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KHABAROVSK REFINERY HYDROPROCESSING PROJECT SRU – CONTROL SYSTEM and ESD

KHABAROVSK REFINERY HYDROPROCESSING PROJECT SRU – CONTROL SYSTEM and ESD. TRAINING COURSE. APRIL 29th – MAY 3rd 2013, MADRID, SPAIN. CONTROL SYSTEM. The purpose of this section is to give a general explanation of how the control system works.

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KHABAROVSK REFINERY HYDROPROCESSING PROJECT SRU – CONTROL SYSTEM and ESD

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  1. KHABAROVSK REFINERY HYDROPROCESSING PROJECT SRU – CONTROL SYSTEM and ESD TRAINING COURSE APRIL 29th – MAY 3rd 2013, MADRID, SPAIN

  2. CONTROL SYSTEM • The purpose of this section is to give a general explanation of how the control system works. • Process variables, that determine a good operation of the Unit, play a different role depending upon the type of operation. • The Combustion Control of Thermal Reactor has been chosen because is the most important control system.

  3. MAIN CONTROL LOOPS OF CLAUS AND TGT SECTIONS • THERMAL REACTOR SYSTEM • TAIL GAS SYSTEM • LEAN AMINE/STEAM FLOWRATE TO REBOILER • THERMAL INCINERATOR SYSTEM

  4. THERMAL REACTOR SYSTEM In sulphur plants, the control of the Thermal Reactor is achieved by regulating the ratio of total air (oxygen) to total acid gases (H2S) entering the plant. Three operating modes are possible: Fuel gas Operation Normal (AAG and/or AAG + SWS) Operation Fuel gas Operation Fuel gas combustion is used during start-up to achieve the minimum flame temperature to start normal (acid gas) operation, during shut down or in hot stand-by operation. Normal Operation During normal operation, about 75% of the amine acid gas and the whole SWS acid gas and the relevant combustion air are fed to the thermal reactor burner (1st zone) and no fuel gas combustion is required. The remaining 25% of the amine acid gas is sent to thermal reactor 2nd zone.

  5. TAIL GAS SYSTEM The Tail Gases from the Claus Sections are fed to the TGT Unit via the valves (HV-3301 and HV-3303) equipped with a controller which allows the gradual opening/closing of the valve during the start-up or shut-down of the relevant section. It is possible to send Tail Gases from Claus Sections to the Thermal Incinerator via the valves HV-3302 and HV-3304; each of these valves is equipped with a controller which allows the gradual opening/closing of the valve during the start-up or shut-down of the relevant section. The operator at DCS will perform the diversion acting on the dedicated valves. During the start-up of the TGT Section, at the end of the diversion procedure all the Tail Gas from the Claus Section is fed to the TGT Section and Tail Gas from TGT Section is fed to the Thermal Incineration Section.

  6. TAIL GAS SYSTEM • If the following conditions are verified by the ESD system, the operator at DCS can activate the diversion procedure, acting on the relevant selector: • Pressure at inlet of the Tail Gas Section less than or equal to 0.15 kg/cm2 g (figure to be defined during commissioning); if the pressure is above 0.15 kg/cm2 g, the diversion procedure cannot start and the operator at DCS has to change the set point of the PC in the Tail Gas Section, until the pressure at inlet of the Tail Gas Section is less than or equal to 0.15 kg/cm2 g; • Acid Gas operation and presence of flame in the Claus Section; • TGT section is not in shutdown

  7. LEAN AMINE/STEAM FLOWRATE TO REBOILER With the purpose to minimize the LS Steam consumption and to guarantee a stable composition of H2S to Claus Section, the lean amine flow rate and the steam flow rate to reboiler will be set on a fixed ratio. The stripping steam flow rate to the Regenerator Reboiler is controlled on the basis of the measured flow of the lean amine solution entering the Absorber column. WLPS_REBOILER =WLEAN_AMINE* KLPS_REBOILER where WLPS_REBOILER is the total steam flow rate fed to the reboiler. KLPS_REBOILER: LP Steam required for lean amine.

  8. THERMAL INCINERATOR SYSTEM • The Thermal Incinerator Burner is Natural draught type; the combustion of the tail gas is achieved with support fuel gas combustion. The combustion air is supplied by Air Intake on the burner. • Thermal Incinerator Burner is lit with a dedicated pilot. • The fuel gas to Thermal Incinerator Burner is regulated in order to get the optimal temperature in combustion chamber for abatement of H2S.

  9. THERMAL INCINERATOR SYSTEM The amount of H2S in the Tail Gas from Absorber is too low to have 650°C in the combustion chamber of the Incinerator; this temperature is developed burning fuel gas. • The temperature in the combustion chamber is controlled by the temperature controller TIC-3340 acting on fuel gas valve TV-3340.

  10. FUEL GAS OPERATION Fuel gas is burnt in the Main Burner during the Claus Section heating-up in order to reach the minimum allowable temperature before admitting the acid gas. THERMAL REACTOR CONTROL SYSTEM COLD MODE: NO SULPHUR ON CATALYST BEDS CLAUS SECTION START-UP HOT MODE: SULPHUR ON CATALYST BEDS

  11. COLD START MODE (No sulphur on the catalyst bed) EXCESS OF OXYGEN TYPE OF COMBUSTION FUEL GAS OPERATION CONTROL SYSTEM The flow rate of combustion air to the thermal reactor burner (WCA_FG ) is calculated as following: WCA_FG = WTOT_FG * KCA_FG_STO * KTRIM_FG where WTOT_FG is the total fuel gas flow rate fed to the burner. The value “Combustion Air required for fuel gas - Stoichiometric” (KCA_FG_STO) is based on fuel gas composition. The trim balancing function (KTRIM_FG) varies between 1-10 and shall be imputed by the operator in order to maintain the temperature under 1350 °C (this value represents the excess of air).

  12. HOT START MODE (Sulphur on the catalyst bed) STOICHIOMETRIC TYPE OF COMBUSTION FUEL GAS OPERATION CONTROL SYSTEM The flow rate of combustion air to the thermal reactor burner (WCA_FG) is calculated as following: WCA_FG = WTOT_FG * KCA_FG_STO * KTRIM_FG where WTOT_FG is the total fuel gas flow rate fed to the burner. The value “Air request for fuel gas” (KCA_FG_STO) is based on fuel gas composition. To maintain the temperature under 1350°C, quench steam shall be used. The trim balancing function (KTRIM_FG) varies between 0.8-1.2 in case of stoichiometric combustion.

  13. CONTROL SYSTEM ACID GASES OPERATION • The SRU Unit can be switched from the Fuel Gas operation to Acid Gases operation only when the plant is heated-up and a minimum temperature of 1000° C is reached. • In particular the following item shall be checked: • Thermal Reactor: temperature (1000°C); • Claus WHB: steam pressure (4.5 kg/cm2 g); • Claus Reactors: inlet temperature (220 and 200°C); • Sulphur Condensers: steam pressure (4.5 kg/cm2 g); • Steam Jacketing/Tracing: in operation. • During normal operation, both main and trim valves on combustion air are used.

  14. CONTROL SYSTEM ACID GASES OPERATION The total flow rate of combustion air to the thermal reactor burner is calculated as followed: WTOT_CA_REQ = WCA_AAG+WCA_SWSAG+WCA_FG Where: WCA_AAG is the Total Combustion Airrequired for amine acid gas. WCA_SWSAG is the Total Combustion Air required for sws acid gas. WCA_FG is the Combustion Air required for fuel gas. The total air flow rate is the sum of the flow through the main and trim valves: WTOT_CA_REQ = WMAIN_CA + WTRIM_CA The Main control valve input is based on total acid gases flow rate and their H2S concentration. The Trim control valve is controlled by the Tail Gas analyzer (H2S / SO2)

  15. CONTROL SYSTEM ACID GASES OPERATION • Control systems will be implemented for the control of the air required for the combustion of Amine Acid Gas, SWS Acid Gas. • The control system is based on the feed-forward control (based on DCS operator inputs and acting of main valve) plus a feed-back control system (signal from Air Demand Tail Gas Analyzer to trim valve). • The output signals of the tail gas analyzer AIT are the H2S and SO2 content of the tail gas.

  16. CONTROLSYSTEM - ACID GASES OPERATION The Main control valve input is based on total acid gases flow rate and their H2S concentration. WTOT_CA_REQ = WCA_AAG+ WCA_SWSAG+WCA_FG The Trim control valve is controlled by the Tail Gas analyzer (H2S / SO2)

  17. The analyzer measures the H2S and SO2 concentrations in the tail gas: a ratio 2:1 of H2S/SO2 corresponds to the max sulphur recovery The trim air demand (AD) can be described with the following equation: AD = H2S - 2 * SO2 where SO2 , H2S = % Vol. CONTROL SYSTEM Trim air

  18. FUEL GAS OPERATION FC FC FC FC FC COMBUSTION CONTROL CONTROL SYSTEM INCREASE FLOW  KEEP THE CORRECT TEMPERATURE! QUENCH STEAM NO FLOW AMINE ACID GAS INCREASE FLOW  FUEL GAS COMBUST. AIR THERMAL REACTOR BURNER

  19. The quantity of air that must be added or subtracted through the trim valve to obtain the desired condition in the Claus section is only a part of the total air. The optimum position is the one at which the trim air flow is about 50% of the nominal range. The gain of the AIC should be modified with the plant load in order to maintain the trim air control valve always open (preferably 20÷50% open). During operation with fuel gas AIC shall not be maintained in operation; the combustion control shall be based on analysis of flue gas to determine O2 and CO concentration. CONTROL SYSTEM Trim air

  20. CONTROL SYSTEMACID GASES OPERATION FC FC FC PC FC PC SWS AAG COMBUST. AIR COMBUSTION CONTROL THERMAL REACTOR BURNER

  21. LOGIC SYSTEM What is the LOGIC SYSTEM? A Logic System is an integrated highly automated safety facility, able to perform several actions: Handling all the start-up and shut-down phases of the Plant; Isolate the whole plant, one unit of the plant from the other, or just a section of one unit; Interact with the DCS system; Keep always the Plant under safe conditions.

  22. LOGIC SYSTEM HOW DOES IT WORK? The working principle of a Logic System is: INPUT: Upset signal from the Plant OUTPUT: One or more actions performed automatically on valves, equipment, … MAIN FUNCTIONS OF A LOGIC SYSTEM • Shut-Down Procedure • Start-Up Procedure • Avoid Upset conditions during normal operation that could damage the personnel or the equipment

  23. LOGIC SYSTEM MAIN FEATURES OF LOGIC SYSTEM All the logics are handled by a central hardware called PLC (programmable logic control) Input/output signals of PLC are usually electrical, not digital. PLC input signals: output signals from instrumentation installed in the plant. PLC output signals: input signals for SD, ESD, control valves.

  24. LOGIC SYSTEM – START UP • When starting-up a section of a plant, the logic system shall strictly follow a well determined procedure. • This procedure is tied to verify the necessary conditions, in order to guarantee the safety of the operation. • These conditions are called PERMISSIVES.

  25. PERMISSIVE DURING START-UP • HOW IS A START-UP PERFORMED? • Start-up is guided and monitored by well competent operators and specialist engineers. • Start-up must be performed both from field and control room. • The operators in control room shall monitor the start-up via graphic pages.

  26. PERMISSIVE DURING START-UP • On the graphic pages, the whole plant is graphically represented: • - Equipment • - Instruments • - SD, ESD, control valves • - Control loops • - Alarms and SD causes • Furthermore, in the graphic pages the operator can check if all the logic conditions necessary for the start-up are met. • If some of the required permissives are not met, the logic system doesn’t give its permission to start-up.

  27. The ESD System helps the operator during start-up. COLD START: NO SULPHUR PRESENT IN THE UNIT PRE-IGNITION PURGE (BURNER LIGHT-ON) HOT START: SULPHUR PRESENT IN THE UNIT • THE MAIN DIFFERENCE BETWEEN THE TWO LIGHT-ON PROCEDURES ARE: • PRE-IGNITION PURGE (AIR/NITROGEN) • TYPE OF COMBUSTION (EXCESS OF OXYGEN/STOICHIOMETRIC) The choice of the type of purge is left to the discretion of the operator

  28. When the logic system starts to operate (with panels and logic system electrically powered) or following a shut‑down, it is necessary to reset it before starting any operation. Some process overrides are necessary in order to proceed with the operation (for instance Amine Acid Gas very low flow rate) in order to get the consent to reset the system. If all the shut-down causes are absent and if the interlocks are verified, the logic shall signal the possibility to reset the system In addition the logic shall check the proper position of the cut-off/cut-in valves verifying the interlocks of the relevant limit switches. The operation of the Thermal Incinerator is a necessary logic consent (permissive). The running of one of the Combustion Air Blowers is necessary before the reset of the system. The consent of running shall be given by the combustion air low pressure switch.

  29. Pre-ignition purge COLD START MODE • The purpose of this operation is to guarantee the absence of explosive mixture inside the combustion chamber by an adequate purge activity. • The operations are the following: • Select COLD START on the HOT/COLD START selector • Push the SYSTEM RESET push-button • The logic response shall be the opening of the main combustion air cut‑off valve while keeping the purge nitrogen valve closed During the pre‑ignition purge the combustion air cut‑off valve and the nitrogen purge cut‑in valve are interlocked; therefore when one is open, the other one is kept closed by the logic; Since at this step, the SYSTEM RESET push-button will activate the opening of the combustion air cut‑off valve and this may result in a sudden pressure decreasing in the combustion air header, an automatic override of PSLL-3112 is required. The override shall be automatically removed after 60 seconds (time adjustable within 0÷120 seconds). • Once the logic has opened the combustion air cut-off valves, the operator shall increase the flow in order to satisfy the minimum air purge flow rate.

  30. Pre-ignition purge COLD START MODE • The logic response to the correct minimum air flow rate is: • Start the computing of the 5 minutes purge time • At the end of the 5 minutes purging time the logic shall: • Light-on the PURGE COMPLETE lamp • The PURGE COMPLETE status light advises the operator that the purge has been completed and indicates that a five minutes light‑on cycle can be initiated. If this light‑on cycle is not initiated, purging shall continue indefinitely if all purge interlocks continue to be verified. • After the pre-ignition purge COLD START the interlock between combustion air and purge nitrogen valves shall be removed by the logic. • The logic input for defeating the interlock shall be the START LIGHT-ON status.

  31. Pre-ignition purge HOT START MODE • The purpose of this operation is to guarantee the absence of explosive mixture inside the combustion chamber by an adequate purge activity. • The operations are the following: • Select HOT START on the HOT/COLD START selector • Push the SYSTEM RESET push-button • The logic response shall be the opening of the nitrogen cut‑off valve while keeping the combustion air cut‑off valve closed. During the pre‑ignition purge the combustion air cut‑off valve and the nitrogen purge cut‑in valve are interlocked; therefore when one is open, the other one is kept closed by the logic. • Once the logic has opened the nitrogen air cut-off valves, the operator shall increase the flow in order to satisfy the minimum nitrogen purge flow rate.

  32. Pre-ignition purge HOT START MODE • The logic response to the correct minimum nitrogen flow rate is: • Start the computing of the 5 minutes purge time • At the end of the 5 minutes purging time the logic shall: • Light-on the PURGE COMPLETED lamp • The PURGE COMPLETE status light advises the operator that the purge has been completed and indicates that a five minutes light‑on cycle can be initiated. If this light‑on cycle is not initiated, purging shall continue indefinitely if all purge interlocks continue to be verified. • After the pre-ignition purge HOT START the interlock between combustion air and purge nitrogen valves shall be removed by the logic. • The logic input for defeating the interlock shall be the START LIGHT-ON status.

  33. CAUSE & EFFECT DIAGRAM • The protective system (ESD) shall be based on the input (causes) and the output (effects) as shown in the Cause & Effect Chart • MAIN ESD SYSTEM • CLAUS SECTION SHUT-DOWN • TGT SECTION SHUT-DOWN • INCINERATOR SECTION SHUT-DOWN

  34. CLAUS SECTION – ESD SYSTEM

  35. TGT SECTION – ESD SYSTEM

  36. INCINERATOR SECTION – ESD SYSTEM

  37. RomeViale Castello della Magliana 7500148 - ItalyPh. +39 06 602161Fax +39 06 65793002info@tecnimontkt.it – www.tecnimontkt.it

  38. THANK YOU FOR THE ATTENTION

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