Introduction
A regulator is a device used to reduce a higher upstream pressure to a controlled, lower downstream set pressure. Because any regulator can fail, an additional “over-pressure protection” (OPP) device is usually installed alongside it to protect downstream piping and equipment from damaging overpressure. The two OPP approaches modeled by this module are the relief valve, which vents gas to atmosphere when pressure rises above its set point, and the monitor regulator, a second regulator placed in series that takes over pressure control without venting any gas.
When a regulator or relief valve is considered as a single, isolated device, calculating its flow is fairly straightforward. Within a complete station, however, performance becomes more complex, because the supply, intermediate, vent/stack and outlet piping all interact with the devices. For this reason, reviewing only a device’s standalone capacity is not a valid method for determining station adequacy; the capacity of the entire station must be evaluated under both normal and failed conditions.
The remaining calculations in this module address the service side and the thermal behavior of a system. Excess Flow Valve and Service Line sizing account for the large equivalent length of fittings installed on short service lines, where items such as taps and EFVs can contribute more pressure drop than the line itself. The Line Heater calculation sizes the heat input needed to offset the Joule-Thomson cooling that occurs across a pressure reduction, keeping the gas above its hydrate-formation temperature.
Module/Application
- Regulator & Relief Valve System
- Use Case: Design and analyze a single-stage regulator station protected by a relief valve
- Limitations:
- Relief valve set pressure typically 5-10 psig above the regulator set pressure
- Standalone device capacity is not a valid measure of station adequacy
- Differentiator: Evaluates the complete station – supply, regulator, intermediate, relief, stack and outlet piping – under normal and failed conditions
- Regulator & Relief Valve System – 2 Stage
- Use Case: Design and analyze a two-stage regulator station, each stage protected by a relief valve
- Special Considerations:
- Shares the single-stage background; differs mainly in available operating modes
- Failed Single mode assumes each regulator fails independently
- First-stage failed capacity sets the second-stage inlet condition
- Differentiator: Models the cascade between the first and second stages
- Regulator & Monitor System
- Use Case: Design and analyze a station that uses a monitor regulator for over-pressure protection
- Special Considerations:
- Monitor set pressure typically 1-5 psig above the controlling regulator
- Supports Downstream Passive Monitor – Upstream Control and Upstream Passive Monitor – Downstream Control configurations
- Differentiator: Monitor takes over pressure control with no gas vented to atmosphere
- Relief Valve & Piping System
- Use Case: Size and evaluate a relief valve together with its inlet and vent/stack piping
- Limitations:
- In-service relief flow can approach sonic velocity
- Choked flow in the stack raises back pressure and build-up pressure on the protected system
- Inlet-piping loss generally limited to ~3%, discharge-piping loss to ~10% of set pressure
- Differentiator: Models sonic/choked flow and checks velocity limits across the whole system
- EFV Sizing / EFV Values
- Use Case: Size and select an excess flow valve for a service line
- Limitations:
- Normal demand must stay below the minimum trip flow
- Severed-line flow must exceed the maximum trip flow to close the valve
- Large equivalent length must be included in service line sizing
- Differentiator: Accounts for EFV equivalent length, trip range and bypass/reset rate
- Service Line Sizing
- Use Case: Size a service line from tap to termination, including all fittings and EFVs
- Limitations:
- Fittings such as taps and EFVs can dominate pressure drop on short lines
- Small diameters can produce high velocities; a velocity limit may apply
- Differentiator: Comprehensive equivalent-length method; can solve for an unknown size, flow or pressure
- Line Heater
- Use Case: Size an indirect inline heater to offset Joule-Thomson cooling at a pressure reduction
- Limitations:
- Approximates the pressure reduction as isenthalpic
- Rated in Btu/hr; confirm detailed design with the heater manufacturer
- Differentiator: Returns the required heat, fuel consumption and outlet gas temperature
- Pressure Test Duration
- Use Case: Estimate the required hold duration for a pipeline pressure test
- Note: Content pending – calculation is in the requirements-gathering stage with no reference file yet
Reference
- ASME B31.8 – Gas Transmission and Distribution Piping Systems (selectable compliance code).
- API Standard 520 – Sizing, Selection, and Installation of Pressure-relieving Devices (basis for the generic relief valve models).
- Pipeline and Hazardous Materials Safety Administration (PHMSA), Pipeline Safety Regulations, 49 CFR Part 192.
- Joule-Thomson (isenthalpic expansion) theory, as applied in the Line Heater and station temperature-change calculations.
- IGT-Improved and related pipe-flow equations used to compute piping pressure drop and equivalent length throughout the module.
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.