Chemical injection – from Africa to Arctic

Two articles in the current issue of Offshore magazine cover very different areas of the world ? West Africa and the Arctic.

They also cover very different scenarios. One is talking about ‘brownfield’ development – extending the life of existing fields and brining new reserves online through existing infrastructure. The other is focused on new technologies and challenges in safely exploiting hitherto unreachable reserves.

The common element is that both articles mention chemical injection as a key technology. This is, of course, an area in which Litre Meter has considerable experience.

Over the past year we have shipped a large number of meters to be used subsea, on a variety of chemicals, over a wide range of flows at high pressure and calibrated at specific viscosities. For example, Litre Meter rotary piston flow meters are part of systems used to control the amount of ‘antifreeze’ injected into pipelines at high pressure (430 bar) in subsea gas exploration on fields in the Caspian Sea and the North Sea.

‘Antifreeze’ fluids like methanol are used as thermodynamic inhibitors, which lower the freezing point of gas hydrate. They are injected into pipelines where there is a risk of hydrates (dew) forming then freezing at low temperature.

The antifreeze prevents gas hydrates solidifying as crystals and blocking pipelines – which can result in a costly shutdown and the risk of explosion or unintended release of hydrocarbons into the environment.

Litre Meter has also recently shipped flowmeters for use in chemical injection skids on a number of fields in the Gulf of Mexico and the Persian Gulf off Dubai. These meters are used in the flow measurement of wax dispersants and pour point depressants (PPD) to control their use to a very high tolerance.

Wax dispersants break apart and prevent the reformation of hydrocarbon sludge deposits and improve flow by reducing the viscosity of the fluid. PPDs are used to reduce the viscosity of oil and to maintain flow rate by preventing the build-up of wax crystals at low temperatures.

Sludge deposits are typically composed of varying concentrations of hydrocarbon, asphaltene, paraffin, water and inorganic materials. They are commonly found in storage tanks and vessels, production and transportation pipelines, process systems and hydrocarbon-producing formations where they have an adverse effect on the flow of crude oil from the well head.

Our expertise is nicely summarised in our new chemical injection brochure which highlights the enormous success of the VFF meter over the past few years in solving chemical injection measurement problems around the world.

With 3,000 VFFs in active use on chemical injection Litre Meter has demonstrated a depth of experience and knowledge that is unmatched in the low flow arena. The brochure provides further detail of the VFF range in one eight-page document.

Positive displacement meters: pros, cons and selection

Positive displacement flowmeters, sometimes known as PD meters, have been around for more than 100 years. They are commonly used in a wide range of applications from domestic water measurement to measuring ultra flow rates of chemical at high pressures subsea.

First off – what is a “positive displacement” meter? Well, as the name suggests it involves the positive displacement of a volume of fluid – this is usually a liquid but there are some units suitable for gas. There is a chamber and inside the chamber, obstructing the flow, is a rotor.
The shape of the rotor and chamber vary greatly with each meter type but they all provide an output for each rotation. Most meter designs therefore lend themselves to being totalisers. Most can have the flow rate calculated from this primary data.
An accurate PD meter will have minimal ‘leakage’ across the rotor seal. This is generally minimised with the use of more viscous liquids and accuracies of ±0.1per cent are sometimes quoted. On the other hand rotary piston flowmeters are used by the water industry in the UK for measurement of water over a normal flow range to accuracies of ±2 per cent.
Because they measure a volume precisely it does not matter if the flow is pulsing. They will follow the increase and decrease of flow found in reciprocating pumps of all types. With higher viscosities the turndown ratio can be high. Even with water 100:1 is not uncommon and 3000:1 is possible at 250cSt. Few applications require this but it does enable measurement of ultra low flow rates without miniature parts or normal flow measurement at minimal pressure drop.
Most meters are simple to maintain as they have only one or two moving parts and are coupled with simple readouts that are easily understood in the field. There is no requirement for straight pipe lengths like that might be needed for electromagnetic or turbine devices. They can be connected directly to elbows or valves and in most cases in a variety of orientations.
Designs are relatively easy to adapt for high pressure applications eg over 100 bar.
All PD meters require clean fluid so a filtration level of 100 micron is usual. Some meters can actually block the flow if a larger particle is trapped in the wrong place. Many meters are not made in high specification materials and therefore corrosion can be a concern. An all plastic or all 316SS meter is the exception rather than the rule. As the application flow rate increases the size of the PD meter seems to increase by a square law! It is rare to find meters over 12-in in size although they exist at these elevated sizes for the prime reason of accuracy – frequently being utilised for custody transfer reasons.
In the author’s opinion, the most common PD meters are as follows:
  • Rotary Piston: As mentioned above these form the basis of domestic water measurement but the design of the rotary piston that oscillates in a circular chamber with a fixed web has been modified and extended to ultra low flows and high flows, as well as high pressures and for food applications. A good all-rounder.
  • Spur gear: The fluid rotates two gears and is forced around the outside of the gears and the inside of the chamber. Depending on the location of the sensor these can yield very high pulses per litre values useful in batching and fast acting processes.
  • Diaphragm (or bellows meter): These are common in many people’s home as their domestic gas meters. When the gas flows through it alternately fills and empties bellows causing levers to crank a shaft providing an output. Very useful for wide-ranging gas totalisation.
  • Oval Gear: Quite similar to the spur gear where two oval gears mesh together and sweep the chamber. The volume displaced is much larger than the round gear. Fairly low cost and some designs available in plastic.
  • Nutating Disc: This meter is the hardest to understand but is effective. The rotor is a circular disc attached to a ball. The shaft on the ball is inclined. As the disc rotates in a spherically sided chamber the disc and therefore the shaft wobble creating an output.
  • Helical Screw: Possibly the most accurate PD: meter two intersecting cylindrical bores are fitted with 2 interlocking helical screws. As the fluid passes through they rotate. On standard applications the author has observed differences of just ±0.37 per cent of reading over 50:1 turndown over annual recalibrations over 10 years – quite an achievement. Also common nowadays fitted on petrol pumps.
  • Slide Vane: Historically the most accurate of PD meters with the rotating element having a number of moving blades that rotate about a fixed cam. Linearities have been claimed of ±0.02 per cent.
  • Others: If we go back to Felix Wankel’s seminal work on rotary machines we see that there are as many designs for PD meters as there are pumps. He explored in a rational way the various shapes of rotor and chamber. While the majority don’t see the light of day in the marketplace this brief essay illustrates the variety in general use, and this is without discussing the Roots meter, wet gas meter and multi rotor designs.
Two decades ago the PD meter was considered to be old technology and likely to be overtaken by more modern electromagnetic and ultrasonic devices. Nowadays the PD meter still represents good value and can provide excellent measurement in a wide variety of duties.

Product labelling – QR codes, documentation access

We also unveiled a new electronic project documentation system. Details of each flowmeter supplied as part of a single project – for example, all the flow meters supplied for a particular chemical injection skid – will be held at a unique URL. The website will include items such as calibration certificates, PMI certificates and material certificates as well as specifications, manuals and instructions.

The address of the website will be printed on a chemical-resistant and wear-resistant label securely attached to each meter. The label will also carry a QR code linking to the website which will make it easy for service personnel on site to call up all the documentation on a smart phone by simply pointing it at the label. We believe most customers will note down the simple address and access it from a control room.

LM QR code
Litre Meter QR code allows remote access to documentation
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