Charles Wemyss discusses positive displacement meters and how they can benefit flow measurement in chemical injection applications.
|VFF meter. Images: Litre Meter|
Keeping up with the global demand for fuel creates intense pressure on producers to ensure oil production continues 24 hours a day, 7 days a week. According to the latest figures from the International Energy Agency’s Oil Market Report, demand for oil in 2014 is expected to increase by more than one million b/d to 92 million b/d. Global demand was 74.8.1 million b/d in 2Q 2013 rising to 77.3 million b/d in the third quarter.
Flow assurance is therefore critical. Systems are designed to best ensure interruption to the production and processing flow is prevented at all times. There are a number of typical chemical injection applications that achieve this. These include, but are not limited to, demulsifiers, wax inhibitors, pour point depressants, corrosion inhibitors and hydrate formation inhibitors.
Chemical injection systems are controlled using multi-discipline modular skids which consist of rotary and mechanical equipment, piping, instrumentation and electronic controls.
Most chemical injection applications face changes in process conditions over the lifetime of the oil field on which they are located. Using flow distribution panels as part of the skid means that substantial cost, space, weight and maintenance savings can be made. Using a robust meter such as a positive displacement flowmeter that can be customized to suit different conditions form an important element of these panels.
Managing ancillaries is essential to meet demand and maximize return on capital investment. Accurately controlling the amounts of chemicals, such as corrosion inhibitors, that are added to crude oil is essential to minimize production costs. If too much is used, they need to be removed when the oil is refined – and this can cost almost 10% of the price of a barrel of oil. If too little is used then the production lines may scale up, corrode through – or worse.
|Litre Meter VVF meters on a Geveke Pompen chemical injection skid|
The chemicals are also expensive. Up to 30% of the running costs of a platform can be associated with the chemicals used to pre-treat crude oil before it is pumped ashore. Many of these chemicals are aggressive or corrosive. The effect of pumping them along tens of kilometers of carbon steel and stainless steel pipes at fluctuating pressures means that particles can erode from inside the pipe. Filters designed to capture these and other particles can become blocked. This can result in jamming the sensitive devices.
Contamination of the fluids can reduce the measurement accuracy of positive displacement flowmeters and lead to the incorrect amount of chemical being used – leading to additional cost to refine the oil.
Therefore, there is increasing emphasis on robust and reliable equipment such as positive displacement (PD) flowmeters. PD flowmeters work using the positive displacement of a volume of fluid. There is a chamber; inside of it, 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 lend themselves to being totalizers. Most can have the flow rate calculated from this primary data.
|A PVD coated meter wafer for Geveke|
An accurate PD meter will have minimal leakage across the rotor seal. This is generally minimized with the use of more viscous liquids and accuracies of ±0.1per cent are sometimes quoted. 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, which may be needed for turbine or other devices. They can be connected directly to elbows or valves and in most cases in a variety of orientations.
As part of a continual process of extraction and refining, accurate and consistent measurement is required to ensure that the consumption of chemicals is monitored accurately to maintain efficiency and control costs. All PD meters require clean fluid so a filtration level of 100μm is usual.
Engineering a flowmeter to meet the NAS1638 standard will enable it to operate reliably when measuring fluids with high particle contamination associated with corrosive chemicals. The NAS1638 standard was originally developed in the US for aerospace components but is now widely used in oil and gas applications. It defines the level of contamination in terms of the maximum allowed particle count for particular particle size ranges.
A VFF meter works by dividing the flow into discrete pockets. This prevents the flow by passing the meter’s rotor and producing errors. Fluid flows through the meter causing a rotor to move within the measuring chamber. A sensor picks up the movement and gives a reading representing an increment of volume flow.
The disc-shaped rotor has an annular groove on its underside, which can hold and transport flow from the chamber inlet to the outlet.
|The integrated flow solutions IRCD skid on Chevron's Tombua Landana drilling and production platform off Angola.|
Some fluid is also transported in a cavity formed between the outside of the rotor and the chamber wall. A center peg under the rotor is constrained to run in a circular groove in the body. A plate is engaged with a slot in the rotor to modify the rotation into an oscillation as the flow passes.
This oscillation allows the fluid to be divided up into positively displaced pockets. The movement of these pockets is detected by a powerful magnet fitted directly above the peg which engages and disengages with a reed switch sensor located in the top of the meter.
A volt-free contact closure output signal is given out for each oscillation, which represents a volume increment.
Because the majority of chemical injection applications tend to use more concentrated fluids, less is needed. Therefore, less is moved and smaller amounts are measured. A more concentrated chemical has a lower flow rate.
The VFF flowmeter is suitable for measuring liquids with flow rates from 0.0004 l/m (0.5l/d) to over 270l/min, at pressure ratings up to 4,000 bar (60,000 psi). It is of intrinsically safe design and manufactured to operate reliably at temperatures ranging from -40 to 100°C.
Research has lead to the development of highly customized versions of standard rotary piston meters. For example, the low-flow capability of Litre Meter VFF meters has been improved by providing the pressure balance chamber and titanium rotor with a physical vapor deposition coating designed to lower the friction properties of the meter to provide extended flow ability.
The additional hardness provided by the PVD coating also improves wear resistance.
Litre Meter’s VFF LF05 is capable of measuring down to 0.03l/hour at viscosities of 2cSt and just 0.02l/hour when the viscosity is 10 cSt.
It has a flow range of zero to 30l/hour, a viscosity range of 0.8 to 2000cSt or greater, an accuracy of ±0.5% of reading and repeatability of ±0.25%. At Offshore Europe 2013 in Aberdeen, Scotland, Litre Meter introduced the VFF LF03.
It takes the capability of the range down to lower flows than ever before – for example, on a fluid with a viscosity of 5 cP the LF03 will measure down to 0.015 l/hour rather than 0.024l/hour for the LF05 and 0.065l/hour for the LF15.
Standard meters are available with a 316 stainless steel body. Customized versions are available in titanium, duplex and super duplex steel bodies.
As a low-flow specialist working with suppliers to major companies such as BP, Chevron, Anadarko, Shell UK and Tyco, Litre Meter has supplied meters for chemical injection in the oil and gas industry for projects around the world for more than 20 years.
This experience in chemical injection applications onshore and offshore has confirmed the instrument’s capability to reliably measure fluids to help maintain flow assurance under extreme conditions of both temperature and pressure.
Charles Wemyss is the chief executive officer of flowmeter specialist Litre Meter, joining the company as chief engineer after graduating with a degree in mechanical engineering from the University of Sheffield in 1982. Overseeing the introduction of the company’s Viscous Fluids Flowmeter in 1986, he was then appointed engineering director in 1990. He purchased the company in 2001, selling it to the TASI Group in 2011.