Comment

01 April 2004

Quantify cost savings

By Mark Menezes

New flow technologies and approaches that allow users to minimize installed costs are here along with software that allows users to figure the savings in their own specific installations.

Here are three technology schemes:

Conditioning primary flow elements that minimize straight pipe: With few exceptions, accurate flow measurement requires a fully developed flow profile, which usually means straight up- and downstream piping around the flowmeter. In the case of orifice meters, the straight pipe requirement can be 30 diameters or more, depending on the beta ratio and the upstream disturbance. Alternatively, users can install a flow conditioner—typically, a tube bundle that straightens the flow, but this increases both capital cost and permanent pressure loss with its associate increases in pumping cost.

Recently, suppliers have introduced primary flow elements that not only measure but condition the flow, minimizing the need for straight pipe. For example, one company has introduced a conditioning plate that provides up to 0.5% accuracy with only two diameters straight pipe for most upstream disturbances. This technology is available in both a traditional paddle-type plate and integrated with a manifold in a wafer-style compact orifice configuration.

Most control valves are located at grade, to make them accessible for maintenance. The new conditioning plate will allow users to install their orifice meters two diameters upstream of these control valves. This will make the orifice plate accessible for maintenance and eliminate the need for long impulse tubing to the transmitter, reducing cost and improving reliability.

Size vortex like an orifice plate with reducer flexibility: Vortex meters offer significant advantages over orifice meters in many applications, especially where wide turndown is required, and where high reliability and safety are key. One key advantage of an orifice meter is its flexibility. If the flow range used to size the meter is incorrect, or if it changes over time, the user can easily accommodate the new flow range, within limits, by reranging the differential pressure (DP) transmitter, and possibly replacing the plate. With a vortex meter, the user would not only need to change the entire body—even this is not too expensive, because presumably the old meter can be reused in another application—but change the piping. The vortex meter requires straight up- and downstream piping, and this piping must be of the same dimension as the meter itself. In most applications, this requires a minimum twenty-four-hour shutdown: remove insulation, cut pipe, weld reducers and expanders, X-ray welds, reapply insulation, install new meter.

With a reducer vortex, the supplier welds on reducing and expanding flanges and in a flow lab calibrates out any impact on accuracy. So to replace an existing meter with a reducer meter (or vice versa), the user could simply replace the meter—no changes to piping.

Integrated flowmeters minimize cost and risk: To obtain a DP flowmeter, the user typically needs to specify, procure, install, and commission a variety of pieces and parts from several different suppliers. This results not only in high cost, but also high risk. If even one part is late or the engineer incorrectly specifies or a technician errantly installs, the entire system will behave poorly. To minimize these costs and risks, some users are adopting "integrated flowmeters." Using supplier software, they specify and validate the entire system, which arrives preassembled, calibrated, and leak tested, ready to bolt on between user flanges.

In many cases, the acquisition cost of the integrated flowmeter will be greater than the sum of the individually procured parts—the benefits come from reduced labor costs, faster start-up, and lower risk. The Total Installed Cost spreadsheet shown can help quantify these savings. It includes default labor times and labor rates from Richardson's Handbook, but allows users to edit any parameter to better suit their specific application and situation.

Because all of these new technologies and approaches minimize installed cost and risk, they provide the greatest benefit during a project. However, users are rarely willing to incur the risk or invest the time needed to evaluate new technology and approaches during project execution. For that reason, the best time to try out, learn, and validate these new technologies and approaches is well in advance of the actual project. DT

Behind the byline

Mark Menezes has degrees in chemical engineering and business. He has fourteen years of experience in process automation and is a past vice president of the Toronto ISA Section. He works at Emerson Process Management as the measurement business manager for Canada. Write him at mark.menezes@emersonprocess.com.