Steady state accuracy, linearity and repeatability test as per API 675 (Positive Displacement Controlled Volume Pumps):

At first following terms shall be defined (as per API 675):

-          Steady state accuracy:

The flow variation expressed as a percentage of mean delivered flow under fixed system conditions. Steady state accuracy applies over the turndown ratio.

-          Flow repeatability:

Expressed as a percent of rated capacity, describes the reproducibility of pump flow rate under a given set of conditions when capacity setting is varied and then returned to the set point being tested.

-          Flow Linearity:

The relationship between the actual volume of liquid discharged at a given capacity setting and a best fit straight line (for example y=0.337 x-6.74 obtained as per a regression analysis in a test. x is percent and y is capacity ) through the plotted points of volume and capacity setting determined during calibration tests of a pump. The deviation from this line is expressed as percent of the rated capacity of the pump.

-          Turndown ratio:

The rated capacity divided by the minimum capacity that can be obtained while maintaining specified steady state accuracy and linearity (as per API turn down ratio shall be at least 10:1.For turndown ratio of 10:1,It means that accuracy cannot be guaranteed by vendor below the 10% of rated flow )

Requirements:

- The steady state flow accuracy shall be within ± 1 %  over a turndown ratio of at least 10:1 (please refer to remark 1).

(Maximum recorded capacity-minimum recorded capacity) / (mean delivered flow) or (rated flow as per remark 1) ≤ ± 1 %

- The flow repeatability shall be within ± 3 % of rated flow over the specified turndown ratio.

(Maximum recorded capacity-minimum recorded capacity)/ (rated capacity) <= ± 3 %

- Deviation from linearity shall not exceed ± 3 % of rated flow over the specified turndown ratio.

|Estimated capacity as per fit straight line –recorded capacity at each set point| / (rated capacity) <= ± 3 %

-  When flow repeatability and linearity tests are specified, they shall be run by using 100 %, 75 %, 50 %, 25 %, and 10 % of rated capacity points in descending and ascending order. The flow variations shall not exceed ± 3 % of the value at rated capacity.

Remark 1:As per clause 6.1.17 of API 675 edition 3, “The steady state flow accuracy shall be within ± 1 %  of rated flow over a turndown ratio of at least 10:1. This clause is inconsistence with the definition of the steady state flow accuracy (refer to definition mentioned above or clause 3.58 of API 675) which is stated that “steady state accuracy is the flow variation expressed as a percentage of mean delivered flow”. However, in my opinion it is logical that for this test in 100% capacity setting the flow variation as percentage of rated flow, and in other capacity setting (75% to 10%),the flow variation as percentage of mean delivered flow is followed.

Moreover, it should be noted that in some specification, it is stated as another requirement for this test that “pump delivery measured in steady state accuracy test at 100% of capacity setting shall be equal or greater than rated capacity” (an upper limit is sometimes imposed, usually when turndown ratio is critical)

Remark 2: All of these tests shall be performed in maximum specified pressure.

Please see following figure from Hydra-cell for more understanding.