1. GAS LEAKS

The measurement of ultrasound is nowadays the most used method for the location of internal gas leaks through valves.

When a gas goes through an orifice, it generates an ultrasound with a frequency and intensity that are related especially with: the pressure and temperature of the gas, the speed of the gas through the orifice and the geometry of the orifice.

The presence of ultrasound in a valve in closed position clearly means the existence of a fissure through which an internal or external gas leak is produced. Now then, it is practically impossible to determine the magnitude of the leak only with the measurement of the ultrasound, because it also 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 the ultrasound and the size of the leak should be established for each valve in each case.

2. STEAM LEAKS. LIMITATIONS OF THE ULTRASOUND METHOD

The application of the ultrasound measurement method has experimented a big apogee in the predictive steam trap maintenance. However, recent investigations made by BiTherm during the development of the SteamWatch™ y SmartWatch™ systems for the predictive maintenance of steam traps, have demonstrated that the traditional method of ultrasound measurement doesn't offer enough reliability when it is applied in big installations with thousands of steam traps in operation (oil refineries, big chemical plants, etc.), wherefore the problem is more complex when the gas is steam and the valve is a steam trap.

The steam trap discharges normally at a temperature near the boiling point. During this process the condensate suffers expansion, which produces a liquid-vapour phase change. The gas phase (flash steam) flows at a high speed generating a certain level of ultrasound, with a frequency and intensity that depend on the pressure variation in the steam trap (differential pressure) and the cooling the condensate suffers before being evacuated by the steam trap. However, in this case, the ultrasound presence doesn't mean existence of life steam leak through the steam trap.

A serious additional problem appears in lines of low pressure (3.5 bar) in installations with thousands of steam traps discharging to a general condensate return collector. In this case, the forming of flash steam pressurises locally the return line, decreasing the differential pressure in the steam trap. This effect is very strong when different steam leaks of some steam traps are added, which is generally very frequent. In conditions of strong local backpressure the steam flow speed through the steam trap, even in case of internal leak, is not sufficiently high to produce the ultrasound level that allows the identification of such leak.

In summary, the reliability of the ultrasound measurement method in the predictive maintenance of steam traps decreases, the higher the number of steam traps in the installation. The main failures of the method are two:

1. Apparent life steam leaks: inexistent, because it is flash steam

2. Non-detected life steam leaks: disguised due to high backpressure

The investigations made by BiTherm during the last four years has allowed the improvement of the method, reaching a high grade of reliability with the SteamWatchÔ system (international patents pending).

3. EXPERIMENTAL RESULTS

During the first phase of the SteamWatch™ method development, multiple leak measurements were made in steam traps with different types of portable ultrasound devices, in order to find correlations between the frequency and ultrasound intensity and the thermodynamic parameters of steam and of the tested steam traps.

When trying to reproduce the results in big petrochemical installations it was observed that the conclusions obtained in the laboratory didn't correspond in any way to reality.

It was found, for example, that installations with an apparently good predictive maintenance and a percentage of life steam leakage in steam traps under 2%, presented an unacceptable operation with strong thermal waterhammering and great amount of steam (apparently flash steam) in the condensate return collectors. After a thermodynamic study of the installations, a big coupling grade between the steam distribution lines and the condensate return lines was found, which demonstrates incorrect steam trap operation.

It was verified that the steam trap is not only a "trap for steam" that allows condensate to go through and closes to avoid the pass of steam, it has to be considered also as a regulation valve that constantly controls the energetic degradation level of the fluid, before being evacuated to the return line. In this way, the steam trap is able to produce the highest possible pressure loss and achieve the optimal operation of the installation.

All the inspected steam traps were exhaustively verified and it was found that the percentage of damaged ones, with life steam leakage, reached a number between 20% and 27%. This had not been detected by any of the portable ultrasound equipment used, due to the local pressurisation in the discharge area of the steam traps, which reduced significantly the speed of steam through them.

4. SteamWatch™ SYSTEM

The SteamWatch™ system achieves its high reliability due to ten basic concepts:

1. Continuous monitoring

2. Operation without human assistance

3. Simultaneous measurement of various steam trap parameters

4. Wide range of ultrasound sensibility

5. Continuous self-inspection

6. Detection in advance of failures in the steam trap

7. Evaluation of the energetic efficiency of the steam trap

8. Differentiated alarm of the steam trap failure

9. Continuous or historical register of the steam trap operation

10. Local steam trap repair without interrupting its operation

Therefore, the reliability of the SmartWatch™ system is extraordinarily high due to its continuous monitoring of various steam trap parameters, with no human assistance, joined to a wide scale of high sensibility which allows the detection of small variations of the monitoring parameters, even before the fail is produced, making the energetic evaluation of the steam trap possible.

Additionally, the system makes a continuous self-inspection, generating different types of alarms in case of failure or if there is tendency of failure.

Finally, the system also allows to repair the steam trap failure, without substituting it, in 90% of the cases (corrective maintenance). This operation is done in a couple of minutes with no need of interrupting the normal operation of the steam trap.

5. PROFITABILITY OF THE SteamWatch™ SYSTEM

SteamWatch™ must be considered a new concept, a new generation of intelligent steam traps or super steam traps capable of eliminating all types of problems in the installations (thermal waterhammering, backpressure, steam leakage, etc.), optimising the energetic efficiency of the installation.

SteamWatch™ globalises the concept of predictive maintenance considering it an additional characteristic of the steam trap. In this way, the user acquires a steam trap with its own predictive and corrective maintenance system incorporated.

SmartWatch™ brings very high add value to any installation. Its payback period is as short as 6 to 14 months depending on the application.

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