Where We Go Wrong In Line Sizing

Dick Hawrelak

Presented to ES317Y in 1999 at UWO

Introduction

31% of large property damage losses are caused by failures in piping systems.Some lines reach 60 inches in diameter - larger than many equipment items.

Mass and Energy BalancesMB & EB balances must include:Normal mass balance - base case.Start-up, shut down or upset devn.Recycle of off-spec products.Equipment bypass conditions.MB for flare, fire water, sewers, steam, condensate and CTW headers.

Poor Line Sizing Guidelines

Limiting line velocities not known or ignored.Velocity greater than 15 fps in CS pipe can cause erosion.Low velocities (< 3 fps) can cause entrained solids to settle and plug lines.

Poor Line Sizing Engineering Discipline

Poor record keeping during Phase 3 prevents detailed follow-up in Phase 4 design stage.PSV lines need to be checked in Phase 4.Pump lines need to be checked in Phase 4.

Poor Pipe Selection

Corrosiveness of fluid unknown.At high temps, degradation may produce acidic components. (La. HumbleTherm example).Pipe not suitable for unexpected cold temperatures.Pipe not suitable for unexpected extreme high temperatures.

Choked Flow Of Compressible Fluids

Compressible fluid flow not well understood by process engineers.Flows at Mach 1 (Sonic Flow) can be destructive.

Choked Flow Example 1

Choked Flow Example 2

Two-Phase Flow

Two Phase Flow not well understood by process engineers.Horizontal Flow - Baker Chart.Vertical Down Flow.Vertical Up Flow.

Baker Chart Horizontal, Two Phase Flow

Two-Phase Flow In Vertical Down Flow Piping

Siphons may form readilyMay have damaging pulsating flow

Vertical Down Flow Piping

Two-Phase Flow In Vertical Up Flow Piping

May have damaging pulsating flow4 ft. dia. Cooling tower inlet pipe experiences severe vibration, fails & floods plant

Vertical Up Flow Piping

Two-Phase Flow Pressure Drop

Two phase flow pressure drops may be higher than expected and thereby limit performance.

Two-Phase Flashing Fluids

Flashing liquids often not considered during process design.

Two phase flow can be a complex, transient problem.

Two-Phase Flow In PSV Systems

PSVs are usually sized for single phase flow.Two phase flow can occur when vessels operated above maximum fill limit.Two phase flow can occur when flows are higher than expected.

Vortex Problems

Draining water from KO Pot to open sewer during rain storm.Flammable vapor entrained due to formation of liquid vortex.Lightning strikes near plant ISBL.Sewer explodes, wrecks cold box in Ammonia Plant.One operator killed.

Maldistribution

Flow in reactor manifolds.Flows in branched piping.Flows in shell-side of exchangers.

Water Hammer

System or vessel design pressures exceeded when EBV suddenly stops flow.EO transfer example.See computer spreadsheet design program.

Water Hammer Results

Typical Exam Problems

Which flow regime should you avoid for horizontal, two phase flow?If a liquid vortex forms on draining from a vessel, what safety problems can this create?When can a PSV which is sized for single phase flow experience two phase flow?

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