Introduction
The sampling techniques employed in extracting a representative sample for crude oil BS&W analysis has received a high level of scrutiny in the last twenty years from concerned oil companies around the world. The revenue implications are considerable, if the sample is not accurate. Whether it is pipeline custody transfer, tanker loading/unloading, or refinery input, a representative composite sample of the oil for laboratory analysis is critical. Plainly put, a 1% error is worth hundreds of thousands of dollars and frequently over a million dollars on one single ship load. That makes sampling a serious matter in the measurement world for crude oil sales.
The design of the equipment should be centered on the concept of extracting a sample from the pipeline, which is truly representative of the product. This is done by taking a sample in basically isokinetic conditions; actually and physically taking a sample from the flowing conditions or replicated flowing conditions. Sampling from dead legs, non-flowing streams, launch tubes, etc., will not provide a dependable quality sample.
Different sampling locations may present negative obstacles which must be designed around, or at least reduced as much as possible to preserve the integrity of the sample accuracy, but these obstacles must be considered and dealt with. The sampling of crude oil is decidedly more important now than it has been in past years and with the increased price of oil, the importance of proper sampling increases in kind.
The object of crude oil sampling is to determine the quality of the oil at the custody transfer point. The amount of basic sediment and water in the oil, a chemical analysis of the oil and shrinkage, and the API gravity should be determined. The process of accomplishing this task is not something to be left to convenience or ease of operations. The process must be accurate.
The purpose of a crude oil sampling system is to withdraw from a pipeline a small representative portion of the product that contains the water, oil, and contaminants in the same proportion as is flowing in the stream.
As an update at the time of this paper, API 8.2/ASTM D4177 is under revision. ISO 3171 will be shortly and the EI document is being used to help with both revisions. API Chapter 8 will be completely revised to not only update the crude oil section, but to separate and expand the chapter to look at not only crude oil, but also the other liquid products that have been overlooked for years. It is the desire of those who are working on this matter, to soon have a comprehensive document that will cover crude oil, refined products, feed stocks and other liquid products in a specific manner. Using the current 8.2 as a guide for refined products is not a proper use of the standard. Hopefully, this will be rectified within a few years.
For now, we look at the current task of sampling crude oil.
Sample Systems
When sampling crude oil, three major items must be present:
- The pipeline must be conditioned so that the point of sample is representative of the composition of the pipeline.
- A sampling device must be able to sample the product under pipeline conditions (pressure, temperature, viscosity and contaminants) and also take a complete sample while not destroying the benefits of the conditioning system.
- Properly designed sample receiver, capable of remixing the contents of the container and providing for an aliquot for the field or laboratory testing.
In addition to these fundamental components, there are additional aspects that are critical to the overall success of a quality system. They include:
- Proper sample handling of the sample
- Properly chosen analytical method and laboratory analysis
- Monitoring of the system performance
- Validation of the system installation
A Sampling System should be designed and developed on these three principles. A conditioning or mixing system should be designed for crude oil service. It must provide a mixture of the pipeline which is adequately dispersed and distributed across the pipeline diameter. A sampler then grabs a sample of this mixture with a collection device designed to allow the mixture to flow through the collection point under flowing conditions without distorting the sample due to creating a flow profile that would alter the mixed stream at the point of sample extraction. Then, this collected sample is directed to a sample receiver designed to maintain the integrity of the sample in all ambient conditions and re-mix the contents so that a small portion of the total sample volume can be removed from the whole and analyzed.
Sampling is not just `another’ function of measurement; it is the heart and soul of the profit figure. Crude oil sampling goes on past the pipeline, into the laboratory, analysis and accounting. It must start correctly if it is to end well! Companies have to seriously weigh the question of perceived convenience versus accuracy in analysis and the monetary savings.
The object of mixing in the line is to ensure the droplets of water are small enough to be sampled and the contaminants are uniformly distributed across the line. If a good mixture is not present, a good sample is not possible.
Piping configurations, pumps and other turbulence producing devices may provide mixing for good representative sampling. There are times when these conditions will allow for a good sample. However, while they do provide some mixing characteristics, they may not consistently provide distribution and dispersion of the pipeline contents adequate for proper sampling. That is a very critical decision that must be made – can we get lucky, or do we need to take a proactive approach to pipeline conditioning. Without adequate pipeline conditioning, getting a representative sample is at high risk of failure.
The sampler should be a positive displacement pump that voids the sample “grab” area with each stroke. This is to ensure there are no contaminants left to alter the next sample or that portions of the sample grab were not directed to the container. This can be accomplished by a variety of positive displacement methods and designs – squeezable collection heads, piston/cylinder designs or piston syringe style designs. The key is to extract a sample and direct the entire sample to the sample receiver.
The positive displacement pump concept also should be used so the sample will be pumped into the container, regardless of the pipeline conditions, ambient temperature, wax content, pour point, sand and sediment content, or other physical properties of the sample that inhibit the free flow into the container.
A properly designed insertion sampler ensures an accurate sample is being withdrawn from the flowing stream. The retractable type sampler allows for pigging, inspection, and maintenance without shutdown and isolation of the main line piping.
The world’s best sampler is useless unless it sees an optimally mixed pipeline. The world’s best mixer is wasted if the sampler cannot properly retrieve the mixture.
And lastly, a very critical point is the sample container. Careful consideration should be taken with the type (pressurized or non-pressurized), style (mixer or non-mixer), and design (features). A poorly designed container will adversely affect your sample analysis. After grabbing a representative sample, it is vital that the container that becomes the depository for the samples is properly designed for the task at hand. A container that allows for water to cling to the sides and avoid being mixed into the sample at a later point, is not properly designed. Lid closures, piping designs, sloping sides, internal lining and coating, the mixing system and similar items of interest are all to be considered in a receiver. The afore mentioned standards all address the various features that should be part of the design of the containers. The containers should be proven and show that they indeed provide for an adequate mixing of the retained sample and providing for the small amount of sample to be used in the lab.
Also, High Vapor Pressure Crudes should be closely looked at, as regards high pressure crude containers. All of these systems should be able to be proven, and certified that with a given product, they truly provide adequate mixing for the lab and the lab tests.
