DRA Operations

9.3.3.1 DRA Facilities

Figure 9-19 below shows an example of a test installation for application of the new ExtremePower™ DRA (warm climate). Some additional equipment would be required for cold climate installation.

A DRA injection facility is constructed of injection and feed pumps, flow meter, stor- age tank, pressure sensors, control equipment, and equipment for safety such as pressure relief valves. Figure 9-20 shows a DRA injection system. Multiple tanks are normally used for DRA storage, and gear pumps driven by variable speed motors for injecting it. The DRA injection rate is measured by a positive displacement flow meter.

9.3.3.2 DRA Injection

The effectiveness of DRA is measured in terms of the reduction in frictional losses in the pipeline. It varies with the DRA concentration, viscosity of the solvent fluid, pipeline temperature, fluid velocity, and pipeline diameter. Since a DRA is composed of long polymer strands, the DRA can be sheared when it passes through the pipe- line and equipment such as pumps and control valves. This results in degradation of its effectiveness. The DRA effectiveness depends on the length of the pipeline and the amount of shearing. Therefore, DRA must be injected downstream of all pumps, meters and valves to prevent shear degradation, requiring the DRA pump pressure be higher than the pump station discharge pressure.

Figure 9-19. Example of field test facilities (courtesy of ConocoPhillips Specialty Products, www.liquidpower.com)

Startup of DRA Injection: The DRA injection system is usually automated and

controlled remotely by an operator from the control center. Refer to Figure 9-20, which illustrates a DRA injection system at a pump station. In anticipation that the remote control system may not work, the facility needs to provide local manual control capa- bility. Described below is a normal DRA injection starting procedure:

Select stations where DRA is to be injected, while checking to ensure that in- ·

compatible product (such as jet fuel) will not be affected by wrongly injecting a DRA into the passing batches.

Determine the DRA flow rate set point based on the target flow rate in order to ·

obtain an optimum drag reduction. The required DRA flow rate is calculated automatically if the line flow rate is known. Normally, DRA injection initially begins at a high flow rate, and then lowers to the required flow rate.

Select and start the DRA pump if there are several DRA pump units. ·

Check if the DRA flow rate agrees with the DRA flow rate set point. ·

DRA injection operation is shown in Figure 9-21. The DRA flow rate is control- led to reach the DRA set point. The pump station is equipped with two variable speed pumps (two VFD drivers) in series, and the DRA injection system is installed down- stream of the station. A booster pump is installed to boost the suction pressure of the mainline pump.

In order to make sure that any jet fuel batch is not contaminated with DRA, the following steps for the startup and shut-down of DRA injection are taken:

DRA injection should not start about one hour after a jet fuel batch has passed ·

the DRA injection station.

DRA injection should be shut-down about an hour before the jet fuel batch ar- ·

rives at the injection station.

Where a batch tracking application is employed, DRA lockout can be triggered ·

by the approach of a jet fuel batch to the DRA injection station with a status returned to SCADA that can be used to lockout the DRA pump.

Shut-down of DRA Injection: When a pipeline operates near the pressure op-

erating limit, shut-down of DRA injection could cause line operating pressure to be exceeded if the flow rate remains constant. Described below is a normal DRA injection shut-down procedure:

Select the station where DRA injection is to be shut down. ·

Check if DRA is no longer required to obtain line flow rate for pending batches ·

or to maintain the line pressure within the operating limits.

Select and stop the DRA pump at the station where DRA injection is shut ·

down.

Check if the DRA flow rate decreases to zero. ·

The above figure shows a DRA injection and shut-down control through the SCADA screen of the pump station, where the DRA injection takes place.

9.3.3.3 DRA Concentration Tracking

The DRA concentration is measured in parts-per-million in the flowing product. The DRA concentration is tracked as it moves down the pipeline, and the concentration in the subsequent section includes the degradation due to moving along the pipeline and passing through running pumps. The inclusion of the DRA will create a new ‘batch’ blended with the DRA when the DRA is being injected.

Both sheared and non-sheared DRA concentrations need to be tracked to prop- erly operate DRA injection. A DRA injection rate is used with measured or calculated product flow rate to calculate the DRA concentration. When a DRA passes through a pump, it is sheared and no longer active. The DRA tracking function tracks the sheared and active DRA concentrations and checks the concentration against the maximum DRA concentration allowable in the product. For example, DRA is not allowed in jet fuel and thus its concentration should be checked against zero concentration level. A graphic view of the DRA contents within a pipeline can show active, sheared and total concentration of DRA in the product as well as the positions relative to DRA injectors or pump stations.

9.3.3.4 DRA Limitations on Operation and Design

If the throughput is restricted by the pipeline capacity, it is generally cost-effective to install DRA facilities at pump stations. However, if the desired flow rate is higher than the pump capacity, the pumping capacity must be increased to accommodate the increased throughput requirements.

Figure 9-22 illustrates the pump operating point change due to capacity increase. Since the throughput increases in the presence of a DRA, the existing pumps may not be able to accommodate the flow rate increase without modifying the pump charac- teristics. Note that the pump does not operate at the best efficiency point (BEP) when DRA is injected into the fluid.

DRA can be used in the transportation of crude oil and refined petroleum products (except jet fuel) in order to increase pipeline throughput. The DRA can accumulate on turbine blades and may damage the turbine. Therefore, it cannot be used for jet fuel transportation, not because of its effectiveness but because of its potential safety concern. The original DRA did not work with heavy crude, but ConocoPhillips has recently developed a DRA [23] that has proven effective for heavy crude.

Since much higher flow rate can be achieved with DRA, the flow velocity can be fast. It has to be noted that the higher velocity can also increase the surge pressure. Therefore, a check must be made to see if the existing pipeline can meet the new tran- sient pressure requirements.

As the DRA is injected into the pipeline section, the throughput increase takes place slowly because the increased rate is linearly dependent on the flow velocity.

The desired throughput can be achieved only after all the liquid in the pipeline section contains the required DRA concentration. For example, it would take about 10 hours to reach the intended throughput in a 100 km section if the flow velocity is in the order of 10 km an hour.

As discussed in the previous section, DRA is sheared as it moves through pipes and particularly pumps. In other words, it has to be injected at every operating pump station. Therefore, it can be very costly for a long pipeline with short pump station spacing.