Continuous and Point Level Control Selection Guide

Drexelbrook Continuous and Point Level Control Products

Based on experience from of thousands of inquiries about level measurement, most questions boil down to "What is the best level measurement instrument for my application? That depends on quite a few application considerations, such as:

• Will the measured media add coating to the probe?
• Can the probe have contact with the media?
• Are there explosion hazards?
• What is the physical type of application, well, tank, open channel or floating roof tank?

This document can help deliver the perfect continuous level transmitter or level switches for your application.

Download a PDF version of the Drexelbrook Continuous and Point Level Control Selection Guide here, or view the embedded document below.

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Drexelbrook Continuous & Point Level Selection Guide from Piping Specialties, Inc and PSI Controls

Floating Roof Tank Spill Prevention

Floating Roof Tank Spill Prevention (Drexelbrook)

One of the most difficult and critical applications is measuring the high alarm or potential overfill condition on a floating roof tank containing liquid petroleum products such as crude oil or refined products such as fuel. It normally is comprised of a cylindrical steel tank equipped with an internal or external floating roof, that floats on the surface of the stored liquid. Floating roof tank systems are especially beneficial in eliminating the evaporative losses of the liquids. As opposed to a fixed roof tank there is no vapor space in the tank. This helps to reduce risk in highly explosive vapor environments. This is an extremely high cost of failure application, and one in which only the safest and most trusted products are accepted.

Drexelbrook's
Intellipoint

Safe operation of the tank farm relies on critical real time continuous level measurements of the liquids in the tank, as well as detecting when a high level condition exists. Products used in this application are typically required to meet the API 2350 Overfill Protection Standards, as well as SIL Safety Integrity Level performance standards to IEC 61508.

The challenges to reliably detect a high level condition on a floating roof tank are long sensor length requirements, and the variability of what is being measured. The floating roof may be dry in which case you need to detect the position of the physical metal roof. Or there may be a few inches of rain water or petroleum liquids on the roof. Measuring instruments need to determine very accurately, usually within a few millimeters, when the position of the floating roof has reached a high level alarm condition.

Drexelbrook's Intellipoint, with its unique floating roof probe, can accurately detect and alarm on the position of the floating roof or the presence of liquid under all these conditions. The safety Intellipoint is a SIL2 fully certified RF admittance point level switch with uncompromising reliability for the most demanding applications.  Drexelbrook has almost 60 years of RF admittance technology experience and is proud to offer this specialized product as the latest in its award-winning portfolio for the level market.

Product features include:

• Adjustable up to 15 feet, or 4.6 meters, to accurately control the alarm point. 
• A trip point accuracy of a few millimeters.
• Fully SIL2 certified to IEC61508. 
• Worldwide hazardous area approvals .
• Meets overfill protection standards API2350.
• A floating roof tank probe that is unique in the industry.

Understanding Guided Wave Radar Level Instruments

Guided Wave Radar (GWR)
level transmitter (Drexelbrook)

One of several technologies used for level measurement in process control is guided wave radar. A Guided Wave Radar (GWR) level transmitter combines time domain reflectometry (TDR), equivalent time sampling (ETS), and low power circuitry with a form factor that includes a wave guide extending into the contained media. TDR measures distance or level using pulses of electromagnetic energy. The pulse travels along the waveguide until it reaches the media surface and is reflected back to the unit. The speed of the pulse is known, so an accurate measure of the travel time for the signal can be processed into a distance measurement. Different media will produce a range of amplitude in the reflection, with a greater dielectric difference between air and target medium producing higher amplitude in the reflection. Industries, such as telephone, computer, and power transmission, have relied on TDR for years in order to detect and pinpoint breaks in wires or cables, making the technology more mature than it may appear by its limited timeline in level measurement applications.

ETS is used to measure the high speed, low power electromagnetic energy, and is typical when applying TDR to level measurement technology, where the signal travel distance and time are very short. The electromagnetic signals are captured by the ETS technology in nanoseconds, and are then reconstructed in the equivalent time of milliseconds. The radar scans the waveguide, collecting thousands of samples to be used in signal processing. Integrating both technologies into a single level transmitter yields an accurate and responsive instrument for process measurement.

GWR instrumentation is useful in the process control industry for its ability to measure levels in a quick, consistent way. GWR transmitters are contact radar level measurement tools, as opposed to pulsed non-contact radar transmitters that emit radar pulses through free air without a waveguide. Probes, inserted into the subject tank or vessel, serve as the waveguide for the pulsed signal. They guide the pulsed microwave vertically into the tank, providing a measure of immunity from disturbance by the tank and surrounding media. Guided wave radar technology differs from non-contact radar in a number of ways. The presence or absence of a probe is only one of them.

GWR level transmitters are used in process measurement applications throughout many industries, such as food and beverage. Tanks, pumps, and piping systems for both storage and transport can utilize GWR to continuously monitor levels. Other vessels, such as reduction, forming, mixing, heating, cooking, and cooling, can utilize GWR for similar reasons. Additionally, other stages of food and beverage manufacturing, such as centrifugation and decontamination, can be good fits for GWR technology. Guided wave radarís previous applicability in industries aside from liquid processing and implementation in a wide range of process settings show the flexibility and reliability of GWR technology.

Selecting the best level measurement technology for an application can be a challenge. Share your project requirements and concerns with a process instrumentation specialist, combining your own process knowledge and experience with their product application expertise to develop effective solutions.

Measurement Solutions for the Water and Wastewater Industries

Measurement Solutions for the Water and Wastewater Industries from Piping Specialties, Inc and PSI Controls

Understanding Hydrostatic Pressure

Hydrostatic level transmitter
(Drexelbrook)

Pressure measurement is an inferential way to determine the height of a column of liquid in a vessel in process control. The vertical height of the fluid is directly proportional to the pressure at the bottom of the column, meaning the amount of pressure at the bottom of the column, due to gravity, relies on a constant to indicate a measurement. Regardless of whether the vessel is shaped like a funnel, a tube, a rectangle, or a concave polygon, the relationship between the height of the column and the accumulated fluid pressure is constant. Weight density depends on the liquid being measured, but the same method is used to determine the pressure.

A common method for measuring hydrostatic pressure is a simple gauge. The gauge is installed at the bottom of a vessel containing a column of liquid and returns a measurement in force per unit area units, such as PSI. Gauges can also be calibrated to return measurement in units representing the height of liquid since the linear relationship between the liquid height and the pressure. The particular density of a liquid allows for a calculation of specific gravity, which expresses how dense the liquid is when compared to water. Calculating the level or depth of a column of milk in a food and beverage industry storage vessel requires the hydrostatic pressure and the density of the milk. With these values, along with some constants, the depth of the liquid can be calculated.

The liquid depth measurement can be combined with known dimensions of the holding vessel to calculate the volume of liquid in the container. One measurement is made and combined with a host of constants to determine liquid volume. The density of the liquid must be constant in order for this method to be effective. Density variation would render the hydrostatic pressure measurement unreliable, so the method is best applied to operations where the liquid density is known and constant.

Interestingly, changes in liquid density will have no effect on measurement of liquid mass as opposed to volume as long as the area of the vessel being used to store the liquid remains constant. If a liquid inside a vessel that’s partially full were to experience a temperature increase, resulting in an expansion of volume with correspondingly lower density, the transmitter will be able to still calculate the exact mass of the liquid since the increase in the physical amount of liquid is proportional to a decrease in the liquid’s density. The intersecting relationships between the process variables in hydrostatic pressure measurement demonstrate both the flexibility of process instrumentation and how consistently reliable measurements depend on a number of process related factors.

Visit PSI-Team.com for more information on pressure and level instrumentation.

Level Measurement Technologies Provide Accurate Level Control & Ignore Foams in Filler Bowl Applications

RF Admittance and Magnetostrictive technologies have a proven performance record in gravity feed flow control for the dispensing of liquids into bottles or containers.

In high speed bottling operations many different filling methods can be used depending on the nature of the product and type of container being filled. For many Food and Beverage, and Pharmaceutical applications the preferred filling method is by using level measurements to control the gravity ow of liquids into bottles or containers from the filler bowl. The level measurement method is very consistent with liquids and slurries to prevent over-filling or under-filling of a bottle or container by keeping a consistent product level in the filler bowl. 

Level filling is the oldest filling method and is still largely favored in specific markets. This is largely due to products being sold in translucent containers. The consumer expects to see that all containers are filled to the same precise level and will reject a container with a level lower than others on the shelf. 

In a gravity feed filler bowl, the natural head pressure of the liquid is used to ll each bottle. The liquid level in the filler bowl must be kept at a constant level so the pressure within the filler bowl remains constant. This permits each bottle or container to ll to the correct level in the same amount of time. 

The Problem: 

Hydrostatic pressure level measurement systems, which have been traditionally used for this application, are found to have errors in level measurements when changing from one process material to the next, which usually has a slightly different specific gravity. As the process fluid’s specific gravity is changed, this leads to either an over- ll or under- ll condition. 

The Solution: 

AMETEK-Drexelbrook provides sanitary 3A approved systems in both RF Admittance and Magnetostrictive technologies for use in filler bowl measurements that remain unaffected by changes in specific gravity, changes in temperature or changes in pressure or vacuum. Both technologies can provide the accuracies that are required for reliable performance in the face of light or heavy liquid viscosities, foaming conditions, and have the ability to ignore process coatings that may develop on the sanitary sensors. The sensors are of rugged construction and will not be affected by the shock or vibration of the bottling process. 

RF Admittance systems are supplied with a Triclover fitting with a rigid Teflon coated sensor the length of the measurement range. Accuracy is ±1% of measured span. Systems are agency approved as intrinsically safe for Class I, Div. 1 hazardous installations. RF Admittance has the ability to measure a wide range of process materials and ignore most foam and process build-up on the sensor. Systems are powered by a two-wire, 24Vdc power source. 

Magnetostrictive systems are supplied with a Triclover fitting and use a 240 grit finished 316SS rigid sensor and oat. Accuracy is 0.1% of measured span. Systems are agency approved as intrinsically safe for Class I, Div. 1 hazardous installations. Magnetostrictive systems can easily ignore foaming conditions as the oat will sink through the foam and rest on the liquid surface. Systems are powered by a two-wire, 24Vdc power source. 

AMETEK-Drexelbrook systems can provide analog 4-20 mA, HART, or Honeywell DE outputs. Sensor lengths can be as small as a few inches to over 10 ft. All systems are maintenance free and can be easily con gured without complex calibration. 

AMETEK-Drexelbrook has successfully supplied filler bowl level measurement systems to many major Food & Beverage and Pharmaceutical customers over the past 40 years and have hundreds of successful applications on products such as milk, fruit and vegetable juices, jellies, baby foods, soups, beer, spirits, ground meat, pet foods, sodas, and more.