ACCURATE LPG ANALYSIS BEGINS WITH SAMPLING PROCEDURES, EQUIPMENT

Chris M. Wilkins
Koch Pipelines Inc.
Medford, Okla.

Proper equipment and procedures are essential for obtaining representative samples from an LPG stream.

Sampling of light liquid hydrocarbons generally involves one of two methods:

Flow-proportional composite sampling by a mechanical device
Physical transfer of hydrocarbon fluids from a flowing pipeline or other source into a suitable portable sample container.

If sampling by proper techniques and equipment supports careful chromatographic analysis, full advantage of accurate mass measurement of LPG can be realized.

EVOLUTION OF TECHNIQUES

Measurement of NGL mixes has evolved over the past several years from the straightforward volumetric form of measurement to the more sophisticated and more accurate mass measurement systems being used in virtually all LPG custody-transfer applications.

In mass-measurement systems, metered volumes of product are multiplied by the product density at flowing pressure and temperature to arrive at the measured mass of product. As the term implies, however, the result of this type of measurement is a mass of product, not its volume.

Because LPG is commonly bought and sold by volume, the measured mass must be converted to volume so that accounting can be completed (OGJ, Sept. 10, p. 66). To make this conversion, a representative sample of the product delivered over the specified accounting period must be secured and analyzed by gas chromatography to obtain a compositional breakdown.

The total measured mass of product may then be separated into individual component masses and, by use of the proper conversion factors, into individual component volumes. The accurate analysis of a representative sample thus is a vital and intricate part of the mass-measurement procedure.

To demonstrate the effect of composition on the total value of a measured mass of product, Table 1 illustrates an example in which the component concentrations determined from a chromatographic analysis have been varied by relatively small amounts. These different analyses have then been applied to the same quantity of measured product to calculate total value.

With January 1989 Gulf Coast spot prices and the example pounds total, which is roughly equivalent to a month's production at a moderately sized demethanizing plant, the difference in product value is more than $3,300 solely because of these very realistic differences in reported composition.

On the assumption that this example is representative of a 1-month accounting period, this would amount to nearly $40,000/year.

Admittedly, sampling would not be the only potential source of error when a discrepancy in stream composition is suspected. The chromatographic analysis itself could also be a source.

But the adage that the analysis can only be as accurate as the sample to be injected is very true. The laboratory analyst totally depends on the discretion of the person securing the samples.

Improper sampling techniques, faulty equipment, leaks or obstructions in sampling loops or plumbing, contaminants, as well as many other potential problems could very easily lead to an inaccurate determination of stream composition.

Knowing the effect of sampling on the overall measurement picture, the person sampling LPG should strive to keep the ultimate objective in mind-to secure, in a suitable container, an adequate portion of an homogeneous single-phase hydrocarbon fluid under pressure which has the same composition as the product stream being sampled.

GUIDELINES

The physical characteristics of liquid hydrocarbon mixes vary widely with composition and with the temperature and pressure of the product.

For example, some mixed streams may have a vapor pressure of less than 30 psi at typical operating temperatures, while others, such as ethane-propane mixes, may have a vapor pressure in excess of 500 psi.

To ensure that the sample being acquired is single phase, the pressure on the sample source must be well above the vapor pressure (at source temperature) of the product being sampled.

Liquefied gas, even under high pressure, will tend to stratify and separate when not exposed to some form of turbulence. Heavy-end components will tend to fall out in tank storage and stagnant lines, especially when exposed to temperature gradients.

To ensure that a homogeneous sample is being secured, always locate the sampling point on a flowing product stream and use sample probes or scoops that are long enough to allow the sample to be taken from the inner one third of the pipeline.

This area of liquid flow is characteristically faster moving and more turbulent.

Also, if at all possible, place the sampling location after a physical obstruction, such as a meter or an orifice plate. This will create a superior mixing action to aide in securing a representative sample.

When transferring product from a composite sampler collection vessel into a portable sample cylinder, make sure the product has been thoroughly mixed with an adequate mixing device before the transfer because stratification and separation will also occur in this situation.

When possible, locate sampling points downstream of existing line filters. Contaminants such as dirt, water, or amine are normally not of interest to the analyst and may damage chromatographic valves, seals, etc.

Another very important aspect of sampling is safety. Never place a sampling point where expected operating pressures exceed the recommended psi ratings of sampling equipment.

Use tubing, fittings, and sample cylinders made of a material which is not reactive to and affected by the product being sampled. The 300 series of stainless steel has proven to work very well in LPG applications.

Personal protective equipment, such as impervious gloves, safety glasses, and Nomex clothing should be used when applicable.

COMPOSITE SAMPLING

Sometimes it is hard to imagine that a sample can be secured, transported to the laboratory, and injected into the chromatograph, and that the resultant analysis can represent the composition of a very large volume of product which has been delivered or received over a period of time.

With the proper sampling instruments and procedures, however, this is exactly what must and does happen.

A composite (or continuous) sampling mechanism is a device which is used to obtain a representative sample from a flowing product stream over a given period of time.

The system should consist of a sample probe, a flowing sample loop or slip stream, an orifice plate or other source of drive to ensure flow through the sample loop, a flow through sample injection valve, and a constant-pressure (floating piston) sample-collection system.

A method is also required for mixing the product sample in the collection chamber before it is analyzed or transferred into a secondary cylinder (Fig. 1).

Sample probe. The length of the sample probe should be calculated to allow sampling from the inner one third of the pipeline. The bevel of the probe should be pointed upstream to aid in gathering samples and in establishing the flow pattern.
Sample loop. The sample loop lines should be of small diameter (usually 1/4 in.) and kept as short as possible.
Precautions should be taken to avoid vaporization in sample loop lines when operations are near the equilibrium vapor pressure of the liquid. In some cases, to prevent product flashing, it may be necessary to insulate the lines to protect them from extreme ambient temperatures.
It is critical that the sample loop be a continuous flow of liquid product. Sampling off a dead loop line or a line that is terminated at the sampling valve will result in a sample unrepresentative of the main flow.
Drive source. To ensure a fresh supply of liquid is continuously flowing, the sample loop must be installed around a device that causes a differential pressure. An orifice plate or small pump has proven satisfactory for this purpose.
Sampling device. The flow-through injection valve takes a small amount of sample from the flowing stream and injects it into the collection vessel. The device is most generally controlled by a flow computer and should be actuated proportional to volumetric flow, not time.
The sampling device should be adjusted so that incremental samples will be obtained at such a rate that the collection vessel will have adequate capacity to hold the sample during its period of sampling. Care should be taken to prevent overfilling of the sample container so that relieving does not occur, thus changing the composition of the sample.
The valve must function independently of line pressure and collection cylinder pressure and should be capable of injecting sample into the collection cylinder from pressures higher and lower than that of the collection vessel.
Collection system. The collection system should consist of a floating piston-type cylinder for sample collection and a source of inert gas to be used for back pressure.
The floating piston cylinder should be connected to the sample-injection valve with small-diameter tubing and should be plumbed so that a minimal amount of dead space is present.
Inert-gas pressure is applied to the precharge side of the cylinder to force the piston to the starting end of the vessel.
The inert gas should be maintained at a pressure which exceeds by 100 psi the equilibrium vapor pressure of the fluid sampled under expected varying temperature conditions.
The cylinder should be filled with liquid product to a maximum of 80%. This type of system accomplishes two purposes:
1. The captured product is maintained in the liquid phase at all times.
2. Because of the cushion of inert gas on the back side of the cylinder, expansion of the product caused by rises in temperature is compensated for.
Mixing device. The product that has been obtained and stored in the collection vessel must be mixed thoroughly by either a mechanical device or a liquid pump and circulatory system before it is analyzed by gas chromatography or transferred into a secondary portable sample cylinder.
Any malfunctions (leaks, equipment failures, etc.) that could result in an inaccurate sample being secured should be noted to the laboratory analyst.
This will assist in the evaluation of analytical data.

INSTANTANEOUS SAMPLING

At many LPG facilities, there is a need for liquid spot sampling off a flowing pipeline to aid in quality control. Also, some models of composite samplers do not have a removable collection cylinder, and portions of the sample contained in the master cylinder must be transferred into other suitable portable containers and transported to the lab for analysis.

There are two types of instantaneous sampling methods available to choose from: the floating piston cylinder and liquid displacement.

FLOATING-PISTON CYLINDER

The floating-piston cylinder method is the choice in most LPG applications. A very indepth description of the method is detailed in Gas Processors Association Publication 2174-83, "Method for Obtaining Liquid Hydrocarbon Samples Using a Floating Piston Cylinder."

The sample cylinder should consist of a metal tube, precisely machined and polished on the inside surface, with removable end caps for easy access to the inside of the cylinder. The end caps should be drilled and tapped to accommodate valves, gauges, and relief devices.

Contained in the metal tube is a moving piston equipped with O-rings, Teflon rings, or other devices to effect a leak free seal between the sample and precharge side of the cylinder.

The piston must be able to move freely within the cylinder while maintaining the seal between the two chambers. The vessel must also include a device for mixing the acquired sample.

The cylinder, piston, and sealing device must be resistant to the sample, the pressurizing material, cleaning solvents, and any expected corrosion.

Some cylinders are fabricated from nonmagnetic materials such as stainless steel. The piston likewise is made of stainless steel but has magnets attached to the precharge side of the piston.

Mounted on the outside of the cylinder is a series of bi colored flags made of magnetic material. As the piston moves the length of the cylinder, the magnetic field generated by the magnets on the piston flips the flags. To the person taking the sample, this process indicates the piston position and volume of product in the cylinder.

Other types of piston cylinders may have a rod attached to the piston which extends through the end cap of the precharge side of the cylinder.

As the piston moves, the length of rod extending out of the back of the cylinder will very. The length of rod showing will indicate piston position and cylinder volume.

The piston position indicator along with the pressure gauges on the end caps of the cylinder are very important. Not only are they convenient to the person taking the sample, these features are also excellent indicators of potential safety problems.

A quick glance at the cylinder will show the person handling it the volume of product in the cylinder and the pressure being exerted on the container by the product.

To obtain liquid samples from a flowing pipeline or to transfer samples from a collection cylinder of a composite sampler, the following techniques must be followed during use of a floating piston cylinder (Fig. 2):

With the sample side of the piston cylinder evacuated (from cleaning operation) and Valve C open, fill the displacement end with inert gas to approximately 10 psi or more greater than sampling pressure. Close Valve D.
Connect the piston cylinder to the sampling source at Valve A.
With Valves B and C closed, open the sample source Valve A to full open position.
Observe the sample pressure on the gauge (L). Crack Valve B and fitting at Valve C to purge the line. Do not allow the pressure (L) to drop below the original sample pressure.
Close Valve B and tighten the fitting at Valve C. Discontinue purging after a sufficient time and only when liquid is present. If the product flashes without leaving a liquid residue, judgment must be used by the operator.
Adjust the pressure on the gauge (N) to equal the pressure (L) by releasing adequate inert gas through Valve D.
With Valve D closed, slowly open Valve C completely. There should be no pressure change on gauge (N), and pressures at the three gauges (L, M, and N) should all be equal.
Partially open Valve D, slowly allowing inert gas to vent to the atmosphere. Do not allow the pressure (M) to drop below the sampling pressure, thus preventing flashing.
Continue operation until the indicator designates that the cylinder contains 80% by volume of product (following manufacturer's instructions).
Close Valves D, C, and A in that order. Open Valve B to release pressure on sample line. Disconnect the cylinder from the sample source.
Do not take outage or reduce pressure on the piston cylinder. Check Valves C and D for leaks, cap the valves to seal and protect the threads, prepare sample information tag, and box for transportation following Department of Transportation (DOT) or applicable requirements.

The floating piston cylinder method for obtaining liquid hydrocarbon samples has several advantages over the liquid displacement method. Primarily, the sample is maintained in the liquid state at relatively the same pressure as the source from which it was taken.

Additionally, as a result of the nature of the equipment and techniques used, the sample is contained in an isolated chamber at all times and does not come into contact with a displacement fluid.

The piston cylinder also offers a superior method to mix the sample. The mixing ball or mixing dasher employed by most types of modern piston cylinders provide excellent ways to ensure a homogeneous sample.

The major disadvantage of the piston cylinder does not come in performance but rather in cost. The typical piston cylinder may cost up to five times more than a cylinder suitable for liquid displacement. This extra cost, however, could be justified by the potential gain in accuracy from utilizing the floating piston cylinder as opposed to the other methods.

These cylinders most commonly have a capacity of 300, 500, or 1,000 cc and may be purchased from various vendors.

LIQUID DISPLACEMENT

Currently, the only light-hydrocarbon liquid sampling method detailed in an industry-accepted standard publication is the floating piston cylinder (GPA-2174).

Recently, a cooperative analytical study under the direction of a GPA work group through Technical Section B was undertaken to establish other acceptable methods of sampling.

As a result of this study, three methods of liquid displacement are tentatively scheduled to be included as acceptable methods, along with the floating piston cylinder, in the upcoming revision of the original 2174 publication.

These liquid-displacement methods are the following:

Water displacement (total removal-80% hydrocarbons/20% displaced outage)
Water displacement (partial removal-70% hydrocarbons/20% displaced outage/10% water remaining in cylinder)
Ethylene glycol displacement (total removal 80% hydrocarbons/20% displaced outage).

It is important to note at this point that the purge method of sampling was deemed unacceptable by this same study and thus should not be used when accuracy is desired or custody transfer is involved.

These methods employ a stainless steel sampling cylinder filled with clean water or, if freezing is a problem, ethylene glycol. These procedures are acceptable only if the displacement liquid does not appreciably affect the composition of the sample of interest.

Following is the general procedure for securing samples in this manner:

The sample cylinder should be completely filled with the displacement liquid.
After purging the sampling line, connect the cylinder to the sample source in the vertical position. If the cylinder is equipped with an outage tube, the tube end of the cylinder should be at the bottom.
With the sample cylinder valves closed, open the sample source valve to full-open position and observe the pressure on the sample header gauge. Slowly crack the fitting between cylinder and source to purge fittings and tubing.
Do not allow the pressure to drop below the original sampling pressure. Discontinue the purge when a liquid product is present.
Open the top valve on the sample cylinder to full open position.
Open the bottom valve on the sample cylinder slowly until a small stream of displaced liquid is allowed to flow from the bottom of the cylinder.
If the cylinder is not equipped with an outage tube, a graduated cylinder or other measuring container should be used to collect the displaced liquid. Drain the displacement liquid slowly, making sure the original sampling pressure is maintained.
Continue to displace liquid until the desired amount of sample has been transferred, which is not to exceed 80% of the volume of the cylinder. Close the bottom valve of the cylinder.
Close the top valve of the cylinder and the sample source valve, in that order. Relieve the pressure from sample line or header.
The cylinder now contains approximately 80% hydrocarbons on the top and 20% displacement liquid on the bottom. The cylinder should be disconnected and the remaining 20% displacement liquid drained from the bottom.
If the cylinder is equipped with an outage tube, invert it so that the outage tube is at the top before draining the displacement liquid from the bottom.
Under no circumstances should the cylinder be transported when full of liquid.
Because of the 20% outage, the cylinder now contains a sample existing in equilibrium. To force the product back into the liquid phase, it must be repressurized with liquid to greater than its vapor pressure before being injected into a chromatograph.
Check valves for leaks, cap valves to seal and protect threads, prepare sample information tag, and box for transport as per U.S. DOT or applicable requirements.

These cylinders, as well as all others, should be equipped with a pressure-relief device.

Exposure of the cylinder to extreme ambient temperatures should be avoided to prevent possible relieving. This not only could create possible safety hazards but may also compromise the sample and render any resultant analysis useless.

REFERENCES

Caffey, Bill R., "Obtaining and Handling Natural Gas Liquid Samples," 14th GPA School of Chromatography, 1987.
McCann, Scripsick, Wilkins, and Hefley, "Work Group Report, Natural Gas Liquids Sampling Project (for Revision of GPA Standard 2174)," 67th Annual GPA Convention, 1988.
Wilkins, Chris M., "Chromatographic Analysis of Natural Gas Liquids," Proceedings of International School of Hydrocarbon Measurement, 1987.
GPA Publication 2174-83, "Method for Obtaining Liquid Hydrocarbon Samples Using a Floating Piston Cylinder."