High LPR suction pressure

[stanthonycold]

Good Morning,

I was wondering if anyone can tell me why the suction pressure is so high (gauge )on our LPR.(seq. suction pressure)Suction pressure is normal on our screw.Compressor suction is at 2-3" hg where the lpr suction on gauge ia always 5" hg higher.I may be wrong but shouldnt lpr seq suction be the same as compressor ??

Thanks

[brian_chapin]

Assuming no significant restrictions between the two and that the LPR is where the compressors get their suction you should see very little difference.

Have you calibrated the gauges and sensors taking the readings? It's not unusual to find them off-calibration and it is my practice to calibrate them annually.

[stanthonycold]

Assuming no significant restrictions between the two and that the LPR is where the compressors get their suction you should see very little difference.

Have you calibrated the gauges and sensors taking the readings? It's not unusual to find them off-calibration and it is my practice to calibrate them annually.

Thats what i thought ,yes i calibrated the sensors yesterday and when i came in today the ccs was saying 15 hg but gauge on lpr read 10 hg.When i calibrate the sensors on both compressor and LPR from gauges there is still a difference of 5 hg than compressor suction and dont know why!

[Terron]

Try re-setting your static set points for your CCS, (enter in the values on the pressure transducer). This will give you a true reading of the pressure transducer for the SEQ pressure. Then calibrate the Dynamic pressure to match your booster suction. If the span starts to vary shortly after, I would replace the pressure transducer, it maybe going bad. When you re-calibrate it should stay very close to the pressure you calibrated it to. If not...it's showing signs of going bad.

[Grizzly]

Where are the 2 readings taken from exactly?

Do you have a suction strainer in-line?

Which could be blocked partially. Or a restriction somewhere.What is your working level like?

Is this a new phenomonen?

Grizzly

[Sandro Baptista]

Where are the 2 readings taken from exactly?

Do you have a suction strainer in-line?

Which could be blocked partially. Or a restriction somewhere.What is your working level like?

Is this a new phenomonen?

Grizzly

Grizzly, that it can be possible as you say or if it's that so the measure error of the values is bigger.

See below what stanthonycold has said:

"Thats what i thought ,yes i calibrated the sensors yesterday and when i came in today the ccs was saying 15 hg but gauge on lpr read 10 hg.When i calibrate the sensors on both compressor and LPR from gauges there is still a difference of 5 hg than compressor suction and dont know why!"

[Terron]

The Seq. Suction pressure is usually taken off the common suction header not the pressure vessel or accumulator it's self. If you have a pump down valve tied into your suction header...try putting a calibrated gauge their and see if it matches your compressors suction readings. Simple things such as defrost cycles, liquid feed, and Evap. loads will make pressures slightly different at the L.P.R. than at the compressors.

[stanthonycold]

Try re-setting your static set points for your CCS, (enter in the values on the pressure transducer). This will give you a true reading of the pressure transducer for the SEQ pressure. Then calibrate the Dynamic pressure to match your booster suction. If the span starts to vary shortly after, I would replace the pressure transducer, it maybe going bad. When you re-calibrate it should stay very close to the pressure you calibrated it to. If not...it's showing signs of going bad.

I dont understand what you mean by the values on the seq suction transducer.Do you mean the reading in the ccs?

[Terron]

The object reading the pressures for the ccs. It should have values on the side of it that read 0 to 200# or 0 to 500#. You are running a computerized system right? If so, if you go into your program get to your calibrate sensors screen, under seq. pressure, (Static sensor adjustment) enter the values on the side of the pressure transducer. This will zero out any offsets and give you a true reading. Then follow the previous reply

[stanthonycold]

The object reading the pressures for the ccs. It should have values on the side of it that read 0 to 200# or 0 to 500#. You are running a computerized system right? If so, if you go into your program get to your calibrate sensors screen, under seq. pressure, (Static sensor adjustment) enter the values on the side of the pressure transducer. This will zero out any offsets and give you a true reading. Then follow the previous reply

ok on the seq transducer it says
input 8-30
ouput 4-20
range 185 + vac.

Whats there now is min. -2.1 and max. 196 psig

enter all this in static sensor adjustment?

[Terron]

Make sure you right down your values before you change anything. My sensor for instance says P51-200A-A-PO-20ma. I know that this is a 0 min and 200 max. So under static adjustments I would enter the values. This would give me the actual pressure the transducer is reading under seq. suction. Now if that value is off from the gauge I would go to dynamic adjustments and calibrate. I would be guessing but it seems your sensor is a min 0 and max 185. Call your parts supplier and ask him what they are for sure and calibrate your pressure transducer from scratch. After you do all of that and it still gets out of sink, I would replace the part. Its a sign its going to fail in the near future. Have you taken a gauge reading from your common suction header?

[Terron]

by the way....anytime you adjust the dynamic value your static value changes as well. So don't get alarmed if you check your static adjustments and its off again after re-calibrating. I am assuming you have a shut off valve or henry valve before your pressure transducer right? If so, remove your pressure transducer add a tee and put back your pressure transducer and add a gauge. That way...you will be sure it's reading the right pressure.

[stanthonycold]

Make sure you right down your values before you change anything. My sensor for instance says P51-200A-A-PO-20ma. I know that this is a 0 min and 200 max. So under static adjustments I would enter the values. This would give me the actual pressure the transducer is reading under seq. suction. Now if that value is off from the gauge I would go to dynamic adjustments and calibrate. I would be guessing but it seems your sensor is a min 0 and max 185. Call your parts supplier and ask him what they are for sure and calibrate your pressure transducer from scratch. After you do all of that and it still gets out of sink, I would replace the part. Its a sign its going to fail in the near future. Have you taken a gauge reading from your common suction header?

Yes i got a gauge on it,like you said it is right on the money for a few days but gradually starts to drift off.I guess i will have to change the transducer and cable.Thanks for your input appreciate it

[Terron]

No problem.

[stanthonycold]

No problem.

1 more question lol

ok let me get this str8

Im fairly new to this profession

So should all the settings in the static sensor page be what is on each transducer for that particular piece of equipment??The max and min i mean ?for example pressure across filters should that be 0 min 200 max(or whatever values are on transducer) for static setpoints?

[Terron]

In a perfect world....yes. For instance, if you install a new pressure transducer. You would want to reset the min and max values for that particular item because, if the previous pressure transducer was ever calibrated via the dynamic adjustments it would offset the static values as well. They are linked together, so one will affect the other. If your readings are accurate I would not worry about adjusting them. Like I said, the static values will change every time you adjust/ calibrate the dynamic values. They are no different then temp. blocks or RTD's adjustments.

[Terron]

I would advise you to right down every adjustment you do or back up/ save your system files before you go tinkering. Do me a favor....pick something that is off line, write down the suction (static) values min and max. Now on the same piece of equipment write down the (dynamic) values. Adjust the dynamic value and check the (static) set points. They will have changed.

[Rcsac]

Before getting to crazy in my past experience I've found oi accumulation in the control line to the transducers. I like to put the compressor in a positive pressure state and purge the lines occasionally. Hope it helps you out.

[sterl]

RCSAC has got the idea...PT's don't often drift, they just crap out. And this one holds a setting for a few days and then reverts to being "off", right?

Check the PT and location for the PRU for the capability to "slop" refrigerant and hold oil...If its on the end of a piece of tubing, see if part of the tubing, the low area, tends to stay colder or warmer than the rest....usually its colder.

If so you may have oil and you may also have water in that section of tubing.

[Terron]

Stanthonycold.......keep us posted on the progress and what your solution was, Thanks.

[brian_chapin]

In a perfect world....yes. For instance, if you install a new pressure transducer. You would want to reset the min and max values for that particular item because, if the previous pressure transducer was ever calibrated via the dynamic adjustments it would offset the static values as well. They are linked together, so one will affect the other.

Quite right. Also, although it should go without saying, be careful to make sure you are using the correct sensor. I recall a facility that had two sensors that looked identical but the ranges were 20"-150PSIG and 0-300PSIG. You wouldn't belive the chaos that caused! Eventually the facility changed brands on one of the sensors to avoid accidents.

[chilldis]

just another idea to throw in to the mix. I have installed a drip leg (like a gas valve) in my suction transducer lines. this catches the oil and gunk that can end up at the transducer fitting.