Emergency Shutdown (ESD) System Design

1. Introduction

The plant protection system provides tiered responses to successive levels of hazard. The most significant hazards (fire) trigger a full plant Emergency Shutdown (ESD). Less significant hazards may limit operation to a single building or unit without full station isolation.

Regulatory Framework

• 49 CFR Part 192: Transportation of Natural Gas by Pipeline - Minimum Federal Safety Standards
• NEC (NFPA 70): National Electrical Code requirements for installations
• State/Local regulations: Municipal, state, and federal regulations as applicable

Applicability

ESD functions shall be implemented for all compressor stations of 1,000 installed horsepower or more. Unattended field compressor stations of 1,000 HP or less are exempt from some requirements.

Protection System Levels

Level	Response	Scope
Plant ESD	Full isolation and blowdown	Entire station
Fuel Gas Shutdown (FGS)	Block/vent fuel, unit stops	Single building
Unit Shutdown	Individual unit stop	Single compressor
Alarm Only	Annunciation, time-limited	Monitoring point

2. Plant ESD System

The Plant ESD System isolates the compressor station from the pipeline and vents the station yard high-pressure piping to atmosphere. It is the response dictated for the most significant hazard expected (fire).

ESD System Functions

• Block gas out of the station (close side valves)
• Blow down station piping to atmosphere
• Shut down gas compressing equipment
• De-energize electrical facilities in gas header area
• Discharge gas at a location where it will not create a hazard

Pneumatic Control System

The ESD operates by directing high-pressure gas from a secure source into a pneumatic control line. The control line is pressurized to activate the ESD.

Component	Requirement
Volume storage vessel	Fed from highest pipeline pressure available
Control gas line size	2" nominal minimum
Valve actuation time	All valves within 10 seconds
Valve operators	Direct acting, no spring return required

Manual Control Stations

At least two manually operated pneumatic control stands shall exist to initiate the system by pressurizing the Control Gas line:

• One located in a safe area accessible during fire near compressor building
• Outside the gas area of the station
• Near exit gates (fenced station) or emergency exits (unfenced)
• Maximum 500 ft from station limits
• Minimum 25 ft from compressor building

Valve Operations During ESD

Valve Type	Action on ESD	Actuation
Side valves (inlet/outlet)	Close	Direct from control gas line
Blow-off valves	Open (vent to atmosphere)	Direct from control gas line
Bypass valves	Position as required	May be electric (non-critical)

Electric ESD Initiation

Two design principles are acceptable for electrically initiated ESD:

Option 1: Master ESD Circuit (Fail-Safe)

• Normally energized solenoid control valve
• Breaking the circuit activates ESD
• Provides true fail-safe operation
• Any wire break, power loss, or button press triggers ESD

Option 2: Supervised Circuit

• Normally de-energized solenoid
• Energizing the circuit activates ESD
• Circuit continuity is monitored
• Failures are annunciated without triggering ESD

3. Fuel Gas Shutdown (FGS)

The Fuel Gas Shutdown is a response to a lesser hazard, limiting the response to a single building without requiring isolation and blowdown of the entire station yard piping.

FGS Components

Component	Function
Fuel block valve	Isolates fuel gas supply to building
Fuel vent valve	Vents fuel piping to atmosphere
Unit controls	Forces suction/discharge valves closed
Case vent	Vents compressor cases
Electrical disconnect	De-energizes non-essential circuits

FGS Operation

• Fuel blocked and vented by pair of flow control valves
• Solenoid control valve operates pilot to position block/vent valves
• Pneumatic connection to ESD control gas line preferred
• ESD activation also causes FGS
• Block/vent valves shall not reposition when ESD control gas is de-pressurized

Unit Shutdown Methods

Stop Type	Description	Application
Emergency Stop	Immediate shutdown, case vented	Safety-critical situations
Fast Stop	Rapid shutdown with case venting	Lesser hazards (with vent provision)
Normal Stop	Controlled shutdown sequence	Routine operations

FGS vs. ESD Comparison

Feature	ESD	FGS
Scope	Entire station	Single building
Pipeline isolation	Yes	No
Yard piping blowdown	Yes	No
Fuel gas blocked	Yes	Yes (building only)
Units affected	All	Building units only

4. Detection Systems

A standard set of instrumentation shall be provided for compressor buildings to protect personnel and equipment. For unattended facilities, this includes flame detection, fire detection, and hazardous gas detection.

Required Detection Types

Detection Type	Technology	Purpose
Flame Detection	UV/IR optical	Detect oil fires at compressor base
Fire Detection	High heat detectors	Detect hot gases near roof
Gas Detection	IR point or open path	Detect hydrocarbon releases

Flame Detection

• UV/IR combination detectors for reliability
• Coverage of all locations around compressor engine base
• Primary purpose: detect oil fires
• Sufficient detectors to sense fire at any location

High Heat Detectors

• Installed near roof to detect hot gas accumulation
• Additional detectors in pits and basements with restricted airflow
• Rate-of-rise or fixed temperature types

Hazardous Gas Detectors

Required in all compressor buildings:

Detector Type	Application	Conditions
Open path IR	Large open areas	Building arrangement allows effective use
Point IR	Localized monitoring	Temperature maintained within limits
Catalytic bead	General purpose	Clean atmosphere required

Detection Response Matrix

Detection	Level 1 Response	Level 2 Response
Flame detected	FGS, alarm	ESD (confirmed fire)
High heat	Alarm, ventilation	FGS (sustained)
Gas 20% LEL	Alarm, ventilation	—
Gas 50% LEL	FGS, unit stops	ESD (automatic for offshore)

5. Control Logic

A control system shall monitor instruments and provide control actions as needed. It shall include event recorder, annunciator, and operator interface functions.

Logic Implementation Options

• Hardwired relay-based controls
• Programmable Logic Controllers (PLCs)
• Dedicated safety processors
• Contact/button strings (simple systems)

Standard Functions Matrix

All new compressor buildings shall implement the standard protection matrix:

Function	Type	Notes
High Heat Detection	Required	With fault detection
Ventilation Control	Required	Start/stop capability
Electrically Initiated ESD	Required	With fault monitoring
Electrically Initiated FGS	Required	With fault monitoring
Manually Initiated ESD	Required	With fault monitoring
Manually Initiated FGS	Optional	With fault monitoring
Shunt Trip - Lights	Required	Non-essential circuits
Unit Emergency Stop	Required	Hard wired
Unit Fast Stop w/ Vent	Required	Hard wired
Fire Horn	Required	Enabled when attended
Blue Beacon	Optional	Visual alarm

Bypass and Testing

• Functions may be bypassed for maintenance with permanently installed switches
• All bypassed functions shall be annunciated
• Bypasses shall be automatically recorded by event recorder
• ESD and FGS test functions required

Monitoring Display Requirements

Protection system shall include dedicated monitoring display in a "safe" area:

• Event logs and operator interface
• At least two displays if merged with automation system
• One display in designated safe area
• Isolated communications circuit for safe area display
• Battery backup and standby generator power

6. Power Supply

A dedicated power supply shall be provided for the protection system with battery backup to ensure operation during power outages.

Power Supply Requirements

Parameter	Requirement
Battery backup (no generator)	Minimum 2 hours
Battery backup (with generator)	Minimum 20 minutes
Master ESD circuit power	Dedicated feed, maximum reliability

Acceptable Power Supply Types

Type 1: DC Battery Bank

• Large bank of DC batteries
• Redundant DC chargers
• Optional inverter for 120 VAC loads

Type 2: Commercial AC UPS

• Commercial grade AC UPS
• External bypass switch for rapid replacement
• External distribution breakers

Type 3: Distributed Small Supplies

• Multiple small DC supplies and AC inverters
• Individual power to selected loads
• Sized for load transfer after single supply failure
• Redundancy through cross-connection capability

Conduit and Wiring

When Master ESD Circuit is required:

• Route through dedicated conduit
• All conduit and fittings dedicated to ESD circuit
• No sharing with non-protection system circuits
• Underground routing preferred
• If through shared pillboxes: isolate with liquid tight (red color-coded)

7. Code Requirements

Applicable Codes & Standards

Code/Standard	Scope
49 CFR Part 192	Gas transmission pipeline safety standards
NFPA 70 (NEC)	Electrical installations
API RP 14C	Analysis, design of safety systems (offshore)
IEC 61511	Safety instrumented systems
NFPA 72	Fire alarm and signaling code

49 CFR 192 ESD Requirements Summary

• Required for stations >1,000 HP (except unattended field stations)
• Must block gas and blow down station piping
• Must discharge gas at safe location
• Must shut down compressors, gas fires, and electrical (except emergency lighting and equipment protection circuits)
• Operable from at least 2 locations outside gas area
• Locations near exit gates/exits, within 500 ft of station

Automatic ESD Triggers (Offshore/Inland Waters)

Condition	Station Type	Action
Pressure = MAOP + 15%	Unattended	Automatic ESD
Uncontrolled fire on platform	Unattended	Automatic ESD
Uncontrolled fire in building	In building	Automatic ESD
Gas ≥ 50% LEL with ignition source	In building	Automatic ESD

Electrical Exceptions During ESD

The following may remain energized during ESD:

• Emergency lighting: Required to assist personnel evacuation
• Equipment protection: Circuits needed to protect equipment from damage

References

• 49 CFR Part 192 – Transportation of Natural Gas by Pipeline (DOT/PHMSA)
• NFPA 70 – National Electrical Code
• NFPA 72 – National Fire Alarm and Signaling Code
• API RP 14C – Analysis, Design, Installation of Basic Surface Safety Systems
• Safety Instrumented Systems
• API RP 500 – Classification of Locations for Electrical Installations