Hazards of pressurized equipment

Almost everything in a facility is under pressure. Separators hold gas at hundreds or thousands of kilopascals, compressors raise it higher, pumps push liquids through piping and pipelines carry both for hundreds of kilometers. Pressure is useful because it moves fluids and makes separation and processing possible. It is dangerous because a pressurized system stores energy, and when containment fails that energy is released in milliseconds. HSE-330 explained why gas stores far more energy than liquid at the same pressure and how pressure releases injure people. This lesson looks at how facility equipment is designed to hold pressure, how it is protected against overpressure, how it fails and what workers can do to keep it inside the pipe.

Pressure terms every worker should know

Several pressures appear on drawings, nameplates and rounds sheets, and confusing them leads to mistakes. The operating pressure is what the equipment normally runs at. The design pressure is the pressure the engineer used to design the vessel or piping, with margin above the operating pressure. The maximum allowable working pressure (MAWP) is the highest pressure the finished vessel may see at the top at a stated temperature; it is stamped on the nameplate and is at least the design pressure. The relief valve set pressure is the pressure at which the relief valve starts to open, normally at or below the MAWP. During a relief event, pressure can rise above the set pressure while the valve is opening fully; the allowed rise is called accumulation, and the codes limit it to small percentages above MAWP. Finally, the hydrostatic test pressure is the higher pressure, typically about 1.3 to 1.5 times MAWP depending on the code, at which a new or repaired vessel is tested with water before it goes into service.

The program below lays out the pressure ladder for an invented separator at the Pinecrest battery. It uses typical code allowances: up to 10 percent accumulation for a single relief valve in an ordinary overpressure case, and up to 21 percent for a fire case. These values are illustrative; the actual limits are set by the applicable code and the engineer's design.

# Pressure ladder for an invented separator (psig). Allowances are typical, illustrative.
operating = 610
mawp = 870
set_pressure = mawp                  # relief valve set at MAWP
accum_single = 0.10                  # single valve, non-fire case
accum_fire = 0.21                    # fire case
hydrotest = 1.3 * mawp               # typical modern code ratio

ladder = [
    ("Normal operating", operating),
    ("High-pressure alarm (example)", 0.90 * mawp),
    ("High-pressure shutdown (example)", 0.95 * mawp),
    ("Relief valve set pressure", set_pressure),
    ("Max during relief, non-fire", mawp * (1 + accum_single)),
    ("Max during relief, fire case", mawp * (1 + accum_fire)),
    ("Hydrostatic test pressure", hydrotest),
]
for name, p in ladder:
    print(f"{name:34s} {p:>7,.0f} psig  ({p/mawp*100:5.1f} % of MAWP)")
print(f"Margin from operating to set pressure: {set_pressure - operating:,.0f} psi")

Normal operating                       610 psig  ( 70.1 % of MAWP)
High-pressure alarm (example)          783 psig  ( 90.0 % of MAWP)
High-pressure shutdown (example)       826 psig  ( 95.0 % of MAWP)
Relief valve set pressure              870 psig  (100.0 % of MAWP)
Max during relief, non-fire            957 psig  (110.0 % of MAWP)
Max during relief, fire case         1,053 psig  (121.0 % of MAWP)
Hydrostatic test pressure            1,131 psig  (130.0 % of MAWP)
Margin from operating to set pressure: 260 psi

The ladder shows the layers of protection stacked between normal operation and failure. Alarms warn operators, shutdowns act automatically, and the relief valve is the last mechanical line of defense before the vessel is exposed to pressures it was not designed for. The hydrostatic test proved that the vessel could withstand a pressure above all of these, but only when it was new or freshly repaired, before years of corrosion and fatigue.

How equipment gets overpressured

Engineers design relief systems for credible causes of overpressure, and knowing them helps workers avoid creating one. The common causes are:

• Blocked outlet: a valve downstream is closed, by mistake or by a control failure, while flow continues to enter.
• Gas blowby: a liquid level controller fails or a drain valve sticks open, so that high-pressure gas flows through a liquid outlet into a lower-pressure vessel or tank that cannot take it.
• Control valve failure: a pressure controller fails open and lets a high-pressure source into a lower-pressure system.
• Fire: an external fire heats a vessel, boils its liquid and raises pressure while weakening the steel.
• Thermal expansion: liquid trapped between two closed valves expands when the sun or process heat warms it, and liquid is nearly incompressible, so pressure rises very fast.
• Reaction or contamination: unwanted chemical reactions or the wrong fluid entering a vessel.
• Utility failure: loss of cooling, power or instrument air changes how equipment behaves.
• Tube rupture: in a heat exchanger, a high-pressure tube fails and pressurizes the lower-pressure shell side.

Gas blowby deserves special attention in field facilities. A separator operating at several thousand kilopascals dumps liquid through a level control valve into a treater or tank operating at a few kilopascals. If the level falls to zero and the dump valve stays open, gas follows the liquid. An atmospheric tank built to hold a few kilopascals can be torn open in seconds by blowby from a high-pressure separator, and several fatal tank explosions have involved this sequence. Low-level shutdowns, properly sized tank vents and keeping level controls maintained are the defenses. Never bypass a low-level shutdown to keep a well flowing.

Thermal expansion is the other overpressure that workers most often create themselves. A short section of liquid-filled pipe isolated by two closed valves on a sunny day can rise from near zero to many thousands of kilopascals, enough to burst a gasket or crack a fitting. Thermal relief valves protect lines that are routinely blocked in, and procedures for isolating liquid lines should say how to avoid trapping liquid.

Relief valves and other relief devices

A pressure relief valve is a spring-loaded or pilot-operated valve that opens at its set pressure and closes again when pressure falls. A rupture disc is a thin membrane that bursts at a set pressure and must then be replaced. Relief devices discharge to a flare header, a vent stack or, for some liquids, a closed drain. Each device is chosen and sized by engineers for a particular service, set pressure and capacity.

Workers protect relief systems by respecting a few rules. Never block in a relief valve: isolation valves under or above relief devices must be locked or car-sealed open and may only be closed under a controlled procedure with alternative protection in place. Never change a set pressure, swap a relief valve for one from another service or replace one with a plug or blind. Report a relief valve that is leaking, chattering or past its test date. Keep relief discharge points pointed away from walkways and ladders, and remember that a vent stack can release flammable or toxic gas without warning when a valve lifts.

Shutdown and blowdown systems

Modern facilities protect equipment with automatic shutdown systems that close emergency shutdown valves on high or low pressure, high or low level, high temperature, fire or gas detection, or when an operator presses an emergency stop. Compressor stations and gas plants also have blowdown systems that depressure equipment to the flare in an emergency, reducing the inventory that could feed a fire. These systems are safety instrumented functions, discussed in HSE-311, and they only work if they are tested and not bypassed. A bypassed shutdown is one of the most common findings after serious incidents. Any bypass must be authorized, recorded, time-limited and backed by other controls.

How pressure equipment fails

Pressure equipment rarely fails at its design pressure when new. It fails because something has weakened it or because pressure has gone far above the design. The common mechanisms include:

Small-bore connections deserve a special mention. Instrument tubing, vents, drains and sample points on vibrating equipment such as reciprocating compressors fail more often than large piping because they are light, often poorly supported and exposed to constant vibration. A broken 0.5 in (13 mm) fitting on a high-pressure line can release gas at a high rate and start a jet fire. Report loose supports, vibration and weeping fittings, and never use small-bore connections as steps or handholds.

Brittle fracture is a cold-climate concern. When a high-pressure gas system is depressured quickly, the expanding gas cools the steel, sometimes to well below −30 °C. Some steels become brittle at low temperatures, and a vessel that is then repressured while cold can crack suddenly. Procedures for depressuring and repressuring, and for working in very cold weather, account for the minimum design metal temperature on the nameplate.

When containment is lost

What happens after a leak depends on the fluid, the pressure, the size of the hole and whether it ignites. A gas leak that ignites at once forms a jet fire, an intense flame that can cut through nearby steel in minutes. A leak that does not ignite at once can form a flammable cloud that drifts until it finds an ignition source, then burns as a flash fire or, if it is partly confined among equipment, explodes. Liquid releases can form pools that burn as pool fires. Sour releases add a toxic cloud.

The most violent event is a boiling liquid expanding vapor explosion, or BLEVE. It happens when a vessel containing a liquid stored above its boiling point, such as propane or NGL, is weakened, usually by fire, and fails. The sudden drop in pressure flashes much of the liquid to vapor, which ignites as a fireball and throws fragments of the vessel. The program below uses widely published empirical correlations to estimate the fireball size and duration for invented masses of propane. These are teaching approximations, not a basis for emergency planning.

# Empirical BLEVE fireball estimates (CCPS-type correlations; approximate, teaching only).
def fireball(mass_kg):
    diameter = 5.8 * mass_kg ** (1 / 3)                 # m
    if mass_kg < 30_000:
        duration = 0.45 * mass_kg ** (1 / 3)            # s
    else:
        duration = 2.6 * mass_kg ** (1 / 6)             # s
    return diameter, duration

for label_, kg in [("100 lb cylinder", 45), ("Small tank", 1_000),
                   ("Truck load", 15_000), ("Storage bullet", 60_000)]:
    d, t = fireball(kg)
    print(f"{label_:16s} {kg:>7,} kg propane: fireball about {d:5.0f} m across, lasting {t:4.1f} s")

100 lb cylinder       45 kg propane: fireball about    21 m across, lasting  1.6 s
Small tank         1,000 kg propane: fireball about    58 m across, lasting  4.5 s
Truck load        15,000 kg propane: fireball about   143 m across, lasting 11.1 s
Storage bullet    60,000 kg propane: fireball about   227 m across, lasting 16.3 s

Even a small tank produces a fireball tens of meters across, and a storage bullet produces one larger than a sports field. Fragments can travel much farther than the fireball. That is why fire services and plant emergency plans tell people to evacuate far from a fire-exposed pressurized liquid vessel rather than fight the fire up close, and why such vessels have fire protection such as water deluge, insulation and spacing.

What workers can do

Most of the defenses in this lesson are designed by engineers, but workers keep them working. Know the MAWP and normal pressures of the equipment you operate, and treat any pressure outside the normal range as a problem to report. Never close a valve without knowing what is upstream and downstream, and check that the downstream equipment can take the pressure. Avoid trapping liquid between closed valves. Keep relief valves free and their isolation valves locked open. Do not bypass shutdowns. Report vibration, leaks, weeping flanges and corrosion. When you open or work on equipment, confirm that it is isolated, depressured and drained, as Lesson 8 and HSE-333 explain.