7. FIRED HEATERS AUTOMATION
7.4. LOGIC SEQUENCE DESCRIPTION
7.4.1.1 The following conditions shall be fulfilled for the initial purge permission, indicated at the panel by the blinking of the “Enable to Purging” indication:
a) Pilot gas individual supply pipelines shall be blocked;
b) All Fuel gas/oil individual supply pipelines shall be blocked;
c) Pilot gas double block valve(s) shall be closed and vent valve shall be opened;
d) Fuel gas/oil double block valve(s) shall be closed and vent valve shall be opened;
e) Control damper shall be opened;
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f) No flame indication.
g) Not high level at fuel gas drum.
I) Atomizing steam valves shall be fail closed.
h) Forced Draft Fan in purge operation condition (flow or pressure indicator), if applicable.
7.4.1.2 Once all these conditions are satisfied, “Enable to Purge” indication shall blink.
7.4.2 Leakage Test sequence
7.4.2.1 Operator shall command to start purging from DCS.
7.4.2.2 The leakage test will be done during purge period for gas pilot header and main gas header, but the purge timer shall be started only at the end of pressurization system test.
7.4.2.3 For a best comprehension, a figure above will be helpful.
7.4.2.4 The test checks the individual valves leakage (valves 4) and the second double block valve leakage (valve 2).
7.4.2.5 Pressure indicator (PI) between double block valves can be used for a double block leakage test during maintenance period.
7.4.2.6 Leakage test is a condition for continuing purge step.
7.4.2.7 The leakage test should be performed according flowchart below. Except when explicitly written, all actions should be automatic.
7.4.2.8 The maximum allowed pressure loss during pressurizing test can be 3 % from operational pressure.
7.4.2.9 All manual valves (5) should be opened for leakage test. If some manual valve (5) is closed before or during leakage test, it should be kept at this position during pilots or burners light-off.
7.4.2.10 If the leakage test for individual valves (4) is not successful, the operator should verify leakage valve (2) closing valve (6), also according flowchart below.
7.4.2.11 If it is proven leakage at one valve (4), this problem should be solved by maintenance immediately.
7.4.2.12 The end of the leakage test occurs after depressurization of the stretch between the valves (2) and (4) by opening the valve (2) by pulse.
7.4.2.13 All leakage test shall be recorded.
7.4.3 Purge sequence
7.4.3.1 If any automatic valve is manipulated during leakage test or during purge time, the start procedure is cancelled.
7.4.3.2 Snuffing steam valve shall be automatically opened and the “Purging” indication shall blink when the snuffing steam valve limit switch (ZSH) indicates full opening and snuffing steam pressure is higher than 20% of normal operation pressure.
7.4.3.3 For forced draft fired heater, forced draft fan should be used instead of snuffing steam.
7.4.3.4 Once the “Purging” indication is noticed, a timer shall be initiated and the purge time shall be started. The purge time shall be T1. The minimum T1 time should be defined to 3(three) trading of volume of the combustion chamber.
7.4.3.5 If the operator closes the snuffing steam valve before the total purge time, the start-up sequence shall be inhibited and a new purge shall be necessary to enable any ignition.
7.4.3.6 If any interlock that causes the general shutdown of the fired heater is triggered during the purge operation, the start-up sequence shall be aborted and, after the end of the purge operation, the ignition shall not be enabled.
7.4.3.7 The closing of snuffing steam valve after the purging time shall be:
a) Automatic, after the operator command for open pilot double block valve and close vent valve.The “Enable to Ignition” indication shall blink during a time interval equal to T2.
b) Automatic, after a delay timer, greater than T1 + T2. In this case, start up procedure is aborted.
c) Manually, at any moment, according operation necessities. Also in this case, start up procedure is aborted.
7.4.3.8 The permission for open double block valve and close vent valve for ignition shall be given only if all shutdown conditions are normalized, exception for this initiators, that should be automatic by passed:
a) Very high pressure for pilots, gas burner and oil burners.
b) Very low pressure for pilots, gas burner and oil burners.
c) No flame at 50 % of pilots.
7.4.4 Ignition sequence
7.4.4.1 In normal conditions, after the purging time, the “Enable to Ignition” indication shall blink during T2. This time, T2, shall be adjusted considering the ignition time of at least 50 % of pilots more one.
7.4.4.2 During “Enable to ignition” indication, operator shall activate the ignition sequence logic by a hand switch. This logic will command the pilot gas double block valves to open and the vent valve to close.
7.4.4.3 When the pilot gas double block valves are commanded to open and the vent valve is commanded to close, the high and low pressure trip signals shall be bypassed during T3 timer or until after 30 seconds from header pilot pressure stabilization in order to enable the start-up.
7.4.4.4 Since the pilot double block valves are opened and vent valve is closed, each pilot shall be ignited individually.
7.4.4.5 After 2 seconds from ignition command, individual pilot valve shall be opened.
7.4.4.6 The ignition command shall energize the igniter during T4. After this time, the energizing shall stop and pilot light off should be proven by flame rod indication.
7.4.4.7 If flame is not detected 5 seconds after T4, the pilot valve shall be closed by the logic implemented at PES. In this case a new ignition will be possible just after T5.
7.4.4.8 The operator should light off the first pilot in a period of T9. If this not happens, a shutdown sequence should be started.
7.4.4.9 In case of expiration of the “Enable to ignition” time and at least 50 % or more of pilots without fire in each row, the fired heater shall be tripped.
7.4.4.10 In order to avoid spurious trips, a timer shall be configured in the logic actuated by the flame detectors. For the “flame detected” condition, the signal must be active for at least 5 seconds to be acknowledged. For the “no flame detected”, it must be active for at least 2 seconds.
7.4.4.11 By operational procedure, it shall be avoided more than two adjacent pilots without flame.
7.4.4.12 By operational procedure, operator should light off the pilots in an equal distribution form. Preferentially, pilots should be light off alternatively.
7.4.4.13 For fired heater with 10 or more pilots, if four unsuccessfully tries of lighting off pilot happens in the same row, logic shall guarantee:
a) If the number of pilots on fire is less or equal than the minimal number (50% of total pilots), the furnace should be shut down.
b) If the number of pilots on fire is greater than the minimal number (50% of total pilots), a timer of T10 should be started, avoiding the try of lighting off any pilot during this time.
7.4.4.14 During ignition time, logic should not permit to open the main gas header or the oil header.
7.4.5 Fuel gas burner alignment sequence
7.4.5.1 Once the ignition sequence is established and, at least, 75% more one of pilots are on fire, operator shall activate the main burners sequence logic by a hand switch from DCS. This logic will command the burners fuel gas header valve to flare (valve 8 ) to open, the burners fuel gas double block valves to open and the vent valve to close., only if any process interlock which cause a “partial trip” on fired heater is not actuated.
Valve (8) should be closed by operator or automatic after timer T9.
7.4.5.2 When the burners fuel gas double block valves are commanded to open and the vent valve is commanded to close, the high and low pressure trip signals shall be bypassed during T6 timer or until after 30 seconds from header fuel gas burner pressure stabilization in order to enable the fuel gas burner alignment.
7.4.5.3 Once the burner’s Fuel Gas double block valves are opened and vent valve is closed, the operator, near the burner, shall be able to open each Fuel Gas individual valve, since there is flame indication at the corresponding pilot. The following items shall be considered :
a) it shall be provided two local hand switches: one for opening, another one for closing;
b) by operational procedure the operator shall verify the quality of flame near the burner immediately after the Fuel Gas individual valve opens.
7.4.6 Fuel oil burner alignment sequence
7.4.6.1 Once the ignition sequence is established and, at least, 50 % more one of pilots are on fire, operator shall activate the main burners sequence logic by a hand switch from DCS. This logic will command the burner’s fuel oil block valve to open, the recirculation valve to open and the bypass valve to close, only if any process interlock which cause a “partial trip” on fired heater is not actuated.
7.4.6.2 When the burners’ fuel oil block valve and the recirculation valve are commanded to open and the bypass valve is commanded to close, the high and low pressure trip signals shall be bypassed during T8 timer or until after 30 seconds from header fuel gas burner pressure stabilization in order to enable the fuel oil burner alignment.
7.4.6.3 Once the burners’ fuel oil block valve and the recirculation valve are opened and the bypass valve is closed, operator, near the burner, shall be able to open each Fuel Oil individual valve, since there is flame indication at the corresponding pilot. The following items shall be considered:
a) it shall be provided two local hand switches: one for opening, another one for closing;
b) fuel oil burner alignment can not be from DCS.
7.4.7 Shutdown Step Sequence
7.4.7.1 The shutdown procedure should guide the equipment to a safe condition.
7.4.7.2 Snuffing steam shall be open only at manual shutdown stop (HS). For this situation, snuffing steam valve should only be closed by operator.
7.4.7.3 For the others cases of general shutdown, it is enough to close double block valves, open vent valve and fully open control damper. The natural draft will purge the equipment. If, for any general shutdown, the control damper does not confirm full open, snuffing steam should be admitted. Also for this situation, snuffing steam should only be closed by operator.
7.4.7.4 For closing double block valve and opening vent valve, the logic should guarantee that valve (2) will close after valve (1) and after the opening of valve (3). The reason for this recommendation is depressurize the headline between valves (4) and (2).
7.4.8 Recommendations For Timers
7.4.8.1 All timers configured on the start-up logic program shall be adjustable according to the following table.
7.4.8.2 Only supervisors shall be able for tuning these timers.
Timer
Identification Description Initial Reference Value
T1 Purging time 15 minutes
T2 Time to ignite at least 50% +1 pilots 30 seconds/pilot T3
Time to bypass pressure trips at the pilot
system 2 minutes
T4 Igniter energizing time 10 seconds
T5
Minimum interval between ignition tries
for each pilot 60 seconds
T6
Time to bypass pressure trips at the main
fuel gas system 2 minutes
T7
Maximum time between the opening of burner individual valve and confirmed burner’s fire
15 seconds
T8
Time to bypass pressure trips at the main
fuel oil system 2 minutes
T9 Maximum time for first pilot light off 1 minute T10
Time interval after 4 consecutive
unsuccessful tries for pilot’s light off 2 minutes
7.5. GENERAL REQUIREMENTS
7.5.1 The combustion chamber pressure transmitter should be protected against wind, dust, dirty and so on, since it operates with a very low range.
7.5.2 The low-low and high-high pilot fuel gas pressure transmitters should be located one upstream to the first pilot and the other downstream to the last pilot.
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7.5.3 The high-high main fuel gas pressure transmitter should be located upstream to the first burner. The low-low main fuel gas pressure transmitter should be located downstream to the last burner.
7.5.4 All block valves should be tight shut off valves (TSO).
7.5.5 The flame rod and igniter should be mounted on the pilot, avoiding the existing problems on external mountings.
7.5.6 All individuals main gas block valves should have a slowly opening characteristic and a quick closing characteristic.
7.5.7 High pressure combustion chamber can automatic open control damper.