Compressor Trip 3

[EEENGRX]

Attached is the shutdown sequence of a process reciprocating compressor. What is the likely cause for this compressor to trip judging by the shutdown relay logic attached?

 

[che12345]

Dear Mr. oosani (Electrical)(OP)11 Nov 21 18:52
"... Attached is the shutdown sequence of a process reciprocating compressor. What is the likely cause for this compressor to trip judging by the shutdown relay logic attached?..."
1. Sorry, I can't follow the shutdown sequence ladder diagramme. Please check it through thoroughly. Otherwise any suggestions would be meaningless/misleading.
2. Please advise whether the RESET is a N.O. [ self-reset to N.O. push-button]?
2.1 When do you press the RESET push-button, a) every time before a healthy normal start (unusual!) or b) only when there is a fault shut-down.
3. Why 1st , 2nd stage temp contacts shown in N.C. ; while two LSHH (13-14) , (17-18) are shown N.O. and lub oil shown N.O. ?
4. BTW for information only: very often, compressor motor (trips on over-load) (or unable to pickup speed or long starting time) during starting; when the [starting by-pass pressure switch failed].
Che Kuan Yau (Singapore)

 

[FacEngrPE]

Compressor trips happen because one of the operating parameters is out of bounds. It can be any of them. As the ladder logic does not contain a first out function, you get to troubleshoot the old school way. Here is an example

http://www.plctalk.net/qanda/showthread.php?t=23974.

 

[itsmoked]

The utter lack of any time delays. There are lots of transients in systems like compressors.

I'd expect the lack of what's called a "blinding" on the oil pressure to be tripping off the system on shutdown.

Blinding is when a timer is used to block alarms from being triggered. For example if a water pump is commanded on there can be a pressure surge during the startup that trips a non-blinded high-pressure alarm. Instead, the high-pressure alarm should be blinded for a second or whatever time it takes the startup transient to pass.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[davidbeach]

People that “know” control systems come up with rather different results than people who build controls for systems they know.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[waross]

That drawing is so incomplete as to be almost worthless.
Most likely cause of tripping based on the information given?
1. Something wrong, out of range or broken on the compressor.
2. Something failed in the control circuits.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[electricpete]

It looks to me that the circuit does provide logic to tell you which trip inputs have been receieved (up until the reset button is pushed). But as FacEngrPE mentionedt, it doesn't tell us which came first, so if there are multiple trip signals received (maybe the trip itself causes other trip conditions to occur like low lube oil pressure if there is a shaft driven oil pump) then it might be harder to interpret.

Here's the way I think that circuit is intended to work. Pushing the reset button energizes CR6 and CR7 which closes a path to energize output relays CR1 through CR5 which closes the associated CR1 thru CR5 "NO" contact, which maintains those relays energized after the reset button is released. This implies to me that the input contacts (1st stage temp, 2nd stage temp, etc) are closed in the safe state and open in the unsafe (needs tripping) state... and the output relays CR1 thru CR5 initiate trip when any of them is deenergized. That's the only way the circuit makes sense to me (otherwise the reset function doesn't make any sense) but you (op) could certainly look at other portions of the circuit to validate or refute my assumptions.

So during the period after the trip and BEFORE anyone presses reset, you should be able to determine which input contacts sent a trip signal by looking for which output relay(s) among CR1 thru CR5 are deenergized. If you can't tell visually by looking at the relays (or something fed by the output of the relays like a light or an alarm) then you can measure the voltage at terminal point 2 of the relay... it should be close to the left bus voltage for the relays that remain energized (never received a trip signal from associated input contact) but close to the right bus voltage for the one (or more) relay(s) that is in the deenergized condition after having received a trip signal from the associated input contact.

Maybe all this is already obvious. But it's the only thing we can say from your drawing. We can't determine the likely cause without inspecting the relays or knowing those voltage measurements. And if the reset button has already been pushed (it probably has to be pushed to restart the machine) then you're out of luck figuring it out from examining this circuit. In that case look for other evidence... maybe the switches or the output relays send signals to other circuits that might create a computer record.

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(2B)+(2B)' ?