To open equipment and line to atmosphere for the inspection, hazardous materials should be completely purged from system. − And also, before restarting the plant, O2 in the system should be purged to the safe level of O2 concentration so as not to form the explosive mixtures with flammable materials. − Although the safe level depends on the process, O2 content is normally reduced to 0.5 ~To open equipment and line to atmosphere for the inspection, hazardous materials should be completely purged from system. − And also, before restarting the plant, O2 in the system should be purged to the safe level of O2 concentration so as not to form the explosive mixtures with flammable materials. − Although the safe level depends on the process, O2 content is normally reduced to 0.5 ~To open equipment and line to atmosphere for the inspection, hazardous materials should be completely purged from system. − And also, before restarting the plant, O2 in the system should be purged to the safe level of O2 concentration so as not to form the explosive mixtures with flammable materials. − Although the safe level depends on the process, O2 content is normally reduced to 0.5 ~
TIÊU CHUẨN THIẾT KẾ P&ID tranhaiung@gmail.com Department of Oil and Gas Processing Piping - Purge-out Operation General ₋ To open equipment and line to atmosphere for the inspection, hazardous materials should be completely purged from system ₋ And also, before restarting the plant, O2 in the system should be purged to the safe level of O2 concentration so as not to form the explosive mixtures with flammable materials ₋ Although the safe level depends on the process, O2 content is normally reduced to 0.5 ～ 1.0 vol% Piping - Purge-out Operation Purge-out Methods (1) Steam-out − This is the most typical purge-out method used in actual plant Steam-out should not be applied to the systems which should be maintained free from moisture, since condensates will be generated after steam-out operation − After steam-out operation, it is necessary to inject N2 or fuel gas into the system to protect the system from vacuum due to steam condensation (2) N2 purge Applied for the systems which should be maintained free from moisture, such as LPG pumps, compressors, reactor circuit, etc Piping - Purge-out Operation Purge-out Methods (3) Fuel gas purge Fuel gases are hazardous and are not used for purge-out; they are used for vacuum break after steam-out operation (4) Evacuation Evacuation using an ejector is the method to purge air from the system effectively It is used for systems which should be maintained free from moisture, such as the reactor circuit in hydrogen desulphurization units After evacuation, air will leak into the system Therefore, the system should be pressurized with N2 or fuel gas to prevent the ingress of O2 Piping - Purge-out Operation Location of Purge-out Connection Purge-out connections are generally provided at the following points in actual plants, in order to purge the hazardous materials from the system effectively (1) Vessels (towers, drums, etc.) bottom (2) Pump discharge line (downstream side of discharge valve) (3) At least one purge-out connection shall be installed for each line (excluding utility lines) Where a line has branches, the purge-out connections should be provided so that the purge-out operation for all branch lines can be carried out Piping - Purge-out Operation Location of Purge-out Connection Around stabilizer (1) Steam-out connection − Tower bottom − Feed surge drum/OVHD receiver side bottom − Feed/LPG pump discharge (2) N2 connection − Feed surge drum/OVHD receiver - for pressurizing − LPG pump suction Piping - Purge-out Operation Location of Purge-out Connection Around battery limit Piping - Purge-out Operation Location of Purge-out Connection Evacuation system The typical evacuation system for the reactor circuit in the hydrogen desulphurization unit *1 N2 purge connection to prevent the ingress of O2 into the reactor circuit Piping - Purge-out Operation Connection Size for Purge-out Minimum connection size for purge-out is as follows: (1) Steam-out a) For vessel Vessel Volume (m3) - ∼ 150 - 150 ∼ 400 - 400 ∼ b) For Line Conn Size (inches) 1-1/2 c) For all others : 1” Line Size (Inches) - ∼ 10 - 12 ∼ 20 - 22 ∼ Conn Size (inches) 1-1/2 Piping - Purge-out Operation Connection Size for Purge-out Minimum connection size for purge-out is as follows: (2) N2 connection − To purge out pump and compressor : 3/4” − To pressurize vessel and line after steam-out : 1” − To pressurize reactor circuit after evacuation : N2 connection size depends on the volume of reactor circuit and pressurizing speed Safeguarding device - Voting System Depressurizing Valve (3) Remote (on/off): critical emergency level Note: To protect the equipment (which is designed based on differential pressure) from over design differential pressure and control the flare load within the design capacity of flare system, it is necessary to limit the depressurizing rate In the case of using on/off valve, a restriction orifice shall be provided at the downstream side of the depressuring valve Though an air cylinder actuator is used in the typical arrangement, the actuator type shall be modified according to the valve selection It generally depends on the size and shutoff pressure of valve Safeguarding device - Voting System Depressurizing Valve Notes on type (2) and (3): a) Inspection and maintenance of depressurizing valve: Block valves shall be provided on both the upstream and downstream side of the depressurizing valve and shall be “locked open” (LO) b) Tight shutoff type (TSO): All depressurizing valves shall be of TSO type c) Failure action in case of an instrument air failure: The failure action of the depressurizing valve shall be “failure open (FO)” from a safety viewpoint When “failure close (FC)” is set in order to minimize the loss of valuable material or restart easily, it is required to confirm that in case of an instrument air failure the system pressure does not rise to the safety valve set pressure d) Secure instrument air supply system, i.e independent instrument air reservoir, is often provided with the emergency shut-down valve for the backup of motive air at the instrument air failure This system should be indicated on P&ID Safeguarding device - Emergency Shutdown Valve Switch for Detecting Emergency Condition • Separately from a connection to the measuring instrument, another connection to the switch for detecting emergency conditions shall be provided The type of switch for detecting emergency conditions shall be one of the following two: (1) Switch mounted directly on the process (2) Dedicated transmitter and switch for detecting emergency conditions • As for the primary selection of the type, the use of a switch mounted directly on the process is normally recommended from the viewpoints of economy and reliability because of the simple construction Safeguarding device - Emergency Shutdown Valve Switch for Detecting Emergency Condition Safeguarding device - Emergency Shutdown Valve Switch for Detecting Emergency Condition Safeguarding device - Emergency Shutdown Valve Shutdown Valve The following considerations are required for the design of shutdown valve − Failure action will be “failure close (FC)” excluding exceptional cases − Tight shut-off (TSO) valve shall be used (Note) − A bypass line shall not be provided for the dedicated shutdown valve − The bypass valve for control valve shall be “locked close (LC)” Safeguarding device - Emergency Shutdown Valve Shutdown Valve (1) Control valve used as emergency shutdown valve Safeguarding device - Emergency Shutdown Valve Secure Instrument Air Supply System • For the backup of motive air at the instrument air failure, the secure instrument air supply system is often provided for the depressuring valve and the emergency shutdown valve as per client’s request • The facilities of this system should be clarified and indicated in P&ID Safeguarding device - Safety Relief Valve Basic Arrangement Safeguarding device - Safety Relief Valve Vent Line to Flare A vent line to flare is required in a system handling hazardous or flammable gas for the startup and shutdown operation Where no vent line to flare is provided in the system, a bypass line is provided to the safety relief valve for this purpose The vent line is of the same size as that of the vent line of the suction vessel Single block Double block Double block shall be applied to LPG, H2 and high pressure service equal to 300# rating or higher Safeguarding device - Safety Relief Valve Flange Requirement for Hydraulic Test Where the vent line is 1-1/2” or less, a welded type valve is used A flange shall be provided for isolation at hydraulic testing Safeguarding device - Safety Relief Valve Safety Relief Valve with Spare (1)Spare safety relief valves (one is spare) : 100% × safety relief valves (one is spare) : 50% × *1: For venting of RV suction line during maintenance *2: To be locked open to avoid overpressure due to upsteam valve leakage Safeguarding device - Safety Relief Valve Safety Relief Valve with Spare (2)Warehouse Spare safety relief valves (one is spare) : 100% × safety relief valves (one is spare) : 50% × *1: For venting of RV suction line during maintenance *2: To be locked open to avoid overpressure due to upsteam valve leakage Safeguarding device - Safety Relief Valve Others Thermal relief valve on equipment with on-stream maintenance facilities (example of heat exchanger) A block valve with car seal type (Open : CSO) shall be provided on the RV discharge line to isolate the RV during onstream maintenance Safeguarding device - Safety Relief Valve Others Thermal relief valve on equipment with on-stream maintenance facilities (example of heat exchanger) A block valve with car seal type (Open : CSO) shall be provided on the RV discharge line to isolate the RV during onstream maintenance