on December 17th, 2013

Dear Colleagues,

I am sure you all have encountered the nightmare and lies often inherent in interpreting vendor specifications. How to determine what is right and what is wrong is one issue. A vital issue. But there is perhaps an even more important issue: do you need these particular specifications for your project – in other words – do they mean much for your application?

The answer often is the specifications do not mean much. Even if they are valid.

A Good Example is the Turndown Ratio
Turndown is the ratio of the maximum flow rate to the minimum flow rate for a specific flowmeter for a required performance.

Typical quoted values for turndown for flow meters are 40 to 1 or even, 90 to 1 (as absolute maximum values). These turndown ratios may sound absolutely fabulous. However, they may not mean much for your application due to the real world impact of actual maximum allowable flow rates in your pipes and costs of pumping.

Energy Costs and Abrasion can be Real World Problems
It is definitely true that when pumping certain slurries, one doesn’t want the flow rate to be too low otherwise the solids tend to settle out. However, a real problem is that energy costs can dramatically increase with higher flow rates and the pipe wear factor may increase astronomically (esp. with slurries). So having significant turndown ratios may be quite irrelevant.

An Example to Illustrate
An ultrasonic flow meter may have a quoted turndown ratio of 40:1.

However, due to the abrasion effects of the fluid, the actual flow rate may actually be less than 1m/s. This means the turndown ratio (maximum to minimum flow rate), will be less than 2:1. So the 40:1 ratio is quite irrelevant.

Tripping the Light Fantastic
And this is why I say: Tripping the Light Fantastic with Vendor Specifications, as these specifications of turndown ratio can be generally irrelevant to your design. Look at the real flow rates that you can achieve and then examine the specifications supplied by the vendor.

And this applies to all specifications. Always consider the real world situation before becoming fixated by incredible specifications proffered by your vendor. You may be dismayed to find these wonderful specifications are actually quite irrelevant for your application.

David Thoreau's comment applies to interpreting specifications in terms of the real application: Be true to your work, your word, and your friend.

Thanks to my good colleague,  David Spitzer, for a thought provoking article on this thorny topic entitled: Flowmeter Turndown

Yours in engineering learning,

Steve Mackay

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