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D2759-00 – Standard Test Method Technical Guide
ASTM D2759-00, reaffirmed in 2017, establishes a standardized practice for collecting gas samples from the gas space above the insulating liquid in transformers operating under positive pressure. This practice is essential for obtaining representative samples to analyze gas composition and detect early fault conditions.
📐 Apparatus and Materials
The primary apparatus includes a gas sampling bottle and an attached plastic bag. The bottle must be made of stainless steel, Monel metal, or glass, with a minimum capacity of 250 cm³, and include valves and hose connectors on both ends. The plastic bag should have a single opening and an expanded volume approximately 15 times that of the sampling bottle. All components must be clean and free from contaminants.
| 🟦 Component | 📏 Material | 📐 Specification |
|---|---|---|
| Gas Sampling Bottle | Stainless steel, Monel metal, or glass | Capacity: ≥250 cm³ |
| Plastic Bag | Plastic | Volume: ~15× bottle volume |
⚙️ Sampling Procedure
The procedure ensures a contamination-free sample. First, purge the transformer outlet valve by venting a small amount of gas to remove moisture and oil. Connect one end of the sampling bottle to the valve and the other end to the plastic bag, ensuring leakproof connections. Purge the bottle and hose by opening both bottle valves and slightly opening the transformer valve. Allow gas to flow until the bag fully expands. Close the bottle valve nearest the bag first, then the other bottle valve, and finally the transformer valve. Detach the hose and bag, and label the sample with an attached tag.
Tip: Always ensure the sample is not diluted by air through proper purging and tight connections.
| 🟦 Step | ⚡ Action |
|---|---|
| 1 | Purge transformer outlet valve |
| 2 | Connect bottle and bag |
| 3 | Purge system with gas flow |
| 4 | Close valves in order: bag-side, bottle, transformer |
| 5 | Attach identification tag |
📊 Significance and Key Considerations
Gas sampling enables early detection of developing faults in transformers. Combustible gases like hydrogen and hydrocarbons form from insulating material degradation under thermal and electrical stress. Elevated concentrations prompt necessary action. It is vital to sample directly from the gas space above the liquid, not from remote compartments, to avoid dilution. Adhering to this practice ensures sample integrity.
Important: Avoid sampling from remote expansion compartments as this can introduce air or nitrogen, diluting the sample.
❓ Frequently Asked Questions
🔍 Why must the sample be taken from the gas space directly above the liquid?
To ensure the gas is representative of the conditions in the transformer, without dilution from other areas like expansion compartments.
💡 What are the requirements for the sampling bottle?
The bottle must be of stainless steel, Monel metal, or glass, with a volume of at least 250 cm³, and have valves on both ends.
⚡ How do you clean the sampling bottle properly?
Wash it with anhydrous isopropyl alcohol or a residue-free cleaner, then dry using dry air, nitrogen, oven baking, or under vacuum.
📌 What is the function of the plastic bag during sampling?
It captures the purge gas, allowing the system to be flushed of air without losing gas pressure, ensuring a pure sample.