How to handle \"shall\", \"should\", and \"may\" in instrument industry specifications

3.2.1. The control room should be located within the plant or complex, and outside any area prone to explosions; the process safety team calculates the explosion intensity to determine whether the building needs to have an explosion-resistant structure. 3.4.1 For chemical plants with explosion hazards, the construction and structure of the control room building shall be designed based on the calculation and analysis results regarding blast resistance. (Many structural engineering specialties cannot do it): It’s not possible; in that case, the structural design should be carried out according to the recommended strength levels, as explained in the appendix. 3.4.2 For chemical plants with explosion hazards, the control rooms and field control rooms shall be designed to be explosion-resistant. (As above): Whether it is blast-resistant or not is not within the scope of the instrumentation field. These two are likely to be difficult to handle, especially in some small renovation projects. II. GB 50770-2013 Code for Design of Safety Instrumented Systems in Petrochemical Industries: 6.2.1 For SIL 1 level safety instrumented functions, the measuring instruments can be shared with the basic process control system. : They should be separated as much as possible. A separate transmitter should be provided for the SIS loop. 6.2.2 For SIL 2 safety instrumented functions, the measuring instruments should be separated from the basic process control system. : They must be set separately. 6.2.3 For SIL3 safety instrumented functions, the measuring instruments shall be separated from the basic process control system. : It must be separated. 6. 3.1 For SIL 1 safety instrumented functions, a single measuring instrument can be used. : Yes. As long as the transmitter is independent, redundancy need not be considered. 6.3.2 For SIL 2 safety instrumented functions, redundant measuring instruments are recommended. : A degree of redundancy is necessary. 7.1.2 Pneumatic control valves are preferred for the final components, rather than electric control valves. : Electric valve – once the power supply is lost, it cannot return to the “defined safe position”. 7.3.1 For SIL I safety instrumented functions, a single control valve can be used. : Adjust +sov. 7.3.2 For SIL 2 safety instrumented functions, redundant control valves are recommended. : One regulator + SOV, one dedicated shut-off valve. 7.3.3 For SIL 3 safety instrumented functions, redundant control valves shall be used. : One control valve for regulation + SOV, one dedicated shut-off valve + valve operation testing (once a year). 7.3.4 For control valve redundancy, either one control valve and one shut-off valve can be used, or two shut-off valves can be employed. : The approach of one regulator plus two cut-offs is rarely used; the SIL 3 treatment method can handle the vast majority of SIL circuits. The above approach is a general rule derived from the verification of SIL circuits. Reading the guidelines ten thousand times is not as good as doing it once in practice. ;P

Thank you for your comment. Although the anti-riot design of the control room is not within the scope of automatic control, some people don’t think so; therefore my response has always been to follow the specifications.

Guidelines for the use of terms: Terms in the regulatory provisions that require varying degrees of strict compliance are explained as follows: 1 indicates a very high level of strictness – terms that mandate action must be used; for positive statements, “must” is used, while for negative statements, “strictly prohibited” is used. 2 indicates a high level of strictness – actions should be taken under normal circumstances; for positive statements, “should” is used, while for negative statements, “should not” or “not allowed” is used. 3 indicates that some flexibility is allowed – actions should be taken first when conditions permit; for positive statements, “it is advisable” is used, while for negative statements, “it is not advisable” or “not allowed” is used. When there is some flexibility and an action can be taken under certain conditions, “may” is used.

In the field of instrumentation, it is not possible to carry out calculations related to explosive shock forces; therefore, in terms of blast resistance design, instrumentation is a supporting discipline rather than a leading one. You can refer to the provisions on blast resistance in the previous version of the \"Control Room Design Code\". In practical design, however, instrumentation cannot provide evaluation methods or detailed implementation rules, which leads to it being overshadowed by other disciplines, especially those related to structure design. In the new codes, instrumentation is no longer given a prominent role.

I can’t answer the first question. As for the second one, which relates to safety instrumented systems, this is how I proceed with design work: “Possible” means that it is acceptable for instruments to be shared between SIS and DCS, while “Preferable” means that the instruments for DCS and SIS should be separate. In special cases, for example, if the pressure in a tank is classified as SIL2 and there are already two pressure transmitters, both of those can be used by the SIS; the DCS can then borrow one of them through a signal distributor with one input and two outputs. “If that’s the case, then it’s necessary to completely separate the DCS and SIS instruments and valves, after all, both are of SIL3 level.

DCS can be left uncontrolled; it definitely won’t work if control is required