At Credence Gas Services, we know firsthand that once you’ve done the hard work of conditioning raw natural gas and dropping out rich liquids, your job is only half done. Storing Natural Gas Liquids (NGLs)—like propane, butane, and natural gasoline—comes with a distinct set of operational and safety requirements.
Because NGLs are stored under pressure as liquefied compressed gases, proper vessel sizing and strict adherence to safety standards aren't just regulatory boxes to check—they are critical to keeping your lease safe, compliant, and profitable.
Here is a practical guide to sizing NGL storage vessels (commonly called NGL bullet tanks) and the key industry safety standards every midstream operator should know.
1. Sizing NGL Storage Tanks: Getting the Capacity Right
Sizing an NGL storage tank isn't just about matching your daily liquid recovery rate. It requires balancing production volume, haul-off schedules, pressure requirements, and regulatory expansion limits.
Production Volume & Off-Take Frequency
The primary driver for storage capacity is your residence time—how many days of production you need to store before truck or pipeline haul-off occurs.
Standard Field Sizing: Most midstream pads size storage for 3 to 5 days of peak liquid production to handle weather delays, transport availability, or weekend road restrictions without shutting in production.
The 80–85% Liquid Fill Rule (Ullage Space)
You can never fill an NGL pressure vessel to 100% capacity. Because liquefied gases expand significantly with ambient temperature changes, regulations (such as NFPA 58) require vapor space—called ullage—above the liquid level.
Maximum Safe Fill Level: Typically 80% to 85% of the vessel's total water capacity (WC).
Takeaway: If your pad produces 10,000 gallons of liquid over your desired hold period, you will need a vessel rated for at least 12,000 to 12,500 gallons of total water capacity.
Pressure Rating Selection
NGL composition determines the vapor pressure inside the vessel. Propane-heavy mixtures require higher design pressures than heavier condensate streams.
250 PSIG Design Pressure: The industry standard for Y-grade NGLs and high-propane mixes, built to ASME Section VIII specifications.
100–150 PSIG Design Pressure: Often used for heavier, low-vapor-pressure condensate liquids.
2. Key Safety Standards for NGL Storage
When handling high-pressure, flammable liquids, safety standards govern everything from vessel fabrication to site layout and emergency shutdown systems.
ASME Code Construction (Section VIII, Division 1)
All midstream NGL pressure vessels must be designed, fabricated, inspected, and stamped in accordance with ASME Section VIII, Division 1. This ensures:
High-strength steel plate construction capable of handling cyclic pressure changes.
100% X-ray / radiographic inspection of critical weld seams.
Hydrostatic pressure testing at 1.3 to 1.5 times the Maximum Allowable Working Pressure (MAWP).
NFPA 58 & API 2510 (Site Layout and Setbacks)
Where you place the tank on your lease matters just as much as how the tank is built. NFPA 58 (Liquefied Petroleum Gas Code) and API Standard 2510 (Design and Construction of LPG Installations) establish minimum safety distances:
Distance from Equipment: Storage vessels must maintain specific clearance distances from fired heaters, gas compressors, ignition sources, and site property lines.
Distance Between Vessels: Multiple bullet tanks installed in battery must maintain spacing to allow adequate ventilation and emergency access.
Pressure Relief Valves (PRVs)
Overpressure protection is the single most important safety feature on an NGL tank.
Dual PRV Assemblies: Tanks are typically equipped with dual safety relief valves mounted on a three-way manifold. This allows one PRV to be tested or serviced without taking the vessel out of service.
Sizing for Fire Exposure: PRVs must be sized to vent vapor fast enough to prevent vessel rupture during a pool fire event, in compliance with API 520 / 521.
Emergency Shutdown (ESD) and Secondary Containment
Internal Valves / Emergency Shutoff: Tank liquid outlet lines feature pneumatically or hydraulically actuated internal valves equipped with thermal fuses. In the event of a line break or fire, the valve closes automatically to isolate the tank's contents.
Earthen Berms & Grading: Site grading must direct potential liquid spills or surface runoff away from the tank foundation, fired equipment, and site roadways.
The Credence Gas Difference: Sized Right and Built for the Field
At Credence Gas Services, we know that no two well pads or gas streams are identical. As a family-owned Texas company, we don't push one-size-fits-all answers. We sit down with you, look at your gas analysis, evaluate your logistics, and deliver storage and gas conditioning equipment tailored to your exact operational footprint.
When you partner with Credence Gas:
Custom Engineering & Sizing: We help you calculate true production hold times and liquid expansion margins to size your vessel correctly from day one.
Code Compliance Built-In: Every vessel we deploy meets strict ASME, NFPA, and API code requirements for peace of mind out in the field.
Field Support: From pad placement advice to commissioning, our specialists bring hands-on experience to every site.
Need help sizing NGL storage or gas conditioning equipment for your next project?
Give us a call or drop us a note—we're always ready to talk shop and work through your project requirements.