LIMITATIONS OF ULTRASOUNDS

ERRORS IN VAPOR LEAK DETECTION IN PURGERS

By Vicente Blázquez (Aeronautical Engineer)

1. ULTRASOUND GAS LEAK DETECTION

Ultrasonic measurement is currently the most widely used method for locating internal gas leaks through valves.

The passage of a gas through an orifice generates an ultrasound whose frequency and intensity are related to several parameters, such as differential pressure on both sides of the orifice, gas temperature, gas density, speed of gas passage through the orifice and hole geometry.

Regardless of the gas flow through the orifice, if the differential pressure is greater than 1 bar, the passage of gas through the orifice generates sound waves in a wide range of frequencies. The wave of greatest intensity occurs around 39 kHz, with multiple harmonics at higher frequencies. Now, experimentally it is verified that the intensity or peak of the frequency wave does not depend on the gas flow through the orifice.

Therefore, the presence of ultrasound in a valve in a closed position clearly means the existence of a crack through which an internal or external gas leak occurs. However, it is practically impossible to determine the magnitude of the leak from the ultrasound measurement since this depends on the geometry of the valve, the geometry of the fissure and the thermodynamic variables of the gas. In any case, the correlation between ultrasound and leakage flow would have to be established for each valve and in each case. There are approximate calculation methods, which are accepted internationally; Among them, one of the most recognized is the Masoneilan formula for offering quite conservative results.

2. DETECTION OF STEAM LEAKS. LIMITATIONS OF ULTRASOUNDS

The application of the ultrasonic measurement method has experienced a great boom in the predictive maintenance of steam traps. However, research carried out by BITHERM during the development of the SmartWatchWeb ™ system for predictive maintenance of steam traps has shown that the traditional method of ultrasound measurement does not offer sufficient reliability when applied in large facilities with thousands of steam traps in service (oil refineries). , large chemical plants, etc.), since the problem becomes more complex when it is not a question of detecting gas leaks but steam, in valves such as steam traps or steam traps, which handle a fluid that can be in the liquid or vapor phase .

Indeed, the trap normally discharges at a temperature close to boiling. During this process, the condensate undergoes an expansion that produces a liquid-vapor phase change or flash steam. The gaseous phase (expansion vapor) flows at high speed generating ultrasound. However, the percentage of flash steam generated in the expansion of the condensate depends on the pressure drop across the trap (differential pressure) and the degree of cooling that the condensate undergoes before being evacuated by the trap. However, the presence of ultrasound in this case does not mean the existence of a live steam leak through the trap. In this case, to make a reliable diagnosis it is necessary to determine the cause of the origin, for which the attenuation of the ultrasound downstream of the trap must be measured, or other variables such as the temperature in front of the trap and the temperature jump across the trap must be analyzed. same.

A serious additional problem occurs in low pressure lines, typical in accompanying steam applications in Refineries and Petrochemicals, with steam at 3.5 bar in facilities with thousands of steam traps discharging to a general condensate return collector. In this case, the buildup of steam build-up locally pressurizes the return line, lowering the differential pressure across the trap. This effect is very intense and typical of installations that use steam traps that do not allow adjusting their discharge temperature (thermodynamics, float, inverted bucket, impulse, calibrated orifice, etc.). The effect is even more intense when there are steam leaks through traps and steam traps, which is generally very frequent since the leak rate in these elements usually ranges between 10% and 15%.

It must be taken into account that although the average pressure in the condensate collector is acceptable, just at the discharge of the trap there is a localized pressure increase in the pipeline. Under conditions of strong local back pressure, the differential pressure can be less than 1 bar in the environment of the trap; This significantly reduces the speed of steam passing through the trap, even in the event of an internal leak, and may not be enough to generate ultrasound to identify the leak.

In summary, the reliability of the ultrasound measurement method in predictive steam trap maintenance decreases as the number of steam traps and their internal steam leak rate increase. The main failures of the ultrasonic leak detection method are:

♦ Apparent live steam leaks, masked by expanding steam.

♦ Real live steam leaks undetectable due to high back pressure.

Research carried out by BiTherm over the years has allowed the method to be refined to a high degree of reliability achieved by the BiTherm SmartWatchWeb ™ intelligent bithermostatic steam trap (international patents).

3. EXPERIMENTAL RESULTS

During the development of the Bitherm SmartWatchWeb ™ intelligent steam trap, multiple tests were carried out in order to find correlations between frequency and intensity of ultrasound with the thermodynamic parameters of the steam and those of the steam traps themselves tested.

When trying to reproduce the results in large petrochemical facilities, it was observed that the conclusions reached in the laboratory did not correspond at all to reality.

Thus, for example, steam installations with apparently correct preventive-corrective maintenance, whose rate of steam leaks in traps and steam traps did not exceed 2%, presented an unacceptable operation with strong thermal water hammer and a large amount of steam (apparently steam). expansion) in the condensate return collectors. Carrying out a thermodynamic study of the facilities, a strong degree of coupling was found between the steam distribution lines and the condensate return lines, a coupling that demonstrates an incorrect operation of traps and / or steam traps.

It was found that the bithermostatic smart trap, unlike the steam trap (which discharges at steam saturation temperature), behaves as an energy control valve, which constantly regulates the level of energy degradation of the fluid, before being evacuated to the return line. In this way, the intelligent drain is able to control the amount of flash steam in the condensate return collector to achieve optimum operation of the installation.

As an example, during the inspection of low pressure traps in a refinery, the ultrasound method detected 3% failures due to steam leakage. Given the doubtful validity of the result, an exhaustive inspection of traps was carried out using several alternative methods, obtaining a real failure rate due to steam leakage of 27%. Finally, it was found that the ultrasound method did not detect steam leaks in those areas where the differential pressure on both sides of the steam trap was less than 1 bar.

4. RELIABILITY OF THE BITherm SmartWatchWeb ™ INTELLIGENT BLEEDER

The BiTherm SmartWatchWeb ™ intelligent steam trap achieves high reliability by analyzing fundamental aspects of its operation, such as:

♦ Continuous monitoring of various parameters: It allows to analyze the evolution of ultrasound, temperature and back pressure, generating a reliable diagnosis

♦ Trend analysis: Premature failure detection

♦ Continuous self-inspection of the monitoring system

♦ Evaluation of the energy efficiency of the steam trap

♦ Differentiated trap failure alarm

♦ Continuous or historical record of the steam trap operation

As a result, the reliability of the BiTherm SmartWatchWeb ™ smart steam trap is extraordinarily high as it allows the detection of small variations in the monitored parameters, even before the failure occurs, making it possible to evaluate the energy performance of the steam trap.

Finally, the Bitherm trap is repairable online without the need for replacement parts. This operation is carried out in a couple of minutes without interrupting the normal operation of the steam trap.