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How to lean P&ID?
5

How to lean P&ID?

How to lean P&ID?

(OP)
Dear all,

I‘m a new technician in a plant, some time i need to real P&ID to find out the sensor or valve, but lot of symbol i don’t understand and i have tried to search form google.but it failed.

Have any P&ID test book for a new? tiphat

RE: How to lean P&ID?

(OP)


On this picture why LSH have not a LE? Please

RE: How to lean P&ID?

Ken 1972,
I'll answer your second post first. This is a classic example of the common practice of using two different technologies to raise a high level alarm (I am assuming the LSH's are both connected to alarms) so that even if one of them fails, there is a high probability that the second will not fail simultaneously and therefore the alarm will still be raised. In the case you've drawn, the right hand hi level comes off a transmitter electrical output - most likely a top entry level transmitter is radar or similar technology - while the left hand one is most likely a mechanical switch such as a float.

In order to learn how to read P&ID's, your company (or the contractor who supplied the P&ID's) will have either a drawing or an engineering standard showing what each symbol on the P&ID represents. If you haven't been given this drawing or standard, I suggest you ask your supervisor to provide you with a copy.

I hope this helps - the ability to read P&ID's is certainly a valuable skill that I wish more technicians/plant operators possessed.

RE: How to lean P&ID?

(OP)
AlmostRetired,
Thank you for your detailed explanation.

Can I interpret it by that If a digital signal form a mechanical float switch on/off in P&ID, it just only a LSH not need LE symbol?
In the other hand, LE and LSH that is a sensor to give a analog signal so it have two symbol.

I have tried to ask my supervisor to get symbol list, but that list is so simple can’t to show all on it. That why i need too get a test book

RE: How to lean P&ID?

2
Hello again Ken,

Your interpretation of the difference between an analog and a digital instrument is more or less correct in this case, but showing them differently is not directly because one is digital and the other is analog. The level switch LSH will be a digital instrument in either case, but the left hand one is physically activated by the tank level raising a float, whereas the right hand LSH is reading a 4-20 mA analog signal and activating if that signal is above set point.

Another way to look at it is to remember that each instrument circle on the P&ID represents a separate device. So where there is just the one device, it generates the high level digital signal by itself, whereas with two separate devices as in your right-hand example, the high level digital signal is generated by a switch that is reading the 4-20 output of another device.

RE: How to lean P&ID?

(OP)
Thank you bow

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