China National Petroleum Corporation www.cnpc.com.cn/en “The first phase of theproject has beencompleted, withCNPC havingsuccessfullydesalinated thousandsof tons of seawater.”
China National PetroleumCorporation Operates EfficientSeawater Desalination System
China National Petroleum Corporation Operates Efficient
Seawater Desalination Systemby Invensys Operations Management
Solutions and Products
• Foxboro Measurement & Instruments.
evaporator, control the flow of seawater andvapor through the system, know exactly when
• Successfully desalinated thousands of tons of
minimize the corrosive effects of the process
• Reduced cost of seawater desalination; • Reduced maintenance time and cost of
• Making seawater usable for industry requires
removal of salt in a precisely orchestrateddistillation sequence and involves accuratemeasurement.
Foxboro Measurement & Instruments. Beijing, China – China National Petroleum
Adding to the efficiency of the system design,
Corporation (CNPC) is China’s largest producer and
seawater passing through the condensation coils en
supplier of crude oil and natural gas, and a world
route to desalination cools them, and lithium
leading integrated energy company. CNPC provides
bromide acts as a desiccant to facilitate
operational services and technical support in such
areas as geophysical prospecting, well drilling,logging, well testing, downhole operations, oilfield
Maintaining the right pressure in each evaporator,
surface facilities construction, pipeline construction,
controlling the flow of seawater and vapor through
the system, knowing exactly when the purity level
manufacturing and supply of petroleum equipment.
has been reached, and minimizing the corrosiveeffects of the process on the desalination equipmentrequires reliable and high performing process
Seawater Distillation Challenge
instrumentation. CNPC has selected Foxboro® as itsprimary supplier of pressure, flow, and analytical
Making seawater usable for industry requires removal
instrumentation used in this important operation.
of salt in a precisely orchestrated distillationsequence. CNPC faces this challenge every day as itdesalinates some 500 tons of freshwater to cool its
Foxboro Pressure, Flow, and Analytical
boilers. They use a time-tested, desalination process
called Multi-Effect Distillation (MED), in which theseawater passes through a programmed series of
CNPC currently uses more than 38 Foxboro pressure,
flow, and analytical sensors in its MED desalinationoperation. For pressure measurement, Foxboro
The MED evaporators, also called ‘effects’, are
differential and gauge pressure sensors are used at
arranged horizontally, vertically, or stacked,
each of its evaporators to balance and regulate
depending on heating requirements. As the first
pressure in the presence of steam, water, and lithium
evaporator receives seawater, the water is heated to
bromide. Each pressure transmitter uses the same
boiling and the resulting vapor is piped to a second
innovative topworks packaging with modular
vessel, where it meets a condenser, which converts
intelligent electronics, which greatly simplifies
installation, operation, servicing, and spare partsrequirements at CNPC.
Although some salt is removed at this step, theprocess must be reiterated until a desirable purity
To measure flow, CNPC uses ten Foxboro magnetic
level is reached. The beauty of the MED process is
flow sensors to monitor the passage of seawater
that it enables multiple distillations using only the
through the system and two vortex flowmeters to
heat that was applied to the first unit. Itdoes this by exploiting two physicalprinciples. The first is that water vaporreleases heat as it condenses back intowater and it is this heat generated by thecondensation that helps boil the water inthe second vessel, which in turncondenses into a third vessel and boilsthe water there.
However, condensation alone does notprovide enough heat to boil the water. Itneeds help from the second principle:reducing the ambient pressure alsolowers the boiling point of water. Therefore, by systematically reducing thepressure in each subsequent evaporator, theseawater is boiled, condensed, re-boiled,and condensed as many times as necessaryto achieve the desired level of purity.
Foxboro Measurement & Instruments.
desalination process, and two Foxboro contactingconductivity sensors to monitor the percentage ofsalt remaining in the water. Designed specifically forchallenging process applications, the DolpHinsensors feature a unique pH glass formulation thatprovides superior measurement stability andaccuracy, and longer service life, which has reducedprobe replacements and maintenance calls at CNPC.
The Foxboro conductivity sensors provide CNPCwith an economical, reliable solution for monitoringsalt content of the water being processed. As theconcentration of salt ions in the water changes sodoes detectable conductivity. The Foxboroconductivity sensors combine advanced sensingtechnology with durable electrode materials to
Foxboro magnetic flow sensors to monitor the passage of
accurately measure the changes in salt ion
concentration, enabling CNPC to monitor andmaintain the desired salt content throughout the
measure gas and nitrogen. With the ability to
withstand the demands of constant exposure toseawater, the Foxboro magnetic flowmeters have
Successfully Desalinated Thousands of Tons
proven themselves as the durable solution for
reliable, accurate measurement needed for thisdemanding application.
The first phase of the project has been completed,with CNPC having successfully desalinated
At CNPC, measuring gas and nitrogen requires
thousands of tons of seawater. CNPC estimates that
precise accuracy and the Foxboro vortex flow meters
the MED process has resulted in substantial cost
are the industry standard, with best-in-class accuracy
savings, while contributing significantly to reliable
of ± 1.0 % of reading in gas and steam, and ± 0.5 %
operation of its boilers. A company representative
has expressed that they are very pleased with thecost savings and downtime benefits CNPC has
To assure the quality of the desalinated water, CNCP
enjoyed from MED, and believes that the Foxboro
relies on two DolpHin pH sensors to monitor the
instruments and the after sales support, were a big
acidity of the seawater as it passes through the
This document was realized thanks to the support of: China National Petroleum Corporation.
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