Orifice primary flow elements, flange unions and holding blocks

Offering lower capital investment and ease of replacement


The orifice plate is one of the most common differential producer type flow elements for measuring clean liquids, gases, and low-velocity vapor (steam) flow. This design offers the optimum flexibility for applications with many diverse existing piping schemes, as well as considerably lower capital investment for the initial purchase of equipment. The orifice plate also has the advantage of relatively easy and low-cost replacement for changes in the nominal flow rates in a given application or when accuracy deteriorates.


The operation of this flow element is based upon the Bernoulli principle, and relies upon the comparison of the pressure upstream and downstream (differential pressure) of the constriction represented by the orifice plate bore to infer the rate of flow of the line fluid through the bore diameter. Unlike other differential pressure flow elements, such as Venturi type meters, where the line fluid upstream and downstream of the constriction are generally and variously conditioned, the energy consumed by the line fluid flowing through the orifice metering section is considerably higher.


By application of and reference to various codes, the performance of orifice plates are quantified and published for the benefit of users. Generally, the accuracy attributable to the orifice plate meter is between +/- 1.0% to +/- 5.0% of full scale.


Of the many design varieties described in technical manuals, codes and standards, the most prevalent configuration in the United States of America is the square-edged concentric paddle type orifice plate, in combination with flange tapped carriers. The accuracy of the orifice plate is directly contingent upon the presence of the sharp edge in the orifice bore. Any deterioration to this bore edge will cause immediate and unpredictable loss of accuracy.


Properly applying this technology and sizing the orifice flow element for a specific application requires proper analysis and consideration of the installation conditions by a knowledgeable resource expert in the field. Primary Flow Signal has an experienced staff of application engineering professionals ready to assist you with any flow measurement needs. Primary Flow Signal is a world-class designer, engineer, and manufacturer of flow meters and flow measurement accessories, with a reputation for high quality products and services.


Consistent with the company’s leadership position within the industry, our line of orifice plates and carriers represent the best in equipment quality, service and support in the marketplace. The design and manufacture of the paddle type orifice plates provided by Primary Flow Signal meet or exceed the standards set forth by the A.G.A., the I.S.A., the A.N.S.I., the A.S.M.E., and A.P.I. recommendations.


Each orifice plate is manufactured with strict adherence to ISO 9001 certification standards thereby applying numerous inspections at key points in the production process, assuring thereby that the “squareness” and “sharpness” of the bore edge remains well within the tolerances permitted by the above referenced codes and standards.


Every plate is stamped with identification data including material grade, bore size, line size, flange rating, and tag number. Typically, material selection for standard application orifice plates are either 304 or 316 stainless steel. Other materials, including but not limited to Monel, Hastelloy, titanium, and brass are available to accommodate special service requirements. Additionally, all equipment is available cleaned for oxygen service, if required.


Typical components of an orifice metering assembly:
While the orifice plate is the key element that by virtue of precise machining to a specific bore diameter creates the desired differential pressure at a specified flow rate, several other components are required to assemble a complete flow meter for service. In installations other than the integral orifice design, the orifice plate is held between a set of flanges, known as an orifice flange union, separated by suitable gaskets.


These flange unions are specially machined to include the appropriate pressure tap sets that in conjunction with the orifice plate, will provide a means of obtaining the high and low pressure signals and thus the differential. The signals are then typically piped into the secondary instrument of choice, most likely a differential pressure transmitter. The transmitter will typically convert the differential into a 4 - 20mA output that can then be used with process control instrumentation and/or local indication. The specific parameters of a proposed installation will generally dictate the type and nature of the components that will provide the best solution.


Contact us for expert installation and application assistance and support.


Paddle-type orifice plate typical key dimensions:
Primary Flow Signal, Inc. offers standard paddle type orifice plates in accordance with the following dimensional tabulation, offered here for your general reference. Specific dimensions and tolerances are confirmed only on the basis of an official Primary Flow Signal, Inc. submittal presented as an integral part of an actual purchase of equipment from Primary Flow Signal, Inc.


Any and all data provided herein is for general information purposes only and as such may be changed, amended or deleted without prior notice by Primary Flow Signal, Inc. Please refer to figure 3 at right to identify key typical paddle type orifice plate dimensional attributes provided in table 1 on page 3 of this bulletin. All orifice bore dimensions “B” are determined by customer specification, or calculated by Primary Flow Signal, Inc. based on application data furnished by customer.


Orifice flange unions:
The orifice plate provides the means of creating a differential pressure, however the flange union, in most cases, will provide the means of securing the orifice plate in the line, as well as provide the tap set(s) to allow the pressure signals to be extracted from the line, and using suitable impulse piping (the piped connection between flow element and secondary instrument) transfer this signal to an appropriate differential pressure transmitter that then compares the signals, determines the differential pressure and further processes the signal inferring flow rate and other appropriate signal transmission or reporting, as required.


The flange union is available in various end connections, and can be manufactured using materials of choice. Standard service for nonharsh environments can be accommodated using carbon steel with piezometer taps sleeved with stainless steel. If demanded by the application, the flange unions can be produced in various grades of stainless steel. For severe service, the flange unions can be manufactured in Monel, Hastelloy, titanium, brass, or nickel to mention a few choices. Naturally, as the nature of the material selected for the flange union becomes increasingly exotic, so increases the acquisition cost of these components. Please review Orifice Holding Blocks (below) to find less expensive severe service flange union solutions.


Orifice flange holding blocks:
Orifice flange holding blocks provide an excellent solution, especially for severe service requirements. The singular advantage with this solution results from using the existing pipe flanges, and simply inserting the holding blocks between which the orifice plate, suitably gasketed can be secured. By so doing, even exotic materials can be provided for special service applications without the high costs associated with fabrication of entire flanges in expensive materials that would conventionally be indicated. For dimensional guidance, please examine figure 6.


The distance between the center of the piezometer tap centerline, and the face of the paddle type orifice plate (including the thickness of the gasket) is 1.00 inches in all cases. The laying length of the holding block is 1-⅞ inches in all cases. Otherwise, the outside diameter of the holding block is identical with plate dimensions D as indicated on figure 3 and “pipe bore” is matched to customer specified pipe I. D. depending on existing process piping. The installation protocol for holding blocks stipulates proper gaskets be used upstream and downstream of each block.


The centering of the orifice block with respect to the centerline of the existing process piping must be performed with the same degree of care as with the orifice plate itself. As such, the holding blocks and orifice plate are held concentrically and, entirely within the bolt circle of the mating process flanges. No gasket material is allowed to protrude into the ID of the pipe line. The torque applied to the flange bolting must be consistent with standard piping protocol and be sufficient to prevent leakage of the line fluid from any of the mating surfaces of the flanges and metering components.