Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
D3700-21 – Standard Test Method Technical Guide
ASTM D3700-21 establishes the standard practice for obtaining representative samples of liquefied petroleum gas (LPG) and other natural gas liquids (NGLs) using a Floating Piston Cylinder (FPC). This method is critical for maintaining single-phase conditions, preserving the sample’s compositional integrity from the source to the laboratory.
📋 Scope and Applicability
This practice specifies equipment and procedures for LPG meeting ASTM D1835 or GPA 2140 standards. It is also generally suitable for other NGL products such as NGL mix and field butane, as well as volatile (higher vapor pressure) crude oils. The standard explicitly cautions against its use for multi-phase products containing significant undissolved gases (N₂, CO₂) or free water.
🧪 Equipment and Terminology
The core device is the Floating Piston Cylinder (FPC), defined as a high-pressure container with a free-floating internal piston that separates the cylinder into two compartments. This design effectively prevents the formation of a gaseous phase, which could alter the liquid sample’s composition.
| 🟦 Equipment Component | 📐 Definition / Requirement |
|---|---|
| Sample Container | High-pressure vessel designed for FPC operation |
| Internal Piston | Free-floating seal isolating sample from pressurizing medium |
| Maximum Fill Volume | Safe liquid capacity expressed as a percentage of total cylinder volume |
| Target Products | LPG (D1835, GPA 2140), NGL mix, field butane, volatile crude |
⚙️ Procedure and Safety Considerations
The standard places the responsibility for representative sample point location on the user. Operational precautions are critical, particularly regarding piston seal tightness for high vapor pressure fluids.
⚠️ High Vapor Pressure Crude Oil: Some FPCs have very tight piston seals that may require a handle to move the piston. For UN Class 3 liquids (vapor pressure under 300 kPa at 52 °C), Practice D8009 (Manual Piston Cylinder) is a recommended alternative.
💡 Maintaining Single-Phase Integrity: The key advantage of the FPC is the elimination of the vapor phase. The floating piston compensates for thermal expansion, ensuring the sample remains entirely liquid and representative throughout transport and storage.
| ⚡ Parameter | 🎯 Guideline / Note |
|---|---|
| Multi-Phase Products | Not covered; requires additional precautions (See Appendix X1) |
| Low Vapor Pressure Fluids | May require MPC sampling per D8009 if seals are too tight |
| Undissolved Gases | Free N₂ or CO₂ can cause sample separation; excluded from scope |
❓ Frequently Asked Questions
🔍 What defines a Floating Piston Cylinder (FPC)?
An FPC is a specialized high-pressure container featuring a free-floating internal piston. This design maintains a barrier between the sample and a driving fluid, preventing the formation of a vapor space and preserving liquid-phase composition.
💡 What specific fluids are covered by ASTM D3700?
The practice is intended for specification LPG (D1835, GPA 2140), NGL mix, field butane, and volatile crude oils. It is explicitly not designed for raw multi-phase streams or those with significant free water or undissolved gases.
📌 What is the Maximum Fill Volume requirement?
The Maximum Fill Volume defines the safe capacity for a liquid sample, expressed as a percentage of total cylinder capacity. This is a critical safety and representativity parameter to prevent over-pressurization and ensure proper piston movement.
⚡ When should D8009 be used instead of D3700?
Tight piston seals in some FPCs can impede sampling of low-vapor-pressure liquids. Practice D8009, using a Manual Piston Cylinder (MPC), is recommended for UN Class 3 liquids under 300 kPa at 52 °C, or whenever a handle is required to move a tight FPC piston.