GB2163438A - Injectable sealant compositions - Google Patents

Abstract

An injectable sealant composition comprising graphite powder dispersed in an oil vehicle contains none of the asbestos fibre conventionally employed but instead incorporates (a) synthetic fibres (eg vitreous fibres such as E-glass) which at a temperature of 250 DEG C neither melt nor decompose and (b) a finely divided mineral silicate, preferably a clay, or mineral sulphate, for example barytes. The composition can be cast or extruded into sticks and injected by conventional means to stop leaks in valves, flange-joint gaskets and the like forming part of plant operating at high pressures and elevated temperatures.

Description

SPECIFICATION Injectable sealant compositions This invention relates to injectable sealant compositions.

Many industrial processes - for example, the generation of electricity and the production of chemical compounds - are operated under high pressure at elevated temperature; and when a leak occurs, as in a valve packing or a flange joint gasket, it is desirable to mend it without interrupting the process concerned. For this purpose it is common to employ an injectable sealant composition based on asbestos fibres and graphite powder, which are dispersed in an oil vehicle containing a small amount of solid lubricant such as calcium stearate. The sealant composition is cast or extruded into the form of a stick.In use, a narrow passage, of diameter up to 5mm, is drilled in the affected element (for instance, the body of a valve) to communicate with the cavity associated with the leak, and the stick of sealant composition is loaded into a hydraulically actuated piston-and-cylinder gun and then injected into the cavity. The pressure applied to the composition has, of course, to be greater than that of the process in operation, which may be 4,000 psi (about 280 bar) or even higher. Since the operating temperature is usually above 1500C, most of the oil vehicle quickly volatilises on injection, with the result that the material injected shrinks. Accordingly, pressure is maintained to force in more sealant composition until the leak cavity is completely filled.Having lost most of its oil, the composition remaining in the cavity loses its fluidity under pressure and forms a resilient barrier of fine particles of graphite held together by asbestos fibres, The present invention provides an injectable sealant composition not containing asbestos but usable generally under the same conditions as the asbestos-containing one. Like the latter the composition contains graphite powder dispersed in an oil vehicle, but instead of asbestos fibres it incorporates (a) synthetic fibres which at a temperature of 250"C neither melt nor decompose (these can conveniently be referred to as "high temperature fibres") and (b) a finely divided mineral silicate or sulphate.

The high temperature fibres are preferably of diameter less than 20 am, a diameter in the range 6-15 m being particularly preferred. They may be of conventional length (3-25mm), but this is of course considerably reduced by the shearing forces involved in the preparation of the composition, which at room temperature is ordinarily a stiff paste. The fibres may be given a conventional dressing to improve their dispersibility in the oil vehicle.

The high temperature fibres may be aramids such as those available under the names KEVLAR and NOMEX, or carbon fibres (preferably of low or medium modulus), but are preferably vitreous fibres such as: E-glass; a chemically more resistant glass such as a ZrO2-containing glass; mineral wool fibres; or ceramic fibres, which may be those available under the names Saffil or REFRASIL or alumino silicate fibres. If the sealant composition is to be used in mending leaks in elements such as valves, with moving parts, then any non-fibrous impurity in the high temperature fibres - such as the "shot" commonly present in mineral wool - should be kept as low as possible to avoid abrasion. Mixtures of high temperature fibres can be used if desired. The high temperature fibres suitably form 2-40%, preferably 15-30%, by weight of the sealant composition.

The finely divided mineral silicate is preferably a clay, for example ball clay, kaolin, bentonite or attapulgite,which may prior to use be treated to render it oleophilic and thus make it more readily dispersible in the oil vehicle. A suitable oleophilic clay is sold under the name BENTONE. Microfibrous natural clays, for example that sold under the name attagel, with a particle size in the 0.1-1 tm range, may also be employed as ingredient (b). So also may talc and (if greater stiffness is required in the composition after injection) plate-like minerals of the mica-vermiculite family. Besides contributing to the sealing function of the composition, the finely divided mineral silicate improves the cohesiveness of the composition before injection, thus helping it to keep its form when cast or extruded.The finely divided mineral silicate suitably forms 5-50%, preferably 10-40% by weight of the injectable composition. A mineral sulphate (eg barytes) behaves similarly to a silicate, and may be used instead.

As to the conventional ingredients of the injectable sealant composition, the oil (which may be a straight mineral oil or a gelled oil ie grease) suitably forms 20-60%, preferably 25-50%. by weight; and the graphite powder may form 15-60%, preferably 20-45%, by weight.

If desired, a wax may be included in the composition to make sticks cast or extruded from it more rigid at room temperature and so more easily fitted into an injection gun.

As already indicated, graphite powder is an essential ingredient of the injectable composition of the invention, just as it is an ingredient of the known asbestos-containing composition. However, there may additionally be included graphite in the form of expanded graphite, suitably in a state partially re-compacted to reduce its porosity and then shredded. The expanded graphite is conveniently compacted to a foil of density 1000 kg/m3, and the foil can then be milled to pass a 10 mesh British Standard sieve and be retained on a 40 mesh British Standard sieve. Such use of a proportion of expanded graphite (up to half the quantity of graphite powder) gives a cast or extruded stick more resistant to damage during handling.

The invention is further illustrated by the following examples.

Examples The ingredients set out in the table below were mixed in a Z-blade mixer, and the mixtures formed were extruded into sticks of length 7.5 cm and of diameter 14mm. The properties of the compositions formed are shown in the lower part of the table.

Example No. 1 2 3 4 (conventional asbestos sealant) % by wt % by wt % by wt % by wr E-Glass fibre; 10 Fm diamter, 6mm length, 23 20 20 asbestos 34% PVA dressing Mineral Oil (Mobil 600W) 32 28 30 36 Synthetic graphite 95% < 75cm; specific 23 22 14.6 30 surface area BET 8.9mug Shredded graphite foil (starting density 1000 kg/m3 shredded to < 2% - 7.4 - retained on 10 mesh; > 89% retained on 40 mesh) Ball clay Calcium (98% < 20 wm; 74% < 2 > m) 22 30 28 stearate 4% Properties Yield stress before heating (load required to induce flow at 10mm/min through 1025N 1220N 1190N 2000N a capillary 45mm long, 2mm diameter *Hardness after heating at 500"C for 175 hrs.

(Penetration in mm of a 2.10 1.60 1.60 2.10 6mm diameter steel ball at a load of 100N Appearance after 500"C No cracks, No cracks, No cracks, No cracks, for 175 hrs cohesive, cohesive, cohesive, cohesive, resilient, no resilient, resilient, resilient, apparent no apparent no apparent some graphite loss graphite graphite graphite loss loss loss * Hardness Test A small sample of sealant is put into cavities in a steel block. The cavities are then closed, by clamping the block against a second block, and heated to 500 C for 175 hours to simulate conditions in a leaking valve or flange. After dismantling, the fired sealant is assessed for general texture, integrity, etc and its hardness is measured by an indentation test using the steel ball specified. This gives an indication of its resistance to being forced out of a leaking joint by the pressure in the system.

Claims (8)

1. An injectable sealant composition comprising graphite powder dispersed in an oil vehicle, the composition being free from asbestos and incorporating (a) synthetic fibres which at temperature of 250"C neither melt nor decompose and (b) a finely divided mineral silicate or sulphate.

2. A composition according to claim 1, in which the high temperature fibres (a) are of diamter less than 20 wm.

3. A composition according to claim 1 or 2, in which the fibres (a) are vitreous fibres.

4. A composition according to claim 1, 2 or 3, in which the finely divided mineral silicate is a clay.

5. A composition according to claim 4,in which the clay is ball clay.

6. A composition according to any preceding claim,in which the proportions of the various ingredients are: High temperature (a) 2-40% by wt Finely divided mineral (b) 5-50% by wt Graphite powder 15-60% by wt Oil Vehicle 20-60% by wt

7. A composition according to claim 6, in which the proportions of the various ingredients are: High temperature fibres (a) 15-30% by wt Finely divided mineral (b) 10-40% by wt Graphite powder 20-45% by wt Oil vehicle 25-50% by wt

8. An injectable sealant composition substantially as herein described with reference to Example 1, 2 or 3.