Gas Line Sealing Method Utilizing Penetrating Flexible Sealant

Leakage of fuel gas from pipelines is a serious problem. In U.S. Pat. No. 4,304,805 there is described a process for sealing leaks in buried gas pipes by polymerization of volatilized amino-silane monomers. This same reference discusses other prior art methods for sealing gas pipes and includes the following statement:

In U.S. Pat. No. 3,507,725 to Hylak et al., there is disclosed a method of repairing gas main fibrous packed joints with a liquid sealing material which may be a liquid monomer such as styrene monomer, by introducing the monomer into the gas main in liquid form, and permitting it to flow to a low point in the system where a leaking joint exists, the packing of which becomes saturated with the sealant which then polymerizes over a relatively long period of time, namely six weeks to three months. Styrene is employed as a liquid, and no specific catalyst is named in the patent.

Recently there has been developed an apparatus for sealing gas pipe leaks which includes means for moving the apparatus through a pipeline to a leak site, inflatable elongated balloon or bladder means having three separate adjacent inflatable chambers, means for selectively inflating and deflating the respective chambers, and means for injecting a curable sealant fluid between the center balloon chamber and the inner pipe wall. The apparatus is operated so as to position the central balloon chamber at a leak, inflate the two outer balloon chambers so as to enclose a central leak containing area within the pipe, inject the curable sealant into the enclosed area and then inflate the central balloon chamber to move the fluid into the porosities causing the leak where the fluid is cured. The apparatus is removed after deflation of the balloon chambers. It has been proposed to use an anaerobic sealant in this process since such sealants have a property, convenient for this application, of being stable on exposure to air but readily and spontaneously curing upon deprivation of oxygen. It has been found however that the anaerobic sealants designed for impregnating rigid articles may not be suitable for sealing leaking gas pipe joints because such sealants produce rigid cured products which are not sufficiently flexible to accommodate the relative movement which underground gas pipes undergo due to gradually applied stresses from soil settling, changes in water table, road vibrations, etc. Thus, an improved anaerobic sealant suitable for sealing leaks in underground gas pipes is desirable.

The present invention uses a novel low viscosity wicking acrylic sealant which readily cures to a flexible solid capable of withstanding substantial movement between substrates. While heat cure or other catalyst systems may be usefully employed, the sealant is preferably catalyzed with an anaerobic type catalyst system. The inventive sealants are mixtures of long chain mono-acrylic esters such as 2-ethylhexyl methacrylate, tridecyl methacrylate, stearyl methacrylate and lauryl methacrylate; hydroxy functional acrylic esters such as hydroxypropyl methacrylate and hydroxyethyl methacrylate, acid monomers such as acrylic and methacrylic acids, plasticizers such as tetraethylene glycol di(2-ethyl)hexanoate, polyfunctional acrylic esters such as trimethylolpropane trimethacrylate, stabilizers and catalysts.

The inventive sealants must give a cured product having a glass transition temperature (Tg) of no more than 0.degree. C. and the Tg must be at least about 5.degree. C. below the minimum pipe joint operating temperature. Preferably the Tg is at least 20.degree. C. below the minimum joint temperature.

The viscosity of the composition is preferably no greater than 100 cps. Suitably the viscosity is less than 50 cps.

The quantities of the respective ingredients are selected to optimize the viscosity, wetting and cure speed of the uncured product while maintaining the requisite cured Tg. Suitable ranges are:

(a) long chain acrylic ester: 20-80%

(b) hydroxy functional acrylic ester: 15-50%

(c) acid monomer: 0.5-5%

(d) plasticizer: 5-30%

(e) polyfunctional acrylic ester: 0.1-5%

Preferred ranges are (a) 50-65; (b) 20-30; (c) 2-4; (d) 10-20 and (e) 0.5-1.5.

The plasticizer is most preferably a low viscosity liquid ester. Soluble rubbers may also be suitable if sufficient plasticization of the cured composition can be achieved at rubber concentrations which do not adversely affect viscosity of the uncured compositions.

An example of such a sealant is given below.

Bulk aeration of this sealant is necessary in order to prevent polymerization. Thus, it is recommended that in applications where aeration is unfeasible, a portion of the catalyst system, e.g. the copper saccharin salt, be separately added only shortly before use. Conveniently the copper saccharin salt may be added as a separate solution in a portion of some or all of the monomer base. Alternatively the copper saccharin salt may be added as a solution in a suitable solvent such as ethylene glycol, propylene glycol, triethanol amine, or N-methylpyrrolidone.

The above exemplified sealant has a viscosity of about 5 cps. Typical cured properties after a 24-hour cure at 23.degree. C. are as follows:

Tensile strength: 30 Kg/cm.sup.2

Percent elongation at break: 300%

Hardness, Shore A: 80

Tg: -20.degree. C.

Tear resistance: 15 N/mm

The catalyzed sealant may be applied to pipe joints by any suitable positive pressure system. It is particularly suited for use as the sealant with the three chambered inflatable device discussed above. Once in the joint, the sealant cures rapidly and will unitize any corrosion scale present to form a leak proof seal. This sealant may also be applied to porous components using either positive pressure or vacuum systems.

Depending on desired cure speed, the hydroperoxide level may be varied between about 0.1% and about 5% and the copper saccharin level may be varied between about 0.01% and 3% without materially affecting the cured properties obtained.

While an anaerobic curing sealant as exemplified above is preferred for underground pipe joint applications, in other applications other catalyst systems may be employed. Other suitable anaerobic cure systems are described, for example, in U.S. Pat. Nos. 3,855,040 to Malofsky, 4,180,640 to Melody et al. and 4,321,349 to Rich. Benzoyl peroxide/amine cure systems may also be employed.

A formulation was prepared as in the previous example except that only 24 parts of hydroxypropyl methacrylate was used, 0.1 parts saccharin was added and copper saccharin salt eliminated. Just prior to curing, a catalyst mixture comprising 12.5 parts of an 8% copper naphthanate solution and 87.5% of tetraethylene glycol di-2-ethylhexanoate is added. Cures were obtained at ambient temperatures unless otherwise stated. Cured products were prepared, evaluated, and the results obtained as set forth in Tables I-III. Dogbone specimens were utilized, prepared from cured sheets per ASTM D638 using a type IV die.

An alternate procedure to that described in the background section above, for sealing leaking joints in cast iron pipes with lead/yarn joints, developed subsequently to the priority date of this application, is as follows. A hole is drilled through the lead into the joint packing, and a short rigid nipple, such as a piece of metal tubing, is pounded into the hole. A length of flexible plastic tubing used to connect the nipple to a reservoir of anaerobic sealant. The sealant is then pumped into the joint and the plastic tubing clamped off and cut above the clamp. Gas pressure in the cast iron pipe drives the injected sealant into the leak paths. The sealant cures anaerobically in the leak path and in the nipple and plastic tubing section. A conventional nitrogen cylinder with 2-stage pressure regulator has been found to be a convenient means for pumping sealant into the joint. Further description of this method is believed to be reported in U.K. patent application No. 84,14544. This alternate method was used to conduct sealant trials on 6 inch diameter cast iron pipe sections.

The joints were prepared by packing the annular ring with jute and poring molten lead on top to form a seal. These joints are pressurized with nitrogen at 3 psi and adjusted by relative movement of the pipe sections to give a leak rate of one liter per minute. Sealant trials were then conducted as set forth in Examples 3-6.

A product formulated as in Example 2 using 2% catalyst was injected at a joint temperature of 17.degree. C. Leakage ceased and after injection of 500 grams of sealant, the injection tube was clamped off. After gellation of the sealant (40 minutes) the internal pressure was increased to 10 psi without further leakage being evident. Dismantling the joint showed good penetration and cure.

To a formulation as in Example 2 was added 18% of a polyester thickener sold under the trandmark Atlac.TM.382E. The product had a viscosity of 80 cps. This product was catalyzed by addition of 2% of the catalyst mixture. The joint temperature was 20.degree. C. 500 cc of this product was injected at 1-2 bar pressure over 18 minutes. Leaking was successfully stopped t throughout the injection period and did not re-establish. Product gelled at 38 minutes and the leaks remained sealed after the gel time. Dismantling the joint showed good even penetration and total cure of injected product.

The formulation of Example 2 was modified by addition of 0.5% fumed silica. 500 cc's of this modified product, catalyzed with 2% catalyst, was injected at a joint temperature of 18.degree. C. and a pressure of 1-4 over a 20 minute period after which the injection tube was clamped. Leaking stopped 10 minutes after injection began and did not re-establish.

A product identical to that used in Example 5 was injected into a joint at 0.degree. C. using 1-3 bar. An identical sealing pattern was obtained to that in Example 5.

As can be seen from Examples 4-6, polymeric thickeners or silicas may be added to the formulation if desired, so long as flexibility is not impaired and the viscosity is not increased to the point where injection into the joint is impaired. Preferably, silicas are utilized at a level of 0.3-2%. Still more preferably, 0.3-1.0%, which is a range in which the silica does not impart substantial thixotropy and yet provides significant sealing benefits especially at low temperatures (i.e., near 0.degree. C.).