Rosemount GWR complies with API 18.2 for Custody Transfer

High-performance version of Rosemount 3308 GWR Wireless Level Transmitter delivers enhanced accuracy that can be verified without opening the thief hatch, thereby increasing safety.

Emerson has introduced a high-performance version of its Rosemount 3308 Guided Wave Radar (GWR) Wireless Level Transmitter that complies with the API 18.2 standard guidance for crude oil custody transfer from small lease tanks. The Rosemount 3308 is therefore said to to be the first standalone wireless radar level device to achieve this. The transmitter delivers enhanced accuracy – and also offers performance verification without having to open a tank’s thief hatch, thereby increasing safety.

“The API 18.2 standard places strict accuracy demands on level measurement instrumentation because any uncertainty during custody transfer can have significant financial implications,” said Christoffer Widahl, product management lead with the Emerson measurement and analytical business. “Measurement precision is essential in these applications, and the enhanced performance of the Rosemount 3308 delivers the high accuracy required to reduce uncertainty and comply with API 18.2.”

This new model uses an upgraded microwave module, which makes the Rosemount 3308 more tolerant to difficult process conditions and therefore able to deliver a more sensitive and repeatable measurement with high accuracy. API 18.2 requires level transmitters to operate with 1/8” (3mm) resolution and 3/16” (4.7mm) measurement accuracy, which the Rosemount 3308 achieves when set up in the new high-performance mode. This then enables it to achieve the installed accuracy of 1/4” (6.3mm) required to comply with API 18.2. In standard mode, the accuracy of the device has been improved to 1/5” (5mm).

Accuracy can be easily verified in just a few minutes using the Rosemount VeriCase mobile verification tool. This straightforward procedure does not require a tank’s thief hatch to be opened or any product to be transferred. [Opening the thief hatch can cause high concentrations of hydrocarbon gases and vapours to be released, putting worker health at risk, so eliminating this requirement is an important safety improvement.]

In addition to providing the accuracy required for custody transfer applications, the Rosemount 3308 also delivers reliability in both continuous surface level measurement and interface monitoring applications. It satisfies many applications across refineries, oil fields, offshore platforms and chemical plants, thereby providing a cost-effective standardised solution across an entire facility. The Rosemount 3308 is a top-mounted device that is virtually unaffected by changing process conditions such as density, conductivity, temperature and pressure, and because it does not have moving parts, no re-calibration is required, and maintenance is minimised. A wide range of process connections, probe styles and accessories ensure application flexibility.

For applications involving interfaces, the high accuracy of the Rosemount 3308 helps to maintain product separation by issuing an early warning if an interface is identified where there should be only one liquid. By eliminating this uncertainty and optimising product quality, the unit can help to produce significant savings for end users.

Wireless technology significantly reduces installation and configuration time for level measurement applications and can typically reduce costs by at least 30 percent compared with a wired solution. The Rosemount 3308 can be installed and operating in less than an hour – reliably transmitting data via a wireless gateway to a control system or data historian. Status information and device diagnostics are easily accessible from the control room, reducing maintenance requirements and enhancing operator safety by eliminating unnecessary field trips.

Confusion over radar level measurement

We have learned not to get too confused over suppliers using buzz-words and clever marketing names, but recently it seems the major level measurement system vendors have been introducing new and higher radar frequency systems as their latest development – and therefore, by implication, maybe the best. We were used to 6 GHz, and then 26 GHz radar frequencies, but why should we suddenly go to 80 GHz? Then, perhaps just to add a little excitement to the mix, Endress+Hauser started talking about 113 GHz!

The E+H radar line up that offers 113GHz!

This article was first featured in the journal South African Instrumentation & Control in September 2017, a journal published by Technews

Let’s dispel a few myths. Firstly, in the same way that lasers for fibre-optic communications systems made the technology available to create infrared optical systems for process gas analysers, and mobile phone technology possibly provided the hardware for the first radar level measurement systems; the 80 GHz versions are a result of measurement technology made commercially viable on the back of production investment in the distance measurement systems and parking sensors used in modern cars. So the suppliers take the available sensors and chipsets to create a new industrial product, and then have to find the best applications – in this case, the ones that might benefit from the 80 GHz.

Secondly, E+H do not have a 113 GHz system, this is a marketing statement, made to catch attention – ‘with a wink’ is their expression. They claim a ‘complete radar competence of 113 GHz’ because this is the sum of the many different frequencies their different sensors use! These are 1, 6, 26 and 80 GHz.

So why have different frequencies?

Possibly the best explanation for the applications suited to the different frequencies has been provided by the Rosemount measurement division of Emerson, in their “Engineer’s Guides”. The Emerson expertise stretches back many years, having acquired the Saab Tank Radar business. Per Skogberg, from the Gothenburg HQ in Sweden, separates the devices into low, medium and high frequency, to generalise.

Radar signals are attenuated, i.e. they lose signal strength as they pass through the air, or vapour, above the liquid. High frequencies are more severely affected than lower. When the air has moisture, steam or liquid droplets (from spray or filling) present, the attenuation is higher. Equally in solids applications, dust particles have the same effect. So low and medium frequency radar are best when there is dust or moisture present.

At lower frequencies, the wavelength is longer (30-50 mm), so surface ripples in a tank have a small effect. At higher frequencies, surface ripples and foam on the surface can be a problem. But the shorter wavelength of the high frequency units (4 mm) allows accurate operation over short ranges, for example in small tanks. The higher frequency units can use a smaller sensor construction, so the unit is easier to install. The beam angle is narrower, so it can be aimed at a smaller target area and therefore can be positioned more easily to avoid any obstructions in the tank. But even this can be a disadvantage, as the installation needs to be exactly vertical and any turbulence of the surface during filling or stirring can cause the signal to be lost temporarily, in larger tanks.

When reading these suggestions, it is important to remember that Emerson does not offer an 80 GHz unit yet, so their marketing approach would naturally bias users to look at low and medium frequency units. The suppliers of high frequency units (Vega, Krohne and E+H) would point out that in many liquid storage tanks the surface is undisturbed, since any foam, turbulence and significant ripples (>2 mm) caused by filling or liquid transfer will only cause short-term interference. Plus the small antenna size and short range performance make 80 GHz units very useful for smaller process vessels and tanks.

Radar system types

There are two types of radar systems, Guided Wave Radar (GWR) and Free Space Radar. The GWR systems use a conducting rod, or similar, extending down into the liquid, often working in a stilling chamber attached to the main process tank. These operate at low microwave frequencies, and are independent of surface turbulence and foam. They are useful for shorter range measurements and interface measurement between liquids, as well as long ranges.

The Free Space Radar systems are more widely used, since they are top-mounted with nothing in the tank: indeed, some can operate through non-conducting windows in the tank roof. Low and medium frequency radar systems generally transmit a signal pulse and measure the liquid distance by the time delay for the returned pulse. High frequency (80 GHz) systems use an FMCW radar measurement, where the frequency of the transmission is swept, and the frequency difference of the returned signal is measured to assess the distance. The FMCW technique is also used at 26 GHz in some recently launched sensors.

Radar systems can transmit their measurement data using 4-20 mA, fieldbus systems like HART, FF, Profibus PA and Modbus, or indeed via wireless systems like Bluetooth. The low and medium frequency pulsed radar systems generally operate over a two-wire interface: some of the higher frequency FMCW systems require more power and use a separate power connection.

Major applications

Simple low-cost radar level measurement sensors have been specifically designed for water industry use, in sewage sumps and flume flow measurement, by Vega and Endress+Hauser. Vega suggest that 40,000 such sensors are now in use in the water industry, mainly in Europe, and claim their total output of such sensors exceeds 550,000 units over the last 25 years.

Several of these devices use simple Bluetooth interrogation and programming from a handheld PDA: E+H demonstrates this at its facility in Maulberg, working on the stream that runs through the factory complex, as seen below.

Both E+H and Vega produce further industrial units for use on process vessels, and storage vessels for solids and liquids. Recently, E+H has extended its capability to add long-range units, such as the 80 GHz FMR62, working at up to 80 m range, with an accuracy of 1 mm. Other units work up to 125 m range, at 3 mm accuracy. These units will eventually be aimed at the large petrochemical industry storage tank markets, and specifically are working towards use for custody transfer duties.

Krohne have similarly announced a new range of its 80 GHz Optiwave sensors. Some of these can even operate at up to 700°C, for example for use on molten salt vessels in solar power plants. Lower specification units rated at up to 150°C can be used through a tank roof made of plastic, or similar materials. Suitable for small or narrow tanks, the unit can measure ranges of up to 100 m. Krohne also offers lower frequency Optiwave systems for use on solids and powders, or to electronically monitor the float position in magnetic level indicator columns attached to process vessels.

Postscript: Krohne is organising a webinar with the title “80 GHz Radar Level – Allrounder or Overrated?” to discuss their recent developments with such systems. This webinar will take place on 18^th October 2017 at 3pm London time/10am New York time.