Regulator & Monitor System

Introduction

This application calculates the flow, pressure, and capacity values for a monitor-style regulator station — two regulators working in series, where one regulator controls the station outlet pressure and the second acts as a monitor that takes over if the controlling regulator fails. The calculation routine sizes and evaluates the upstream regulator, the downstream regulator, and the supply, intermediate, downstream, and outlet piping as a single connected system, under both normal and failed operating conditions, and can perform overpressure compliance checks against a selected regulatory code.

There are no formal industry standards for regulator-station design beyond providing adequate capacity and overpressure protection; the routine applies common “rules of thumb” rather than a prescriptive design code. The temperature change across the regulators is estimated using the Joule-Thomson method, which is only valid for high-methane-content gases, and no temperature change is calculated for the pipe sections. When using the US DOT 192 code, the 75% SMYS limit of part 192.201(a)(2)(i) is not supported — confirm borderline cases with the Hoop Stress calculator.

Regulator & Monitor Stations

A regulator reduces a higher upstream pressure to a lower downstream set pressure. To protect downstream piping and equipment if the regulator fails to maintain a safe outlet pressure, an over-pressure protection (OPP) device is commonly installed alongside it. A monitor regulator is one of the more widely used \OPP device.

A regulator and monitor station consists of two regulators in series: a controlling regulator that normally maintains the station outlet pressure, and a monitor regulator that watches the controlling regulator’s performance. The monitor’s set pressure is usually set slightly higher — perhaps 1 to 5 psig — than the controlling regulator’s set pressure. If the controlling regulator fails to hold the sensed pressure below the monitor set pressure, the monitor takes over pressure control.

Because the performance of each device depends on the rest of the station, the routine evaluates the whole assembly. The upstream piping and regulator must have enough capacity for the downstream regulator to perform, and the downstream and outlet piping must avoid excessive pressure drop before connecting to the supplied system. In-service flow can approach sonic velocity; when a pipe section reaches sonic flow it is “choked” and its flow cannot increase without raising upstream pressure, which restricts the capacity of the entire station.

Station Configurations

Three monitor-station configurations are supported. In a passive monitor the monitor regulator’s valve sits in a static wide-open position and only senses downstream pressure; a working monitor additionally performs a first-cut pressure reduction during normal operation.

Operating Modes

The Operating Mode determines what the calculation solves for.

Design Considerations

Several practical guidelines inform station design:
• Velocity / noise — piping velocity is often kept below 100 ft/sec to limit noise, though some recommendations allow up to 300 ft/sec. With pressures fixed, increasing pipe size is usually the only way to reduce velocity. Where noise is not a concern, velocity may be allowed to reach choked conditions.
• Regulator sizing — the required capacity of a regulator is usually kept between roughly 20% and 80% of its rated capacity.
• Minimum differential — each device must achieve its minimum differential pressure to fully open; otherwise its capacity is reduced. Required differential varies by device type and configuration.
• Outlet velocity — some manufacturers limit the regulator outlet velocity to ensure proper device performance.

Calculation Method

The regulator and monitor system calculation is a method rather than a single equation. It combines the pipe and fitting components of each piping section into single equivalent segments, assumes the pressures at the supply and sense points are constant, calculates flow through the pipe sections using the same methods as the Pipe Flow routine, and calculates flow through the regulators using the same methods as the Regulator Values routine.

Using an iterative process, the flow rate through the station is computed and the pressure difference across the entire system — upstream piping, upstream regulator, intermediate piping, both regulators, and the downstream piping — is computed and compared to the known values. When the flow rate produces the appropriate overall system pressure drop, the solution is complete.

Pipe Strength Rating

In the Design data calculations, the required minimum pipe strength rating is determined as follows.

S_{REQD} = \frac{P_{MAX} \times OD}{2\,T_{WALL} \times DF}

S_{REQD} = \frac{P_{MAX} \times OD}{2\,T_{WALL} \times DF}

Where:
S_REQD − Required strength for the pipe segment (psi)
P_MAX − Maximum required pressure of the associated segment group (psig)
OD − Outside diameter of the pipe segment (in)
T_WALL − Wall thickness for the pipe segment (in)
DF − Overall design factor, dimensionless

Flange-Body Pressure Rating

In the Design data calculations, the required minimum pressure rating for a flange or device body is determined as follows.

P_{REQD} = \frac{P_{MAX}}{DF}

P_{REQD} = \frac{P_{MAX}}{DF}

Where:
P_REQD − Required pressure rating for the flange or device body (psig)
P_MAX − Maximum required pressure of the associated segment group (psig)
DF − Specified design factor, dimensionless

Compliance Checks

The selected Regulatory Code determines how the allowable pressure values on the Compliance tab are calculated; values that exceed the allowable limits are shown in red. Comparisons are made segment-by-segment using the maximum required pressure for each segment group (Supply-Inlet, Intermediate, Downstream-Outlet, and Instrument Piping). The supported codes are listed below.

Case Guide

Part 1: Create Case

Input Parameters

Part 2: Outputs/Reports

Results

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.
• GASCalc 6.1 – Regulator Values Calculation Reference.
• GASCalc 6.1 – Pipe Flow Calculation Reference.
• GASCalc 6.1 – Station MatchMaker Specifications Help Reference.

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