Personal engineering reference · Piping, hydraulics, and equipment
Working notes assembled from public codes and widely published industry practice. Not a substitute for the governing code edition, client standard, licensor package, or project basis of design.
Primary public references
Client standards, licensor requirements, and the project BEDD govern when they exist. These notes are starting criteria for new design and a checklist when reviewing an existing plant.
ASME B31.3 §301: design pressure is the most severe coincident internal or external pressure and temperature that governs wall thickness and flange class. It is not automatically “normal operating + 10%.”
Maximum design pressure — typical process-engineering construction
Use the most stringent of:
Design pressure generally extends from the source to the farthest block valve that can see that pressure, regardless of parallel paths.
Minimum design pressure
Maximum design temperature
B31.3 §301.3: metal temperature at the coincident design pressure.
Typical process construction:
Minimum design temperature
Governing cases: auto-refrigeration on depressuring, ambient winter, Joule–Thomson across a valve, flare-header blowdown, and stagnant lines in cold weather. This sets MDMT and impact-test requirements, not just insulation.
Stress design temperature
Flexibility analysis uses the metal temperature range that produces the governing expansion stress. That range is not always identical to the pressure-design temperature. Record both.
Relief-system piping
API 521 / 520 govern inlet and outlet losses:
Corrosion / erosion allowance
ASME B31.3 requires an allowance c in the wall-thickness equation. Typical starting values used in many process plants (confirm with corrosion engineering):
| Service (carbon steel, general) | Typical CA |
|---|---|
| Non-corrosive organics, dry gas | 1/16 in (1.6 mm) |
| Cooling water, steam, general process | 1/8 in (3.2 mm) |
| Sour water, wet acid gas, some crude | 1/8–1/4 in (3.2–6.4 mm) |
| Severe corrosion or unknown | Set by corrosion rate × life + inspection margin |
Alloy systems often use 0–1/16 in if the alloy is the corrosion control. CA is not a substitute for the right material.
B31.3 hoop-stress wall thickness (straight pipe):
where is design pressure, outside diameter, allowable stress, quality factor, coefficient (0.4 for most ferritic steels below 900 °F), sum of mechanical and corrosion allowances. Mill undertolerance (often 12.5% for seamless ASTM pipe) must be applied to nominal wall before the pressure calculation.
Designation. State purpose on the line list: heat conservation (H), process stability, personnel protection (PP), freeze protection, condensation control, fireproofing, or acoustic.
Heat conservation. Thickness from a heat-loss calculation (ASTM C680-type) or owner spec tables. Driving cases: maintain viscosity, prevent freeze, keep a bubble-point liquid from flashing, or limit steam condensing in a header.
Personnel protection. OSHA does not publish a single metal-temperature number. Many owner specs treat an uninsulated surface above about 140 °F (60 °C) in an accessible area as requiring PP insulation or a guard. PP insulation is thinner than conservation insulation; do not assume they are the same.
Trace when the fluid can freeze, wax, hydrate, or drop below a minimum viscosity at the lowest ambient plus wind.
Typical decision order:
Electric tracing is common for long runs and precise control. Steam tracing is common in plants with spare LP steam. Do not trace across expansion loops without a plan for the tracer. Maintenance temperature is a process number (pour point + margin, hydrate + margin), not a tracer-vendor default.
Velocity is a screening tool. The design is a pressure-balance from source to destination at the governing flow case (normal, design, start-up, recycle, turndown).
Typical liquid screens (widely published; SI and FPS):
| Service | Typical velocity | Typical ΔP screen |
|---|---|---|
| Pump suction, subcooled | 3–5 ft/s (0.9–1.5 m/s) | 0.25–0.5 psi/100 ft |
| Pump suction, near bubble point | 2–3 ft/s (0.6–1.0 m/s) | ≤ 0.25 psi/100 ft |
| Pump discharge / general process | 6–12 ft/s (1.8–3.5 m/s) | 1–3 psi/100 ft |
| Gravity flow | ≤ 3 ft/s | keep submergence |
| Reboiler trap-out / thermosiphon | 1–4 ft/s | very low; close the circuit |
Typical gas / vapor screens
| Service | Typical velocity | Notes |
|---|---|---|
| Compressor suction | 30–60 ft/s (9–18 m/s) | noise, pulsation, ΔP to suction drum |
| Compressor discharge | 50–100 ft/s | check aftercooler ΔP |
| Column overhead / process vapor | 40–80 ft/s | higher in vacuum if ΔP allows |
| Relieving / flare laterals | API 521 / acoustic | not a process-line velocity |
Steam (Spirax / common process practice)
API RP 14E erosional velocity (often used as a screen, not a code limit for onshore plants):
where is ft/s, is mixture density lb/ft³, and is commonly 100 for continuous non-corrosive service and 80 for intermittent / corrosive / sand-bearing service. Many onshore owners use other values. Treat this as a starting check.
Darcy / Crane pressure drop
Fittings by equivalent length or Crane :
Use Crane TP-410 or equivalent data. Do not mix equivalent-length tables from different sources on the same line without checking the assumed .
Compressible flow: isothermal or adiabatic Darcy, or isothermal Weymouth/Panhandle for long gas lines. Check Mach number. In-plant gas lines are usually kept well below 0.3–0.5 Mach except relief paths.
Two-phase flow
Limiting velocities
Initial estimates. For early hydraulics, a control-valve allowance of 10–15 psi or 10–30% of the frictional system drop (excluding static) is a common starting point. Close it later with a valve-sizing calculation (ISA 75.01).
Control-valve allowance. The valve needs enough ΔP to control at minimum, normal, and maximum flow. A valve that is barely dropped at design flow will not control at turndown. A valve that takes most of the system drop will be noisy and oversized at low load.
Hydraulic design basis — close these items
ASME VIII Div. 1: the process engineer sets design pressure and design temperature. The fabricator calculates MAWP from actual thickness. MAWP ≥ design pressure.
New vessels — common process practice (Pressure Vessel Handbook and many owner specs; not an ASME rule):
Maximum design temperature: hottest metal coincident with design pressure — process, steam-out, regeneration, fire case only if the project treats fire as a design case rather than a relief case.
Minimum design temperature: coldest metal — auto-refrigeration, winter, depressuring. This is the MDMT discussion with materials.
Existing equipment. Compare the new operating envelope with stamped MAWP / MDMT and the relief set. “It has run this way” is not a design basis.
Vessel corrosion allowance. Typical CS starting point 1/8 in (3.2 mm) on process vessels; internals may differ. Confirm with corrosion.
PWHT for carbon steel. Driven by ASME VIII UCS-56 thickness / material and by service (wet H2S / sour service per NACE / owner). Process does not waive PWHT.
Sparing is a reliability and operating decision: spared pumps on essential services, spared filters, sometimes spared exchangers on fouling duties. Write the basis (API availability class, owner sparing philosophy). Do not spare a piece of equipment in the P&ID and then size the spare for a different hydraulics case than the main.
Typical new-design rotating-equipment margins (published owner/practice ranges; confirm project BEDD):
| Equipment | Common capacity margin on process design flow |
|---|---|
| Centrifugal pump | 10% |
| Reciprocating pump | 10% |
| Centrifugal compressor | 0–10% (curve and recycle decide) |
| Reciprocating compressor | per rod load and clearance, not a blanket % |
Heat exchangers: dirty-duty design with TEMA fouling, not an extra hidden area margin stacked on top of an already conservative fouling factor unless the project says so.
Liquid surge / residence
Define the volume between alarm and trip, not only “minutes of holdup.”
| Function | Typical starting residence |
|---|---|
| Feed surge to a unit | 5–20 min on normal flow |
| Reflux drum / accumulator | 5–10 min on reflux + product |
| Knockout / compressor suction | 2–5 min liquid, plus mist-eliminator design |
| Steam drum | level control + swell, not a generic minute count |
Use LLLL / LLL / NLL / HLL / HHLL and say which volume sits between which marks.
Liquid levels. Keep NLL away from inlet nozzles and vortex. Minimum bottom-to-nozzle dimensions come from vortex / NPSH / fabricator standards, not from a sketch.
Manways and inspection. 18 in (450 mm) manway is a common minimum for vessel entry. Smaller handholes for instruments only.
Vents, drains, steam-outs. Size to actually drain and steam the vessel in a turnaround window. 2 in drains on large vessels are often too small. Provide a high-point vent to a safe location.
Nozzle flange ratings. Match the piping spec at that design P/T. Do not drop a 300# nozzle onto a 150# line because the vessel MAWP “looks low.”
GPSA Section 7 is the usual public method.
Souders–Brown allowable vapor velocity:
Typical published (ft/s) for a vertical vessel with a mesh pad:
| Pressure | Approximate |
|---|---|
| ~0 psig | 0.35 |
| 100 psig | 0.35 |
| then subtract ~0.01 per 100 psi | |
| 900 psig | ~0.27 |
| 1500 psig | ~0.21 |
GPSA notes commonly repeated in public sources:
Vertical vs horizontal
Mist eliminators. Mesh pads: high efficiency to ~5–10 µm, limited turndown and slug tolerance. Vane packs: higher , better slugs, coarser cutoff. Cyclones: still higher , vendor-specific.
L/D. Vertical two-phase drums often land around 3–5. Horizontal separators often 3–6 on TAN/TAN, set by liquid residence and vapor disengagement area above the liquid.
Tray efficiency. Start with published typicals (Glitsch / GPSA / Kister ranges): 60–80% for well-designed hydrocarbon trays; lower for high viscosity, high vacuum, or dirty service. Do not use 100%.
Flooding. Design below flood. Common new-design targets:
System factor (foaming): apply GPSA / vendor factors for amines, glycols, sour water, and hot-carbonate systems.
Internals. Inlet distributors, draw pans, chimney trays, and reboiler returns make or break the tower. Recirculating thermosiphon, once-through thermosiphon, and forced circulation are different hydraulic circuits — draw the circuit before sizing the tower.
Packed towers. HETP or HTU from vendor / packing type. Pressure drop limits are a process number (vacuum columns especially).
API 610 / ISO 13709 for hydrocarbon and chemical service when the project specifies it.
NPSH
Evaluate at the pump datum, lowest credible suction level, hottest liquid, and governing flow (often runout, not only rated).
Public margin practice:
Suction specific speed (US customary, BEP at max impeller):
Many specifications watch above about 11,000. High is not automatically rejected; it needs a stable operating range.
Suction piping. Short, one size up from the pump nozzle is common, with a straight run into the suction (often 5–10 D). Velocity 3–5 ft/s.
Minimum flow. Continuous service needs a recycle or a minimum-flow valve sized for the vendor’s stable minimum, not for “a little bit back to suction.”
API 660 + TEMA class (R for most process, C for commercial, B for chemical).
Design P/T. Each side independently, including blocked-in liquid on the cold or hot side if that case exists.
Configuration. Fluid allocation: dirty / fouling / corrosive fluid on the tube side when mechanical cleaning matters. High-pressure fluid often on the tube side. Condensing / boiling allocation follows the hydraulic circuit (thermosiphon, kettle, forced circ).
Fouling. Use TEMA published fouling resistances as a starting table, then replace with plant data when you have it. Typical public TEMA-order-of-magnitude values:
| Fluid | Typical (hr·ft²·°F/Btu) |
|---|---|
| Steam (clean) | 0.0005 |
| Cooling-tower water | 0.001–0.002 |
| Light hydrocarbons | 0.001 |
| Heavy hydrocarbons / residue | 0.003–0.005 |
| Crude (depending on end) | 0.002–0.005 |
Approach and LMTD
is the LMTD correction for configuration. If falls below ~0.75–0.8, change the configuration (add a shell in series, 1-2 vs 2-4, etc.) rather than inflating area.
Temperature approach: 10 °F (5–6 °C) is a common process minimum on many services; tighter approaches are bought with area and are legitimate when energy matters.
API 661.
API 617 (axial/centrifugal), 618 (recip), 619 (screw) as specified.
Essential pumps usually get suction and discharge block valves and a check valve on discharge. Emergency isolation (ESD) is a safety-case decision, not a default on every pump.
API 520 Part II:
The upstream design pressure can appear at the valve inlet on failure. Body rating, flange class, and downstream design must be checked for that case. A downstream spec break is a process decision, not a drafting convenience.
Block-isolate-bypass is standard on important loops. Bypass size is a start-up / fail-in-place decision; a full-size bypass defeats the valve, a too-small bypass cannot start the unit.
For every control valve, orifice, PSV, and analyzer, issue the process data that the specialist actually needs: fluid, flow range, P/T at the device, density / MW / viscosity / vapor pressure, allowable ΔP, and the failure position. A P&ID bubble without a process data sheet is unfinished.
Process cases (normal / design / upset / start-up / blocked-in / steam-out)
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Design P/T and materials on every envelope
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Equipment sizing (duty, surge, NPSH, K-factor, UA)
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Line list + hydraulics close
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Relief load and device list
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P&ID and instrument process data
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Reconcile with mechanical, piping stress, and control
Methods reference only. Confirm every numerical screen against the current code edition and the project basis of design.