If you ask 10 construction-project managers to define commissioning, you are likely to get 10 different answers—sometimes overlapping, sometimes divergent. Rather than attempting to definitively define what commissioning is “supposed to be,” this article focuses on a specific and highly effective aproach to commissioning a commercial boiler system.
Today’s boilers/burners are the end result of 150-plus years of industrial development and are among the most widely applied and robust pieces of heating equipment in the world. However, although the basic boiler may stay much the same, it comes in a number of designs, is asked to play a variety of roles, and often serves as the “base” for a wide array of supporting or dependent equipment. These supporting pieces and components must be closely matched to a specific job, and it is with these pieces that much of our commissioning work is done. The goal is to provide a building owner the most stable, efficient, and maintainable system possible given the overarching goals and constraints of a project. The process requires spending a great deal of time onsite, examining components as they are being installed and studying component manuals to determine how they can best be operated, controlled, and interfaced.
When the process works well, it is a win for everyone: The owner gets a quality system that works efficiently and can be maintained easily, the commissioning agent’s close relationship with the engineer of record and links to the builder’s request-for-information process ensures the designer’s true intent is realized, the craftsmen have an on-site technical representative who can provide guidance, everyone has a point of contact with whom they can coordinate subsystem startups and initial testing, the construction contractors and equipment vendors have definitive dates as to when the system will be installed and working properly, and efficiencies that may have been overlooked during the design process are discovered and provide the owner/operator significant operating savings over the life of the system.
Unfortunately, because of time and/or financial constraints, the full commissioning process often is cut short. The most common example of this occurs when the commissioning agent is brought into the process late in the game, sometimes seemingly as an afterthought. In reality, the agent can provide the most useful input early in the design process, preferably before significant equipment and interface selections are made.
The following sections detail the major steps of a commissioning process.
Design development, including equipment submittal, installation, operation, and maintenance review. This phase is of equal importance to the actual testing that is commonly thought of as commissioning. The goal in this phase is not to second-guess the designer, but rather to truly understand how the designer and owner intend for the overall system of systems to operate.
Steps in this phase include:
Load estimation and boiler sizing. Boilers must be appropriately sized not only for the full design load, but all reasonably expected turndown states. The goal is to prevent problems that could affect reliability and efficiency caused by boiler short-cycling during periods of low demand. Bigger boilers are not necessarily better.
Staging and control. Are the boilers to be automatically staged by a central “master control” system, will additional boilers simply be manually added (and removed) as required, or will the system be some type of hybrid of the two?
Redundancy. There must be sufficient redundancy (in numbers) to meet the designer’s and owner’s concept of operations and requirements for maintenance and/or failover. For example, the owner might prefer to operate two smaller boilers firing in parallel, rather than risk a total failure of a single larger boiler, or he might want to always have one “extra” boiler to allow planned maintenance and unplanned repair.
Standby efficiency. If standby boilers are expected to perform when needed, how warm must they be maintained to allow a quick cycle-up when called upon? What method is provided to most efficiently maintain this temperature, and is that method compatible with the boiler firing-rate-control method?
For example, we recently commissioned a 190°F-outlet-temperature firetube hot-water-boiler system that, by design, had water returning from the building at 160°F. A small flow of return water sufficient to maintain the offline boiler at return temperature was provided. Unfortunately, this concept had not been worked out in sufficient detail with the boiler manufacturer. In practice, when the boiler was started at the return temperature, the boiler control could take up to 90 min before it would allow full-rate firing. This limitation was imposed by the manufacturer to allow sufficiently gradual warming of the refractories. Clearly, this did not meet the owner’s requirements for failover, and an alternative boiler-staging scheme and operating concept had to be implemented.
Piping and pump sizing. Pumping and piping subsystems must be sufficient to support the boiler design and meet the owner’s operating concept and any redundancy requirements. They also must match and meet the building heating system’s design intent.
Flowmetering. Flowmeters (e.g., steam, feedwater, and/or makeup water) must be appropriately selected and sized to provide reliable metering at maximum design flows and also at maximum turndown. Much like boiler selection, bigger is not necessarily better; a flowmeter simply “matching the size of the pipe” almost always is too large and often has difficulties reading at a lower demand level.
As these devices generally are used for performance monitoring and/or billing—and sometimes for actual boiler control (e.g., as in a three-element steam-drum water-level control system)—their proper selection is essential. Equally critical is the need for required upstream and downstream piping diameters. In many “tight” boiler plants, these devices may drive the need for straight run(s) of piping specifically installed to allow accurate flowmetering.
Control valving. Much like flowmeters, a control valve “sized to match the pipe” is generally too large. It should be sized to match the flow of the boiler.
Relief-valve coordination. Are the boiler pressure-relief settings sufficiently separated from the normal boiler operating point? A general rule of thumb is to avoid normal boiler operation in excess of 80 percent of the unit’s pressure-relief-valve setpoint. Are the feed pump(s) capable of providing sufficient flow into the boiler at at least 103 percent of the boiler relief setting? The intent of the boiler-code requirement is to be able to flow feedwater into the boiler as fast as the boiler reliefs can dump steam, thereby preventing a high-pressure condition from cascading into a dangerous low-water casualty. Are all relief valves in the feedwater system coordinated to fit into this scheme?
Test connections. Is there a way to operationally test the boiler(s) across the full range of operation? It may be possible to connect the boiler to the actual load, but will that reliably provide sufficient load to fully “flex” the boiler from its maximum turndown up to its design maximum operating point during the timeframe(s) the testing needs to be performed? Assume the testing might need to be repeated several times.
A steam-boiler system may need a steam blowoff/silencer or other means of creating an artificial steam demand. Depending on the configuration (e.g., a high-condensate-return-percentage design) a temporary auxiliary makeup-water system might also be required to support a steam dump or blow-off. Another option would be dumping steam to heat exchangers to allow condensate recovery while rejecting the heat elsewhere.
Hot-water HVAC boilers often can have their heat rejected into a chiller system via manual air-handler-control-valve manipulation. This can provide a load for chiller testing, too. Regardless, any special testing connections need to be thought out in the design phase.
In this phase of commissioning, construction progress is monitored onsite as the various components are installed. The goal is early detection of any problems that would cause issues during startup and operational testing.
Examples of items that should be reviewed in detail include:
Piping installation and supports. Is the piping and hanger system going in as designed and per any stress analysis to allow adequate room for thermal expansion without imposing excessive stresses at equipment connections? Are the various check valves installed in the proper direction? Are steam reducers being installed right-side up? (This seems to be a too-common problem; some installers think steam reducers appear upside-down when installed properly.) Is the steam piping system properly “trapped” to avoid water hammer? Are the necessary piping accessories being installed to facilitate routine maintenance? For example, are unions (or flanges) fitted on relief valves to allow easy removal for routine pressure-bench testing? Are relief valves and their associated drains properly routed to either the specified location or another safe location? Is remote operating gear attached to all specified valves to allow operation—including emergency isolation—without the need for a ladder or climber safety gear?
Boiler accesses. As the core of the boiler becomes buried in piping and the wire raceway, are all of the necessary maintenance and repair accesses left open? Pay particular attention to door-opening swing arcs, which often are prime targets for encroachment.
Are the boiler’s own thermal expansion mechanisms set up properly? This generally is accomplished with loosened, double-nutted bolts on slotted or sliding feet. Is there adequate allowance around the boiler for expansion? The bigger and hotter the boiler, the more it expands. The physical expansion of larger boilers can be quite significant. It must be ensured that all of the piping, the raceway, and platforms being installed on and around the boiler move in concert with the way the boiler itself is designed to expand.
Chemical treatment. Is the chemical-treatment provider actively involved in the construction process? This person should be involved before the first drop of water is placed in a vessel and manage water chemistry throughout the entire startup, testing, and commissioning process.
Testing connections. The construction-installation phase is when we ensure that various taps and connections are in place to allow complete commissioning testing. The most common examples of these connections are devices that allow pressure and temperature measurements to be made at critical points, or where there is no designed-in instrumentation.
Commissioning plans. This is the last element of this phase. These plans can be as simple or as complex as the project itself. Generally, the overall boiler system is broken down into its main subsystems (e.g., feedwater, fuel supply, condensate, boiler proper, etc.). Each plan clearly identifies what commissioning testing will be performed, what the expected design outcomes should be, what pre-requisites are required, and what parties are involved in the testing.
These plans are best written in conjunction with the key providers and installers of the entire boiler system. This helps achieve buy-in and promotes teamwork for a successful effort.
Manufacturer startup reports are a key piece in these commissioning plans, but considerable emphasis is placed on tailoring each plan to realistically exercise each component as designed given the context of its actual operational function. For example, a manufacturer’s startup report might simply verify that Pump Set A starts and runs properly, has correct and balanced amperage, does not leak, and operates on its designed performance curve. However, the balance of the commissioning plan might test that Pump Set A activates as called for by System B, distributes its properly balanced discharge to System C, and accurately reports its status and alarms to System D via System E.
This phase of commissioning nearly always significantly overlaps with construction and installation. It is best to test equipment as soon as it’s ready to be tested. If only 60 percent of a particular sub-system’s capability is ready for testing, that’s fine: Test that 60 percent now, and finish the rest when the rest is ready. The earlier each test is performed, the earlier problems will be discovered and corrected, all of which contributes to keeping the schedule on-track. This is why pre-written commissioning plans clearly defining all the testing to be accomplished are crucial in tracking commissioning testing completion.
Commissioning testing and demonstration covers five areas:
Equipment installation and pre-checks. Ensure the system is installed in accordance with the design and the manufacturer’s instructions. Most of this checking is completed during the installation and construction phase described above and often is well-itemized in a manufacturer’s startup checklist and report.
Equipment safety checks. Ensure that the subsystem operates safely and all of its own safety interlocks function as designed.
Equipment operational sequencing testing. Does the system start, stop, and control as intended in the design? Does the system properly respond to all of its designed inputs, and does it provide the proper monitoring, control, and alarm outputs? Is any startup/shutdown sequencing appropriate to the overall system operation, and does it adhere to the overall design intent? Does the plant sequencing support the owner’s operating concept?
Equipment performance testing. This is the heart of what normally is considered commissioning. Does the boiler produce its design Btu output at design pressure, efficiency, and emission levels? Does it smoothly transition across its entire operating range down to its maximum design turndown? Does it properly stage up and down, and does it “behave well” with any other boilers that ight also be in the plant, sharing the load as per design intent? Does the boiler properly flex, providing adequate responses to expected load-demand changes consistent with the design?
Equipment off-normal testing. Does the boiler and its related subsystems place itself and the plant as a whole into a safe condition during failures? Does a trip result in a controlled shutdown or a series of cascading failures? Does the standby boiler system start and come online with sufficient rapidity to maintain the load in accordance with the design intent and the owner/operator’s operating concept? Does the system make its status clearly apparent to the operator so that he may take any immediate or controlling actions and make appropriate notifications? Is there sufficient data logging to allow reasonable post-event reconstruction?
As with any formal construction process, a final report is required. This report outlines the various findings and provides detailed documentation in the form of startup reports, field notes, performance testing data, equipment data sheets, etc.
There should not be any surprises in the report; any equipment that is not performing per the design intent already will have been the subject of considerable discussion by the project team.
These reports represent a valuable source of information during the life of the plant as it nearly always becomes necessary to compare current performance with original as-built performance.
Rick Botto, ME, PE, is the founder and owner of Cornerstone Automation LLC. Cornerstone specializes in HVAC and mechanical/electrical/plumbing-system design, installation, and integration in the health-care, hospitality, and medium-industrial sectors. He holds a bachelor’s degree in mechanical engineering from Christian Brothers University and has worked as a mechanical and automation contractor since 1985. Jim Wyant joined Cornerstone in 2007 after 24 years of active-duty service as a surface-warfare officer in the U.S. Navy, with a specialty in mechanical engineering. He holds a bachelor’s degree from Texas A&M University and a master’s degree from The Naval War College.