Coefficient of Discharge must be controlled during production.

To produce accurate, repeatable orifices, all the variables that might influence the Cd Value (Coefficient of Discharge) must be controlled during production. This includes the orifice hole length, edges, surface finishes, roundness and the elimination of all tool marks, burrs, ragged edges and irregularities. If any one of these areas is not perfectly managed, the orifice flow rates will vary from piece to piece thereby making it impossible to predict flow with any accuracy (Figure 1).

Figure 1 illustrates that changing the edge profile of a Sharp Edge Orifice will result in large fluctuations in Cd values. A True Sharp Edge Orifice has a Cd value of .61. Introducing a chamfered edge can boost Cd to as much as .9, and introducing a radius may result in Cd’s up to .98. This illustrates how significant small alterations in orifice edges can be.

Figure 1: Source: Design Engineers Handbook, Parke Bulletin 0224-B1, Pg. g-9

Chamfers and radius’s are even more difficult profiles to repeat in production. Chamfering or radiusing the orifice edge introduces more variables than a simple sharp edge. In a sense, you have added another set of edges and surfaces into the equation. As a result, the .9 and .98 values will be hard to maintain repeatedly. Sharp Edge Orifices with a Cd of .6 are preferred by engineers because they are more consistent if produced correctly.

Precision machine drilling of perfect Sharp Edge Orifices is next to impossible. Drilling inherently can be a brute process of cutting metal to generate a hole. Drills leave behind spiral tool marks, burrs, poor surface finish and out of round or elliptical holes due to drill wobble.

If the burrs are removed by chamfering then, as illustrated above, the Cd value is altered. If they are not removed, then you have ragged edges. It is a Catch 22. Secondary procedures, such as electro polishing, while helpful, cannot remove all the irregularities that are created by the drilling process.

Because of the pitfalls of drilling, many manufacturers resort to individually flow checking each orifice. This can be labor intensive and expensive. The smaller the orifice, the more and more difficult it becomes to produce large quantities of repeatable orifices.

Keeping an orifice sharp after it is installed is also a key factor, especially in high-pressure gas and liquid applications that can wear down the effective edge.

If the instrument has been calibrated, it may now significantly under value the flow once the edge begins to radius from wear or damage. This is why orifices need to be made of hardened steel or other durable materials to reduce the wear factor. Making a True Sharp Edge Orifice and keeping it is paramount.

True Sharp Edge Orifices are successfully being produced economically by a nontraditional three-step process using a combination of laser drilling and wire lapping in conjunction with surface lapping. Ruby is the material of choice, since it is next to diamond in hardness yet economical. This is a man-made crystal grown from alumina powder, which is melted at high temperature to crystallize on a seed crystal to form a single crystal of either ruby (which is red because of a chromium .05 percent dopant) or clear sapphire (no dopant) that is almost chemically inert, 9 mohs hardness (diamond being 10), with five times the abrasion resistance of carbide.

Some applications in water jet cutting approach 40,000 psi using sapphire orifices. Where other materials, such as hardened steel, would quickly erode away, the sapphire keeps its edge under such extremes for longer duration.

Figure 2: illustrates the ruby blanks being vibratory fed to the laser for piercing. This is the best method for quickly getting through this hard material. (Photo courtesy Hrand Jeva Suisse by Paul Dubois).

Figure 3: illustrates how the pierced ruby blanks are strung thousands at a time on long tapered wires. The wires are then connected to two large spools. The spools slowly advance the taper in an oscillating motion through the strung rubies using fine diamond slurry until the exact size orifice is achieved. The resulting holes are all identically round. Imparted from the roundness of the wire, they all will have a 2 micro inch polished surface finish or better. Tolerance within a lot will vary less than .0001”, and overall tolerances of .0002” are the norm.

The final finishing operation is face lapping both sides of the orifice to both control the orifice throat length as well as achieving the desired sharp edges. The faces will also have 2 micro inch finish. The result of this three-step process is a True Sharp Edge Orifice.

The Cd value will always be the same, since all the variables that could influence it are controlled. The orifice edges will remain sharp longer in applications of high pressure, chemical or abrasive conditions. See Figure 4: .0100” Orifice 440X Power

Thousands of cloned Sharp edge Orifices are simultaneously and economically produced in sizes from .0016” and up. They are easily inserted into a variety of stainless, brass or plastic fittings, connectors or inserts.

Finding application in analytical instruments, gas chromatographs, gene splicing tools, medical gas metering, chemical metering, sand blasting nozzles, critical flow restrictors for sampling instruments, pace makers, anesthesia orifices, nitrous oxide boosters, ink jet printers, leak detection masters, water jet and various pneumatic regulator and hydraulic restrictor applications. See Figure 5.

For more information visit www.birdprecision.com