Design a pumping system for controlling water quality being recycled within an i

Question

Design a pumping system for controlling water quality being recycled within an industrial process. The water leaves the process and is collected in a large holding tank (10 m high). When the water reaches or exceeds 9m in the tank, a valve opens at the bottom of the holding tank and water is dumped into the drainage system until the level in the holding tank is 7m. In the holding tank, a chemical is added to adjust the pH down to a particular set point. This is currently performed manually. Water is irregularly pumped back into the process from the holding tank but only after it is confirmed that the pH of the water in the tank is below a certain pH value. Typically, the water flow required for the return line from holding tank varies from 100 to 200 litres per minute (depending on changes in demand from the plant). A pump capable of delivering 200 litres per minute with 300 kPa of head is available for pumping water from the holding tank back into the factory. There is approximately 150 kPa of losses in the pipe network (excluding the control valve) associated with the water return line at 200 litres per minute. A ball valve is currently positioned manually to control this flowrate back from the tank but this does not provide very satisfactory control. The company wants to automate the system using a PLC based control system and feedback control loops where necessary. a) Select a suitable device for measuring the height of the water in the holding tank. b) Select a suitable device for measuring the pH of the water in the holding tank. c) Select the appropriately sized control valve, using the procedure described by Bolton. What sort of valve would you suggest? d) If this system is fully automated, identify what feedback control loops would be required and draw the relevant block diagrams that show the basic elements of these loops. e) A PLC has been selected to co-ordinate the sequences required for operating the system. Propose a ladder diagram that could be used for programming the PLC. f) How would you propose to tune the controller that will be used to control the delivery of the chemical for modifying the pH in the holding tank? g) Assuming that you have successfully automated the flow back into the process from the holding tank, can you foresee any problems with the current arrangement, in terms of process stability, and environmental controls. Discuss these issues and propose possible solutions.

Explanation / Answer

A public water system (PWS) is defined by the Safe Drinking Water Act (SDWA) as “a system for the provision to the public of water for human consumption through pipes or other constructed conveyances, if such system has at least fifteen service connections, or regularly serves at least twenty-five individuals". Thus, individuals on wells and systems that serve fewer connections or people are not captured under Federal regulations, though some States regulate smaller systems. Federally regulated systems are called “public water systems” because they serve water to the public, not because they are publicly-owned. A public water system may be publicly owned (e.g., owned by a municipality) or privately owned (e.g., owned by an investor-owned utility or by the owner of a mobile home court). SDWA further divides public water systems into community water systems (CWSs) and non-community water systems (NCWSs). o CWSs include public water systems that serve 25 people or 15 connections yearround. Examples of CWSs include municipal water systems or water systems that serve a mobile home park or other groups of residents. o NCWSs are PWSs that do not serve a permanent resident population. This latter category is further defined, and includes two water system types. – The first, non-transient, non-community (NTNCWSs) includes systems serving at least 25 people (the same people) at least six months of the year, such as some churches, schools, and factories. – The second, transient non-community (TNCWSs), includes facilities such as roadside stops, commercial campgrounds, hotels, and restaurants that have their own water supplies and serve a transient population at least 60 days per year. o Each of these types of PWSs can be publicly or privately owned. The majority of PWSs are transient non-community water systems. While these systems are numerous, they serve a small percentage of the population because each system serves a small number of people. Nearly everyone is frequently served by transient non-community water systems. Remember that TNCWSs include roadside stops, commercial campgrounds, hotels, and restaurants that have their own water supplies and serve a transient population at least 60 days per year. Therefore, it is important to regulate these systems even though each one generally serves a small populationat any one time. The Public Water System Supervision program is authorized by SDWA. SDWA regulatory requirements for drinking water systems are implemented through the PWSS program. These regulations help ensure that the public receives safe and adequate supplies of drinking water. In this way, the program supervises public water systems as the title of the program suggests. EPA, along with States and Tribes, regulate approximately 162,000 public water systems. Of these, community water systems provide drinking water to more than 90 percent of Americans. The number of regulated systems is very large. Of those 53,437 systems that meet the definition of a CWS, 93 percent are considered to be small systems—serving fewer than 10,000 people. Even though these small systems are numerous, they serve only a small fraction of the population. o For example, systems that serve 3,300 people or fewer make up 84 percent of CWSs nationwide, yet serve 10 percent of the population. o On the other hand, the approximately 800 systems (about 1.6 percent of systems) that serve more than 50,000 people each provide water to more than 56 percent of the population served by community water systems. Small systems face the greatest challenges with SDWA compliance. For this reason, the 1996 SDWA Amendments include provisions that allow for additional flexibility in regulatory.implementation and monitoring requirements for small water systems.

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