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SOO Hot Water System

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Glycol Hot Water System (HWS) - Sequence of Operation

Version: 1.0 Date: 2025-12-29 Author: Gemini Agent Related SSO: SSO Temp Sensor, SSO Pressure Sensor, SSO Valve Actuator, SSO Relay Output

1. Overview

This document outlines the sequence of operation for the Glycol Hot Water System (HWS), which provides heating capacity to the facility. The system initiates operation either automatically based on outside air temperature (OAT) conditions or via manual activation from the User workstation.

Key functions of this system include:

  1. System Activation & Temperature Control: Hot water pumps are started, and upon proving operation, heat exchanger valves modulate to maintain a hot water supply temperature setpoint. This setpoint dynamically resets as a function of OAT to optimize energy usage.
  2. Differential Pressure Control: A differential pressure transmitter located in the main hot water supply piping modulates a bypass valve to ensure a constant differential pressure is maintained across the supply and return mains.
  3. Glycol Make-up Automation: An integrated glycol make-up pump system automatically monitors and maintains the system pressure, activating the pump when necessary and ensuring safe operation with various pressure and level alarms.

The system is controlled by an ECL-650 controller, which monitors and manages all associated points and alarms.

2. Point List

Inputs (Physical)

  • Hot Water Pump 7 Status (P-7-S): BI (ECL-650, UI1), Dry Contact
  • Hot Water Pump 8 Status (P-8-S): BI (ECL-650, UI2), Dry Contact
  • Glycol Tank Low Level Alarm (GT-LL-A): BI (ECL-650, UI3), Dry Contact
  • Hot Water Pump 9 Status (P-9-S): BI (ECL-650, UI4), Dry Contact
  • Glycol Hot Water Differential Pressure Transducer (HWS-DP): AI (ECL-650, UI5), 0-10VDC
  • Hot Water Supply Temperature (HWS-T): AI (ECL-650, UI6), 1K RTD
  • Hot Water Return Temperature (HWR-T): AI (ECL-650, UI7), 1K RTD

Outputs (Physical)

  • Hot Water Pump 7 Command (P-7-C): BO (ECL-650, UO1), 12 VDC
  • Hot Water Pump 8 Command (P-8-C): BO (ECL-650, UO2), 12 VDC
  • Hot Water Pump 9 Command (P-9-C): BO (ECL-650, UO3), 12 VDC
  • Glycol Hot Water Bypass Valve Output (HW-BYP-VLV-O): AO (ECL-650, UO4), 0-10VDC
  • Heat Exchanger Control Valve 1 Output (HX-1-O): AO (ECL-650, UO5), 0-10VDC
  • Heat Exchanger Control Valve 2 Output (HX-2-O): AO (ECL-650, UO6), 0-10VDC

Software Points & Setpoints

  • Heat Exchanger Differential Pressure Setpoint (HX-1-DP-SP): Setpoint (Software)
  • Hot Water Supply Temperature Setpoint (HWS-SP): Setpoint (Software)
  • Hot Water System ON/OFF Command (HWS-RUN): Software (from User Workstation or OAT)
  • Outside Air Temperature (OAT): AI (Networked or Local)
  • Hot Water Supply Temp High Limit Alarm (HWS-T-HILMT): Setpoint (Software)
  • Hot Water Supply Temp Low Limit Alarm (HWS-T-LOLMT): Setpoint (Software)
  • Glycol System Normal Operating Pressure (GCHW-Press-SP): Setpoint (Software)
  • Glycol System Low-Low Pressure Alarm Limit (GCHW-Press-LOLMT): Setpoint (Software)
  • Glycol System High-High Pressure Alarm Limit (GCHW-Press-HILMT): Setpoint (Software)

3. Safety Devices & Interlocks

  • System Activation Interlock: Heat exchanger valves (HX-1-O, HX-2-O) will only be activated upon confirmation of operation (P-7-S, P-8-S, P-9-S) of the associated hot water pumps.
  • Glycol Pump Pressure Safety: The glycol make-up pump (P-9) will be de-energized if the system pressure drops below a low-low limit setpoint (GCHW-Press-LOLMT) or rises above a high-high limit setpoint (GCHW-Press-HILMT). An alarm is generated in both cases.
  • Glycol Pump Start Failure: If the make-up pump (P-9) is commanded ON (P-9-C) but its corresponding status contact (P-9-S) does not prove operation within a defined time, the pump command will be stopped and a malfunction alarm will be generated.
  • Glycol Tank Low Level: If the glycol tank low level float switch (GT-LL-A) activates, the associated make-up pump (P-9) will be stopped, and an alarm will be generated to prevent the pump from running dry.

4. Modes of Operation

Hot Water System Start/Stop

  • System Start Conditions:
    • Automatic: Outside Air Temperature (OAT) drops below 60°F (adjustable).
    • Manual: Activated via the User workstation (HWS-RUN).
  • System Stop Conditions:
    • Automatic: Outside Air Temperature (OAT) rises above 60°F (adjustable) AND HWS-RUN is OFF.
    • Manual: Deactivated via the User workstation (HWS-RUN).
  • Start Sequence:
    1. Upon start condition, hot water pumps (P-7-C, P-8-C, P-9-C) are commanded ON.
    2. Upon confirmation of pump operation (via P-7-S, P-8-S, P-9-S auxiliary contacts), heat exchanger valves (HX-1-O, HX-2-O) are activated.

Hot Water Supply Temperature Control

  • Objective: Maintain HWS-T at HWS-SP.
  • Control Action: The one-third two-third steam control valves (implied by HX-1-O, HX-2-O) will be modulated, in sequence, to maintain the desired HWS-SP.
  • Setpoint Reset Schedule: The HWS-SP will be reset as a function of OAT in accordance with the following schedule (user adjustable):
    • OAT < 20°F: HWS-SP = 180°F
    • OAT = 40°F: HWS-SP = 160°F
    • OAT > 60°F: HWS-SP = 140°F (Minimum setpoint, system likely OFF)
    • Linear interpolation between these points.

Hot Water Differential Pressure Control

  • Objective: Maintain a constant differential pressure (HWS-DP) across the hot water supply and hot water return mains.
  • Control Action: The HWS-DP transmitter modulates the normally open hot water differential pressure bypass valve (HW-BYP-VLV-O).
    • If HWS-DP drops below its setpoint, the bypass valve modulates closed.
    • If HWS-DP rises above its setpoint, the bypass valve modulates open.

Glycol Make-up Pump (P-9) Control

  • Monitoring: An analog pressure transmitter (assumed part of HWS-DP or a separate sensor) in the expansion tank feed line continuously monitors system pressure.
  • Pump Activation: If system pressure drops below the GCHW-Press-SP, the glycol make-up pump (P-9, commanded via P-9-C) is activated.
  • Pump Deactivation: The pump runs continuously until system pressure rises to meet or exceed the GCHW-Press-SP.
  • Safety Stop: The pump is stopped if system pressure drops below GCHW-Press-LOLMT or rises above GCHW-Press-HILMT.

5. Alarms & Fault Conditions

All alarms shall be annunciated at the User workstation.

  • Hot Water Pump Failure:
    • Condition: Status contact (P-7-S, P-8-S, P-9-S) indicates a pump is off when it should be running.
    • Priority: High.
  • Hot Water Supply Temperature Out of Range:
    • Condition: The HWS-T reading exceeds the high (HWS-T-HILMT) or low (HWS-T-LOLMT) alarm limits.
    • Priority: Medium.
  • Glycol Pump Pressure Alarm:
    • Condition: System pressure drops below GCHW-Press-LOLMT or rises above GCHW-Press-HILMT.
    • Priority: High.
  • Glycol Pump Malfunction:
    • Condition: The glycol make-up pump (P-9) fails to prove status after being commanded ON.
    • Priority: High.
  • Glycol Tank Low Level:
    • Condition: The low level float switch (GT-LL-A) is activated.
    • Priority: Critical. Prevents pump operation.

6. Commissioning & Verification Checklist

  • Verify the hot water system starts automatically when OAT drops below 60°F and stops when OAT rises above 60°F (and manual command is OFF).
  • Verify the hot water system can be manually started and stopped from the User workstation.
  • Verify all hot water pumps (P-7-C, P-8-C, P-9-C) activate upon system start.
  • Verify heat exchanger valves (HX-1-O, HX-2-O) activate only after pump operation is confirmed (P-7-S, P-8-S, P-9-S).
  • Verify HWS-T setpoint resets according to the OAT schedule (e.g., 180°F at 20°F OAT, 160°F at 40°F OAT, 140°F at 60°F OAT).
  • Verify heat exchanger valves modulate to maintain the HWS-SP.
  • Verify the HWS-DP reading is accurate. With pumps running, adjust load conditions and observe HW-BYP-VLV-O modulating to maintain the HWS-DP setpoint.
  • Simulate a hot water pump failure (e.g., disconnect P-7-S) and verify a “Hot Water Pump Failure” alarm is generated.
  • Adjust the high and low alarm limits for HWS-T and verify that an alarm is generated when the temperature goes out of range.
  • Simulate a drop in system pressure and verify the glycol make-up pump (P-9) activates.
  • Verify the make-up pump deactivates once the GCHW-Press-SP is achieved.
  • Simulate a system pressure drop below GCHW-Press-LOLMT and verify the make-up pump is stopped and an alarm is generated.
  • Simulate a system pressure rise above GCHW-Press-HILMT and verify the make-up pump is stopped and an alarm is generated.
  • With the make-up pump commanded on, disconnect its status input (P-9-S) and verify a “Glycol Pump Malfunction” alarm is generated.
  • Manually activate the GT-LL-A float switch and verify the make-up pump (P-9) is de-energized and a “Glycol Tank Low Level” alarm is generated.

Document Owner: Technical Training Department
Last Updated: December 2025
Next Review: March 2026
Feedback: training@example.com

“A well-controlled hot water system is the backbone of comfort and energy efficiency.”

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