Leaks

Another potential source of trouble is gas leaks in the GC. Diluted detergent solutions may be used to detect them, but tightening the fittings will squeeze the liquid from between the front ferrule and the conus into the gas supply system. This causes sharp peaks as well as baseline drift and variations in detector sensitivity and thus is not encouraged. 

A better method to search for leaks is dripping about 0.5 ml of a volatile chlorinated solvent, e.g., dichloromethane slowly from a syringe onto the suspected leak site, starting from near the detector (ECD). A clear peak signals the presence of a leak. The time lag between solvent application and signal detection increases with increasing distance between the leak and the ECD (e.g., from seconds to minutes). 

The connections of the capillary to the detector, injector and the insert are additional and frequent trouble spots. Injeetion of 2 mL of dichloromethane into the closed oven is a useful overall leak test. A rapid response of the detector (10-20 s) indicates a leak near the detector, a signal delayed by the residence time in the column (eg., 60s for a 30m column length) indicates a leak near the injector. The more specific signal test is then applied as discussed above. Ferrules for the capillary are frequent sources of problems with some equipment. Graphite or vespel ferrules must have a 0.5 mm bore for a 0.32 mm i.d. column. 

A human hair is trapped across an O-ring on a vacuum system where the internal pressure is 10 4mbar and the external pressure is 103mbar. The temperature is 20 °C. Leakage of air into the vacuum system is occurring. 

It is probable that, at the leak exit, because the pressure is so low, choked flow will be established. For air at 20 °C, the critical pressure (p ) at which this is established is given by Equation (2.17)  

The pV throughput under choked flow conditions is given by Equation (2.18)  

This is almost identical to the above value, therefore choked flow can ignored. 

If it is assumed that molecular flow occurs in the leak then, according to Equation (2.37)  

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Vapor cloud explosions. Explosions which occur in the open air are vapor cloud explosions. A vapor cloud explosion is one of the most serious hazards in the process industries. Although a large toxic release may have a greater disaster potential, vapor cloud explosions tend to occur more frequently. Most vapor cloud explosions have been the result of leaks of flashing flammable liquids. 

This eliminates the vapor space but sealing the edge can be a problem. Double seals can help and sometimes a fixed roof is also added above the floating roof to help capture any leaks from the seal. However in this case, the space between the fixed and floating roof now breathes and an inert gas purge of this space would typically be used. The inert gas would be vented to atmosphere after treatment. 

Distillation. There is a large inventory of boiling liquid, sometimes under pressure, in a distillation column, both in the base and held up in the column. If a sequence of columns is involved, then, as discussed in Chap. 5, the sequence can be chosen to minimize the inventory of hazardous material. If all materials are equally hazardous, then choosing the sequence that tends to minimize the flow rate of nonkey components also will tend to minimize the inventory. Use of the dividing-wall column shown in Fig. 5.17c will reduce considerably the inventory relative to two simple columns. Dividing-wall columns are inherently safer than conventional arrangements because they lower not only the inventory but also the number of items of equipment and hence lower the potential for leaks. 

Use the divided wall column shown in Fig. 5.17c to reduce the inventory relative to two simple columns, and reduce the number of items of equipment and hence lower the potential for leaks. 

What you don t have, can t leak. If we could design our plants so that they use safer raw materials and intermediates, or not so much of the hazardous ones, or use the hazardous ones at lower temperatures and pressures or diluted with inert materials, then many problems later in the design could be avoided. 

Even if all of the elements described so far have been present within a sedimentary basin an accumulation will not necessarily be encountered. One of the crucial questions in prospect evaluation is about the timing of events. The deformation of strata into a suitable trap has to precede the maturation and migration of petroleum. The reservoir seal must have been intact throughout geologic time. If a leak occurred sometime in the past, the exploration well will only encounter small amounts of residual hydrocarbons. Conversely, a seal such as a fault may have developed early on in the field s history and prevented the migration of hydrocarbons into the structure. 

To detect surface anomalies caused by hydrocarbon accumulations often very small amounts of petroleum compounds have leaked into the overlying strata and to the surface. On land, these compounds, mostly gases, may be detectable in soil samples. 

In many cases faults will only restrict fluid flow, or they may be open i.e. non-sealing. Despite considerable efforts to predict the probability of fault sealing potential, a reliable method to do so has not yet emerged. Fault seal modelling is further complicated by the fact that some faults may leak fluids or pressures at a very small rate, thus effectively acting as seal on a production time scale of only a couple of years. As a result, the simulation of reservoir behaviour in densely faulted fields is difficult and predictions should be regarded as crude approximations only. 

Tubing corrosion due to FIgS (sour corrosion) or COg (sweet corrosion) may become so severe that the tubing leaks. This would certainly require a workover. Monitoring of the... 

The failure of a concrete structure is of course not confined to catastrophic collapse. A concrete structure has failed or reached the end of its serviceability life when it is no longer capable of fulfilling its design functions, e.g. leak-tightness or as a barrier against deleterious elements which may cause corrosion. 

Leaking fi om process flows may pose operational risks and cause environmental problems as well as economic losses. Two examples of tracer methods for testing, localising and quantifying leaks are given below. 

Leak size determination at a refinery heat exchanger... 

The combined result of two such determinations yielded a leak size figure of 8.8% of the feed flow (with a relative standard deviation of less than 5%). This figure could sufficiently explain the product quality problems experienced, whose alternative explanation in turn was catalyst poisoning. 

Figure 4. Principle for radiotracer leak detection in pipelines.

In the demonstration survey leaks were simulated by placing minor amounts of tracer outside the pipeline at difterent positions. 

The methodology and the pigging tool have proven to be capable of identifying very small leaks. In oil pipelines like the Danish carrying 1500 m /h leakages down to 1 litre per hour can be detected without injection of large quantities of tracer. Leakages can be positioned with an accuracy of less than 1 metre. 

In electron spin echo relaxation studies, the two-pulse echo amplitude, as a fiinction of tire pulse separation time T, gives a measure of the phase memory relaxation time from which can be extracted if Jj-effects are taken into consideration. Problems may arise from spectral diflfrision due to incomplete excitation of the EPR spectrum. In this case some of the transverse magnetization may leak into adjacent parts of the spectrum that have not been excited by the MW pulses. Spectral diflfrision effects can be suppressed by using the Carr-Purcell-Meiboom-Gill pulse sequence, which is also well known in NMR. The experiment involves using a sequence of n-pulses separated by 2r and can be denoted as [7i/2-(x-7i-T-echo) J. A series of echoes separated by lx is generated and the decay in their amplitudes is characterized by Ty. ... 

In general it is difficult to construct a calorimeter that is truly adiabatic so there will be unavoidable heat leaks q. It is also possible that non-deliberate work is done on the calorimeter such as that resulting from a change in volume against a non-zero external pressure / Pk i dk>, often called /iFwork. Additional work w ... 

All calorimeters consist of the calorimeter proper and its surround. This surround, which may be a jacket or a batii, is used to control tlie temperature of the calorimeter and the rate of heat leak to the environment. For temperatures not too far removed from room temperature, the jacket or bath usually contains a stirred liquid at a controlled temperature. For measurements at extreme temperatures, the jacket usually consists of a metal block containing a heater to control the temperature. With non-isothemial calorimeters (calorimeters where the temperature either increases or decreases as the reaction proceeds), if the jacket is kept at a constant temperature there will be some heat leak to the jacket when the temperature of the calorimeter changes. 

Hence, it is necessary to correct the temperature change observed to the value it would have been if there was no leak. This is achieved by measuring the temperature of the calorimeter for a time period both before and after the process and applying Newton s law of cooling. This correction can be reduced by using the teclmique of adiabatic calorimetry, where the temperature of the jacket is kept at the same temperature as the calorimeter as a temperature change occurs. This teclmique requires more elaborate temperature control and it is prunarily used in accurate heat capacity measurements at low temperatures. 

Figure Bl.27.9. High-temperature heat-leak calorimeter. (Reproduced by pemiission from Cliristensen J J and Izatt R M 1984 An isothemial flow calorimeter designed for high-temperature, high-pressure operation...

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