Level Measurement Using Isolating Air Flow

Bubbler method of liquid level
measurement is simple and reliable
Courtesy Yokogawa

Remotely utilized measurements of tank liquid level are common throughout the fluid processing industries. Various means of inferential or direct measurement are available, each with its own set of performance, maintenance, and cost attributes that may make it the preferred choice for a particular application.

Inferring liquid level using a hydrostatic pressure measurement is a simple, easy to implement strategy for delivering a continuous level signal to the process operating and control system. There can be situations where mounting the measuring gear near the bottom of a tank or other vessel may be impractical or undesirable. A pressure transmitter mounted low on a tank may expose it to damage from plant traffic or other physical hazards. It is also possible that the tank may not have a suitable fitting located low enough to provide the needed measuring range. Having a potential leakage point at a fitting low on the tank may also be undesirable. Another, and certainly not final, scenario would be an application involving a corrosive liquid which must not come in contact with the pressure sensor.

The bubbler method of liquid level measurement employs an arrangement that overcomes all of the previously mentioned impediments. It can utilize connections at the top of the tank, above the liquid level. The way in which the method works will keep the pressure sensor out of contact with the process liquid, providing isolation from potential corrosive effects.

The apparatus for level measurement using the bubbler method employs a simple dip tube that extends from the pressure sensor or transmitter to nearly the bottom of the tank or vessel. A small amount of purge air or gas continually flows through the dip tube and will bubble out the bottom of the tube. This dip tube arrangement essentially transfers the hydrostatic pressure at the bottom opening of the tube to the pressure transmitter, while the purge gas keeps the liquid from advancing up into the pipe. The bubbler can be used on atmospheric or pressurized tanks with a properly configured pressure or differential pressure transmitter.

Probably the most significant application point of the bubbler method that will distinguish its use from many other level measurement methods is the importance of maintaining air or gas flow in the dip tube. The flow provides isolation of the sensor, but the flow must also be set to a level that will not impact the pressure measurement in the tube itself. If the flow is excessive, backpressure in the tube can be offset from the level imparted by the tank contents, with the result being an incorrect measurement.

Below is an application note from Yokogawa, showing how their pressure transmitters and rotameters can be used to create the setup. Share your process measurement challenges and requirements with instrumentation experts, combining your own process knowledge with their product application expertise to produce effective solutions.

Liquid Level Measurement Using the Bubbler Method from Miller Energy, Inc.

Yokogawa Differential Pressure Transmitter from Miller Energy, Inc.

Yokogawa Rotameter Flowmeters for Industrial Applications from Miller Energy, Inc.

Case Study From CSB: Industrial Plant Heat Exchanger Explosion

Shell and tube heat exchangers

Industrial accidents, whether minor or catastrophic, can serve as sources of learning when analyzed and studied. Operators, owners, and technicians involved with industrial chemical operations have a degree of moral, ethical, and legal responsibility to conduct work in a reasonably and predictably safe manner without endangering personnel, property, or the environment. Part of  a diligent safety culture should include reviewing industrial accidents at other facilities. There is much to learn from these unfortunate events, even when they happen in an industry that may seem somewhat removed from our own.

The U.S. Chemical Safety Board, or CSB, is an independent federal agency that investigates industrial chemical accidents. Below, find one of their video reenactments and analysis of an explosion that occurred at a Louisiana chemical processing plant in 2013. A portion of the reenactment shows how a few seemingly innocuous oversights can combine with other unrecognized conditions that result in a major conflagration.

Check out the video and sharpen your senses to evaluate potential trouble spots in your own operation.

Accurate Level Measurement Contributes to Heat Rate Reduction

Industrial steam turbine

Steam production is a costly operation in any facility, but is of paramount importance in power generation plants. The bottom line of a combustion based power generation facility is sensitive to the cost of input fuel. Measures that can be taken to reduce fuel input for a unit of power output (called heat rate) can translate directly into profitability. An additional benefit of reducing heat rate is a commensurate reduction in emissions.

A major contributor to heat rate reduction is the recovery of heat from the process and transference of that heat into the boiler feedwater. A sizable feedwater preheater of the shell and tube type is used to recover the heat. Shell and tube heat exchanger efficiency can be maximized with accurate control of liquid level.

Magnetrol, globally recognized leader in level measurement technology, makes the case for using guided wave radar level measurement technology as the most advantageous means for this application. The video below describes the process and how the guided wave radar level transmitter can provide the best performance.

Magnetrol has an information kit devoted to heat rate reduction. Share your steam system and level measurement challenges with a product specialist, and ask how you can get the Heat Rate Reduction Kit. Combining your facility and process knowledge with the product application expertise of a specialist will result in effective solutions.

Applying Coriolis Flowmeters

Coriolis Flowmeter
Courtesy Yokogawa

Coriolis mass flow meters are widely used throughout the process measurement and control field. Their basic operating principle, combined with modern sensor and signal processing technology, provide a list of potential application advantages.

• Directly measure mass flow based on the principle of measurement.
• No "moving" parts (except the slightly vibrating sensor tubes).
• Coriolis sensors have the ability to measure flow of non-conductive fluids.
• Measure the mass flow rate with high accuracy.
• Instruments can be applied across a wide usable measurement range.
• Deliver density measurement based on oscillating frequency.
• Not materially impacted by fluid viscosity or density.
• Coriolis flow meters also do not need lengthy straight pipe sections upstream or downstream of the sensing unit.

SELECTION AND INSTALLATION RECOMMENDATIONS

Some considerations for product selection and installation of Coriolis mass flow meters:

• Rapid temperature changes within the measured medium can impact measurement accuracy.
• Wide ranges of connection types are available, specify the best match for your system.
• Select sensor tube construction material to accommodate media characteristics.
• For liquid measurement, the sensor tube will extend downward from a horizontal pipe run. 
• As with most industrial products, there may be restrictions on where the device may be used. For example, not for use in aircraft. Assure that your application conforms with the intended usage of the instrument.
• Install at least one tight shutoff valve adjacent to the instrument for purposes of zeroing. Isolation valves for inlet and outlet are preferred.
• Follow manufacturer recommendations for pipe supports, if required, at inlet and outlet sides of the unit
• Avoid installation at locations with high levels of vibration or strong electrical fields.
• Locate the sensor unit at the lowest practical level in the piping system. This helps avoid collection of gas bubbles in the sensor tubes.
• Sensor tubes need to be completely filled with subject medium to obtain accurate measurement.
• Avoid the use of flexible connections and reducers at the unit connection points, unless in accordance with specific manufacturer guidelines.

The Coriolis flow meters of different manufacturers may have some particular recommendations, all of which should be followed to insure the best available performance, safety, and longevity of the unit.

Share your process instrumentation challenges and requirements with process measurement specialists, combining your process knowledge and expertise with their product application experience to develop effective solutions.

Quick Reference Guide for Pressure and Flow Instrumentation

Mass Flow Controller
Courtesy Brooks Instrument

Brooks Instrument is a globally recognized manufacturer of flow and pressure instrumentation for scientific and industrial use. The company's product line ranges through:

Variable Area Flowmeters - Armored metal, glass tube and plastic for reliable measurement of liquids and gases

Mass Flow Controllers - Coriolis and thermal mass flow technology for precision fluid measurement and control

Pressure Controllers - Digital and mechanical pressure regulators and controllers deliver high precision gas control

Pressure and Vacuum Products - Pressure transducers, gauges, and capacitance manometers

Vaporization Products - Deliver controlled high purity vapor to processes from source liquid

There are many products and variants. The company developed a summary document that provides an overview of the various product types, enabling potential users to focus quickly on the instruments that will meet their requirements. The document is included below.

Share your pressure, vacuum, and flow measurement and control challenges with product application specialists, combining your process knowledge and experience with their product application expertise to develop effective solutions.

Flow, Pressure & Vapor Instrumentation from Miller Energy, Inc.

Thermal Mass Flow Meter Q&A From Magnetrol

Thermatel® thermal mass flow meter
Courtesy Magnetrol®

Sometimes you discover that others do something better than you. When that happens, watch and listen.

Tom Kemme, from Magnetrol®, expertly fielded some questions about thermal mass flow meters in a recent blog post. Mr. Kemme's responses were so useful and clear that I decided, with all the credit flowing his way, to share them here for those of you that may not closely follow the Magnetrol® Blog.

Question: What is the difference between the flow units Nm3/h, Sm3/h, and actual m3/h?

Answer: Actual m3/h is a flow rate at operating temperature and pressure. Normal or standard m3/h (Nm3/h = Sm3/h) is a flow rate at standard temperature and pressure (STP). I tend to reference the natural gas industry, where it is not possible to compare flow rates at every operating condition, so it is preferable to reference all flow rates back to a set of base conditions, such as 60°F and 1 atm. STP is not universal so it may be unique based on the region or industry.

Most flow meters output a flow rate at operating conditions and need to correct this measurement. This may be accomplished with a multivariable transmitter or external to the device. A few examples that do not need to correct the measurement are thermal mass flow meters, such as the ones produced by MAGNETROL, and Coriolis flow meters.

Question: Do you have any certified failure rate data on your units to perform an SIL verification?

Answer: A Failure Modes, Effects, and Diagnostics Analysis (FMEDA) is completed during development to determine failure rates and Safe Failure Fraction (SFF). The SFF is utilized to determine Safety Integrity Level (SIL), which is often the published value.

Question: What should my meter be reading with no air flow in the pipe?

Answer: At zero flow and a dry pipe, a thermal mass flow meter should measure zero. Different thermal meters may have varying stability at no flow due to differences in operation.

There are two different types of operation: constant temperature (CT) and constant power (CP). CT devices start with a low power and this power increases with the flow rate to maintain the constant temperature difference (ΔT) between the RTDs. CP devices start with a high ΔT between RTDs at low flow and the ΔT decreases as the flow rate increases. CP may lack stability at zero flow due to possible convection currents associated with the high ΔT. CT will hold zero better, particularly devices that add less heat. For example, the maximum surface temperature of a TA2 probe is 4 C above process temperature. This is extremely low heat, eliminating convection currents due to the sensor. Convection currents could also occur through the pipe due to temperature variations.

It is also possible for a thermal meter to measure above zero during a no flow condition when there is pressure buildup in the line (typically a valve closed downstream). There may be low flow cutoff settings that can be changed to ignore nuisance measurements.

You can easily tap into Magnetrol® expertise to solve your flow measurement challenges. Reach out to a product specialist and combine your process knowledge with their flow measurement expertise to develop effective solutions.

Mounting Options For DP Transmitters With Universal Mounting Bracket

This short video illustrates the ease with which numerous mounting configurations of DPharp transmitters can be accomplished in new and existing operations. The DPharp line of pressure transmitters utilize Yokogawa's top flight digital sensing technology for accurate process measurement of pressure and differential pressure in a multitude of applications.

Share your process measurement challenges with application experts, combining your process knowledge with their product application expertise to develop effective solutions.