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Pump ON/OFF Control P&ID symbols

Pump ON/OFF Control P&ID symbols

(OP)
I'm not a controls engineer, and my company doesn't have one on staff, so I need some help. I have a scenario where I have a pump suction tank. The tank has 4 float switches installed. One switch is LSL which turns the pump off on low level. The next switch is LSH which turns the pump on on high level. The third is LSH which turns on an indicator light on a pump panel. The last switch is a LSHH which turns on an indicator light on a pump panel and also sends a signal to an autodialer. We currently show the LSL and LSH that turns the pump on or off with an LSL bubble that has a dashed line going to one side of the motor with the word "OFF" and an LSH bubble that has a dashed line going to other side of the motor with the word "ON". My manager thinks that this is not the correct way to show this. Is he right? If so, what the correct way to show this on a P&ID?

Thanks very much for your help.

RE: Pump ON/OFF Control P&ID symbols

(OP)
Forgot to mention that my manager's point is how can we have a LSH that turns the pump on, and another LSH that illuminates a light indicating a possible problem. He seems to think that we should show a LSH that turns the pump on, a LSHH that illuminates the light and a LSHHH that illuminates the light and dials the autodialer. I told him that I didn't think there was such a thing as an LSHHH bubble on a P&ID.

RE: Pump ON/OFF Control P&ID symbols

Hello jkate,

I'm not a controls engineer either, and I have no intimate knowledge of P&IDs...but I still hope to contribute something of value notwithstanding.

Within my discipline an LSH typically delivers a 'high level' alarm of some description, and an LSHH delivers a 'high high level' alarm; similarly for LSLs and LSLLs.

My company generally takes the view that it would be confusing to have a switch labelled as LSHH - meaning with two aitches - deliver a 'high level' alarm with only one aitch; similarly a three-aitch device delivering a two-aitch alarm...which seems to agree with your manager's viewpoint.

The approach we use in such situations is to choose words that have different last letters, and use the last letter of those words. For instance: since the first letters of In and Out can easily be confused with the numbers 1 and 0, we instead use their last letters, such in the case of power flows from a generator where the real power output is denoted, by way of example, as 250 MW T and 20 MX N.

I no longer have access to our control elementary diagrams, and can't confirm what we do in fact use, but since Start and Stop both begin with the same letters, and a hand-written S can be misinterpreted as the number 5, and since On and Off both begin with the letter O, easily misread as the numeral zero, LSN and LSF could be used as Level Switch ON and Level Switch OFF....but this is only a suggestion; if there's a prevailing standard out there that addresses all of this, I'd certainly like to know what it is.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

RE: Pump ON/OFF Control P&ID symbols

(OP)
Crshears:

Thanks for your reply. That's a good suggestion and I'll present it to my manager and see what he thinks.

Any other thoughts out there?

RE: Pump ON/OFF Control P&ID symbols

You can use an LC with a line labeled START for the low level switch, another LC with a line labeled STOP for the first high level switch, with both lines feeding into a diamond with an I (for interlock) in it, and the output of the diamond feeding to the pump. This is how differential control of a pump is shown as an example in the superseded version of ISA 5.1 (1984-R1992). That leaves you free to use LSH and LSHH for the other two switches. The older version of the standard says you should use no more than four letters for the description in the tag, but doesn't say you can't use more. I'd use no more than four, however.

In the 2009 version of ISA 5.1, there is no example to show differential pump control. If you want to use letter suffixes, you could use LSL-01A, LSH-01A, LSHH-01A, and LSHH-01B to tag your devices. In that case, I'd use the LSL for the low level switch and the LSH for the first high level switch, with both lines from each bubble feeding into a diamond with an I (for interlock) in it, and the output of the diamond feeding to the pump. That would be clear enough for most instrumentation and control engineers.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Pump ON/OFF Control P&ID symbols

(OP)
Xnuke:

Great suggestion as well. Thanks very much!

RE: Pump ON/OFF Control P&ID symbols

Looks okay. Since ANSI/ISA 5.1-2009 shows "N" is a user's choice modifier, and "F" is undefined as a modifier and therefore LSN and LSF are nonstandard abbreviations, I'd be sure to put them on a legend somewhere as meaning Level Switch On and Level Switch Off.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Pump ON/OFF Control P&ID symbols

If it were me and it isn't, I would follow xnuke's example from the 2009 version of ISA 5.1. I would take care of the on/off details on a loop sheet or wiring diagram of the motor controls. The switches will be normally open or normally closed and should be fail safe. All that should be documented on loops and wiring diagrams.

To me, it's best to follow standards and not take care of too many details in the legend. If you follow ISA, LSL has a definite meaning as do the other nomenclatures. The position of the characters have meaning as do the characters. You want your P&IDs to be as clean as possible. But that's my training, from years ago.

I did not look at your file. Apologies.

Pamela K. Quillin, P.E.
Quillin Engineering, LLC

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