Reciprocating compressor pressure control 1

[Abu najad]

We have make up gas compressor reciprocating with three stages and three trains.suction pressure 22 bar and discharge pressure 185 bar
It has common and individual spillback.second stage is fed to diesel unit.
What is the purpose of individual spillback and how we can control the pressure to diesel unit?and can you explain control scheme in attached figure?

 

[georgeverghese]

Pressure control to DHT seems to be ok at first glance. High(??) pressure on supply line to DHT results in recycle to stage 1 suction. Low pressure on this line results in individual spillback at each stage.

Individual spillback is only for stage 2 - for low suction pressure - this is required to account for possible high draw rate from DHT. There is an error in your sketch here - spillback from stage 3 suction drum should be to stage 2 suction drum, not to stage 2 discharge. But this spillback line may not be necessary.

I can see a problem with controller interaction with low pressure recycle since you have individual recycle at each compressor, which could result in unstable control when DHT draw rate is high.

Can you clarify "??" marked above. The control scheme for supply to DHT needs to be modified.

 

[georgeverghese]

Based on what I see now on this sketch, would suggest
a) delete the recycle line across stage 2 for all 3 machines
and
b) Add a new PCV on the common feed line to the DHT - this PCV will be controlled by 2 controllers - an upstream low set PIC which maintains stage 2D pressure, and a high set PIC downstream to throttle supply to the DHT if consumption rate is too low. Both controller outputs directed to a signal selector which then feeds the PCV.

 

[Abu najad]

Dear georgeverghese
There is to pressure controller located at make up gas compressor KOD drum .PC100A set with low set point and PC100B set with low low set point . If the first stage compressor suction pressure falls too low, suction line pressure controller PC100A will open the common spillback PV105and recycle the gas to the inlet of the Make-Up Hydrogen 1st Stage Suction Drum. If the first stage compressor suction pressure still continues to fall too low, suction line pressure controller PC100B will open the 2nd stage spillback valves. To prevent compressor first stage from overpressuring, the simple protective control loop PC600shall be utilized. If compressor first stage suction pressure rises above the set point of the protective pressure controller PC600,then the pressure control valve on vent line opens. This is a simple control loop and it is not developed at this document.
Compressor third stage discharge pressure is maintained by pressure controller PC200.If compressor third stage discharge pressure rises above the set point of the pressure controller PC200then the pressure control valve on 3rd stage spillback line opens. The higher value of PC200 output and first stage protective pressure controller PC100A output is selected via high selector PY-200The purpose of the selector block is to prioritize compressor protection over delivering the hydrogen to the HCU Unit reaction section loop.

Compressor second stage discharge pressure is maintained by pressure controller PC225 .If compressor second stage discharge pressure rises above the set point of the pressure controller PC225,then the pressure control valves on 2nd stage spillback line open. The higher value of PC225 output and first stage protective pressure controller PC100B output is selected via High selector PY225.The purpose of the selector block is to prioritize compressor protection over delivering the hydrogen to the DHT Unit reaction section loop. Hydrogen delivery pressure to the DHT Unit .

To prevent compressor second stage discharge from overpressuring, the simple protective control loop PC400 shall be utilized. If compressor second stage discharge pressure rises above the set point of the protective pressure controller PC400,then the pressure control valve on vent line opens. Setpoint of the protective controller PC400 shall be set higher than the setpoint of the controller PC225.
Why we select high selector?
If PC225 PV increases then controller output OP has to increase or decrease? And PC100A if Process value increases or decreases what will happen to output should be increases or decreases?
Regarding your comments I think the individual spillback back is given to third stage suction because we drawn H2 to DHT from second stage incase of suction pressure for third drop.

 

[georgeverghese]

Think there is some typo errors here:

"To prevent compressor second third stage discharge from overpressuring, the simple protective control loop PC400 shall be utilized. If compressor second third stage discharge pressure rises above the set point of the protective pressure controller PC400,then the pressure control valve on vent line opens. Setpoint of the protective controller PC400 shall be set higher than the setpoint of the controller PC225 PC200."

Re your query
"Why we select high selector?
If PC225 PV increases then controller output OP has to increase or decrease? And PC100A PC100B if Process value increases or decreases what will happen to output should be increases or decreases?"

Though your sketch is not complete, I can guess the following:
High selector is used because of the controller behavior selected. You can change to low select also, but controller behaviour then has to be changed.
In your case, if PC225 PV increases, then controller PC225 is DIRect acting and output will increase. For PC100B, if PV decreases, output is increasing - controller PC100B is REVerse acting. Then the output from high select PY225 is reversed and used to operate fail open PCV 225 on stage 2.

If you implement the changes I had suggested earlier, then PC100B and PC225 and PCV 225 can be deleted.

And all of this not required: " Compressor second stage discharge pressure is maintained by pressure controller PC225 .If compressor second stage discharge pressure rises above the set point of the pressure controller PC225,then the pressure control valves on 2nd stage spillback line open. The higher value of PC225 output and first stage protective pressure controller PC100B output is selected via High selector PY225.The purpose of the selector block is to prioritize compressor protection over delivering the hydrogen to the DHT Unit reaction section loop. Hydrogen delivery pressure to the DHT Unit".

 

[Abu najad]

Hi georgeverghese
I agree with you PC100B should be reverse action and PC225 should be Direct acting.And what about PC100A and PC200 which one is must be Direct action ? Because PC200 set point is output of recycle gas compressor suction (Hydrocraker system pressure PC305)
For example if Hydrocraker system pressure drop two bar first priority should be for make up gas compressor to avoid compressor trip.if suction pressure not healthy will not allowed the spillback to close if suction pressure is healthy spillback can be close to Hydrocraker system pressure same while for DHT.
You suggested before to delete PC100B,PC225 and PCV225.
Please can you draw sketch for your suggestion?
Thanks a lot dear

 

[georgeverghese]

"And what about PC100A and PC200 which one is must be Direct action ?"

Since recycle valve PV105 is most likely FO, then it is likely the arrangement is same as that for stage 2 recycle i.e. PC200 will be Direct acting, PC100A will be reverse acting, and selector will be high select.

Deletion of PC225, PC100B and PCV 225 is straightforward enough. Attached is the drawing for new controls to be included on common supply line to DHT.

 

[Abu najad]

Hi Dear,
Thanks for your fast response
Which I understand from sketch if PC20 pressure falls below set point output has to be increased and PV for PC10 also PV will come down because both PC10 and PC20 are taking PV from same line .So if PV for PC20 falls down same while PC10 which is controlling second stage discharge pressure will fall also.please can you explain me more in different scenarios?

 

[georgeverghese]

Initial condition : PCV wide open
If this line pressure drops, then output of PC20 is high. Output from PC10 is low. PC10 will be selected and low signal will tend to close PCV in order to maintain minimum permissible 2nd stage discharge pressure as set at PC10
If this line pressure is high, output from PC20 is low, output from PC10 is high. PC20 will be selected and PCV will close based on PC20 control in order to prevent high pressure in H2 supply to DHT. I dont know if there is a case for high pressure in this line(downstream of this PCV), since I dont know what is the range of operating pressure in H2 feed to DHT in comparison to range of operating pressure on 2nd stage discharge. If there is such a case, then a common cold recycle controller PC226 is required and existing PCV 225 may be retained to send excess gas back to stage suction when DHT consumption is low. See attached modification "A" to previous sketch. The action of this recycle controller is similar to that for PC225, except that this controller is on the common DHT supply line.

The alternative to "A" is to retain your existing PC225 and PCV225 controls on each stage, but there will be instability due to these 3 controllers interacting with each other - is this the case in actual operations?

Further, I think we need to insulate the 3rd stage discharge for this makeup H2 compressors from variations in hydrocracker reactor pressure. Right now, there is no low pressure control protection on stage 3D. This is necessary to prevent stage 3D pressure fluctuations ( originating from hydrocracker reactor) from affecting stage 2. High pressure protection control at stage 3D is already enabled by PC200. See sketch B for this. I suspect such low pressure protection is not required for your recycle gas compressors since there is no intermediate draws or feeds to these recycle gas compressors.

 

[Abu najad]

Thanks Dear
I am so happy with your answers
But there is protection controller for 3rd stage suction by individual spillback .

 

[georgeverghese]

3rd stage suction spillback is PC225? That is meant for high pressure at this location for stage 2D, as you described earlier.

There is no low pressure protection for stage 3D existing. This will be required when hydrocracker draws more H2 than the compressor can deliver at 3D, pressure here will collapse. This in turn will collapse the pressure on 2D also, so there is knock on effect at DHT supply when there is fluctuation in hydrocracker H2 demand. This will not happen if you install this new PC205 on sketch B.

Regarding the protection controls for high pressure at stage 2D, there is another alternate to the PC226 that I included in modification A. This alternate is better than this modification A and also better than the existing PC225 arrangement. I can sketch this out tomorrow morning if you are interested.

 

[Abu najad]

Thanks dear
Appreciate your efforts to help me
Tomorrow I am waiting for your sketch
Please in your sketch you have to mention individual spillback for third stage and PC located at third stage suction.

 

[georgeverghese]

So this sketch C is preferred alternate to modification A also. PC100B also not required

 

[Abu najad]

Thanks a lot Dear
So I have to combine Sketch B and C.

 

[georgeverghese]

Yes. Sketch B will protect the compressors when hydrocracker pulls more gas than is produced by makeup gas compressors. This will prevent knock on effect of low pressure at stage 2D for DHT supply.

And sketch C is for the case when DHT pulls less gas than produced at stage 2D (same as stage 3S) - note this is for recycle of COLD H2 back to common stage 1 suction KOD

But you also need the low pressure and high pressure controls shown for HOT H2 to DHT (PC10, PC20)shown on the first sketch (but without the clouded portion marked A - PC226):-
a)for low pressure protection of stage 2D ( by action of PC10) for the case when DHT pulls more gas than is available at common stage 2D collection line. This will also prevent knock on effect of high compression ratio for stage 3 which could lead to high discharge temp trip for stage 3.
b)PC20 MAY be required if the setting of PC 226 (in sketch C) is higher than the max permissible normal operating pressure of the hot H2 feed to the DHT.

 

[Abu najad]

Thanks a lot georgeverghese for your response on the topic. I could understand and feel the concept now.

 

[georgeverghese]

In sketch B, the location for new PC205 and PCV 205 should be downstream of existing PC200.