all of the heavy dust and waste sandblasting media shall be removed and then a second and final vacuum to remove all of the fine dust.

APPLICATION OF RUBBER LINING

All procedures should be followed as recommended by the rubber lining manufacturer. After blasting is complete Cements will be applied in a consistent manner to give a smooth, uniform coverage. Surfaces that are cemented will not be exposed to sunlight and/or weather. Applying cement to blasted surfaces consist of four steps: Metal Primer Intermediate Cement Tack Cement (vessel) Tack Cement (rubber) Cements will be allowed enough drying time to form a dry or tacky film. All cemented parts or areas will be kept free from all contamination during the drying period. After metal preparation is completed, the rubber lining is cut to the desired size and shape on a clean table. Once the desired size and shape is achieved, one side of the rubber lining is coated with tack cement and applied to the blasted and cemented surface. After the rubber lining material is applied, it is rolled down using steady and firm overlapping strokes with a roller of 2-1/2″ maximum width. Care is taken not to stretch or apply tension to the rubber lining. The rolling action starts in the center of the lining material and is worked toward the edges. This is essential to remove any air out from behind the lining and ensure a proper bond is achieved. Rubber linings will be seamed together, if necessary, using either skive butt joints or lap joints. All seams will be offset.

CURING PROCESS

After application, the un-vulcanized rubber linings must be vulcanized or cured to ensure proper adhesion to the metal. Depending upon the composition of the lining and the size of the vessel, vulcanization can be achieved through one of these five methods: Autoclave Cure Internal Pressure Cure Exhaust, or Atmospheric, Steam Cure Hot Water Cure Chemical Cure

AUTOCLAVE CURE

An autoclave cure provides the fasted, best and most uniform cure and should be used whenever possible

INTERNAL PRESSURE CURE

This method is used when a rubber lined component is too large to be placed in one of our autoclaves

EXHAUST, ATMOSPHERIC OR STEAM CURE

Exhaust, atmospheric or steam cure is normally used for field vessels that have open tops and/or bottoms or for vessels that will not withstand pressure.

HOT WATER CURE

This method of curing rubber involving filling a vessel with water and using steam to heat the water mass.

CHEMICAL CURE

This method involves applying Chemcure activator to chemical cure rubber and allowing it to dry. It is typically used for field repairs. Steam or other heat sources may be used to accelerate the curing process.

INSPECTIONS

Our rubber linings are subject to numerous inspections and testing, both pre-cure and post cure. At a minimum, all rubber lining will be tested and inspected for the following:

PRE-CURE INSPECTION

All linings will be visually inspected for proper adhesion and loose joints. All linings will be visually inspected for blisters (trapped air), pulls, or surface defects. All linings will be 100% spark tested using a properly calibrated spark tester to locate any possible pin-holes leaks.

POST CURE INSPECTION

All linings will be visually inspected for proper adhesion and loose joints. All linings will be visually inspected for blisters (trapped air), pulls, or surface defects. All linings will be 100% spark tested using a properly calibrated spark tester to locate any possible pin-holes leaks. All linings will undergo durometer checks using a properly calibrated durometer gauge to ensure the rubber is cured per the specification.

TRADITIONAL NOVALAC EPOXY LINERS FOR TREATED WATER MATERIAL

DESCRIPTION

All the values for the heat transfer on coated tubes were measure by Bridger Scientific Inc. Six sections of copper tube were tested for heat transfer performance. Three of these tubes were lined with the optimal new tube lining material described above, two were lined with our standard tube lining material, and one was a reference tube with no lining. The test was set up in conditions similar to duty in a power plant’s main steam condenser. A five inch split aluminum heater block is clamped securely to each tube. A microprocessor based data acquisition system continuously measures and records test data. Maintaining a constant applied heat, the total heat transfer resistance of the tube is calculated over a range of flow velocities, 1.4 to 2.3 m/s. This method was originally developed to quantify the thermal performance loss due to fouling of the inside diameter of condenser tubes. It provides an excellent way to directly compare, in a simulated service condition, the loss in thermal conductivity of a coated condenser tube against a reference uncoated tube. Surface Preparation After Lining Removal (Rubber Liner): The surface is inspected prior to sandblasting for contamination; grease, oil, dirt, rust or other foreign matter. All foreign matter will be removed by buffing, burning or washing with solvent and will conform to the SSPC-SP1 specification. All surfaces to be epoxy lined are sandblasted to a clean gray white metal surface finish with a 3.0 mil minimum profile for proper metal to epoxy bond. All sandblasted areas will be checked for the proper surface profile using our Surface Profile Comparator. The blasted surface will conform to SSPC-SP5 (or NACE 1) specification for a white metal blast. All sandblasted areas will be checked for the proper surface profile using our Surface Profile Comparator. The blasted surface will conform to SSPC-SP5 (or NACE 1) specification for a white metal blast. The surface is also inspected for pits, crevices and pin-holes. If any are found, the surface has to be re-surfaced. The entire area to be coated will be vacuumed to eliminate any dust being trapped between the coating and substrate. The vacuuming process is done in 2 stages, first all of the heavy dust and waste sandblasting media shall be removed and then a second and final vacuum to remove all of the fine dust. III.

APPLICATION NOVALAC EPOXY COATING SYSTEM

Any detectable pitting, air chisel impressions and all welds shall be stripe coated with a paste grade epoxy to create a flat and or smooth transition for the epoxy liner . The coats of the paste grade material are trowel-applied at a thickness of 60-125 mils. The vessel is now ready for the Novalac Liner to be applied in a three to four-coat system, for a final dry film thickness of 50 mils +. All coats of Novalac Liner are traditionally roller applied in alternating colors and applied in a cross hatch pattern to insure a uniform application. All coating will be forced cured at air temperatures of 100F + for a period of 24 hours. Forced Cures may be used either to enhance chemical resistance or to allow the coated component to be placed in service before seven (7) days. To prevent slumping the products will have taken a firm set. See below a Duromar force cure graph. FIGURE 1: Heat Transfer Resistance of Coated Tubes 70% Clean 80% Clean 85% 90% 95% 97% 99% 60% Clean FIGURE 2: Overall Thermal Performance with Various Designed Heat Transfer Coefficients

PRE-CURE INSPECTION

All linings will be visually inspected for pinholes & surface defects. All linings will be 100% spark tested using a properly calibrated spark tester to locate any possible pin-holes leaks.

POST CURE INSPECTION

None.

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