Dave Curtis (Liquid Sampling)

Measurement & Product Validation


Cash Register


Liquid compositions are becoming more and more crucial because of the wide range of hydrocarbon liquid products we produce, sell, and buy.

Condensates, Natural Gas Liquids, High Vapor Pressure Oil’s, “Dead” Oil’s.

Accurate liquid compositional measurement is the key to success because it affects all aspect of the industry.

Production, Operations, Plant Optimization, Environmental Reporting, Production Allocation, Fiscal Allocation, and Sales.

Rocky Mountain Measurement Society Liquid Sampling


Accurate liquid compositions start with proper sampling.

The sampling system has to be matched to the product you are collecting. Once size does not fit all.

Liquid Sampling


Representative Sampling The delivery of a representative liquid sample to the analyzer is critical to producing accurate and reproducible results.

A representative sample is a single phase sample, collected from the proper location, at the pressure and temperature of the collection point, which has remained unchanged from the sample point to the analytical system. There is no technology available to collect a representative sample of a multi-phase

product. If compositions are altered due to an inadequate sample collection system, the accuracy of the analyzer is inconsequential, the results will still be biased.


Sample Collection - Sample Probes A sample probe is mandatory to ensure the sample is extracted from the most representative portion of the sample stream.

Located in the center third of the pipe. Just off center (not specified in guidance document.

Static Mixer

• GPA 2174 • API MPMS 8.1 • API MPMS 8.2


What Do You Need to Know When Designing the Sampling System

Product type

Analysis required

Expected composition Static Dynamic

Amount and types of contaminants Oxygenates, etc…

Source pressure and temperature

Ambient temperature conditions

Required analyzer flow rate (on-line GC)


Sample Collection There are three primary types of sample collection.

Spot Composite On-line gas chromatograph


Spot Samples Spot samples represent a moment in time.

Typically collected monthly, quarterly, semi-annually, or even annually. Any changes in compositions due to factors such as daily and seasonal changes in

ambient temperatures, product temperatures, and product pressures are not captured and therefore the spot may not be representative of the time period of interest.

Spot values are applied to the entire time period of interest. Typically used for lower volume locations or products with static compositions.

Spot Collected 1/18/2015 Methane – 94.08 Mole % Hexanes Plus – 0.0599 Mole %


Composite Samples Composite samples represent an average for the time period of interest.

Typically collected weekly or monthly. Average concentrations for the specified time period are typically used for accounting

purposes. It does not provide the level of detail needed for plant and process optimization.

Methane Average = 90.49 Mole %

Hexanes Plus Average = 0.191 Mole %


On-Line Gas Chromatograph On-line gas chromatographs provide a data point every six to ten minutes.

Average concentrations for the specified time period are typically used for accounting purposes.


Condensates and NGL’s are the lighter hydrocarbon liquids. It is a condensate until it has been processed, then it becomes an NGL. Marketing may tell you something different.

Condensate and NGL Compositions

Compound ConcentrationName (Mole%)Nitrogen 0.00Methane 1.07Carbon Dioxide 0.07Ethane 43.25Propane 26.91i-Butane 5.81n-Butane 7.97i-Pentane 3.31n-Pentane 2.72Hexanes Plus 8.88




Light Hydrocarbon Liquid Spot Sample Collection Samples MUST be maintained under back pressure to ensure the sample remains in the liquid phase.

Floating piston cylinders (typically 1000 to 1200 psi back pressure)



Light Hydrocarbon Liquid Composite Sample Collection Maintaining backpressure is critical to ensuring the sample remains in the liquid phase and does not get distorted.

Pressurized sampler. Collected proportional to flow.

Typical method for NGL and Condensate analysis. GPA 2177. May not be applicable for heavy condensates.



Light Hydrocarbon Sampling for On-Line GC’s Lighter compositions (low bubble points).

Stainless steel, un-heated sample line. Be careful, the pressure reduction could make the product go multiphase.

Heated vaporizing regulator just prior to injection.

Heavier compositions (high bubble points). Stainless steel, un-heated sample line. Liquid inject (vaporized in high heat inject point).

If the liquid hydrocarbon product goes multiphase at low temperature, it needs to be vaporized at source and treated as a gaseous sample.

Vaporize at collection point. Heated sample line. Be sure the vaporizer and heated line maintain sufficient temperatures.

Speed loops are necessary in all applications to ensure a representative sample.

Be Careful !!!!! A single GC may not work when a plant changes conditions such as recovery to rejection.

• GPA 2174 • API MPMS 8.1 • API MPMS 8.2 • API MPMS 10.4


Crude Oil vs NGL Crude Oil -10 to 100 API

Natural Gas Liquids 74.5 to 272.8 API 350.0 to 688.0 (kg/m3) Relative Density


Oil compositions can vary greatly.

“Dead Oil” Live Oil High Vapor Pressure Oil

Sampling system has to be matched to the product and the application

Oil Compositions




Oil Spot Sample Collection High vapor pressure oils.

Samples MUST be maintained under back pressure to ensure the sample remains in the liquid phase. Floating piston cylinders (typically 1000 to 1200 psi back pressure) Liquid displacement

Stabilized oil. Sample Bottles.



Oil Composite Sample Collection High Vapor Pressure Sampling

Pressurized piston sample cylinder Water Displacement Collected proportional to flow

Dead Oil Sampling Traditional low pressure oil sample pots BS&W and API Gravity Be careful

• GPA 2174 • API MPMS 8.1 • API MPMS 8.2 • API MPMS 10.4


Oil Analysis High Vapor Pressure Oils & Heavy Condensates

GPA 2186M

Stabilized Oils GPA 2103

Things to know about the analyses Only lights can be analyzed by standard Gas Chromatographs. Heavies will need to analyzed by an extended method. Complex and expensive. Additional analyses required for physical properties for plus fraction. Distillation / Weathering

► Shrink Factor & Flash Gas Factor Dew Point Depression for Molecular Weight Densitometer for Density


Reid Vapor Pressure (RVP) and True Vapor Pressure (TVP) RVP and TVP values can vary greatly from source to source.

Incorrect values can lead to shut-ins or equipment damage.

Discrepancies can occur due to several factors. Analytical technique Sampling technique Value reported Understanding the value requested


ASTM D323 Standard Test Method for Vapor Pressure of Petroleum Products (Reid Method).

Sampling Technique Canister at atmospheric pressure Pressurized liquid chamber

Analysis Technique The liquid chamber is attached to a vapor chamber (4:1 V/L

ratio) The assembled apparatus is placed into a water bath, which is

maintained at 100º F (37.8°C), and rotated for a specified period time, typically 1 hour.

Biases Loss of light hydrocarbons in atmospheric pressure canisters. The amount of bias between this test vapor pressure and true

vapor pressure is unknown.


VPCRx - The pressure exerted in an evacuated chamber at a vapor-liquid ratio of X:1, where X may vary from 4 to 0.02.

Sampling Technique Canister at atmospheric pressure Pressurized floating piston cylinder

Analysis Technique The sampling apparatus is connected to the analytical instrument which draws the sample and analyzes it in accordance with manufacturer specifications.

► The calibration of the analytical instrument is routinely verified against a reference standard of known properties. ► TVP – The analytical equipment provides a measured TVP (VPCR0.02) value at a user specified temperature. ► RVPE – The analytical equipment will calculate an RVP equivalent value from the measured VPCR0.02. ► RVP – The analytical equipment provides a measured RVP (VPCR4) value.

The RVP (VPCR4) at 100°F (37.8 °C) value is recommended by D6377 for reporting for crude oil samples.

Biases Loss of light hydrocarbons in atmospheric pressure canisters. Determination of value to report.

ASTM D6377 Standard Test Method for Determination of Vapor Pressure of Crude Oil: VPCRx (Expansion Method)


Recommend RVP/TVP Sampling and Analysis Techniques Composite samples should be collected, if the appropriate equipment is in place.

►If the installed composite sampling equipment does not maintain the product at flowing pressure throughout the ticketing period, spot samples should be collected from appropriate sampling locations.

Samples should be collected in pressurized cylinders. Backpressure on the cylinder should be maintained at a minimum of 20 psi

above flowing pressure of the product throughout the analytical process to ensure the product remains in liquid phase. The sample should be introduced to the analytical equipment at cylinder

pressure to ensure the light hydrocarbons are not liberated, biasing the vapor pressures low. Analytical procedures should follow ASTM D6377 to obtain a TVP (VPCR0.02)

value at flowing temperature, a TVP (VPCR4) value at flowing temperature, and an RVP (VPCR4) value at 100º F (37.8°C). ►The TVP (VPCR0.02) value at flowing temperature is recommended for engineering

purposes. ►The VPCR4 at 100° F (37.8°C) value is considered to be related to the vapor pressure

value obtained by ASTM D323 and is recommended by ASTM D6377 for crude oil vapor pressure reporting.

►The TVP (VPCR4) value at flowing temperature is considered to be most representative of the values used by the regulatory agencies and recommended for TVP reporting purposes.


Third Party Labs Don’t get me started…

Documents

Dave Curtis (Liquid Sampling)