PSV Sizing Report Review Guide

How to Use This Guide

Use this as your checklist when reviewing pressure safety valve (PSV) sizing reports from vendors. Work through each section systematically—this catches 90% of typical errors. For quick screening before deep dive, skip to the Quick-Screen Checklist at the end. All references to API standards are normative; cite the specific edition your company uses.

Typical Report Contents

A complete PSV sizing report should include: - Service description — fluid properties, normal operating conditions, design/MAWP - Relief conditions — accumulation scenario, relieving flow rates, inlet/discharge pressures - Valve selection — orifice designation, set pressure, valve model/manufacturer - Sizing calculations — capacity verification against required flow, factor-of-safety - Discharge piping analysis — back pressure evaluation, reactive force assessment - Installation notes — inlet/discharge geometry, outlet stacking, pressure taps

Missing sections are a red flag. Push back and ask for them.

Section-by-Section Review Checklist

1. Set Pressure vs. Design Pressure

• [ ] Set pressure ≤ MAWP (Maximum Allowable Working Pressure)
• [ ] Set pressure accounts for system accumulation requirements (typically 10% overpressure allowed per ASME Section VIII Div. 1)
• [ ] Set pressure > normal operating pressure by safety margin (typically 5–15%)
• [ ] Justification provided if set pressure is near MAWP (unusual, requires explanation)

Red flags: Set pressure exceeds MAWP. Set pressure too close to normal ops without margin.

2. Relieving Pressure Calculation

• [ ] Relieving pressure = Set Pressure + Accumulation (% of MAWP)
• [ ] Accumulation percentage cited per ASME code or client requirement
• [ ] Back pressure accounted for in relieving pressure (back pressure added to set point)
• [ ] Inlet pressure drop deducted if system-critical

Red flags: Accumulation exceeds code limits. Relieving pressure undefined or inconsistent with set pressure.

3. Back Pressure Assessment

• [ ] Superimposed back pressure calculated (constant outlet pressure, e.g., atmospheric)
• [ ] Built-up back pressure estimated (increases as flow increases through discharge header)
• [ ] Total back pressure = superimposed + built-up
• [ ] Valve type selected appropriately:
• Conventional (accepts ~10% back pressure max)
• Balanced bellows (handles higher back pressure, ~50%+ relief pressure)
• Pilot-operated (for tight outlet control, capacitive)
• [ ] Back pressure effect on capacity factored in (reduces effective relieving pressure differential)

Red flags: Back pressure ignored. Wrong valve type for application. Back pressure > valve rating.

4. Inlet Pressure Drop

• [ ] Inlet line pressure drop calculated or measured
• [ ] 3% rule: if inlet drop > 3% of relieving pressure, valve oversizing required or piping upgraded
• [ ] Inlet piping sized per API 520 Part I guidelines (velocity limits, fitting losses)
• [ ] Pressure tap location documented (taken upstream of any restrictions)

Red flags: Large inlet drops unaccounted for. No reference to inlet sizing standards.

5. Reactive Force on Discharge Piping

• [ ] Discharge reaction force (thrust) calculated: F = m_dot × V (momentum-based) or API 520 tables
• [ ] Piping/supports sized to withstand reaction force without excessive deflection
• [ ] Discharge elbows supported externally (can’t rely on valve body alone)
• [ ] Installation geometry matches calculations (e.g., if piping routed differently, recalc needed)

Red flags: No mention of reaction force. Piping support details missing. Geometry changes post-sizing.

6. Orifice Selection

• [ ] Orifice designation matches capacity requirement (per API 526 tables)
• [ ] Orifice area sufficient for required flow rate at relieving pressure differential
• [ ] Orifice sizing factors applied correctly (see next sections)
• [ ] Valve model consistent with orifice (not all models available for all orifices)

Red flags: Orifice undersized. Orifice/model mismatch. Capacity just barely meets requirement (no margin).

7. Gas/Vapor Sizing Factors

For compressible flow, verify all factors applied: - [ ] W — required flow rate (lbm/hr) - [ ] T — inlet gas absolute temperature (°R) - [ ] Z — compressibility factor (≠ 1 for real gases, especially high-pressure) - [ ] M — molecular weight - [ ] Kd — capacity correction for back pressure (reduces capacity, < 1.0) - [ ] Kb — capacity correction for inlet pressure drop (if significant) - [ ] Kc — capacity correction for viscosity (high-viscosity gases, viscosity index used) - [ ] Kw — capacity correction for valve design (manufacturer-specific, from performance data)

Calculation check: W = (P₁ × C × A × Kd × Kb × Kc × Kw) / (T × √Z × M) (API 520 form; verify constant C used matches your unit system)

Red flags: Z set to 1.0 for non-ideal gases. Viscosity corrections missing. Kd ignored (back pressure present). Capacity calculated without all factors.

8. Liquid Sizing

For incompressible flow: - [ ] Q — required flow rate (gallons per minute) - [ ] G — liquid specific gravity (at relieving temperature) - [ ] Kd, Kw — same as gas factors above - [ ] Kc — capacity correction for viscosity (liquid viscosity in SSU or cSt) - [ ] Kv — viscosity correction factor (from API 520 viscosity charts; significant if ν > ~20 cSt)

Calculation check: Q = (P × C × A × Kd × Kw × Kc) / (G)^0.5 (for liquids, temperature effects on viscosity are critical)

Temperature note: Viscosity is temperature-sensitive. Verify relieving temperature used matches actual relief scenario (e.g., worst-case hot oil case).

Red flags: Viscosity ignored. Relieving temperature not specified. Oversized orifice used to “compensate” for viscosity (bad practice).

9. Two-Phase Flow Methodology

If two-phase relief is required (e.g., heat exchanger upset with liquid flashing): - [ ] Methodology cited (Omega method per API 520, homogeneous equilibrium model HEM, or proprietary) - [ ] Quality (vapor fraction) at relief conditions calculated thermodynamically - [ ] Sizing performed using two-phase capacity correlation (not simple single-phase calculation) - [ ] Conservative assumptions stated (e.g., maximum expected quality) - [ ] Discharge piping analyzed for erosion/vibration (two-phase flow is aggressive)

Red flags: Two-phase scenarios ignored when steam/flash possible. Single-phase calculation used for two-phase service. No reference to API 520 Annex C.

Common Mistakes & Red Flags

Quick-Screen Checklist (5-Minute Review)

Use this before digging into calculations:

• [ ] Set pressure ≤ MAWP and > normal ops + margin
• [ ] Relieving pressure = Set + Accumulation; back pressure method stated
• [ ] Valve type (conventional/balanced/pilot) matches back pressure estimate
• [ ] Orifice designated (API 526 letter) and capacity verified
• [ ] For gas: W, T, Z, M, Kd, Kb, Kc, Kw all shown; Z ≠ 1.0 if non-ideal gas
• [ ] For liquid: Q, G, Kc, Kv shown; viscosity table or EOS referenced
• [ ] Inlet piping sized or drop < 3% of relief pressure
• [ ] Discharge reaction force addressed; piping support noted
• [ ] Manufacturer and model listed; orifice/model combo available
• [ ] References cite API 520 Part I/II, API 526, relevant to application

If 8+ items checked: Report likely sound. Review calcs in detail. If < 8 items: Request missing sections before approving design.

Sources

• API RP 520 Part I (2015) — “Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries — General Principles”
• API RP 520 Part II (2015) — “Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries — Sizing Equations and Effective Areas”
• API STD 526 (2017) — “Flanged Steel Safety Relief Valves” (orifice designations, performance data)
• API RP 521 (2014) — “Design and Installation of Pressure Relief Systems in Refineries” (system-level context, discharge header design)
• ASME Boiler and Pressure Vessel Code, Section VIII, Division 1 — Pressure vessel design rules, accumulation limits, MAWP definitions