Coriolis and Aerationl

J. Reizner Page 1 of 19 Date: 4/16/2001

THE EFFECTS OF AERATION ON CORIOLIS MASS FLOW METERS

Guidelines for Installing Meters to Avoid Problems Related to Air Entrainment

(and how to diagnose and resolve problems if they do occur)

Table of Contents

Topic Page

1. Purpose .............................................................................................. 2

2. Definitions ........................................................................................... 2

3. Background ......................................................................................... 3

4. Meter Sizing and Selection ................................................................. 6

5. Aeration Testing .................................................................................. 8

6. System Design to Reduce the Effects of Air Entrainment .................. 9

7. Piping and Process Design to Minimize Air Entrainment .................. 11

8. Diagnostic / Troubleshooting Techniques ......................................... 16

9. Micro Motion Transmitter Parameters ............................................... 18

10. Meter Zeroing .................................................................................... 18

11. Recommendations ............................................................................ 19


Acknowledgements I would like to acknowledge the input and critique that was so generously given to me by Bob Catlin of P&G. I would also like to thank Micro Motion and their people who contributed many comments and ideas to this document. Purpose The purpose of this document is to explain the effects of aeration of liquids on Coriolis mass flow meters and how to deal with these issues. This document will provide information regarding:

1. The affect of entrained air on Coriolis flow meters 2. Concepts to minimize the effects of entrained air on Coriolis meters:

a. Meter sizing and selection b. System design c. Meter installation

3. How to recognize the symptoms of entrained air on existing systems, and how to resolve the problems when they exist.

Similar issues may be experienced when powders are entrained in liquids that are metered with Coriolis technology. In this document the terms air entrainment and gas entrainment are used interchangeably. Definitions Drive gain: A relative indication of the amount of power the transmitter is delivering to drive

the sensor. When drive gain is saturated the transmitter is delivering the maximum amount of power it has available.

Amplitude: An AC voltage measured at one of the sensor’s pickoffs (PO), directly

proportional to the sensor tube displacement. Amplitude is often referred to as PO amplitude; furthermore, there are two pickoffs on Micro Motion Coriolis meters, referred to as left-pickoff (LPO) and right-pickoff (RPO). The LPO and RPO have the same amplitude.


Background For a background on the operating principals of Coriolis mass flow meters refer to the Micro Motion software program Tutor

TM. This program is available from your local Fisher-Rosemount /

Micro Motion representative, or from the Micro Motion web site at: www.micromotion.com To generate accurate flow and density signals the Coriolis meter has a drive coil that vibrates the flow tube to a specified displacement (see Figure 1). This displacement is large enough to produce high-resolution signals at the sensor’s pickoffs. The transmitter provides the power required to produce this vibration. Direct feedback from the pickoffs is used to automatically adjust the amount of power that the transmitter provides and to maintain the proper tube displacement. Thus if the transmitter detects the PO amplitude decreasing, it will increase the drive gain and deliver more power to the sensor, allowing the sensor to drive to the correct displacement.

Figure 1 (Copyright Micro Motion, Inc.)

Under nominal conditions the sensor’s material properties, tube geometry and size determine the power that is required to drive it to the specified displacement. In general, a larger sensor requires more power than a smaller one, and a straight tube sensor requires more power than a similarly sized curved tube sensor. The amount of energy required by the sensor to maintain nominal vibration displacement varies from sensor size to sensor size. The primary reason for this difference in required energy is that the force required to deflect the tubes varies with the stiffness of the flow tube (and the stiffness of the flow tube varies with size). In general, the energy required to drive the Coriolis meter increases with the size of the Coriolis meter, e.g. a 2 inch Micro Motion ELITE meter requires more energy than a 1 inch Micro Motion ELITE meter. Another general statement is that a straight-tube Coriolis meter is stiffer (and requires more power) than a like size bent-tube Coriolis, e.g. a 1 inch straight-tube meter requires more power than a 1 inch bent-tube meter. However, it should also be noted that the power required by a straight tube is not always greater than that required by a bent tube, i.e. smaller straight-tube meters can require less power than larger bent-tube meters.

http://www.micromotion.com/


For various reasons (including the laws of physics, concern for tube and weld integrity and intrinsic safety power limitations), the amount of power that a transmitter has available to drive a sensor is limited. Figure 2 depicts the relationship between the energy required by a sensor and the energy available from the transmitter to maintain nominal vibration displacement yielding an accurate measurement in non-aerated water. Defined by this figure is the term energy overhead. Energy overhead is typically expressed as the ratio of the maximum amount of energy supplied by the transmitter to the energy required by the sensor in non-aerated water. It can be seen in this figure that the energy overhead varies from sensor design to sensor design (by size and sensor shape, metallurgy, etc.).

When a fluid is aerated the entrained air (or gas) bubbles absorb some of the vibrational energy from the sensor’s tubes that makes it more difficult to drive the sensor. This requires the transmitter to provide more power to maintain the proper tube displacement, but the transmitter has only a limited amount of power available. A relatively small percent volume of air entrained in the fluid will cause the drive gain to become saturated. When the sensor requires more power than the transmitter can deliver the tube amplitude will decrease below the desired value. This can result in less accurate flow and density measurements. Since the percent volume flow of the entrained air is likely to be erratic compared to the overall fluid flow, entrained air can also cause unstable flow and density measurements.

Figure 2

Time

En

erg

y L

ev

el

Maximum amount of energy the transmitter can deliver

Energy Required for accurate measurement in non-Aerated Water

Energy Overhead

Increases with meter size

Increases for straight tube


Figure 3 depicts the relationship between the energy required by a sensor monitoring an aerated fluid and the energy available from the transmitter.

Accuracy and performance of a Coriolis meter will degrade as the volumetric quantity of entrained air increases. For example, testing performed by Micro Motion using their ELITE meters with water show that drive gain becomes saturated with the introduction less than 2% (by volume) of entrained gas. However, as long as the pickoff amplitude (LPO or RPO) remains greater than 50 mVrms, an accuracy of much better than 1% can generally be attained. Once the pickoff amplitude drops below 50 mV the measurement is considered to be unreliable. As with drive gain, both pickoff amplitudes are available as diagnostic information from the transmitter. This information can be accessed digitally with a Rosemount 275 Handheld Communicator or ProLink. The specific levels of aeration that cause these problems are unknown. The aeration problem appears to be exacerbated by viscous fluids, i.e., the same level of entrained air in a viscous fluid appears to create more dampening effect than in a thinner fluid. The size of the bubbles also appears to affect the meter performance. Very small bubbles may not effect the operation whereas larger bubbles may. Another factor that influences the amount of aeration tolerated by the sensor is the level of backpressure on the meter. Increasing backpressure tends to decrease bubble size or, in some cases, eliminate the bubbles by keeping the air in solution. Thus, in general, a sensor will be more tolerant of entrained air as the meter backpressure increases.

Figure 3

Time

En

erg

y L

ev

el

Maximum amount of energy delivered by the transmitter

Energy Required for accurate for Accurate Measurement in Highly Aerated Liquid

Energy Shortfall


Experienced-based “rules of thumb” for levels of entrained air are:

5% entrained gas by volume will generally be a problem

Recommend keeping entrained gas below 3% by volume Micro Motion has laboratory test data on entrained air in water. Since their flow lab is limited to centrifugal pumps, they have no laboratory test data on the effects of entrained air on viscous fluids. MMI is in the initial stages of designing and building a flow lab to obtain data on the effects of aeration on viscous fluids. MMI’s limited data on entrained gas in viscous fluids is from customer installations. Meter Sizing and Selection High flowing velocities through the meter are generally advantageous to the free movement of gas through the meter. A “rule of thumb” is that meter velocities should be kept above 3 feet per second to aide gas removal in thin water-like fluids, and above 1 foot per second in higher viscosity fluids (lower flow rates are required with higher viscosity fluids to keep meter pressure drops reasonable. Generally these lower flow rates are sufficient to keep entrained gas flowing in the more viscous products). Low velocities may more readily allow air to become trapped in the meter, preventing successful meter operation. This must be balanced vs. meter pressure drop considerations. As noted previously, smaller meters require less power to drive the tubes. Given that the available power from the transmitter is limited, smaller meters have more drive overhead. Therefore, everything else being the same, smaller meters are a better choice for entrained gas applications. As noted above, the Micro Motion ELITE meter has the largest amount of headroom, and therefore is generally the preferred choice for aerated fluids. Within the ELITE line there are two styles: the “Omega” shape (Figure 4) which is used through 1 inch size, and the “U” shape (Figure 5) which is used in sizes above 1 inch.


Figure 4 Micro Motion ELITE Omega-Style Meters

(Copyright Micro Motion, Inc.)


Figure 5 Micro Motion ELITE U-Style Meters

(Copyright Micro Motion, Inc.)

The U-style is easier to mount in a way to prevent air pockets from forming than is the Omega style. Therefore the U-style is preferred for this reason – but since the U-styles are larger they will result in lower fluid velocities for a desired mass flow rate. Where such tradeoffs are presented, it is suggested that the user select the smallest meter that meets the pressure drop requirements. Additionally, if it is suspected the fluid aeration will be present in the process the user should consider ways to increase the back-pressure on the sensor - e.g. sensor location (low-point in the process), down stream valves and constrictions, etc. Aeration Testing Fluid aeration levels can be tested simply. First, extract a small sample of fluid from the process at the point where the meter is currently installed or will be installed. Place approximately 200 mL into an appropriate container. (Note: There are many potential safety issues associated with obtaining a sample, including but not limited to: high pressure, high / low temperature, hazardous fluid properties such as corrosive, etc. The person obtaining and handling the sample must learn and follow all pertinent safety standards.) Put about 100 mL of this sample into a graduated cylinder. Record the time, along with the starting sample volume and fluid temperature. Let the sample sit for a period of time sufficient to insure that all entrained gas is removed. For viscous samples, this could be as long as 48 hours. Higher temperatures generally reduce viscosity and speed up removal of entrained gas (but remember that heating up the sample may be hazardous). After the sample has sat for a length of time sufficient in time to remove all entrained air, record the ending volume of the sample.


This sample volume must be temperature corrected, which may be done in one of two ways.

The easiest way is to record the ending volume, then using density vs. specific gravity charts for the fluid make the temperature correction.

If such density correction charts are not available, temperature correction may be done by heating the fluid sample so that it approximates the fluid starting temperature. Once again, remember to consider any potential safety hazards associated with heating the fluid.

Divide the temperature corrected ending volume by the temperature corrected starting volume. Subtract this number from 1, and express this number as a percent. This is the percent of entrained air in the sample. System Design to Reduce the Adverse Effects of Air Entrainment In systems where entrained gas is inevitable, several methods exist to minimize the potential effects. Increasing the amount of back-pressure on the sensor will reduce the gas volume and possibly keep the gas in solution. This higher backpressure may be accomplished with a restriction device such as a pipe reducer, valve, orifice plate or equal vent device downstream of the flow meter. The preferred solution is to prevent air from entering the system in the first place. The following offers details on good system design practices to prevent the introduction of air:

1. Supply Piping into Tanks Generally the supply piping bringing fluids into tanks enters the tank from the top. In large tanks the fluid can fall significant distances to the fluid level in the tank. This can cause a large amount of air to be entrained into the fluid. Solutions to this problem include placing the entrance pipe below the normal tank level, continuing the entrance pipe inside the tank to a point below the tank low level, or running the tank at higher levels to make the drop less severe. Make sure that the material entrance is kept as far away from the outlet piping as possible to minimize the possibility of air-entrained product being sucked directly into the pump. Do not direct the inlet pipe discharge towards the pump suction. To do so could result in cavitation, water hammer or other similar problems.

2. Tank Agitators When tank agitators are operated with the agitator blades at the fluid level the blades will entrain substantial quantities of air. A possible solution to this involves electrically interlocking the tank agitator to the tank level so that the agitator does not run when its blades are at the fluid-air interface.


3. Tank Discharge Piping to Pump Suction a. Don’t pump when the fluid level is below the top of the pump suction line. To do

so will suck substantial quantities of air into the fluid. Interlocking with a tank level-measuring instrument can automatically perform this function.

b. Insure that the pump suction line connection at the tank is located high enough above the tank bottom so that tank sediment is not sucked into the fluid and passed through the system.

c. Insure that there are no high spots in the pump suction line for air to accumulate. If there are, this can cause slugging of the flow that will adversely affect meter performance.

d. Do not put filters in the pump suction line. They belong in the pump discharge line. An appropriately sized strainer may be located in the pump suction line to remove large objects. Strainers are often required in the suction line to various types of positive displacement pumps to prevent debris from mechanically damaging the close mechanical passages of a positive displacement pump.

e. Make sure that pump suction piping is tight and leak-free. Since this suction line may be under vacuum when the pump is operating, an ultrasonic vacuum leak detector may be required to find these leaks. A vacuum leak on the pump suction can suck in large amounts of air, and is generally non-observable.

4. Air Removal Devices

Devices are available to remove entrained air from fluids. The simplest device to remove air is a tank of relatively large size (to give a long residence time for the air bubbles to rise to the top). The tank can be MILDLY agitated to facilitate the air bubbles coming to the surface. The tank should have as large a surface area as possible and not be very deep. This will make it easier to remove the air bubbles. The tank should be put under vacuum if this is possible and if this can be done safely. The vacuum will speed up the process of air bubble removal. Custom-built devices are available from various companies to remove entrained gases from fluids. These devices are generally called deareators. These custom-made devices are generally used for more viscous products and where higher levels of air removal are required. Basically these devices cause the product to fall on a rotating plate, where the product is wiped into a thin film by centrifugal force and a squeegee. The device is under a vacuum. The large surface area, thin surface film and high vacuum result in high amounts of deaeration for viscous products.

5. Operating Parameters Generally raising the system operating temperature will decrease the viscosity of the fluid, and increase the ability of entrained air to be removed in any given time from the fluid.


Piping and Process Design to Minimize Air Entrainment 1. Pressure Piping

a. Don’t create high spots in the piping system that can trap gases. b. Slope horizontal lines up at a slight angle. The exact slope is a function of the

viscosity of the fluid. Thin fluids may only require a 2-degree slope; thick fluids may require a 10-degree slope. This may be difficult for construction crews to accomplish, so discuss this with them. The following sketch shows an easy way to accomplish this.

Copyright Krohne, Inc.

c. Use reducers correctly so that they do not trap air. Use concentric or eccentric reducers as appropriate.

When using eccentric reducers, install the flat so that air can most easily travel through the system. The exact orientation of the flat of an eccentric reducer is a function of the piping orientation, but should be obvious when one thinks about minimizing points where air can be trapped.

Concentric Reducer

Eccentric Reducer (shown with flat up)


2. Meter Installation to Minimize the Effects of Entrained Gases Accurate measurement with any type of flow meter requires that the meter be completely full of process fluid at all times in order to provide an accurate flow measurement. Therefore the following meter locations must be avoided:

a. Installation at high points in the piping system (this allows air accumulation) b. Installation in a downward flowing line, especially where the line discharges

immediately downstream of the meter. The lack of backpressure on the line will cause slug flow in the meter.

Downward Piping When the piping downstream of the meter is in a downward direction, follow the sketch below. Install a “goose neck” immediately downstream of the meter, with a siphon air vent valve located at the high point in the piping to break the vacuum. This installation will prevent partial vacuum from occurring in the line.

Copyright Krohne, Inc. Open Discharge Install the flow meter at a low point in the line for open discharge piping systems to insure that the meter is always full of fluid.

Copyright Krohne, Inc. Open Discharge

Control Valves Always install control valves and shutoff valves downstream of any flow meter to prevent vacuum from developing in the flow meter.


Pumps Never install a flow meter on the suction side of a pump.


a

.

b.

a.

Syphon



Preferred Installation a. ELITE U-Style

b. ELITE Omega-Style

Concentric Reducers (if required)

ELITE Omega-Style will allow air to be trapped in pocket

Flow

Flow


2. T-Meter

Concentric Reducers (if required)

Flow


Alternate Orientation

a. U-Style and Omega-Style ELITE Flow

b. T-Meter

Slope away from inlet for air removal

Eccentric Reducers, flat up on the inlet, flat down on the outlet (where required)

Flow


Diagnostic/Troubleshooting Techniques

Due to the complexity of the variable interactions, currently-available Coriolis mass flow meters cannot automatically diagnose an air entrainment problem. Fortunately the transmitters do provide enough information for someone who understands the operation of the sensor and the process to properly make that diagnosis. Air entrained in a system generally manifests itself in one of two ways:

As a semi-steady stream of bubbles

As a large pocket of air that collects in a system high point. This will occasionally allow a large bubble to “burp” through the sensor, typically called slug flow. When this occurs, the transmitter will report a Slug Flow alarm and the transmitter’s outputs will go to fault conditions until the condition clears, usually within a few seconds.

We will concentrate on the first item, the stream of bubbles. Some key points:

Entrained air does not always result in a foamy, frothy fluid. While this is possible, in the majority of cases no characteristic foam is present in the process fluid. Bubble size does matter but no one is sure to what degree because lab testing has not been done on this subject. It appears that larger bubbles may be worse than smaller bubbles. Having said that, microscopic bubbles can also be very detrimental to sensor performance. Smaller bubbles are more difficult to see when a sample is drawn. Be prepared to allow a sample to sit for a period of time (as long as 48 hours) if visual confirmation of entrained air is required. Entrained air shows up as a decrease in sample volume over time, as described in more detail elsewhere in this document. Entrained air causes the density measurements to be lower than expected in bent-tube sensors (Micro Motion U-tube and Omega style). This makes sense, since fluid with entrained air is less dense than the fluid itself. Entrained air causes the density measurements to be higher than expected in straight-tube sensors (Micro Motion T-Series). In straight-tube meters the entrained air makes it difficult to drive the tube and therefore the tube vibrational frequency goes down, resulting in a false higher density reading. This is counterintuitive. More air means less mass in the tube, which suggests that the density reading should be lower - but the difficulty in driving the tube is the overriding factor here. If the sensor is operating under an entrained air condition the drive gain will be saturated.


Typically, the first indications of an air entrainment problem are inaccurate flow and density readings. Usually the sensor is assumed to be at fault, and a calibration check is called for. Prior to recalibrating or replacing the sensor, perform the following:

1. Monitor drive gain and density under normal flowing conditions. If the drive gain is saturated and the density reading is in error per the previous statements, air entrainment is the most likely cause. The following steps can confirm this diagnosis.

2. Stop flow. Allow time for any air bubbles to rise out of the tubes. (Note: a more

viscous fluid will require more time for the bubbles to rise out.) Monitor drive gain and density. If there is no air in the sensor, the density reading should be accurate and stable, and the drive gain should be at a normal value (not saturated.)

3. If possible, under normal flowing conditions draw a sample of the process fluid.

Be sure to follow all necessary safety precautions. Visually check the sample for air bubbles.

4. If doubt remains as to whether the sensor or system is the source of the problem,

perform a calibration check of the sensor using non-aerated water and a bucket or drum on a scale. Note: If the calibration check is performed by just diverting the normal process fluid to a scale, all that will be confirmed is that the sensor is not accurately measuring the flow. This cannot determine if the problem originates with sensor or the process.

As noted previously, density and drive gain voltage can be monitored using either a Rosemount 275 Handheld Communicator or ProLink. Pickoff amplitude below 50 mV is considered to be unreliable.


Micro Motion Transmitter Parameters Micro Motion transmitters contain two parameters that can be adjusted to minimize the effects of slug flow (gas slugs in a liquid flow stream). The Slug Flow parameter allows the user to set high and low process fluid density limits at which the meter will report an error signal. Above the high or below the low process density limits, the transmitter output signal will go to its minimum value (zero for frequency, 4 mA for a 4 to 20 mA signal), and an error message (SLUG FLOW) will be displayed on the transmitter. The Slug Flow Duration allows the user to program a time between 0 and 60 seconds into the transmitter. If the fluid density goes outside the high or low slug flow limits, the flow outputs will hold their last measured value for the period of time established by this parameter. After that time, if the product density is still out of limits, the outputs go to their minimum value as described in Slug Flow above. Meter Zeroing Capturing a good zero is essential for optimum sensor performance. It is best to zero the meter at conditions approximating actual fluid operating conditions. Fluid temperature and pressure should be representative of actual process operating conditions for an accurate zero and accurate flow measurement. Check the stability of the zero capture by performing the zeroing procedure a couple of times to check the consistency of the zero captured.


Recommendations Following are a list of recommendations on how to deal with entrained air:

1. Use the best Coriolis meter possible, which is generally the Micro Motion ELITE. In general do not use the T-Meter, due to its lower headroom as discussed previously.

2. Size the meter correctly as discussed previously.

3. If at all possible, determine the percent of entrained air expected (use the procedure

discussed previously). Discuss this data with Micro Motion. 4. Experience is that, depending on fluid viscosity, bubble size, etc. entrained air of 5% by

volume or more can be a problem for Coriolis mass flow meters. It is recommended that entrained air be kept below 3% by volume. Recognize this is simply a “rule-of-thumb” and is not a precise value for all situations. Knowledge and testing does not exist to give more specific direction.

5. When possible, lower the amount of entrained air using techniques noted above. 6. Design the system, piping and installation to minimize possible problems with entrained

air. See elsewhere in this document for more details.

7. If the amount of entrained air is more than a Coriolis meter can deal with, consider alternate flow meter technologies such as magnetic flow meters, vortex meters or positive displacement flow meters. Recognize that each meter type has its drawbacks, and that none can deal with an unlimited amount of entrained air. Slug flow also creates a problem with most meter types. For more assistance with alternate meter selection refer to the author.

8. If the entrained air cannot be dealt with, and no flow metering technology is appropriate,

load cell tank weighing systems may be the only option. The advantage of load cell tank weighing systems is that they will read mass regardless of quantity of entrained air. The disadvantages of load cell tank weighing systems are that they are very expensive and are difficult to design, install, calibrate and maintain. They also generally require that the process be designed as a batch process rather than as a continuous process, so retrofitting load cell tank scales into existing continuous processes can be extremely expensive.

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