Issues with the high, high limit, low, and low-low limits for DCS production units
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Issues regarding the high-high limit, high limit, low limit, and low-low limit settings for DCS production units: should these be determined based on process specifications? For example: the operating pressure for the process parameters is 0.1~0.35. How should this value be set? Let’s discuss this, as the expert inspection team has conducted several inspections and given different opinions.
For example: Process parameter – operating pressure: 0.1~0.35, with an upper high limit of 0.4, an upper limit of 0.35, a lower limit of 0.1, and a lower low limit of 0.05; or an upper high limit of 0.35, an upper limit of 0.3, a lower limit of 0.15, and a lower low limit of 0.1? Also, this upper limit cannot be exceeded based on the device’s design
The upper and lower limits are adjusted according to the operating conditions; initially they can be set based on the process parameters, and later they can be modified and optimized based on actual operation. The high and low limits are set in accordance with safety requirements and based on adjustments for process and equipment safety; they are generally not modified and are determined during the plant design phase. Additionally, the high-high limit and low-low limit are SIS parameters; they do not share instruments with the DCS, and the safety level of the corresponding instruments is also higher.
The high high limit and low low limit should be related to interlocking. However, a hierarchical management system should be adopted: those related to interlocking are classified as level 1, other process parameters are classified as level 2, and those that fall outside levels 1 and 2 and are not key monitoring points are classified as level 3.
Generally, setting high and low limits as interlock values is sufficient; it’s enough to set the high and low limits as alarm values.