Knowledge / Risk and reliability

HAZOP pressure and flow deviations: separating causes from consequences

Separate blocked-outlet, reverse-flow and trapped-liquid heating scenarios with pressure-head and thermal-expansion examples, keeping causes and safeguards specific.

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“High pressure” and “no flow” are deviations, not complete causal explanations. A closed discharge valve, a reversed pressure gradient and heating of an isolated liquid segment can produce different combinations of pressure and flow, with different protective needs. A useful HAZOP keeps those mechanisms separate long enough to identify the actual energy source, affected boundary and credible consequence.

Start with the intended direction and pressure boundary

Define the node as a specified liquid line from the pump discharge to a receiving manifold, with normal direction toward the receiver. State the operating flow, source pressure capability, fluid density, elevation changes and component limits. Identify whether pressure is gauge or absolute and whether a quoted pump head is a differential rather than an outlet pressure.

The IEC HAZOP catalogue summary supports the guide-word examination framework. The worked hydraulic scenarios here are original examples within that framework. They do not reproduce the paid standard or define equipment settings. Meaningful deviations depend on a declared design intent and on the actual physical boundary.

Analyze a blocked centrifugal-pump outlet

A closed outlet can bring external flow toward zero while a running centrifugal pump develops its shutoff differential head. Assume an invented shutoff head of 60 m, density 900 kg/m³ and g=9.81 m/s². Then Δp=ρgH=529,740 Pa≈5.30 bar. Units reduce to kg m⁻¹ s⁻², which is a pascal.

If inlet pressure is 2.0 bar gauge at the same elevation reference, the simplified discharge pressure is about 7.30 bar gauge. The pump adds a pressure difference; it does not erase the inlet pressure. Actual evaluation needs the pump curve, speed, fluid properties, elevations and any alternate flow path. This arithmetic does not establish that the pipe or pump can safely sustain the condition.

Do not stop the consequence at static pressure

No external flow can also leave input energy heating liquid within the pump. Seal, bearing and fluid conditions may deteriorate even when the maximum static discharge pressure stays below a pipe rating. The DOE mechanical-science handbook discusses dead-heading and distinguishes centrifugal from positive-displacement pump behavior.

A positive-displacement pump does not generally provide the same self-limiting shutoff-head behavior. Continued displacement into a blocked route can increase pressure until another mechanism limits it, such as slip, drive limitation, protective action or failure. A safeguard suitable for one pump type cannot be credited for the other without examining the actual arrangement and required capacity.

Treat reverse flow as a pressure-gradient problem

When a pump stops, the receiver may retain greater pressure or elevation head than the source. If an open path remains, flow can reverse. For a liquid of density 900 kg/m³ and a free-surface elevation difference of 8 m, the static driving difference alone is ρgΔz=70,632 Pa≈0.706 bar, before other gas-space pressures and flow losses are considered.

That pressure difference does not determine reverse flow rate without a resistance model and a valid path. A check valve can be part of the intended prevention, but its leakage, sticking or installation condition must be considered. Reverse flow may empty the receiver, overfill the source, contaminate another system or rotate machinery backward; select consequences that the actual connections support.

Examine trapped-liquid heating with the pump stopped

Now isolate a liquid-filled segment between closed boundaries and apply heat. No pump needs to run. Let liquid volume be 0.050 m³, volumetric expansion coefficient β=7×10⁻⁴ K⁻¹ and temperature rise ΔT=20 K. Unrestrained expansion would be ΔV=βVΔT=0.0007 m³, or 0.7 L.

For a fully liquid-filled, ideally rigid segment with constant bulk modulus K=1.5 GPa and no relief, leakage or phase change, balancing thermal expansion against compression gives Δp≈KβΔT=21 MPa, or 210 bar. These deliberately simplified properties illustrate the mechanism; they are not data for a named fluid or a prediction of an installed line's final pressure.

State why the thermal estimate can change greatly

Pipe-wall compliance, temperature-dependent properties, a gas pocket, leakage and protective relief alter the result. A small compressible gas volume can change the pressure response substantially, but its existence and persistence cannot be assumed as a safeguard. At large pressure changes, constant-property linearization is especially limited. A detailed assessment needs the actual fluid and containment behavior.

The CSB-hosted thermal-expansion technical appendix, inspected through its public indexed text, distinguishes fluid expansion and compressibility. It supports the physical mechanism, not the invented 210 bar result. The relevant HAZOP question is whether a credible heating source acts on an isolated liquid volume and what valid protection remains connected to that volume.

Keep cause, deviation and consequence in separate fields

For the blocked-outlet case, the cause might be an incorrectly closed valve, the deviations low flow and increased discharge pressure, and the consequences pump damage or loss of the receiving service. For reverse flow, the cause is a reversed driving gradient with an ineffective nonreturn path; the deviation is wrong direction. For trapped heating, the cause combines isolation and heat input; the deviation is rising pressure without intended flow.

Writing “high pressure causes high pressure” adds no mechanism. Writing “valve failure” without the failed position is equally weak. State what changes physically and how it crosses the consequence threshold. One guide word can generate several scenarios; do not merge them merely because they share the same deviation label.

Credit a safeguard only for the connected scenario

A pump stop can remove pump energy but may not prevent gravity-driven reverse flow or heating of a trapped segment. A nonreturn valve can address direction while having no suitable thermal-relief role. A relief device isolated from the threatened volume cannot protect it merely because it appears on the same drawing. Effective capacity, destination and response conditions matter.

A low-flow alarm also depends on where flow is measured and whether time remains for useful action. An automatic stop that creates a new trapped volume can change another scenario. HAZOP should record those interactions and request the necessary engineering verification, without inventing settings or assuming that every safeguard is an independent protection layer.

Recognize the transient mechanisms left outside the simple balances

Rapid valve movement can create pressure transients; cavitation, gas release and fluid inertia can change both direction and magnitude. The steady head and constant-volume thermal calculations above do not estimate surge pressure or relief sizing. Adding their numerical maxima without a compatible time history can be as misleading as ignoring them.

Identify which mechanism controls the credible event: steady dead head, thermal pressurization, reverse-flow inventory transfer or a rapid transient. Use an appropriate calculation and assumptions for that mechanism. If several interact, a combined model must preserve the relevant timing and boundary conditions rather than assemble unrelated worst-case numbers.

Use observations to challenge the cause without closing it prematurely

A low-flow indication can result from a blocked route, loss of suction, pump-speed reduction, gas binding or a measurement problem. An elevated discharge pressure can support one explanation but does not prove it without a consistent pressure reference and known upstream condition. HAZOP explores credible causes; troubleshooting later compares those causes with evidence. The two activities should share clear definitions without being confused.

For a recorded event, preserve simultaneous inlet pressure, discharge pressure, flow, speed, valve-state evidence and the operating lineup where available. A command and a physical position are different observations. A trend averaged over a long interval can hide a short reverse-flow period. The useful question is which proposed mechanism remains consistent with the whole observation set, including uncertainty and timing, rather than which single reading looks most persuasive.

Check the threatened volume after every isolation change

A thermal scenario belongs to the volume enclosed by the actual closed boundaries. Moving one isolation point can divide that volume into smaller segments, disconnect a relief path or expose a previously unheated section. The source of heat may be tracing, sunlight, an adjacent hot line or residual process heat. Its credibility and duration need a defined operating state.

A drawing note saying thermal relief provided is incomplete unless the relevant volume remains connected to that protection in the scenario being examined. Follow the path to its discharge destination and identify any intervening isolation or backpressure assumptions. This is a boundary and evidence check, not a relief-device sizing procedure. It explains why a safeguard valid during transfer may not remain effective during shutdown, maintenance or a temporary hose arrangement.

Leave an action that can be technically closed

A useful action asks for a defined missing result: confirm maximum source pressure against the affected boundary, establish a credible reverse-flow path and inventory consequence, or verify protection of the isolated heated volume. Record the required evidence and the scenario it resolves. “Check safety” does not give the next reviewer a closure criterion.

The examples show how mechanism separates superficially similar HAZOP rows. They are not pressure-test instructions, relief-device selections or operating limits. Real conclusions need the installed drawings, fluid properties, equipment data, approved protective arrangements and applicable engineering requirements. The calculation should clarify the question before it claims to settle it.

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