Vapor valves

Due to the extremely small pilot flow, the pilot on gas/vapor valves normally discharge to atmosphere through a weather and bug-proof fitting. Pilots for liquid service valves have their discharge piped to the main valve outlet. 

Figure 3.5. Vacuum control with steam jet ejectors and with mechanical vacuum pumps, (a) Air bleed on PC. The steam and water rates are hand set. The air bleed can be made as small as desired. This can be used only if air is not harmful to the process. Air bleed also can be used with mechanical vacuum pumps, (b) Both the steam and water supplies are on automatic control. This achieves the minimum cost of utilities, but the valves and controls are relatively expensive, (c) Throttling of process gas flow. The valve is larger and more expensive even than the vapor valve of case (a). Butterfly valves are suitable. This method also is suitable with mechanical vacuum pumps, (d) No direct pressure control. Settings of manual control valves for the utilities with guidance from pressure indicator PI. Commonly used where the greatest vacuum attainable with the existing equipment is desired.

Figure 3.10. Condensers, (a) Condenser on temperature control of the PF condensate. Throttling of the flow of the HTM may make it too hot. (b) Condenser on pressure control of the HTM flow. Throttling of the flow of the HTM may make it too hot. (c) Flow rate of condensate controlled by pressure of PF vapor. If the pressure rises, the condensate flow rate increases and the amount of unflooded surface increases, thereby increasing the rate of condensation and lowering the pressure to the correct value, (d) Condenser with vapor bypass to the accumulator drum. The condenser and drum become partially flooded with subcooled condensate. When the pressure falls, the vapor valve opens, and the vapor flows directly to the drum and heats up the liquid there. The resulting increase in vapor pressure forces some of the liquid back into the condenser so that the rate of condensation is decreased and the pressure consequently is restored to the preset value. With sufficient subcooling, a difference of 10-15 ft in levels of drum and condenser is sufficient for good control by this method.

Open vapor valve from pressure building coils. 

Step 2. There are 26 control degrees of freedom in this process. They include three feed valves for oxygen, ethylene, and acetic acid vaporizer and heater steam valves reactor steam drum liquid makeup and exit vapor valves vaporizer overhead valve two coolers and absorber cooling water valves separator base and overhead valves absorber overhead, base, wash acid, and liquid recirculation valves gas valve to CO removal system gas purge valve distillation column steam and cooling water valves column base, reflux, and vent valves and decanter organic and aqueous product valves. 

Figure 3.15. Control modes for the upper sections of fractionators, (a) Pressure control by throttling of the overhead vapor flow. The drawbacks of this method are the cost of the large control valve and the fact that the reflux pump operates with a variable suction head. The flow of HTM is hand set. (b) Applicable when the overhead product is taken off as vapor and only the reflux portion need be condensed. Two two-way valves can replace the single three-way valve. The flow of HTM is hand set. (c) How rate of the HTM is regulated to keep the pressure constant. One precaution is to make sure that the HTM, for example water, does not overheat and cause scaling. The HTM flow control valve is small compared with the vapor valve of case (a), (d) Pressure control is maintained by throttling uncondensed vapois. Clearly only systems with uncondensables can be handled this way. The flow of the HTM is manually set. (e) Bypass of vapor to the drum on PC ...

A vaporizer is typically used in a process to provide a vapor feed to downstream equipment. In that case, it may be desirable to set the flow of vapor directly with the setpoint of a flow control loop. Then the heat input is adjusted for pressure control and the liquid level is maintained by adjusting the inlet flow, in a reverse material balance manner, as shown in Figure 3.13(A). If heat transfer limits throughput, then both the vapor valve and the steam valve will operate fully open and the pressure will droop to an equilibrium point, where heat transfer equals the flow through the downstream equipment. 

The pressure in the first column is controlled by vapor flow rate to the auxiliary condenser. A low-pressure override pinches the vapor valve to the second column leboi ter. 

LIQUID-VAPOR VALVE REFRIGERANT CYLINDER - Dual hand valve on refrigerant cylinders, which is used to release either gas or liquid refrigerant from the cylinder. 

An alternative simulation was developed using the Flash3 model, as shown in Figure 8.40b after exporting and installing a control structure. In Aspen Plus, a vapor line with a valve is added. The decanter is specified to be adiabatic and at a fixed pressure (0.6 atm). The temperature specified in the upstream condenser HX2 is adjusted to 320 K to give a very small vapor flow rate (3% of the feed). After the file is exported, a pressure controller is inserted on the decanter, but it is put on manual and the vapor valve is closed. Now decanter pressure varies with temperature and composition. Its steady-state value is 0.366 atm with the decanter temperature controller set point set at 313 K so that a direct comparison with the previous case can be studied. 

When switching feed out of a full coke drum into an empty drum,. the pressure on the full drum will drop. Experienced operators carefully observe the pressure on the full drum during a switch. Allowing the pressure to drop too rapidly will start a foamover. A drop of 1 or 2 psi every five minutes is about right. The top gamma-ray level indicator will show when a foamover starts during the feed switchover. To stop this foamover, the vapor valve on the full drum should be pinched closed until the drum pressure rises a few psi. 

Once the switch has been completed and a small flow of steam (4,000-5,000 Ib/hr for a 20-ft diameter drum) has been estabished, the vapor valve on the full drum will be slowly reopened. Meanwhile, the operators should be lining up the cooling water to the coke drum. For the short coke drum cycles to be maintained, it is necessary to skip the standard "big steam" portion of the cycle. Within 30 minutes of the switch, the flow of water to the coke drum should be established. The flow of water must be controlled automatically as follows ... 

The quench rate is about 3% on resid feed, or enough to lower the drum vapor temperature by around 15°F. In addition to reducing coke buildup, this is also an energy-free method to dispose of refinery slop oil. An added bonus is that shift operators report that slop injection makes the overhead vapor valves easier to turn. 

Coking cycle time, 49-70 human factors, 50 time-saving techniques, 50-51 switching feed between drums, 51-53 vapor valve operation, 53 water feed drain, 53 water filling, 54-56 decoking crew, 56-57 unheading, 57-59 resid line to bottom head, 59 bot-... 

The proper method to control drum foamovers during a rapid switchover is to control coke drum pressure by pinching back on the outlet vapor valve to maintain the pressure in the full coke drum. While the operator at the switch valve is diverting the feed to the empty drum, he observes the full coke drum pressure. As this pressure falls during the switch, he closes off the vapor outlet valve from the full drum to hold the pressure within 5 psig of normal operating pressure. Using this technique, the switchover can be accomplished in about 20 minutes. 

Coke drum overhead vapor valves are massive affairs. Switching a pair of 20-ft diameter drums may require the manipulation of four 18-in. and four 12-in. valves. Size aside, the vapor valves are difficult to open and close because of coke deposits on valve internal parts. Removing the insulation on piping upstream of the vapor valves or injecting a small liquid quench (typically 3 vol%) into the vapor line at the first 90° turn will help. 

To minimize the amount of hard physical labor required to switch drums every 12 hours, the vapor valves should be fitted with locally controlled air operators. Such a device is little more than an air gun permanently mounted to the stem of each valve. The speed of the valve movement is controlled with a y4-in. air valve. 

The pressure controller on the decanter is on manual with the vapor valve shut. 

Usually, there are two valves in line with the railroad track. These are connected to liquid eduction pipes that extend to the bottom of the tank cars. They are referred to as liquid valves or eduction valves. Another valve is mounted toward the side of the car, terminating in the tank s vapor space. Sometimes an additional valve is located on the opposite side. Such valves are referred to as vapor valves. A pressure relief valve is always present, usually in the center of the cover plate. 

A Liquid eduction valve B Vapor valve C Pressure relief valve... 

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