Why Turndown Ratio is Important when Selecting a Flow Meter ?

Understanding Turndown Ratio in Flow Meters: Importance, Calculation, and Application Across Flow Meter Types

When choosing flow meters, the turndown ratio is an essential criteria, particularly in situations where flow rates fluctuate significantly. The range over which a flow meter can reliably detect flow rates is defined by this ratio, which is often referred to as rangeability. This article provides a thorough analysis of turndown ratio, including its significance, calculation, common turndown ratios for different types of flow meters, and more.

What is Turndown Ratio?

The turndown ratio of a flow meter is the ratio of its maximum measurable flow to the minimum measurable flow, where the meter maintains accuracy within specified limits. It indicates how flexible the flow meter is in handling varying flow rates while still delivering accurate readings. 

What is the formula for turndown ratio?

Turndown ratio is expressed as:

For instance, the turndown ratio of a flow meter with a minimum flow capacity of 0.4 L/min and a maximum flow capacity of 20 L/min would be:

This demonstrates that the flow meter is capable of accurately measuring flows as low as 1/50th of its maximum range, which allows it to provide flexibility in measuring both large and low flows.

Why is Turndown Ratio Important?

Flow rates can fluctuate greatly and are not constant in many industrial applications. Without requiring frequent recalibration, a flow meter with the right turndown ratio ensures precise readings under all flow circumstances. Inaccurate readings at low flow rates or insufficient coverage of large flows may result from choosing a flow meter with an inadequate turndown ratio. The following are the main advantages of taking turndown ratio into consideration:

1. A high turndown ratio ensures that the meter can handle a wide range of flow rates without losing accuracy. This is especially important for processes with fluctuating flows.
2. Processes with varying production loads or seasonal demands require meters that can operate effectively at both peak and low flow conditions.
3. With a suitable turndown ratio, a single flow meter can cover a wide range, reducing the need for multiple meters or frequent calibration.
4. A high turndown ratio allows for better control of flow rates, which is crucial for precise process management and quality assurance.

What is the Turndown Ratio for the Types of Flow Meters?

Depending on the design and measurement technique of the flow meter, each type of flow meter has a characteristic turndown ratio. An overview is as follows:

In many cases, the difference in turndown ratios can be related to the different working principles of each technology. For instance, Coriolis and ultrasonic flow meters, which do not contain any moving parts, typically provide higher turndown ratios in comparison to orifice meters, which are more restricted due to their reliance on differential pressure.

Calculating Turndown Ratio for Flowmeter

Calculating the turndown ratio is straightforward. Simply divide the maximum measurable flow by the minimum measurable flow.

For example, if a flow meter has a minimum flow detection limit of 2 gallons per minute (GPM) and a maximum flow rate of 20 GPM, its turndown ratio would be:

In this case, the flow meter can accurately measure flows as low as 2 GPM and as high as 20 GPM.

Examples of Turndown Ratio in Flow Meters

Consider a flow meter with a full-scale measurement range of 1–10 GPM. If the meter has a turndown ratio of 10:1, it can measure down to 1 GPM. 

However, if the minimum flow required for accurate measurement is 0.5 GPM, then a meter with a 20:1 turndown ratio would be necessary.

In some cases, a specific turndown ratio may limit the range within which the meter can be used accurately. For instance, a mass flow controller with a 50:1 turndown ratio calibrated for a maximum flow of 25 liters per minute (L/min) can accurately measure down to 0.5 L/min (1/50 of the maximum). 

If lower flows need to be measured accurately, either a different meter with a higher turndown or dual-range meters might be required.

Selecting Flow Meters Based on Turndown Ratio

It is essential to have a thorough understanding of the turndown ratio requirements of your application before making a choice about a flow meter. Factors to consider include:

Flow Variations

• Systems with high flow variability benefit from meters with a higher turndown ratio to ensure accuracy across the full range of flows.

Measurement Accuracy

• For precise measurements at low flow rates, ensure that the minimum flow rate is well within the meter’s turndown ratio.

Cost and Practicality

• Higher turndown ratios can sometimes mean more expensive equipment, so it’s essential to balance performance requirements with budget constraints.

Flow Meter Technology

• Each flow meter technology has inherent advantages and limitations with respect to turndown ratio. For instance, magnetic or ultrasonic flow meters generally offer higher turndown ratios and can measure a wider range of flows compared to orifice meters.

Click here for Streamlining Your Flowmeter Selection Process: Tips and Insights

What is a Good Turndown Ratio?

A “good” turndown ratio depends on the application. 

For instance:

1. Low-Flow Applications: Coriolis or ultrasonic meters with ratios of 100:1 or more are ideal.
2. Medium-Flow Applications: Vortex or turbine meters with ratios between 10:1 and 30:1 might be suitable.
3. High-Flow Applications with Minimum Variations: Orifice or positive displacement meters with ratios as low as 5:1 can be adequate.

Turndown Ratio in Differential Pressure Flow Meters

In meters that measure flow using differential pressure (e.g., orifice plates or venturi tubes), turndown ratio affects the measurable range due to the square relationship between flow rate and differential pressure (Bernoulli’s principle). 

Because of the reality that the differential pressure signal becomes weaker at low flow rates, accuracy is reduced.  Hence, orifice meters typically have a lower turndown ratio of 5:1 to 10:1.

Turndown Ratio and Measured Signal Strength

The signal strength in differential-pressure meters decreases as the flow rate drops, affecting measurement accuracy at low flow levels. High turndown ratios may compress the signal at low flows, making it difficult to capture accurate readings. This limitation underscores the need to match turndown requirements with application needs.

What Does 30% Turndown Mean?

If a flow meter has a 30% turndown, it means it can operate accurately down to 30% of its maximum flow capacity. For instance, if a meter’s maximum flow capacity is 100 GPM, a 30% turndown means it can accurately measure flows as low as 30 GPM.

Typical Turndown Ratios for Various Industrial Applications

1. Oil & Gas Industry: High turndown ratios are often required to measure both low flow in production lines and high flow in transport.
2. Water Treatment: Turndown ratios of around 10:1 or higher are usually sufficient.
3. Pharmaceuticals: Precision dosing and process flexibility often require turndown ratios of 100:1 or more.

What is the turndown ratio in flow meters?

The turndown ratio in flow meters, also known as rangeability, is the ratio of the maximum measurable flow rate to the minimum measurable flow rate. It defines the range over which the flow meter can accurately measure flow, from the lowest to the highest flow values within its specifications.

What is the Minimum Turndown?

The minimum turndown refers to the lowest flow rate of gas that a plant must process to sustain continuous operations without interruptions. It represents the minimal capacity required for the system to function effectively.

What is the Turndown Ratio of a Flow Control Valve?

A flow control valve’s turndown ratio, which is commonly represented as the ratio of maximum Cv (flow coefficient) to minimum Cv, is the range between its maximum and minimum controllable flow rates.

• For a typical control valve with an equal-percentage flow characteristic, the turndown ratio is approximately 30:1 under normal sizing conditions.
• If the valve is oversized and throttling at the low end of its range, its effective turndown ratio can drop significantly to around 3:1 or less, limiting its control accuracy.
• This highlights the importance of proper valve sizing to maintain a broad and effective turndown ratio for precise flow control.

Click here for Comprehensive Guide to Flow Measurement: Principles, Installation, Calibration, and Troubleshooting