The Regulator & Relief Valve System calculator analyzes the hydraulic performance and regulatory compliance of a single-regulator, single-relief-valve gas pressure regulation station. It evaluates the full station as an integrated system — including supply piping, the regulator, intermediate piping, the relief valve, vent stack piping, and outlet piping — under both normal and failed operating conditions. Results are based on nominal device properties and the specified pipe flow equations; always consult applicable regulatory codes and manufacturer’s literature before finalizing a station design.
Background
A regulator (also called a governor, control valve, or pressure-reducing valve) reduces gas pressure from a higher upstream value to a lower downstream set pressure. Because a failed regulator can expose downstream piping and equipment to damaging overpressure, an over-pressure protection (OPP) device is typically installed alongside it. A relief valve is one of the most common OPP devices used in the gas distribution industry.
In a regulator and relief valve station, the two devices work together: under normal conditions the regulator controls outlet pressure and the relief valve remains closed. The relief valve set pressure is usually 5 to 10 psig above the regulator set pressure. If the regulator fails wide-open and outlet pressure begins to rise, the relief valve opens to vent gas and limit the build-up pressure in the intermediate piping.
Why System-Level Analysis Matters
Reviewing the regulator and relief valve as standalone devices — without considering the associated piping — gives only a rough indication of adequacy. The capacity of every piping section affects overall station performance:
- Supply piping must have sufficient capacity to feed the regulator at both minimum normal flow and maximum failed flow rates.
- Intermediate piping must pass both the downstream demand flow and the relief valve venting flow simultaneously during failed conditions.
- Stack (vent) piping must not restrict the relief valve’s venting capacity. Excessive back-pressure on the discharge side reduces effective relief capacity.
- Outlet piping must deliver the required maximum station capacity to the downstream system without excessive pressure drop.
Choked Flow Considerations
Under failed conditions, flow rates through a regulator station can be very large, and velocities in the piping and devices can approach or reach sonic velocity. When a pipe segment reaches sonic (choked) flow, its capacity cannot be increased without raising upstream pressure. Choked flow anywhere in the station limits overall station capacity and must be considered during design.
Regulator Sizing Guidelines
As a general rule of thumb, the required capacity of the regulator should fall between 20% and 80% of its rated capacity under normal operating conditions. This range roughly corresponds to a Factor Ratio of 20%–80% as reported by the calculation. Operating outside this range — either oversizing or undersizing the regulator — increases the risk of instability, noise, or inadequate relief protection under failed conditions.
Relief Valve and Vent Piping Guidelines
Industry practice generally recommends that pressure loss across the relief valve inlet piping not exceed 3% of the relief valve set pressure, and that pressure loss across the vent (stack) piping not exceed 10% of the set pressure. An oversized relief valve relative to the failed regulator capacity leads to chattering or popping, which damages the valve seat and can prevent the valve from fully closing. Field experience indicates that pipe efficiency values of 0.4–0.6, using the Colebrook equation with the sonic velocity limit enabled, best match measured venting results for stack piping calculations.
Calculation Method
The Regulator & Relief Valve System routine combines the pipe flow, regulator, and relief valve calculation methods into a single iterative station model. Pipe sections are reduced to equivalent segments. An iterative process varies the station flow rate until the computed pressure drop across the full system — supply piping, regulator, intermediate piping, and outlet piping — matches the known boundary pressures. Depending on the selected operating mode, the calculation may or may not route flow through the relief valve and vent stack.
Temperature change across the regulator and relief valve is estimated using the Joule-Thomson method, which is valid for high-methane-content gases. No temperature change is applied to the pipe sections. This behavior can be suppressed via the Ignore Temperature Change In Device Calculations preference option.
Operating Modes
| Mode | Description |
|---|---|
| Failed — Regulator Failed Wide Open, Relief Valve Open | Regulator assumed failed wide-open; relief valve assumed open in relief mode. Pressure values are calculated based on the flow present if the relief valve remains wide-open. Uses Maximum Inlet Pressure and Minimum Downstream Flow. |
| Normal — Regulator Controlling Normally, Relief Valve Closed | Regulator operating normally; relief valve closed. Pressures calculated using the Maximum Downstream Flow specified on the Outlet Piping tab. Uses Minimum Inlet Pressure. |
| Maximum Station Capacity — No Velocity Limit | Regulator normal, relief closed. Maximum station flow calculated without a velocity cap (or limited only to sonic if the Limit Pipe Velocity To Sonic preference is enabled). |
| Maximum Station Capacity — Velocity Limited | Regulator normal, relief closed. Maximum station flow calculated while limiting all piping velocities to at or below the Normal Velocity Limit preference value. |
Supported Regulatory Codes
| Code | Reference |
|---|---|
| ASME B31.8 – 2007 | American Society of Mechanical Engineers, Gas Transmission and Distribution Piping Systems, B31.8-2007 |
| US DOT Part 192 – 2019 | United States Department of Transportation, Pipeline and Hazardous Materials Safety Administration, Pipeline Safety Regulations, Part 192, October 2010 |
| None Selected | No compliance checks performed; hydraulic calculations still run. |
Note: Codes and regulations change over time. The compliance checks in GASCalc are based on the referenced version of each code. Requirements in other versions may differ. When using the US DOT Part 192 code, the 75% SMYS limit under §192.201(a)(2)(i) is not supported — verify this condition separately using the Hoop Stress calculation routine if failed pressures may approach that limit.
Station Piping Sections
The station is divided into five piping sections, each with its own data tab. Components (pipes and fittings) in each section are drawn from the Pipe and Fitting Property Tables. Each section’s equivalent diameter, length, and roughness are used to compute pressure drop and velocity.
| Section | Description |
|---|---|
| Supply (Upstream) Piping | Pipes and fittings between the lateral tap or tee and the regulator inlet. |
| Intermediate Piping | Pipes and fittings between the regulator outlet and the relief valve inlet. Normally carries both downstream demand flow and (during relief) venting flow combined. |
| Stack (Vent) Piping | Pipes and fittings between the relief valve outlet and the vent exit. Outlet pressure is typically set to atmospheric. |
| Outlet Piping | Pipes and fittings downstream of the intermediate piping to the connection with the downstream system. |
Relief Branch Components for Intermediate Piping
In most stations, a tee connects the relief valve branch to the intermediate piping. To accurately model the flow split, two special fitting components are available on the Fittings tab of the Piping Components screen:
- Relief Branch – First: Marks where the vent piping branches from the intermediate piping. Piping upstream of this point carries combined relief valve flow plus downstream demand flow. Piping downstream carries only relief valve venting flow. Use this component for single relief valve configurations.
- Relief Branch – Second: Used in addition to Relief Branch – First when multiple identical relief valves are present. Marks where the common vent run splits to each individual valve. Piping between the First and Second branches carries total relief valve flow; piping downstream of the Second branch carries only the flow through a single valve.
Case Guide
Part 1: Create Case
- Select the Regulator & Relief Valve System application from the Regulators & Meters Module.
- Click the Clear command button to reset all data items to blank (null) values.
- Click the Base Conditions command button. Set the base pressure, base temperature, gas properties file, and atmospheric pressure method, then click Apply.
- On the General tab, select the appropriate Operating Mode. Optionally fill in station identification, location, and review date fields.
- On the Supply Piping tab, use the Add button to build the component list for upstream piping. Enter the pipe flow equation, efficiency, minimum and maximum inlet pressures, elevation, and temperature.
- On the Regulator tab, click the ? button next to Size/Type to select the regulator model from the Device Selection screen. Enter the regulator set pressure.
- On the Intermediate Piping tab, add components between the regulator outlet and the relief valve inlet. Set the pipe flow equation and efficiency. Add a Relief Branch – First fitting component if the relief valve branches from this run.
- On the Relief Valve tab, click the ? button to select the relief valve model. Enter the set pressure, minimum build-up pressure, and number of installed valves.
- On the Stack Piping tab, add vent piping components. Set pipe flow equation and efficiency. Select Set To Atmospheric Pressure if the vent discharges to atmosphere.
- On the Outlet Piping tab, add downstream piping components. Enter the minimum and maximum station flow rates.
- On the Compliance tab, select the Regulatory Code and enter the MAOP for each piping section (Upstream, Intermediate, Outlet).
- Click the *** Calculate *** command button.
Input Parameters
| Parameter | Description |
|---|---|
| Operating Mode | Specifies the calculation scenario: Failed (regulator wide-open, relief valve open), Normal (regulator controlling, relief valve closed), or Maximum Station Capacity (with or without velocity limit). |
| Station Identification | Optional text field for the station ID used in reports and record-keeping. |
| Station Description | Optional text field for a descriptive name or notes about the station. |
| District Identification | Optional text field for the district or system identifier. |
| Legal Description | Optional text field for the legal location description of the station. |
| Location | Optional text field for the physical address or location description. |
| Review Date | Date of the most recent station review. Used with the selected regulatory code for compliance tracking. |
| Reviewed By | Name or initials of the engineer who performed the review. |
| Previous Review | Date of the prior review cycle, for reference. |
| Next Review | Calculated or manually entered date for the next required review. Use the Calculate Next Review Date button to auto-populate based on the regulatory code. |
| Include When Performing Station Matching | When checked, this station will be included in station matching operations. |
Part 2: Outputs/Reports
- Review each data tab for calculated pressures, flow rates, velocities, and temperatures.
- Check the Compliance tab — values displayed in red exceed the allowable limit for the selected regulatory code. Adjust regulator orifice size, relief valve model, or piping as needed and recalculate.
- Switch the Operating Mode between Failed and Normal (or Maximum Station Capacity modes) and recalculate to verify both scenarios.
- To SAVE the current calculation, click the Save button. Saved files use the .rrs extension.
- To open a previously saved calculation, click the Open button.
- To PRINT a report, click the Print button to access Print Settings. Three formats are available: Standard Report (full data + schematic), Short Report (schematic + relief valve + compliance items), or Inspection Form (field inspection checklist).
- To create a duplicate calculation for side-by-side scenario comparison, use Additional Actions → Open Duplicate Calculation.
- To close without saving, click Cancel.
Results
| Output | Description |
|---|---|
| Inlet Pressure (Supply / Intermediate / Stack / Outlet) | The calculated pressure at the upstream end of each piping section, after the calculation is complete. |
| Outlet Pressure (Supply / Intermediate / Stack / Outlet) | The calculated pressure at the downstream end of each piping section. |
| Flow Rate (all sections) | The calculated flow rate through each piping section, in standard volume units. |
| Maximum Velocity (all sections) | The calculated maximum gas velocity through each piping section, based on flow rate, temperature, outlet pressure, and smallest inside diameter. |
| Flowing Temperature (Intermediate / Stack / Outlet) | The average flowing temperature of the gas in each piping section. |
| Wide-Open Valve Factor (Regulator) | The rated wide-open (failed) valve factor for the selected regulator. Calculated in Failed operating mode only. |
| Controlling Valve Factor (Regulator) | The rated controlling valve factor for the selected regulator. Calculated in Normal operating mode only. |
| Required Valve Factor (Regulator) | The calculated valve factor required to satisfy the specified flow conditions. Not calculated in Failed mode. |
| Factor Ratio (Regulator) | The ratio of the Required Valve Factor to the Controlling Valve Factor, expressed as a percentage. Not calculated in Failed mode. Recommended range: 20%–80%. |
| Outlet Temperature (Regulator / Relief Valve) | The estimated gas temperature at the outlet of the regulator or relief valve, calculated using the Joule-Thomson method. |
| Flow Mode (Regulator) | The type of flow through the regulator (e.g., Sub-Sonic – Normal Flow, or Sonic). |
| Outlet Velocity (Regulator / Relief Valve) | The calculated gas velocity at the outlet of the regulator or relief valve. |
| Sizing Factor (Relief Valve) | The rated sizing factor or orifice area for the selected relief valve. |
| Inlet (Build-Up) Pressure (Relief Valve) | The calculated pressure at the upstream side of the relief valve — the system build-up pressure under relief conditions. |
| Outlet (Back) Pressure (Relief Valve) | The calculated pressure at the downstream (discharge) side of the relief valve. |
| Operating Status (Relief Valve) | The calculated operating status of the relief valve (e.g., Continuously Open, Continuously Closed, Popping). |
| Failed Regulator Capacity (Compliance) | The calculated failed capacity of the regulator. Displayed in Failed operating mode only. |
| Relief Valve Capacity (Compliance) | The calculated relief valve and stack capacity. Displayed in Failed operating mode only. |
| Maximum Station Capacity (Compliance) | The calculated maximum capacity of the station. Displayed in Normal or Maximum Station Capacity operating modes. |
| Maximum Calculated Pressure (Compliance) | The maximum calculated pressure for each piping section (Upstream, Intermediate, Outlet). |
| Maximum Allowable Pressure (Compliance) | The calculated allowable pressure for each piping section, based on the operating mode and selected regulatory code. Values shown in red exceed the allowable limit. |
| % of MAOP (Compliance) | The ratio of the Maximum Calculated Pressure to the specified MAOP for each piping section. |
Notes & Considerations
- All pressure values displayed or entered on this screen are in gauge pressure.
- All flow rate values are in standard volume units, adjusted to the base pressure and temperature specified in Base Conditions.
- The regulator set pressure is sensed immediately downstream of the device. The relief valve set pressure is sensed immediately upstream of the valve.
- Minimum Inlet Pressure is the lowest inlet pressure the station can experience and is used in the normal calculation. Maximum Inlet Pressure is the highest inlet pressure and is used in the failed calculation.
- Minimum Downstream Flow is used during failed calculations — enter zero (0) for conservative results. Maximum Downstream Flow is used during normal calculations.
- Minimum Build-Up Pressure is the minimum additional pressure above relief valve set pressure required to fully open the valve. If unknown, enter zero (0) for conservative results.
- Any piping section can be ignored by leaving its Components list empty. A confirmation message will appear during calculation — click Yes to proceed. To suppress this message, enable the Allow Empty Component Lists preference option.
- The on-screen schematic provides a clickable overview of the station. Clicking any label or icon on the schematic navigates directly to that component’s data tab.
- Some regulatory codes require periodic inspection reviews. The information on the General tab (Review Date, Reviewed By, Previous Review, Next Review) can be used to document those reviews. The Calculate Next Review Date button computes the required next review date based on the selected regulatory code.
- For vent stack calculations, field data suggests that the Colebrook equation with a pipe efficiency of 0.4–0.6 and the Limit Pipe Velocity To Sonic preference enabled best correlates with actual venting results.
- The US DOT Part 192 code’s 75% SMYS overpressure limit (§192.201(a)(2)(i)) is not supported by this routine. When failed pressures may approach that limit, verify using the Hoop Stress calculation routine.
- For velocity-related noise considerations, station piping velocities are commonly maintained below 100 ft/sec, though some guidance allows up to 300 ft/sec. When noise is not a constraint, velocities may be allowed to approach sonic.
References
- B3PE LLC — GASCalc™ 6.1 Calculation Reference: Regulator & Relief Valve System, Revision 005, Copyright 2025
- American Society of Mechanical Engineers — ASME B31.8-2007: Gas Transmission and Distribution Piping Systems
- United States Department of Transportation, Pipeline and Hazardous Materials Safety Administration — Pipeline Safety Regulations, 49 CFR Part 192, October 2010
FAQ
-
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.
-
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.
-
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.
-
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.