GB2050607A

GB2050607A - Peak Pressure Indicator

Info

Publication number: GB2050607A
Application number: GB7919154A
Authority: GB
Original assignee: British Gas Corp
Current assignee: British Gas Corp
Priority date: 1979-06-01
Filing date: 1979-06-01
Publication date: 1981-01-07
Also published as: GB2050607B

Abstract

A peak pressure indicator device comprises a cylindrical gas container (6) sealed at one end (7) and open at the other (8) to a source of fluid pressure. Within the cylinder are an actuating piston (10) which compresses the enclosed gas in response to external pressure changes and a vented follower piston (9) which indicates the maximum distance of travel of the actuator piston. <IMAGE>

Description

SPECIFICATION Peak Pressure Indicator Device This invention relates to devices for the measurement of pressure and, in particular, to devices suitable for monitoring the absolute pressure to which a pipeline inspection vehicle is subjected when traversing a pipeline.

Instruments used for the in situ inspection of underground pipelines are carried on such a vehicle commonly known as a pig which is caused to traverse the pipelines. Pigs may be driven by an inboard motor, but usually they are propelled by means of differential pressure of the fluid in the pipeline acting across them. The usual arrangement is for a flexible cup which fills the pipeline cross-section to be mounted on the body of the pig. Fluid pressure acts on this cup and generates the motive force for the vehicle.

The instruments are usually carried in a pressure vessel forming the body part of the inspection pig and may be sensitive to the effects of excess pressure should the seals of the vessel fail. During the course of an inspection run, the pig is inaccessible and the vessel containing the instruments may experience abnormal pressure build-ups, possibly causing failure of the pressure vessel seals and damage to the instruments. It is therefore desirable to provide some indication of the peak pressure experienced by the instruments.

A typical device used to monitor changes in pressure during the course of a measuring run is described in U.K. Patent 1,262,609. This patent discloses a peak pressure gauge used for monitoring periodically varying gas pressures and has a housing with an inlet conduit which is attached to the source of pressure to be measured. The gas pressure acts on a spring-loaded piston which frees a rotating indicator cap. The angle of rotation of the cap depends on the pressure applied to the piston and, when the piston is subjected to periodic fluctuations in pressure, provides an indication of the maximum pressure during the course of a run. This device, however, has a very complex construction and requires very precise assembly. (In a typical embodiment, the movement of the piston is 0.1 mm).

It is therefore expensive to construct and is not suitable for routine measurements on pipeline inspection pigs. It was therefore considered desirable to design a device which would be simple to construct, but would provide a reliable indication of the pressure to which the instruments in a pipeline inspection pig had been subjected.

According to the present invention, there is provided a device for indicating the peak pressure within a fluid comprising, cylinder means communicating with a closed vessel containing a gas, first piston means movable in a first direction in response to increase in pressure in said fluid and in the opposite direction in response to decreases in said pressure and having a surface exposable to said fluid, and second piston means movable by said first piston means to indicate the movement of said first piston means in said first direction.

An embodiment of the Invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows schematically a pipeline inspection pig having a number of instruments and peak pressure indicating device, and Figure 2 shows an embodiment of a peak pressure indicating device in accordance with the Invention.

Referring now to the drawings, a pressure vessel body 1 of an inspection pig is mounted by means of rubber cups 2 in a pipeline 3. Carried inside the body 1 are measuring instruments 4 and a device 5 in accordance with the Invention to monitor the pressure to which the instruments have been subjected. The cups 2 form a seal with the internal surface of the pipeline and force the pig along the pipeline in the direction indicated by the arrow.

The device 5 for indicating the pressure is illustrated in Figure 2. It comprises a transparent glass tube 6 sealed at one end 7 and open at the other end 8. Within the tube are two pistons 9, 10 each having a flexible peripheral ring 11 which forms a seal with the glass tube. The inner, follower piston 9 is provided with a vent hole 12 so that it will move to the left as illustrated in the diagram when the gas in the tube is compressed as a result of external pressure on the outer, actuator piston 10, but when the gas expands again as the pressure is reduced, the follower piston will remain in position, thus providing an indication of the maximum pressure to which the gas has been subjected.

Example A pressure measuring device was constructed from a glass tube sealed at one end and having a column of gas of effective length 1 70 mm enclosed by two pistons of plastics material. The distance moved by the follower piston was measured at different external pressures and the pressure calculated assuming no static friction. The results were as indicated in the table.

Table Distance moved by Gauge Calculated follower pressure pressure piston (mm) (KNIT2) (KNIT2) 33 138 32 108 276 249 120 414 369 125 552 448 130 689 558 Variations in the manner of construction can be made to achieve desired variations in response.

For example, by making the diameter of the tube small and the walls of the tube of metal with only a window for observation of the follower piston, isothermal compression of the gas and hence greater movement of the follower piston can be achieved. If the frictional resistance on the actuator piston is increased, perhaps by coupling it to a dashpot, the response can be damped and the device made insensitive to brief pressure increase. Furthermore, a plurality of devices each having a different response time could be incorporated on an inspection pig.

A one-way valve may be provided to release internal pressure prior to removal of the device from the pipeline.

Claims

1. A device for indicating the peak pressure within a fluid comprising cylinder means communicating with a closed vessel containing a gas, first piston means movable in a first direction in response to increases in pressure in said fluid and in the opposite direction in response to decreases in said pressure and having a surface exposable to said fluid and second piston means movable by said first piston means to indicate the movement of said first piston means in said first direction.

2. A device as claimed in claim 1 wherein said second piston means has an internal vent to permit the passage of said gas therethrough.

3. A device as claimed in claim 2 wherein means are provided to increase the frictional resistance on the first piston means.

4. A device as claimed in claim 3 wherein the means to increase the frictional resistance comprises a dash pot.

5. A device as claimed in any one of the preceding claims wherein the cylinder means are of glass and said second piston means is visible therethrough.

6. A device as claimed in any one of the preceding claims 1 to 4 wherein the cylinder means is of metal having a window therein.

7. A device substantially as herein described with reference to the Example and as illustrated in the accompanying drawing.

8. A pipeline inspection pig incorporating a device as claimed in any one of the preceding claims.