Tank Designs

One facility was constructed with several tanks which consisted of steel shells with rigid plastic "inserts" that were supposed to be liners. Because of large differences in coefficient of expansion (in this case 5:1), they failed by stress cracking before ever being used. Not enough allowance for expansion caused the plastic to buckle inward and crack in the corners. If too much allowance for expansion is made, the same failure can occur because the liner then buckles outward. It is best to avoid this type of tank design altogether. The failed tanks were replaced with tanks made of "composite" construction consisting of 1/4-inch PVDF bonded to 3/8-inch fiberglass. No further problems have been reported with the new tanks which have now been in service over a year.

Chromium plating tanks are often manufactured from steel with a bonded PVC liner. When this liner is applied to steel that has not been blasted to near white metal, or if the bonding agent (glue) has not been properly applied, blistering of the liner can result. The blistering usually appears above the surface of the plating solution because the forces of expansion and contraction are greatest there. Also, below the surface, the weight of the plating solution tends to keep the liner pressed to the tank wall. The best "fix" weve been able to recommend is addition of a drop-in liner. In fact, many newer installations are using drop-in liners in steel tanks as the original equipment. This allows installation of leak detection systems which sound an alarm if the liner is perforated.

We have seen plating tanks made of steel with sprayed-on PVC coating and cant help feeling that they will become problems in the future. How does one make a repair in such a tank? A flexible sheet liner will blister at a pinhole leak. I dont think that sprayed-on PVC will behave similarly.

As long as we are on the subject of chromium plating tanks, why is it that many new tanks are installed with the bus work directly in front of the slot in the exhaust duct? The bus is subjected to continuous attack by the chromium mist exhaust, resulting in contamination of the plating solution and shortened bus life. We visited one plant in which the bus work for the chromium plating tanks was installed above the exhaust duct and high enough above the lips of the tanks that workers could easily place parts in them. Also, the bus work was placed in a "beehive" fashion so that adjustments could easily be made for different load sizes.

Black Oxide Process

Weve been in some new metal finishing shops that operate high temperature black oxide processes. Apparently, little thought was given to safety measures for these processes. High-temperature black oxide is essentially boiling concentrated caustic soda. The temperature is controlled by judicious addition of water. If water is added too fast, steam pockets can be formed within the liquid which may erupt as the steam expands, sending the boiling caustic into the air and possibly onto a worker. Experienced operators of high temperature black oxide processes invariably state that they have never experienced an eruption. However, we have read news articles about inexperienced operators being killed by erupting black oxide.

Safety shields that block the path of eruption of a black oxide process are a good idea. Other design parameters that should be considered are:

• If heating is electrical, use a proportioning device to turn off selected heaters as the temperature approaches the set point. This limits the potential for overheating which may cause addition of too much water.
  
• Water should be added by an automatic valve which opens as the temperature increases above the set point. A deadband of +3° F should be used.
  
• A high level limit switch or cut-off should be provided to ensure that overflow of the boiling caustic does not occur should the water addition valve fail in the open position. This device should turn off the water and the power to the heaters.
  
• A low level sensor should be used to turn off the heaters if the level of the boiling caustic goes below the set point. This will protect the heaters from burn-out.
  
• A diffuser plate and splash shield should be added at the water inlet to help prevent formation of steam pockets.
  
• We visited one new metal finishing shop which offered a black oxide process that had an exhaust hood made of rigid PVC! This material cannot tolerate exhaust air temperatures in the neighborhood of 250° . The hood had to be replaced with one made of stainless steel.
  

Electroless Nickel Plating Tanks

Weve noted the following problems when reviewing process designs for electroless nickel plating:

• One plant had tanks far too large for the anticipated loading. Tank size should be somewhere near the recommended loading level of 0.25 square foot per gallon.
  
• Another plant utilized disposable Iiners to eliminate the need for periodic passivation of the tank walls. Unfortunately, the liners were held in place by agitators, filters, probes, heating coils, shields and assorted other obstructions. If you intend to use disposable liners, install tank accessories so that they do not interfere with easy removal.
  
• Even if disposable liners are used, keep in mind that all other accessories which touch the process solution will need to be stripped at some time. Provide a tank wherein this operation can be performed.
  

Where to Put the Rectifiers

This is a question with no good answer. Rectifiers are typically placed as close to the process tank as possible without being in a location that subjects them to corrosive fumes or poor maintenance. Keep in mind that you may need to remove a rectifier from service at some point. is a hoist available to do this? Will the rectifier clear super structure and other potential obstructions?

In most new plants weve seen rectifiers placed on platforms located directly behind the plating tanks. This keeps bus runs short and provides for ready access by maintenance personnel. It does subject the rectifiers to higher levels of humidity (corrosion) which must be overcome by the plant air handling system.

We dont favor locating rectifiers in "the basement" below the process tanks because they rarely get proper attention from maintenance personnel who dont like going down there (and we cant blame them!). Lower level locations also are susceptible to chemical splash problems unless the rectifiers are shielded or placed in discrete rooms.

Plant Noise

Metal finishing shops tend to be noisy. Designs should incorporate noise reduction measures wherever possible. One source of noise readily eliminated is the use of mixers driven by compressed air. Even with mufflers, they add to the general noise level. With an inadequate muffler (or one needing service), weve measured decibel levels around 92 dBA! Compressed air mixers also spew a fine oil mist from the mufflers. This mist can cause process contamination and create OSHA problems. Breathing oil mist is no fun for the workers, either. The solution is obvious: use electric mixers wherever feasible.

Another source of noise is the make-up air handler. Loose nuts, bolts, blades, etc. rattle and add to plant noise. Measurements we have made on make-up air handlers approach 85 dB Answer:

Some designs use fan blades noisier than "squirrel cages." Locate these handlers as far from the work area as possible and incorporate sound isolation measures in fan housings and diffuser blades. Torque all nuts, bolts and screws, and add rubber or plastic washers.

The last noise source well cover is the low pressure blowers for push-pull ventilation systems or air agitation of process tanks. These things whine like banshees! Weve measured dBA ratings as high as 95. Locate the blowers outside the building if at all possible. If they must be located inside, employ sound deadening measures.