Corrosion and erosion sensor

A sensing probe having a hollow tube for disposition in a flow stream is provided with an internal stiffening rod. A volume between the stiffening rod and the inner surface of the tube communicates with an open end of the sensing probe. In one embodiment, oppositely directed spiral grooves are formed in the surface of the stiffening rod to define the volume. A method of manufacturing the sensing probe includes first inserting a stiffening rod into a tube and then turning the tube to a desired wall thickness.

The invention relates generally to corrosion and erosion sensors for 
pipelines transmitting fluids and more particularly concerns a corrosion 
resistant, thin walled, closed end tubular sensing probe which may be 
suspended in a flow stream to act as a sacrificial corrosion or erosion 
sensor. 
Sensing probes such as the sensing probe disclosed in Kelly U.S. Pat. No. 
3,630,216 are known. Sensing probes, which may also be called sand probes, 
also include Otis Nos. 70P209, 70P225 and 70P236, Baker CAC Nos. 877-47, 
877-64 and 877-65, and HLR Controls No. 7620. Outer Continental Shelf 
Order No. 5 of the U.S.G.S. permits the use of sand probes in erosion 
control programs. Typically, these probes have an elongated corrosion 
resistant metallic or plastic tubular member closed at one end and 
provided with a coupling member at the other end through which the bore of 
the tubular member communicates to a pressure sensitive indicator. The 
material and the wall thickness of the tubular member may be selected for 
the particular application. The material selected is often the same 
material used for the pipeline. The tubular member is suspended in the 
flow stream of a pipeline to act as a sacrificial sensor. Corrosive, 
caustic, acid or alkali fluid within the pipeline may act upon and corrode 
the tubular member. Similarly, particulate matter such as sand suspended 
in fluid flowing through the pipeline may impinge upon and erode the 
tubular member. The material and the wall thickness of the tubular member 
may be selected so that the corrosive and erosive effects of the fluid 
will breach the wall of the tubular member before the same corrosive and 
erosive effects damage or break components of the pipeline. When the 
tubing wall of the probe is penetrated by any means such as impingement 
erosion caused by abrasive fluid flow, pressure entering the bore of the 
probe tubing is transferred to the pressure sensitive valve or indicator, 
which may operate an alarm system. 
A specific usage in oil field service is to sense excessive sand production 
into a petroleum product flowstream that can lead to erosion failure of 
production equipment. High sand content fluids erode production flow 
conduits at a greater rate than do low sand content fluids. A thin-walled 
sand probe is designed to erode and fail prior to the thick-walled 
production equipment, thereby providing an alarm signal or equipment 
shutdown reaction prior to equipment failure. This characteristic has 
resulted in regulatory acceptance of sand probes as an economical means of 
monitoring erosion in oil field production operations. 
Problems associated with the manufacture and practical application of sand 
probes are based in the fragile construction of the probes. The thin walls 
are easily flattened, bent, and broken. Fabrication is difficult for thin 
walled tubes, and minimum wall thicknesses may be determined by 
fabrication limitations rather than actual field requirements. 
High pressures that envelop the outside of the sand probe impart collapse 
forces on the thin walled tube, limiting its service pressure. The limited 
resistance of a thin walled sand probe to physical force and pressure 
force failure is decreased as erosion thins the wall. As the tubing wall 
is eroded, it becomes more susceptible to collapse forces. Any flattening 
or bending of the fragile tube dramatically affects its collapse 
resistance and its ability to remain extended into the flowstream. The 
outside diameter of the sand probe is limited to very small diameter (in 
the range of 3/8", 9.5 mm) to withstand collapse pressures. 
The reach or length of the small diameter, thin walled tube is limited 
because flow of viscous fluids such as those produced in petroleum 
applications can flex and bend the tube to a position parallel to the pipe 
wall, thereby reducing its effectiveness. Flexure can also cause breakage 
of the tube producing false alars in the erosion monitoring system. 
Flexure work hardening of fragile tubes induces premature failure, and 
these flexure forces are additive to the collapse forces and stress risers 
at the base of the tube attachment to its threaded adapter. These 
undesirable concentrations of abuse are concentratred at a point that is 
often a zone of weld attachment of the probe tube to its threaded adapter. 
This attachment zone is therefore a common failure zone. 
Accordingly, an object of this invention is to increase the physical 
strength and pressure capabilities of a sand probe to enhance the 
likelihood that the probe can perform its intended function by minimizing 
false alarms and premature failures. 
A further object of this invention is to more easily, accurately and 
economically fabricate a sensing probe. 
An even further object of this invention is to provide thin-wall sensing 
probes usable in higher pressure applications. 
SUMMARY OF THE INVENTION 
A sensing probe having a hollow tube for disposition in a flow stream is 
provided with an internal stiffening rod. A volume between the stiffening 
rod and the inner surface of the tube communicates with an open end of the 
sensing probe. In one embodiment, oppositely directed spiral grooves are 
formed in the surface of the stiffening rod to define the volume. A method 
of manufacturing the sensing probe includes first inserting a stiffening 
rod into a tube and then turning the tube to a desired wall thickness.

While the invention will be described in connection with a preferred 
embodiment, it will be understood that the description is not intended to 
limit the invention to that embodiment. On the contrary, the description 
is intended to cover all alternatives, modifications and equivalents as 
may be included within the spirit and scope of the invention as defined by 
the appended claims. 
DETAILED DESCRIPTION OF THE INVENTION 
Turning first to FIGS. 1, 2, 3 and 4, there is shown a sensing probe 10 
including a thin-wall hollow tube 12, a longitudinally continuous 
stiffening rod 14 within the tube 12 and a threaded coupling 16 at a first 
end of the tube 12. Two oppositely directed, intersecting spiral grooves 
I8 are formed in the outer cylindrical surface 20 of the rod 14. The 
multiple intersections of the grooves 18 provide multiple fluid paths 
along the surface 20 within the dimensional envelope of the rod 14 as the 
ungrooved portions of the surface 20 engage the inner surface 22 of the 
tube 12. The grooved portions of the surface 20 of the rod 14 together 
with the inner surface 22 of the tube 12 define a continuous volume 24 
along the length of the tube 12 and the rod 14, which volume 24 
communicates to the first end 26 of the tube 12 and the first end 28 of 
the rod 14, which first ends 26 and 28 are both within the threaded 
coupling 16. 
Alternatively, a single spiral groove 18 may be prov[ded in the surface 20 
of the rod 14, either alone or in conjunction with one or more axial 
grooves, not shown, in the surface 20. As will be apparent, other grooves 
may also be used. 
A passage 38 is formed to connect the grooves 18 in the surface 20 of the 
rod 14 to the face 40 of the first end 28 of the rod 14. Preferably this 
passage 38 in an axial passage 42 drilled into the face 40 and intersected 
by a transverse passage 44 drilled through the rod 14 so that the ends 46 
of the transverse passage 44 lie within the grooves 18 formed in the 
surface 20 of the rod 14 Alternatively, an external passage, preferably 
axial, such as a flat or an axial groove 47 may be ground or otherwise 
formed into the surface 20 of the rod 14 and the face 40 of the first end 
28 of the rod 14. The axial groove 47 may be deeper than the spiral 
grooves 18. 
The second end 30 of the tube 12 is closed. The second end 30 of the tube 
12 extends slightly beyond the second end 32 of the rod 14, defining a 
recess 34 within the second end 30 of the tube 12. Weld metal 36 is 
deposited within the recess 34 to both seal the second end 30 of the tube 
12 and to fix the rod 14 to the tube 12. The second end 30 of the tube 12 
and the weld metal 36 may be ground smooth if desired. 
The threaded coupling 16 includes a passage 48 therethrough containing 
within a first end 52 of the passage 48 the first end 26 of the tube 12. 
Preferably, the first end 28 of the rod 14 is flush with the first end 26 
of the tube 12 to abut a shoulder 50 within the passage 48. Alternatively, 
the first end 26 of the tube 12 extends beyond the first end 28 of the rod 
14 to abut the shoulder 50. The passage 38 within the first end 28 of the 
rod 14 communicates the grooves 18 with the passage 48 of the coupling 16 
beyond the shoulder 50 and with the exterior of the coupling 16 at a 
second end 54 of the passage 48. The tube 12 is fixed and sealed to the 
coupling by weld metal 56 deposited about the tube 12 at the surface of 
the coupling 16. The depth of the shoulder 50 exceeds the depth of the 
axial passage 42 within the rod 16 within the passage 48 of the coupling 
16, so that the transverse passage 44 intersects the grooves 18 beyond the 
zone where the weld metal 56 is deposited, insuring communication in the 
event that the welding operation partially blocks the grooves 18 in the 
weld zone. As shown in FIG. 5, means suct as external threads 58 and 60 
are provided on the coupling for securing the sensing probe 10 
respectively to the pipeline 62 and to a suitable indicator 64, such as a 
Sigma Enterprises, Inc. 29SP24 Sand Probe Indicator. 
Both the wall thickness and the material of the tube 12 may be selected for 
the specific application of the sensing probe 10. The incorporation of the 
rod 14 into the tube 12 allows the wall thickness of the tube 12 to be 
reduced at any subsequent point in the assembly process. A sensing probe 
may commonly use 0.035", 0.9 mm wall thickness, 0.375", 9.5 mm outside 
diameter 316 stainless steel tubing for the tube 12, although sensing 
probes for use in large pipelines may be 3", 76 mm or more in outside 
diameter. The tube 12 may then be turned on a lathe or ground to a wall 
thickness as small as 0.010", 25 mm or even 0.005", 0.13 mm along a 
substantial portion of its length preferably excluding the first end 26 
and second end 30 of the tube 12, which ends 26 and 30 are preferably left 
at their full original diameters in the zones of the weld metal 56 and 36, 
respectively. The first end 26 may be inserted into a 0.375", 9.5 mm 
diameter opening in the coupling 16 either before or after the tube 12 is 
turned. The turning on a lathe of the tube 12 may even be deferred until 
after the sensing probe 10 is otherwise complete. Alternatively, a 
completed sensing probe 10 which has already been turned down to a 
specific wall thickness of the tube 12 may later be further turned to a 
smaller wall thickness. 
The spiral grooves 18 may be formed to a typir:al width of 0.020", 0.5 mm 
and a typical depth of 0.010", 0.25 mm at a typical pitch of 0.125", 3.2 
mm. 
Thus, it is apparent that there has been provided, in accordance with the 
invention, a corrosion and erosion sensor that fully satisfies the 
objects, aims and advantages set forth above. While the inventicn has been 
described in conjunction with a specific embodiment thereof, it is evident 
that many alternatives, modifications and variations will be apparent to 
those skilled in the art in light of the foregoing description. 
Accordingly, it is intended to embrace all such alternatives, 
modifications and variations as fall within the spirit and broad scope of 
the appended claims.