High liquid level shutdown

The vapor-flow area is calculated as tire area available above the high liquid-level shutdown. Once tire vapor-flow area is known, the vapor residence time can easily be calculated, using the actual vapor velocity. The required length of the KO drum is calculated by multiplying the droplet velocity and vapor residence time. This length is then checked with tire available length. 

Setting of a high liquid level shutdown (HLLSD) depends on the other internals present in the separator. Generally, the highest possible liquid level... 

Allow 3 minute liquid surge time from LLL to low low liquid level (LLLL), and 3 minute su e time from ULL to high high liquid level (HHLL). It is assumed that at HLL or LLL, liquid level alarm will be sounded to alert the operator, and at HHLL or LL1.L, the feed flow or liquid outlet flow will be stopped by an emergency shutdown valve. HHLL and LLLL is user s option to have them or not. 

In this accident, the steam was isolated from the reactor containing the unfinished batch and the agitator was switched ofiF. The steam used to heat the reactor was the exhaust from a steam turbine at 190 C but which rose to about 300°C when the plant was shutdown. The reactor walls below the liquid level fell to the same temperature as the liquid, around 160°C. The reactor walls above the liquid level remained hotter because of the high-temperature steam at shutdown (but now isolated). Heat then passed by conduction and radiation from the walls to the top layer of the stagnant liquid, which became hot enough for a runaway reaction to start (see Fig. 9.3). Once started in the upper layer, the reaction then propagated throughout the reactor. If the steam had been cooler, say, 180 C, the runaway could not have occurred. ... 

Variations In liquid level causing nuisance high and low level alarms and shutdowns. ... 

The effect of this on the reactor operation would have to be considered. This would be brought out in the operability study on the reactor it would be a possible cause of no chlorine flow. Since the flow controller does not know that steam flow has been lost, chlorine will continue to be pumped into the vessel until the high-level alarm sounds and the high-level shutdown closes the control valve. A secondary consequence is that the vessel is now filled with liquid chlorine that must be drained to a safe level before operation can be resumed. The operating procedures must include instructions on how to deal with this scenario. 

The restricting orifice and the liquid knockout pot both guard against entrainment of liquid chlorine, which might create a serious hazard in the process. The orifice, sometimes replaced by a flow control valve, limits the flow to no more than the vaporizing capacity and so helps to avoid entrainment. Other possible measures include a high-level shutdown on the vaporizer and gas temperature control or low-temperature shutdown. The knockout pot is the second line of defense. 

The separator design using API 521 method uses a high-level shutdown (instead of normal liquid level, as in other cases) to calculate the separator length. 

Vaporizer floods, liquid to reactor fit high-level alarm on LICl with automatic pump shutdown. Add independent level transmitter and alarm LT2. 

The maximum level of the activity concentrations in the coolant is probably reached at the moment when the gap of the failed fuel rod is filled with water and when there is no further movement of the liquid front and no convection within the liquid phase. After this point, additional fission products may reach the leak position and escape to the coolant by diffusion in the liquid phase only, which within the gap is probably a comparatively slow process and does not cause a significant further increase of the activity concentrations in the coolant, which are already high at this moment. Therefore, the activity concentrations in the coolant begin to decrease at a rate which corresponds to that effected by the action of the purification system. Following a reduction in the coolant pressure, however, an additional fraction of the liquid phase can be transported from inside the rod to the coolant by the action of temporary pressure differences, leading to the formation of the secondary depressurization spikes as shown in Fig. 4.9. When the reactor is started up again after the shutdown period, water which still remained in the gap of defective fuel rods, containing dissolved fission products, is transported out to the coolant, forced by the increase in temperature of the fuel pellets. 

Shown here are two overrides (pneumatic devices illustrated). One has a latch-up circuit with a gain 25 relay, which provides a high signal the first time the reboiler is filled with liquid the steam valve is held closed until the level covers the tubes. Once latched up this circuit does not function again until it is unlatched by switching to shutdown. ... 

A review of an alarm history has identified that the bad actors include level, pressure, and temperature alarms that are associated with a liquid-phase, continuous, stirred-tank reactor. The chemical reactions are exothermic, and the CSTR is used at different times to make two products, A or B. The low-level and low-pressure alarm violations occur mainly during shutdown operations, whereas high temperature alarms for the jacket cooling water occur primarily when product B is produced. It is desirable to devise a strategy for reducing these bad actor alarms. 

The use of high or low limits for process variables represents another type of selective control called an override, where a second controller can override or take over from the first controller. This is a less extreme action than an interlock, which is used for emergency shutdown of the process (see Chapter 10). The anti-reset windup feature in feedback controllers (cf. Chapter 8) is a type of override. Another example is a distillation column that has lower and upper limits on the heat input to the column reboiler. The minimum level ensures adequate liquid inventory on the trays, while the upper limit exists to prevent the onset of flooding (Buckley et al., 1985 Shinskey, 1996). Overrides are also often used in forced draft combustion control systems to prevent an imbalance between air flow and fuel flow, which could result in unsafe operating conditions (Singer, 1981). 

One of the most serious failure modes for a compressor is for liquid to enter the casing. The result of this failure could be a catastrophic failure of the casing and/ or the compressor internals. Liquid knockout pots and scrubbers are ordinarily provided in suction lines for this purpose. High-level alarms and compressor shutdown devices should be installed on knockout pots and scrubbers. 

A simple instrumented shutdown device would require the features shown in the above diagram i.e. a level switch set to detect extra high level in the tank causes an automatic shutoff valve to close off all liquid feed to the tank. The shutoff valve remains closed until the defect in the process control system has been rectified. 

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