Last month, in Part I of this series, I defined the lead/lag control strategy as it applies to parallel pump operation and discussed the need for identifying the designer’s intent as to whether or not it is appropriate. In this article, I will discuss the sequence of operation and some items a commissioning provider should look for during their review.
Example lead/lag sequence of operation:
Sidebar: Example lead/lag sequence of operation:
1. Secondary chilled water pumps operation
a. Lead pump enable/disable
i. Lead secondary chilled water pump will be enabled anytime the chilled water (CHW) system is enabled and will remain enabled for 30 minutes after the CHW system is disabled.
b. Pump speed control
i. Lead pump’s speed will be modulated to maintain system differential pressure at CHW dP Set Point (initially 8 psid, adj.).
c. Lag pump operation
i. Lag pump will enable when lead pump speed signal increases above Lag Pump Stage On Set Point (initially 95%, adj.) for Lag Pump Stage On Delay (initially 5 minutes, adj.).
ii. When enabled, lag pump will modulate off same speed signal as lead pump.
iii. Lag chilled water pump will disable when pump speed signal falls below Lag Pump Stage Off Set Point (initially 60%, adj.) for Lag Pump Stage Off Delay (initially 5 minutes, adj.).
iv. Lag Pump Stage Off Set Point will not be allowed to be adjusted higher than 20% below the Lag Pump Stage On Set Point.
d. Pump failure response
i. If lead pump is enabled but status is not proved for 30 seconds
1. Lag pump enables
2. Lead pump remains enabled
3. Non-latching pump failure alarm is generated for lead pump
ii. If lag pump is enabled but status is not proved for 30 seconds
1. Lead pump remains enabled
2. Lag pump remains enabled
3. Non-latching pump failure alarm is generated for lag pump
e. Lead pump designation rotation
i. Lead pump designation will be rotated via a user adjustable schedule, initially set to rotate the 4th Tuesday of each month at 9 am.
ii. During rotation, outgoing lead pump remains enabled for 1 minute after incoming lead pump is enabled. During this period, they will receive the same speed signal.
The example above provides an example lead/lag sequence of operation. The remainder of this article will refer to this example to highlight areas of interest one may want to pay attention to when reviewing sequences by others.
Set Point Nomenclature Consistency
Defining set point names within the sequence (as can be seen by the italicized set points in the example) makes it easier for building operators to compare the sequence to point labels on a BAS graphic. It also makes the functional testing process go smoother as the commissioning provider attempts to determine if the programmed sequence matches the documented sequence.
Initial Set Point Values
Reasonable, initial set points should be provided by the designer. This provides the temperature controls contractor the chance to start in the right ballpark as he or she works out the kinks of getting the automation of the system zeroed in. An inappropriate initial value for a set point may lead to controllability and tuning issues for the chilled water system as well as its served loads.
Final Set Point Values
Final set point values established by the testing, adjusting, and balancing (TAB) consultant (e.g. CHW dP Set Point) and the tuning process (e.g. Lag Pump Stage Off Set Point) should be updated in the as-built sequence of operation to ensure the appropriate set points are sustained well into the occupancy phase of the building.
Our firm was brought in after a museum had a control system upgrade to opine on several disputes between the owner and the contractor that were lingering from the project. One of the complaints was an excess of humidity in the galleries. All AHUs in the building were found to have their cooling valves wide open, but their supply air temperatures were well above their respective set points. Yet, the secondary chilled water pump was maintaining the differential pressure set point quite easily at about 33 Hz speed. The lack of cooling/dehumidifying at the AHUs was a result of too low of a differential pressure set point in the chiled water system. The contractor who performed the retrofit did not know the set point that was used by the previous control system, as that system had “gone black.” Luckily, our firm had retro-commissioned the building several years prior and had screenshots from the previous BAS, which confirmed the set point needed to be raised substantially. The as-builts from the previous control system did not specify any value. Had they, this issue may have been thwarted much earlier.
Some of the set points denoted as “adjustable” within the sequence should eventually be made “overridable,” meaning they will be flagged when they deviate from the appropriate values determined by TAB and tuning. This will be discussed in greater detail in Part III.
Paragraph 1.c discusses the staging of the pumps based off the speed command. The conventional approach of specifying 95% instead of 100% for the Lag Pump Stage On Set Point is that engineers don’t want to wait until the pump is maxed out and probably failing to meet the CHW dP Set Point prior to enabling the lag pump. Every system is different, but, generally speaking, there will come a time when it is actually more efficient to run two pumps instead of one due to improvements in pump efficiency. And, the speeds at which that occurs are usually well below 100%. So, paragraph 1.c. is really written in a manner to provide consistent performance out of the pumping system, not necessarily optimizing energy conservation. That Lag Pump Stage On Set Point is specified as adjustable for a reason, though, and I will discuss how to zero in on the optimal value for that set point in Part III of this series. Staging off a speed command is never going to be as energy efficient as wire-to-water efficiency-based staging logic1 or even staging off of measured system flow, which ASHRAE Guideline 362 recommends. In my experience, both of those approaches are still rare and require additional hardware and/or more sophisticated programming logic for their implementation.
Paragraph 1.c.ii specifies that if both pumps are operational, they receive the same speed command. That is important to maintain evenly distributed flow between the operational pumps. Though I have seen sequences leave this part out, I have never seen something written in direct contrast to it. It’s a pretty noncontroversial concept; however, there are several scenarios that can inadvertently create a situation where differing speed commands are sent to the operating pumps. Part III of this series will walk through such scenarios.
Paragraph 1.c.iii discusses staging off the lag pump. It’s common for such stage-off logic to be omitted from the sequence. When this omission is pointed out, the sequence author often states the controls programmer should know this is needed. Last winter, my firm recommissioned a building where the design engineer’s sequence had no stage-off logic discussed. The building operator, during our initial interview, stated he had never seen the lag secondary pump stage off. Review of the control logic confirmed the stage-on programming was in there, but no stage-off programming. Once the lag pump was enabled, it remained enabled until the system as a whole was disabled. A competent temperature controls contractor should have caught this issue; however, in the controls contractor’s defense, he did exactly what the design called for.
When stage-off logic is specified (paragraph 1.c.iii), what is often not programmed is provisions to prevent the Lag Pump Stage Off Set Point from being adjusted to a value too close to, or above, the Lag Pump Stage On Set Point (paragraph 1.c.iv). For example, if the Lag Pump Stage Off Set Point was ever allowed to be adjusted above the Lag Pump Stage On Set Point, the result would be an unstable situation where the lag pump is repeatedly cycled on and off. In this case, that cycling would occur at five-minute intervals, long enough for a building operator to walk away from the BAS and notice the consequences of his or her inappropriate set point adjustment.
Incorporating the Concept of Standby
Paragraph 1.d incorporates the “standby” concept, despite this being a lead/lag sequence. If the lead pump fails, the lag pump will double as a standby pump.
Paragraph 1.d.i.3 references a non-latching alarm. An article planned for later this year will discuss latching versus non-latching alarms in great detail. My general position is that if an alarm can stably be non-latching, meaning that it automatically clears once the alarm situation has gone away, then it should be. Most building operators are plagued by too many alarms coming from the BAS, so let’s help them when we can.
Lead Pump Designation
Paragraph 1.e.i has the pumps rotating lead designation on a day and time when it is expected someone will be on-site to assist if something goes wrong or improves the chances of a mechanical service provider arriving within normal business hours to prevent the owner from paying increased labor rates. In a recent project, a hot water system was reprogrammed on a Wednesday. The functional testing was not scheduled until the following week. The pumps went through their first scheduled lead rotation at midnight on Sunday morning. The incoming lead pump had its power unintentionally left off at its electrical disconnect. Standby logic (like that depicted in Paragraph 1.d) was not correctly programmed, thus the newly designated lag pump did not come back online when the newly designated lead pump failed to prove operation. The building was left without heating hot water distribution pumps until the service contractor could arrive three hours later.
This column, the second of a three-part series, aimed to dive deeper into the lead/lag control sequence for variable-speed distribution pumps, provide some best practices, and offer a few items to be aware of. Part III will provide examples of what commissioning providers are to look for during the functional testing of such systems.
1Rishel, J. 2001. “Wire-to-Water Efficiency of Pumping Systems.” ASHRAE Journal (April).
2ASHRAE Guideline 36-2021, "High-Performance Sequences of Operation for HVAC Systems."
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