1. TBS Overview & Components
A town border station (TBS) is the point where gas transfers from a transmission pipeline to a local distribution system. It is one of the most critical infrastructure elements in a gas utility, serving as the gateway that controls pressure, measures volume, adds odorant, and provides overpressure protection for the entire downstream distribution network.
TBS facilities range in size from small above-grade installations serving rural communities (10,000-50,000 SCFH capacity) to large below-grade or building-enclosed stations serving major cities (500,000 to 5,000,000+ SCFH). Regardless of size, all TBS installations must comply with 49 CFR 192 requirements for design, construction, testing, and operation.
Major TBS Components
A complete town border station includes the following systems arranged in order from upstream to downstream:
- Inlet piping and isolation: Connects to the transmission main via a tee or hot tap. Includes a full-bore mainline valve (typically ball or plug) rated for transmission MAOP. The inlet section must be designed to ASME B31.8 for Class 3 or 4 locations (near buildings and populated areas).
- Filtration and separation: A coalescing filter or filter-separator removes pipeline liquids, compressor oil carryover, and particulate matter. Essential for protecting downstream regulators and meters. Typical rating is 10-micron absolute filtration with automatic drain.
- Gas heating: For high-pressure drops, an indirect-fired water bath heater or electric line heater raises gas temperature before regulation to prevent hydrate formation, ice blockage, and regulator freeze-up. Sized based on Joule-Thomson cooling calculations.
- Pressure regulation: Primary and monitor regulators reduce pressure from transmission level to distribution level. Dual-run configurations with active and standby regulators provide redundancy. Each run includes an active regulator and a monitor regulator in series.
- Overpressure protection: Relief valves or slam-shut valves prevent downstream overpressure if regulators fail. Must meet 49 CFR 192.195 and 192.199 requirements. Relief valve vent pipes must discharge to a safe outdoor location.
- Metering: Turbine meters, rotary meters, or ultrasonic meters measure gas volume for custody transfer billing and system balancing. Equipped with flow computers, pressure and temperature compensation, and telemetry to the SCADA system.
- Odorization: Gas entering a distribution system must be odorized per 49 CFR 192.625. Odorant injection equipment adds mercaptan or THT (tetrahydrothiophene) at a rate sufficient for detection at one-fifth the lower explosive limit (LEL). Typical rate is 0.5 to 1.0 lb per million standard cubic feet.
- Outlet piping and isolation: Downstream piping connects to the distribution main. Includes outlet isolation valve, pressure gauges, and test connections.
Station Configurations
Single run
Basic configuration
One regulator set with monitor. Suitable for non-critical, small loads where brief outage during maintenance is acceptable. Common for farm taps and rural communities.
Dual run
Standard for most TBS
Active and standby runs. Allows maintenance without service interruption. Each run has its own active regulator and monitor. Standby run is normally isolated.
Triple run
High-reliability
Two active runs plus one standby. Used for large cities or critical loads where maintenance on one run cannot reduce capacity below peak demand.
Below-grade vault
Noise & aesthetics
Station installed in concrete vault below grade. Reduces noise by 10-15 dBA and visual impact. Requires ventilation per 49 CFR 192.189. Higher construction cost.
Design basis: TBS design is driven by three key parameters: the maximum transmission inlet pressure (MAOP), the required distribution outlet pressure, and the peak hour demand. The station must handle full peak demand at the minimum expected inlet pressure while maintaining outlet pressure within the distribution system tolerance (typically plus or minus 5% of set point). All components upstream of the regulator must be rated for transmission MAOP.
2. Pressure Regulation Design
Pressure regulation at a TBS must reduce gas pressure from transmission levels (typically 200-1000 psig) to distribution levels (typically 30-100 psig for HP distribution, or 0.25-2 psig for LP distribution). This large pressure reduction creates significant challenges including choked flow, noise, vibration, and Joule-Thomson cooling.
Regulator Selection for TBS
TBS applications almost always use pilot-operated regulators due to their superior performance characteristics:
- Tight lockup: Outlet pressure at zero flow within 1-2% of set point, preventing downstream overpressure during low-demand periods.
- Low droop: Outlet pressure variation of only 2-5% from no-flow to maximum flow, maintaining consistent delivery pressure across the entire operating range.
- High capacity: Large Cv values available in compact body sizes, reducing station footprint and cost.
- Wide turndown: Can handle 10:1 or greater flow range while maintaining outlet accuracy.
Monitor Regulator
Monitor Regulator Set Point:
P_monitor = P_active + (5% to 10% of P_active)
Example:
Active regulator set point = 60 psig
Monitor set point = 60 + (0.10 × 60) = 66 psig
During normal operation:
- Active regulator controls at 60 psig
- Monitor is wide open (seeing 60 psig, set for 66 psig)
If active regulator fails OPEN:
- Downstream pressure rises above 60 psig
- When pressure reaches 66 psig, monitor begins to close
- Monitor controls downstream pressure at 66 psig
- Operator is alerted by pressure rise
The monitor provides overpressure protection without venting gas
to atmosphere (unlike a relief valve), which is preferred for
environmental and safety reasons.
Two-Stage Regulation
When the inlet-to-outlet pressure ratio exceeds approximately 3:1, two-stage regulation should be considered. Two-stage regulation splits the total pressure drop across two regulators in series, reducing noise, vibration, and JT cooling at each stage.
Intermediate Pressure Selection:
P_intermediate = sqrt(P1 × P2) (geometric mean)
Example: P1 = 500 psig, P2 = 60 psig
P_int = sqrt(500 × 60) = sqrt(30,000) = 173 psig
Stage 1: 500 → 173 psig (ratio = 2.89:1)
Stage 2: 173 → 60 psig (ratio = 2.88:1)
Benefits:
- Each stage operates below choke conditions
- Noise reduced by 15-25 dBA compared to single-stage
- JT cooling distributed across two stages
- Smaller heater may be sufficient
- Longer trim life (lower velocity at each stage)
Slam-shut valves: Some TBS designs use slam-shut (fast-closing) valves instead of or in addition to relief valves. A slam-shut valve is installed upstream of the regulator and closes rapidly (within 1 second) when downstream pressure exceeds a preset limit. Unlike relief valves, slam-shut valves do not vent gas but instead shut off the gas supply entirely, requiring manual reset. They are common in European practice and gaining acceptance in US operations.
3. Joule-Thomson Cooling & Heating
When gas expands through a regulator, it cools due to the Joule-Thomson effect. This cooling is one of the most important design considerations for TBS installations, as it can lead to hydrate formation, ice blockage of regulator internals, and freeze damage to instrumentation.
Joule-Thomson Coefficient
Joule-Thomson Coefficient for Natural Gas:
μ_JT = (1/Cp) × [T × (dV/dT)_P - V]
Simplified empirical approximation:
μ_JT ≈ 7°F per 100 psi pressure drop
(Valid for natural gas near ambient temperature and moderate pressures)
Temperature drop through regulator:
ΔT = μ_JT × ΔP / 100
Example: P1 = 800 psig, P2 = 60 psig
ΔP = 740 psi
ΔT = 7 × 740 / 100 = 51.8°F
If ambient (inlet) temperature = 40°F:
Outlet temperature = 40 - 51.8 = -11.8°F
This is well below freezing and hydrate formation temperature!
Gas heater is REQUIRED.
Note: The JT coefficient varies with pressure, temperature, and gas
composition. At higher pressures (>500 psig), μ_JT can be 5-8°F/100 psi.
At low pressures (<100 psig), it decreases to 4-6°F/100 psi.
Hydrate Formation
Natural gas hydrates are ice-like crystalline structures that form when water combines with methane, ethane, or propane under specific pressure and temperature conditions. In TBS applications, hydrate formation is the primary concern because hydrate crystals can block regulator trim, seats, and sensing lines, causing complete loss of pressure control.
- Formation conditions: Hydrates form at temperatures above 32 degrees F when gas is at pressure. For typical pipeline gas at 500 psig, hydrates can form at temperatures up to 60-65 degrees F if free water is present.
- Prevention: Maintain gas temperature above the hydrate formation temperature (typically above 40-50 degrees F) by heating gas before pressure reduction.
- Dehydration alternative: If the transmission gas is dehydrated to a water dew point below the minimum expected outlet temperature, heating may not be required.
Heater Types for TBS
| Heater Type | Capacity Range | Advantages | Disadvantages |
|---|
| Indirect water bath | 100,000-10,000,000 BTU/hr | Simple, reliable, constant outlet temp | Large footprint, requires glycol in cold climates |
| Electric line heater | 10,000-500,000 BTU/hr | Compact, no combustion, easy to control | High operating cost at large flows, limited capacity |
| Direct-fired (catalytic) | 50,000-2,000,000 BTU/hr | Efficient, moderate size | More complex, requires combustion air management |
| Heat exchanger (ambient) | Varies | No fuel cost, uses ambient heat | Only works in warm climates, limited capacity |
Heater Sizing
Heater Duty Calculation:
Q_heater = m_dot × Cp × ΔT_required
Where:
m_dot = Mass flow rate (lb/hr)
= Q_scfh × P_std × MW / (R × T_std)
Cp = Gas heat capacity at constant pressure ≈ 0.50 BTU/(lb·°F)
ΔT_required = Target outlet temp - Actual outlet temp (without heater)
Target outlet temperature: typically 40-50°F (above hydrate formation)
Example:
Q = 200,000 SCFH, SG = 0.60, MW = 17.37 lb/lbmol
m_dot = 200,000 × 14.696 × 17.37 / (10.73 × 519.67)
m_dot = 51,072,000 / 5,576 = 9,160 lb/hr
P1 = 500 psig, P2 = 60 psig, T_inlet = 40°F
JT drop = (440) × 7/100 = 30.8°F
Actual outlet = 40 - 30.8 = 9.2°F
ΔT_required = 50 - 9.2 = 40.8°F
Q_heater = 9,160 × 0.50 × 40.8 = 186,864 BTU/hr
Select: 200,000 BTU/hr indirect water bath heater (with 10% margin)
Design for minimum conditions: Heater sizing must be based on the coldest expected inlet temperature (winter design day), maximum pressure drop, and maximum flow rate. Many stations are designed for a worst-case ambient temperature of 0 degrees F or lower, which can require heater capacities 2-3 times the typical operating duty. Include a 10-20% safety factor on the calculated heater duty.
4. Metering & Odorization
Gas metering at the TBS serves two primary purposes: custody transfer measurement (for billing between the transmission company and the distribution utility) and system balancing (tracking gas volumes entering the distribution system for leak detection and lost-and-unaccounted-for analysis). The choice of meter type depends on accuracy requirements, flow range, and maintenance capabilities.
Meter Selection for TBS
| Meter Type | Accuracy | Turndown | Pressure Rating | Best Application |
|---|
| Turbine | ±0.5-1.0% | 10:1 | Up to ANSI 600 | Medium to large TBS, custody transfer |
| Rotary (PD) | ±0.5-1.0% | 80:1 | Up to ANSI 300 | Small to medium TBS, wide flow range |
| Ultrasonic | ±0.5% | 20:1 | Up to ANSI 600 | Large TBS, no moving parts |
| Orifice | ±1-2% | 3:1 | Any | Legacy installations, backup measurement |
Meter Installation
Meters are typically installed downstream of the regulator where the gas is at distribution pressure. This approach uses smaller, less expensive meters rated for the lower downstream pressure and simplifies maintenance. The flow computer applies pressure and temperature corrections to convert measured actual volume to standard volume.
Odorization Requirements
49 CFR 192.625 - Odorization:
Gas in distribution systems must be odorized so that at a
concentration in air of one-fifth (1/5) of the lower explosive limit,
the gas is readily detectable by a person with a normal sense of smell.
Typical odorant: Tertiary butyl mercaptan (TBM) or THT
Injection rate: 0.5 to 1.0 lb per MMSCF (varies by operator)
Daily odorant consumption:
Odorant_lb/day = Q_MMSCFD × injection_rate
Example: 2.4 MMSCFD station at 0.75 lb/MMSCF
Daily consumption = 2.4 × 0.75 = 1.8 lb/day
Tank sizing: 30-day supply = 54 lb ≈ 8 gallons of TBM
Odorant must be injected UPSTREAM of the first customer connection.
Most TBS include odorant injection downstream of the meter but
upstream of the station outlet isolation valve.
SCADA and telemetry: Modern TBS installations are connected to the gas utility SCADA (Supervisory Control and Data Acquisition) system. Typical monitored parameters include inlet pressure, outlet pressure, flow rate, gas temperature, heater status, odorant tank level, and valve positions. Alarms are set for high and low outlet pressure, heater failure, loss of odorant, and filter differential pressure. Remote monitoring allows 24/7 oversight of station operation without requiring on-site personnel.
5. Design Examples
Example 1: Small-Town TBS
Given:
Transmission pressure: P1 = 400 psig
Distribution pressure: P2 = 60 psig
Peak hour demand: Q = 50,000 SCFH (serves ~1,000 residential customers)
Gas gravity: SG = 0.62
Minimum winter temperature: T_min = 10°F
Configuration: Dual run (active + standby)
Step 1: Regulator sizing (per run)
P1_psia = 414.7, P2_psia = 74.7
x = (414.7 - 74.7) / 414.7 = 0.820
xT = 0.75, so flow is CHOKED
Y = 2/3
Cv = 50,000 / (1360 × 414.7 × 0.667 × sqrt(0.75/(0.62 × 469.67 × 1.0)))
Cv = 50,000 / (1360 × 414.7 × 0.667 × 0.0455)
Cv = 50,000 / 17,097 = 2.92
Select 2" pilot-operated (Cv = 66) with reduced trim
Step 2: JT cooling (worst case at 10°F)
ΔT = (400 - 60) × 7/100 = 23.8°F
Outlet temp = 10 - 23.8 = -13.8°F (VERY COLD)
Step 3: Heater sizing
MW = 0.62 × 28.97 = 17.96
m_dot = 50,000 × 14.696 × 17.96 / (10.73 × 519.67) = 2,367 lb/hr
ΔT_required = 50 - (-13.8) = 63.8°F
Q_heater = 2,367 × 0.50 × 63.8 = 75,500 BTU/hr
Select: 100,000 BTU/hr indirect water bath heater
Step 4: Meter sizing
At 60 psig, 50,000 SCFH » 10,000 ACFH
Select 3" rotary meter (capacity 75,000 SCFH at 60 psig)
Step 5: Relief valve
Full open Cv = 66, at P1 = 414.7 psia
Relief capacity = 1360 × 66 × 414.7 × 0.667 × 0.0455 = 1.13 MMSCFH
Select 2" spring-loaded relief valve
Step 6: Odorization
Daily volume = 50,000 × 24 / 1,000,000 = 1.2 MMSCFD
Odorant rate = 1.2 × 0.75 = 0.9 lb/day
Install 50-gallon TBM tank (60-day supply)
Example 2: Large City TBS
Given:
Transmission pressure: P1 = 800 psig
Distribution pressure: P2 = 100 psig
Peak demand: Q = 500,000 SCFH (major distribution feed)
SG = 0.58, T_min = 0°F
Configuration: Triple run (2 active + 1 standby)
Step 1: Regulator sizing (per active run, 2 active)
Q_per_run = 500,000 / 2 = 250,000 SCFH
P1_psia = 814.7, P2_psia = 114.7
x = (814.7 - 114.7) / 814.7 = 0.859 (CHOKED)
Cv = 250,000 / (1360 × 814.7 × 0.667 × sqrt(0.75/(0.58×459.67×1.0)))
Cv = 250,000 / (1360 × 814.7 × 0.667 × 0.0530)
Cv = 250,000 / 39,195 = 6.38
Select 3" pilot-operated (Cv = 150) — significant margin for growth
Consider two-stage regulation due to high pressure ratio (7:1)
Step 2: Two-stage intermediate pressure
P_int = sqrt(800 × 100) = 283 psig
Stage 1: 800 → 283 psig
Stage 2: 283 → 100 psig
Step 3: JT cooling (worst case)
Stage 1: ΔT1 = (800 - 283) × 7/100 = 36.2°F
Stage 2: ΔT2 = (283 - 100) × 7/100 = 12.8°F
Total: 49°F
Outlet temp = 0 - 49 = -49°F without heating!
Step 4: Heater sizing
MW = 16.80, m_dot = 500,000 × 14.696 × 16.80 / (10.73 × 519.67) = 22,126 lb/hr
ΔT_req = 50 - (-49) = 99°F
Q_heater = 22,126 × 0.50 × 99 = 1,095,237 BTU/hr
Select: 1,200,000 BTU/hr water bath heater
Step 5: Meter
Select 6" turbine meter (capacity 500,000 SCFH at 100 psig)
This large station requires careful noise analysis, community
impact assessment, and likely below-grade vault construction.
Commissioning and testing: Before placing a TBS in service, 49 CFR 192 requires pressure testing of all piping at 1.5 times MAOP, testing of all regulators for set pressure and lockup, testing of relief valves for pop pressure and reseat, verification of odorant injection rate, and SCADA communication checkout. A new station typically requires 2-4 weeks of commissioning before gas flows to customers.