Hot work and gas testing in hydrocarbon and sour service

Hot work in an oil and gas facility brings an ignition source into a place built to contain flammable gas. HSE-216 covers hot work in general, including how hot work starts fires, protecting combustibles, gas testing near tanks and containers, and the fire watch. HSE-330 covered gas detectors: how sensors work, their limits, bump testing, calibration and alarm set points. This lesson puts the two together for the conditions of a battery, compressor station or sour gas plant, and shows how gas testing becomes a control within the permit rather than a box on the form.

First question: does it have to be hot work?

The safest hot work is the hot work that never happens. Before issuing a hot work permit in a process area, the planner should ask whether the job can be done another way. Many tasks that were once welded can now be done cold:

• cutting pipe with cold-cutting saws, abrasive water jets or pipe cutters instead of torches;
• joining with mechanical couplings, flanged connections or bolted clamps instead of welds, where the design and code allow;
• removing bolts with hydraulic tools or nut splitters instead of cutting them off;
• prefabricating spools and supports in a shop or outside the classified area and bringing them in for bolting;
• scheduling the work during a planned shutdown when the area can be cleared of hydrocarbons.

If hot work is still needed, the next question is whether it can be moved out of the classified area or whether the area around it can be made gas-free, isolated and kept that way for the whole job.

Where gas comes from during hot work

A gas test at the start of a job only shows conditions at that moment. In an operating facility, gas can arrive at the hot work site from many sources while the work is going on:

• equipment being worked on that was not fully drained, purged or cleaned, including scale, sludge and liners that release vapour when heated;
• passing valves, open drains and vents on nearby equipment;
• tank hatches, vapour recovery headers and connected tanks, as at Partridge-Raleigh;
• sewers, drain hubs, catch basins and trenches, where heavier-than-air vapour collects;
• relief and blowdown discharges and flare knockout drums;
• leaks from flanges, packing and seals that change with pressure and temperature;
• process upsets elsewhere in the plant, which can release gas with little warning.

Wind matters too. A light wind can carry gas from a leaking flange 20 m (65 ft) away onto the welding bench, and a calm, cold morning can let vapour settle in low spots that were clear at noon the day before.

Where and in what order to test

Gas tests for hot work are taken at the work point and at every place gas could come from: inside the equipment if it has been opened, at nearby drains and openings, at low points and inside any nearby enclosure or shelter. Tests inside equipment use a sampling pump and tube so the tester does not have to enter or lean in. The usual order is oxygen first, because a low oxygen reading means flammable and toxic readings may be wrong; then flammable gas as percent of the lower explosive limit (LEL); then hydrogen sulphide and any other toxic gas expected, such as benzene near condensate tanks or sulphur dioxide near the sulphur plant. The detector should be bump-tested before use, and the tester should allow enough sampling time for gas to reach the sensors through the tube.

Acceptance criteria

Each company sets its own acceptance criteria in its hot work and permit standards, usually stricter than the legal minimum. The table shows criteria of the kind many oil and gas companies use. They are illustrative only; always use the values in your site's standard.

How often to retest

Retesting is where many hot work permits break down. Company standards usually require a test immediately before work starts, a retest after any break longer than a set time, retesting at set intervals during the job, continuous monitoring for open-flame work in process areas, and an immediate retest when conditions change: a wind shift, a process upset, a new job starting nearby, or anyone smelling gas. The program below works through a day of readings from the hot work site at Kettle Creek and flags when a retest was missed or a reading failed.

# Check a day of hot work gas tests against illustrative criteria and a 2-hour retest rule.
LIMITS = {"O2_low": 19.5, "O2_high": 23.0, "LEL": 0.0, "H2S": 5.0}   # illustrative
MAX_GAP_MIN = 120
tests = [  # (time hh:mm, O2 %, LEL %, H2S ppm, note)
    ("07:40", 20.9, 0, 0, "pre-start, work point and drains"),
    ("09:35", 20.9, 0, 0, "after coffee break"),
    ("12:20", 20.9, 0, 1, "after lunch"),
    ("13:05", 20.8, 3, 2, "wind shifted to the south"),
    ("13:30", 20.9, 0, 0, "after leaking flange tightened"),
    ("15:20", 20.9, 0, 0, "mid-afternoon"),
    ("16:10", 20.9, 0, 0, "end of welding"),
]

def minutes(t):
    h, m = map(int, t.split(":"))
    return 60 * h + m

prev = None
for t, o2, lel, h2s, note in tests:
    problems = []
    if not LIMITS["O2_low"] <= o2 <= LIMITS["O2_high"]:
        problems.append("oxygen out of range")
    if lel > LIMITS["LEL"]:
        problems.append(f"LEL {lel} % - stop hot work")
    if h2s > LIMITS["H2S"]:
        problems.append("H2S above action level")
    if prev is not None and minutes(t) - prev > MAX_GAP_MIN:
        problems.append(f"gap of {minutes(t) - prev} min exceeds {MAX_GAP_MIN}")
    status = "; ".join(problems) if problems else "ok"
    print(f"{t}  O2 {o2:4.1f}  LEL {lel:>2}  H2S {h2s:>2}  {note:<36} {status}")
    prev = minutes(t)

07:40  O2 20.9  LEL  0  H2S  0  pre-start, work point and drains     ok
09:35  O2 20.9  LEL  0  H2S  0  after coffee break                   ok
12:20  O2 20.9  LEL  0  H2S  1  after lunch                          gap of 165 min exceeds 120
13:05  O2 20.8  LEL  3  H2S  2  wind shifted to the south            LEL 3 % - stop hot work
13:30  O2 20.9  LEL  0  H2S  0  after leaking flange tightened       ok
15:20  O2 20.9  LEL  0  H2S  0  mid-afternoon                        ok
16:10  O2 20.9  LEL  0  H2S  0  end of welding                       ok

Two problems stand out. The gap between the after-lunch test at 12:20 and the 09:35 test was 165 minutes, longer than the two-hour rule, so either a test was missed or it was not recorded. And at 13:05 the wind shifted, the work point read 3 percent LEL and the crew had to stop welding until the source, a leaking flange upwind, was found and tightened. On a continuous monitor, that reading would have sounded an alarm without waiting for the tester. This is exactly the kind of change that a single pre-start test cannot catch.

Sour service and hot work

Hot work on equipment that has been in sour service has extra hazards. Iron sulphide scale inside piping and vessels can be pyrophoric, glowing or igniting when it dries out in air, and heating it can release sulphur dioxide. Scale, sludge and porous materials can hold H2S and release it when heated or disturbed, so a vessel that tested clear before welding can produce H2S once the arc starts. Welders working on or near sour equipment need personal H2S monitors, a plan for breathing apparatus, and in many companies a requirement that the equipment be steamed, water-washed or chemically cleaned before any hot work on it. HSE-332 covered pyrophoric iron sulphide and the chemicals of sour plants.

Hot work in live plants: habitats and in-service welding

Sometimes hot work must be done in an area that cannot be shut down, such as a tie-in beside a running compressor. Two specialized methods are used under strict procedures.

• Pressurized welding enclosures, often called habitats, are fire-resistant enclosures built around the work and kept at a slight positive pressure with air drawn from a safe location. Gas detectors at the air intake and inside the enclosure automatically shut off the welding power and alarm if gas is detected. They allow welding in a classified area, but only when designed, installed, tested and monitored by trained people.
• In-service welding and hot tapping weld onto pipe or vessels that are still under pressure and flowing. They require engineering of the weld procedure to avoid burn-through and cracking, verification of wall thickness, and specialist crews. As Lesson 2 noted, hot taps under documented procedures are excluded from OSHA's lockout standard in the US; in both countries they are a specialized activity, not ordinary hot work.

The fire watch in oil and gas

The fire watch's job does not change from HSE-216: watch for sparks and smouldering, keep extinguishing equipment ready, stop the work and raise the alarm if needed, and stay after the work ends. In oil and gas facilities, the fire watch also often carries a multi-gas monitor, watches for gas alarms and process changes, and knows how to activate the emergency shutdown or plant alarm. Fire watches should have no other task, and in many companies they must remain for at least 30 to 60 minutes after hot work ends, longer where insulation, sludge or scale could smoulder.