Pressure Valve and Code

🧩 1. API 520 – Sizing, Selection, and Installation of Pressure-Relieving Devices

Full title: API Standard 520, Parts I & II

🔹 Part I – Sizing and Selection

• Focuses on how to correctly size and select a PSV or PRV (Pressure Relief Valve).

• Ensures the valve can release sufficient flow to prevent system overpressure.

• Covers:

  • Sizing formulas for gas, vapor, steam, and liquid service

  • Determination of set pressure and overpressure

  • Backpressure effects and correction factors

  • Capacity correction for different fluids and conditions

  • Selection of valve type (spring-loaded, pilot-operated, etc.)

✅ Application: Used during design stage of boilers, pressure vessels, and process lines to calculate the right PSV or rupture disc capacity.

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🔹 Part II – Installation

• Provides guidance on how to install the PSV safely and effectively.

• Covers:

  • Piping configuration for inlet and outlet

  • Reaction forces and supports for discharge lines

  • Minimizing pressure drop in inlet lines (≤ 3% of set pressure)

  • Ensuring drainage, venting, and safe discharge locations

✅ Application: Used during construction and commissioning — ensures installation follows best practices to maintain reliability and accuracy of PSV performance.

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🧩 2. API 521 – Pressure-Relieving and Depressuring Systems

Full title: API Standard 521, Pressure-Relieving and Depressuring Systems

🔹 Purpose:

• Focuses on system-level design of relief and flare systems — not the valve itself.

• Ensures the entire relief system (valves, headers, flare stacks) can handle the worst-case scenario.

🔹 Key Topics:

• Identification of overpressure scenarios, e.g.:

  • Fire exposure

  • Blocked outlet

  • Thermal expansion

  • Control valve failure

• Relief load calculation methods

• Depressuring systems design (for emergency shutdown)

• Flare and vent system design, including:

  • Flare tip sizing

  • Knockout drum sizing

  • Noise, radiation, and dispersion control

✅ Application: Used during HAZOP, relief load study, or flare network design — ensures the plant can safely relieve excess pressure to atmosphere or flare.

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🧩 3. API 576 – Inspection of Pressure-Relieving Devices

Full title: API Recommended Practice 576, Inspection of Pressure-Relieving Devices

🔹 Purpose:

• Provides inspection, testing, and maintenance procedures for PSVs and rupture discs.

• Ensures the device remains functional and reliable throughout its service life.

🔹 Key Sections:

• Inspection intervals (based on service conditions and history)

• Testing methods: bench testing, pop testing, seat leakage testing

• Common failure modes: corrosion, seat leakage, spring fatigue, blockage

• Documentation & tagging requirements for traceability

✅ Application: Used during plant maintenance or turnaround (TA) — guides inspection intervals, testing frequency, and certification process of PSV.

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⚙️ Summary Table

API Code	Focus Area	Stage of Use	Key Application
API 520	Sizing, selection, and installation	Design & commissioning	To select the right PSV and install properly
API 521	System design & relief scenarios	Design & HAZOP	To size flare systems and determine relief loads
API 576	Inspection, testing, and maintenance	Operation & maintenance	To ensure reliability and compliance of PSV

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