4 replies to this topic

[nybrla]

Greetings,

 

I'm designing a system where n-C4 is injected to a natural gas pipeline to achieve a certain mol% of C4 in the natural gas stream. GC reading (call it AE-1) will be taken 30 miles upstream of the injection point to determine the required injection rate, and a VFD on a diaphragm metering pump will be speed controlled to that flow. Another GC sample (AE-2) will be taken downstream of the injection point, to "verify" that the injection resulted in the correct mol%. I have the following questions regarding how to design a control loop of this system:

 

1) Since the flow rate is directly proportional to the VFD speed, is there a need to install an FIC on the discharge flow? Or can the speed of the pump directly controlled by a PLC logic based on AE-1 readings?

 

2) How can AE-2 readings be incorporated into the control loop? I'm thinking AE-1 going into a feedforward control of the motor speed that's somehow linked to a feedback control of AE-2 with the mol% setpoint. If someone could explain how this loop would actually work and shown on a P&ID, that would be very helpful.

 

Thank you in advance!

 

 

 

,      (AE-1)                                                                        (AE-2)

.           |                                                                                |

==============================================================> NG Pipeline

.                                                                 ^

.                                                                  |

.                                                                  |

.                                                             C4 Injection

.                                              (Metering Pump VFD controlled)

[ankur2061]

Nybria,

 

In normal process plant controls, combined feed forward and feed back controls are quite rare. However, a special application as described by you probably warrants such a complex control. Refer the links below which shows a combined feed forward and feed back control system. What you need to do is do some control engineering adaptation from the control loop diagrams shown to suit your application:

 

http://www.bgu.ac.il...s/Chapter_9.pdf

 

http://blog.opticont...k&search=Search

 

http://controlguru.c...r-architecture/

 

Hope this helps.

 

Regards,

Ankur

Edited by ankur2061, 05 June 2016 - 06:42 AM.

[fallah]

nybria,

 

1) No need to control the flow at metering pump discharge line...the flow will be controlled by a control logic based on AE-1 reading...

 

2) Reading of AE-2 can be terminated to local and/or remote alarm through a dedicated transmitter when there would be higher absolute difference value (between the reading and the set point) than a pre-specified +/- tolerances...

[nybrla]

Thank you for your replies.

 

Ankur - The links you provided are very helpful in understanding the combined feedforward-feedback control structures. I will review these and attempt to build a control loop that suits my application.

 

fallah - Thanks for confirming that flow measurement does not need to be directly linked to the VFD speed control. Triggering an alarm when AE-1 deviates from the setpoint by a prescribed tolerances is a good recommendation and we will put that in place. That will be in addition to using AE-1 reading as an input to the controller for the feedforward-feedback loop.

 

Hopefully I can share where I get in several days. In the mean time, if there are any more pointers, I'd appreciate it. Thanks.

[Bobby Strain]

But you are still missing a critical control element. You must also measure the gas flow and pressure. Use this to control the injection pump speed. Then use the analyzer to adjust the ratio of injection to gas flow.

 

Bobby