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 the 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. LDH 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 1380bar/20,000psi/20ksi in modern systems injecting sub sea. Topsides injection is at lower pressures typically from 200 to 1000 psi.
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, 690bar but also with external pressures up to 345bar/5,000psi. Listed below are all the flow meters within our range that are suitable for chemical injection, including the VFF which is often called the Chemical Injection Flow Meter with over 5,000 installed on Chemical Injection worldwide.
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Rotary Piston Meter
Positive displacement flowmeters, also known as PD meters, measure volumes of fluid flowing through by counting repeatedly the filling and discharging of known fixed volumes. A typical positive displacement flowmeter comprises a chamber that obstructs the flow. Inside the chamber, a rotating/reciprocating mechanical unit is placed to create fixed-volume discrete parcels from the passing fluid. In the VFF the rotor is basically a disc shape with an annular groove on its underside capable of holding and transporting flow from the chamber inlet to the outlet i.e. a rotary piston. The units are manufactured in 316 stainless steel or titanium. They can measure pulsing flows. Suitable for a wide range of fluids from Methanol to Epoxy resin. 0.01 to 33,000 l/hr. +/-0.5% accuracy, +/-0.1% repeatability.
Inline ‘axial’ turbine flowmeters are velocity measuring devices – they measure the average velocity of a fluid flowing through the body of the meter. Mounted within the body of a liquid turbine flowmeter is a vaned rotor. The rotor is centred on a shaft and allowed to rotate on bearings. The shaft is supported in the housing by tube bundles that also provide a measure of flow conditioning for the fluid stream. Good for low viscosity epoxy and catalyst flow metering. Wide size range and excellent accuracy. 0.24-17 up to 3028-56775 litres/min. Overrange 150% intermittent. ±0.25% reading – typical. ±0.1% repeatability.
Helical Screw Meter
Positive displacement flowmeters, also known as PD meters, measure volumes of fluid flowing through by counting repeatedly the filling and discharging of known fixed volumes. A typical positive displacement flowmeter comprises a chamber that obstructs the flow. Inside the chamber, a rotating/reciprocating mechanical unit is placed to create fixed-volume discrete parcels from the passing fluid.
The helical screw principle uses 2 counter rotating gears to form a continuous constant volume cavity. The SRZ is a stainless steel version suitable for high viscosity and abrasive media flow metering. 0.04 to 400 litres per minute, +/-0.1% accuracy, +/-0.05% repeatability. Housing in 316L and 303 with screws in 1.4122 (430), tungsten carbide bearings and FKM or PTFE seals. Pressures to 400 bar, -40°C to 150°C.
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