A new fix up.

A new fix up

Water is abrasive. Steam is even worse. Companies that use steam to heat presses, autoclaves and other equipment have to deal with the problems of scale build-ups, properly operating steam traps, blow-down of equipment and other maintenance problems. Most engineers and technologists in these plants are aware of the major problems. However, there are others that get little attention.

One of these occurs in "hot wells." Hot wells typically are concrete pits into which bleed off steam is released and where blow down occurs. Water and steam released in these pits is hot - 212 [degree] F - and contains a variety of abrasive elements collected in the system. This can include bits of metal, scale and other debris. In addition, during the release, the flow is very turbulent.

This hot steam flow does not react well with the concrete in the hot well. In fact, it causes significant degradation to the concrete. Cracking, exposure of the aggregate and loss of the concrete matrix are the normal results.

While not desired, this type of degradation has largely been accepted as part of doing business. Because of continuous operations and relatively short shut-down periods, repairs have been limited to "quick fixes" such as epoxy repairs.

Unfortunately, these have even less resistance to the abusive environment and must be frequently re-done. Normal epoxy materials soften at temperatures over 200 [degrees] F and are blown out by the high velocity, particulate loaded hot water. However, something new is now available.

What's new?

Last year, the Bridgestone/Firestone plant in Oklahoma City decided it was time for a change. Quick fixes were just too expensive over the long run. Still, they could not shut down curing operations for longer than their normal maintenance shutdown. In this case, all work had to be completed within seven days, with the new material ready to accept the hot steam and abrasion at that time.

Fortune smiled with the appearance of a pre-packed cementitious microsilica mortar - Sikachem 103. This material was developed by the Sika Corp. of Lyndhurst, New Jersey for quick application using shotcrete equipment. It is reportedly capable of application to horizontal, vertical or overhead surfaces in either thin layers (minimum of 1/4 inch) or with relatively high rates of build-up. In addition, it cures rapidly to very high compressive strengths.

Because it is designed for gun application, hand labor is greatly reduced. In addition, it is a simple system - water addition to the mixing equipment is all that is required. No mixing of various ingredients. It has low rebound (a characteristic needed for the shotcrete application) and it allows passage of water vapor in the same way as normal concrete.

The shotcrete technique applies a layer of concrete to a surface by pneumatically "hosing" a layer of material onto the appropriate surface. Mixing of the concrete with water occurs at the nozzle as the material is applied, resulting in a continuous rather than a batch process. Mixing with water is done at a predetermined ratio at the mixing head. This allows for use of only the amount of material needed for the job as well as rapid application.

There are several important factors in successful shotcrete application:

* Distance of nozzle from surface (2-6 ft.).

* Angle of nozzle to the surface (90 [degrees] optimal).

* Consistency of the mortar (should be somewhat plastic).

This method reduces the high labor factor required with normal mortar. Because of the high impingement velocities, it is also able to fill cracks and voids to a much greater degree. Finally, the process also allows (and somewhat requires) use of a fast curing material.

Prior to application of this type of material, the surface to which the new material will be applied should be thoroughly cleaned. This can be done using a high pressure water blast (6,000 psi) or sandblasting. With this material, no additional primer or surface preparation was needed.

The Sikachem 103 is reported to behave very much the same as normal concrete except it is stronger and cures faster. Table 1 shows typical values for strength of the cured material. Reports indicate that there is no problem bonding to other cured concrete or to itself, whether cured or not.

Table : Table 1 Compressive strength (ASTM C-109)

  2 days                6,000 PSI
  7 days                8,100 PSI
  28 days               9,000 PSI

Flexural strength 

  7 days                1,000 PSI
  28 days               1,400 PSI

Tensile strength 

  7 days                  600 PSI
  28 days                 750 PSI

How did the material work?

With the Sika material, it was possible to apply up to a 1-1/2 inch layer of material in one pass with no sag problems. Application was done using 300-400 psi air.

All work was done in a closely confined space, requiring low dust material. With the shotcrete method, the material was applied and was able to be tooled right away on all surfaces. According to the applicator, the material had the consistency of a firm jelly when applied. As the data in table 1 indicates, compressive strength develops to over 6,000 psi within two days. (Normal concrete has a compressive strength of 4,000 psi when fully cured). From the beginning, the work proceeded smoothly. In approximately one 12 foot by 12 foot chamber by cleaning to bare aggregate. Two inch by two inch wire mesh was applied to floor, walls and ceilings. Then 1-1/2 inches of Sikachem 103 was applied, tooled and cured.

After almost a year of use, the surface is reported to be in good shape. Good enough that Bridgestone/Firestone is planning on using the same material and methods to repair hot wells in other plants.

Any potential problems?

Virtually none. Minimum temperature for application is 45 [degrees] F. Surfaces should be clean and free from deteriorated material. The surface to which the material is applied should be "saturated surface dry."

As with normal concrete, pH extremes (particularly acid) should be avoided. No problems would he expected with use in the range of pH 5-9.

Application is typically done by commercial applicators who are trained and approved by Sika. Shotcreting is done per standards established by ACI 506-R85.

Finishing is done by normal methods. The surface can be broomed to a rough texture, wood floated to a granular texture or steel troweled to a smooth finish. Finishing should begin about 5-10 minutes after initial set of the material.

If local conditions are likely to cause premature surface drying, a fine water mist spray, wet burlap or a curing compound should be applied. Conditions likely to cause this include high temperatures, low humidity and strong winds.

The material is a microsilica filled cement. As such, normal safety procedures for handling dry and wet concrete need to be observed: use dust masks, safety goggles and rubber gloves. Skin and eye contact should be avoided.

Summary

Many of the maintenance functions and requirements go unnoticed by engineers and technologists who are busy making products. General maintenance is an unsung hero in our industry. Without attention to problems so mundane as hot wells, we wouldn't be able to make very many acceptable products.

The report on this product looks very promising for one aspect of maintenance that certainly deserves attention. It obviously seems to be of benefit to Bridgestone/Firestone.