The principle of the USS is that it is a simple stand alone system with a minimum number of I/O which will provide a back-up to a small number of manual ESD actions. Its inputs will be ESD pushbuttons located in the control room defined by safety philosophies to back-up ESD1 & 0. These shall be the same buttons that are connected to the ESD system. The USS functionality will be transparent to the operator. Upon reception of a trip command the USS will initiate a timer to allow time for the ESD system to carry out the ESD function. Upon timeout the USS shall trip its outputs which will generally be connected to power supplies for safety valves (ESDV’s, BDV’s) and main process equipment, thus initiating the fail safe shutdown of the facilities. The USS system shall be based on a non-programmable system. It shall be of a fail safe design built using high reliability, high availability and fault tolerant equipment. It may interface with the PCS via serial links in order to report status and diagnostic information to the ICSS HMI. A minimum number of I/O shall be utilised and the logic treatment kept to a minimum. The USS will provide individual maintenance inhibit and isolate functions for each of its loops and shall be fully self-testing; all status and faults concerning these facilities will be reported to ICSS.
Posted on: Mon, 09 Sep 2013 08:26:39 +0000
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