Compressor and Receiver Pressure issue 2

[nkotari]

Hi,
We have 4 screw compressors of approximately 1200 SCFM, one compressor on VFD.
We have continuous variable demand downstream of receivers from 5000 SCFM to 1500 SCFM.
When we start the compressors with variable setpoints for all compressors 7 bar max pressure for one of the compressor, all the compressors which are in load condition at that time, are showing a pressure reading of 6.5 to 6.4 bar when in load condition, whereas receiver is showing a pressure of 7 Bar constantly, we have two receivers connected in parallel to discharge header of all 4 compressors.
Generally, when there is a pressure drop, the compressor side which is the generating end should be having higher pressure and receiver should have lower pressure. Is there any logic for this reverse scenario happening?
The media is air and each compressor set point measures the value at the individual compressor discharge before check valve.
If there is any data needed, please let me know.

 

[1503-44]

A diagram would help to explain your arrangement more clearly. The exact locations of where your gages and check valves are is very important.

Otherwise, I dont think it would be unusual to expect that your 7 barg compressor will charge until the receiver has 7 barg pressure.

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I take no responsibility for any damages or injuries of any kind that may result.

 

[LittleInch]

Calibrate your instruments using a dead weight tester.

Then see what they say.

A 10% drift isn't uncommon or the setting on the input parameters into the DCS is incorrect for one or more of your transmitters if the dead weight test comes back OK.

Happened more than once that when someone loop checks the instrument they find some setting is different between the what the instrument is reading and what the control system thinks it is.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

Ahhh yes. As they say, "With two watches, you never know the correct time."

If you think I am wrong about anything I say, then please do correct me, but please also refrain from lecturing myself, or others here, in the preschool level details of chem, math and physics. Thank you. I will try to do the same.

 

[horacio Torres]

I will recommend the following

1. Check the pressure transmitters instruments. . 0,5 bar differential pressure is high for a compressor header. Always instrument workers say that is fine, and is it not 100% true.

2. Use a calibrated PI manual indicator to measure all pressure points and compared with the reading in the DCS, someone is giving a false reading.

3. I have seen in some process plants that the PM set in the pressure transmitters configuration was not set at the right number, giving wrong pressure reading.

If you share a process drawing and pressure place reading, we could help you more accurate

Horacio

 

[nkotari]

All our gauges are regularly calibrated. There is no chance of instrument error.
There is no DCS and each compressor is controlled by its own OEM inbuilt PLC and HMI.
Each compressors transmitters have to be verified, this point is noted.

Refer the sketch.

Set points: Compressors numbered from top to bottom
Compressor 1:- Load Pressure 6.4 bar Unload Pressure 6.8 bar
Compressor 2 (VFD):- Load Pressure 6.5 bar Unload Pressure 6.8 bar
Compressor 3:- Load Pressure 6.3 bar Unload Pressure 7 bar
Compressor 4:- Load Pressure 5.5 bar Unload Pressure 6.2 bar

Constant Pressure in Receivers always:- 7 bar

As per my knowledge, no compressor has to have lesser pressure than receiver, by which there will not be any push from compressor to receiver. Strange that it is in reverse scenario.

inlet Header-

 

[1503-44]

LittleInch, Maybe I have to take your star away. No. Still a good answer.
But I think could be what I thought, The 7 barg Compressor is always topping off.
The gages are reading pressure delivered at each compressor's discharge, not header pressure, because the gages are upstream of the NRVs.
The gage pressures are slightly higher than the compressor setting because the compressors have to work a bit harder to try to keep up with the 7bar compressor. When the lower pressure compressors can not reach higher pressures of the 7barg compressor, the NRVs close and the 7bar compressor tops off the receivers at 7 bars.

Nkotari,
Can you please take the 7 barg compressor offline and let us know what the gages read then.

If you think I am wrong about anything I say, then please do correct me, but please also refrain from condescending lectures to myself and others here, in the preschool level details of chem, math and physics. Thank you. I will try to do the same.

 

[LittleInch]

Well it might sound a bit odd but have you actually personally checked that the isn't an NRV in the line into the air receivers or hidden inside the receiver?

Often receivers will often have an inlet and outlet line and are intended to be a reserve and hence prevent backflow of the inlet line.

The fact that your receiver pressure doesn't vary tells me they are isolated from the header fluctuations

At the moment I can't see any other reason unless someone has closed the valve on either the air tank OR the pressure guage on the receiver??

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.