Posts Tagged ‘Pressure measurement’

On the level: A guide to level measurement

March 19, 2014

ABB offers an extensive range of level measurement instrumentation. Les Slocombe explains about the recent developments in technology and what the future holds for level measurement applications.

What are the key recent developments in level measurement?

Being able to transmit level data via 4-20 mA, HART, Foundation Fieldbus and the other commonly used communication methods is a key advance in level measurement. An example is our magnetostrictive level transmitters, which come ready to support communication protocols, allowing transport of level measurement information.

How has this changed the way level measurements are performed?

In the past, level measurement meant an operator going round to check the level at the vessels. Today’s level transmitters can relay this information to the control system so it appears on screen in an easily readable format.

What benefits have these developments brought?

As well as being far more convenient, it can also be much safer, keeping personnel away from potentially hazardous materials.

As well as the convenience of easy communication, many of today’s level measurement systems are also easier to install, configure and maintain. Previous generations of level technology often required a fair amount of technical knowledge to install and configure properly, today’s software rich level measurement systems allow just about anybody to commission the device.

There has also been a significant increase in the accuracy of interface measurement. The latest magnetostrictive technology is capable of accuracies 0.010 per cent to 0.25 per cent. This compares extremely well with older methods like hydrostatic tank gauging, which could only offer accuracies ranging from 1 per cent to 25 per cent.

How should level measurement solutions be chosen?

One of the most important considerations, particularly when specifying magnetic level gauge technology, in is in the manufacture of the float. A magnetic level gauge relies on a float to give a level measurement, which must be designed specifically to suit the process medium being measured. A magnetic level gauge can give 30 to 40 years of reliable service, but a poor choice of float construction could cause performance and reliability to suffer.

Avoid the temptation to go for level measurement systems that can do everything. Concentrate on the core goals of the application and keep in mind that the product itself is less important than the solution you’re trying to achieve.

To guide your selection, keep some essential question in mind:

• What kind of accuracy does the application require?
• What is the temperature, pressure, media type?
• What is the temperature range of the application?
• What is the budget?

Overall, keep three key factors in mind when evaluating your level measurement systems:

• Safety
• Efficiency & reliability
• Environmental

What should I look for in a level measurement vendor?

Look for a supplier with a broad range of level measurement solutions, as they will be able to identify the appropriate solution for your application. They might also offer online tools to make commissioning, installing and troubleshooting the level measurement system easier and more convenient.

Also, keep in mind that a low cost product may not offer the performance and safety levels provided by current generation level measurement solutions.

Talk to the manufacturer and give them as much application information as possible. For example, if the application involves a dirty fluid with the possibility of build-up, then a float-based approach will require special considerations. Again, if the application is susceptible to a high vibration levels, a reed switch technology would probably not be ideal.

What does the future hold for level measurement?

There will be more support for protocols in level measurement systems, including 4-20 mA, HART and Foundation Fieldbus, together with more emphasis on functional safety with products certified to standards such as SIL2.

The future is sure to see level instruments incorporating more digital features, as well as using wireless in creative ways to provide application visibility and flexibility for level measurements.

ABB now offers an extensive range of level measurement products across a variety of applications in pulp and paper processing; mining and construction; food and beverage manufacturing; oil and gas production; and chemical processing. For more detailed information on ABB’s level measurement offering please phone 0870 600 6122 re: ‘level measurement’ or email moreinstrumentation@gb.abb.com.

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Top tips for pressure transmitter selection

November 4, 2009

Les Slocombe, Pressure Temperature UK Business Manager at ABB Instrumentation, gives his top tips on how to choose the right pressure transmitter for the job at hand.

The variety of potential applications and installation locations for pressure measurement means that it’s difficult to set any hard and fast rules. At ABB Instrumentation we have developed six top tips to help you shape and narrow down the choice of pressure measurement device for your application.

1. Transducers or transmitters?

Many people give confused over the difference between these two devices.

A transducer creates a low-level electronic signal in response to changes in applied or differential pressure and features an internal sensor that converts the applied force into an electric signal, from which the measurement is derived. Transducers are generally unsuitable for the harsh environments typical of many industrial applications as they are poorly protected against the effects of overpressure or damage caused by sudden variations in process conditions.

Transducers also have inferior stability and measurement accuracy compared to transmitters. They offer limited compensation for variations in process or ambient conditions such as temperature and are often fixed range devices, which can only measure within a set span.

Transmitters for measuring pressure or differential pressure, on the other hand, comprise two basic parts. A primary element directly or indirectly in contact with the process collects the measurement, while a secondary electronics package translates the output from the primary element into a standard 4-20mA dc output signal.

These secondary electronics are highly sophisticated and perform many functions that transducers cannot. Variations in process or ambient conditions measured by the primary sensor can be automatically compensated for before being converted into a 4-20mA signal.

This minimises unwanted measurement errors and gives the transmitter a very stable output. Operators can also calibrate the transmitter over a range of input pressures, enabling one unit to be used to measure a range of spans.

2. Consider your operating environment

Modern pressure transmitters should be able to comfortably handle adverse temperatures, humidity and vibration conditions provided they are within design specification limits. Minimising the effects of such conditions will help to maximise the transmitters operational life.

  • Temperature

Most electronic transmitters are suitable for conditions ranging from lows of  –20°C to –40°C to highs of 60°C to 85°C, although this may not always be the case for certain types, for example where special filling materials have been specified for the transmitter. An application’s ambient temperature conditions can significantly affect transmitter efficiency; high temperatures can have a detrimental effect, potentially causing premature component failure. Ideally, the temperature of the transmitter should be kept as low as possible for maximum life expectancy.

Careful consideration also needs to be exercised when installing a transmitter outdoors. Atmospheric conditions such as direct sunlight or high winds can cause heating or cooling of transmitters, which can adversely affect their operation.

  • Humidity

Vapour caused by humid conditions can sometimes penetrate the transmitter housing and attack sensitive components. Prolonged exposure to high humidity can also result in corrosion of the transmitter housing and mountings. Transmitter housings are designed to protect electrical components against the ingress of moisture caused by humidity.

Some manufacturers employ various methods such as using potting material to protect transmitter electronics against humidity. Whilst they can delay humidity problems, these methods do not provide a long-term solution. The only true prevention against humidity is for the transmitter housing to be hermetically sealed.

  • Vibration

Avoid installing a transmitter in an area subject to prolonged or unnecessary levels of vibration, as this can reduce the service life of the transmitter. To protect against potential damage or malfunction caused by vibration, transmitters should be mounted in a location that will be unaffected by vibration when a process is in operation.

 3. Two wire, four wire or digital?

 There are many different types of transmitters, each of which use different techniques to transmit a signal. The role of the transmitter is to amplify and condition the signal so that it can be relayed over long distances to devices such as indicators, recorders and controllers without deterioration or interference.

 For the majority of applications where power is readily available, the two-wire transmitter, which uses a 4-20mA current both to operate its circuitry and to relay a signal, is often the most practical choice. Because the current is protected against the effects of changing resistance along the line, signals can be relayed over long distances.

For applications where a ready supply of power is not available, such as remote installations, then a low powered four-wire electronic transmitter is the best choice.

Four-wire devices use two wires to power the instrument and two wires to transmit the signal. These low powered transmitters typically consume about one-tenth of the energy of standard two-wire transmitters under normal operating conditions. Despite consuming less energy, they are only really suitable for applications where the devices to which they are to be connected are close by. This is because long transmission distances produce a loss of voltage signal due to the internal resistance drop in the line, affecting measurement accuracy.

Where high-speed transmission of accurate data is essential, the best choice is to opt for digital transmitters, which transmit both digital and analogue signals via a two-wire link. Digital signal transmission is faster and more accurate than analogue and allows much more data to be relayed between the instrument and the control room.

ABB’s pressure transmitters are available with a choice of communications standards including HART/4-20 mA, Profibus, Modbus and Foundation Fieldbus. All ABB pressure measurement equipment is also based on ABB’s IndustrialIT platform, enabling it to be used with other ABB IndustrialIT enabled products to form part of a complete process system.

4. Is the application hazardous?

Any electronic instrument either stores electrical energy or is a source of electrical energy. In certain conditions, this energy, if discharged could ignite any accumulated mixtures of flammable gases, combustible dusts and ignitable fibres that may be present.

Caution needs to be exercised when locating any electronic pressure transmitter in a hazardous or potentially hazardous location.

The recently introduced ATEX Directive sets out various measures for assessing the risk posed by hazardous environments and the steps that need to be taken to minimise the risk of ignition.

There are three main approaches that can be taken when installing a pressure transmitter in a potentially hazardous area. The first is to locate the transmitter in a safe area, where there are no ignitable gases or combustible dusts present. A remote seal is then used to perform the sensing in the hazardous area. This prevents the device’s energy source from being in the hazardous area.

The second approach is to specify a transmitter with an explosion-proof housing, which can contain the ignition source and prevent it from discharging outside of the device.

The third, and potentially best approach, is to use an intrinsically safe pressure transmitter device. These instruments do not have sufficient energy to cause ignition, making them ideal for use in hazardous locations. 

ABB’s Safety Pressure Transmitters provide a single range of pressure measurement devices for hazardous applications that call for SIL2 and SIL3 integrity, such as in the chemical, petrochemical, pharmaceutical and oil and gas industries. In a SIL2 environment, a single safety transmitter can provide the same level of protection as two conventional devices, eliminating the need for two devices and effectively reducing operating and lifecycle costs by 50%.

 5.   Do you need a remote seal?

Remote seals are used to isolate pressure transmitters from conditions that will either shorten their operational life or dramatically affect their performance. Comprising a transmitter body, a capillary and a seal element incorporating a liquid filled diaphragm, remote seals are ideal for a range of pressure measurement applications involving process fluids which are:

  • Highly corrosive
  • Dirty, viscous or laden with solids that can block or foul the impulse lines
  • Likely to solidify in the impulse lines or the transmitter body
  • Too hazardous to enter the control area where the transmitter is located

 Remote seal transmitters can also provide an ideal solution for hygienic processes, such as in the pharmaceutical and food and beverage industries, where it may be undesirable to have a pressure measurement device in direct contact with the product.

 6.    What else do you want to measure?

 For certain applications involving the measurement of gases or fluids subject to rapid density changes, it may be advantageous to use a multivariable pressure transmitter device. These devices offer a three-in-one solution for the measurement of flows of liquids, steam or gas with absolute pressure and temperature compensation, ideal for calculating changes in flow density.

Previously, the main method of calculating flow density involved deriving measurements against known standard conditions. Though fine for applications with constant or relatively minor deviations in process conditions, this approach is less effective for installations where high accuracy measurement is required or where fluctuations in the flow medium are likely to occur.

For ABB’s multivariable transmitter, the combination of three different measurements into one device enables users to select appropriate accuracies for their applications ranging from 0.04% to 0.075%.

Incorporating three different forms of measurement into one unit can also significantly reduce installation costs savings on installation through the need for fewer devices and a reduction in the amount of cabling and I/O devices required.

Although there is a lot to take on board in these top tips bearing all these factors in mind when specifying a pressure measurement device is a guaranteed way to ensure you select the right transmitter for the job!”