Due to the fact that the main problem arises with the disposal offshore we consider this case in detail. Disposal onshore is not problematic as storage facilities are available, partially the HCCs are also separated and
further processed.

Offshore the HCC is separated from the gas and intermediately stored in so-called knock-out drums. Usually the re-injection resp. - for several platforms - the transportation to a central gathering station for re-injection
resp. for further transportation onshore and for processing is carried out via the float switches at the knock-out drums.

This means that the flow can be somewhat variable (depending on the selection of the pump), however pressure and availability are important aspects. The pumps must withdraw a sufficient volume so that the LSHH of the knock-out drum does not trigger otherwise the production must be stopped.

Additionally the following substances are contained:

• H₂S: corrosive and highly toxic
• CO₂: high vapour pressure, non-lubricating, freezes when decompressed (leakage)
• Mercury: highly toxic and hazardous to water in some gas production places up to 250 ppb
• Water that was also separated from the gas and which is very aggressive resp. corrosive together with H₂S; small fractions of diesel are not dangerous apart from the potential contamination of water
• Note: water + H₂SO₄ => acid = very corrosive
• Sand particles in HCC (low, normally < 1%), which are highly abrasive at the operating pressures applied of up to 250/300 bar

Due to the mixture of corrosive and abrasive contents the wetted parts must be selected very carefully. Especially the valves, for which hardness and corrosion resistance must be brought in line, require particular attention. 

The pump supplier must be able to determine and calculate the following points:

• Pipeline design
• Nominal diameter of pipeline
• Strainer data
• Pressure drop and mesh size i.e. strainer area, double strainer with switch-over differential pressure display / signal
• Shut-off valves with full area
• Pulsation dampers and their location
• Position of the knock-out drum: possibly 2 decks above the pump so that with a normal deck height of 3 m a suction height of 6m is given

Why should diaphragm metering (process) pumps be used?

Plunger pumps are less expensive initially, however, especially for the usually intermittent operation of HCC installations, have major disadvantages. The only technical advantage is the somewhat lower NPSHr (Net Positive Suction Head required). This however is compensated without problems by LEWA’s M900
diaphragm pump heads.

Advantages of the LEWA diaphragm pump in offshore-applications:

• Absolutely resistant PTFE diaphragm
• Diaphragm pump body consists of resistant solid material
• Optimal valve design due to the modular system
• Safe sandwich diaphragm with monitoring. No leakages of toxic and environmentally hazardous substances into the atmosphere
• Quick and easy diaphragm replacement, no special knowledge required. A service life of over 16 000h is normal
• No wetted plunger seal
• Low drive power due to negligible friction loss in the sealing area - plunger works in the oil bath
• Optimal service life at lowest maintenance costs: Depending on the pump size the diaphragm pump can pay off within 2-3 years just by the low spare parts costs.

If down times and production losses are taken into account, there is no alternative to the process diaphragm pump.

The stand-by pump can be started-up without any problems and it is a huge advantage that it can remain switched into the system under suction or even discharge pressure because LEWA‘s diaphragm support allows long downtimes under pressure without any problems.