Choking

Cavitation will start as soon as the throat pressure has reached the local vapour pressure. Theoretical considerations suggest that the throttling process as far as the throat will be isentropic, which will lead to a small decrease in liquid temperature. Hence the vapour pressure in the throat will be slightly less than the vapour pressure corresponding to the inlet temperature. This effect is small for a liquid, and the throat vapour pressure is unlikely to be reduced by more than 5% below the vapour pressure at the inlet to the valve at the liquid pressures normally found on process plant. For practical purposes, we may assume that the temperature of the liquid in the throat of the valve is the same as it was at the valve inlet. Thus the condition of cavitation onset may be written  

It has been found in practice that choking does not always occur immediately bubbles form, but that the onset of choked flow may be delayed until the mean throat pressure has fallen to a fraction, r p, of the vapour pressure at the valve inlet  

When choking occurs in the valve, the upstream and downstream pressures are related by a condition very similar to the equation pair (7.11) for cavitation, but now using equation (7.16) for the throat pressure, rather than (7.IS), and replacing the cavitation coefficient, Kc, by a new choking coefficient, K  

K is sometimes referred to as simply the pressure recovery coefficient . The two coefficients are identical for globe valves K = Kc, and often only K Is quoted. The choking coefficient is somewhat larger than the cavitation coefficient for a rotary valve, with the full-open value, Ku, taking a value of typically 

rather than the value 0.25 typical for Kc- The ratio K /Km for a rotary valve follows a similar path to that of Kc/Kc, rising as the valve closes according to the approximate equation  

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If a situation arises whereby formation fluid or gas enters the bore bole the driller will notice an increase in the total volume of mud. Other indications such as a sudden increase in penetration rate and a decrease in pump pressure may also indicate an influx. Much depends on a quick response of the driller to close in the well before substantial volumes of formation fluid have entered the borehole. Onoe the BOP is closed, the new mud gradient required to restore balance to the system can be calculated. The heavier mud is then circulated in through the kill line and the lighter mud and influx is circulated out through the choke line. Once overbalance is restored, the BOP can be opened again and drilling operations continue. 

Having reached the wellbore, the fluid must now flow up the tubing to the wellhead, through the choke, flowline, separator facilities and then to the export or storage point each step involves overcoming some pressure drop. 

The end of field life is often determined by the lowest reservoir pressure which can still overcome all the pressure drops described and provide production to the stock tank. As the reservoir pressure approaches this level, the abandonment conditions may be postponed by reducing some of the pressure drops, either by changing the choke and separator pressure drops as mentioned, or by introducing some form of artificial lift mechanism, as discussed in Section 9.7. 

Gas is sometimes produced at very high pressures which have to be reduced for efficient processing and to reduce the weight and cost of the process facilities. The first pressure reduction is normally made across a choke before the well fluid enters the primary oil / gas separator. 

The most basic subsea satellite is a single Subsea Wellhead with Subsea Tree, connected to a production facility by a series of pipelines and umbilicals. A control module, usually situated on the subsea tree, allows the production platform to remotely operate the subsea facility (i.e. valves, chokes). 

The manifold is typically a tubular steel structure (similar to a template) which is host to a series of remotely operated valves and chokes. It is common for subsea tree control systems to be mounted on the manifold and not on the individual trees. A complex manifold will generally have its own set of dedicated subsea control modules (for controlling manifold valves and monitoring flowline sensors). 

Sensors on the tree allow the control module to transmit data such as tubing head pressure, tubing head temperature, annulus pressure and production choke setting. Data from the downhole gauge is also received by the control module. With current subsea systems more and more data is being recorded and transmitted to the host facility. This allows operations staff to continuously monitor the performance of the subsea system. 

A separator is fed with a condensate/gas mixture. The condensate leaves the bottom of the separator, passes a flowmeter and is followed by a choke valve, after which the condensate is boiling. The flow can not be measured using the transit time method, due to the combination of short piping, the absence of a suitable injection point and the flow properties of the condensate, which is non-newtonian due to a high contents of wax particles The condensate can not be representatively sampled, as it boils upon depressuratioh... 

B) A hot solution has to be filtered to remove traces of insoluble impurities, and kept hot meanwhile to prevent crystallisation of the main solute, which would otherwise choke up the filter. 

If the organic compound which is being steam-distilled is freely soluble in water, an aqueous solution will ultimately collect in the receiver F, and the compound must then be isolated by ether extraction, etc. Alternatively, a water-insoluble compound, if liquid, will form a separate layer in F, or if solid, will probably ciystallise in the aqueous distillate. When steam-distilling a solid product, it is sometimes found that the distilled material crystallises in E, and may tend to choke up the condenser, in such cases, the water should be run out of the condenser for a few minutes until the solid material has melted and been carried by the steam down into the receiver. 

The reaction is carried out in a 2-litre long-necked round-bottomed flask, to which is fitted an efficient reflux water-condenser, capable of condensing a sudden rush of vapour without choking. For this purpose, a long bulb-condenser, similar to that shown in Fig. 3(A) (p. 9) is best, but the inner tube must be of wide bore (at least 12 mm.). Alternatively, an air-condenser of wide bore may be used, an.d a short double-surface water-condenser fitted to its top. A steam-distillation fitting for the flask should also be prepared in advance, so that the crude product can subsequently be steam-distilled directly from the flask. The glj cerol used in the preparation must be anhydrous, and should therefore be dehydrated by the method described on p. 113. 

Wider passages are provided for vapours and the comparatively narrow tubes, which are usually fitted through holes bored in cork or rubber stoppers, are absent this considerably diminishes danger in violent reactions and also tends to give better results in distillation under reduced pressure as well as diminishing the hazard of choking. ... 

Automatic choke heater Early fuel evaporation heater... 

Injection Well Considerations. Eluid injection rate can have a significant effect on oil recovery economics. Elow is radial from the wellbore into the reservoir. Thus the region near the injection wellbore acts as a choke for the entire reservoir. 

Sometimes conveying (qv) velocities, typically 20—25 m/s, exceed the flame-propagation rate as determined by tests. Moreover, velocities vary throughout the system. If the flame-propagation rate exceeds the conveying velocities, consideration must be given to the isolation of parts of the system by choking, ie, the use of rotary values, screen conveyors, bins, etc. 

Transport Properties. Viscosity, themial conductivity, the speed of sound, and various combinations of these with other properties are called steam transport properties, which are important in engineering calculations. The speed of sound (Fig. 6) is important to choking phenomena, where the flow of steam is no longer simply related to the difference in pressure. Thermal conductivity (Fig. 7) is important to the design of heat-transfer apparatus (see HeaT-EXCHANGETECHNOLOGy). The viscosity, ie, the resistance to flow under pressure, is shown in Figure 8. The sharp declines evident in each of these properties occur at the transition from Hquid to gas phase, ie, from water to steam. The surface tension between water and steam is shown in Figure 9. 

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