5.2 LIQUID bATCHING TRANSPORTATION
5.2.4 Products batching Definitions and Terms
The following terms are commonly referred to in batching of multi products pipelines and in batched pipeline operations and reporting:
Figure 5-16. Typical batched products pipeline
Pipeline Operation and Batching n 249
Batch: Batches are the means by which product movements can be tracked. A
batch always starts out as a defined continuous volume of product. It may be split, partially delivered, or stored in one or more location product pools before it is finally delivered to its nominated destination.
Batches also exist for accounting and shipper/transport purposes. A single batch must be unique with respect to a shipper, product, origin, “carrier from,” and ratability.
Fungible Batches: A “fungible batch” is defined as a batch of petroleum product
meeting carrier’s established specifications, which may be commingled with, or delivered in substitution for, other quantities of petroleum product meeting the same specifications. (i.e., they are interchangeable)
Fungible product specifications are established based on industry standards, federal and state requirements, and the pipeline operator’s ability to handle various products. Fungible products usually provide shippers with a significant degree of flexibility for scheduling lifting and delivery times.
Segregated Batches: A “segregated batch” is defined as a batch of petroleum
product meeting carrier’s established specifications, which may not be commin- gled with other quantities. A batch may be segregated because it has properties that differ from the fungible specifications.
5.2.4.1 batch Sequencing
Liquid pipeline operators transport various liquid petroleum products or grades of the same product in sequence through a pipeline, with each product or “batch” distinct from that preceding or following. Once a refined product or crude oil grade is injected and begins its journey, subsequent products may be injected and shipped. It can be fungible, in which case same products from different shippers can be allowed to be shipped in as one batch. As a consequence of batching without a separation pig, inter- face development occurs between batches. This interface is a petroleum mixture which occurs during normal pipeline operations between adjacent batches of petroleum prod- ucts having different specifications. It may also be called “slop” or “transmix.”
Factors that affect the order of batches include: Compatibility with adjacent batches ·
Batch cycle requirements ·
Buffering requirements ·
Transmix or contamination levels ·
Capacity and power availability and scheduling and operational requirements ·
Ratability ·
Product availability ·
A typical batch sequencing is shown in Figure 5-18.
TAbLE 5-2. typical batched liquid quality specification
Test Parameter Refined Products Synthetic buffer Natural Gas Liquids
Sediment and water or particulates density @ 15°C @STP, IP 216 < 5 mg/L. particulate (< 5%) < 10 mg/L. particulate (< 10%) < 5 mg/L. particulate (< 5%) Viscosity @ lower of 0.4–2 cs < 20 cs –Receipt temp. –Reference temp.
Vapor Pressure, kPa < 103 kPa Reid vapor Pressure
< 103 kPa Reid vapor Pressure
< 1100 kPa absolute @ 37.8°C
ATM copper strip corrosion test (ASTM D130)
Figure 5-18 is representative of a typical distillate and gasoline cycle; however, it may be noted that in observing environmental concerns, low-sulfur and high-sulfur diesels (LSD/HSD) are no longer produced in North American refineries.
Also, it may be noted that another factor that impacts batch sequencing is the prod- uct availability. Sometimes, a batch may be scheduled to be shipped, but it is not avail- able in sufficient volume when a shipment is required. Thus, the batching sequence must be modified to account for this factor.
5.2.4.2 batch Cycle/Slug
A batch cycle is a set pattern of batches of similar commodity type that takes into account:
Interface contamination of batches ·
Injection and delivery patterns of batches ·
Repeated usage for production of batch sequences ·
Incompatible batch cycles are buffered to minimize contamination. A cycle is a defined period (generally, 1 to 10 days, short term or 10 to 45 days long term) where a prescribed set of products is transported, generally in a particular order and in particu- lar batch sizes. Cycles simply repeat one after the other throughout the month.
A typical batch cycle for transportation is shown in Figure 5-19 and consists of the following products and their sizes, butane (352 m3), gasoline (regular, 1500 m3; premium 5200 m3), kerosene (4200 m3) and diesel (8200 m3). The sizes given are examples only.
A slug/batch train identifies a continuous stream of a single homogenous product within a pipeline. A slug may contain one or more batches, so long as the batches con- sist of the same product. Slugs are operational movement tools, and are defined purely for the convenience of the scheduler. At any time, one or more batches, or even one or more partial batches, may be contained within a slug.
5.2.4.3 buffers
A buffer is a petroleum commodity that physically separates dissimilar or different commodity types so as to minimize contamination. Some liquid pipeline companies use a synthetic crude oil, a semi-refined, “clean” crude product as a buffer between liquids such as propane gas (LPG) and refined products. Use of buffers ensures that the
Figure 5-18. Typical batch sequencing [10]
Pipeline Operation and Batching n 251 lighter product does not migrate into heavier products and affect their vapor pressure and hence flash point.
When used, buffers are delivered to separate facilities (interface or “slop” or “transmix” tanks) or are mixed in with the contaminated batch when it is delivered.
5.2.4.4 batching Travel Time
The time that it takes for a batch to travel depends on factors such as product flow rate, the number and type of preceding and sequential batches, compressibility dif- ferences between batches, pipeline elevation, etc. However, knowing the flow rate, a rough estimate can be made as to the time for batch travel from the point of injection to delivery location.
Long-distance liquid pipeline companies usually create a batch transit table or a chart that shows the approximate time required for a batch to move from one terminal location in a system to another.
The charts are most useful when batch transit times are long and batches are pumped in one month and delivered in the following month. Shippers utilize the tran- sit table to estimate batch delivery times, even though the carrier may have yet to issue a new monthly schedule.
For pipeline shipment of varied products, a seasonal or other upheaval in delivery or intermediate terminals logistics can be disruptive. The shipment depends on deliv- ering each product to each terminal in sequence. A batch travel chart together with delivery terminal restriction will assist in scheduling deliveries into facilities, diverting a portion of flow to an available terminal and sending the remainder to more distant delivery locations.
5.2.4.5 batch Interface Marking and Detection
Batch interface detection is necessary to send the incoming batch to the proper tank or to perform other necessary operations. Batch interfaces can be detected by a den- sitometer if batches have different densities or by other types of detector if the batch densities are similar. Densitometers are most frequently used for detecting batch in- terfaces. Interface detectors are normally installed upstream of the delivery locations. If batches are separated by a sphere, then the sphere has to be inserted at the time of batch launch, bypassed when it passes a pump station, and removed when it arrives at the delivery point.
Some pipeline companies use a fluorescent dye to mark refined products’ batch interfaces and use fluorometers to detect the arrival of batch interfaces. Others use opti- cal interface detectors (OID) [11].
For interface detection with fluorescent dye, a single shot of dye is injected into the interfaces between refined products. To indicate the beginning and end of a new batch cycle, two shots of dye may be injected into the leading synthetic crude buffer as well as the last refined products batch. One injection is at the refined product (RP)/ buffer interface, and the other is upstream of the interface, to provide early warning of the arrival of the following RP/buffer batch.
Advantages and disadvantages of such marking are: Advantages:
·
Allows the use of one pipeline for different products ·
Lower capital and operating costs ·
Disadvantages: ·
Complex dye injection design and operation ·
Potential for missing an interfacial cut due to a misplaced or missing dye ·
shot
Possible contamination and degradation ·
Possibility of lower-quality product impacting higher-quality product ·
With OID, the change in product is determined by calibrating and measuring the OID output as a function of product gravity. In this way, interfaces can be detected easily and accurately [11]. One commercially available dye product that is used in pipeline batch interface marking is Fluorescent Yellow 131 SC, produced by the Rohm and Hass company [12]. This is a liquid, concentrated fluorescent solvent soluble dye. It contains Solvent Red 175 in a high flash, severely hydro-treated naphthenic solvent system with the following characteristic/properties (Table 5-3):
The disadvantage is that it is completely miscible in all petroleum fuels and many other related hydrocarbon solvents. Also, a disadvantage of interface dye marking is the increased risk to batch cuts due to the potential for failures in the marker injection/ detection.
The dye emits an intense white-yellow fluorescence when energized with a high intensity black light, making the smallest of leaks detectable. Because of this property, it makes for a good leak detection application for a variety of process fluids.
Another method that may be used in interface detection involves the use of a sen- sitive and precise viscometer. Typically, the same product coming from two different refineries will have slightly differing viscosities. Therefore, such a viscometer can be used effectively in determining and differentiating interfaces between two batches.
Calculating batch interface positions requires the following information: Pipeline system configuration
·
Current storage levels ·
Injection/receipt and delivery volumes ·
Batch sequences ·
5.2.4.6 batch Injection, Transportation, and Delivery
Liquid batches are injected and transported through the pipeline for delivery into ter- minals or refineries in the following fashion:
Full stream injection ·
Side stream injection ·
Straight injection and delivery ·
In full stream injections the fixed portion of the main line is completely filled with the fluid, displacing its current contents.
Once batch sequences are determined, the pipeline is then scheduled from the first injection into the system through to the final delivery out of the system. Midline break- out tankage facilities are usually provided to be able to schedule start and end times for deliveries and injections. Breakout tankage is also used to manage differing flow rates in pipeline segments of differing diameters. This, in turn, allows for proper tankage/ storage facility management.
In side stream injections, the liquid pumped into the main line shares the line with the same type of liquid already flowing in the line (Figure 5-20), side stream injection operations are useful when:
TAbLE 5-3. characteristic/properties of batch interface marking,
fluorescent yellow 131 Sc (manufactured by ref. [12])
Physical Form Dark-Colored Liquid
Specific Gravity 0.92 Flash Point, ASTM D-3278 > 85 °C Excitation wavelength 494 nM Emission wavelength 535 nM
Pipeline Operation and Batching n 253
The allowable flow rate downstream of the injection supply/terminal is greater ·
than the allowable flow rate upstream of a terminal.
The maximum injection flow rate at a supply terminal is lower than the required ·
pipeline flow rate.
With this method, batch injections can be achieved with minimum impact on over- all pipeline flow rates. If there are flow rate or pressure restrictions, then the upstream flow rate can be reduced in order to allow the side stream injection.
Side stream delivery operations allow the pipeline to be used more efficiently, by increasing throughput or capacity, and are therefore advantageous when
the upstream allowable flow rate is higher than the downstream flow rate and ·
the delivery rate at a terminal/delivery location is low. ·
With straight injection/delivery, one liquid is delivered to a terminal from the main line, simultaneous with a second liquid being injected from that same location into the main line. The injection and withdrawal may be of same volume and product or may be of different volumes and products. This method allows for optimizing and increasing pipeline through- put, through tank/storage usage optimization when the product batching cycle is created.
A big advantage of side stream injection is that it facilitates the increase of the size of a fungible batch, thereby reducing the numbers of interfaces required, and any associated contamination or transmix costs.
5.2.4.7 batch Reporting
Batching reports are used for the following purposes:
Assist control center operators in operating the pipeline, and ·
Help pipeline schedulers to schedule and coordinate batch movement ·
Help maintenance staff schedule pipeline maintenance ·
As an indicator of pipeline capability/integrity ·