Section 9.4.1 (CSA B149.3 – 10) refers to “Boilers." Does this interpret as all fired heaters in hydrocarbon service?
In CSA B149.1-10 on page 6 under definitions: "Boiler — an appliance intended to supply hot liquid or vapour for space-heating, processing, or power purposes." This definition covers pretty much all field process heaters filled with any type of liquid (glycol, oil, water, amine, molten salt, etc.) Besides the definition we use a simple test to determine if a given permissive must be present. If you can obtain a letter from the heater manufacturer of this or similar appliance saying that you can run burner at maximum fire for extended periods of time with heater empty without liquid then you do not need a level switch. Obviously no heater manufacturer will issue such letter.
When it comes to permissives used in BMS systems the intent of the CSA B149.3 Code is that they are directly connected to the BMS without the involvement of another logic solver in between. If you take an analog signal from a level transmitter to a PLC then take an output from the PLC then effectively this PLC becomes a part of the safety circuit and must be designed/evaluated as a safety PLC according to the CSA B149.3-10 Section 9.7. The concern here is that in a general purpose PLC, which is also used for other processes in the plant the safety permissive signal may be too easily affected or overwritten by other processes or by online programming activities. There are also specific requirements related to the PLC design, software, functionality and wiring within the safety circuit. In general if a PLC is used for safety purposes it cannot be used for any other function.
Since the above requirements are difficult if not impossible to meet with multi function general purpose PLC, it is recommended that the analog signal from the transmitter be handled outside of the PLC. Following are some of the options:
1) Use a BMS controller with built in 4-20 mA input such as Profire 2100 (Profire 4-20 mA card has two LSHH outputs: one turns of the pump jack the other turns the alarm on, it also has an LSLL trip of the burner. 2) Use a 4-20 mA signal splitter such as Moore Industries MIX or MIT multi channel converter (one channel out to BMS with 4-20 mA, the other channel to PLC if you need more alarm points) 3) Use a 4-20mA programmable current safety trip alarm such as Moore Industries STA (set mA alarm an wire relay to BMS without 4-20 mA input) 4) Use a separate level switch in the tank wired directly to BMS. If 2" port is available you could use a Kimray, SOR, Murphy or similar switch. For 1" or 3/4" ports there is a Vega Swing 63 switch available.