Duty Standby Logic and controls for Chillers and Pumps.

abdulsattarmd

Member
Join Date
Sep 2009
Location
Riyadh
Posts
3
Hello everyone,

This is to request you all to help me out with the logic of the program in "Control logic programming" for the system explained below.


The system consists of the following:
  • Six chillers (4 Duty + 2 Standby)
  • Six primary chilled water pumps (4 Duty + 2 Standby) - DOL Starter
  • Four secondary chilled water pumps (2 Duty + 2 Standby) - VFD Control

Chillers and Chilled Water Pumps:

Each chiller consists of water temperature sensor in the outlet header. Also on the common water supply (two groups) and return pipes, there are immersion temperature sensors.
Two pressure bypass valves are installed between the common water supply and return pipes; they will open when the AHU two-way valve is closed. This feedback is coming from the two differential pressure sensors installed between the common supply and return pipes.

Chillers are controlled through a programmable controller (DDC); whereby loading and unloading of chillers will be based on the cooling demand of the water, as measured by the temperature sensor at the common return, the controller shall initiate a command to start the chiller (enable the control circuit).
To insure water will flow through the chillers supply line prior to chillers start up, hardwire interlock between the pumps, water flow switches and the chillers circuits should be provided.


System Operation:
  • On a signal from the BMS, the associated duty primary water pump shall be energized.
  • After proof of water flow through the chiller is established by the water flow switch (located at each chiller outlet), a signal will be transmitted to the control panel to enable the chiller to operate.
  • If after an adjustable time delay, the temperature sensor located in the common chilled water return header does not satisfy a BMS adjustable reset value (6.6oC), an additional chiller shall be started by the BMS in the same manner as the leading chiller, and so on until the set-point is achieved.
  • When the chilled water return temperature drops below the BMS adjustable preset value of 6.6oC, the leading chiller shall stop through signal from the BMS. After an adjustable time delay, the chiller associated pump shall stop.
  • If any set of duty pump and chiller is tripped, the standby pump and chiller set shall takeover and an alarm is energized in the BMS for the defective pump/chiller.
  • Operation of variable speed secondary chilled water pump is through a start/stop control as follows:
  • Start/stop of secondary chilled water pumps is through a selector switch Auto/Off/Hand located in the control in the MCC panel:
  • At Auto position the respective unit shall operate under the control of DDC controller with predefined time scheduling.
  • At Off position, the respective unit shall stop.
  • At Hand position the respective unit shall operate without any time schedule.
  • Operation of secondary chilled water pumps shall vary in response to signals received from pressure sensor installed in the supply pipe line to satisfy the building cooling loads.
· The lead pump will be activated with gradual speed increase and shall respond to the pressure set point to satisfy the cooling load of the building.
· The standby pump shall operate under one of the following conditions any of the Lead / Lag pumps failed to operate. Abnormal operation or shutdown condition of any pump.
· Temperature monitoring: The current values for the chilled water supply and chilled water return temperatures can be observed on the BMS screen.
· Pumps Alarm: In case any motor Trip, the respective pump shall stop and this will be monitored on the BMS workstation.
· Chillers Alarms: In case any alarm from the chillers is observed, the DDC shall remove from service that specific chiller along with associated equipment. This will be monitored on the BMS workstation.
· Chiller Monitoring: The water flow switches in the discharge of each chiller shall ensure that water flows through the chiller. Chillers shall only be enabled when the water flow is detected through the flow switches. If there is no water flow detected by the water flow switch, immediately the DDC shall disable the respective chiller and associated equipment and enable the standby unit.
· Cyclic Time Schedule: Cyclic Scheduling is programmed in the DDC. It shall operate all the chillers and pumps on equal run time.
 
Consider yourself fortunate that you have spare capacity to allow maintenance.

I think the answer to your question at least simply and generically is to determine which chiller is on line or additional to place on line should be determined by
1. Load - add chillers with increasing load. Take off line as load drops.
2. Units are prioritized by availability and by hours run to enable wear balancing.

Dan Bentler
 
I think people here would be a lot more interested in helping if you were showing some work that you did so far. Otherwise it looks like you are trying to get someone to write the program for you. That's what a lot of these guys do for a living, write programs. Except they do it for money. You cannot honestly expect someone to write a program, especially a program that in-depth, and post it here for you to use for free...?? Can you??
 
If you look at the top of the page, there is a banner that says downloads.
In there under allen bradley, are some program that might give you assistance to understand the concept you describe.

They are mainly .rss files, that will open under logix 500 I think,
and Logix pro which I think is the PLC simulator available at the top of the page

The PLC you are using is Control Logix and runs, Logix 5000 .acd files
 

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