Technology TIP: Measurement of Pulsating Flow

INTRODUCTION

TechTIP

Like most companies producing gas pumps, KNF uses glass tube and float type flowmeters to measure flow during pump production testing. This type of flowmeter has been used for several decades as they are fast-acting, reliable, and accurate. The normal industry practice is to calibrate this class of instrument using laminar flow. Unfortunately, the pulsating flow from reciprocating pumps produces an artificially high flowrate reading compared to the laminar flow calibration. As a result, all diaphragm and piston pump manufacturers using traditional flowmeters will end up promoting higher flow rate values than what the pumps actually provide.

OUR GOAL

measurement-flow-blogAt KNF, we are passionate about meeting the engineering design challenges of our customers. Our goal is to provide our customers with pumps that meet the actual needs of the system in which they are installed. Along with this goal comes the responsibility to provide data that best represents the performance capabilities for each pump produced at KNF. Simply stated, we want our customers to know the flow rate values we provide will accurately correspond to the actual flow produced by the pump — the true amount of gas delivered; not just an artificially inflated reading.

The flow measurement discrepancy manifests during system-level performance comparisons between continuous flow (non-pulsating) and pulsating pump types, reported to provide the same flow rates. The non-pulsating pump winds up delivering greater flow performance than the pulsating pump that was erroneously thought to be equivalent, skewing results in favor of the non-pulsating pump type.

TechTIP_MoPF_fig1

Figure 1: KNF pulsation-compensated flowmeter

THE SOLUTION

To address this situation, several years of research and development by our flow experts at the KNF Gas Pump Design Center in Freiburg, Germany has culminated in an advanced system for the measurement of pulsating flow. KNF has made an investment to implement this new technology. The resulting pulsation-compensated flowmeters (see Figure 1) are tuned and calibrated to measure pulsating flow more precisely than the traditional glass tube and float type flowmeters.

 

The more accurate flow readings from our pulsation-compensated measurement standard show lower values for flow than the laminar flow based systems used in the past. This document describes why your pump is still providing the same flow performance even though the measured and recorded flow value is lower. Glass tube and float type flowmeters are also called variable area flowmeters as the cross sectional area of the tube varies from smaller at the bottom to larger at the top (see Figure 2 below).

Figure 2

Figure 2: Cross-section illustration of a float type flowmeter

Pulsating flow always creates a higher reading in a float type flowmeter. The reason is that the float cannot move downward quickly enough between pulses. The float will remain on the top of the flow wave (see Figure 3 below). KNF has been aware of this phenomenon for quite some time and has been continually investigating better ways to attenuate the effect of the pulsation. The pulsation-compensated KNF flowmeter assemblies include physical components to minimize the effect of the pulsations.

Figure 3

Figure 3: Impact of pulsation on flowmeter readings

Simply put, our pulsation-compensated flowmeters are dampened to reduce the effect of the pulsations — these values are represented by the green line in the figure below. A dampened flowmeter may read pulsating flow too high if it was calibrated using laminar flow — that is why we use pulsating flow to calibrate our pumps at KNF. Flowmeters with little or no dampening will read artificially high as shown by the red lines.

SUMMARY

This advanced flow measurement system combines variable area flowmeters with a mass-flow calibration system. Optimized for pulsating flow, the system provides the most accurate measurement of flow available today. The improved accuracy is shown in Figure 4 (below).

Figure 4

Figure 4: KNF flowmeter reading (optimized for pulsating flow).

The chart shows flowmeters with a range of 2 – 10 liters per minute calibrated using pulsating and laminar flow compared to a target flow established by a mass flow meter. While this improvement is typical, actual results may vary across the flowmeter size ranges.

To learn more about KNF’s advanced flow measurement system please contact a KNF applications engineer.

One thought on “Technology TIP: Measurement of Pulsating Flow

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s