Pilot exhausts

When the pilot exhausts to the atmosphere, a pilot-operated PR valve is fully balanced. Like the balanced bellows valve, therefore, its opening pressure is unaffected by back pressure, and high built-up back pressure does not result in chattering. 

The pilot exhaust is normally vented to the main valve oudet. Set pressure and operability are unaffected by backpressure up to 70% of set pressure, provided that a backflow preventer is used whenever backpressure is expected to exceed inlet pressure during operation (consult the manufacturer for backpressures greater than 70% of set pressure). The capacity is affected, however, when flow is subcritical (ratio of absolute backpressure to absolute relieving pressure exceeds 55%). In this case, the flow correction factor Kb (see Appendix B) must be applied. If the ratio of absolute backpressure to absolute relieving pressure is less than 55%, no correction factor is required, Kb = 1. 

A final consideration when installing a POSRV on an LNG application is the exhaust of the pilot. On some installations, the exhaust can be done to the atmosphere, either directly or via a mast. But on other installations, even the small volume relieved per opening cycle by the pilot (the volume of the main valve dome) cannot be accepted. Therefore, the pilot exhaust must be piped to a safe place. 

If the pilot is fully balanced against the backpressure and the valve is connected to a dry system, then the pilot exhaust can be safely piped to the main valve outlet so that all emissions go to the exhaust system. As the pilot is kept out of the cold, the problems which occur on balanced bellows spring-loaded SRV in the same situation will not occur. 

Remote Depressuring - A pilot operated valve is sufficiently positive in action to be used as a depressuring device. By using a hand valve, a control valve or a solenoid valve to exhaust the piston chamber, one can open the pilot-operated valve and close it at pressures below its set point from any remote location, without affecting its operation as a pressure relief valve. 

In the case of a pilot-operated valve, provided that the pilot valve exhausts to the atmosphere, the main piston is independent of back pressure and is thus also considered as a balanced valve. Balanced PR valves can be characterized by the following ... 

If the pilot valve exhausts to the atmosphere, a pilot-operated valve may be considered as a balanced valve. 

For a new process plant, calculations can be carried out using the heat release and plume flow rate equations outlined in Table 13.16 from a paper by Bender. For the theory to he valid, the hood must he more than two source diameters (or widths for line sources) above the source, and the temperature difference must be less than 110 °C. Experimental results have also been obtained for the case of hood plume eccentricity. These results account for cross drafts which occur within most industrial buildings. The physical and chemical characteristics of the fume and the fume loadings are obtained from published or available data of similar installations or established through laboratory or pilot-plant scale tests. - If exhaust volume requirements must he established accurately, small scale modeling can he used to augment and calibrate the analytical approach. 

The crystallization step is generally studied quite exhaustively at the laboratory scale and often at the pilot scale. The reaction chemistry should be properly understood to access effects, if any, of the synthesis step on the impurity profile. In batch cooling crystallizers attempts have been made to create optimum conditions by on-line turbidity analysis (Moscosa-Santillan et al., 2000). Physicochemical characterization of the products should be done rigorously (Tanguy and Marchal, 1996). 

It is demonstrated in the pilot tests that TCE can be removed by 99% for the direct contact exposure route within 3 to 5 yr using the vapor extraction system. The potential for fugitive losses of air contaminants would be minimal under good control conditions. A countercurrent packed tower air stripper (13.72 m tall and 1.22 m in diameter) would be used to treat the extracted groundwater to meet the performance goal of 5 pg/L TCE concentration. The exhaust air would be discharged through carbon beds for adsorption. 

MSO is unsuited for treating materials with high inert content, such as asbestos, concrete, soils, and rubble. There is concern over emissions from MSO relating to particulate mercury content and radioactivity. MSO is inappropriate for wastes with high tritium levels. MSO pilot programs have encountered problems with carbon monoxide (CO) emissions. The corrosion of reactor materials by molten salt has remained a concern for the long-term operability of the system. The viscosity and volatility of the melt have to be controlled. There have been problems with material from the melt plugging air exhaust and feeder systems. 

Figure 6. Block drawing of the pilot installation for the production of trichloromethyl chloroformate by exhaustive photochlorination [39] 1 Dryer for gaseous Cl2 (H2S04 cone.). 2 Safety tank. 3 Thermoregulated immersion-type photochemical reactor. 4 Raschig column. 5 Cl2 detection system (1,2,4-trichlorobenzene). 6 Neutralization tank (20% NaOH). 7 Reservoir of 20% NaOH. 8 Buffer to atmospheric pressure (20% NaOH). 9 Active carbon filter. 10 Reservoir of crude trichloromethyl chloroformate. 11 Buffer to normal atmosphere via CaCl2 filter and direct entry for trichloromethyl chloroformate to be distilled. 12 Distillation flask with Vigreux column. 13 Exit to vacuum pump. 14 Solid NaOH filter before pump. 15 Cooling water alarm linked to power supply of the light source. 16 Medium pressure mercury arc. 17 Heater for distillation apparatus. 18 Magnetic stirrers. /T thermometer /P manometer.

In the plant used for academic purposes [4], both the solvent and exhausted CO2 are wasted. In an industrial plant both streams should be recycled after purification, for obvious economic reasons. The precipitator size and plant-flow-rates are obtained by increasing 80-fold the relative quantities used in the pilot plant [4]. This scale factor was suggested by the company that supplied the drug. Two vessels, P, in parallel are needed while the former is running, the latter can be cleaned and the solid product can be recovered. Cleaning and product-recovery expenses are not directly evaluated in this example. In the pilot plant, the flow of THF-polymer-drug solution was 0.072 kg/h, and the CO2 flowed in the quantity of 1.08 kg/h (the ratio CO2 to solution equals 15). The precipitator was a 0.4-liter vessel. The actual precipitator scale-up is not considered here. The main factor to consider in scaling-up the precipitator is the nozzle scale-up. The nozzle-size, nozzle-shape, and number of nozzles per reactor volume, determine the precipitate size in a complex and still incompletely understood way [5-8], It is assumed that issues related to the injectors are already solved. 

The feasibility of hydrotreating whole shale oil is demonstrated by means of several long pilot plant tests using proprietary commercial catalysts developed by Chevron. One such test was on stream for over 3500 hr. The rate of catalyst deactivation was very low at processing conditions of 0.6 LHSV and 2000 psia hydrogen pressure. The run was shut down when the feed supply was exhausted although the catalyst was still active. 

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