1.07.031 LOGIX panel - How to adjust the Logix PID control settings (PID fine tuning for smooth control)

This Policy is under construction and is not completed Please Call Area Manager or Field Service Director for Verification of contents

Purpose

The purpose of this procedure is to establish the minimum requirements for fine tuning the Logix panel PID settings to provide smooth control of the Logix panel. This procedure provides an example using real time data from an actual compressor to illustrate how it can affect the controls in order to make speed and capacity changes extremely smooth.

Definitions

PID - Proportional-Integral-Derivative Controller is a control loop feedback mechanism. In this application, the PLC continuously compares a target with the actual value. the difference between the desired value and the actual value factors into a calculation to derive the proper adjustment. Large variances result in large adjustments, small variances result in small adjustments, thus resulting in smooth changes by the controller.

Cascading PID's - Engine speed governors are PID devices. the PLC output to control to the governor is a PID function. The speed target for the speed control PID comes from various other PID's. These include suction pressure target, discharge pressure target, tank pressure target, and motor current target. The use of one PID to provide a change target to another PID , which provides a control signal is a cascade of control loops. A third level in the cascade is that control signal adjusting the engine's speed governor. Cascading PID's can require some tricky tuning, but they also can reduce the effect of tuning errors, giving excellent control characteristics.

Proportional term - A factor in a PID calculation that effects how much change to make in relation to how much error is detected between desired target and actual value. A high proportional gain results in fast reaction, but can make it unstable. A low proportional gain results in slow reaction and possibly inadequate control.

Integral term - A factor in a PID calculation that factors the error that should have already been corrected. This factor affects how fast the calculation evolves as it gets closer to a perfect output. Higher integral gains result in achieving stability faster, but can result in overshooting the target.

Derivative term - A factor in a PID calculation that predicts system behavior by calculating the slope of the error over time. This factor can mitigate the overshoot caused by the Integral gain and can work together to get stable faster. The derivative gain is rarely used in practice and most defaults values in Logix panel are set to the lowest value

Procedure

This procedure applies to all units that are currently in service in the Field that have a Logix control panel. PID tuning is more art than in science. Default settings are generally good, but not perfect for all scenarios. Because it is easy to make instability worse, this feature requires access with a random password, which opens a line of communication to Engineering and technical support. Field Serve Technicians performing this procedure should not hesitate to call for assistance or guidance from Engineering or the Training support personnel.

This procedure needs to be read all the way before any adjustments are made to the PID loop setting in the Logix panel.

Before any work is performed a JSA needs to be done according to Flogistix procedure along with any Customer requirements before any work is started.

The settings for the PID Loop are not a set formula and are a general setting for unit conditions running on the Test Loop in Pampa Texas. If the RPM of the unit is holding steady and the PLC controls the unit for conditions the PID Loop does not need to be adjusted.

PID Loop settings should only be changed if RPM fluctuates and when changes are made the changes need to be in small increments. It is a good idea the record the original PID settings before any other changes are made to the PID Loop.

To initially set up the PID loop on a Gas Engine Drive go to the GAC Governor box in the Logix panel (see Example "A")

example "A"

Remove the top black plug cover. This plug covers the Speed, Gain, and Stability adjustment screws. With the plug cover off, inside the governor control box there will be a large rectangular Blue Box with two white adjustment screws. (see Example "B")

example "B"

Both of the white adjustment screws need to be turned all the way down in a counter clockwise position. Be careful when turning these adjustment screws they have a fragile stop point so do not force them past the stop position. When turned all the way down the both adjustment screws should be in the position as in Example "C"

example "C"

With the GAC Governor control box set go to the PID loop page in the Logix panel to make any needed adjustments.

PID overview page:

With the PID Loop screen visible as in this case, the Suction Pressure is the Primary controller and that is illustrated by having an Orange indicator needle on the gauge image as indicated for this example with a circle around the Suction pressure Speed Ref. (see example "D")

If the Discharge Pressure takes control because it needs to override the Suction Pressure, that PID gauge will have the orange needle and it will will be the function that commands the speed PID (Output to the engine governor)

example "D"

To adjust the PID function of any of the PID reference gauges simply touch the PID gauge that is needing checked or adjusted. For the next example the Suction Pressure PID will be used. It will be listed at the top of the PID page as Suc_Press_PID_Loop.

Proportional Gain C+1

This is the amount of change.

The larger the number the less amount of change. A larger number in the proportional gain results in a smaller reaction to variance.

The smaller the number the more amount of change. A small number in the proportional gain results in a larger reaction to variance.

The number can range from .1 to 999.9

Default is 10, but in a lot of cases if an engine is surging, start by changing the Proportional Gain to 100 to 125

Intergral Gain C+2

This is the dead band above the set point.

**Note in this example the Dead Band is not .1 pounds of suction pressure. the .1 is for a Speed Reference Dead Band

Derivative Gain C+3

This is the dead band below the set point.

Sample Period C+4

This is the sample rate in seconds

Direction Setpoint C+5

This is the formula to tell the PLC what direction for change

Default is 11

Range Setpoint C+6

This is the Physical Range

Default is 6296 ** This needs to be left at 6296

Lower Limit C+7

This is the lower limit of the RPM range or Pressure range

Upper Limit C+8

This is the Upper limit of the RPM range or Pressure range

In all cases, change one value first and watch it react, then try different operating scenarios before changing changing another value.

After making any changes toggle the on/off switch on the PID Loop Set up Programming Switch.

Do Not adjust any value other than the proportional and derivative unless instructed by Engineering.

The next examples are of a unit before any adjustment were made to the PID settings.

Notice the engine RPM speed fluctuating and not holding steady.

Engine speed trending before PID tuning

Also notice the Suction pressure fluctuating and not holding steady.

Suction pressure trending before PID tuning

Notice the Proportional Gain C+1 is set at 12.0

The proportional Gain C+1 was changed to 150.0

Notice how the engine RPM speed (example "E") and the Suction pressure (Example "F") is holding a more steady line

Engine speed trending after PID tuning

example "E"

Suction pressure trending after PID tuning

example "F"

Responsibilities

Management - Management is responsible for implementing, supporting, and enforcing the requirements of this procedure to their respective locations/area.

EHSS - Responsible to assist in the implementation of this procedure and to review for effectiveness.

Field Service Technicians are responsible for commissioning all gas compressors, new or used. This process includes properly setting all systems to function as needed based on the application.

Scope

This procedure applies to all Flogistix Field Service employees, contractors, and temporary employees that have access level to make changes in a Flogistix Logix panel.

Hazards

Employees should be aware that units could contain flammable and toxic gasses. Potential for the possible hazards of gas ignition or fire, hot surfaces & fluids, rotating equipment. Risk of possible concentration of H2S. Hazardous gases could be in the units pressure vessels, cooler, or associated piping and risk of inhalation of hazardous gas fumes.

Safety

If this procedure is performed in the field the following safety requirements are required.

All Personal Protective Equipment (PPE) for normal Field Mechanical duties apply for this procedure.

Four Gas Monitor, which is outlined in the Flogistix EHSS Manual, PPE Section 8, Procedure 8.3

Insure Flogistix personnel have proper safety certification or site orientation if required by the Flogistix Customer.

Insure Flogistix personnel adhere to all Customers safety practices.

Training

All employees shall be initially trained on the following topics:

• Employee Personal Protective Equipment (PPE)

• Emergency and First Aid procedures

• Methods and Observations that may be used to detect the presence or release of a hazardous chemicals or gas in the work place

Tools & Equipment Required

All employees performing this procedure shall be required to have the listed tools to perform job tasks:

• Access to Flogistix decoder for password to access PID Loop functionality screens.

Approved by:

Field Service approved by:

Date:

Engineering approved by:

Date:

EHSS approved by:

Date: