Lining a Steel Pipe Internally With a Cement Mortar

The present invention relates to a method of making a corrosion-proof and corrosion-proofing layer on the inside surface of steel pipes.

Corrosion-proofing of water pipes by means of a cement mortar lining has been known for many years. This lining is usually applied by means of centrifugal force action. The resulting lining has, for instance, a water-to-cement ratio of 0.4 at a mixing ratio of sand to cement of approximately 2.5. The German Industry Standard, laid down in Paper W 342 of December 1978, sets forth requirements and test particulars for such a material, particularly when used in combination with drinking water.

One may use other methods of applying such a cement lining; decisive is that such a resulting cement mortar lining provides sufficient protection against corrosion. This is, indeed, true for most types of water; and pipe lines can in that manner be deemed completely corrosion-proof for most cases. However, water rich in carbon dioxide; industrial, acid-rich waste water; or salt water which contains additives being particularly aggressive to cement (e.g., magnesium salts or sulfates); all will corrode the cement and will slowly destroy the protective properties of steel pipes. The corrosion process involves, in particular, gradual dissolving of the calcium so that the mortar structure is disturbed and destroyed. This phenomenon was particularly observed where partially decalcinated mortar was permitted to dry. Temporary dryness is inevitable when, for instance, repairs and other maintenance is required. Here then, the mortar will loose directly some of its sand. Repetition of the wet-dry situation will gradually destroy the protection.

It has been suggested to improve the corrosion resistance of such mortar by adding, for example, organic polymer in a watery dispersion. However, a centrifugal application of the substance to a rotating pipe as per German paper W 342 mentioned above is not feasible because the synthetic dispersed in water will be removed by the centrifugal action.

German printed patent application No. 26 20 669 suggests casting the cement mortar against the inside wall of a non-rotating pipe by means of air pressure and under utilization of a rotating disk or distribution head. The mortar contains a synthetic dispersion, particularly for ensuring adhesion of the fresh mortar in the wall. This method, however, requires that, subsequently, the surface of the mortar layer be smoothed. Moreover, the material itself, as applied, is not adequately compacted. In the case of significant porosity with little or no compression, all reactions tending to dissolve the calcium are actually enhanced. A mortar blended with the dispersion of a synthetic will still decalcinate as fast as it would without the additive, but resistance against mechanical wear was noticably improved. See, for example, "MANNESMANN FORSCHUNGSBERICHTE" 770/1978, pp. 448-459.

It is a surprising discovery that significant compression of a synthetically alloyed mortar improves greatly its corrosion resistance, whereby particularly the strength of the mortar as well as its decalcination are involved.

It is an object of the present invention to provide a new and improved method of lining the interior of pipes with a compressed or compacted, highly corrosion-proof and corrosion-proofing material, particularly for protecting the pipe against acidic fluids.

In accordance with the preferred embodiment of the present invention, it is suggested to provide a blend of cement mortar with a sand-to-cement (weight) ratio of from 1 to 3, and a water-to-cement (weight) ratio of from 0.25 to 0.35, and of a dispersion of synthetic material in the water and including a synthetic polymer and a surface-active synthetic resin, the solid component of the dispersion being present at approximately 3 to 20 parts, preferably 10 parts, by weight per 100 parts by weight of cement. This blend is deposited inside the pipe, and the pipe is subjected to rotation, including high-speed rotation, in order to distribute and compact a mortar layer on that inside surface.

The preferred synthetic material is to be comprised of melamine resin and acrylic resin. Preferably, a puzzuolanic filler is to be included in the blend in order to enhance fluidity without the need of excessive amounts of water. The pipe should be rotated at first at a relatively low speed of from 140 to 240 RPM's (depending upon the diameter of the pipe) for providing an initial distribution, and later the speed is increased to 400 to 750 RPM's in order to obtain compacting. It was found advantageous to superimpose a radial vibration upon the pipe during rotation to further enhance compacting.

The synthetic polymer, particularly the acrylic resin if included, bonds the unorganic components of the mortar to each other, fills any voids and gaps, and is highly resistive (chemically) to alkaline reaction of the cement. Surface-reacting i.e., reactive resins are those which are absorbed on the surface of the cement and sand particles, causing the mortar to remain fluid, even if the water-to-cement ratio is rather small.

The conventional lining methods process cement mortar which, initially, has a water-to-cement ratio of at least 0.5. Such mortars cannot be blended with synthetic materials, as outlined above. It was found surprisingly that surface-active substances, such as melamine resin, permits the making of mortar with a water-to-cement ratio of well below 0.5, even below 0.35, for a sand-to-cement ratio of 2.5. These mortars are deposited initially by centrifugal action; and they contain already the synthetic dispersion. During centrifugal action, very little water (and, therefore, very little of the synthetic additive) is removed.

In order to obtain a very high compacting and compression action, with little or no separation of the components of the blend, one should superimpose a radial vibration upon the pipe. This shaking action combined with strong compacting forces minimizes any voids in the layer. The resulting water-to-cement ratios of the centrifugally distributed and compacted mortar is below the ratio of 0.35.

A particular example for practicing the invention will be described next.

First, cement and sand are mixed, at a weight ratio of 1:2.5. The cement and the sand have grain sizes of from 0.1 mm to 1.5 mm. Water is added at a quantity corresponding to a water-to-cement ratio (weight) of 0.32. The water, or a portion thereof, was previously used for providing a dispersion of synthetics, including a surface-acting component and an alkaline-resisting polymer. A puzzuolamic filler may also be included at a weight content of 5 parts to 30 parts per hundred parts cement.

This kind of filler generally improves the fluidity of the blend, prevents separation, and thus improves workability of the mortar. The filler, in fact, absorbs the synthetic components. In the preferred form, a so-called EFA filler is used which is captured flying dust from high-temperature refineries, and having been produced at temperatures of from 1000.degree. C. to 1700.degree. C. They contain glass of approximately 90% and exhibit puzzuolanic effect of 115%.

After the blend has been prepared, it is fed to the pipe, which is at rest or rotates slowly. The rotational speed is gradually increased during a period of from 1 minute to 3 minutes. At the end of the speed-increasing period, the maximum speed has been obtained and is maintained for a few minutes.

The maximum rotational speed for a pipe of particular diameter should be such that the centrifugal force in the mortar layer is at least 20 g (g being the per unit mass of gravitational acceleration), preferably more. Particularly during the startup and speed increase, but also during high-speed rotation, the pipe is subjected to radial vibrations which cause the particles of the material to settle in the compactest disposition in order to obtain maximum density. The centrifugal compacting action is terminated by slowing the pipe down over a period of approximately 1 minute, or thereabouts. Generally speaking, the rate of initial speed increase should at least be about the same as the (negative) rate of slowdown.

The example above is based on a sand-to-cement weight ratio of 2.5. In the case of a deviation (which should, nevertheless, remain within the range of from 2 to 3), correspondingly smaller or larger minimum values for the water-to-cement ratios should be observed. In other words, the proposed ratio ranges are preferably observed in a corresponding fashion; that is to say, smaller (larger) sand-to-cement ratios should preferably be used for smaller (larger) water-to-cement ratios.

The following particular composition was found to be very resistant to acidic and salty waste waters (all parts are given by weight):

The invention is not limited to the embodiments described above; but all changes and modifications thereof, not constituting departures from the spirit and scope of the invention, are intended to be included.