US3686732A

US3686732A - The method of making a positive choke device

Info

Publication number: US3686732A
Application number: US35976A
Authority: US
Inventors: James H Mcclure; James W Mccrary
Original assignee: Materials Technology Corp
Current assignee: Materials Technology Corp
Priority date: 1970-05-11
Filing date: 1970-05-11
Publication date: 1972-08-29
Anticipated expiration: 1989-08-29

Abstract

Disclosed is a positive choke device having bore liner of silicon carbide. Also disclosed is a method of fabricating flow beans and the like having bore coatings of corrosion resistant materials having a hardness of at least 2,700 on the Knoop scale without machining of hard materials.

Description

United States Patent McClure et al. 1 Aug. 29, 1972 [54] THE METHOD OF MAKING A [56] References Cited POSITIVE CHOKE DEVICE UNITED STATES PATENTS [72] lnventors: James II. McClure, Dallas; James l Mccmry, Richardson both of 2,640,503 6/1953 Millngan ..138/ 141 T ex himary Examiner-John F. Campbell [73] Assignee: Materials Technology Corporation, Assistant Examiner Donald p Rooney Dallas Attorney.lack A. Kanz [22] Filed: May 11, 1970 ABSTRACT 211 App]. No.: 35,976

Disclosed is a positive choke device having bore lmer of silicon carbide. Also disclosed is a method of [52] US. CL ...............29/l57 C, 29/157 B, 138/141, fabricating flow beans and the like having bore 184/ 5 2 coatings of corrosion resistant materials having a hard- [51] Int. Cl.....B2ld 53/00, B2lk 29/00, B23p 15/26 ness of at least 2,700 on the Knoop Scale without [58] Field of Search ..29/l57 C, 157 B; 184/52;

machining of hard materials.

SCIaims, 3 DrawingFigures PATENTEU M1829 m2 F'ICLZ INVENTOR JAMES H. MCCLURE JAMES W MCCRARY CURRENT SOURCE w mzm THE METHOD OF MAKING A POSITIVE CHOKE DEVICE This invention relates to flow regulating devices. More particularly it relates to devices commonly known as positive chokes which have an orifice of fixed dimension and are secured within a line of high pressure fluid to control the rate of flow of fluid through the line.

in the petroleum producing industry it is often required to accurately control the rate of flow of crude petroleum and the like under high pressures through a particular conduit. For this purpose constriction devices, commonly referred to as positive chokes or flow beans, are inserted in the conduit. Such devices have a predetermined orifice passing therethrough which is determinative of the rate of flow of fluid under fixed conditions of temperature and pressure. Accordingly, the rate of flow of petroleum through a conduit can be controlled by affixing within the conduit a choke or flow bean of reduced diameter which restricts the flow through the conduit and regulates the flow of fluid therethrough.

Since the rate of flow of fluid through the choke is determined by the size of the aperture, precise control over the size of the aperture in the choke must be maintained at all times. However, ordinary crude petroleum may contain large amounts of hydrogen sulfide gas or other corrosive materials which attack and corrode conventional materials such as stainless steel. Furthermore, crude petroleum often contains large concentrations of abrasive materials such as sand and the like. Since the crude petroleum may be forced through the choke at pressures as high as 10,000 psi, the choke may be seriously abraded by foreign materials contained in the petroleum stream. Conventional materials, such as stainless steel and the like, are seriously corroded and abraded when used under these conditions and rapidly lose their effectiveness for precisely controlling the rate of fluid flow therethrough.

Attempts have been made to overcome these problems by placing liners of corrosion resistant materials within the choke orifice. However, it is difficult to accurately machine suitable materials to the dimensions required, since most corrosion and abrasion resistant materials are quite brittle.

In accordance with this invention, a positive choke is provided which has a liner of extremely hard, corrosion and abrasion resistant material in the orifice. The unique choke is formed without mechanical milling or machining of hard or brittle materials. The liner, however, has the uniform dimensions required to effectively operate as a positive choke. The liner is formed by chemical vapor deposition of silicon carbide on a graphite rod. The graphite is removed leaving a dense, impermeable liner of silicon carbide which is then secured within the choke orifice.

Because of the unique characteristics of silicon carbide and the manner in which the liner of the invention is fabricated, the choke device of this invention is substantially immune to attack by corrosive materials commonly encountered in crude petroleum streams, and is also substantially immune to abrasion by the abrasive materials contained in the crude petroleum. Accordingly, the liner protects the orifice of the choke and prevents deterioration thereof.

A particular advantage and feature of the invention is the provision of a positive choke which is resistant to both corrosion and abrasion. The choke may be formed with the required uniform dimensions without machining of hard materials. Other features and advantages of the invention will become more readily understood from the following detailed description taken in connection with the appended claims and attached drawing in which:

FIG. 1 is a sectional view of one embodiment of a positive choke made in accordance with the invention;

FIG. 2 is a schematic drawing of the process system for producing the liner of the invention; and

FIG. 3 is a perspective view of a graphite rod coated with silicon carbide in accordance with the process described with reference to FIG. 2.

In accordance with the present invention a liner is inserted within the orifice of the choke or flow bean to protect the orifice from both corrosion and abrasion by foreign materials contained in the crude petroleum stream. However it should be noted that in accordance with this invention the liner is silicon carbide produced by vapor deposition of silicon carbide on a rod of graphite. Accordingly, the silicon carbide liner of the flow bean is not machined in any manner to produce the precision orifice required. inserts of silicon carbide having precise internal dimensions are produced by depositing silicon carbide on precision ground rods of graphite as described hereinafter with reference to FIGS. 2 and 3.

In order for a positive choke device to precisely control the flow of fluid therethrough, the bore passing through the device must be of substantially uniform diameter. Furthermore, the bore must be substantially smooth to avoid causing unnecessary turbulance in the choke.

While it is not difiicult to form smooth uniform bores in conventional materials such as stainless steel, it is quite difficult to bore uniform holes in extremely hard materials. However, in order to prevent abrasion of the walls of the bore, the walls must be at least as hard as the abrasive materials contained in the fluid stream. Furthermore, the bore walls must be extremely dense to prevent abrasive particles from becoming embedded in the bore walls and causing abrasion thereof as the fluid is forced through the choke under high pressures.

in accordance with the method of this invention, a bore liner is produced which has a surface hardness of at least 2,700 on the Knoop scale and has smooth uniform internal dimensions without machining, milling or polishing of hard materials. The liner is produced as a coating on a relatively soft rod which is then removed leaving only the coating. The coating is then inserted as a liner in the bore of the choke device. Since the internal dimensions of the liner conform precisely to the outer dimensions of the rod upon which the coating was formed, the bore of the liner is made smooth and uniform by depositing the coating on a precision ground smooth graphite rod.

in the preferred practice of the invention silicon carbide is deposited on graphite rods since graphite is quite readily machined to the desired dimensions and silicon carbide may be formed on graphite without excess difficulty. However, since the reaction occurs at elevated temperatures, care must be taken to select a material for the rod which has a coefficient of thermal expansion which closely approximates that of silicon carbide to avoid cracking of the coating. For this reason a high grade, high expansivity graphite, such as Speer Grade 9345 is preferred. This grade graphite has a thermal coefficient or expansion of about 5 X cm/cm/C. Silicon carbide produced by the method described herein has a thermal coefficient or expansion of 4.5 X 10' cm/cmC.

Silicon carbide may be formed by several known methods. ln the preferred method of the invention a dense impermeable coating of silicon carbide is formed by reacting silicon halide with a hydrocarbon containing gas on a heated graphite surface. While it will be understood that various reactant combinations may be used to deposit silicon carbide, the preferred process is described herein with reference to FIG. 2.

The apparatus of FIG. 2 includes a scalable deposition chamber 20 comprised of upper portion 21 and lower portion 22. The upper and lower portions are removably secured together by conventional means such as bolts 23, clamps or the like. Chamber 20 has an exhaust port 24 connected to exhaust line 25 which is in turn connected to a conventional vacuum pump or the like for removing gases from the deposition chamber.

A rotatable table 26 is mounted on a shaft 27 which passes transversely through the bottom of the deposition chamber 20 and is adapted for rotation by conventional means. Rotatable table 26 is preferably constructed of a relatively inert material such as graphite or the like while the deposition chamber 20 may be constructed of stainless steel or any other suitable material.

A radiant heater 28 is secured below rotatable table 26 and interconnected to a suitable power source 29 for heating materials in the deposition chamber. Reactants are injected into the reaction chamber through

lines

30 and 31 by way of

control valves

32 and 33, respectively, and into nozzle 34. Nozzle 34 projects into the upper portion of the deposition chamber and directs the reactants toward the surface of the rotatable table 26.

Precision ground graphite rods 35 of uniform diameter are positioned substantially vertically on table 26 for rotation therewith and directly below the nozzle 34. The deposition chamber 20 is then closed and sealed.

The sealed chamber is evacuated by withdrawing the atmosphere therefrom through line 25 and the chamber refilled with dry hydrogen through inlet 30, valve 32 and nozzle 34. When the chamber 20 is filled with dry hydrogen, the exhaust port 24 is opened and hydrogen allowed to flow through the chamber 20 at the rate of about 25 to about 50 liters per minute.

With hydrogen flowing through the chamber at essentially atmospheric pressure heater 28 is activated by passing current therethrough from current source 29. The graphite rods 35 are heated to a temperature between about 1,000 C and about 1,400 C and maintained at this temperature under flowing hydrogen for about to about 30 minutes to assure complete cleaning and outgasing of the graphite rods. Throughout the cleaning and following deposition process, rotatable table 26 is rotated at a rate of about 1 to 15 rpm. It is therefore uniformly heated by radiant energy from the radiant heater 28 and all graphite rods 35 are maintained at a relatively constant temperature.

After the graphite rods 35 have been thoroughly cleaned as described above, the flow of hydrogen through the deposition chamber is stopped and hydrogen containing silicon tetrachloride (SiCL) substituted therefor. Simultaneously with the introduction of hydrogen and silicon tetrachloride containing gas, a mixture of hydrogen and toluene is introduced through line 31, valve 33 and into nozzle 34.

In the preferred practice of the invention, the composition of the reactant gas entering chamber 20 through nozzle 34 is approximately 2 to about l0 mole percent silicon tetrachloride, about 0.3 to about l.5 mole percent toluene, and the remainder hydrogen, the total gas flow through the system being about 50 liters per minute to about 100 liters per minute. It will be understood that within the gas composition ranges given an approximate stoichiometric relationship should be maintained with respect to silicon and carbon in the reactant gas stream. Therefore it will be apparent that the mole ratio of SiCl, to C H CH, should be approximately 7:1. Throughout the deposition process the temperature of the graphite rods is maintained at a constant temperature between about l,000 C and about l,400 C. Under these conditions a uniform dense coating of silicon carbide is formed on the surface of the graphite rods at rates up to 40 mils per hour.

When the desired thickness of silicon carbide is formed on the surface of the graphite rods 35, the flow of reactants through nozzle 34 is stopped and pure dry hydrogen substituted therefor. The flow of current through radiant heater 28 is then stopped and the rods 35 allowed to cool to substantially room temperature in flowing hydrogen. The deposition chamber 20 is then flushed with nitrogen and the coated rods removed.

Upon removal from the deposition chamber the coated rods are cut into suitable lengths such as illustrated in FIG. 3. The rod produced comprises a graphite rod 35 having a coating 40 of dense impermeable silicon carbide adherently bonded to the surface thereof. lt will be observed, however, that vapor deposited silicon carbide formed as described above forms a coating 40 which conforms precisely to the external dimensions of the rod 35.

The coating 40 may be recovered and removed from the rod 35 by chemically removing the graphite rod. A solvent must be selected, of course, which dissolves the graphite without affecting silicon carbide. A solution comprising 1 part BNO, and 9 parts H by volume, has been found suitable for this purpose. In the preferred practice of the invention, part of the graphite is first removed by boring a hole axially through the center of the rod. in this manner approximately 80 percent of the graphite rod can be quickly removed and the remaining graphite rapidly dissolved in the HNO :H,SO solution.

Since the HNO :H SO solution dissolves graphite but does not attack silicon carbide, the rod -35 is completely removed leaving only the coating 40. Furthermore, since the coating formed a mirror image of the surface upon which it was deposited, the bore through coating 40 is precisely the same dimensions as the surface upon which it was formed.

Vapor deposited silicon carbide coatings 40 produced as described above have been found to have a density of 3.22 grams per cubic centimeter and a surface hardness of 2,740 on the Knoop scale, using a 100 gram weight. The material is also found to have an excellent resistance to oxidation up to temperatures as high as l,500 C and has a compressive strength of 119 X psi and essentially zero porosity with a coating thickness of at least 0.005 inch. Accordingly, liners having the above characteristics are resistant to attack by any corrosive materials contained in fluid streams such as crude petroleum and the like. Furthermore, because of the extreme surface hardness and smoothness of the surface produced, abrasive materials contained in a fluid stream passing through such a liner have little effect on the silicon carbide surface.

A positive choke constructed according to the invention is illustrated in FIG. 1. The choke comprises a stud having a hex head 10 with a threaded shank 11. A bore 12 passes axially through the full length of the choke. Liner 40 is positioned within the bore 12 and secured therewithin as by brazing or with a suitable adhesive or other conventional means. A suitable adhesive for securing the liner 40 within the bore of the bean has been found to be an adhesive epoxy cement manufactured by the Armstrong Company of Warsaw, lndianna marketed under the trade name of Armstrong A-2 Adhesive.

Conventional positive chokes are generally fabricated from stainless steel or the like, have bore dimensions ranging from 6/64 inch up to 1 inch, and are usually about 1 inch to about 6 inches long. Chokes made in accordance with this invention can be made from conventional choke materials with oversized bores into which the liners are inserted. The liners 40 can be made to have any desired internal diameter by selecting graphite rods of the desired diameter. Accordingly, chokes made in accordance with this inven tion may be readily substituted for conventional chokes. Furthermore, liners having any desired internal diameter may be produced without machining hard materials by depositing silicon carbide on a graphite rod having outside dimensions of the desired bore. Since graphite is comparatively soft and readily machined, rods may be precision ground to any desired size. Consequently, the liners, which conform to the configuration of the rod, can be formed which have smooth, uniform bores with variations in internal diameter of less than i- 1 mil throughout the length of the bore.

While the invention has been described with particular reference to a positive choke, other similar devices may be fabricated by the same method. For example,

flow control is sometimes maintained using a master bean of a first internal diameter and having a threaded socket at one open end. Flow beans of the general configuration illustrated in FIG. 1 may then be secured within the socket to reduce the aperture of the choke. Flow beans having larger or smaller apertures may thus be readily substituted to change the flow characteristics of the choke. Such flow beans may readily be fabricated in accordance with this invention as described above with reference to positive chokes whi i t e t he invention has been described with specific reference to specific embodiments thereof, it is to be understood that the forms of the invention shown and described in detail are to be taken as preferred embodiments of same, and that various changes and modifications may be resorted to without departing from the spirit and scope of the invention as defined by the appended claims.

We claim:

I. The method of producing a positive choke device comprising the steps of:

a. forming a rod of graphite having external dimensions conforming to the desired dimensions of the bore of the positive choke to be produced;

b. heating said graphite rod to a temperature between about 1,000 C and 1,400" C;

c. depositing a coating of dense impermeable silicon carbide on the surface of said graphite rod;

d. removing the graphite rod from the silicon carbide leaving a tubular body of silicon carbide having internal dimensions conforming to the desired bore dimensions of the positive choke to be produced; and

e. securing said tubular body of silicon carbide within the bore of a positive choke device.

2. The method set forth in claim 1 wherein said coating of silicon carbide is deposited on said graphite rod by reacting a hydrocarbon and a silicon halide on the surface of said graphite rod.

3. The method set forth in claim 2 wherein said hydrocarbon is toluene and said silicon halide is silicon tetrachloride.

4. The method set forth in claim 3 wherein said silicon tetrachloride and said toluene are entrained in a hydrogen stream in a ratio of approximately 7. l.

5. The method set forth in claim 1 wherein said graphite rod is removed from said silicon carbide coating by boring a hole axially through the center of said graphite rod and dissolving the remaining graphite in a solution comprising about 1 part HNO, and 9 parts H by volume.

Claims

2. The method set forth in claim 1 wherein said coating of silicon carbide is deposited on said graphite rod by reacting a hydrocarbon and a silicon halide on the surface of said graphite rod.
3. The method set forth in claim 2 wherein said hydrocarbon is toluene and said silicon halide is silicon tetrachloride.
4. The method set forth in claim 3 wherein said silicon tetrachloride and said toluene are entrained in a hydrogen stream in a ratio of approximately 7:1.
5. The method set forth in claim 1 wherein said graphite rod is removed from said silicon carbide coating by boring a hole axially through the center of said graphite rod and dissolving the remaining graphite in a solution comprising about 1 part HNO3 and 9 parts H2SO4 by volume.