Field Installation of Rubber Linings
The following technical bulletin presents an overview of the field installation of rubber linings. Blair Rubber’s technical staff is available to provide further assistance with Blair’s products and questions/answers that are not addressed in the following bulletin.
Rubber linings have been used successfully in equipment for the manufacture, storage and transportation of phosphoric acid for many years. It is the most cost-effective material for preventing corrosion when the correct material is selected and the application is done properly. With current world market pressure, manufacturers have to maximize plant and equipment use to remain competitive in the world economy.
It is necessary to have a good understanding of the various service conditions, rubber linings suitable for these conditions, and proper procedures for application and curing in order to obtain a long and trouble-free service life. The purpose of this technical bulletin is to give some minimum standards, point out potential problem areas and share information that may help accomplish that goal.
TYPICAL SERVICE CONDITIONS
Temperatures can range from 140°F (60°C) to over 200°F (93°C). The concentrations can be from 28% to over 70%. The solution may be under pressure, static storage or under full vacuum. Sometimes there are heating coils in the storage tanks for higher concentrations of corrosive solutions.
Under these conditions, the acid is highly corrosive and requires the best efforts of all parties to yield a rubber lining that will give the desired service life. The following sections will go into some detail to explain the steps required to accomplish a successful rubber lining project.
RUBBER LINING MATERIALS
All the service conditions must be considered when selecting the rubber lining. Factors to consider are:
- Chemical composition and make-up
- Acid concentration
- Vacuum service or static storage
- Solids in the system that may cause abrasion
- Heating coils in the tank
- Defoamer type or other additives
- APPLICATOR SELECTION
Make certain the applicator you choose has experience in your industry. For the rubber lining you select to withstand the hostile environment, the applicator must properly install, cure and inspect the lining. If you are not familiar with an applicator, ask for references and check those references. Determine the experience of the supervisors and lining personnel. Ask for a safety manual to be certain they have the ability to meet the plant requirements. Ask for and review their quality control manual so you can reach an agreement on all quality standards.
THE PURCHASER’S JOBSITE REQUIREMENTS
The bid documents should include, as a minimum, the following:
- Any safety or drug testing required
- Union requirements, if any
- Any restricted or mandatory hours of work
- A schedule for completion
- Disposal of blast media and other materials
- Specialized equipment requirements
- Limitations on use of plant equipment and resources
- Plant entrance and exit procedures
- Vehicle restrictions
- Any requirement that is unique to the plant
THE PURCHASERS LINING SPECIFICATION
Include the following:
- The rubber lining that is to be installed
- The thickness of the lining
- Any areas of extra thickness
- Dehumidification equipment
- The required cure method
- Required inspections
- Repair methods
- Acceptance and/or rejection criteria
The applicator should clearly state in his proposal exactly what he is providing and what he expects the owner to provide. Some of these items are:
- Lay down area, state the size and proximity to work area
- Electrical requirements
- Handling equipment provided by you and by the owner
- Steam for cure
- Blind flanges for cure when required
- Type of blast media to be used
- Compressed air
- Toilet facilities
- Disposal of waste
- Time required for completion
As you can see, there are a lot of opportunities for items to be missed by both parties. The obvious solution is for there to be a pre-bid meeting and walk through so all the bidders have the same information and the successful bidder should be fully prepared to accomplish the work.
When practical, the successful applicator should make a second visit to the site to make certain all his requirements are being met and there have not been any significant changes that could affect his ability to perform the quoted work. This also allows the applicator to refine his approach to the work and arrive better prepared for the task at hand.
The following is a breakdown of the steps that are taken during a field rubber lining project. It is not intended to be all-inclusive. The intent is to give some minimum requirements and offer advice that may help improve quality, timely completion and/or reduce the cost of the job.
The same steps are required for shop lining with exception of scaffolding, insulation and some differences in curing the lining. The advantages of lining in the shop are closer supervision, better environmental control, better quality control and lower cost. Any equipment small enough to be shipped over the road should be lined at the applicators premises.
Some amount of insulation can be beneficial for several phases of the rubber lining. The insulation blankets that are commonly used for the curing of concrete work well. They are inexpensive, lightweight, and reusable, allow better control of the environment inside the tank and shorten the cure times. Insulation is one of the most cost effective things one can do when installing rubber linings in the field.
The entire vessel should be covered to prevent wind, rain or snow from coming in contact with the vessel surface. Moisture of any kind on the surface can lower temperatures up to 50°F and thus prevent the tie gum or adhesives from curing.
Cool weather: If steel is wrapped with 6 mil polyethylene and an outside plastic tarp is 6″ to 4′ away from vessel providing a warmed area of 60°F, then the steel temperature under the polyethylene will go up to 180° to 200°F when the inside vessel temperature is at 250°F. Vessels with steel gauges at 3/4″ thick, it is recommended that the outside of the vessel be tarped, not only for low outside temperature, but also if there is a chance for rain to come in contact with steel surface.
The applicator must make certain he knows all the safety requirements for the erection and dismantling of scaffolding. There are OSHA requirements, and possibly site requirements, that must be met. It may be advisable to sub-contract the scaffolding if you are not experienced in this area.
For larger tanks, one should consider a full scaffold instead of rolling towers. Rolling towers cannot be moved with personnel on the tower, which means experienced rubber liners spend much of their time pushing a scaffold around. By the time the lining is complete, the rolling scaffold would have been pushed around the tank six (6) to eight (8) times. This is not only expensive, but can delay the completion of the job. Invest the money for a full scaffold and one should save time and money in the end.
The sandblast cannot be compromised. It is necessary to have a white metal finish in accordance with NACE No. 1 (or SSPC-SP5) specifications. The blasting media used must be adequate to achieve a surface profile of 1.5 to 3 mils. If any rusting occurs before the primers are applied, the rusted area must be re-blasted. The blasted surface should be cleaned and primed within eight (8) hours of blasting. Dehumidification equipment can extend the time before cleaning and priming is required but extreme care should be taken to assure there is no oxidation of the steel.
For the ultimate adhesion, all dust or other foreign material must be removed from the surface before the primers are applied. Compressed air is useful for removing large quantities of dust or heavy particles but it just moves the lightweight dust from one place to another. After blowing down with compressed air, it is a good practice to brush the steel with a soft bristle brush, starting at the top and working down, so gravity can help remove the dust from the area to be primed. Vacuuming is also acceptable. For the ultimate in cleanliness, use a tack cloth – like those used by automotive painters – to remove the last remnants of dust. Cleanliness is crucial to obtaining maximum adhesion.
PRIMING AND CEMENTING
Use only primers recommended or approved by the lining manufacturer. The commonly used primers are designed for application as a very thin coat. Drips or runs can be detrimental to the adhesion. Primers are not a paint that should hide the blasted steel, follow the manufacturers’ instructions on the dried film thickness. In large tanks it can take several days to grit blast and primer may be applied to clean steel after blasting sections of the tank. When the primer is dry, usually by the next day sand blasting may resume and the process repeats it self until the blasting and priming is complete. Since, it may have been several days since the first section of primer was applied. It is advisable to apply another coat of the first primer to the entire tank to make certain there is full coverage producing a fresh surface for the next application of primer, intermediate or tack cement.
The primers are extremely sensitive to sunlight and should be protected from direct exposure to it. Areas, such as nozzles, that get long-term exposure to sunlight, should be solvent wiped and another coat of the first primer applied before proceeding with the lining.
After all the primers are applied, a coat of tack cement should be applied to both the primed steel and the surface of the rubber that will be placed against the steel. With some linings, a solvent wipe may be adequate for adhesion, but a coat of cement is always best. High adhesion values are critical to the success of rubber linings in vacuum equipment and a coat of cement should always be applied to both the steel and the rubber except in the case of a “Tacky Back” rubber lining. Rubber linings with “Tacky Back” do not require rubber adhesive application.
CUTTING OF THE RUBBER
There are any number of ways to cut rubber, but the motor driven, water lubricated, circular blade knife works the best. It is fast, has a fixed base for a very uniform skives (or bevel) and inexperienced people can become proficient very quickly. Other commonly used methods of cutting rubber are hand cutting with a long knife, a razor knife or a heated knife.
There should be a plan before the cutting starts. First, decide if the skives will be open or closed. All linings constructed of different materials must have either closed skives or cap strips to protect the underneath materials from direct contact with the acid. Closing the skives is the preferred method. Make patterns for difficult areas, such as, domed or coned roofs, coned bottoms and baffles. Use chalk lines or straight edges to mark the rubber sheets prior to cutting so the cuts will be straight and the finished lining will look professional.
After the cement on the rubber sheet has been allowed to dry, a liner cloth is placed over the cemented side to act as a separator to prevent premature sticking of the rubber to the steel. Wet cement will result in blisters during the cure and repairs will be necessary. Using rollers and stitchers, the tools of the trade, all air must be eliminated between the rubber and steel and between the layers of rubber at the overlap. This sounds simple but it requires skilled personnel to do the work properly. Two-inch overlaps of the rubber are the standard method of placing the sheets and each overlap should be marked to assure uniform and straight seams. Stagger the seams so there are not more than three (3) layers of rubber. Do not specify “no more than two (2) layers of rubber”. This requires the trimming of one layer at the overlap and results in less than a full width overlap.
Any trapped air must be removed. This is commonly accomplished by using an awl or a hypodermic needle to puncture the lining and stitchers are then used to roll the air out. Anytime a hole is made in the rubber, it should be marked and an overlay of the same material should be cemented and placed over the hole. It is inevitable that some air will be trapped during the lining of large tanks due to rough welds or irregular steel surfaces. An overlay should not be considered a defect or a repair. These “button patches”, as they are sometimes called, should all be the same size and all oriented the same way. By using round “buttons”, the finished product will have a better appearance. If there are several required in a small area, such as along a rough weld, don’t use multiple “buttons”, use an overlay strip across the width of the rubber sheet to make it look like it belongs there.
Neatness counts! Rubber lining is a craft and the work of true craftsmen will be evident.
INSPECTION BEFORE CURE
Visually inspect the entire tank. In large vessels, this will have to be done before the scaffolding is removed. Inspect for trapped air by holding a light at an oblique angle to the rubber. Any trapped air will show up as a shadow. Remove any air and overlay as mentioned in the previous section.
Inspect for any loose seams. Apply cement and stitch as necessary. A hot knife can be used to seal down loose seams.
Look for “channel leaks”. Too many layers of rubber in an overlap can result in an open path to the steel or “channel leak”. The spark test will not find this type of leak. Repair these areas, even if it requires the removal of all the overlaps and applying an overlay.
If there are any thin places, caused by the scaffolding or other objects resting against the rubber, overlay them to maintain the minimum thickness called out in the specifications.
There has been much written and many discussions about the proper voltage for spark testing of rubber. In actuality, the spark length should be used to determine the spark tester setting. The length of the spark should be at least twice the thickness of the lining and the voltage required to accomplish this is irrelevant and will vary with the type of tester used. An easy way to set the spark length is to have a piece of steel with two (2) layers of lining and adjust the tester until it jumps across the thickness of the rubber to the steel. Spark testers that give an audible alarm can give false alarms on some linings that have lots of carbon black in the compound. Visual observation of the spark is only real test for holes in the rubber and the audible alarm should be turned off or ignored unless it can be established that each alarm is a hole in the rubber. The spark will turn from blue to very white when there is a hole in the lining. The spark test will not find air pockets under the rubber; it will only find holes to the steel that do not exceed the spark length.
Neoprene and graphite filled linings are subject to burning holes from the spark tester when excessive voltage is used but they are not commonly used in phosphoric acid. Consult the manufacturer of the lining for the correct spark length to use with these materials.
The spark test should only be done by experienced personnel. A properly calibrated spark tester should be moved over the surface of the lining at a speed that will allow the operator to observe any spark leaks. Any leaks should be marked for an overlay as described in the “RUBBER APPLICATION” section. There is no need to go extremely fast. Don’t keep trying to make a suspicious looking spot leak by testing over and over. Wet or dirty surfaces cannot be properly tested because they will give false readings. The spark test cannot find leaks that are longer than the length of the spark such as a lap joint.
CURING OF THE RUBBER LINING
Always follow the manufactures recommendations for cure time and temperature. The purpose of this section is to explain the different methods for curing rubber linings and some do’s and don’ts that may improve the final product.
METHODS OF CURING (VULCANIZING)
Autoclave. This is the preferred method of curing because the pressure increases the bond strength and the density of the lining and the heat from both sides gives a uniform cure. Due to the limitations on the sizes of autoclaves available, this type of cure is normally restricted to smaller items lined in the applicators shop.
For all types of curing the outside temperature and weather does have a significant influence on the time required to cure a vessel. In cool temperatures (60°F), it is imperative that the vessel be insulated in some manner to effectively carry out the cure, either by tarping or direct insulation to the tank’s exterior. A foul weather factor cannot be compensated for by internal steam alone.
Internal Pressure. The next best method is an internal pressure cure. It is often used for curing pressure rated and vacuum vessels. Any time an internal pressure cure is to be used, it must be determined what pressure the vessel can withstand. This is the preferred method for vacuum vessels too large for an autoclave. Exceeding the calculated pressure rating of a vessel can result in severe damage to the tank and death or injury to personnel.
In order to pressurize the vessel, the openings must have blind flanges that are bolted tight enough to prevent excessive loss of steam. Unvulcanized rubber will not allow the bolts to be tightened enough to hold the pressure after the vessel gets hot. Therefore, the flanges should be lined with pre-cured or chemical cure rubber. If chemical cure rubber is used, the curing process should be started by applying heat or allowing it to remain at ambient temperatures until curing can take place.
It is extremely important to remove all condensate from the vessel during the cure. Pockets of water in the vessel will result in cold spots, which will not fully cure.
Complete cure is extremely important for all rubber lined equipment. Many parameters come into play such as the lining involved, vessel design, metal thickness, steam supply and above all, the external weather conditions. In low temperatures, it is mandatory to shroud the vessel and heat with external dry heat. The heat is also beneficial for improved working conditions and it certainly enhances the evaporation of solvent from cements and aids in controlling the moisture content within the vessel.
When curing with pressure, it is best to place the vessel under 15 – 20 psi air pressure for at least one hour before the introduction of steam. This aids in allowing any trapped air to dissipate before cure.
To cure, it is recommended that at least two 2-inch steam lines be used in case one malfunctions. The piping arrangement should be such that the steam is directed downward and not impinging on the rubber lining itself.
Thermocouples shall be maintained on both the interior and exterior to monitor the cure temperature and time required to cure the lined vessel. Thermocouples are required in the vessel bottom, middle and top areas both inside and out.
Pressurized steam shall be used to displace the air while holding the pressure and temperature to cure the lining (More on this below).
Condensate drainage is accomplished by using a sump pipe connected to a steam trap or by using a thermometer and steam trap on a bottom outlet. The steam and air inlet pipes should be 2″ diameter and all nipples should be bled with 1/4″ petcocks. Sufficient boiler capacity should be available to raise the temperature from ambient to cure temperature in a relatively short period of time. A thermometer and pressure gauge should be placed in a blind flange near the bottom of the vessel and when the thermometer registers the temperature corresponding to the gauge steam pressure, the air has been fully evacuated.
If the temperature on the exterior cannot be obtained, then more insulation is required along with a longer cure time. A minimum external temperature of 170°F is required for curing.
The lining inside next to the steel shall be no less than 180°F for curing the tie gum, with the cure time as specified in the lining specification.
It is advisable to use surface temperature thermometers or an infrared hand held device to read the external metal temperature. Temperatures on the outside shell should read a minimum of 170°F to 190°F or higher when curing at 250°F/15 psi. For example, the bottom of the vessel may show temperatures of 170°F whereas the top area would have temperatures in the 185°F to 195°F range. The external temperature provides additional data to insure proper cure of the vessel.
Temperature Records: On the outside, if using a recorder, the best is to place them every 30″ @ 12 foot intervals high all around exterior. This may be cost prohibitive, therefore; the use of new magnetic temperature gauges that can be read using flash light or best, a good handheld infrared temperature device and then record the data. Rounds should be made every 20-30 minutes. A thermocouple temperature recorder is good for checking heat on the inside of the vessel and must have a minimum of 3 or 4, one toward top, middle and one about 4 foot from bottom if possible on a large vessel.
Also, the steel thickness needs to be taken into account. Heavy, thick steel needs additional lining cure time to compensate for the heat sink and heat transfer. This is where external temperature gauges are quite valuable in monitoring the time/temperature so one can judge and insure themselves that a positive cure is being obtained.
Pressure Cure Note: When there is a small vessel on top of an other vessel, such as some evaporators, which has a 4 ft. dia. elbow feeding into them from main body, do not start injecting steam in upper area until main body is at least up to 180°F. Then evenly raise both steam hoses to the 250°F or more when permitted. Generally start injecting steam into the upper area when the main body is at 200°F.
Various curing conditions are encountered; thus the following temperature equivalents are provided for information. Sea level elevation is assumed.
(115°C) 240°F = 10 lbs. /sq. in.
(121°C) 250°F = 15 lbs. /sq. in.
(125°C) 258°F = 20 lbs. /sq. in.
(134°C) 274°F = 30 lbs. /sq. in.
Atmospheric, or Exhaust, Steam. Equipment that will not fit in an autoclave and cannot be pressurized should be cured using this method. It is a relatively simple method but requires a larger volume of steam and more time to complete the cure. It is very controllable because steam without pressure can only be 212°F (100°C) maximum.
All the openings must be covered in some manner. Plastic sheeting is often used to cover the nozzles but be careful not to press the plastic against the rubber because it may cause wrinkles that could require repair. There should be a hole in the plastic to allow condensate and steam to escape except on the top openings. These need to be sealed tight to prevent a flu effect that could draw cold air in at the bottom openings and prevent a full cure. Blind flanges can also be used but it is difficult to get enough heat on the face of the flange to get a full cure. Pre-cured or chemical cure rubber should be applied on these flange faces to prevent damage to the lining during the piping installation. Very tall tanks that have nozzles that are near the bottom will require steam injected at these points to ensure the nozzle cure.
Condensate will insulate the bottom and prevent a full cure in that area. If there is a bottom drain, use it to drain all the condensate. A pump may be required to remove the water periodically if there is no nozzle near the bottom.
It is very important to have enough volume of steam going in the tank to force some steam out all the openings and, in particular, the lower ones. It is important to check and record external temperatures to be certain there is an even distribution of heat. Never direct the steam directly on the rubber at close range. It will cause blistering. It is best to use some kind of sparger system to get uniform steam and heat distribution.
There is no substitute for conscientious persons being present during the entire time a vessel is curing. They must constantly check to verify the cure temperatures and pressure inside the vessel. These individuals must have the know-how and be able to take appropriate action to maintain the inside and outside temperatures during the cure. To adjust the amount of steam whenever weather conditions changed to maintain the cure temperature, care must be taken on a vessel or one could cause the tank to collapse if steam is lost. The loss of steam could occur when the weather changes enough to condense the steam inside an atmospheric curing tank.
We all know that proper selection of the tank lining is first in primary importance, application second and curing third. Maybe curing should be listed first, for without a correctly cured rubber lined vessel, one has nothing.
There are many factors to consider in cool weather application as to the amount of shrouding, solvent evaporation, possible condensation and hot tables for good rubber application.
With proper shrouding to maintain an interior surface temperature in the range of 60° – 80°F on the steel, if that is not possible an indirect heat source is mandatory. No steam shall be used to heat the exterior during application because the steam moisture causes improper drying of solvent adhesive and condensation can build up inside the vessel. Also, it’s mandatory that the temperature always be at least 5°F above the dew point. Therefore, a dehumidifier may be required for proper moisture control within the vessel.
Many applicators know the importance of shrouding evaporators and even field storage tanks in cold weather conditions to guard against the dreaded “cold wall effect”. The degree of insulation required depends on the severity of the climatic conditions as to the amount of protection or R factor.
Even in summer time weather, it’s best to wrap the tank and/or evaporator to protect against a sudden rainstorm that causes rapid temperature changes to occur. For example: a steel surface condensation exposed to the weather elements can have a temperature loss of 50°F or more in one minute of time.
You may say that you can’t afford to spend the extra cost or time to shroud the tank or have external indirect heat available along with humidity control inside the vessel. Let me put it this way, no-one can afford not to! It’s always better to do the job correctly and not run the risk of having failures or potential disbonding problems in future service.
Monitoring your cure with thermocouples and an infrared gauge is an effective way to record both internal and external temperatures and serious consideration should be given to shrouding every field vessel to ensure a positive cured lining.
INSPECTION AFTER CURE
This inspection is very much like the inspection before the cure and it must be thorough. This is the last opportunity to find and repair any problems in the lining. At this stage the visual inspection may be the most important. Only the visual inspection will find trapped air, loose seams and “channel leaks”. The spark test should still be done in the same manner. Any defects found after cure will have to be repaired in accordance with the manufacturer’s recommendations. Any repairs of linings must be fully cured before placing in service. Any uncured rubber placed in service may come off the wall and result in a shutdown and significant damage to the steel.
Success should be the first consideration for everyone involved in every phase of the lining process. The ultimate responsibility for a successful rubber lining installation lies with the applicator and the lining manufacturer. They must use all their experience and knowledge of the industry, be aware of all the potential problems and take action to prevent premature failures. The specifier and the end user have important parts to play as well. By working together toward the goal of long and trouble free service life, rubber lining in the phosphoric acid industry will continue to be another “Success Story”.
For additional product information or installation questions or concerns, please contact our technical staff at tel. 330.666.3600 fax. 330.666.9334 or e-mail: firstname.lastname@example.org.