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1.1. Sequence of Operations Overview

The sequence of operations is the set of instructions programmed into the automation system which outlines how the equipment is to operate. A fully developed sequence of operations must take into account all possible operational states for the controlled equipment. This is required in order to allow the automation system to detect operational states that are out of the ordinary and to notify the operator and/or take immediate action to correct the condition or shut the equipment off in order to prevent equipment damage.

The sequence of operations is typically determined by the LSS or process system designer and is based on the recommendations of the equipment manufacturer as well as the requirements of the customer or end user. This will typically take the form of control notes on the initial system design drawings at first and will later be turned into a narrative document during final design and system construction.

Here is an example of  control notes from an LSS Piping & Instrumentation Diagram:

Here is an example excerpt from a sequence of operations narrative:

Section 3.04 Recirculation Filter Backwash Sequence (Two Valve)

A. Perform a recirculation filter stop sequence.

B. Turn on open output for the backwash discharge valve. Wait for the valve opened status input to be on, 35 seconds. Turn off open output for the valve if the opened input is on and 35 seconds have elapsed. If opened input status is not on at the end of the 35 seconds,

1. Set filter operating state to “failure”.

2. Do “stop” recirculation filter.

3. Log message to the event log, backwash discharge valve “failed to open”.

C. Set filter operating state to “backwashing” and log message to event log that the filter is “backwashing”. Pulse on close output for the system throttle valve, 10 seconds. Wait for the valve ‘opened’ input point to indicate off, if the opened input status point is not off after the initial 10 second pulse close command times out:

1. Set filter operating state to “failure”.

2. Do “stop” recirculation filter.

3. Log message to the event log, the system throttle valve “failed to close”.

D. The system throttle valve will be pulsed open or closed depending on the optimum backwash flow rate for filter. A software PID control loop will be used to control the optimum backwash flow. The default time duration for a backwash cycle is 8 minutes (variable).

E. Remove filter from backwash queue and do a recirculation filter “stop” sequence.

RCK Controls can easily tailor the programming logic to fit the needs of our customers. If you have any questions regarding the sequence of operations programming at your site, please contact support@rckcontrols.com and let us know.