NEC 500.7(K) "shutdown criteria"

[Natfuelbilll]

Any defined explanation of what the "shutdown criteria" or "shutdown procedure" (NECHB) might mean in NEC 500.7(K) ?

This has ben asked before, but I still wrestle with what the heck I REALLY need to do to satisfy this Code and the other FPN references, if this protection technique is used.




API 500 recommends a remedial action at low LFL levels to avoid increasing levels of gas. Presumably a remedial action can be the starting of a ventilation system.

Wikpedia defines a "remedial action" as "a change made to a nonconforming product or service to address the deficiency."​

At high LFL levels, or GDS malfunction, API 500 recommends a corrective action to immediatlely reduce gas concentration. I presume this means to automatically [?] remove the cause of the problem, ie: get rid of all the gas.

BusinessDictionary.com defines "corrective action" as "Identification and elimination of the causes of a problem, thus preventing their recurrence."​

 

[weressl]

Any defined explanation of what the "shutdown criteria" or "shutdown procedure" (NECHB) might mean in NEC 500.7(K) ?

This has ben asked before, but I still wrestle with what the heck I REALLY need to do to satisfy this Code and the other FPN references, if this protection technique is used.




API 500 recommends a remedial action at low LFL levels to avoid increasing levels of gas. Presumably a remedial action can be the starting of a ventilation system.

Wikpedia defines a "remedial action" as "a change made to a nonconforming product or service to address the deficiency."​

At high LFL levels, or GDS malfunction, API 500 recommends a corrective action to immediatlely reduce gas concentration. I presume this means to automatically [?] remove the cause of the problem, ie: get rid of all the gas.

BusinessDictionary.com defines "corrective action" as "Identification and elimination of the causes of a problem, thus preventing their recurrence."​

Re-establishing the safe state level is the objective and it can be accomplished by various means and could take several steps. Shut of the supply, dillute the mixture, etc. Should these actions fail and the unsafe condition persist, or escalate shutting of the power remains a choice. Of course if there are heated elements in the proceess that may not be a sufficient measure and you would shut off the power to the heaters immediately as you attempt to re-eastblish the safe conditions, then turn the heat source powers back again.

 

[rbalex]

Again, I?m reluctant to respond to this. Not so much that I don?t like gas detection but because the ?rules? are still too vague and it is difficult to design and inspect; e.g., how can one reasonably determine that API RP 500 Subsection 6.5.2.c has been met? Purging or pressurizing is often a better option.

Assuming that the location is completely consistent with either API RP 500 Subsection 6.5.1.a or 6.5.1b, I can offer some opinions that I believe are consistent with Subsections 6.5.2.d and 6.5.2.e.

It must be recognized that both Subsections 6.5.2.d and 6.5.2.e require action well below the lower explosive limit (LEL); i.e., 20% and 40% respectively. It must also be recognized that Section 6.5.2 applies to both Subsections 6.5.1.a and 6.5.1.b.

Subsection 6.5.1.a already assumes the ventilation is inadequate. For Subsection 6.5.1.b ventilation may or may not be inadequate. It is basically something similar to Section 6.4.2 and Figure 3 except there may be an occasional opening to only the Division 2 location.

?Remedial action? may indeed be initiating a ventilation system; but it can be anything that will keep the gas or vapor below 40% LEL. In my opinion, ?Corrective action? is basically limited to ?initiat[ing] automatic disconnection of power from all electrical devices in the area that are not suitable for Division 2.? (Although it could just be closing the door) My experience is that, once it hits 40% LEL if ?remedial action? has been initiated, it will hit 100% fairly quickly.

 

[rbalex]

... Of course if there are heated elements in the proceess that may not be a sufficient measure and you would shut off the power to the heaters immediately as you attempt to re-eastblish the safe conditions, then turn the heat source powers back again.

Your point is well taken especially about temperature retention; however, API RP 500 Subsection 6.5.2.h already prohibits ??electrically heated parts or components (not enclosed in explosionproof enclosures) that may operate at a temperature equal to or above 80 percent of the ignition temperature (expressed in degrees C) of the gas or vapor involved unless the component has been verified by a nationally recognized testing laboratory (NRTL) to operate below the ignition temperature of the gas or vapor.? In other words, they must be suitable for Division 2 also.

It also notes:

"Electrically heated parts and components could remain at or above the ignition temperature for some time after deenergization."​