Chemical injection is the process designed to assist in the production of oil where various chemicals are injected into the crude oil to provide a degree of protection. For example; a scale inhibitor will prevent the build-up of scale on the pipes and fittings used to transport the oil on its journey to the refinery. Flow meters are used to monitor and control the amount of chemicals added to crude oil. and biocide prevents the build-up of organisms in the pipe.
Corrosion inhibitor is the most popular additive to be measured but there are various types depending on the type of main liquid. Pour point depressant is added to reduce the pour point, thereby making the oil easier to flow through the main pipelines. Most of the chemicals added tend to be at low flow rates of the order of 0.01 to 40 litres per hour. LDHI or Low Dosage Hydrate Inhibitor, has the highest injection rate, sometimes up to 17,000 litres per hour. It is designed to inhibit water-based mixtures which would otherwise have a tendency to freeze.
Methanol is often used with the same effect, but more exotic chemicals are used for similar purposes, all with the intention of reducing cost on the way to the refinery. It is a less known fact that flow meters are used to measure these chemicals, which contribute 30% of the cost of running an offshore platform. Control systems are often manually set from the flow meter displayed value (either locally or from the SCADA system), and there is a trend towards automation of this process using control valves.
A typical turndown ratio of 500:1 is measurable with some Positive Displacement flow meters. Often the systems are relatively slow dynamically and need only slight adjustments from day to day, which is why manual controls are still popular. It is important that the flow measurement system for the chemicals is able to cope with the pulsing nature of the pumps used and also with the high pressures that are encountered. These can be up to 1380 bar/20,000 psi/20 ksi in modern systems injecting sub sea. Topsides injection is at lower pressures, typically from 200 to 1000 psi. Litre Meter units are rated at 530 bar, 690 bar, 1035 bar, 1380 bar and 2500 bar.
There is also a trend towards measuring the chemicals sub-sea, i.e. underwater rather than when the crude oil reaches the surface. Not only does the flow meter have to work at high internal pressures of over 10,000 psi, 690 bar but also with external pressures up to 345 bar/5,000 psi. The flow meter suitable for chemical injection is our VFF which is often called the Chemical Injection Flow Meter, with over 5,500 installed on Chemical Injection lines worldwide.