Introduction
This application calculates the flow and pressure values associated with a two-stage regulator and relief valve style regulator station. It models the complete station — the supply, intermediate, stack, and outlet piping, two regulators in series, and a relief valve at each stage — and reports the flow rate, inlet and outlet pressure, and maximum velocity for every piping section, along with code-compliance checks for the various pipe sections.
Only the Operating Mode on the General data tab is required to perform a calculation; the remaining General items are optional documentation fields. Pressure values are entered and displayed as gauge pressures, and flow values represent “standard” volumes adjusted to the base pressure and temperature set in the Base Conditions. This two-stage routine shares its background and methods with the single-stage Regulator & Relief Valve System calculation, differing chiefly in the available operating modes.
Background
A two-stage regulator station reduces supply pressure to delivery pressure in two steps, with each stage protected by its own relief valve and vent stack. The station is divided into a sequence of piping sections separated by the two regulators and two relief valves. The supply piping generally refers to the pipe and fittings between the lateral tap or tee and the first-stage regulator. The intermediate piping refers to the pipe and fittings between the outlet of a regulator and the inlet of its associated relief valve. The stack piping refers to the pipe and fittings between the outlet of a relief valve and the vent exit. The outlet piping refers to the pipe and fittings downstream of the second-stage intermediate piping, and generally should extend only to the connection to the “system” supplied by the station.
The set pressure for each regulator is assumed to be sensed immediately downstream of the device, while the set pressure for each relief valve is assumed to be sensed immediately upstream of the valve. The inlet pressure to a relief valve is the same as the system “build-up” pressure — the pressure the protected system would experience under relief conditions. Any piping section can be ignored by leaving its Components list empty.
Operating Modes
The Operating Mode determines which failure scenario is analyzed and therefore how the inlet pressures and section flows are established. The two-stage configuration supports the following modes:
Failed Single — Each regulator is assumed to fail independently. The first-stage supply piping inlet is set to the Maximum Inlet Pressure, and the first-stage upstream and intermediate piping carries the failed capacity of the first-stage regulator and relief valve plus any downstream flow. The second-stage supply inlet equals the first-stage regulator set pressure less the pressure drop across the first-stage intermediate piping; the second-stage stack piping carries the failed capacity of the second-stage regulator and relief valve, and the outlet piping carries the minimum outlet flow.
Failed Double — Both regulators are assumed to fail simultaneously. The first-stage upstream and intermediate piping carries the failed capacity of the first-stage regulator and relief valve plus the second-stage regulator and relief flow plus any downstream flow. The second-stage inlet equals the first-stage regulator set pressure less the first-stage intermediate pressure drop, and the outlet piping carries the minimum outlet flow.
Failed Upstream — Only the upstream regulator is assumed to fail; the second-stage regulator operates normally and the second-stage relief valve is assumed closed. The first-stage supply inlet is set to the Maximum Inlet Pressure, and the second-stage supply inlet equals the calculated build-up pressure at the outlet of the first-stage intermediate piping. The second-stage upstream piping, intermediate piping, and outlet piping carry the minimum outlet flow.
Failed Downstream — Only the downstream regulator is assumed to fail; the first-stage regulator operates normally and the first-stage relief valve is assumed closed. The second-stage stack piping carries the failed capacity of the second-stage regulator and relief valve, and the outlet piping carries the minimum outlet flow.
Normal — Both regulators operate normally. The first-stage supply inlet is set to the Minimum Inlet Pressure, and all non-stack piping carries the maximum outlet flow. The second-stage supply inlet equals the first-stage regulator set pressure less the first-stage intermediate pressure drop.
Normal Maximum — The maximum capacity of the combined regulators is calculated with both regulators operating normally. The first-stage supply inlet is set to the Minimum Inlet Pressure, and the non-stack piping carries the calculated maximum station capacity. The maximum station capacity equals the lower of the maximum calculated capacities of the first- or second-stage regulators.
Relief Branch Components
In most installations a tee or branch connection ties the relief valve into the intermediate piping, and that branch only carries flow when the relief valve is venting. To model this accurately, two special fitting components are available on the Fittings data tab of the Piping Components screen. The Relief Branch – First component marks where the vent piping branches from the intermediate piping; flow upstream of it equals the relief valve flow plus any downstream flow, while flow downstream of it equals the relief valve flow only. When multiple identical relief valves are present, the Relief Branch – Second component additionally marks where the common run splits to each individual valve. Flow upstream of the First Relief Branch equals the flow through all relief valves plus downstream flow, the common run between the First and Second branches carries the relief valves’ flow only, and the run downstream of the Second branch carries a single relief valve’s flow.
Code Compliance
The selected Regulatory Code establishes how the allowable compliance limits are calculated for each piping section; refer to the referenced code for details. Supported codes are ASME B31.8 – 2007, US DOT Part 192 – 2019, and None Selected (no checks performed). When using the DOT 192 code, the 75% SMYS limit of part 192.201(a)(2)(i) is not supported; where failed pressures are suspected to approach this limit, the resulting hoop stress should be checked separately using the Hoop Stress calculation. Comparison fields are displayed in red when a value exceeds its allowable limit. Because codes and regulations change over time, compliance checks reflect the specific referenced edition for each code.
Calculation Method
The regulator and relief valve system calculation combines equations from several other Technical Toolboxes calculation routines into a single iterative “method” rather than relying on one defining equation. The pipe and fitting components of each piping section are combined into a single equivalent segment, and the pressures at the supply and sense points are assumed constant. Flow through the various pipe sections is calculated using the same methods as the Pipe Flow calculation; flow through the regulators uses the same methods as the Regulator Values calculation; and flow through the relief valves uses the same methods as the Relief Valve Values calculation.
Using an iterative process, the station flow rate is computed and the pressure difference across the entire system — upstream piping, regulator, intermediate piping, and downstream piping — is compared against the known values. When the flow rate produces the appropriate overall system pressure drop, the solution is considered complete. Depending on the operating mode, flow is calculated either through only the upstream piping, regulator, intermediate piping, and downstream piping, or through the entire station including the relief valve and vent stack piping. The process mirrors the single-stage station calculation, differing in how the inlet pressure to the second-stage supply piping is established for each operating mode. The maximum velocity reported for each section is derived from the section flow rate, the average temperature, the section outlet pressure, and the smallest inside diameter of the section’s components.
For the underlying equations and parameter definitions, refer to the Pipe Flow, Regulator Values, Relief Valve Values, and single-stage Regulator & Relief Valve System calculation references.
Case Guide
Part 1: Create Case
- Select the Regulator & Relief Valve System – 2 Stage application from the Valves & Fittings Module.
- Click the Clear command button to set all values to an empty (null) value.
- Click the Base Conditions command button, enter an appropriate base pressure and temperature, select a Gas Properties File (or “None”), select an Atmospheric Pressure Method, then click Apply.
- On the General data tab, select the appropriate Operating Mode (required) and optionally enter the documentation items.
- On the 1st Stage tabs, add the Supply Piping 1 components and enter its data; select the Regulator 1 Size/Type and Set Pressure; add the Intermediate Piping 1 components; select the Relief Valve 1 Size/Type, Set Pressure, Minimum Build-Up, and Number Of Valves; then add the Stack Piping 1 components.
- On the 2nd Stage tabs, repeat the same sequence for Supply Piping 2, Regulator 2, Intermediate Piping 2, Relief Valve 2, and Stack Piping 2.
- On the Outlet Piping tab, add the outlet components and enter the Minimum and Maximum Flow Rates.
- On the Compliance tab, select a Regulatory Code and enter the MAOP for each pipe section.
- Click the CALCULATE command button to overview results.
Input Parameters
| Parameter | Description |
|---|---|
| Operating Mode | Specifies the operating mode of the station (Failed Single, Failed Double, Failed Upstream, Failed Downstream, Normal, or Normal Maximum). This is the only required item on the General data tab. |
| General Documentation Items | Optional items used for code-compliance documentation: Station Identification, Station Description, District Identification, Legal Description, Location, Review Date, Previous Review Date, Next Review Date, and Reviewed By. Dates use MM/DD/YYYY format. |
| Supply Piping Components | The pipe and fitting components upstream of each stage’s regulator. Edited with the Add, Insert, Delete, and Clear command buttons below the Components list. |
| Minimum Inlet Pressure | The lowest inlet pressure the station can experience at the first-stage upstream piping. Used during the normal calculation process. |
| Maximum Inlet Pressure | The highest inlet pressure the station can experience at the first-stage upstream piping. Used during the failed calculation process. |
| Temperature | The flowing gas temperature at the inlet to the first-stage upstream piping (°R / °F). |
| Elevation / Atm Pressure | Elevation above mean sea level at the station, shown when the Atmospheric Pressure Method is not “None.” Atm Pressure replaces it and is shown only when the method is set to “None – Entered Value.” |
| Pipe Efficiency | The hydraulic efficiency value for the components of the associated piping section. |
| Pipe Flow Equation | The pipe flow equation used for the associated piping section (e.g., IGT-Improved, Colebrook). |
| Regulator Size/Type | The regulator Size/Type Code for each stage. Click the ? button to choose a device on the Device Selection screen; the controlling and wide-open valve factors are entered automatically. |
| Regulator Set Pressure | The set pressure for each stage’s regulator, assumed sensed immediately downstream of the device. |
| Relief Valve Size/Type | The relief valve Size/Type Code for each stage. Click the ? button to choose a device; the sizing factor (or orifice area) is entered automatically. |
| Relief Valve Set Pressure | The set pressure for each stage’s relief valve, assumed sensed immediately upstream of the valve. |
| Minimum Build-Up | The minimum pressure above the set pressure required to fully open the relief valve. Enter the manufacturer’s value if known; otherwise enter zero. |
| Number Of Valves | The number of installed relief valve and vent stack combinations. When more than one is present, each valve/stack is assumed identical in size, type, and configuration. |
| Stack Outlet Pressure / Set To Atmospheric Pressure | The downstream pressure of the stack piping. Selecting the Set To Atmospheric Pressure checkbox forces the outlet to local atmospheric pressure and disables the field. |
| Outlet Minimum Flow Rate | The lowest downstream (system) flow the station can experience. Used during the failed calculation process; enter zero for conservative results. |
| Outlet Maximum Flow Rate | The highest downstream flow the station is required to supply. Used during the normal calculation process. |
| Regulatory Code | The regulatory code used for compliance checks (ASME B31.8 – 2007, US DOT Part 192 – 2019, or None Selected). |
| MAOP | The maximum allowable operating pressure for each piping section, used as the basis for the compliance comparison. |
Part 2: Outputs/Reports
- If you need to modify an input parameter, click the CALCULATE button after the change.
- Review the calculated flow, pressure, and velocity results on the individual stage and piping data tabs, and the compliance results on the Compliance tab.
- To SAVE the calculation, click the Save command button (calculation files use the .rr2 extension).
- To print results, click the Print command button and choose an output format on the Print Settings screen: Standard Report, Short Report, or Inspection Form.
- To compare results by changing a value without re-entering all data, use the Open Duplicate Calculation Additional Action.
- Optionally add a title or notes for the calculation using the Notes command button.
Results
| Output | Description |
|---|---|
| Flow Rate (per section) | Displays the flow rate through each piping section (supply, intermediate, stack, and outlet) for both stages. Reported as a standard volume adjusted to the base conditions. |
| Inlet Pressure (per section) | Displays the pressure at the inlet (upstream) end of each piping section and device. |
| Outlet Pressure (per section) | Displays the pressure at the outlet (downstream) end of each piping section and device. |
| Maximum Velocity (per section) | Displays the calculated maximum velocity through each piping section, based on flow rate, average temperature, outlet pressure, and smallest inside diameter. |
| Controlling Valve Factor | Displays the rated “controlling” valve factor for each selected regulator, set when the device is chosen. |
| Wide-Open Valve Factor | Displays the rated “wide-open” (failed) valve factor for each selected regulator, set when the device is chosen. |
| Sizing Factor | Displays the rated sizing factor or orifice area for each selected relief valve. When the factor is an area, the dimensional units list is enabled. |
| Operating Status | Displays the operating status of each relief valve (e.g., Popping, Continuously Closed). |
| Maximum Calculated Pressure | Displays the maximum calculated pressure for each associated pipe section. |
| Maximum Allowable Pressure | Displays the calculated allowable pressure for each pipe section, based on the operating mode and selected regulatory code. |
| % MAOP | Displays the ratio of the maximum calculated pressure to the specified MAOP for each pipe section. Shown in red when beyond the allowable limit. |
| Regulator 1 / Regulator 2 (Capacity) | Displays the calculated failed capacity of the first- and second-stage regulators. Only shown when a Failed operating mode is selected. |
| Relief Valve 1 / Relief Valve 2 (Capacity) | Displays the calculated capacity of the first- and second-stage relief valve and stack combinations. Only shown when a Failed operating mode is selected. |
| Max Of Regs (Capacity) | Displays the maximum capacity of the combined regulators. |
| Max Required (Capacity) / Total Station (Capacity) | Displays the required maximum capacity and the calculated maximum capacity of the station. Only shown when the Normal Maximum operating mode is selected. |
Note: Pressure values are gauge pressures and flow values are standard volumes. The number of decimal places shown for any calculated item can be set under File > Preferences > Decimals, so displayed values may differ slightly but should match when rounded to the least common decimal place.
References
- American Society of Mechanical Engineers, Gas Transmission and Distribution Piping Systems, ASME B31.8-2007.
- United States Department of Transportation, Pipeline and Hazardous Materials Safety Administration, Pipeline Safety Regulations, Part 192, 2019.
- Technical Toolboxes, GASCalc 6.1 Calculation Reference — Regulator & Relief Valve System (single-stage).
- Technical Toolboxes, GASCalc 6.1 Calculation Reference — Pipe Flow.
- Technical Toolboxes, GASCalc 6.1 Calculation Reference — Regulator Values.
- Technical Toolboxes, GASCalc 6.1 Calculation Reference — Relief Valve Values.
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.