4 . I S S U E J U N E 2 0 0 1 / 17

APPLICATIONS TECHNOD I G E S TD I G E S T

The year 2001 will see the introduction of some fundamental changes in the rules that

govern the performance of acceptance tests on pumps. DIN 1944, the German standard

that has remained valid and been in global use since 1968, is being replaced by the

new international ISO 9906 standard. The latter has been in force internationally since

1999 and is now being transferred into DIN ISO 9906. That being so, we are taking

this opportunity to discuss

the resultant changes with

respect to DIN 1944, par-

ticularly with regard to tol-

erances and how to deal with them. The new standard will not be wholly new at all,

because it is based on the old international ISO 3555 and 2548 codes, which governed

the testing of pumps to different degrees of accuracy. These two standards have ceased

to be in effect.

New Rules for Pump Acceptance Tests –

DIN ISO 9906 Supersedes DIN 1944

Dr. Gerhard Mollenkopf

ically correct) distinction between con-structional tolerance and measuring tol-erance (cf. Diagram 1), which entailed occasional representation problems, ISO

CHANGES IN COMPARISON WITH DIN 1944

As far as the application of tolerances is concerned, the new rules differ significantly from DIN 1944. While the lat-ter drew a (phys-

Constructional tolerance: Unavoid-

able dimensional variation in the

hydraulic components of a pump,

which is caused by the production

process and liable to affect the

pump‘s performance. Appropriate

cost-intensive remachining can

reduce the constructional tolerance

to zero.

Measuring tolerance: Measuring

always involves a certain extent

of deviation from the true value.

Despite supreme efforts, the exact

measuring tolerance will always

remain unknown, but certain config-

urational and procedural measures,

including the use of suitably cali-

brated measuring instruments, can

minimize this tolerance.

APPLICATIONS

18 / 4 . I S S U E J U N E 2 0 0 1

TECHNOD I G E S TD I G E S T

9906 combines those two quantities into a single tolerance factor. This is aimed at making comparison between the guar-anteed values and the measured values less complicated and ambiguous. In for-mulating the new standard, the authors attempted to narrow down the latitude of interpretation in connection with the performance and analysis of acceptance tests by including detailed instructions of procedure.

INSTALLATION, TEST ARRANGEMENT AND

PROCEDURE

The installation arrangement of the pump to be tested is described, as are the appropriate test set-up and condi-tions. Special aspects (tolerances) to be considered in the testing and evaluation of pumps produced in series and pumps of very small size are described. In addi-tion, a valid procedure for applying the

new standard is specified, which can on-ly be altered by way of express stipula-tion in the contract of sale. This means that the mere mention of ISO 9906 will ensure contractual security.

Lack of useful details:Unfortunately, some of the useful details

Tolerance factor: Newly introduced

in ISO 9906, this term unites the con-

cepts of constructional tolerance and

measuring tolerance to simplify test-

ing in accordance with specified cri-

teria.

H

η

Q

H (Q)

Q

η (Q)

HG

ηG

QG

Constructionaltolerance

Measurement pointwith measuring tolerance

Measurement pointwith measuring tolerance

Diagram 1: DIN 1944

HG

ηG

+tH • HG

-tQ • QG -tQ • QG

-tH • HG

H(Q)

-tη • ηG

η(Q)

QG Q

H

Measurementpoints

Diagram 2: ISO 9906

Characteristic curves:

Characteristic curves:

Efficiency:

Efficiency:

Diagram 1 (DIN 1944) Diagram 2 (ISO 9906)

Characteristic The guarantee has been The guarantee is deemed

curve met, if the lower limit of to habe been met, if

the tolerance range the curve through the

around the measuring measurement points cuts

points touches the at least one branch of the

constructional tolerance tolerance cross.

range.

Efficiency The guarantee has been The guarantee is deemed

met, if the upper limit of to habe been met, if the

the tolerance range curve through the

around the measuring measurement points at the

points reaches the newly determined operating

guaranteed value. point at least touches the

negative tolerance range.

4 . I S S U E J U N E 2 0 0 1 / 19

APPLICATIONS TECHNOD I G E S TD I G E S T

missible measuring tolerance. The cited tolerance factors also include – to some extent – consideration of manufacturing uncertainties. They are primarily intend-ed to simplify handling and to rule out ambiguity in the verification of guaran-teed values (Table 1 and Diagram 2).

on centrifugal pumps, was designed to combine well-known and time-tested facts and data with simplicity of appli-cation.

The determined and established toler-ance factors range within the scope of the old DIN 1944, which is to be super-seded. However, the latter‘s concept of constructional tolerance has been delet-

ed and is now only ac-counted for to a minor extent in the new toler-ance factors, which go beyond the mere meas-uring tolerance.

Note: An information event devoted to the de-tails of the standard and its impact on guaranteed pump data will be held sometime during the sec-ond half of this year. Please address your in-quiries to the CT Secre-tariate.

in DIN 1944 have been left out of the new standard. For example, a sample error analysis is dispensed with, and the achievable grades of accuracy of various test methods are not quoted.

COMPARISON

The following compar-ison of the guarantee verification criteria indi-cates the magnitude of the differences to be an-ticipated in applying the new standard.

1) Grades of accuracy:While DIN 1944 de-scribed three accuracy classes, ISO 9906 speci-fies two. Grades 1 and 2 in ISO 9906 corre-spond to Classes 2 and 3 in DIN 1944. DIN 1944‘s Class 1 has been incorporated in a sepa-rate ISO standard (5198) that deals with the con-duct of laboratory tests.

2) No differentiation according to char-acteristic curve:Unlike DIN 1944, the new standard does not differentiate between pumps with flat head curves and pumps with steep head curves.

3) Test arrangement:ISO 9906 describes in all detail the re-quirements regarding the test arrange-ment and the quality of both the meas-uring instruments and actual measure-ments. This effectively defines the per-

Quantity Symbol Grade 1 Grade 2[%] [%]

Flow rate tQ ± 4.5 ± 8Pump total head tH ± 3 ± 5Pump efficiency tη - 3 - 5

Error analysis: To assess whether a

test result satisfies the requirements

of a standard, and/or to evaluate

the result‘s reliability the composite

error arising from all elements of the

measuring chain (test arrangement

to test instruments) needs to be ana-

lysed. The relevant importance of

the individual measured data as part

of the overall result must be given

special attention.

Table 1: Values of tolerance factors

Illustration : Modern, standards-compliant test bay for multistage pumps

4) New chapter on small pumps and se-ries-produced pumps:The new standard includes an addition-al chapter specifying increased tolerance factors to account for special aspects of testing pumps produced in series and pumps with very low input power rat-ings.

SUMMARY

ISO 9906, the new standard governing hydraulic performance acceptance tests