Building Management System Control Strategy for various pieces of equi

[RobsVette]

Hey guys, A situation has come up were I work and I want to make sure I have the proper thought process for the use of a Building Management System before we move forward.

The issue that has come up has to do with manual operation of equipment vs the BMS (building management as opposed to burner management, I know this is confusing for many people.)

I would like you guys to review my thoughts here and make sure my understanding of why BMSs are used the way they are is correct. Here goes.

- BMS systems are supposed to provide supervisory control to any type of complex piece of equipment that would have many variables to control. If this is the case typically the piece of equipment (chiller / boiler) will have its own PLC based controller. With a piece of equipment like this, the BMS should provide supervisory control meaning set point adjustment, start stop commands and readouts of what is going on with the machine to be provide to a remote operator. Typically a complex piece of equipment like this will be able to be operated without the direct intervention of the BMS if an operator desires. Ie... if the BMS computers are damaged or not functional etc..

- The opposite of that would be simple pieces of equipment that usually only require one variable to be modulated. An example of this would be a cooling tower or air handler, which would only require the modulation of 1 VFD in either case. Ie... if the condenser water temperature leaving the tower is to warm drive the cooling tower motor faster to flow more air through the tower and drive down the temperature. Air handlers are similar where you a usually only driving the fans and opening the valves on the coils to either heat or cool. These would be no where near as complex as operating a chiller or hot water/ steam boiler. CTs / air handlers also would not require any where near the amount of safties that a chiller or boiler needs.

The problem that has arisen is that at a building in manhattan we have a cogen plant that supplements heating hot water, domestic hot water and chilled water production for a building. The BMS system is required to be active in order to operate any of the equipment, including boilers for heating and domestic hot water chillers, cogen producers etc... There is no way for an operator to use the equipment in manual mode if need be.

Personally I feel this type of control strategy is incorrect. As stated above the BMS system is to provide supervisory control for efficiency and stop operators from running around like crazy. I have always operated under the premise of "get everything operating manually, then use the BMS system to integrate the operation of the whole system for efficiency." Even with this said, in areas like cooling towers and air handlers an operator can still use the equipment manually by selectiong "hand" control for motor starters and manually opening valves, etc... For the situation we are dealing with this is not a possiblity. I guess my thought process can be summed up as saying "BMS are installed to minimize the number of operators required to provide utilities to a building."

What do you guys think and what is the correct thought process for implenting a BMS control system in a building?

Thanks in advance for any help

 

[KiwiMace]

I think you have a valid basis of thought in that paragraph 1 and 2 both look right, but what you are missing is that supervisory control also includes lots of other individual components.

Sure the chillers/boilers/cogen will all have a factory controller running the unit, but there are typically a lot of motorized valves, fans and pumps that are controlled directly from the BMS on the DDC control. eg. The chiller will not control the primary and secondary chilled water pump, there will be a plant controller (part of the BMS) looking after this.

To have an effective manual override (ie. the BMS is dead), all of these components need to be hard wired to a manual control panel with a schematic drawing handy.

On all these individual components, you need to specify hardwired HOA (Hand/Off/Auto) and start/stop buttons on all the fan/motor starters (coordinated with the electrical specification). Motorized valves similar.

This will give you a base level of operation, and you would need default setpoints in the VFD's and someone trained to run the plant like this.

 

[Nabrxx]

I too work in Manhattan, and have opened a number of new buildings there. I have yet to work with a BMS system that I could not get around if I needed to and in every building that I have worked in one of my initial self assigned tasks is considering what I need to do if the BMS crashes and a reboot or j2 of a controller won't fix it. Manual operational insight is key to being prepared for the inevitable.

 

[RobsVette]

Ok, thanks for the help guys.

I realize the BMS is going to perform some tasks directly. As stated these would be driving a pump or opening a valve, but all equipment should have a hand/off/auto position and currently we do not.

Thanks for your insights.

 

[Nabrxx]

Usually at the BMS controller board in the field, there are toggle switches inside the controller panel itself to over ride the controller command. Usually, depending on what is being controlled , it is at least on the same floor. For instance, the controller managing 30 VAV boxes in "occupied" or "unoccupied" on a certain floor can be manually switched inside the controller box on that specific floor, how else could they install it and test it? It would be a logistical nightmare as well as a source of potential tenant disatisfaction NOT to have it set up like that, particularly considering the likelihood of equipment upgrade and mods.