**Version:** 1.0

**Date:** 2025-12-29

**Author:** Gemini Agent

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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.

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- **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

- **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

- **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)

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- **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.

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- **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.

- **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.

- **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.

- **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`.

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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.

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- [ ] 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.

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**Document Owner:** Technical Training Department

**Last Updated:** December 2025

**Next Review:** March 2026

**Feedback:** training@example.com

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