Preliminary water discharge unit

General data

Preliminary water discharge unit (PWDU) serves to separate and discharge formation water, as well as to treat it from oil and mechanical impurities in order to achieve required values for cluster pads, oil treatment units and booster pump station platforms.

Unit components are designed based on technical assignment for equipment design and delivery.

Preliminary water discharge units can be operated in areas with average temperature of the coldest 5 days period reaching up to minus 60°С.

Climatic category of the equipment is moderate-cold and cold climatic execution (UHL, HL respectively), category of location is “1” as per GOST 15150 – 69.

PWDU operational characteristics


fluid, m3/day

up to 15 000

gas ratio, m3/m3

up to 250


at the inlet of PWDU, MPa

up to 4,0

design, MPa


at the outlet of PWDU, MPa

at the customer’s request

Operating medium

oil, associated gas, formation water

Oil density at 20 °С, kg/m3


Formation water density, kg/m3


Water cut, % vol.

up to 90

Products treatment quality at the outlet

Water content in water-gas-oil mixture, % vol., max.


Oil content in water to be separated, mg/l, max.


Mechanical impurities content in water to be separated, mg/l, max.


Equipment composition

Depending on customer’s requirements to oil and water quality at the outlet of PWDU, unit package can be completed with the following equipment:

  • Oil and gas separator with water discharge,НГСВ type V=25…200 m3;
  • Oil and gas separator V=12,5 m3…100 m3;
  • water decanter V=50…200 m3;
  • gas and oil metering unit;
  • surge vessel;
  • oil heaters;
  • oil pumping station;
  • chemical injection unit;
  • flare unit;
  • drain unit;
  • piping and service platforms package;
  • shutoff and regulating valves, instrumentation package;
  • low voltage switchgear;
  • control station.

Operation description (see process flow diagram)

By-pass pipeline is provided at the inlet of PWDU in case of energy shutdown at the station providing streaming of supplied liquid-gas mixture to unit’s outlet. At the inlet of by-pass pipeline and inlet pipeline of PWDU there are gate valves with electrical drive and uninterruptible power supply unit provided in case of blackout. Tie-in for de-emulsifier injection unit is designed next to the gate valve at the inlet pipeline

Liquid-gas mixture flows to oil and gas separator, which separates liquid gas at ~ 1,6 MPа pressure and discharge gas into outlet pipeline of Liquid-gas mixture from PWDU and direct it to OTU. Pressure inside apparatus is maintained by back pressure control valve “РД1”, fluid level is maintained inside oil and gas separator via level control valve PP1.Oil and gas separator is also equipped with safety valves (spring safety valve) unit with control station.

Partially degasified oil and gas mixture is delivered through pipeline from oil and gas separator to Oil and gas separator with water discharge. Further degasification of liquid-gas mixture and separation of formation water from oil occur inside oil and gas separator with water discharge at pressure inside the apparatus of ~ 1,0 MPа, maintained by back pressure control valve “РД2”.

Fluid level inside the apparatus is maintained with level control valve “PP2”. Separated formation water is supplied from Oil and Gas Separator with Water Discharge to Water metering unit and after that to unit’s outlet. Level control valve PP3 is designed for water outlet line from Oil and gas separator with water discharge.

Separated oil is fed to PWDU outlet to OTU.

Gas from Oil and gas separator with water discharge is forwarded to outlet pipeline for oil and gas mixture from PWDU to OTU at the point of gas discharge from Oil and gas separator.

Swing check valve is designed for oil and gas mixture PWDU outlet after gas discharge from Oil and gas separator with water discharge.

Oil and gas separator and Oil and gas separator with water discharge completed with service platforms, piping, shut off valves and instrumentation are placed on an open skid (base-frames). Water metering unit is installed on a base-frame (skid) in the open air or inside shelter-unit. Control station and low voltage switchgear are inside shelter-units. After gaseous flows and gas condensate are metered they are fed together in one pipeline.

A decanter is designed in order to improve separation of oil and gas mixture at the inlet of Oil and gas separator with water discharge. Liquid-gas mixture is fed to Oil and gas separator with water discharge which performs separation of gas from liquid-gas mixture and preliminary separation of oil and water. Water is collected in a lower part of the apparatus before the gate of oil gathering section and is discharged with water outlet nozzle to water decanter. Oil with gas and water residuals is supplied to oil collecting section and then discharged from of Oil and gas separator with water discharge through gas outline pipeline and fed to oil treatment unit. Pressure control valve maintains pressure in Oil and gas separator with water discharge, level control valve maintains oil and water level.

Water from Oil and gas separator with water discharge flows to decanter under the pressure, which performs final water and oil separation. Oil is collected in apparatus upper part and then supplied from Oil and gas separator with water discharge to gas outlet pipeline. Level control valve regulate oil level in the upper part of water decanter. Separated formation water is fed from filtration unit to water metering unit, then to unit’s outlet. In case if apparatus complete drawdown is required it is foreseen a liquid discharge into closed drain system is foreseen for the PWDU area.

Process flow diagram for Preliminary Water Discharge Unit

Process flow diagram

Process flow diagram for Oil and gas separator with water discharge, PWDU


Electrical part

Power supply for PWDU is performed from external source.

Electrical equipment for PWDU includes local manual operating station of ventilation system, electrical heating and lighting. PWDU is equipped with full set of cable structures and cable products.

Technological process automation

Technological equipment for PWDU is completed with local instrumentation, primary and secondary transducers for automatic control of all technological parameters: pressure, temperature, level inside separators, water flow rate.

Unit includes automated control system based on Direct Logic controller or Siemens with software for PWDU control.

Modules structure description

Modular shelters structure represents a skid welded out of steel closed profile and encased in three-layered sandwich panels with non-flammable heating material. Shelter unit base represents a welded metal framing made of steel hot-rolled profile covered with sheet metal and basalt heat-insulating material.

Heating in the equipment module and in the control unit is performed by industrial electrical heaters of general use, which create indoors temperature of not less than plus 8°С.

Ventilation in the equipment module and control station is of suction and exhaust type with natural inducement. The natural inflow ventilation is from upper area, designed for one-time air exchange and as for the exhaust one it is designed for 2,5-multiple air removal from lower area as per full volume of the building.

Modules’ structure allows different types of transportation: railroad, water and vehicle transportation.

Mounting, demounting and operation of PWDU is performed in accordance with the requirements of the project, realized by a specialists group from design organization, PWDU operation management as well as «Safety rules in oil and gas industry» and «Rules of technical maintenance of the consumers’ electrical installations and safety rules when operating consumers’ electrical installations», approved by Gosgortekhnadzor and Gosenergonadzor respectively.

The production and acceptance of activity related with the installation of process equipment and pipelines are performed as per requirements of SNiP 3.05.05-84.

Under a normal process mode or in case of minor short-time operational faults, PWDU equipment shall not contaminate the environment with emissions of hazardous substances (air, water, soil) that exceed the rates, stipulated in standards and sanitary norms:

- GOST «Nature protection. Atmosphere. Rules of setting of admissible emissions and discharge of hazardous substances by industrial enterprises».

- GOST «Nature protection. Hydrosphere. General requirements to the protection of superficial and underground waters from the contamination with oil and oil products».