Corrosion inhibitor: diaphragm metering pump skids for chemical injection
For the exploration and the transportation of oil and gas corrosion inhibitors are used.
The use of corrosion inhibitors is strongly increasing, since the high oil and gas prices favour the exploitation of older fields, in which the oil and gas portion is decreasing. And even fields with low oil and gas portion, but higher water -CO2 / H2S content can be exploited profitably.
The material to be conveyed contains water, CO2 and H2S portions, which can form a sulphurous acid depending on the share of free oxygen and sulphur. This leads to corrosion and pitting even in stainless steel pipelines. The corrosion inhibitors used, either absorb the oxygen or they form a protective coating on the inner pipe wall and thus reduce resp. prevent corrosion.
The effect is measured with so-called corrosion test strips: metal strips with a defined weight, which are weighed in regular time periods in order to determine the erosion.
For smaller fields (low exploitation rates) and short pipelines possibly pipes, valves and fittings of Duplex stainless steel are installed and the corrosion protection treatment is carried out centrally at the gathering centre only.
Slug or regular metering:
Especially offshore the exploitation points are more and more moved into deeper water so that the discharge pressure increases. Whereas onshore 160-25 bar are sufficient, the pressures offshore go up to 900 bar and even more, usually up to 400 bar. The flows usually are around 50 l/h or less, the injection is carried out continuously.
For start-up of an installation the so-called slug metering can be required, i.e. for a short period of time an up to ten times higher amount is injected in order to achieve a faster protection. For this partly the installation of 2/6 pole motors has been successfully used to allow economically both, slug and regular metering.
Among others there are anodic and cathodic inhibitors, in the oil and gas production. The latter are predominantly used.
Due to the high pressures and the fact that some kinds of inhibitors are carcinogen (e.g. chromate/hydrazine) the hermetical LEWA diaphragm pumps are predestined for these applications. We have reference lists dating more than 50 years back, worldwide a huge number of metering pumps and packages have been installed.
Moreover undiluted (lean) corrosion inhibitor can be very corrosive and the use of e.g. Hastelloy valve seats and oxide ceramic valve balls can be required. At lower temperatures the viscosity can be higher.
The pumps for inhibitor metering must be designed for extreme temperatures such as in winter at - 20°C in the North Sea or during the night in desert areas up to -4°C below zero.
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Central distribution of corrosion inhibitors: In case several injection points are supplied with corrosion inhibitors at a central point the use of so-called flow dividers is considered sometimes. The alleged advantage assumed is, that only one big pump for providing the total flow, a safety margin and a multiple distribution system must be installed. Via small valves and pressure balancing equipment the flows are divided and injected at the different injection points.
The space requirement is only allegedly smaller as a separate control is required for each injection point. Moreover the fluid to be injected has to be ultra clean. Otherwise the pressure balancing valves, which are working with a very tight tolerance, get stuck. Ultra clean fluids and smallest components in the field always mean big problems! Additionally, the majority of these systems, provided by different manufacturers, are highly sensible against pressure and viscosity fluctuations. It has been tested that there are flow deviations of up to 25%!
The simple, safe, space saving and, in most cases, even the most economic solution is an easy to maintain multi-headed metering pump with e.g. one stand-by pump head, which can be switched on fast and safely into each injection line by operation of a valve.
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How to know and consider chemical reaction: In some cases the chemical components in the crude oil lead to corrosion and scaling. The "natural" solution is the injection of corrosion and scale inhibitors. The success will be very limited as the effects of these two chemicals practically neutralize each other (base and acid) and thus have no effect.
If this “double problem” occurs, it should be checked onsite, if the injection volume’s reduction of the corrosion inhibitor leads to a residual acid content in the crude oil which dissolves the scaling. In rare cases the injection from two injection points far apart might be successful. |
