VOLUMETRIC METERS FIND DONALD J . KULLMANN
The so-called water meter can accurately measure total volumetric flow
of
clean streams
in many chemical processes uring the past 10 years, there has been a sizable in-
D crease in the number and variety of liquid-measuring meters that have become available to the industrial user. The selection now includes sonic and magnetic meters, mass flowmeters, velocity, turbine, and volume meters-all of which have been somewhat loosely described by the term flowmeters. Each of these devices has its proper place in industrial chemical metering applications. Yet the very novelty of some of the newer metering instruments, and perhaps the complexity of their design, has tended to make some industrial users forget the time-proved workhorse of the metering industry. This workhorse is the positive displacement, or volumetric, meter. A typical example is the water meter, which provides long, reliable service in millions of homes, commercial establishments, manufacturing plants, and other installations throughout the world. According to industry estimates, the number of water meters used in the United States alone exceeded 26 million in 1760. Most people take their water so much for granted that they overlook the features that have gained it such widespread acceptance. These features are typical of volumetric meters and are worth considering when choosing a meter for an industrial application. 34
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Furthermore, in recent years, great strides have been made in developing volumetric meters especially designed for industry and equipped with remote instrumentation and a variety of accessories or controls. The modern chemical plant can, for example, monitor volumetric metering systems from central reading stations or handle batching operations by remote control. Other applications include additive proportioning, ticket printing, and automatic control over cyclic metering operations such as water conditioning. Materials used in these meters-e.g., stainless steelalso are being chosen with the industrial market in mind. And design refinements, such as the magnetic drive, have enabled chemical and paper-coating plants to meter succcssfully such hard-to-handle liquids as latex and clay solutions. The so-called water meter, in other words, has reached a new stage of maturity and found a home in the chemical industry. Deflnition and Description
A positive displacement or volumetric meter may be defined as “a liquid volume-measuring device which, by positive or capillary means and powered only by the liquid stream, divides a flowing liquid into equal segments, integrates the segments, and provides a true total of the volume measured.” Meters of this type are generally divided into four classifications : -Nutating disk (or wobble plate) -Oscillating piston -Rotary vane -Reciprocating piston
NEW INDUSTRIAL USES
While all of these meet the broad definition, this article will be devoted primarily to the nutating disk and oscillating piston meters. These are the only two types which have all the following fcatures : --Simple
construction
---Good over-all accuracy -Wide useful range --Self-powered operation -Variety of accessories -Low cost -Ease of maintenance -Durability and long life -Availability of service parts Simple Construction
T h e nutating disk and oscillating piston meters are complete, self-contained, metering packages. They have a housing to contain the liquid, a measuring chamber and sensing element, reduction gearing, and a readout device. Details of typical meters are shown above. While variable head, variable area, and obstructionless meters appear at first glance to contain fewer parts, they are not complete volumetric devices until integrating and readout means are added. Adding these electric, electronic, or electromechanical accessories creates a complex package. T h e nutating disk and oscillating piston elements need no external bearings, as are required with rotary vane
AUTHOR Donald J . Kullmann is a development engineer for
types, and they eliminate the complex valving gear of reciprocating piston meters because they are self-valving. Mass-flow meters yield a rate output and require rather complicated external integration, resulting in a more complex and costly package. Where true volumetric or linear total output is desired, nutating disk and oscillating piston meters are the ultimate in simplicity. High Over-ail Accuracy
One of the prime considerations governing the choice of a liquid meter is the degree of accuracy obtainable. The term “accuracy” in liquid metering is of primary importance and can be misleading unless fully understood. Where volumetric readings are desired, it is important to remember that the true accuracy of the entire package is the on!y one that really means anything. When several units or components must be combined to yield a volumetric reading, the errors in each component have a cumulative effect on the over-all accuracy. The prospective meter purchaser should scrutinize accuracy claims carefully because they can be quite misleading due to the varying definitions and meanings of the terms used. For one thing, accuracy is commonly specified as 1 or 2%. Obviously what is meant here is percentage of error, not percentage of accuracy. This may seem rather elementary but this misuse of terms is common. What is important to the meter user is the real accuracy, or percentage error. As a case in point, consider two meters, one having an error of 1% of reading,
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NEW INDUSTRIAL USES
While all of these meet the broad definition, this article will be devoted primarily to the nutating disk and oscillating piston meters. These are the only two types which have all the following features : -Simple construction ---Good over-all accuracy -Wide useful range -Self-powered operation -Variety of accessories -Low cost -Ease of maintenance -Durability and long life -Availability of service parts Simple Construction
The nutating disk and oscillating piston meters are complete, self-contained, metering packages. They have a housing to contain the liquid, a measuring chamber and sensing element, reduction gearing, and a readout device. Details of typical meters are shown above. While variable head, variable area, and obstructionless meters appear a t first glance to contain fewer parts, they are not complete volumetric devices until integrating and readout means are added. Adding these electric, electronic, or electromechanical accessories creates a complex package. The nutating disk and oscillating piston elements need no external bearings, as are required with rotary vane
AUTHOR Donald J . Kullmann is a development engineer for
types, and they eliminate the complex valving gear of reciprocating piston meters because they are self-valving. Mass-flow meters yield a rate output and require rather complicated external integration, resulting in a more complex and costly package. Where true volumetric or linear total output is desired, nutating disk and oscillating piston meters are the ultimate in simplicity. High Over-all Accuracy
One of the prime considerations governing the choice of a liquid meter is the degree of accuracy obtainable. The term “accuracy” in liquid metering is of primary importance and can be misleading unless fully understood. Where volumetric readings are desired, it is important to remember that the true accuracy of the entire package is the only one that really means anything. When several units or components must be combined to yield a volumetric reading, the: errors in each component have a cumulative effect on the over-all accuracy. The prospective meter purchaser should scrutinize accuracy claims carefully because they can be quite misleading due to the varying definitions and meanings of the terms used. For one thing, accuracy is commonly specified as 1 or 2y0. Obviously what is meant here is percentage of error, not percentage of accuracy. This may seem rather elementary but this misuse of terms is common. What is important to the meter user is the real accuracy, or percentage error. As a case in point, consider two meters, one having an error of 1% of reading,
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and the other an error of 1% of full scale. These are the two common methods of specifying accuracy. For purposes of comparison, assume that both meters have a flow rate range between 10 and 100 gallons per minute. The meter rated with an error of 1% of reading w7ill maintain a maximum 1% error at all rates of flow from 10 to 100 gallons per minute. However, the meter rated with an error of 1% of full scale may have a 2% error at 50 gallons per minute and a 10% error at 10 gallons per minute. From this it is obvious that, while the percentage error may appear the same in print, it is vastly different in actual meter operation. The accuracy (or percentage error) of nutating disk and oscillating piston meters is always expressed as a percentage of the reading and is, therefore, the true accuracy over the entire useful range of the meter. Wide Accuracy Range
Not only do nutating disk and oscillating piston meters have the highest accuracy of all types of liquid meters, but they also have the ability to retain .their accuracy over the widest useful range. Accuracies within a n
LOW
CQSt
Nutating disk and oscillating piston meters cost less than any other type of meter. There is an added cost advantage when the self-powered feature can be used for local or remote totalization. Once again, it should be noted that meters yielding a flow rate require additional integrating and readout equibment, to provide the totalized output produced automatically by nutating disk and oscillating piston meters.
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distances up to one fifth of a mile without batteries or external power of any type and does it at extremely low cost. An industrial plant could use it, for example, to read a great number of meters from one central location. Other standard accessories include devices used in conjunction with the meters to adapt them to specialized operating conditions. These include meter-operated switches, meter-actuated single and multistage shutoff valves powered by mechanical, electrical or pneumatic means: air eliminators, temperature and density compensators, and electromagnetic pulsing switches for coded telemetric output to automatic data processing centers. Figure 1, page 39, shows four types of metering equipment for batching operations. Most meter manufacturers have additional accessory equipment under development and are constantly exploring special applications of existing devices to broaden meter uses. Some of the typical totalizers and standard accessories which are available are shown in the chart opposite.
-I 2
3
UTE of FLOW, GAL.
m
MIN.
error of only 1% are readily obtained over flow rate ranges of 10 to 1. This is a typical accuracy curve for an industrial oscillating piston meter. Self-Powered Operation
Nutating disk and oscillating piston meters provide instantaneous total volume readings without external power. All power required for the sensing, integrating, and totalizing functions of these meters is furnished by the fluid stream passing through the meter. Variety of Accessories
Accessories available for these meters span the field of instrumentation and can be mechanically, electrically,
pneumatically, hydraulically, or self-powered. Functions of these accessories include local and remote totalization, ticket printing, demand recording, single and repetitive batching, additive proportioning, and mechanical control outputs. Combinations of these functions are also used in some cases. Badger Meter Manufacturing Go., for example, offers a device which can be mounted on volumetric meters to provide self-powered remote electric indication. This system gives instantaneous remote totalization at
Ease of Maintenance
Any discussion of costs should, of course, also include maintenance expenses, which can be quite a factor when complex accessory equipment is involved. Cost is not always the primary consideration when choosing industrial equipment, but it can hardly be ignored. The simple construction and lack of complex external readout equipment provide a bonus in the area of maintenance. A properly installed meter constructed of well chosen materials normally will require only a minimum of maintenance. Routine maintenance. when needed, will usually consist of disassembling and cleaning the measuring chamber and disk or piston. In extremely severe service, it may be necessary occasionally to replace the disk or piston. These routine procedures can be performed by the users’ own service people with very little instruction and training. Some meter manufacturers make special models for hard-to-handle or corrosive solutions used in industrial applications. These contain magnetic couplings to remove the reduction gearing from the liquid being metered and to isolate the liquid entirely within the measuring chamber. The measuring chamber can be quickly cleaned by removing several thumb screws on the cover plate and lifting out the piston, magnet assembly, and control roller, as shown at the front of this article. (Continued on next page) VOL. 5 4
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Many meter components are available to fit specialized uses
Durability and Long Life
Oscillating piston and nutating disk meters, when constructed of the correct materials and properly installed, have an extremely long service life. It is not unusual for the household water meter to be in continuous round-the-clock use for 20 years or more. This durability is the result of simple, rugged design combined with materials that are correctly engineered and applied to suit the application. T o a large extent, this know-how in meter manufacturing is the inevitable result of long experience in this field. Most manufacturers of nutating disk and oscillating piston meters are old-line American companies with a background of 50 to 75 years in the meter business and the experience to design equipment for specific industrial needs. In most instrument applications, the ratings and materials used are important factors in determining whether the equipment will operate effectively. Positive displacement meters are instruments, of course, and must be properly engineered for specific applications so the user can realize the full advantages of the long service life inherent in this type of device. Availability of Service Parts
No matter how carefully any product is designed, built, and applied, it will inevitably require service parts. In industrial applications, the need for such parts usually is immediate and urgent. Manufacturers of nutating disk and oscillating piston meters have nationwide distribution networks to expedite parts orders to customers. Some industrial applications require custom-made parts constructed of special materials. In such cases, the meter manufacturer may advise the customer to carry a small stock of selected parts so there will not be a prolonged interruption of metering service. Special Design Features
As mentioned previously, industrial versions of nutating disk and oscillating piston meters are designed to capitalize on the accuracy and durability inherent in these measuring instruments. T h e measuring chamber, or heart, of the meter can be strengthened or otherwise modified to meet the demands of industrial use. The chambers, while rather simple in appearance, are extremely critical to manufacture and are machined to exacting tolerances. Interiors of these elements are usually machined and gaged on highly specialized custom made machinery. Operating clearance between the interior of the chamber and the measuring disk or piston is determined and controlled by many factors. For example, mate38
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
rials used in the measuring elements are governed by the characteristics of the liquid being measured such as viscosity, lubricity, pH, and corrosive properties. Tolerances arf: then determined according to the tempe;ature range of the application. Some industrial meters now utilize a magnetic coupling to transmit the output of the measuring element through the wall of the meter housing to drive a dryrunning output gear train. This design reduces friction and eliminates the eventual leakage that occurs in meters with packing glands. T h e magnets used in the coupling usually are constructed of inert ceramic materials which have permanent high energy properties and produce a synchronous coupling with relatively high pullout or slip torque. T h e gear trains are of prime quality materials to provide long and trouble-free life under full-load conditions. Meter housings or cases are especially designed for industrial use and include a variety of piping and accessory mounting options. Some features which were rather specialized a few years ago are now considered standard for certain applications. These include all-stainless steel construction (with certain plastic parts) to meter corrosive, complete analysis, liquid fertilizers. This construction also is suitable for a wide variety of chemicals. Minor changes in materials, plus the addition of vapor control valves and lines, have resulted in almost universal use of oscillating piston and nutating disk meters for liquefied petroleum gas measurement by volume for commercial and retail delivery. Where required, these meters are designed to conform with API standards, NBS specifications, and Underwriter Laboratories’ listing requirements. These are but a few of a wide variety of special features designed into these meters to meet the broad and diversified requirements of industrial and chemical applications. A complete picture of the special design features presently available may be obtained from the various manufacturers’ bulletins and catalogs. Highly specialized or unique requirements can best be evaluated by contacting the manufacturers directly or through their industrial representatives. General limitations
Based on the features and capabilities already enumerated, it is quite apparent that nutating disk and oscillating piston meters have a definite place in industry. Like any other instrument or measuring device, however, their design imposes some limitations for certain specific applications. As noted previously, these meters have the best overall accuracy of any type available. But the tight clearances and precision tolerances of the measuring elements restrict them generally to relatively “clean” liquids-
C
A
D
B
although the use of a magnetic drive now makes it possible to meter viscous solutions such as latex. Liquids which tend to “salt out” or become granular can be metered if adequately controlled to prevent these undesirable effects. In general, liquids containing entrained solids or foreign matter can not be metered unless they are thoroughly filtered or strained. Because these meters measure true volume, additional equipment is necessary when the application involves highly volatile liquids such as anhydrous ammonia and liquefied petroleum gases. Liquids of this type can be metered successfully, but vapor separators, check valves, and vapor return lines are required to maintain a completely liquid stream. And, of course, there is a limitation that is common to any type of meter-the effect of long-term wear on meter accuracy. In nutating disk and oscillating pis-
Here are four types of equipment f o r batching operations with volumetric meters. They include: A , meter with strainer, air release, and back pressure valves, and a batching register with ticket printer; B, electrical repeating batch register to actuate a valve or pum$; C, remote controller which prooides pushbutton control over batching or mixing operations at another location; and D , meter with a register for presetting the batch and equipped with a multistage quantity control valve
ton meters, this limitation will involve wear of the measuring element. The temperature range of the liquids being metered also has a bearjng on accuracy. If the temperature range, the coefficient of expansion of the liquid, or both are high enough, they will affect the volumetric accuracy. However, meter manufacturers now have automatic temperature correction or compensation devices available as meter accessories. These automatically correct the meter reading to room temperature or similar basic thermal conditions. Under certain conditions, other factors such as lack of lubricity or extremely low viscosity may appear to present limitations. If volumetric readings are desired under these conditions, the makers of oscillating piston and nutating disk meters can often develop a modified design to handle the intended application. VOL. 5 4
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