When does API 2000 low-pressure safety valve sizing apply?

API 2000 sizing formulas apply specifically to low-pressure venting applications where gas pressures do not exceed 15 psig (1.034 bar g) and for vacuum protection on atmospheric or low-pressure storage tanks.

What is the difference between direct-acting and pilot-operated low pressure vents?

Direct-acting weight-loaded or spring-loaded pallet valves require up to 100% overpressure to reach full rated lift capacity. Pilot-operated relief valves achieve full opening capacity at much lower overpressure (typically 10%), allowing operation closer to the tank design pressure.

Why is the discharge coefficient K critical for valve selection?

The discharge coefficient K accounts for flow resistance and valve body geometry restrictions. Actual flow capacity equals theoretical flow multiplied by K. Manufacturer certified K values must always be verified during detailed design.

Low Pressure Safety Valve Calculator (Online + Excel)

1. Introduction and warning

This design guide aims the reader at understanding the basic concepts behind the design and sizing of pressure relief valves for low pressure venting but not at designing a valve for operational purposes. It is based on published sources but should not be considered for the detail design and selection / ordering of a pressure safety valve. Indeed, pressure safety valves are of utmost importance for the safety of the process, as they are most often the last resort to avoid an explosion or tank collapse, their design must therefore be done only by reputable companies. Only after having clearly defined the application, the position of the valve, etc., with the valve supplier, can he advise properly the plant operator and finalize the design of the valve.

The calculations below are derived from API 2000 5th edition and adapted from various sources. Those calculations are valid for gas only and for pressure < 15 Psig (1.034 bar g) and vacuum, they do not apply for external floating roof tanks or free vented internal floating roof tanks. Other types of services will require a different calculation code. API for example has other standards for higher pressure gas service or liquid (e.g. API 520). ASME or ISO have their own guidelines.

Note that this page is not discussing the choice and calculation of the design scenario, which is the process events leading to the maximum flow released through the valve. The required flow must be defined thanks to a risk analysis and process calculations (API 2000 for example explains how to determine the required flow for thermal breathing and liquid movement). Newer versions of API 2000 have been published since the 5th edition; the reader must consult them to get a full overview of sizing pressure safety valves for low pressure applications.

2. Interactive Low Pressure Relief Valve Calculator (API 2000)

⚠️ ENGINEERING NOTICE & EDUCATIONAL DISCLAIMER: This interactive calculator is provided exclusively for preliminary estimation and educational purposes based on API 2000 5th Edition. It is not intended for detailed mechanical design, final sizing, or equipment procurement without certified vendor rating. No warranty, expressed or implied, is provided, and no liability is assumed.

Unit System: Metric / SI (bar abs, °C, Nm³/h, cm²) US Customary (psia, °F, scfh, in²)

Calculation Mode:

Inlet Pressure P₁ (bar abs):

Backpressure P₂ (bar abs):

Inlet Temperature T (°C):

Molecular Weight M (g/mol):

Specific Heat Ratio k (Cp/Cv):

Compressibility Factor Z:

Discharge Coefficient K:

Required Flow Q (Nm³/h):

Installed Discharge Area A (cm²):

Calculation Results

Click "Calculate Vent Capacity / Area" to compute results.

💡 Plant Engineering Rules of Thumb & Safety Limits

Pressure Limits: Standard API 2000 low-pressure models strictly apply to set pressures below 15 psig (1.034 bar g). Exceeding this pressure transitions flow into compressible sonic choking regimes governed by API 520.
Overpressure Margins: Direct weight-loaded or spring-loaded pallet vents require 100% overpressure above set pressure to reach full certified lift capacity. For tight tank design limits, pilot-operated valves reach rated capacity at 10% overpressure.
Vacuum Protection: Vacuum vent lines must account for atmospheric condensation and potential freezing/icing at low ambient temperatures, which can freeze valve seats shut.
Backpressure Limits: Atmospheric vents releasing directly to air have $P_2 = 1.013\text{ bar abs}$ ($14.696\text{ psia}$). Header discharge backpressure drastically reduces flow capacity due to smaller $\Delta P$ ratios.

3. Theory & Governing Equations

For low pressure tanks, the following formula derived from API 2000 5th Edition can be used (WARNING - This formula is not valid for gas at pressure > 1.034 barg and not valid for liquid, please refer to the high pressure PSV page):

\[ Q = 12503 \cdot P_1 \cdot A \cdot \sqrt{ \frac{k}{M \cdot T \cdot Z \cdot (k-1)} \times \left[ \left(\frac{P_2}{P_1}\right)^{\frac{2}{k}} - \left(\frac{P_2}{P_1}\right)^{\frac{k+1}{k}} \right] } \]

Equation 1 : Flow through low pressure safety valve

With :

$P_1$ = pressure at inlet in bar abs

$P_2$ = backpressure at outlet in bar abs

$A$ = minimum discharge area required of the safety valve in cm²

$Q$ = theoretical flow through the valve in Nm³/h

$k$ = ratio of specific heats ($C_p/C_v$)

$T$ = absolute temperature at inlet in K

$M$ = molecular weight of gas (g/mol)

$Z$ = compressibility factor at inlet conditions

4. Practical Selection & Overpressure Characteristics

The actual flow through the valve is lower than the theoretical flow. This non-ideality is represented thanks to a coefficient of discharge of the valve, $K$.

K = Actual flow / Theoretical flow

The coefficient K is defined by the manufacturer. API 2000 5th edition gives an example coefficient table:

Coefficient of discharge K of low pressure / vacuum safety valves

Figure 1 : Coefficient of discharge K of low pressure / vacuum safety valves

Depending on the technology of the valve chosen, either direct operated (weighted pallet valve) or pilot operated, an overpressure is required to reach the maximum flow. API 2000 provides the following graph to estimate the overpressure per type of valve. It is very important to consider, especially for direct acting valves that will only reach their rated capacity at 100% overpressure. The protected tank must therefore be able to hold this overpressure safely.

Capacity / Overpressure characteristics of low pressure vents

Figure 2 : Capacity / Overpressure characteristics of low pressure vents

Practical Valve Sizing

This kind of valve flow is highly dependent on the valve design and actual allowable overpressure. It is strongly recommended to refer to the manufacturer's flow capacity tables for a given valve model, calculated according to API 2000. An example table is shown below:

Example of commercial low pressure safety valve capacity table

Once the valve is selected for the required flow application, the actual flow considering the overpressure must be determined. Another table with a corrective coefficient is then applied. If the acceptable overpressure differs from standard test benchmarks, the flow must be corrected accordingly.

Example of correction factor commercial low pressure safety valve

WARNING: www.MyEngineeringTools.com cannot be held responsible for the use of the explanations, calculations, and tools presented here. The use of this information is strictly at the user's and organization's own risk and cost.

Source: Various sources based on API 2000 5th edition.

Pressure Safety Valve Basic Design guide - Low pressure venting

Pressure Relief Valve sizing for atmospheric, low pressure tanks (pressure / vacuum)