SH/T 0078-1991 Determination of Trace Water in Liquefied Petroleum Gas (Electrolytic Method)

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Card type: Test method

1. Method number / full name / English name / replacement relationship

• Standard number: SH/T 0078-91
• Full name: Determination of Trace Water in Liquefied Petroleum Gas (Electrolytic Method)
• English name: not provided in the available source
• Replacement relationship: not provided in the available source
• Other: 2001 confirmed in the year; approved by China Petrochemical Corporation 1991-11-26, implemented on 1992-12-01

2. Scope of application and non-applicability

• Applicable to liquefied petroleum gas.
• Determination range is 1～1000 ppm(V/V).
• Not applicable: not provided in the available source.

3. Method principle

Liquefied petroleum gas is vaporized at constant temperature and passed through the electrolytic cell of the water determinator at a fixed flow rate. The water therein is absorbed by the phosphorus pentoxide film in the electrolytic cell. The water content of the gas is determined by the equilibrium current value after electrolysis of the water absorbed by phosphorus pentoxide. The water content [ppm(V/V)] can be read directly from the determinator.

4. Reagents and materials (purity / preparation)

• 4.2.1 Polytetrafluoroethylene filter membrane: thickness 0.2～1 mm, pore size less than 10 nm (requires confirmation against the source standard (source-text extraction), original text is “l0nmo”).
• 4.2.2 Stainless steel tube: outer diameter 3 mm, wall thickness 0.5 mm, with necessary fittings.
• 4.2.3 Molecular sieve: 5A.
• 4.2.4 Drying tube: stainless steel tube, filled with 5A molecular sieve.
• 4.2.5 Nitrogen: ordinary nitrogen.
• 5.1 Anhydrous ethanol: analytical reagent.
• 5.2 Phosphoric acid: analytical reagent, prepared as 10%(m/m) phosphoric acid solution.
• 5.3 Acetone: analytical reagent.
• 5.4 Hydrochloric acid: analytical reagent.

5. Instruments and equipment

• 4.1.1 Water determinator: any type of electrolytic water determinator meeting the following conditions:
  • a. The flow rate of gas entering and leaving the instrument is adjustable;
  • b. A suitable range can be selected according to different water contents;
  • c. The instrument time constant is: to reach 63% of the change in water content of the gas sample, both rise and fall are not greater than 5 min;
  • d. Measurement accuracy: better than ±5% (based on full-scale value).
• 4.1.2 Sample vaporizer: heating by a constant-temperature water bath or by winding the vaporization tube with temperature-controlled heating wire may be used.
• 4.1.3 Soap film flowmeter.
• 4.1.4 Sampling steel cylinder: meets the requirements for samplers in SH/T 0233. The sampler volume is not less than 250 mL.
• 4.1.5 Instrument flow diagram: see “Instrument flow diagram” in the original text; the diagram is labeled: 1— sample steel cylinder; 2— nitrogen steel cylinder; 3— drying tube; 4— bypass flowmeter; 5, 7— needle valves; 6— main path flowmeter; 8— electrolytic cell; 9— total flowmeter; 10— flow control valve; 11— four-way valve; 12— vaporizer (at numbers 9, 10 there is source-text extraction doubtful point, requires confirmation against the source standard (source-text extraction)).

6. Recommended operating conditions

• System leak test: main path flow controlled at 50 mL/min, bypass flow controlled at 1 L/min.
• Flow calibration: main path flow is 100 mL/min.
• Background value determination: main path flow 100 mL/min, bypass flow 1 L/min; first set the range to the 1000 ppm position, and switch range positions according to the decrease in water content.
• Sample determination: main path flow 100 mL/min, bypass flow 1 L/min.
• Electrolytic cell inspection (A1.2): sample flow from 100 mL/min down to 50 mL/min.
• Electrolytic cell drying (A2.1.2): oven 80 ℃, 25 min.
• Electrolytic cell vacuum drying (A2.2.3): evacuate for 2 h.

7. Sampling and sample preparation

• 6.2.1 Before filling, the sampling steel cylinder should be evacuated and dried. Keep the interior clean and dry.
• 6.2.2 During sampling, first open the valves at both ends of the sampler, purge the sampling steel cylinder with the sample, then close the discharge valve. When the sample fills about three quarters of the volume of the sampling steel cylinder, close the sampling valve.
• Reference SH/T 0233 Liquefied petroleum gas sampling method.

8. Analytical procedure

• 7.1 System leak test: purge the entire system with dry nitrogen, control the main path flow at 50 mL/min, control the bypass flow at 1 L/min, block the outlet and perform the leak test. When the main path flowmeter and bypass flowmeter indicate zero, it is considered leak-free.
• 7.2 Flow calibration: use a soap film flowmeter to calibrate the height of the rotameter float when the main path flow is 100 mL/min.
• 7.3 Determination of background value:
  • 7.3.1 Turn the four-way valve to the nitrogen position, adjust the main path flow to 100 mL/min, and the bypass flow to 1 L/min.
  • 7.3.2 Turn on the power switch, first set the range to the 1000 ppm position, then switch range positions according to the decrease in water content.
  • 7.3.3 When the instrument indication is basically unchanged within 5 min, the background value can be read. The background value should be less than 5 ppm(V/V); otherwise, the nitrogen should be replaced or the molecular sieve desiccant should be reactivated.
• 7.4 Determination of water content in the sample:
  • 7.4.1 Place the four-way valve in the sample position, adjust the main path flow to 100 mL/min, and the bypass flow to 1 L/min. When the instrument indication is basically unchanged within 5 min, read the measured value.
  • 7.4.2 After the sample determination result, switch the four-way valve to the nitrogen position, purge the electrolytic cell for 5 min, then turn off the power and gas source.
• Appendix A Cleaning and coating of the electrolytic cell (supplementary item):
  • A1 Inspection of the electrolytic cell
    • A1.1 Inspection with a multimeter: connect the test leads of the multimeter (×100 position) to the two electrode terminals of the electrolytic cell respectively. If the multimeter pointer clearly changes gradually from low resistance to high resistance, this is called a charging phenomenon, proving that the electrolytic cell is normal; otherwise it is called no charging phenomenon, and it needs to be cleaned and coated or replaced. An electrolytic cell with a charging phenomenon should also be cleaned if it is seriously contaminated (brown spots can be seen).
    • A1.2 Inspection during use: after the electrolytic cell is contaminated, measurement errors will be caused. The inspection method is to reduce the sample flow from 100 mL/min to 50 mL/min; at this time the water content reading should also drop to half of the original. If the maximum relative deviation does not exceed 10%, it proves that the electrolytic cell is normal. Otherwise, it indicates that it has been contaminated and should be recoated.
  • A2 Cleaning and coating of the electrolytic cell
    • A2.1 Cleaning
      • A2.1.1 Remove the electrolytic cell from the instrument, connect one end of the electrolytic cell with a rubber tube. Insert the other end of the rubber tube into a beaker containing distilled water, rinse the electrolytic cell by siphon action for 3～4 h, then depending on the contamination, rinse the electrolytic cell with acetone, anhydrous ethanol or other suitable acid solution to remove contaminants.
      • A2.1.2 Place the cleaned electrolytic cell in an oven (80 ℃, 25 min) to dry, or blow dry with a small dry gas flow. Immediately after drying, measure the insulation; if the resistance is not lower than 10 MΩ, it is acceptable. Otherwise, it should be cleaned again or replaced.
    • A2.2 Coating
      • A2.2.1 Use a syringe to inject 10%(m/m) phosphoric acid solution from one end of the electrolytic cell; the injection amount should be sufficient to fill the empty glass part at this end and slightly excess. Rotate the electrolytic cell body slowly in one direction, allowing the acid liquid to flow along the wall of the electrolytic cell until it reaches the end of the platinum wire at the other end, then rotate in the opposite direction very slowly to discharge the excess acid liquid from the injection end, and absorb it clean with a filter paper strip.
      • A2.2.2 Clean the acid liquid in the glass tube at the parts of both ends of the electrolytic cell where there is no platinum wire: fill a small beaker with distilled water, invert the electrolytic cell, slowly insert the end to be cleaned into the water, let the water surface rise gradually along the inner wall of the glass tube, and when it is about to approach the platinum wire, quickly lift the electrolytic cell body. Repeat this three times, and finally use filter paper to absorb the water droplets adhering to the inner wall of the glass tube. If the inner wall of the glass tube at the part of the other end where there is no platinum wire is also contaminated with acid liquid, treat it in the same way.
      • A2.2.3 Drying of the electrolytic cell: connect one end of the electrolytic cell to a vacuum pump through a dryer, and seal the other end. Start the vacuum pump and evacuate for 2 h. When stopping the evacuation, immediately remove the electrolytic cell and inspect the performance of the electrolytic cell with a multimeter according to A1.1.

9. Qualitative and quantitative determination and calculation formula

The water content A [ppm(V/V)] of the sample is calculated according to the following formula:

A = A₂ − A₁

Where:

• A₁—instrument background value, ppm(V/V);
• A₂—sample measured value, ppm(V/V).

10. Precision

• Measurement accuracy (instrument requirement): better than ±5% (based on full-scale value).
• Criterion for checking electrolytic cell contamination: maximum relative deviation not exceeding 10%.
• Other precision clauses: not provided in the available source.

11. Report

Not provided in the available source.

12. Referenced documents

• SH/T 0233 Liquefied petroleum gas sampling method

13. Source and verification

This page is an AI translation of the available Chinese method card. Human verification is pending. It is not an official translation of the standard. Check the source standard and product certificate before use.

14. Quick reference Q&A