International Pipeline Conference Volume 2 ASME 1996 IPC1996-1869 DRAG REDUCING AGENT: A MULTI-USE TOOL FOR LIQUID PIPELINE COMPANIES David H. Rew and Steven R. Sandman Lakehead Pipe Line Co. Duluth, Minnesota A B S TR A C T The role of Drag Reducing Agents (DRA) at Lakehead Pipe Line (LPL) has expanded considerably in recent years. It is now a multi-purpose tool that helps manage electric power usage as well as having traditional capacity increase uses. Indeed, DRA is now an important part of operating the LPL system. This paper examines the multiple uses of DRA as used by LPL. It also describes the structure of the DRA program within the company and provides a summary of lessons learned since the DRA program started at LPL in 1991. This information should be useful for anyone who is considering implementing a DRA program. W H A T IS DRA? DRA is the name given to a group of chemical products that reduce friction loss (pressure drop) in liquid piping systems. Most, if not all DRA products are comprised of an active ingredient suspended in a base material. The active ingredient in most DRA products is a long chain polymer or high molecular weight polymer. Base materials include water, kerosene and alcohol, although others may exist. For specific details on any given DRA product, the manufacturer is the best source of information. H O W DOES DRA W O R K? Different DRA products may work in slightly different ways. The exact drag reducing mechanism of any DRA product is best explained by the manufacturer. One manufacturer indicates that its products reduce drag by slowing down the formulation of eddy currents/turbulence. Another manufacturer merely states that drag is reduced by reducing the internal friction of the fluid. Whatever the precise mechanism(s) may be, DRA does indeed work in many applications. DRA degrades and loses effectiveness in anything other than straight (cross country) pipe. DRA s usefulness is destroyed when going though a pumping station because of pipe fittings, check valves, reduced bore valves, and pumps. Copyright 1996 by ASME
APPLICATIO NS DRA is currently used for two different purposes in the LPL System, de-bottlenecking and power savings. The primary use is for de-bottlenecking particular line sections. By injecting DRA into the bottleneck sections, the bottleneck is reduced or eliminated and overall flow through the line is increased. For debottlenecking applications, various DRA products are utilized depending on the line and the type of crudes they contain. In general DRA is more effective in lighter^ products. Refined products and natural gas liquids (NGLs) can easily see over 50% drag reduction at reasonable DRA injection levels. De-bottlenecking has been done in the LPL system for a number of reasons. DRA has been injected to increase capacity to recover from lost throughput during hydrotesting or other maintenance activities as well as when loops have been out of service. Throughput varies with the season and DRA has been an economical way to increase capacity during these periods. The LPL system is comprised of batched crude shipments. A range of different density products are shipped down the same line. The heavier density crudes will cause the bottlenecks so DRA has been injected into these but not the lighter crudes. An example of representative data for two types of crude in the same pipeline with die same DRA injection concentration is shown in Table 1. LPL has not utilized DRA in light crude since the line loss before DRA is lower than the medium crude line loss after DRA. In other words the light was not causing a bottleneck so DRA was not necessary. Although energy would have been saved if DRA had been injected into die light this was not done since it would not have increased the throughput which was the goal for this application. Table 1 Example Drag Reduction in Different Crude Type s Crude Type Line Loss (psi/mile) Drag Reduction at same DRA Concentration Reduction in Line Loss (psi/mile) Line Loss after DRA (psi/mile) Light 14 30% 4.2 9.8 Medium 20 20% 4 16 The other application, which is relatively new, is to use certain DRA products for managing power usage. DRA is used in two ways to manage power usage - station by-pass and baseload shaving applications. The station bypass application will be discussed first A station is taken off-line by injecting DRA at the closest upstream station. The viability of this depends on the pipeline hydraulics of a given section and the price of electricity at a particular station. The hydraulics may make bypass impossible but more commonly a relationship exists between required quantity of DRA and the price of electricity. For example a given station may be economical to bypass if electricity is over 5 0/kwh but not at 3.5 c/kwh. When a station is bypassed, the station valves are reconfigured so oil does not go through station piping. Pressure control through use of the station pressure control valve is therefore not possible. This is an important factor when deciding to bypass a station. LPL s Line 5 is a 30" line which runs from Superior, WI to Sarnia, Ontario through the Upper Peninsula of Michigan. There are 15 stations on this 742 mile long line which transports various crude oils and NGL
This line is well suited for DRA use and we have bypassed three stations. Target throughput rates can usually be maintained but achieving maximum line rate has proven difficult. To date, we have not bypassed two stations in a row, although this may work in some situations. One station cannot be by-passed due to line operating concerns. The elevation drop of the line upstream of the station could cause column separation in NGL batches under certain circumstances. Therefore this station will not be bypassed as the safe operation of the line would be compromised. This same station bypass feature is useful in conjunction with interruptible electric power contracts. The cost savings of interruptible power carries a trade off of service interruption or curtailment. Most interruptions are weather related and total LPL interruptions have been under 40 hours/year. In these situations DRA can be utilized to maintain throughput when a station is off line. A consideration is that DRA must be present in all the oil between stations before the full pressure drop reduction is seen. This takes about 6 to 8 hours in Line 5, but half the benefit is there in half that time. 1 LPL demonstrated to more than one electric utility that we did have alternatives to electric power. We shut down some stations for up to six months and did influence and motivate the utility to agree to price concessions in our electric power contracts. In crudes where DRA is effective, it is almost always economical at some concentration. As seen in Figure 1, a low ppm concentration can achieve noticeable drag reduction, but increased ppm concentrations achieve a diminishing return. For example it may take 4 times more DRA to increase drag reduction from 20% to 40%. LPL has not fully utilized low ppm concentrations to date, but we are planning to inject DRA in more locations to reduce energy requirements at many of our stations. Even if it is impossible to shut down a station, energy usage can be reduced and cost savings achieved. The use of DRA depends on many variables. A great deal of testing must be done to produce data on the different circumstances in the LPL system It has been said within LPL that use of DRA is more of an art than a science. Future study will allow optimization of DRA usage within LPL as well as use of DRA combinations of the applications discussed above. As yet LPL has concentrated on one application at a time for each injection site. DRA still has a ways to go before it becomes a fully integrated tool for use in the LPL system. DRA PROGRAM IM PLEM ENTATION The DRA program at LPL requires the active participation by at least 4 different departments. The four different departments are Energy Management, Pipeline Control, Technical Services (Engineering) and the Operations Districts. Each department has an important role to play to insure that the program functions smoothly. The responsibilities o f each department as they relate to the DRA program are outlined below: Energy Management has the lead role and determines the following items : 1. Which sites need DRA 2. What should be the injection rate at each site 3. The DRA product to be used at each site 4. The specific injection plan (every batch, select batches only, etc.) 5. Conduct the economic analysis to determine product effectiveness. One of the main reasons that Energy Management takes the lead role is that they have access to and
FIGURE 1 - EXAMPLE GRAPH Drag Reduction vs Concentration of DRA Injection 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 ILLUSTRATES RELATIONSHIP! (% Drag Reduction) ONLY - NOT ACTUAL DATA DRAG R ED UCTIO N
expertise in operating the hydraulic computer model for the pipeline system. The Engineering Department installs the actual DRA facilities (see Fig. 2 for schematic of a typical facility.) The Operations Departments are responsible for maintaining and operating the DRA facilities. The Pipeline Control group issues start/stop orders for DRA injections via the field messaging system. The Technical Services group coordinates all the parties involved and establishes procedures such as usage reporting. DRA usage is reviewed each week in a Power Issues Group meeting which is comprised of members from the above departments. Among other things, this group takes into consideration the following items when making weekly decisions on DRA injections : a. Inventories at various tank farms b. Throughput goals on each line c. Upcoming outages (loop outages, station outages, substation outages, etc.) LESSO N S LEARNED LPL has been actively using DRA since 1991. The following items summarize the lessons we have learned and may be of interest to anybody contemplating using DRA. 1. Running a DRA program is a group effort Buy-in from the various departments involved is important for program success. 2. Don t assume that results in one pipeline are directly applicable to another unless the operating conditions (crude type, viscosity, temperature, diameter, flow rate, etc.) are very similar. 3. A capital investment is required. Installing DRA facilities is oftentimes cheaper than the alternatives but nonetheless a monetary investment is required. A typical DRA facility* will cost approximately $30,000 (U.S. dollars). This includes a concrete slab, power & control wiring, pump/piping hookup, and commissioning. 4. If the pipeline system has any complexity to it (looping, multiple tank farms, etc.) usage of DRA needs to be reviewed on a weekly (and possibly more often) basis to optimize benefits received. 5. Educate/inform all personnel involved with the DRA program, especially and including management D on t make DRA the mystery product - tell them what you know about it and the benefits that it will produce. 6. Be sure to establish procedures for usage reporting, spill cleanup & disposal.
SCHEMATIC - TYPICAL DRA FACILITY FIG. 2