PSV Sizing Services

1. Overpressure Scenario Assessment

The first step is to identify credible conditions that may cause pressure to exceed the allowable limit of the equipment being protected.

Potential scenarios, where applicable, include:

1. Blocked outlet or downstream flow restriction.
2. Relevant control valve or control system failures.
3. Loss of cooling or other essential utilities.
4. External fire exposure.
5. Thermal expansion of blocked-in liquid.
6. Heat exchanger tube rupture.
7. Gas blow-by from a higher-pressure system.
8. Abnormal process conditions that may generate excessive pressure.

The applicable scenarios depend on the equipment configuration, process design, operating conditions, and credible failure mechanisms. Each scenario must be assessed on its own merits; not every scenario applies to every item of equipment.

2. Relief Load Calculation

Relief load calculation determines the flow rate that must be discharged to address the applicable overpressure scenario.

The calculation method depends on the initiating event and the physical behaviour of the process fluid. For example, the relief load associated with external fire exposure is evaluated differently from thermal expansion, blocked outlet, or gas blow-by.

The assessment may require:

1. Verified process and equipment data.
2. Fluid composition and relevant physical properties.
3. The applicable relieving pressure and temperature.
4. Scenario-specific heat input, inflow, or other contributing factors.
5. The required discharge rate for the governing case.
6. Evaluation of the assumptions and limitations affecting the result.

The resulting relief load provides the basis for the subsequent valve sizing calculation.

3. Pressure Safety Valve Sizing Calculations

Using the established relief load and relieving conditions, the required valve capacity and sizing area are calculated in accordance with the applicable methodology.

Depending on the process service, this may include:

1. Gas and vapour relief calculations.
2. Liquid relief calculations.
3. Steam relief calculations.
4. Specialised calculations for applicable flashing or two-phase flow conditions.

The calculation method must be appropriate to the fluid behaviour and design basis. Two-phase relief may require specialised methods and additional process information, so it should not automatically be treated as conventional single-phase flow.

The objective is to establish the required relief capacity and provide a documented engineering basis for evaluating a suitable pressure relief device.

4. PSV Selection Basis Review

Valve sizing establishes the required capacity, but the selected device must also be suitable for its intended service.

Where included in the agreed scope, the review may consider:

1. Required capacity and available certified valve orifice sizes.
2. Valve type and operating characteristics.
3. Set pressure and applicable overpressure requirements.
4. Fluid compatibility and material considerations.
5. Relieving temperature and phase.
6. Superimposed and built-up backpressure.
7. Suitability of conventional, balanced-bellows, or pilot-operated designs, where applicable.

Final valve selection should be checked against the manufacturer’s certified capacity, applicable code requirements, and the actual process and installation conditions. A calculated orifice area alone does not establish the suitability of a specific valve.

5. Existing PSV Review and Re-Sizing Assessment

An existing pressure safety valve may need to be reassessed when the original design basis no longer reflects the current process conditions.

A review may be appropriate following:

1. Plant capacity expansion or debottlenecking.
2. Changes in fluid composition or process service.
3. Changes in operating conditions.
4. Equipment replacement or modification.
5. Changes to upstream or downstream process connections.
6. Findings from a process hazard review.
7. Identification of missing or outdated sizing calculations.

The assessment compares the available sizing basis with the revised conditions and applicable relief scenarios. Where important information is unavailable, the data gaps and assumptions should be identified before drawing conclusions about adequacy.

6. Relief System Interface Considerations

PSV performance can be influenced by the connected inlet piping, discharge piping, and relief disposal arrangements.

Where included in the project scope, relevant considerations may include inlet pressure loss, discharge line pressure drop, superimposed and built-up backpressure, and interaction with a common relief header.

These checks are important because an individually sized valve does not, by itself, demonstrate that the entire pressure relief system is adequate. Relief header, flare, vent, and disposal-system analyses may require separate calculations and a clearly defined engineering scope.