Method of testing pipes for leakage

IS 2 173 249 T3

DESCRIPTION

Checking procedure for leaks in pipes.

The invention relates to the testing of insulated buried thermoplastic pipes to locate the existence of leaks.

Detection of leaks from distribution pipes is carried out by measuring any rate of pressure drop during a pneumaotic pressure test. The facility is pressurized to 1.5 times its maximum working pressure (typically 3, 6 or 7 bar pressure), and regular pressure readings are taken over time until the engineer is satisfied that there is no significant leak. .

The "pass / fail test" criterion applied in British Gas PLC is that the significant leak is one that represents a leak of 0.0028 standard cubic meters per hour of gas at the maximum working pressure of the supply network. .

For a constant temperature and internal volume, the rate of pressure drop due to a leak is proportional to the flow rate of the leak, and inversely proportional to the internal volume of the pipe under test. In large volumes, very small pressure drop regimes have to be resolved. For example, at 100 cubic meters in a 6 bar test, the "pass / fail" pressure drop rate is 36 microbars per hour. Using conventional instrumentation, for example, capable of a resolution of the order of millibars at a measured pressure of 7 bar, several times of test duration are required in order to obtain the record of such small regimes of pressure drop.

During such extended periods of time, other influences such as changing ambient temperature have to be taken into account, both from the point of view of drift of instrumentation readings as well as change in test pressure.

For modern polyethylene gas pipe systems, in addition to the change in temperature, the other main influence on the behavior of the pressure drop is the behavior of progressive deformation of the pipe when subjected to pressure, which causes a time-dependent increase in volume, leading in turn to an additional drop in pressure.

For a system without leaks and at a constant temperature, the behavior of the pressure drop caused by progressive deformation is independent of the volume of the pipe, but it depends on the class of material, on the pressure (stresses) of the standard dimension ratio. (SDR) (which is the outside diameter of the pipe divided by the wall thickness), and the recent history of pipe stresses. The fall regime curve is exponential in nature, eventually stabilizing at a constant pressure as volume changes become small in decreasing fashion. Therefore, for small test volumes, the flow rate due to a significant leak dominates over the flow rate due to progressive deformation, but in large volumes the flow rate due to progressive deformation dominates in the former. days of the test.

US-A-536797 discloses a process for checking a tank for leaks, in which it is established that influencing factors on pressure other than gas leakage have already stabilized before checking the integrity of the tank.

According to the invention, the procedure for checking the existence of leaks in an isolated thermoplastic buried pipe comprises the qualification of the pipe leak with respect to a “pass / fail test” leak criterion, elimination of the effects of temperature change and of changes in the phosphorus volume of the pipe due to the viscoelastic behavior of the plasty pipe, the procedure including temperature measurement, to eliminate the effects of temperature change.

The leak checking device for use in the method of the invention comprises:

a temperature sensing means for sensing the temperature within or adjacent to an insulated plastic pipe to check for leaks;

pressure determination means to detect the internal pressure of the pipe; and a processing means to create the progressive deformation prediction information from the sensed temperature and the sensed pressure, while the pipe is pressurized and to qualify the measured leak of a pressurized pipe, with respect to the information of the prediction of progressive deformation created to eliminate the effects of temperature change and changes in the physical volume of the pipe, due to the viscoelostic behavior of the plastic pipe.

Preferably, a computer and a differential pressure transducer are used to measure the rate of pressure change caused by the combination of pipe leakage, by temperature change, and by change in the physical volume of the pipe.

Preferably, the method comprises:

(i) make pressurized air pass into the pipe and into a reference chamber connected to the pipe by means of a valve, meanwhile recording at intervals of a few seconds the temperature in the reference chamber, the temperature in the pipe or in the land adjacent to the pipe, and the absolute pressure in the reference chamber;

(ii) when the test pressure has been reached, the mentioned valve was closed and the differential pressure between the pipe and the reference chamber was repeatedly measured;

(iii) open the valve to equalize the pressures in the pipe and in the reference chamber;

ES 2 (iv) repeating steps (ii) and (iii) several times at intervals of less than half an hour; and (v) use a computer to calculate:

(a) the prediction curve with respect to the pressure drop in the pipeline (due to creep and leakage due to allowable leakage) with respect to time; and for each set of differential pressures measured in stage (ii):

the rate of change with respect to time of the differential pressure between the pipe and the reference chamber (the measured rate of the differential pressure drift);

(b) the rate of change with respect to time of the temperature of the pipe or adjacent ground (the rate of drift of the test temperature);

(c) the rate of change with respect to time of the reference chamber temperature (the rate of drift of the reference temperature);

(d) the rates of change with respect to time of the pressure drift rates corresponding to (c) and (d) (the test pressure drift prediction rate and the test pressure drift prediction rate). the reference pressure, respectively);

(e) the difference between the pressure drift regimes calculated in step (e) to obtain the differential pressure drift prediction regimen;

(f) the difference between (b) and (f) to obtain the pressure drift rate due to progressive deformation and leakage only in the pipe, independent of any change in pipe temperature;

^Y (vi) compare the results obtained in stage (g), and determine whether or not the results tend towards the intersection with a curve that represents a progressive deformation prediction model for an allowable leak.

The invention will be described below by way of example with reference to the accompanying drawings, in which:

Figure 1 shows the apparatus used in the execution of the procedure according to the invention, and Figure 2 shows a graphic illustration of how the results obtained can be evaluated according to a curve of the pressure drop prediction regime.

The apparatus

Figure 1 shows a typical excavation to expose one end of a buried polyethylene gas pipe 10. Pipeline 10 ha

249 T3 4 has been insulated and its end has been closed by means of a pressure-proof gasket.

12, as well as the remote end (not shown).

The excavation contains the following elements: a reference chamber 14; a valve 16 by which the reference chamber 14 can be connected to the pipe 10 or isolated from it when the valve 16 is closed; a differential pressure transducer 18, which is connected to measure the differential pressure between pipe 10 and reference chamber 14; an electronic unit 20 (containing a computer); and the platinum resistance devices 22, 24 to continuously monitor the temperature in the pipe 10 and in the reference chamber 14. These devices 22, 24 are connected to the unit 20. Instead of measuring the temperature in the pipe 10, It may be more convenient to measure the temperature in the adjacent terrain, as shown in 26.

Unit 20 controls the opening and closing of valve 16. Unit 20 also contains an absolute pressure transducer connected to pipe 10. Unit 20 also contains electronic circuits which locally condition the temperature signals from the control devices. platinum resistance, executing an analog-digital conversion and storing the data.

The nominal absolute pressure is also measured by means not shown in Figure 1.

The operation of the equipment in the excavation is automated, and when convenient and required, data is transferred through a non-fossil-contact interface to a portable interrogation computer 30 located outside the excavation. The excavation is closed by a layer 32 of provisional insulation.

The equipment measures the pressure drop regimes with great precision, and in a relatively short period of time, at measured pressures of up to 7 bar. Differential pressure transducer 18 is a commercially available instrument that has a full scale deflection of ± 0.1 millibars (± 1 mm H₂OR).

The pipe 10 is pressurized by passing air inside it, using a compressor (not shown) and depending on the capacity of the compressor used, and the volume of the pipe 10, pressurizing it can take several hours. The reference chamber 14 is also pressurized.

The procedure

Prior to the pressurization of the pipe 10, the equipment in the excavation is activated, and during the pressurization the absolute temperature of the pipe 10 and the absolute pressure in the pipe 10 are continuously monitored. Information regarding the length of pipe 10 and its SDR will also have been entered into the computer. The recording of the absolute temperature and the absolute pressure of the pipe is continuous during pressurization, and the data obtained are used by the computer to create a prediction model of progressive deformation. This model corresponds to curve 40 of the pressure drop in the pipeline (drop at the test pressure) with respect to time, due to the allowable leakage of 0.0028 standard cubic meters per

ES 2 173 249 T3 hour at the maximum working pressure of the supply network and the progressive deformation of the pipe, as shown in Figure 2.

During pressurization, the equipment closes and automatically opens valve 16, typically every 20 minutes, so that reference chamber 14 is also pressurized at the same pressure as pipe 10.

The equipment measures the temperature in the reference chamber 14 by means of the device 24 in every 5 seconds. It also measures the temperature in pipe 10 (or in the adjacent ground) by means of device 22 (or device 26) in every 5 seconds. The absolute pressure in the reference chamber 14 is also measured with the same time regimen.

The differential pressure between the pipe 10 and the reference chamber 14 is measured by the transducer 18 in every 5 seconds, although these are useful data only when the valve 16 is closed, being of no value during pressurization.

Upon completion of pressurization, a period of several hours is allowed to elapse to allow the temperature in pipe 10 to become normal.

In what follows, it will be understood that the equipment in the excavation automatically obtains the data in each instant in which the valve 16 is closed.

The interrogating computer 30 requests data from the unit 20 at 4 hours, for example, after the pressurization has finished.

Unit 20 has been closing valve every 20 minutes and opening valve 16 if the transducer generates a positive or negative full scale output. Alternatively, valve 16 opens if it has been closed for more than 5 minutes.

The data sent to the computer 30 are those corresponding to the following operation of the valve 16 under the control of the unit 20:

Every 5 seconds the data is collected in the manner mentioned above. The interrogating computer 30 uses said data to derive the rate of drift from the differential pressure.

Valve 16 opens and pressure in line 10 and reference chamber equalize.

Valve 16 closes and differential pressure is recorded every 4 seconds. Valve 16 is opened and the pressures in line 10 and reference chamber 14 are equalized. Valve 16 is closed again and the differential pressure is recorded again. This process is repeated several times.

All data recorded during the period of the aforementioned various closures of valve 16 is transferred to computer 30.

In each period during which valve 16 closes, computer 30 calculates:

(a) the drift rate of the measured differential pressure;

(b) the rate of change with respect to time (all subsequent references to the rate of change mean "with respect to time") of the temperature of pipe 10 (or of the ground adjacent to pipe 10). This is referred to as the drift rate of the test temperature;

(c) the rate of change of the temperature of the reference chamber 14 (the rate of drift of the reference temperature):

(d) the pressure change rates corresponding to (b) and (c) (the drift regimen of the test pressure and the drift rate of the reference pressure, respectively);

(e) the difference between the test pressure drift rate and the reference pressure drift rate for obtaining the differential pressure drift prediction rate;

(f) the difference between (a) and (e) for obtaining the pressure drift rate due to progressive deformation and leakage only in pipe 10, regardless of the change in temperature in pipe 10;

(g) comparing or qualifying the results obtained in step (f), and determining whether or not the results tend to intersect with curve 40 that represents the progressive deformation prediction model for the aforementioned permissible leakage at beginning of the description of the Procedure.

One way to carry out step (g) is to graphically represent the results with respect to model 40 as shown in Figure 2. The results are indicated by the Xs. Next, for each result, the height at₁ of the result above a nominal horizontal line drawn on the graph comprising model 40, determining the depth b₁ result below curve 40, and deriving the quotient a₁/ b₁. The curve or model 40 may represent a worst-case prediction.

If the values ​​of the quotients a₁/ b₁, to₂/ b₂, to₃/ b₃, ... are decreasing, the test shows that the pipe 10 is leaking less than the allowable leak, and therefore "passes" the test. If the values ​​of the quotients are increased, the test shows that the pipe 10 has a leak greater than the allowable leak, and therefore "fails" the test.

The results represented graphically may be subject to factors that alter them, such as noise, which makes them highly variable. In such cases, graphical procedures will have to be applied in order to deduce the aforementioned quotients.