LZIT what is it?

[leomiranda]

I have one P&ID in my hands and there is one vessel with one instrument that has the Tag LZIT.
I don’t know what the “Z” means! I know LI, LIT, LG, but LZIT…. I don’t know what this level instrument does exactly. Can somebody help me?

 

[JLSeagull]

I would expect this to be associated with the control valve position. Z is often used for position. (Think X Y and Z coordinates.) If the level control valve included a position transmitter I would expect the tag to be LZT or perhaps LZIT. Few credible ideas in the typical level process variable measurement come to mind.

 

[controlnovice]

Sounds redundant to me.

L - Level
Z - Position
I - Indicating
T - Transmitter

Since it is a Level transmitter, a LZ seems redundant. Position of the level?

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[JLSeagull]

Many upstream applications rely more on API RP 14C nomenclature than ISA. API RP 14C has been moving toward ISA but many shutdown valves are tagged XV with the position switches tagged ZSO and ZSC for open and closed. This nomenclature is frequently used in refinery and chemical projects too. So, LZIT as a control valve position transmitter on the LV seams reasonable enough to me.

 

[controlnovice]

JLSeagull - seems reasonable.

Why not just ZIT?

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[JLSeagull]

This is where ISA and loop numbering gets interesting. Some projects dedicate the loop sequence number. For large facilities the loop numbers are duplicated for each first alpha such as F-101, P-101 or T-101. It is desirable to keep the loop together using the first alpha for all components in the loop.

 

[leomiranda]

The guy responsible for the execution of the P&ID told me this is used to indicate that the signal will be used in shutdown. If is one LZIT, it means that the signals generated (LSLL and LSHH, for example) must be used to generate shutdown. This is new to me.

 

[JLSeagull]

Typically the system symbology reflects whether the signal is used for regulatory control (basic process control system such as the DCS); or safety shutdown control system also known as a safety instrumented system (SIS). Nothing related to the tag name LZIT would suggest that it is a shutdown instrument. However tags such as LSHH and LSLL would suggest digital contacts that open to initiate a shutdown. Most companies are moving away from discrete devices to analog signals for shutdown system inputs.

Perhaps you could obtain a broader explanation from the person developing the P&ID's.

 

[KiwiMace]

Or using the Occam's razor approach; the Z key is located in close proximity to the shift key...