Corrosion process fluid

Maintenance costs depend on whether processing fluids are solids on the one hand or gas and liquid on the other. Solids handling tends to increase maintenance costs. Highly corrosive process fluids increase maintenance costs. Average maintenance costs tend to be around 6 percent of the flxed capital investment. ... 

During the materials selection procedure isothermal corrosion testing may indicate the suitability of a material for handling a corrosive process fluid. In many cases where heat transfer is involved the metal wall temperature experienced in service is higher than the bulk process fluid temperature. This, and the actual heat transfer through the material, must be taken into account since both factors can increase corrosion rates significantly. 

Cascade coolers Cooling water flows over series of tubes. Special cooling applications for very corrosive process fluids. 

Le Carbone Lorraine, Brochures about ARMYLOR G (PTFE). ARMYLOR G is used to protect the internal surfaces of metal piping (carbon steel) against attack by chemically corrosive process fluids. 

In corrosive process fluids one has to use a corrosion resistive measurement cell. Such systems use nickel or platinum instead of tungsten as filament material. 

Factors affecting the choice of level measurement include corrosive process fluids (requiring exotic materials), viscous process fluids which may cause blockages, hazardous atmospheres, sanitary requirements, density changes, dielectric and moisture changes, and the required degree of accuracy and durability. 

If it is necessary to use a thick metal or ceramic well because of corrosive process fluid, then the response time can increase to 10 times that of the bulb alone. In addition, a large well creates a static error as a result of conduction along the waU of the thermal well. The addition of a large well can increase the measuring device plus well time constant by approximately 1.5 min. This increase can be detrimental in certain processes, i.e. in an exothermic reactor, but is not significant in others. 

In other designs, a diffused siUcon sensor is mounted in a meter body that is designed to permit caUbration, convenient installation in pressure systems and electrical circuits, protection against overload, protection from weather, isolation from corrosive or conductive process fluids, and in some cases to meet standards requirements, eg, of Factory Mutual. A typical process pressure meter body is shown in Figure 10. Pressure measurement from 0—746 Pa (0—3 in. H2O) to 0—69 MPa (0—10,000 psi) is available for process temperatures in the range —40 to 125°C. Differential pressure- and absolute pressure-measuring meter bodies are also available. As transmitters, the output of these devices is typically 4—20 m A dc with 25-V-dc supply voltage. 

Potential for releasing process mateiials to the environment. Fugitive emissions are receiving ever increasing attention. Exposure considerations, both immediate and long term, for maintenance personnel are especially important when the process fluid is either corrosive or toxic. 

Pinch The industrial equivalent of controlling flow bv pinching a soda straw is the pinch valve, Mves of this tvpe use fabric-reinforced elastomer sleeves that completely isolate the process fluid from the metal parts in the valve. The valve is actuated bv applying air pressure directly to the outside of the sleeve, causing it to contract or pinch. Another method is to pinch the sleeve with a linear actuator with a specially attached foot. Pinch valves are used extensively for corrosive material service and erosive sliirrv senice. This type of valve is used in applications with pressure drops up to 10 bar (145 psi),... 

For measuring pressures of corrosive fluids, slurries, and similar process fluids which may foul Bourdon tubes, a cfiemical gauge, consisting of a Bourdon gauge equipped with an appropriate flexible diaphragm to seal off the process fluid, may be used. The combined volume of the tube and the connection between the diaphragm and the tube is filled with an inert liquid. These gauges are availabTe commercially. 

In addition to fluid velocity, other characteristics of the eroding fluid can exert a marked influence on the erosion-corrosion process. Among the important factors are the following ... 

Impediments to water flow resulting from inadequate equipment design or lodgement of foreign objects in the tubes can exercise a dramatic effect on the erosion-corrosion process. Much of this influence is linked to the creation of turbulence and the simple increase in fluid velocity past obstructions. The importance of these factors is quickly recognized when the phenomenon of threshold velocity is considered. 

Use of inhibitors. Because corrosion is such a vital aspect of the erosion-corrosion process, inhibitors that will reduce corrosion under conditions of high fluid velocity have been a cost-effective method of dealing with erosion-corrosion. For example, injection of ferrous sulfate either intermittently or continuously has been successful in inhibiting erosion-corrosion, especially with copper-base alloys. 

We can aggravate the corrosion effect if misiiligned parts have relative movement, sueh as loose fit bearings or rapid changes in the system. Cavitation, erosion and high fluid velocity advance the corrosion process. 

The service is fouling or corrosive, since the bellows shields the spring from process fluid. Note, however, that the bellows convolutions could also foul in extremely viscous service, such as asphalt, limiting the lift of the valve unless the valve is heated and insulated. 

Balanced bellows type valves are normally used where the relief valves are piped to a closed flare system and the back-pressure exceeds 10% of the set pressure, where conventional valves can t be used because back-pressure is too high. They are also used in flow lines, multiphase lines, or for ptu affinic or asphaltic crude, where pilot-operated valves can t be used due to possible plugging of the pilot line. An advantage of this type of relief valve is, for corrosive or dirty service, the bellows protects the spring from process fluid. A disadvantage is that the bellows can fatigue, which will allow process fluid to escape through the bonnet. For HjS service, the bonnet vent must be piped to a safe area. 

Care should be given in selecting the shaft material. It must be resistant to the corrosive action of the process fluids, yet possess good strength characteristics for design. For some designs it is preferable to use a shaft sleeve of... 

Quenching Outside Seals (only) For oxidizing and corrosive liquids, seal liquid washes process fluid, for high temp. 3-29... 

For the corrosion process to proceed, the corrosion cell must contain an anode, a cathode, an electrolyte and an electronic conductor. When a properly prepared and conditioned mud is used, it causes preferential oil wetting on the metal. As the metal is completely enveloped and wet by an oil environment that is electrically nonconductive, corrosion does not occur. This is because the electric circuit of the corrosion cell is interrupted by the absence of an electrolyte. Excess calcium hydroxide [Ca(OH)j] is added as it reacts with hydrogen sulfide and carbon dioxide if they are present. The protective layer of oil film on the metal is not readily removed by the oil-wet solids as the fluid circulates through the hole. 

The corrosion conditions can be different at the fluid line from the bulk condition. Aqueous liquids have a concave meniscus, which creates a thin film of liquid on the vessel wall immediately above the liquid line. Some corrosion processes, particularly the diffusion of dissolved gases, are more rapid in these conditions. Additionally, the concentration of dissolved gases is highest near the liquid surface, especially when agitation is poor. Locally high corrosion rates can therefore occur at the liquid line, leading to thinning in a line around the vessel. This effect is reduced if the liquid level in the vessel varies with time. Any corrosion tests undertaken as part of the materials selection procedure should take this effect into account. 

However, in the case of stress-corrosion cracking of mild steel in some solutions, the potential band within which cracking occurs can be very narrow and an accurately known reference potential is required. A reference half cell of the calomel or mercury/mercurous sulphate type is therefore used with a liquid/liquid junction to separate the half-cell support electrolyte from the process fluid. The connections from the plant equipment and reference electrode are made to an impedance converter which ensures that only tiny currents flow in the circuit, thus causing the minimum polarisation of the reference electrode. The signal is then amplifled and displayed on a digital voltmeter or recorder. 

The process conditions pressure, temperature, corrosive nature of the process fluid. 

Because of plugging or corrosion problems, it is sometimes necessary to keep the process fluid out of the AP transmitter. This is accomplished by mechanical diaphragm seals or by purges (introducing a small amount of liquid or gas into the connecting lines which flows back into the process). 

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