| name | heat-exchanger-field-issues |
| description | Heat exchanger field troubleshooting — fouling (Kern-Seaton model, fouling resistance R_f, cleaning intervals), tube-to-tubesheet joint failures (rolled vs. welded, ligament cracking), flow-induced vibration (Connors criterion, vortex shedding, acoustic resonance, TEMA guidelines), tube erosion and impingement attack, corrosion under deposits, shell-side bypassing (baffle seal leakage, bundle-to-shell gap), differential thermal expansion (U-tube vs. fixed tubesheet expansion joint), floating head seal failures, pass partition gasket failures, nozzle load effects, TEMA R/C/B classification, API 660 heat exchanger field issues. |
| metadata | {"priority":8,"promptSignals":{"phrases":["heat exchanger fouling","tube failure","heat exchanger vibration","HX performance","fouling resistance","tube leak","heat exchanger issue"],"minScore":2}} |
Heat Exchanger Field Issues — Complete Troubleshooting Guide
Fouling
Types and Mechanisms
Particulate fouling: suspended solids deposit on surface; build-up rate ∝ concentration × velocity (settling)
Crystallization fouling: inverse solubility salts (CaCO₃, CaSO₄) → deposit at hot surface; worst at T > 50°C
Corrosion fouling: surface corrodes → corrosion products deposit (magnetite Fe₃O₄ in water circuits)
Biological fouling (biofouling): algae, bacteria, Legionella in cooling water → biofilm → insulating layer; also MIC
Polymerization/coking: hydrocarbon streams at high temperature → thermal cracking → coke deposit
Kern-Seaton fouling model:
R_f(t) = R_f* × (1 − e^(−t/τ)) [fouling resistance builds toward asymptotic value R_f*]
R_f* = asymptotic fouling resistance [m²·K/W]; τ = time constant [h]
Cleaning interval: clean when R_f(t) reaches design fouling resistance R_f_design (TEMA tabulated values)
TEMA fouling resistance design values (typical):
Seawater (velocity > 1 m/s): R_f = 0.0001 m²K/W
Cooling tower water (treated): R_f = 0.0002 m²K/W
River water: R_f = 0.0003–0.0006 m²K/W
Heavy fuel oil: R_f = 0.001 m²K/W
Crude oil (above 120°C): R_f = 0.0005–0.001 m²K/W
Symptoms of excessive fouling:
- Reduced heat duty (outlet temperatures diverging from design)
- Increased pressure drop across HX (shell or tube side)
- Q_actual / Q_design = U_fouled / U_clean; U_fouled = 1/(1/U_clean + R_f_total)
Overall heat transfer coefficient degradation:
U_fouled = 1 / (1/U_clean + R_f,i + R_f,o × (A_i/A_o)) [R_f,i = tube-side fouling; R_f,o = shell-side fouling]
When U drops 30% → cleaning required for most services
Cleaning methods:
- Chemical cleaning (acid circulation): for CaCO₃, biological; HCl 5–10% for calcium; biodegradable enzyme for biological
- Mechanical cleaning (rodding): for soft deposits in straight tubes; pig in tubes
- High-pressure water jetting: 700–3000 bar; removes hard deposits; safe for all metals
- On-line cleaning balls (Taprogge): sponge balls circulate continuously through tubes → prevent deposit buildup
Tube-to-Tubesheet Joint Failures
Rolling vs. Welded Joints
Mechanical rolling:
Tube expanded into tubesheet hole by roller → metal-to-metal contact → no seal weld
Failure mode: tube pulls out (tension from thermal cycling) or leaks (inadequate expansion percentage)
Expansion percentage = (D_i_final − D_i_original) / D_i_original × 100% [target 5–8%; over-rolling weakens tube]
Re-rolling: possible for loose joints; limited number of times before tube work-hardens excessively
Welded + rolled joints:
Seal weld + full-strength weld per ASME VIII; best for high pressure, hazardous fluids
Cracking at weld: thermal cycling → fatigue at weld toe; especially for temperature difference > 100°C between shell/tube fluid
Ligament cracking in tubesheet:
Thermal cycling → stress in tubesheet ligaments (thin web between tube holes)
Minimum ligament width: TEMA specifies (t/P ratio where t = ligament width, P = tube pitch)
Failure: crack propagates from tube hole to tube hole → shell-to-tube side leakage through tubesheet
Flow-Induced Vibration (FIV)
Connors Criterion (Fluidelastic Instability)
Most severe vibration mechanism in tube bundles:
Connors critical velocity:
V_critical = K × f_n × D × √(2πζ m / (ρ D²)) [m/s]
K = Connors constant (stability constant); depends on tube layout; K = 3–6 for typical square/triangular pitch
f_n = tube natural frequency [Hz]; D = tube OD [m]; ζ = damping ratio (typically 0.01–0.03); m = tube mass per unit length [kg/m]; ρ = shell-side fluid density
Design requirement:
V_crossflow < 0.8 × V_critical [TEMA FIV guideline; stay 20% below critical]
Tube natural frequency:
f_n = (π/2L²) × √(EI/m_total) [first mode cantilever; L = unsupported span; m_total includes added mass from surrounding fluid]
Added mass: m_a = C_m × ρ_fluid × π D² / 4 [C_m = virtual mass coefficient ≈ 1–2 for tube bundles]
Vortex shedding:
f_vs = St × V_crossflow / D [St = Strouhal number ≈ 0.2 for circular tubes]
Resonance if f_vs ≈ f_n → large amplitude oscillation → tube-to-baffle hole wear, fatigue
Acoustic resonance:
Standing waves in shell cavity (cross-flow direction) → acoustic mode
f_acoustic = n × c / (2W) [W = shell inside diameter; c = speed of sound in shell-side fluid; n = 1, 2, 3]
If f_acoustic ≈ f_vs → severe acoustic resonance → loud noise + vibration
Solution: anti-vibration baffles (solid baffles or longitudinal baffles interrupt acoustic path); detuning by changing tube pitch
Field symptoms:
- Tube-to-baffle hole wear (fretting; tubes loose in holes; debris in shell side)
- Tube cracks at baffles (fatigue from repeated bending)
- Loud noise (acoustic resonance)
- High shell-side pressure drop increase (damaged/loose tubes)
Mitigation:
- Add intermediate support baffles (reduce L → raise f_n)
- Use anti-vibration baffles (AVB, lattice baffles) in inlet zone
- Reduce shell-side velocity (larger nozzle, increased pitch)
- Change tube pitch (triangular ↔ square) to shift vortex shedding frequency
Erosion and Impingement
Inlet Impingement Attack
Mechanism: High-velocity two-phase flow or droplets at inlet nozzle → impinge on first tube rows → erosion
Common in: steam condensers, kettle reboilers, cooling water coolers with high velocity
Protection devices:
Impingement plate: solid plate in front of inlet nozzle → deflects flow away from tubes
Annular distributor: distributes flow circumferentially before entering bundle
TEMA requires impingement protection when: ρ_v × V² > 2230 kg/(m·s²) for non-corrosive; 740 for corrosive
Tube entrance erosion:
Turbulent flow at tube entrance (calming length ≈ 10D) → erosion-corrosion on tube ID at inlet
Prevention: tube inlets slightly chamfered or bell-mouthed; ferrules in inlet tubesheet
Shell-Side Bypassing and Short-Circuiting
Bundle-to-shell gap (bypass stream C):
Gap between outer tube bundle and shell ID → bypass stream bypasses tubes → reduces effective heat transfer
TEMA tolerance: bundle OD vs. shell ID clearance = 12–25 mm
Excessive bypass → U_effective drops significantly
Sealing strips: metal strips welded to bundle to block bypass lanes; typically 1 pair per 5 tubes width
Baffle seal failures:
Tube-to-baffle hole clearance: TEMA specifies 0.8–1.2 mm; excessive clearance → leakage (A-stream)
Baffle-to-shell clearance: typically 1.5–3 mm; excessive → bypass (E-stream)
Window bypass (B-stream): good; design controlled by baffle cut and window area
Pass partition gasket failure:
In multi-pass shell-side HX: gasket between passes seals pass partition
Failed gasket → hot and cold fluid short-circuits → LMTD collapses → performance loss
Symptom: output temperatures converge (both streams at same outlet temperature)
Thermal Expansion Issues
Fixed vs. Floating Tubesheet
Fixed tubesheet HX:
Both tubesheets welded to shell → ΔT between shell and tube causes differential expansion
σ_thermal_ligament = E × α × ΔT_s-t / (1 − ν) × geometry factor [must stay below fatigue limit]
Expansion joint on shell: bellows or slip joint allows differential movement
Without expansion joint: max ΔT(shell vs. tubes) typically 40–50°C for standard CS design
Field failure: Tubesheet cracking (excessive ΔT during startup transients); expansion joint bellows fatigue (too many pressure/thermal cycles)
Prevention: slow startup procedures; limit ΔT ramp rate to < 30°C/min; expansion joint design review with cycle count
U-tube HX:
Free to expand (U-bend allows differential movement); no shell expansion joint needed
Problem: cannot clean tube ID mechanically (U-bend inaccessible); only chemical cleaning
Problem: U-bend fatigue in high-cycle thermal services (power plants)
Floating head HX (API 660):
One tubesheet floating; allows differential expansion
Floating head seal failure: high temperature + pressure on shell side → floating head cover leaks
API 660: inspects floating head seal regularly; use metallic spiral wound gasket for high P-T
Nozzle Loads
Excessive nozzle loads from pipe stress:
Piping expansion forces on HX nozzles → bending moment + shear at tubesheet and shell nozzle
TEMA: allowable nozzle loads tabulated by shell diameter
Exceeded loads → nozzle distortion → tubesheet warping → tube joint leakage
Fix: add pipe flexibility (expansion loop); pipe support near nozzle; check pipe stress analysis
Standards
| Standard | Scope |
|---|
| TEMA (Standards of Tubular Exchanger Manufacturers) | Design classifications R/C/B; fouling allowances; FIV guidelines |
| API 660 | Shell-and-tube heat exchangers for general refinery services |
| ASME Section VIII | Pressure design of HX components |
| API 662 | Plate-and-frame HX for petroleum industry |
| API 661 | Air-cooled HX for general refinery services |
| ISO 16812 | Petroleum/gas — HX selection and design |
| HEI (Heat Exchange Institute) | Steam surface condenser standards |
Output
Identify HX type (TEMA designation: BEM/AES/AEU etc.) and failure mode. For each:
Fouling: R_f_measured [m²K/W] vs. R_f_design; U_clean vs. U_fouled [W/m²K]; duty loss [%]; cleaning method; interval
Tube joint: rolled or welded; expansion %?; ligament cracking: inspect tubesheet by RT or UT; re-roll feasibility
FIV: V_crossflow [m/s]; V_critical from Connors [m/s]; V < 0.8×V_crit?; f_n [Hz]; tube wear at baffles?; AVB or support baffles added?
Erosion: ρv² at inlet [kg/m·s²]; > 2230? impingement plate present?
Bypass: bundle-to-shell gap [mm]; sealing strips present?; baffle hole clearance [mm]; pass partition gasket condition
Thermal expansion: ΔT shell-tube [°C]; expansion joint present?; fatigue cycles vs. design
Nozzle loads: actual forces/moments vs. TEMA allowables; pipe flexibility adequate?
Applicable TEMA class + API 660 + corrective action.
