Bypass valves

Hand-crimp the hose in back of the test leaf, and then turn on the vacuum pump and regulate the bypass valve on the pump to give the desired vacuum level in the receiver. 

Any set of guidelines must be tempered by the analysts experience. This is an investigative process. The explanations are rarely simple. However, many exhaustive tests have been run to identify that a bypass valve or alternative feed valve had been mistakenly left open. Plant resources were misused because the simple was overlooked ... 

Operations The QRA team will need specific data on how the system is actually operated. For example, are the bypass valves normally left open to increase throughput, what happens when the high level alarm sounds, or do operators bypass interlocks to continue production Human actions/errors are usually dominant contributors to the real-world risks, and truthful data on actual process operations are vital to credible QRA results. Expect to commit one full-time equivalent for the life of the project. 

At high pressure experiments the reactor should be installed in a pressure cell. All check valves before it, and the filter with the flow controller after it, can be kept in the vented operating room. As a minimum, the bypass valve and the flow controller must be accessible to the operator. This can be done by extended valve stems that reach through the protecting wall. Both the operating room and the pressure cell should be well ventilated and equipped by CO alarm instruments. 

The unit was built in a loop because the needed 85 standard m /hour gas exceeded the laboratory capabilities. In addition, by controlling the recycle loop-to-makeup ratio, various quantities of product could be fed for the experiments. The adiabatic reactor was a 1.8 m long, 7.5 cm diameter stainless steel pipe (3 sch. 40 pipe) with thermocouples at every 5 centimeter distance. After a SS was reached at the desired condition, the bypass valve around the preheater was suddenly closed, forcing all the gas through the preheater. This generated a step change increase in the feed temperature that started the runaway. The 20 thermocouples were displayed on an oscilloscope to see the transient changes. This was also recorded on a videotape to play back later for detailed observation. 

Temperature ehanges for the flue gas to the expander produee the effeets shown in Figure 4-63. The expander inlet temperature at design is 1,200°F. As the expander inlet temperature rises, the expander horsepower eurve moves to the left and upward while the ehange in the blower eurve is insignifieant. The results are that the lower horsepower balanee point moves to the left and down, the peak of the expander eurve moves to the left and up, the peak generator load inereases to G, and the expander bypass valve opens at a lower feed rate. 

Instead of using the expander main bypass valve for regenerator pressure eontrol, it is also possible to install a small bypass valve, say 30%, downstream of the expander inlet butterfly valve, bypassing to tlie expander exhaust line as shown in Figure 4-73. Although tills valve may not appear neeessary, it ean provide additional flexibility and proteetion. 

An example is shown in Figure 4-84, whieh represents the regenerator pressure following a malfunetion of the expander bypass valve. The bypass valve trips open in 1 see while the expander inlet valve maintains eontrol. Tlie regenerator pressure drops approximately 2.0 psi before starting to reeover. The inlet valve stroking time was 10 see and the eontroller settings were 5% proportional band and 30 see reset. 

Figure 4-84. Regenerator pressure deviation during expander bypass valve malfunction.

Figure 4-85. Power balance between string components during malfunction of bypass valve.

A TPG block diagram is shown in Figure 4-86. It is similar to the FCC diagram except a second inlet valve is added to assure trip action and a bypass valve is added to reduce overspeed and aid in startup. The only rotating elements are the expander and generator and, possibly, gear (Figure 4-87). 

The lube oil pumps are now ready to be turned on. The pump and filter bypass valves should be open to avoid pressure pulses in the filter eartridges. Strong pressure pulses may eause filter eartridges to eollapse. Typieal filter elements will withstand 35-100 psi differential. If the pumps are turned on with eaeh bypass elosed, an instantaneous pressure of approximately 150 psi will hit the filters. This is due to the setting of the relief valves. For this reason, it is important to have on hand several extra seal gas and lube oil eartridges. 

The expander train supplier shall assist with the selection of the expander inlet and bypass valve configurations and closure rates by performing an open loop dynamic simulation study. For a main air blower train dais could be closed loop. 

The gas quality feeding the dry faee seal should be elean and dry. Due to the possibility of eondensation of the proeess gas in the seal eavity, it was deeided to use a seal gas heater. The heater eontrol was set to provide warm gas at 15°C above the dew point to ensure no eondensate entered the seal eavity. Also, a dual filter in series with 5 and 2 p filtration elements was ehosen to provide an ideal sealing environment and maintain the optimum performanee of the seal. To reduee the risk of seal damage during reverse rotation of the turboexpander, programming logie was set to open the eompressor bypass valve whenever a shutdown impulse was initiated. 

The addition of a seeond expander main bypass valve (Figure 6-38) in parallel with the initial valve ean provide eloser proeess eontrol and flexibility. Both valves may be identieal 50% eapaeity valves or, one valve may be a 100% eapaeity main bypass valve and the other a smaller 30% eapaeity valve. Either approaeh inereases flexibility and provides more preeise regenerator pressure eontrol. 

A typieal TPG valve arrangement is the five valve system (Figure 6-41). The two expander inlet valves provide regenerator pressure eontrol, overspeed proteetion, and flue gas shut-off to the expander. The expander exhaust valve enables expander isolation. The full and partial expander bypass valves permit aeeurate eontrol of the FCC proeess when the expander is not in operation. 

The three valve aiTangement (Figure 6-42), using one expander inlet valve, one expander exhaust valve, and one expander bypass valve, has been tried with disappointing results. Some users have had to add... 

The bypass valves control the differential pressure between the reactor and regenerator by varying the flowrate in the expander bypass. 

A differential pressure controller acts in split range on the inlet control valve and the bypass valves. The differential pressure governor is retained as the standby and backup system. 

All valves are equipped with hydraulic actuators, electronic positioning controllers for precise positioning, and solenoid valves for rapid (trip) opening of bypass valves and emergency closure tripping of trip and inlet control valves. 

The actuating time for a quick-closing/quick-opening operation effected by the solenoid valves is 0.6 sec, and the actuating time under normal control is 5 sec for the inlet valves and 0.6 sec for the bypass valves. These actuating times apply to the full valve stroke outside the end position damping range. 

At the rated duty point, the differential pressure eontroller is aetive. The inlet eontrol valve and trip valve are eompletely open. The main bypass valve is eompletely elosed and the small bypass valve eontrols the differential pressure. Approximately 96%-98% of the flue gas flows through the expander, with the rest passing through the small bypass valve, orifiee ehamber, and double slide valve to the expander outlet to rejoin the main flue gas flow. 

Fluetuations in the flue gas flowrate (typieally less than 3% of nominal) are deteeted by the differential pressure eontroller and eompensated for by adjusting the small bypass valve. The main bypass valve is eompletely elosed during normal operations, but ean open in the event of sharp inereases in the flue gas flowrate. Similarly, in the event the flue gas flowrate deereases, the small bypass valve eloses to a meehanieally preset minimal opening, and the inlet eontrol valve also partially eloses. The minimum opening is neeessary to keep the bypass lines to the minimum requisite operating temperature. 

The ECC unit operating requirements make it neeessary for the bypass eontrol valves to open as soon as the expander inlet valves start to elose. Optimum eontrol response is aehieved if the bypass valves open so that the pressure at the outlet of the regenerator remains eompletely unaffeeted by the expander shutdown. 

The manufaeturer of this system has developed a eontrol strategy by whieh the bypass valves are eontrolled in the event of an expander trip. This enables the valves to preeisely assume the position in whieh, in eonjunetion with the downstream orifiee ehambers, they preeisely emulate the flow loss eaused by the expander prior to the trip event. A properly adjusted and ealibrated eontrol output jump funetion is applied so that the pressure fluetuations that oeeur at a given duty point are kept to a minimum. 

If, however, the FCC unit should be operated at a different duty point, whether this is due to a different flue gas flowrate or a different regenerator pressure, the bypass valves would either open too wide or not wide enough. The result is fluetuation in the regenerator outlet pressure. 

Generally, butterfly valves are used for the inlet eontrol and bypass valves. Tliey are inexpensive to manufaemre, and their aemators are able to operate in aeeordanee witli tlie requisite response times. Butterfly valves do, however, have the disadvantage of a tightly eurved eharaeteristie. 

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