Lower Pressure Drops Translate into Energy Savings

by | Oct 23, 2009 | Measurement Instrumentation

Jim Cahill

Chief Blogger, Editor

In an earlier post, I recapped a podcast on ultrasonic flow meters and their use in custody transfer applications. Gerard Hwang and Dave Seiler, at Emerson’s Daniel Measurement and Control business shared an interesting energy analysis with me. It was an energy calculation for various flow measurement technologies in a liquid (oil tanker) loading application.

Typically, liquid loading applications such as crude oil custody transfer between parties, involve large volumes and sizable flow rates. It takes energy to pump these liquids from their source to their destination. Any pressure drop caused by the flow measurement can be directly translated into an energy cost.

In addition to diagnostics around fluid flow phenomena (e.g., cross-flow, asymmetry and swirl) which can increase flow measurement uncertainty, a fundamental design advantage ultrasonic flow meters have in high-volume flow measurement applications is that they are “full-bore” meters. This means that there are no restrictions, internal obstructions, or bends, which will cause a pressure drop across the meter–the loss incurred is that of an equal length of straight run pipe.

The example was from a large U.S. pipeline company, which transports oil, gas, and refined petroleum products. In this case, the liquid was crude oil and its viscosity was 23.3 centipoise (cPs). The flow rate ranged from 1,800 to 22,000 barrels per hour (BPH), and the crude oil had a specific gravity (S.G.) of 0.86. The maximum design pressure was ANSI 300# and a temperature range from -10 degC to 40 degC.

At maximum flowrate, a 16″ ultrasonic flow meter resulted in a 0.12 psi pressure drop. Other flow measurement technologies required multiple meters in parallel to accommodate the same volumetric throughput and caused any where from 5-12 psi drop. This multi-meter design also increased the number of block and control valves required around the meters.

The energy cost calculation for loading operations 90 days per year at 22,000 BPH is:

(0.12psi x 22,000BPH / 2,448*) x $0.07 USD/KWh x 24 hours x 90 days = $163

*2,448 is a conversion factor from PSI and BPH to hydraulic horsepower (HP) divided by the pump and motor efficiency to get electrical HP multiplied by the conversion from HP to KW to get power.

In other words, the cost of the energy loss from pressure drop across the ultrasonic flow meter is almost negligible. With the differential pressures across other flow meter technologies, energy costs range from $6,800 to over $15,000 per year.

Now if you multiply this cost by all the flow meters performing these large custody transfer applications in your organization, daily, these energy cost savings add up to reduce operational costs.

Update: I received some feedback on the post and cleaned up a few of the paragraphs above.

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