VWS Westgarth Office Contact

Orbital House
3 Redwood Crescent
East Kilbride
G74 5PR

+44 (0)1355 588 000
Fax: +44 (0)1355 588 001


Veolia Water Oil & Gas Angola Contact:

Centro Logistico de Talatona
Via C3A Sector de Talatona
Gleba GU - No. 4278 
Andar - RC - No./Letra
Belas - Luanda
República de Angola

Tel: +244 222 711 799


Wash Water

In situations where the crude oil has to be desalted or dehydrated, the processes employed require supplies of high quality water, commonly referred to as washwater. This washwater is in contact with the crude oil and therefore must be free of solids and salts as these would contaminate the crude and would require downstream removal. The most practical method of producing this water offshore is by seawater reverse osmosis (SWRO). This process ensures that the washwater supply is constant and the quality is consistent.

VWS Westgarth has supplied many SWRO systems fully integrated within our overall water treatment package, which offers the benefits of a reduced footprint, commonality of equipment and a combined control system.

How Reverse Osmosis Works

The phenomenon of osmosis occurs when pure water flows from a dilute saline solution through a membrane into a higher concentrated saline solution.

The phenomenon of reverse osmosis

Figure 1

The phenomenon of osmosis is illustrated in Figure 1.

A semi-permeable membrane is placed between two compartments.

'Semi-permeable' means that the membrane is permeable to some species and not permeable to others. Assume that this membrane is permeable to water but not to salt. Then, place a salt solution in one compartment and pure water in the other compartment. The membrane will allow water to permeate through it to either side but salt cannot pass through the membrane.

As a fundamental rule of nature, this system will try to reach equilibrium. That is, it will try to reach the same concentration on both sides of the membrane. The only possible way to reach equilibrium is for water to pass from the pure water compartment to the salt containing compartment to dilute the salt solution.

Figure 1 also shows that osmosis can cause a rise in the height of the salt solution. This height will increase until the pressure of the column of water (salt solution) is so high that the force of this water column stops the water flow. The equilibrium point of this water column height in terms of water pressure against the membrane is called the osmotic pressure.

If a force is applied to this column of water, the direction of water flow through the membrane can be reversed. This is the basis of the term Reverse Osmosis. Note that this reversed flow produces pure water from the salt solution, since the membrane is not permeable to salt.

Reverse Osmosis in Practice

With a high-pressure pump, pressurised saline feed water is continuously pumped to the module system. Within the module, consisting of a pressure vessel (housing) and a membrane element, the feed water will be split in a low saline product called permeate and a high saline brine called concentrate or reject. A flow regulating valve called concentrate valve, controls the percentage of feed water that is going to the concentrate stream and the permeate which will be obtained from the feed.

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