Instrumentation project of 3rd desalination plant at Tuas (Singapore)

Abstract

This project consists on the description of instrumentation used in Tuas III desalination plant at Singapore and its flow process description.
The project has been developed as part of the work of the instrumentation department of the responsible company of the engineering design of Tuas III desalination plant.
First of all is important to know the water problems which suffer all the people who live in Singapore. Singapore is a city-state and is home to 5.5 million residents. The country has one of the highest per capita incomes. Unfortunately Singapore has few rivers, only 1.4% of its surface is fresh water.
For that reason, PUB (Public Utilities Board) a national water agency of Singapore, has built a water supply plant based on four water sources known as the Four National Taps: Local Catchment Water, Imported Water, Reclaimed water (known as NEWater) and Desalinated water. The four taps are explained into the project.
The objective of this project is defining a real desalination project from the point of view of instrumentation.
Tuas III desalination plant consist on a Sea Water Reverse Osmosis (SWRO) plant which will have a total net design capacity of 47,45 Hm³/year of product water in an operational period of 365 days a year, with a net daily design capacity of 130.000 m³/day of product water.
One important part of this project is the instrumentation because this entire project has been described by instrumentation point of view. Instrumentation is the development or use of measuring instruments for observation, monitoring or control. An instrument is a device that measures a physical quantity variable, such as flow, temperature, level and pressure, these are the most important variables that should be controlled to get a safety and efficient process. Instruments may be as simple as direct reading hand-held thermometers or as complex as multi-variable process analyzers. The part of instrumentation is described in chapter 2. The instruments used in Tuas III are listed below.
Pressure: bourdon manometers and membrane pressure and differential pressure transmitters. For assembly pressure transmitters is used a two valves manometer and for differential pressure is used a five valve manifold.
Flow: electromagnetic flowmeter, vortex, rotameters and thermal flow switches.
Level: magnetic level indicator, hydrostatic level transmitter, float level switches, ultrasonic transmitter, radar transmitter and vibrating fork switches.
Temperature: Resistive Temperature Detector (RTD) sensors with thermowell and transmitter.
Analyzer: Oxygen reduction potential (ORP), O2, Turbidity, Conductivity, pH, Particle counter, total organic carbon (TOC), Chlorine, Hydrocarbon, Algae, Fluoride and Ammonia.
Desalination is a separation process used to reduce the dissolved salt content of saline water to a usable level. All desalination processes involve three liquid streams: the saline feedwater (seawater), low-salinity product water, and very saline concentrate water (brine).
A desalination plant transforms seawater into drinkable water. This plant uses reverse osmosis (RO) which retires all the salts and ions dissolved in seawater. The RO membranes consist on some special membranes which allow the water pass, but not the ions. The problem is that any solid matter could obstruct these membranes and for that reason previously the water is filtered and ultra-filtered.
In the RO process, water from a pressurized saline solution is separated from the dissolved salts by flowing through a water-permeable membrane. The permeate (the liquid flowing through the membrane) is encouraged to flow through the membrane by the pressure differential created between the pressurized feedwater and the product water, which is at near-atmospheric pressure. The remaining feedwater continues through the pressurized side of the reactor as brine. No heating or phase change takes place. The major energy requirement is for the initial pressurization of the feedwater.
In practice, the feedwater is pumped into a closed container, against the membrane, to pressurize it. As the product water passes through the membrane, the remaining feedwater and brine solution becomes more and more concentrated. This brine solution is adequate treated before sending this steam to shoreline.
A desalination process can be resumed as follows:
Pretreatment: The incoming feedwater is pretreated to be compatible with the membranes by removing suspended solids, adjusting the pH, and adding a threshold inhibitor to control scaling caused by constituents such as calcium sulphate.
Pressurization: The pump raises the pressure of the pretreated feedwater to an operating pressure appropriate for the membrane and the salinity of the feedwater.
Separation: The permeable membranes inhibit the passage of dissolved salts while permitting the desalinated product water to pass through. Applying feedwater to the membrane assembly results in a freshwater product stream and a concentrated brine reject stream. Because no membrane is perfect in its rejection of dissolved salts, a small percentage of salt passes through the membrane and remains in the product water.
Stabilization: The product water from the membrane assembly usually requires pH adjustment and degasification before being transferred to the distribution system for use as drinking water.
This process is controlled by instruments assembled on it. For this reason, instrumentation is an important part of engineering process.
In this project there are two different solutions which shall be evaluated; the first one is about instrumentation used to this project and the second one is about desalination as drinking water solution.
The instrumentation design has been developed to be the most efficient, safe, productive and environment friendly as possible.
The most efficiently is referred to the power is correctly used and like example; the energy recovery system is controlled by two differential pressure transmitter, four pressure transmitter and three pressure transmitter.
The safest is referred to control all the process values like pressure and make sure that an alarm is switch on when the pressure is out of range.
The most productive is referred to the production is controlled by a flowmeter (FIT-15601) which measure the flow sent to the network.
The most environmental friendly is referred to control the quality of brine which is sent back to the sea; this part of the plant is controlled by six analyzers.
To resume the second important point of this project, the list of pros and cons of construction of a desalination plant is included.
-List of pros:
1. It is proven and effective. The method of reverse osmosis to remove salt from seawater has been proven to be effective in creating a fresh source of drinking water that is needed to increase the health benefits to people who has not other water source. In fact desalination plants can create water that is good quality and drinkable.
2. It has the massive ocean water as source. Even if all water would come from desalination plants, seawater can serve as almost inexhaustible source. This implies that people would have sufficient access to freshwater needed for growing crops, for everyday living and other needs, even in times of drought.
3. Its method is highly understood. The desalination method is backed up by scientific data and is highly understood. The technology used is so reliable that it allows for high-quality water, which means that using such method should allow for great results and could help eliminate water shortage crisis that the world might face in the future.
4. Its plants are built in safe locations. Desalination plants are and will be located away from large residential communities. Some of those already existing today are located in industrial facilities, which mean that people are not put at risk. Companies that are planning to construct desalination plants have plans in place to make the projects safer in the long run.
-List of cons:
1. It can be a very costly process. For the average desalination plant these days, it takes 2 kWh of energy in order to produce 1 cubic meter of fresh water. Though this would translate to a cost of just under 2 dollars on a lot of power grids, the real production cost comes from the expenditure of fossil fuels that are needed to create electricity for its process.
2. Its plants are expensive to build. Though most plants have operations cost reasonably, building them is not always available for a country or a community. The cost is so very high that some authorities prevented the technology to be developed because they just cannot afford its initial investment.
3. It demands high energy costs. One big problem with desalination is the enormous amount of energy it consumes. The process includes reverse osmosis which demands a high amount of energy to reverse. This plant has been designed with a energy recovery system to reduce the energy consumption but this consumption is still a big problem.
In conclusion, I think that the costs of desalination are worth the gains because desalination means an alternative to fresh water. All we need to survive is fresh water and desalination is a good method to do it possible.
At the end of the project there are five annexes which show the most of the work done in the company during the period of work time. These annexes are the habitual deliverables in a general project.