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Sequences of Operation
An objective look at specified
descriptions of control
Steven R. Calabrese
Division 15 (Mechanical) of any project’s specification manual will often include the temperature controls specification, if not found separately under its own section (Division 17). Integral to this portion of the specification are the all-important Sequence of Operation and Points List for the project. This part of the temperature controls specification is extremely significant, and an essential part of any successful project. The Sequence of Operation and the Points List together are very powerful tools for specifying a DDC control system.
This writing will focus on how an engineer’s specified Sequence of Operation is typically written, how it should be written, what it should include, what’s important, etc. Fundamentally, the Sequence of Operation should outline the engineer’s basic intentions on how the mechanical system(s) should operate, how they were designed to operate, and how they should be controlled to meet that end. The Sequence should be a direct reflection of the mechanical design. A Sequence of Operation failing to convey the engineer’s basic design intentions will only serve to complicate the procurement process and the control systems design and submittal processes.
A typical Sequence of Operation will typically take the form of one of two styles: point style or paragraph style. A Sequence written in point style would be as follows:
Programmable thermostat signifies occupied mode.
Rooftop unit supply fan runs continuously.
Thermostat cycles heating and cooling as required to maintain occupied space temperature setpoints.
The same Sequence written in paragraph style would be as follows:
When the programmable thermostat indicates the occupied mode the rooftop unit’s supply fan runs continuously, and the thermostat cycles the heating and cooling as required to maintain occupied space temperature setpoints.
Specified Sequences will be in long or short form, as dictated by the particular engineer and his intentions. A short form Sequence of a thermostatically controlled exhaust fan may read as follows:
Generator room exhaust fan to be controlled by a space mounted thermostat.
The long version may read like this:
Space mounted thermostat controls the operation of the generator room exhaust fan. Upon a rise in temperature above the setpoint of the thermostat (80 degrees F. adj.), the exhaust fan is energized. Once the space falls back below setpoint and through the differential of 2 degrees (adj.), then the exhaust fan de-energizes.
The long version provides more detail and more insight into the intended operation, and is acceptable for the T/C Sequence of Operation, especially if this is to be turned over to the customer and to the service company. The short version is typically more appropriate for specification purposes.
Writing good Sequences of Operation is an art, one that not all of us in the mechanical systems industry are necessarily suited for (including yours truly!). The specifying/consulting engineer, in addition to knowing the “ins and outs” of the mechanical design, needs to be a good writer as well. As not all of us have the natural ability to describe in writing our design intentions, what we often wind up with is specified Sequences that are either inaccurate and incomplete, or are more or less “cut and paste” versions of previously written Sequences from past projects.
A common shortcoming in the typical engineer’s Sequence is that, either all equipment is not accounted for, or equipment is described that is not part of the specified project. This stems from the use of generic specs or frameworks, in which the engineer starts with a basic skeleton, and then tailors it to the present project. This is an acceptable method of writing a Sequence, if implemented correctly, however one that takes a considerable amount of attention to detail, so as not to “leave out” important details, or “leave in” inapplicable content.
A specified Sequence of Operation should ideally be clear, concise, and complete. The main goal of the Sequence is to convey the engineer’s basic intentions for the operation of the mechanical systems, in alignment with the design of the mechanical systems. To keep it from becoming too lengthy for anyone to want to read it, the Sequence need not spend a lot of time on details; it should get to the intents, first and foremost. An engineer should set out to write a Sequence with this in mind, and add details (as necessary) once he has developed the framework of the sequence, one that accurately and concisely reflects his mechanical systems design.
Safeties and limits should be included in the Sequence, in addition to the basic operating schemes. These will typically take the form of hardware devices, and will need to be accounted for.
The use of exact setpoints should be avoided. Generally the commissioning parties will have the best insight to this. It is virtually impossible to predict exact setpoints, as many variables factor in to the end requirements for any given system. Baselines are acceptable (ex. 55 deg. F., adj.) and are typically included in the Sequence.
The Sequence should not detail the operational characteristics of packaged equipment. Specifying a piece of packaged equipment takes care of this already, and the Sequence does not need to spend time replicating the O&M manual of a packaged piece of equipment. Along the same lines as this, the Sequence should use industry accepted terms to specify certain descriptions of control (as possible), in lieu of completely describing a scheme of operation (ex. Morning Warmup).
The engineer’s specified Sequence of Operation should not belabor basic control fundamentals, as already accepted by the industry. For example, an engineer should not waste his time describing how a programmable thermostat should operate a packaged rooftop unit, or how a pressure-independent cooling-only VAV box should operate in the occupied mode. Alternatives to the above examples could be…programmable thermostat operates RTU in typical manner, and…cooling-only VAV box operates in typical pressure-independent fashion. The point is, the engineer’s Sequence should “get to the point”, in terms of what’s important for the procurement and pre-design stages of the project.
When reviewing an engineer’s specified Sequence of Operation, the important thing is to be able to know how to read the Sequence. How to read between the lines. How to get into the engineer’s head, to understand the mechanical systems thoroughly, to understand the equipment selection parameters, performance criteria, etc. so as to be able to align your interpretation of the Sequence with the engineer’s basic design intents.
If you are reading a Sequence of Operation for the purpose of bidding on a project, remember that a well-written Sequence will define (expressly or implied) all of the required points for the project. For estimating purposes, it is more important to have knowledge of the required input and output points, than it is to know how the systems are to operate. If a separate Points list is provided, then this list will give upwards of 90 percent of what is required for the project, control-wise. Having a Points List does not exempt you from reading through the Sequence of Operation, however, for the Sequence may include miscellaneous additional items, such as safety and limit devices. Items that probably should be included in the Points List, but sometimes aren’t.
If you are reading a Sequence of Operation for the purpose of designing the control systems for a project, then the actual control descriptions are extremely important. A consulting/specifying engineer may go the distance of writing a clear, accurate, and concise (yet detailed) Sequence of Operation, for the control systems engineer to design from. More likely, the specifying engineer will leave that up to the controls engineer to develop, so it’s important for the controls engineer to fully understand what the specifying engineer has written, and perhaps even more importantly, what the engineer has drawn up (mechanical plans, details, and equipment schedules).
Sometimes an engineer will work from a manufacturer’s specification, in order to build his control systems specification. Occasionally this will filter in to the Sequence. It is important for the control systems designer to differentiate what is relevant to system operation, from what is perhaps just an attribute of the specified vendor.
Maybe the most important thing in “developing” a specified Sequence of Operation is to account for all equipment specific to a project. A Sequence should be project-specific. If the Sequence lists out equipment that is not a part of the particular project, then the purpose that it serves is diminished. Likewise if equipment is left out of the Sequence. This is typical of what can happen with the “cut and paste” method of putting together a Sequence.
Once all equipment is accounted for, the specified Sequence should be clear and concise with the intended operation of each piece of equipment, without dragging on and listing every minute detail. There needs to be a certain balance struck, between listing out what’s fundamentally important, and rambling on with operational details that will “come out in the wash”, so to speak. Given this, there needs to be a certain amount of responsibility put onto the shoulders of the commissioning team, and a certain amount of faith put into them as well.
The quasi-dimensional engineering firm may write a Sequence and Points List so accurate and so reflective of their mechanical design, that there is no need for the controls engineer to revise, regenerate, or elaborate on what the engineer has provided. Often this is not the case, and the specifying engineer will “pass off” the task of generating a detailed Sequence of Operation on to the controls engineer. This is the way it works, and is generally in the best interests of all parties. It may seem like the specifying engineer is “passing the buck”, but this forces the control systems engineer to thoroughly understand the mechanical design intentions, and it allows the specifying engineer to evaluate the controls engineer’s comprehension of the specifying engineer’s intentions for the operation of the mechanical systems. On a lesser frequency, the competent, forthright specifying engineer may simply recognize that he is not in the best position to write the full-blown Sequence of Operation, for whatever reason.
The specified Sequence of Operation should not waste its time on commissioning details. A Points List and a concise Sequence shall suffice for the purpose of bidding a job and designing it. A “Description of Control” should be developed for each piece of equipment by the temperature controls contractor, with assistance and/or input from the specifying engineer, to be submitted to the engineer after the job has been awarded. When the project is completed, a full-blown, detailed Sequence of Operation can and should be developed and provided, along with the “as-built” temperature control drawings. This is generally in everyone’s best interests. It keeps the specification short and concise, it allows the project to be awarded without “up-front” creation of a document that is subject to substantial change throughout the design process, it forces the controls contractor to understand the mechanical design concepts of the project, and it allows the consulting engineer to retain control over the final design of the HVAC control system. All of which is in the best interests of the customer (who?).
|Tip of the Month: This month’s tip is simply to “stay tuned” for next month’s column, which is a continuation of the topic, offering some useful information on how to quickly and accurately generate your own Sequences of Operation.|
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