Introduction
The EFV Sizing / EFV Values calculator computes the key operating and sizing parameters for an Excess Flow Valve (EFV) installed on a gas service line. Given the inlet pressure, required capacity, selected EFV size/type, and downstream pipe data, it calculates the pressure drop across the valve, the minimum and maximum trip flow ranges, the maximum bypass rate, the equivalent length, outlet velocity, and the maximum length of service line the EFV can effectively protect.
Important: The Minimum EFV Trip Flow must always exceed the Required Capacity. If the required demand equals or exceeds the minimum trip flow, the EFV may activate and interrupt service under normal operating conditions. A warning will be displayed if this condition is detected.
Background
An Excess Flow Valve (EFV) is a safety device designed to close — or “trip” — automatically when flow through it reaches or exceeds a specified range. EFVs are most commonly installed on service lines serving residential and small commercial customers. Their purpose is to shut off gas flow if the downstream service line is severely damaged or severed, limiting the amount of gas released to the atmosphere.
Proper EFV selection and service line sizing must account for three key considerations:
Pressure Drop
Due to their internal design, most EFVs produce a significant pressure drop — reflected as a large equivalent length value. Unlike fittings in main lines (where equivalent lengths are small relative to the pipe run), the equivalent length of an EFV can exceed the physical length of the service line itself. Omitting the EFV from service line sizing calculations can substantially underestimate the total pressure drop and overestimate capacity.
Trip Flow Range
An EFV does not trip at a single precise flow rate — it activates within a range bounded by the Minimum and Maximum EFV Trip Flow values. Two design checks are required:
First, the anticipated downstream demand must remain below the Minimum EFV Trip Flow to prevent the valve from closing under normal full-load conditions. Second, if the service line were severed at its farthest downstream point (such as at the meter), the resulting flow through the damaged line must exceed the Maximum EFV Trip Flow to guarantee that the valve closes.
Bypass Rate and Reset
After an EFV trips, a small amount of gas continues to flow through a built-in bypass. This bypass allows downstream pressure to gradually equalize with upstream pressure, which eventually causes the valve to reset and reopen automatically once the downstream facilities have been restored to a gas-tight condition. The bypass rate determines how quickly pressure equalization — and service restoration — occurs.
Equations
Pressure Drop and Outlet Pressure
The pressure drop across the EFV is calculated using the equivalent length and wall roughness values read from the EFV Property Table, combined with the selected pipe flow equation, pipe efficiency of 1.0, and the specified operating conditions. This is equivalent to computing the pressure drop across a pipe segment whose length equals the EFV’s equivalent length. The outlet pressure is then:
P_{\text{outlet}} = P_{\text{inlet}} - \Delta P_{\text{EFV}}P_{\text{outlet}} = P_{\text{inlet}} – \Delta P_{\text{EFV}}
Where:
Poutlet − Pressure at the outlet (downstream) side of the EFV (psig)
Pinlet − Pressure at the inlet (upstream) side of the EFV (psig)
ΔPEFV − Pressure drop calculated across the EFV equivalent length (psi)
EFV Trip Flow, Bypass Rate, and Maximum Protected Length
The Minimum EFV Trip Flow, Maximum EFV Trip Flow, and Maximum Bypass Rate values are read directly from the EFV Property Table and adjusted for the specified dimensional units. These values originate from manufacturers’ published literature referenced in the property table.
The Maximum Protected Length is the longest service line that can sustain a flow rate at least equal to the Maximum EFV Trip Flow under the specified inlet conditions. It is determined by solving the pipe flow equation for the length at which the flow through a completely severed line equals the maximum trip point.
Outlet Velocity
The outlet velocity is calculated from the specified required capacity, the inlet temperature, the computed outlet pressure, and the equivalent diameter associated with the selected EFV:
V = \frac{Q}{A} \times \frac{P_B}{P_{\text{outlet}}} \times \frac{T_F}{T_B}V = \frac{Q}{A} \times \frac{P_B}{P_{\text{outlet}}} \times \frac{T_F}{T_B}
Where:
V − Gas velocity at the EFV outlet (ft/sec)
Q − Volumetric flow rate at base conditions (cfh)
A − Cross-sectional flow area based on the EFV equivalent diameter (ft²)
PB − Base pressure (psia)
Poutlet − Outlet pressure (psia)
TF − Inlet gas temperature (°R)
TB − Base temperature (°R)
Case Guide
Part 1: Create Case
- Select the EFV Sizing / EFV Values application from the Regulators & Meters Module.
- From the Valves & Fittings menu, select the EFV Values item. The Excess Flow Valve Values calculation screen will be displayed.
- Click the Clear button to set all values to blank (null).
- Click the Base Conditions button. Enter the appropriate base pressure and temperature, select or enter gas property values, and choose the Atmospheric Pressure Method. Click Apply to save and return.
- In the Operating Data section, enter the Required Capacity (maximum expected flow through the EFV), Inlet Pressure, Elevation, and Inlet Temperature. Select appropriate dimensional units for each field.
- In the Excess Flow Valve Data section, click the ? button next to Size/Type to open the EFV Selection screen and choose the desired valve. Select the preferred units for Equivalent Length and Outlet Velocity.
- In the Pipe Data section, click the ? button next to Pipe Size/Type to select the pipe in which the EFV is installed. Select the Pipe Flow Equation to use for the pressure drop calculation.
- Click the Calculate button to compute all results.
Input Parameters
| Parameter | Description |
|---|---|
| Required Capacity | Specifies the maximum required flow rate through the EFV. |
| Inlet Pressure | Specifies the pressure at the inlet (upstream) side of the EFV. |
| Elevation | Specifies the average height above mean sea level at the EFV location. Displayed when the Atmospheric Pressure Method in Base Conditions is not set to “None” or “None – Entered Value.” |
| Inlet Temperature | Specifies the temperature at the inlet (upstream) side of the EFV. |
| Size/Type | Specifies the EFV Size/Type Code. Click the ? command button to select a device using the EFV Selection screen. |
| Pipe Size/Type | Specifies the Size/Type Code of the pipe in which the EFV is installed. Click the ? command button to select a pipe size using the Pipe Selection screen. |
| Pipe Flow Equation | Specifies the pipe flow equation used during the calculation. |
| Atm Pressure | Specifies the atmospheric pressure at the EFV location. Displayed only when the Atmospheric Pressure Method in Base Conditions is set to “None – Entered Value.” |
Part 2: Outputs/Reports
- If you need to modify an input parameter, update the value and click the CALCULATE button again.
- To SAVE, click the Save command button. Provide a file name and location (.efv file).
- To open a previously saved calculation, click the Open command button and select the .efv file.
- To generate a REPORT, click the Print command button to access the Print Settings screen.
- To compare results for different EFV types or pipe sizes, use Additional Actions > Open Duplicate Calculation.
- To add a title or notes to the calculation, click the Notes command button.
Results
| Output | Description |
|---|---|
| Outlet Pressure | The pressure at the outlet (downstream) side of the EFV (psig or millibar). |
| Equivalent Length | The total equivalent length of the EFV expressed in terms of the selected pipe size (ft or m). |
| Pressure Drop | The calculated linear pressure drop across the EFV (psi or millibar). |
| Outlet Velocity | The gas velocity at the outlet (downstream) side of the EFV (ft/sec or m/sec). |
| Min EFV Trip Flow | The calculated lower bound of the flow range that will cause the EFV to activate (cfh or m³/h). |
| Max EFV Trip Flow | The calculated upper bound of the flow range that will cause the EFV to activate (cfh or m³/h). |
| Max Bypass Rate | The calculated maximum bypass flow rate through the EFV after it has been activated (cfh or m³/h). |
| Max Protected Length | The calculated maximum service line length that the selected EFV can effectively protect (ft or m). |
References
- B3PE LLC — GASCalc 6.1 Service Line Sizing Calculation Reference, Revision 005, Copyright 2025. (EFV trip point, bypass rate, and protected length values derived from manufacturers’ literature referenced in the EFV Property Table.)
FAQ
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What type of station does the Regulator & Monitor System calculator model?
The calculator models a two-stage (monitor-style) regulator and relief valve station in which gas pressure is reduced sequentially across two independent regulator stages. Each stage has its own relief valve and vent stack. This configuration is commonly used when codes require automatic overpressure protection at both stages of pressure reduction.
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Which regulatory codes does this calculator support for compliance checks?
The calculator supports two regulatory codes for compliance checks: US DOT 49 CFR Part 192 (2019 edition) and ASME B31.8 (2007 edition). When a code is selected, the calculator compares the maximum calculated pressure in each piping section against the user-specified MAOP for that section using the allowable limits defined by that code. Results that exceed the allowable limits are highlighted in red on the Compliance data tab.
You can also select “None” to perform the hydraulic and flow calculations without any code-based compliance checking.
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What information do I need before running this calculator?
Before running the calculator, you will need the following for each stage of the station:
For the supply piping, you need the minimum and maximum inlet pressures, the flowing gas temperature, the pipe and fitting specifications (size, wall thickness, length), and the pipe flow equation and efficiency. For each regulator, you need to select the manufacturer and model from the device database and enter the set pressure. For each relief valve, you need to select the model, enter the set pressure, minimum build-up pressure, and number of installed valves. For the vent stacks, you need the pipe and fitting specifications and whether the outlet discharges to atmosphere.
You will also need the gas composition or a gas properties file, base conditions (pressure and temperature), the minimum and maximum outlet flow rates, and — if performing compliance checks — the MAOP for each of the five piping sections: Upstream (Supply 1), Intermediate 1, Upstream (Supply 2), Intermediate 2, and Outlet.
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What operating modes does the calculator support, and when should I use each one?
The calculator supports six operating modes, each representing a different assumption about which regulators are operating normally and which have failed.
Use Failed Upstream or Failed Downstream when you want to evaluate a single regulator failure while the other stage operates normally — these are the most common code-required failure scenarios. Use Failed Single to evaluate each regulator failing independently in two separate analyses. Use Failed Double to evaluate simultaneous failure of both regulators, which is the most conservative failure case. Use Normal to verify pressures and velocities under steady-state operation, and Normal Maximum to calculate the maximum flow capacity the combined station can deliver.
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How does the calculator determine the inlet pressure to the second-stage supply piping?
The inlet pressure to the second-stage supply piping is not a direct user input — it is calculated internally based on the selected operating mode.
In most failure modes (Failed Single, Failed Double, Failed Downstream, Normal, and Normal Maximum), the second-stage inlet pressure is set equal to the first-stage regulator set pressure minus the pressure drop across the first-stage intermediate piping. In the Failed Upstream mode, however, the second-stage inlet pressure is instead set to the calculated build-up pressure at the outlet of the first-stage intermediate piping. This build-up pressure may be higher than the set pressure, since the first-stage relief valve is actively venting and the intermediate piping is operating under overpressure conditions. This distinction is important for correctly sizing the second-stage components in a failed upstream scenario.
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What does the Relief Branch – First fitting do, and when should I use it?
In most physical installations, the relief valve is not installed inline with the intermediate piping — it is connected via a branch tee. This means the piping upstream of the tee carries both the relief valve flow and any downstream system flow, while the branch piping leading to the relief valve carries only the relief valve flow.
The Relief Branch – First component is a special fitting you add to the intermediate piping component list to tell the calculator where this branch point occurs. The calculator automatically splits the flow at that point: combined flow upstream of the component, relief-valve-only flow downstream. A Relief Branch – Second component is also available for installations with multiple identical relief valves sharing a common header, and is used to mark the point where the header splits to each individual valve.
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What does the Operating Status field on the Relief Valve data tab mean?
The Operating Status field shows how the relief valve is responding under the calculated conditions. A status of Popping means the inlet pressure has reached or exceeded the relief valve set pressure and the valve is actively venting gas through the vent stack. A status of Continuously Closed means the inlet pressure is below the set pressure and the valve remains shut — no flow passes through the stack piping under those conditions.
In a failure scenario, you want to see Popping on the stage whose regulator has failed, which confirms the relief valve has opened and is handling the failed-regulator flow. If the relief valve is Continuously Closed when it should be venting, the relief valve may be undersized or the set pressure may be too high relative to the build-up pressure.
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Is the 75% SMYS limit from DOT 192 §192.201(a)(2)(i) checked automatically?
No. When using the US DOT Part 192 regulatory code, the calculator performs MAOP-based compliance checks for each piping section, but it does not automatically evaluate the 75% SMYS hoop stress limit associated with §192.201(a)(2)(i).
If your calculated failed pressures are approaching the MAOP of any section — particularly on higher-pressure upstream piping — you should independently verify the resulting hoop stress using the Hoop Stress calculation routine in the Design & Stress Analysis module to confirm compliance with that limit.