Automatic Idle Time

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					                                                                                                 Automatic Idle Time




                        Automatic Idle Time
Table of Contents
Automatic Idle Time Introduction & Overview .....................................................................1
    Rod Pump Control Approach.........................................................................................1
    Idle Time Configuration..................................................................................................1
    Determining Idle Time....................................................................................................2
Provision for Over-Pumped Wells.......................................................................................3
Programming Parameters ...................................................................................................4
Activate Automatic Idle Time...............................................................................................5
Disable Automatic Idle Time................................................................................................6
Table of Tables
Table 1       Default Idle Time Settings...................................................................................1
Table 2       Parameter 37 Settings and Definitions...............................................................4
Table 3       Idle Time Abnormal Situation Settings ...............................................................5




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Automatic Idle Time Introduction & Overview
Automatic idle time features are commonly available in various rod pump controllers
(RPC). The problem is that none have proven to be effective in a majority of well
configurations. Existing methods involve cycling the well while varying the length of idle
time, noting the length of time the well runs until pump-off for each value of idle time.
The process then becomes trying to determine the value of idle time when production
starts to decrease by analyzing the non-linearity of the relationship between idle time
and run time (cf. Patent Number 5,064,348).

This technique is ineffective because the relationship between idle time and run time
does not indicate the same non-linear tendency as the relationship between fluid
production and idle time. Consequently, the idle time determined by this method is not
very accurate and is often too long, causing a corresponding loss of production.

Rod Pump Control Approach
The method used in the RPC stabilizes the pump-off cycle time. It does this on a
continuous basis by adjusting the idle time according to the run times preceding so that
the desired cycle time is maintained. If there is an increase in run times, there is a
decrease in idle time and vice versa.

While this method cannot optimize the idle time applied to a rod pumped well with the
same results that would occur with a closely monitored and hand-tuned pumping
system, it will quickly and reliably stabilize the vast majority of pumping systems to a
very reasonable operating condition. As this method gives a continuous stabilization of
cycle time, it also automatically adjusts for the effects of:
    • Power outages
    • Production “kicks”
    • Pumping wear
    • Tubing leaks.

Idle Time Configuration
The CAC automatic idle time function uses two preset values, desired cycle time and
minimum idle time (the preset cycle time also serves as the maximum idle time). Table 1
lists the RPC factory default values.

 Default Description                   Value
 Cycle Time                          20 minutes
 Minimum Idle Time                   2 minutes
Table 1 Default Idle Time Settings




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Determining Idle Time
The previous run time is calculated as follows:
1. If there is sufficient history for the average run time to be determined as for software
   timed operation, the previous run time variable is set to this value, or
   If there have been two or more undisturbed pump-off cycles immediately preceding,
   the run times for those cycles are averaged and the previous run time is set to this
   value, or
   If there is one undisturbed pump-off cycle immediately before, the previous run time
   variable is set to this value, or
   If the immediate preceding cycle was disturbed or the controller has just been
   powered up, the previous run time variable is set to the preset cycle time value.
2. The previous run time is subtracted from preset cycle time to determine the next idle
   time.
3. If the resulting idle time is less than the minimum value allowed or is negative, the
   RPC uses the minimum idle time value.




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Provision for Over-Pumped Wells
The default cycle time may be excessive when the pump used is larger in capacity than
the production from the well. This results in a relatively short run time and correspondingly
long idle time. In such cases, it is desirable to run shorter cycle times to avoid loss of
production.

In order to accommodate these situations, there is a provision where, if it is detected that
the run time is less than 50% of the cycle time, the cycle time is automatically reduced
by one-half. This feature may be enabled or disabled and may also be set to illuminate
the fault lamp when this condition occurs, regardless of whether an adjustment is made
to the cycle time.




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Programming Parameters
P20 (Idle Time) is defined as the minimum idle time. Values from 00:00:01 to 99:59:59
are considered valid. A value of 00:00:00 causes a “PARAMETER 20 MISSING” error
message.

P36 (Target Cycle Time) serves as a “long time” parameter. Values ranging from
00:00:00 to 99:59:59 are valid. When set to a non-zero value, the automatic idle time is
activated for operations. A value less than the value in P20 disables the automatic idle
time feature. The factory default value is 00:00:00.

P37 controls the provision for reducing cycle time if the run time percentage is less than
50%. P37 is a byte parameter. Table 2 describes the values for this parameter.

     P37           Cycle Time Adjustment          Illuminate Fault Lamp
    Value            < 50 % Run Time                  < 50% Run Time
       0         Disabled                       No
       1         Disabled                       Yes
       2         Enabled                        No
       3         Enabled                        Yes
Table 2 Parameter 37 Settings and Definitions

The factory default value is set to 2. This instructs the RPC to adjust the cycle time
without illuminating the fault lamp.




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Activate Automatic Idle Time
For most applications, the controller is set-up as described in the existing manuals with
the exception of P20, P36, and P37 which should be left at the factory default values.
Table 3 describes set-up guidelines can be used for abnormal situations.

 Condition                                      Recommended Settings
 Low reservoir pressure and/or small            Reduce cycle time
 diameter casing
 High reservoir pressure and/or large           Increase cycle time to reduce rod stress
 diameter casing
 Very low production wells known to main-       Set P37 to zero (0) to increase cycle
 tain production with longer idle times         time if appropriate
Table 3 Idle Time Abnormal Situation Settings




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Disable Automatic Idle Time
Set P36 to 00:00:00 and P20 to the desired idle time to disable automatic idle time. This
instructs the RPC to use a fixed idle time.

As there is no parameter which displays the actual idle time currently in use, it must be
calculated by subtracting the average run time in P205 (P207 if P205 is zero) from the
cycle time in P36. If the average run time is larger than the value of P36, the value of
P20 is the idle time.




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