Sorting out the truth about pneumatic standards

Oct. 11, 2001
Japanese test methods better predict pneumatic valve performance.

N. Oneyama
Manager Technology Research Div.
SMC Corp.
Indianapolis, Ind.

Edited by Kenneth Korane

SMC follows the latest ISO standards when rating pneumatic valve flow in terms of sonic conductance.

Full stroke time of a pneumatic cylinder is inversely proportional to the maximum exhaust flow rate.

Effective area S represents the maximum constant flow at choked flow. It also expresses valve size in direct relation to the cylinder drive speed.

ISO 6358 and JIS B 8390 express valve-flow characteristics in terms of sonic conductance C and critical pressure ratio b.

CV provides water flow rate through a valve with a 1-psi pressure differential. Gas-flow measurements require a density conversion factor.

Measurements on solenoid valves using ISO test equipment show that maximum flow rate varies with valve construction and critical pressure ratio, even if CV is identical from valve to valve.

CV(ANSI) presents only one point on the flow-rate curve at the extreme end of the subsonic flow region.

Confusion and misinformation seem to be the order of the day when it comes to rating pneumatic valves. For instance, in a recent MACHINE DESIGN article ("Clearing the Air on Pneumatic Valve Ratings," Oct. 5, 2000) the authors assert that flow specifications based on Japanese test standards tend to exaggerate performance. This is simply not the case.

The article claims that U.S. and European companies generally rate the flow capacity of pneumatic valves based on ISO test standards, while manufacturers in the Far East tend to use JIS (Japanese Industrial Standards) and then apply conversion factors to present the data in a more widely accepted format. The article further asserts Far East manufacturers that use the JIS method publish flow ratings that "substantially overstate actual performance."

SMC relies on JIS standards that are based on the latest ISO standards, and its flow specifications do not overstate performance. JIS and ISO standards do differ in terminology, but otherwise they are essentially the same. In fact, the JIS rating system more accurately predicts actual valve performance than do NFPA/ANSI methods. Here's why.

Effective area
To gain a better understanding of valve-rating standards, it is useful to look at how test methods have evolved over the past few decades. One important flow-rating tool is effective area, S, an expression defined by previous versions of JIS that appeared in older SMC publications.

It is based on a typical pneumatic system using solenoid valves to actuate cylinders. Researchers conducted detailed experiments to determine the most suitable way of expressing airflow characteristics that directly correspond to the drive characteristics of cylinders. Results of this research showed that the full stroke time of a pneumatic cylinder is inversely proportional to the maximum exhaust flow rate — literally, the maximum flow rate of the drive circuit.

While the system supplies compressed air to a valve at a constant upstream pressure, the ratio between downstream and upstream pressure (P2 /P1) decreases (the differential pressure increases) as the flow rate increases. Flow rate will increase and pressure ratio decrease until the system reaches sonic velocity and the flow rate stabilizes. Beyond this point, called choked flow, downstream pressure drop continues without increasing flow rate. Thus, a distinctive value for representing the maximum constant flow at choked flow (the sonic velocity flow) was established by the Japanese Industrial Standard, JIS C 9312 (1964) as a way of expressing flow characteristics of solenoid valves. Termed the effective area S, this value also expresses a valve's size (or performance) in direct relation to the cylinder's drive speed. SMC traditionally followed JIS by rating valves in terms of effective area, S (mm2).

At the same time, the process-control industry was applying the term "coefficient of volume flow" or CV to gases, after applying a density conversion factor. CV is a measure of water flow rate at a minimum differential pressure (1 psi) through the valve.

Calculating flow rate using the CV factor is comparable to calculating the subsonic flow rate using S. Mathematically, CV(ref) = S/18, and SMC began to publish CV(ref) catalog data.

Sonic conductance
In 1989, the International Organization for Standardization published ISO 6358, a standard that succeeded in expressing the entire area flow characteristics for pneumatic equipment using sonic conductance C and critical pressure ratio b. Sonic conductance represents the maximum flow rate at choked flow, and has much the same definition as that of effective area S. In fact, S = 5C.

Also, critical pressure ratio can vary from one type of valve to another, which shows that subsonic-flow characteristics depend on valve construction. Thus, ISO and JIS standards are comparable and JIS B 8390 (June 2000) is consistent with ISO 6358.

The National Fluid Power Assn. takes a different approach. Technical committee ANSI/NFPA T3.21.3 adopted CV as a standard measure of the flow coefficient in pneumatic equipment in 1990, a year after ISO 6358 was established. Unfortunately, CV (ANSI) presents only one representative flow-rate value, and calculates it by using a small differential pressure (pressure ratio 0.98 to 0.99) at the extreme end of the subsonic flow regime.

Correlating C and CV
The best way to get a handle on different rating methods is through actual testing. Results of C and CV(ANSI) measurements taken from many solenoid valves using ISO testing equipment (similar to ANSI/NFPA), shown in the accompanying graphic, indicate that the ratio of 5C/CV(ANSI) is "large" if the valves have a small b value, and "small" if b is large. It is not identical. The reference conversion S/CV(Ref) = 18 which had been applied by SMC corresponds to valves with b = 0.3 to 0.5.

Therefore, maximum flow rate can vary with valve construction even when different valves have identical CV(ANSI) ratings. That is because CV(ANSI) does not correspond exactly to C (or S).

A good example is to look at the characteristics of an actuating cylinder driven by valves with identical CV but different b values. Characteristics of an actuating cylinder shows flow behavior of three solenoid valves, all with measured CV(ANSI) = 0.2. The maximum flow rate differs more than 30% between valve A with b = 0.1 and valve C with b = 0.5.

The response time is also significant when the same three valves connect directly to a cylinder. The heavy line in the lower figure expresses piston-rod displacement and pressure at each side of the cylinder for valve C. Lines for valves A and B indicate rod displacement only. This shows that a valve with a larger maximum flow rate (A) permits quicker cylinder response. Even when ANSI methods measure the same CVs it is apparent there are big differences in a cylinder's full stroke time. This clearly shows that CV alone is insufficient as a selection criterion for specifying components in a pneumatic actuating system.

Because ISO 6358 offers the most accurate way to express flow-rate characteristics, JIS B 8390 adopted this definition in June 2000. Since the beginning of 2000, SMC has been testing the flow-rate characteristics for all series of valves.

To help engineers specify products, SMC now publishes measured sonic conductance C and critical pressure ratio b as defined by ISO 6358 (and JIS B 8390). Also, as the need for reference conversions has diminished, SMC now catalogs CV(ANSI) and the JIS measure S as secondary measurements.

Engineers should also keep in mind that it is rare for published flow rates to exactly equal actual valve performance in the field. Flow-rate characteristics appearing in any catalog are only representative values for a particular valve series. All of the standards mentioned — ISO, JIS, and ANSI/NFPA — prescribe an allowable variation of 15%. Thus, it is necessary to include this variance in any design. SMC feels the best way to find flow-rate characteristics for a valve is to use test equipment and methods according to ISO 6358. The company can also provide up-to-date information if current catalogs do not reflect the latest information.

Characteristics of an actuating cylinder
This example of a typical actuating cylinder shows the flow characteristics of three solenoid valves with CV(ANSI) = 0.2. The maximum flow rate varies more than 30% between valve A with b = 0.1 and valve C with b = 0.5.


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