Diaphragm metering pump skids for methanol injection in gas dehydration
In the oil and gas industry methanol is mainly used for dehydration and de-icing.
The product is relatively cheap as it is a by-product of the refining process. Sometimes ethanol is used but this is not very common and so will not be addressed separately. Ethanol is less dangerous than methanol and can be separated by the human body (alcohol). Methanol itself also is not poisonous but the enzymes removing it from the human body are forming formaldehyde and formic acid, which are highly reactive (formaldehyde) or extremely sour (formic acid). The reaction time is 15 hours or more so that the problem is not connected to the cause anymore. This effect is one of the major reasons to treat methanol with care and process it as tight and free from leakage as possible!
Methanol is very inflammable and burns with a bright flame - hardly visible during daylight and therefore extremly dangerous. As mentioned earlier, it leads to poisoning when inhaled or swallowed, even skin contact is dangerous. It forms ignitable gas/air mixtures, electrostatic ignition is possible. Decomposition products are carbon dioxide, carbon monoxide and formaldehyde, a concentration of only 25% in water and above can still ignite and burn!
Methanol is injected both, intermittently (i. e. slug dosing) or continuously. It prevents hydrate formation predominantly in offshore wells and/or reduces the freezing point of low water contents during oil and gas transport. Contrary to
glycol used for dehydration, methanol does not have to be removed from the main product.
Safe and zero leakage operation even at high pressures:
Operating pressures usually are 350 bar and above. Some North Sea and Gulf of Mexico installations operate at 900 bar, pressures of up to 1200 bar are possible. LEWA can offer safe diaphragm pumps that are free from leakage to handle this. The flows range from several hundred (continuous injection) to several thousand litres/hour (slug dosing when e.g. offshore gas wellheads are opened).
In rare cases methanol substitutes are used, so called LDHIs (low dosage hydrate inhibitors). This is the case when the refinery processing the production of the platform has a zero methanol tolerance because its final water treatment stage is biological. The microbes handling the last removal of oil residues would be killed by the methanol traces. As the LDHIs are much more expensive than methanol, they are used only when actually specified.
If this is the case one should know that the LDHI injection rates are much lower, usually 1/10 or 1/20 only of the methanol flows and also that, at higher pressures (>350 bar), it can crystallize and become very abrasive (e.g. requiring tungsten carbide valves).
Another very common problem offshore is the fact that methanol usually is stored and transported in steel vessels. Aluminium or copper lining or zinc coating corrodes under the ambient conditions and paint coatings are attacked or dissolved by the methanol so that the steel is exposed to salty and humid air when empty or partially filled only. This leads to rust built-up and its washing-off by the methanol movement in the tank and so to very hard and abrasive rust particles in the methanol. These solids are small and difficult to filter out. They damage valves and - much worse short-term - the plunger and packing of plunger pumps. Valve wear can be controlled by suitable materials, plunger and packing wear by a high quality and high pressure diaphragm pump from LEWA.
LEWA diaphragms are metal sealed:
Other specific advantages of LEWA diaphragm metering pumps should also be mentioned:
- The M900 design has suction lift capabilities like plunger heads because
of their spring supported diaphragm return - The pump crank cases are splash lubricated, no separate oil pumps including the control and monitoring equipment required for these pumps are necessary
- All moving parts of the pump head including the plunger and plunger rod are moving well protected in an oil bath – a big advantage offshore and even under adverse onshore site conditions
As methanol is also attacking many rubber and plastic materials LEWA can offer properly phased multi-head arrangements to achieve low pulsation and smooth injection even without pulsation dampers. This also solves another frequent and often not thought about problem - how does one charge a damper to approx. 70% of the high operating pressure on site? Here expensive and difficult to handle pressure boosting equipment must be available.
For smaller wells and lower flows requiring slug and continuous methanol injection the use of multipole motors should be looked at so that both duties can be handled by one pump only.
Very low maintenance requirements:
So these pumps are designed for continuous or intermittent operation in low, medium or high pressure injection applications. Because of their extremly low maintenance requirements and safe monitoring they are suitable for both, manned or unmanned operation. Diaphragm service life has proven to be up to and in excess of 10000 hours in North Sea and Gulf of Mexico installations. LEWA's reliable and fast diaphragm monitoring system has provided a considerable increase in process safety at this extreme pressure range. State-of-the-art machining procedures, high fatigue strength materials and laser technology have made this possible.
Should an alarm occur, e.g. by wear caused by mis-operation (the diaphragm size actually is such that it operates far below its fatigue limit so that damage caused by material fatigue is very unlikely) the diaphragm condition monitoring sensor (pressure switch or pressure transmitter) would signal this without delay. Because of the sandwich diaphragm the high pressure pump does not require immediate shutdown, causing an expensive interruption of the process. It allows continued operation of the pump until the maintenance crew is onsite and repair is convenient. The pump remains hermetically tight until then and contamination of the environment and danger to the operating personnel or to the whole installation is safely prevented.
Additional safety thanks to proven solutions:
Due to their design LEWA pumps can easily handle high suction pressures and/or shut down at high discharge pressure. As mentioned earlier the plunger operates in the hydraulic oil and either matched bushings with fitting plungers or plungers with piston rings are used depending on the pressure rating. The high friction losses of packed plungers are avoided and the energy consumption is optimized. Leakage collection and disposal or re-injection like for plunger pumps do not have to be considered.
Once more the LEWA company so has exceeded previous technical limits and is offering yet another reliable and proven solution to the users of high pressure, diaphragm type, chemical injection pumps and packages. The costs for replacing a sandwich diaphragm are much below the costs for replacing a set of packing and plungers, the diaphragm service life is a multiple of the service life of a packing, diaphragm replacement does not take much more than a torque wrench and common sense.
The additional safety and reduced power consumption offered by LEWA pumps and packages make their use economical after a short time period. LEWA offers the biggest diaphragm metering pumps available worldwide, the biggest quadruplex pump has a drive power of 1000 kW!
The decision to opt for LEWA’s high-pressure diaphragm pumps is not merely prompted by their compact modular design, allowing them to fit into the space available for example on FPSOs – also because LEWA technology has already proved its worth for years in many worldwide applications.
Make LEWA your choice to increase safety, profitability, up-times and production!
