Propylene Brine Chiller Package | EPC Playbook

What This Playbook Solves

A propylene brine chiller package is not “just a chiller.” In real EPC projects it is a process-critical refrigeration train (often skid-mounted) that must hold a stable brine supply temperature across steady load + transients + upset cases, while meeting flammable refrigerant (A3) safety expectations and the owner’s documentation requirements. Propylene (R-1270) is widely referenced as an A3 (high flammability) refrigerant in safety classification frameworks, which immediately changes how EPC teams should treat layout, ventilation, detection, electrical area classification, and shutdown logic.

This playbook is written for EPC procurement, technical, and decision teams who want two things at the same time: schedule certainty and audit-proof evidence. If you have lived through “late TBDs,” rework loops, and vendor clarifications that arrive after fabrication starts, you already know the pattern: delays are rarely one big failure—more often they are a chain of small misses across datasheets, interfaces, and hold points.

What you will get (practical outcomes)

• A scope-first RFQ logic: what to freeze early so bids are comparable and redesign loops are minimized
• A systems view: refrigerant loop + brine loop + battery limits + control ownership
• A safety-and-evidence mindset: not just “compliant,” but “provable” via ITP, FAT, and VDR (Vendor Data Record) structure
• A decision toolset you can reuse: duty table template, interface list, and bid leveling checklist

Who this is for (and who it is not)

• For: EPC teams in petrochemical, chemical, LNG/offshore modules, and energy projects where brine distribution serves multiple consumers (reactor jackets, exchangers, condensers, cold boxes, trim coolers, etc.).
• Not for: comfort HVAC selection guides or “catalog-style” chiller shopping. This is about project execution under EPC constraints.

The typical failure chain (why projects slip)

1. RFQ inputs remain TBD (duty split, transient loads, brine temperatures, ΔP limits).
2. Interfaces are vague (“by others,” “as required”), leading to scope disputes and late design changes.
3. Safety philosophy is not frozen (A3 implications, detection, ventilation, ESD), creating late rework and approvals churn.
4. Evidence is discovered late (ITP hold points missed, FAT becomes a “paper FAT,” data book gaps).

If we fix this chain early, delivery risk drops sharply—and vendor performance becomes easier to verify.

How to use this playbook (step-by-step)

• Step 1: Read Part 1 (decision tools) and align internally on architecture + safety philosophy.
• Step 2: Use Parts 2–3 to build a shared systems language across process, mechanical, and E&I.
• Step 3: Before issuing RFQ, build a one-page Freeze-First sheet (we provide the structure in Part 4).

Executive Summary & Quick Decision Tools

EPC buyers do not need more theory. You need a repeatable decision method that produces comparable bids and prevents schedule creep. A propylene (R-1270) package is usually chosen when the project needs high-performance refrigeration and is prepared to manage flammability-driven safeguards (A3).

The commercial truth is simple: vendors can quote anything if inputs are vague. Your job is to convert uncertainty into frozen decision constraints early—then vendors must compete on execution quality, evidence, and lead time, not on assumptions.

The “5 things” that protect schedule

1. Freeze duty inputs: base + peak + transient + upset cases (not only one design point).
2. Freeze brine supply/return temperatures and allowable approach/ΔT philosophy.
3. Freeze battery limits and tie-ins (mechanical + electrical + instrument + control ownership).
4. Freeze safety philosophy for A3: HazArea, detection, ventilation, ESD actions.
5. Freeze ITP + FAT evidence: define what must be witnessed and recorded, not just “FAT done.”

Bid-leveling table (what Procurement should compare)

Fundamentals: What “Propylene + Brine Loop” Means in Real Plants

A secondary coolant (brine) loop is an EPC-friendly way to distribute cooling to multiple users without placing refrigerant-containing equipment everywhere. The brine loop provides temperature stability, simpler distribution piping, and often clearer ownership boundaries between the refrigeration package and process consumers.

Propylene (R-1270) is commonly grouped under hydrocarbon refrigerants and referenced as A3 (highly flammable) in safety classification discussions, which is why “mechanical room philosophy,” detection, ventilation, and ignition control become central topics—not optional add-ons.

Key terms (plain English)

• Refrigerant loop: compressor → condenser → expansion control → evaporator (chiller) → back to compressor
• Brine loop: brine tank/surge → pumps → consumers → return → chiller HX
• Cooling consumer: any equipment that demands heat removal (often with varying duty and flow)
• Turndown: how low the package can operate stably without hunting/tripping
• Transients/upsets: load steps, start-up surges, shutdown cool-down, fouling drift, seasonal ambient changes

Why EPC projects like brine loops (and where they go wrong)

Brine loops are popular because they centralize risk: you contain the refrigerant system within a defined area and distribute a stable secondary coolant to users. But brine loops go wrong when EPC teams do not freeze:

• brine supply/return temperatures and allowable ΔT
• minimum flow and bypass philosophy
• distribution pressure drop and balancing approach
• brine quality requirements (corrosion control, contamination limits, filtration)

The minimum “fundamentals” you must align on internally

1. Temperature level: what brine supply temperature must be guaranteed at the header
2. Load map: which users drive base load vs peak vs transients
3. Control ownership: vendor PLC vs DCS, and who owns permissives/trips
4. Safety frame: A3 implications (where refrigerant is contained, detection zones, ventilation expectations)

System Architecture: From Refrigerant Loop to Brine Consumers

Think of the package as two coupled systems. The refrigerant loop creates cold; the brine loop delivers it. EPC success comes from defining the coupling points clearly: temperatures, flows, ΔP limits, controls, and battery limits.

A practical architecture description should be bid-ready: a vendor must be able to point to your RFQ and say, “we understand the duty map, the interfaces, and the safety expectations.” Anything less becomes a clarification cycle that eats weeks.

Where EPC must be explicit (interfaces and “battery limits”)

Below is the interface list that prevents 80% of scope disputes:

What equipment is typically inside the package (scope reality)

Common packaged elements

• Compressor train (with oil system, seal system as applicable)
• Condenser (air cooler or cooling water exchanger)
• Receiver / suction drum / separator (as required by design)
• Brine chiller evaporator (heat exchanger)
• Brine pumps (duty/standby) and brine tank (if included by vendor)
• Instrumentation (P/T/flow/level/vibration) and local control panel

Common EPC/Owner-side elements (often excluded)

• Long-distance brine distribution headers and consumer-side control valves
• Plant-wide vent/flare header connection (depending on philosophy)
• Building/mechanical room civil works and HVAC (if not in vendor scope)

RFQ Inputs to Freeze Early (Prevent Redesign Loops)

A propylene (R-1270) brine package becomes “hard” when EPC issues an RFQ with uncertain duty, unclear interfaces, and an un-frozen safety philosophy. Vendors will still quote—but you will pay later in redesign loops, vendor clarifications, and schedule slips.

Because propylene is widely referenced as a highly flammable (A3) hydrocarbon refrigerant, the RFQ must explicitly state the project’s hazardous area and safeguards expectations (detection, ventilation, ignition control, and shutdown objectives), not treat them as optional add-ons.

4.1 The #1 delay driver: “TBD” inputs (and how to close them)

If your datasheet still contains TBD on the items below, the quote is not truly comparable—and the vendor’s lead time is not truly defendable.

Your objective is not perfection; it is data closure discipline: confirm what is known, bracket what is uncertain, and lock “must-not-change” constraints early (temperatures, interfaces, and safety objectives).

4.2 Freeze-first RFQ checklist (the minimum EPC must lock)

Use this as a one-page “freeze sheet” that Procurement can attach to the RFQ and later to the PO.

4.3 RFQ duty table (bid-ready format)

This table structure forces clarity and makes vendor assumptions visible.

4.4 Interfaces (battery limits) EPC must define—without ambiguity

Most disputes are not about engineering—they are about boundaries. Put these into the RFQ as a numbered list.

• Mechanical: nozzle schedule, flange rating, allowable loads, tie-in coordinates, drainage/vent routing
• Electrical: incoming feeder scope, MCC/VFD boundary, earthing/grounding
• Instrumentation: signal list, I/O count, instrument air/N2 needs, calibration standards
• Controls: PLC vs DCS ownership, permissives/trips, remote start/stop philosophy, communications protocol
• Safety: gas detection zones, ventilation triggers, ESD actions (what trips what)

Because hydrocarbon refrigerants are commonly treated as highly flammable A3 under safety classification discussions, EPC should ensure the RFQ includes the project’s safeguards expectations rather than leaving them to vendor interpretation.

Process & Thermodynamic Design (Duty Mapping → Turndown → Transients)

A propylene brine package is successful when it holds stable brine supply temperature through real plant behavior: load steps, seasonal ambient changes, start-ups, and upset cases. That requires EPC to define what ‘stable’ means (tolerance bands and response expectations) and to request a control strategy that can actually deliver it.

5.1 Translate duty into “control reality” (what the package must regulate)

EPC should specify three bands:

1. Normal band: typical load and steady operation
2. Transient band: load steps (e.g., a consumer valve opens)
3. Upset band: abnormal but credible conditions (e.g., partial fouling, reduced cooling medium)

A vendor who only guarantees one design point is often pushing risk back to EPC. Your RFQ should ask vendors to state performance at these bands, even if some are “best effort” rather than guaranteed.

5.2 Key process parameters to define (and why)

5.3 Brine loop design: what “good” looks like in EPC language

A brine loop is not just “water with salt.” EPC must confirm:

• Freeze margin: the brine composition must remain safely above freezing at the coldest operating condition
• Materials and corrosion strategy: confirm compatibility and filtration/cleanliness expectations
• Hydraulics: ensure pump NPSH margin and distribution pressure drops are realistic
• Minimum flow protection: define bypass philosophy to protect the chiller evaporator at low load

If EPC leaves brine quality and minimum flow undefined, vendors may quote minimal-cost assumptions that later become operational problems.

5.4 Capacity control and turndown (plain-English view)

For EPC, the key question is: Can the package hold temperature without tripping when load drops?

Ask vendors to describe:

• How capacity is regulated (staging, slide valve, VFD, hot-gas bypass, etc.)
• Minimum stable operating point and what happens below it
• Anti-hunting strategy (especially when multiple consumers cycle)

5.5 What to require as “performance verification” (so FAT has meaning)

Mechanical Package Design (Skid Layout, Vessels, HX, Piping, Materials)

Mechanical design is where EPC projects win or lose maintainability, safety execution, and schedule reliability. A good skid is not only compact—it is serviceable, inspectable, and transportable without rework.

Because propylene (R-1270) is widely referenced as a hydrocarbon refrigerant and commonly treated as highly flammable (A3) in safety-oriented references, mechanical layout must support the safety philosophy (segregation, ventilation paths, detection coverage, ignition control, and safe access).

6.1 Skid modularization: EPC constraints you must state early

6.2 Pressure equipment and heat exchangers: what EPC should demand

At minimum, EPC should require:

• Clear code basis for pressure parts (project-required code and stamping expectations)
• Material traceability approach (MTRs, heat numbers, PMI where required)
• NDT scope and acceptance criteria tied to ITP hold points

For the brine chiller evaporator (HX), EPC should specify:

• Design brine inlet/outlet temps and allowable approach
• Fouling factors (if applicable)
• Maintenance philosophy (cleaning access, spare bundle concept if relevant)

6.3 Materials and insulation: prevent cold-service surprises

Cold service projects often fail from “small” misses:

• Wrong gasket selection for temperature and media
• Insulation details that trap moisture and cause corrosion under insulation (CUI)
• Instrument impulse lines freezing or drifting
• Inadequate supports leading to vibration fatigue

6.4 Piping and valves: the maintainability test

A practical EPC test is: Can a technician service the skid without dismantling half the pipework?

Ask for:

• Valve reach and maintenance access confirmation
• Removable spools where needed
• Clear drain/vent points with safe routing
• Strainers and filters placed for easy cleaning
• Permanent tags aligned to data book indexes

6.5 Rotating equipment: vibration and alignment (non-negotiable basics)

For compressor trains and large pumps:

• Require an alignment method statement
• Define vibration acceptance standards (project-specific)
• Request evidence of shop run / functional checks (tied to FAT and ITP later)

This is not “extra.” It is the cheapest way to avoid start-up failures and repeated commissioning trips.

6.6 Mechanical deliverables EPC should require in the bid stage

Safety & Compliance (A3 Refrigerant Reality, HazArea, Relief, Detection, SIS)

Propylene (R-1270) is commonly referenced as a hydrocarbon refrigerant and classified as A3 (lower toxicity, high flammability) in the ISO refrigerant safety framework. For EPC teams, that single fact changes the project posture: you are not “buying a chiller,” you are managing a flammable-gas risk with engineered safeguards, documented evidence, and auditable operating logic.

Most owners will require hazardous area classification to follow recognized methods/standards (e.g., IEC 60079-10 series for explosive gas atmospheres) and to treat ignition source control as a systematic discipline. Your RFQ and vendor design review should therefore focus on a simple objective: make the risk controls explicit, verifiable, and testable before fabrication is locked.

7.1 EPC safety deliverables to “freeze” early

Use this table as a pre-award checklist. It is not about “more documents”—it is about preventing late redesign and late approvals.

7.2 “Top 10” A3 risk questions EPC should ask vendors (bid stage)

1. Where is the refrigerant-containing equipment located (open air vs enclosed)?
2. What is the ventilation basis (natural/mechanical) and how is it triggered?
3. Where are gas detectors installed and what coverage logic is used (leak scenarios)?
4. What are the alarm and trip setpoints (and the rationale)?
5. What is the ESD action list (what trips what, and in what order)?
6. How are ignition sources controlled (equipment rating, hot surfaces, static, motors, panels)?
7. How are drains/vents handled (where can gas accumulate; where can it vent safely)?
8. What is the maintenance access strategy without violating HazArea assumptions?
9. How is operator response supported (alarms, annunciation, reset logic, re-start permissives)?
10. What is the evidence package that proves the above (not just statements)?

7.3 Compliance anchors you can cite (non-legal, practical)

• ISO 817:2024 defines refrigerant designation and safety classification (A3 = high flammability, lower toxicity).
• ASHRAE Standard 34 maintains refrigerant designation and safety classification tables.
• IEC 60079-10-1 addresses classification of areas where flammable gas/vapour hazards may arise.
• IEC 60079-29-2 provides guidance for selection/installation/use/maintenance of detectors for flammable gas and oxygen.

E&I + Controls (Interfaces, C&E, Trips, Start-Up Stability, Integration)

Controls and electrical scope are the number-one source of “we assumed you would do it” disputes. The fix is to define control ownership and interface boundaries at RFQ stage, then require vendors to submit a Cause & Effect (C&E) and I/O list that are consistent with the safety philosophy and operating philosophy.

Because the package involves a flammable refrigerant (A3 context), the controls must also support the safety functions—especially detection-driven actions and restart permissives—rather than treating them as optional “site works.”

8.1 Define control ownership (simple, explicit, non-negotiable)

Pick one of these models and state it in the RFQ:

RFQ must include:

• Command list (Start/Stop/Load/Unload/Setpoints)
• Permissives list ownership (who validates what)
• Trip reset philosophy (manual/automatic; re-start conditions)

8.2 Minimum E&I interface package (what vendors must provide)

8.3 Controls that protect start-up and avoid nuisance trips

EPC should request vendors to document:

• Start-up sequence (warm-up, permissives, ramp logic)
• Minimum stable operating logic (avoid hunting at low load)
• Alarm rationalization for key variables (suction/discharge P/T, brine supply T, flows, vibration)
• Restart permissives tied to safety state (especially after gas detection events)

QA/QC, ITP Hold Points, FAT/SAT (How EPC Avoids “Paper-FAT”)

For EPC, QA/QC is not a department—it is a delivery risk control system. The goal is to make quality verifiable through ITP hold points, and to make FAT meaningful by proving function, safety logic, and evidence completeness—before the skid is shipped.

A3 context amplifies this requirement: owners are far less tolerant of “trust us” when a system involves flammability-driven safeguards. Gas detection systems have dedicated IEC 60079 guidance and must be supported by documented selection/installation practices and traceable test evidence.

9.1 ITP structure (practical EPC hold points that matter)

9.2 FAT that proves performance (not a meeting with signatures)

A high-value FAT for an EPC buyer should demonstrate:

1. Functional control tests (start/stop logic, permissives, alarm/trip actions)
2. C&E verification (simulate causes and confirm actions; latch behavior; reset conditions)
3. Instrument and I/O proof (I/O forcing logs, calibration certificates)
4. Safety function tests (gas detection inputs simulated; ventilation and ESD actions verified)
5. Documentation closure (Vendor Data Book index complete; redlines resolved)

9.3 “Paper-FAT” red flags (what Procurement should reject)

9.4 Evidence pack (Vendor Data Book / VDR) — minimum index EPC should require

• Equipment list + tag register
• Approved drawings (GA, P&ID, wiring, termination, loop diagrams)
• Material certificates (MTR) + traceability records
• Welding/NDT reports + pressure test reports
• Instrument calibration certificates + loop check sheets
• FAT procedure + FAT report + test logs + punch list closure
• Preservation and packing records
• Spares list + recommended commissioning spares

Project Execution & Schedule Control (Documents, Reviews, Critical Path)

In EPC delivery, a propylene brine chiller package fails on schedule for one reason: decisions are made late and get disguised as “clarifications.” Your job is to convert the project into a sequence of document gates that freeze the right items in the right order—then tie those gates to procurement commitments and ITP/FAT evidence.

Because propylene (R-1270) is commonly classified as A3 (highly flammable), safety deliverables (HazArea basis, detection/ventilation concept, ESD objectives) are not optional add-ons—they must be integrated into the early document gates to avoid late redesign.

10.1 The critical path (what actually drives lead time)

For packaged refrigeration systems, the longest poles are typically:

• Compressor train (compressor, motor, coupling, baseplate, oil system)
• VFD / MCC equipment (if applicable), critical instruments
• Air coolers / condensers or CW heat exchangers
• Pressure parts with code stamping and NDE requirements
• Control panels and communications hardware
• Safety items (gas detectors, certified Ex/ATEX/IECEx components as required)

10.2 Document gates (the “freeze order” that prevents redesign loops)

Standard EPC gating sequence: PO Award → Datasheet Freeze → P&ID Freeze → Battery Limits Freeze → Safety Freeze → Controls Freeze → IFA Vendor Docs → AFC/Approved Docs → Fabrication Start → ITP Hold Points → FAT → Data Book Release → Release-to-Ship

What each gate must contain (practical):

• Datasheet freeze: duty bands, brine temps, min flow, ambient cases, electrical philosophy
• P&ID freeze: all valves, instruments, drains/vents, safety tie-ins, tag register alignment
• Battery limits freeze: mechanical/electrical/I/O/controls boundaries signed by EPC
• Safety freeze: gas detection zones, ventilation triggers, ESD actions, HazArea assumptions
• Controls freeze: C&E matrix, I/O list, comms protocol, restart permissives

10.3 Bid evaluation that prevents “cheap quote, expensive project”

A robust bid evaluation scores risk, not just price.

10.4 Change management (kill scope creep early)

Rule: no change enters fabrication without a documented impact:

• Cost impact
• Schedule impact
• Safety impact
• Evidence impact (what re-tests are required)

Tool: an “Assumptions & Deviations Register” controlled by EPC Procurement and Engineering jointly.

Logistics, Preservation, Installation, Commissioning (From Shop to Stable Operation)

A well-built skid can still fail in the last mile. Logistics and preservation are not clerical tasks; they are risk controls against:

• Corrosion in transit
• Contamination of brine loop
• Rotating equipment degradation
• Electrical moisture damage
• Alignment drift after transport

11.1 Preservation and packing (what EPC should require before shipment)

11.2 Site readiness checklist (what must be ready before delivery)

11.3 Installation and pre-commissioning (sequence that prevents trip cascades)

1. Mechanical inspection + preservation removal check
2. Alignment verification (compressor/pumps)
3. Piping tie-ins + leak tests per procedure
4. Brine loop flush/cleanliness confirmation
5. Instrument calibration verification and loop checks
6. Controls interface test (I/O, comms, permissives)
7. Dry-run sequence test (simulate trips and safe-state)
8. Controlled startup and ramping to normal band

11.4 Commissioning acceptance criteria (make success measurable)

EPC should define acceptance as evidence-based outcomes:

• Brine supply temperature stability within defined tolerance band over a test window
• No nuisance trips under defined load steps
• Alarms and ESD actions behave per C&E matrix
• Data book delivered and closed (as-built documentation)

Operations & Lifecycle (Brine Management, Reliability, Spares, KPI)

This section helps EPC decision makers justify the package based on operability and total cost of ownership, not only capex. A brine loop is operationally stable when brine quality, filtration, and minimum-flow protection are treated as routine discipline.

12.1 Brine management (the reality that impacts reliability)

Operational problems often come from:

• Contamination (solids, oil ingress, process leaks)
• Corrosion under insulation (CUI) and poor vapor barrier details
• Poor filtration and lack of sampling discipline
• Drifting concentration leading to insufficient freeze margin

EPC recommendation: define an O&M routine covering sampling frequency, filtration maintenance, concentration verification, and corrosion inhibitor strategy (as applicable).

12.2 Reliability discipline (what to monitor and why)

12.3 Spares strategy (avoid “we forgot it” downtime)

Define spares in three tiers:

1. Commissioning spares: seals, gaskets, sensors, fuses, filters
2. 2-year ops spares: key instruments, valves, critical pump parts
3. Insurance spares: long-lead items (subject to owner philosophy)

EPC tip: require vendors to propose spares as a structured list with recommended quantity, lead time, storage requirements, and interchangeability notes.

12.4 KPI view (how management measures success)

• Availability (uptime %)
• Unplanned shutdown count per quarter
• Brine temperature deviation events
• Energy consumption trend (kW/ton or kW per duty unit)
• Maintenance cost trend vs baseline

Case Configurations & Reference Patterns (Typical P&ID Blocks)

EPC teams move faster when they reuse proven configuration patterns instead of reinventing architecture every project. The goal of this section is not to prescribe one “best” design—it is to give you standard reference blocks you can copy into RFQs, bid leveling, and 30% design reviews.

Because propylene (R-1270) is widely treated under safety classification frameworks that include flammability classes (A3 context), each pattern below assumes you will align early on hazardous area basis, detection/ventilation concept, and ESD objectives.

13.1 Pattern A — Single Package + Dual Brine Pumps (Duty/Standby)

Best when you have a stable base load and acceptable downtime risk (or short repair windows).

• Specify minimum flow protection (bypass or recirculation) to avoid unstable control at low user load.
• Require GA drawing to confirm maintenance clearance for pump cartridge/seal replacement.

13.2 Pattern B — N+1 Redundancy (Two Packages, One as Standby/Assist)

Best when uptime targets are strict, or production loss cost is high.

• Require vendor to define operating modes: lead/lag rotation, standby warm state, and switchover logic.
• In bid leveling, compare switchover time and restart permissives—these often drive real availability.

13.3 Pattern C — Split Battery Limits (Vendor Skid + EPC Brine Tank/Headers)

Best when EPC wants more control of brine inventory, chemical dosing, filtration, or distribution design.

This pattern only works if you freeze battery limits and assign control ownership clearly (who owns brine temperature loop, who owns pump VFDs, who owns trips).

13.4 “Reference P&ID blocks” EPC should standardize

• Drains/vents and safe routing philosophy
• Minimum flow/bypass arrangement
• Sampling points (brine quality and contamination control)
• Instrumentation set (P/T/flow/ΔP, vibration, levels)
• Isolation strategy for maintenance

RFQ Toolkit (Templates, Checklists, Tables, Download Pack)

This toolkit is designed to convert your project requirements into high-quality, comparable RFQs. It forces input closure, clarifies interfaces, and locks evidence requirements early.

Your “RFQ pack” should reference recognized safety classification and hazardous area principles. ISO 817 establishes a system for refrigerant safety classification, and IEC 60079-10-1 covers classification of areas where flammable gas/vapour hazards may arise.

14.1 What to include in the download pack (practical list)

14.2 Safety-related toolkit notes (keep it practical)

• IEC 60079-29-2 is the standard addressing selection/installation/use/maintenance of flammable gas detectors.
• IEC 60079-10-1 covers area classification for explosive gas atmospheres.
• Include HazArea assumptions in the toolkit so bidders share the same zoning basis from day one.

Glossary & Appendix (Standard Terms + Red-Flag Definitions)

This appendix reduces disputes by defining terms the same way across EPC, Owner, and Vendor.

16.1 Glossary (high-impact terms)

16.2 Red-flag phrases in quotations (and what they usually mean)

16.3 Standards references (how to cite without overclaiming)

• ISO 817:2024 provides a system for refrigerant designation and safety classification.
• ASHRAE Standard 34 publishes refrigerant designation tables tied to safety classification of refrigerants.
• IEC 60079-10-1 addresses classification of areas where flammable gas/vapour hazards may arise.
• IEC 60079-29-2 provides guidance for selection/installation/use/maintenance of detectors for flammable gas and oxygen.