Steam Distribution Metering Solved
Pacific Klamath Energy (PKE) operates a 500 MW electric co-generating station for the City of Klamath Falls, OR, providing electricity for about 450,000 residents. The plant also serves as steam host to a local wood products manufacturing facility that employs about 100 people. PKE is a subsidiary of PPM Energy, based in Portland, Oregon. PPM Energy is also involved in natural gas resources, wind, and solar energy projects throughout the U.S.
The co-generation station is located on 25 acres in Klamath Falls next to the boiler facility. The plant’s co-generation source, which provides greater than 95% reliability, is augmented by three different steam systems.
Problems with turndown range
Providing accurate and reliable steam consumption data for the steam host is necessary to ensure accurate billing with the co-generation facility and other users. Unfortunately, the previous differential pressure flow metering system could not measure the low end of the steam flow scale without major changes to the piping and purchasing a new insert probe. On average, steam flows into the co-generating facility at a rate of 70,000 pph, but it can peak at 150,000 pph. Steam conditions are controlled to 320 psig with 10 °F of superheat or greater at the customer’s delivery point about 1 mile away.
Sensor improves low-end accuracy
To solve the problem, PKE contacted McCrometer about its V-Cone flowmeter, which also relies on differential pressure (DP) technology but uses a more sophisticated device inside the pipe. After examining the application, McCrometer recommended replacing the existing instrument with a V-Cone design. Due to its compact configuration, changing to the new unit required minimal changes to the existing piping.
With accuracy of +0.5%, and repeatability of +0.1%, over a 10:1 flow range, the V-Cone typically requires only straight pipe equivalent to three diameters upstream and one diameter downstream from the sensor for accurate flow measurement. Nearly all other types of flowmeters require as many as 10 pipe straight diameters upstream and 5 pipe diameters downstream to prevent swirl and other flow-profile disturbances from affecting measurement accuracy.
The V-Cone sensor typically reduces plant real estate needs, piping material, associated pipe support structure and installation labor when compared to other sensing technologies. This is particularly helpful when dealing with large pipe diameters. For example, when a V-Cone is installed in a typical 36-inch steam process line, it requires at most 148 in of straight pipe. Other technologies could require 360 in upstream and 180 in downstream.
Unlike traditional DP instruments, such as orifice plates and Venturi tubes, the V-Cone design is typically more accurate because the flow-conditioning function is built into the basic instrument. The V-Cone conditions fluid flow to provide a stable profile that supports accurate measurement. The centrally located cone interacts with the fluid flow and reshapes the velocity profile to create a lower pressure region immediately downstream.
The pressure difference, exhibited between the static line pressure and the low pressure created downstream of the cone, can be measured via two sensing taps. One tap is placed slightly upstream of the cone and the other is located in the downstream face of the cone itself. The pressure difference can then be incorporated into a derivation of the Bernoulli equation to determine fluid flow rate.
The cone’s central position in the line optimizes the velocity of the liquid flow at the point of measurement, forming very short vortices as the flow passes the cone. These short vortices create a low-amplitude, high-frequency signal for signal stability, resulting in a stable flow profile that is repeatable for continuously accurate flow measurement.
Precision with minimal disruption
McCrometer matched both ends of the flowmeter to the existing steam system piping dimensions and provided multiple taps for flow readings. Once installed, the device provided guaranteed flow accuracy of +0.5% that the customer could verify using the same calculations as the initial factory calibration. The installation and third-party calibration was accomplished on time and under budget.
|Bruce Willard is a plant engineer for Pacific Klamath Energy. Reach him at email@example.com|