Operational cracks

Gas oil, which is heavier than kerosene, is the raw material of choice in cracking and other refinery operations. Cracking of gas oil produces a variety of fuels for automotive, industrial, and domestic (furnace) use. 

Next came the use of pipestills and bubble towers where the vapor in the tower bubbles through the liquid on the plates in the tower. Bubble towers allow fractionation to take place while a mixture of rising vapor is scrubbed by a stream of falling oil. Bubble towers are used in topping plants, rerun operations, cracking plants and natural gas stabilization. 

A distinguishing characteristic of concrete, and in particular SPC, is a lack of soundness and equally strong bond between its structural elements. In addition, hardened concrete contains numerous micro and macro defects arising both during manufacture of the product (technological cracks) and in service (operational cracks). 

Stress corrosion cracking of various steels and stainless steels by caustic (sodium hydroxide) is also fairly common in refinery and petrochemical plant operations. Cracking is promoted by small amounts of dissolved oxygen. 

All modern refineries have conversion units, designed to transform black effluent streams into lighter products gas, gasoline, diesel fuel. Among these conversion units, coking processes take place by pyrolysis and push the cracking reaction so far that the residue from the operation is very heavy it is called coke . 

H2S is found with the reservoir gas and dissolved in the crude (< 50 ppm by weight), but it is formed during refining operations such as catalytic cracking, hydrodesulfurization, and thermal cracking or by thermal decomposition of sulfur[Pg.322]

Catalytic cracking is a key refining process along with catalytic reforming and alkylation for the production of gasoline. Operating at low pressure and in the gas phase, it uses the catalyst as a solid heat transfer medium. The reaction temperature is 500-540°C and residence time is on the order of one second. 

Mercaptans are naturally present in crude oil (Chapters 1 and 8), or they result from the decomposition of other sulfur compounds during thermai or catalytic cracking operations. 

These water streams contain mainly dissolved salts ammonium chloride and sulfide, sodium chloride, traces of cyanide, phenols for water coming from catalytic and thermal cracking operations. 

An additional benefit was the detection of big cracks in the balance weights. These weights were hold together only with the fixation screws, (picture 2) This defect can result in big damages of the drums in the following operational period. 

Thermal power plant components operated at high temperatures (>500°C) and pressures, such as superheater headers, steamline sections and Y-junctions, deserve great attention for both operation safety and plant availability concerns. In particular, during plant operation transients -startups, shutdowns or load transients - the above components may undergo high rates of temperature / pressure variations and, consequently, non-negligible time-dependent stresses which, in turn, may locally destabilize existing cracks and cause the release of acoustic emission. 

This header has undergone repair after through-thickness cracking of an assembly weld after 108.000 hours of service, it is presently in operation and periodically submitted to ultrasonic inspection during planned maintenance shutdown. Continuous on-line AE monitoring of the integrity of critical welds is expected to contribute supporting continued and safe operation of the header until its replacement, platmed to take place in 1999. 

In the near future the technique will be further evaluated using ultrasonic signals from natural defects, e.g., fatigue cracks. The performance measure and the parameter optimization procedure wilt also be refined in order to obtain a computationally efficient implementation, easy to use for a trained operator. 

A SQUID [2] provides two basic advantages for measuring small variations in the magnetic field caused by cracks [3-7]. First, its unsurpassed field sensitivity is independent of frequency and thus dc and ac fields can be measured with an resolution of better than IpT/VHz. Secondly, the operation of the SQUID in a flux locked loop can provide a more than sufficient dynamic range of up to 160 dB/VHz in a shielded environment, and about 140 dB/>/Hz in unshielded environment [8]. 

When heat-electric generating station turbine rotors are under operation there is a probability of forming surface cracks in axial canal and heat grooves. Evolution of the above defects can cause a serious crash. Therefore, in accordance with safety standards periodic inspection of the rotor component parts is required. 

The equation system of eq.(6) can be used to find the input signal (for example a crack) corresponding to a measured output and a known impulse response of a system as well. This way gives a possibility to solve different inverse problems of the non-destructive eddy-current testing. Further developments will be shown the solving of eq.(6) by special numerical operations, like Gauss-Seidel-Method [4]. 

Undercuts and cracks are represented in the digitised radiograph as local greyvalue minima (see Fig. 2). This motivates the application of edge-detecting operators. 

Detection of this particle accumulation has so far been done visually. To make the particles more easily visible, they have been chemically treated in order to make them light up or flouresce when struck by an ultraviolet light. The operator sits in a darkened room in which the test pieces are illuminated by ultraviolet light. Cracks show up very clearly and in principle this method of inspection is acceptable. Despite the effiency of this method it is well known that a large number of defective pieces pass this test. Why is it so ... 

The Scania plant at Falun in Sweden manufaetures bus and truck steering knuckles. These are safety-critical components that are specified as having to be crack-free. For this reason, every component is now inspected using an automatic process that ensures the appropiate inspection is consistently canied out to the required standards of quality. Photos of the system are shown in Fig 1. The principle of operation is as follows. 

An operator puts initial data through K into, in particular, information of crack length, object thickness, transformer parameters. After that the device are prepared to work. 

Shear Horizontal (SH) waves generated by Electromagnetic Acoustic Transducer (EMAT) have been used for sizing fatigue cracks and machined notches in steels by Time-of-Flight Diffraction (TOED) method. The used EMATs have been Phased Array-Probes and have been operated by State-of-the-art PC based phased array systems. Test and system parameters have been optimised to maximise defect detection and signal processing methods have been applied to improve accuracy in the transit time measurements. 

The intelligent magnetic pig KOD-4M-1420 was developed and passed trials. This system is designed to provide corrosion and cracks detection in the operating underground gas pipelines at the distances up to 150 km. 

The onus for the organisation of any inspection programme beyond that required by Class rests with the owner or operator of the ship. Under certain circumstances, for example, when the propagation of cracks could lead to pollution through the loss of cargo, an owner of an oil tanker operating in US coasted waters may spend a considerable amount of money on preventative inspections. 

Duduchava R., Wendland W. (1995) The Wiener-Hopf method for system of pseudodifferential equations with applications to crack problems. Integr. Eqs. and Oper. Theory 23, 294-335. 

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