In today's laboratories, water environment, as the basic environment of most laboratories, plays an important role in experiments. In the field of laboratory scientific research, tap water is the starting point of purification. If there are distilled water, deionized water or reverse osmosis water in the laboratory, only the ultra-purification process should be paid attention to. Water quality often determines the authenticity and repeatability of many experimental results. For most experts who do experiments, they usually require the concentration of impurity elements and compounds in pure water to be at ppb level or even lower. Therefore, the front design of pure water system is particularly important, and it is necessary to consider any adverse effects caused by the overall layout, material selection, installation and maintenance, and give an overall solution.
 
 
 
Laboratory pure water supply is changing from the traditional "single machine" mode to "multiple single machines" mode
 
 
 
Generally speaking, the application range of the whole pure water system in the laboratory is between 200 liters and 8000 liters per day. The total pure water consumption of most laboratories or laboratory buildings is between 500 and 2000 liters, and only a few laboratories or laboratory buildings use more than 2000 liters of water every day. In these rare laboratories or laboratory buildings, more than 2000 liters or more of pure water are used due to the large number of fully automatic testing equipment or flushing equipment with high water consumption and possible special applications (such as electronics and materials industry). In view of these situations, it is necessary to carefully define the water consumption and carry out special design. Generally speaking, too much redundancy will cause insufficient startup of the system, the purification module will not fully play its role, and the potential pollution risk of the system will be caused due to insufficient liquidity.
 
Because there is a certain distance between the central pure water treatment room and each experimental instrument, in order to make the pure water reach each water point smoothly, a booster pump is usually used to ensure a certain outlet pressure. The pressure is related to the water consumption of each experimental instrument and its distance from the pure water treatment room in the center of the towel. The greater the water consumption, the farther the distance, the greater the required pressure, and vice versa. In the process of use, the water pressure should be adjusted properly, and the water pressure is too low, which can not ensure the smooth use of water for the instrument and cause the alarm of water shortage for the instrument; If the water pressure is too high, it will lead to gas in pure water. When it is added to the reaction cup of water instrument, a large number of bubbles will be released, which will affect the inspection quality, especially for some projects using concentrated reagents.
 
 
 
The physical purity of Grade III pure water is generally less than 50 μ S/cm, and single distilled water, double distilled water, ordinary deionized water and reverse osmosis water all belong to this level. It is generally purified from tap water. The main uses of three-grade pure water are water for cleaning bottles and dishes, water for high-pressure disinfection devices, water for artificial environment rooms and water for ultra-pure water meters.
 
 
 
26 Secondary pure water Secondary pure water is a vague range, which is often expressed by 5 ~ 5M-cm. However, the second-class pure water is by no means strictly limited to this range, and it can be said that the range of 1 ~ 17M-cm is considered as the second-class pure water. Secondary pure water is generally made by ion exchange or electrodialysis of tertiary pure water. It is mainly used for the preparation of general reagents, water for general chemical experiments and water supply for ultrapure water meters.
 
 
 
17 first-class ultra-pure water first-class ultra-pure water refers to water with physical purity greater than 18M-cm, and it is customary to say that resistivity of 18.2 M-cm is the index of first-class ultra-pure water. The first-class ultrapure water must be purified from the third-class or second-class pure water through nuclear ion exchange resin. It is mainly used in high-precision analysis experiments and life science experiments with high purity requirements.
 
 
 
What are the supply modes of pure water in laboratory?
 
 
 
Laboratory pure water supply mode is divided into central pure water supply mode and decentralized pure water supply mode.
 
 
 
What is the central pure water supply mode? The central pure water supply mode refers to setting up pure water production equipment, and the laboratory water is transported to each laboratory water point through the water supply pipeline, whether it is a single laboratory or a laboratory building, so as to directly obtain laboratory pure water or ultra-pure water from the pure faucet of the laboratory water point.
 
 
 
Advantages: (1) Low operation cost and centralized management. (2) Collective use, there is no possibility of idle machines. (3) The output is large, the water is pipe network, and the water is taken from multiple points in the same laboratory.
 
 
 
Disadvantages: The system must ensure long-term safe operation, otherwise there is a risk of water cut-off.
 
 
 
What is the decentralized pure water supply mode? Dispersed pure water supply mode refers to setting pure water machines or finished water at each water point in the laboratory.
 
 
 
Advantages: The instrument has a separate right to use, and the utilization rate is high.
 
 
 
Disadvantages: (1) High operating cost, decentralized management and relatively high consumption cost. (2) Desktop fixed-point desktop installation, fixed-point water intake, small output, small flow and low working efficiency. (3) If each experimental group is purchased separately, the total investment of the owner in this kind of products is very high, which may lead to the increase of vacancy rate due to the different working conditions of each experimental group, which is not conducive to the maximization of investment efficiency. With the development of laboratory equipment, the pipe network and centralization of laboratory water supply has become the development direction of pure water supply in large laboratory buildings. Not just "central pure water system" and "stand-alone pure water system"
 
 
 
Simple central pure water system or single-machine pure water system at water point are only two extreme cases in the design and selection of overall pure water system. According to different situations, the whole pure water system can have many different designs and choices. Clearly defining the requirements of a project in the early stage of design can help design and select the results that are most suitable for practical use.
 
 
 
How to design the whole pure water system in newly built or renovated laboratories.
 
 
 
Newly built laboratories or old laboratories with renovation needs often face comprehensive challenges from many practical problems, such as system design, product selection, management and maintenance, when laying out the overall pure water system. The selection of the whole pure water system from water production module, water storage module, distribution module, pipeline purification and monitoring equipment to the final terminal purification module, It involves many factors, such as understanding of experimental application, thinking of laboratory management, control of system input cost, requirements of operation and maintenance, installation operation and engineering implementation, etc. Users often involve a lot of energy, but the effect is not very good.
 
 
 
Pure water system pipe selection is very important to see the European and American high-purity water pollution reasons, one is from the introduction of external impurities, the second is the system of various materials contained in the dissolution of pollutants. There are two main reasons for the decline of pure water quality caused by pipeline materials: (1) The impure substances in pipeline materials dissolve in high-purity water, which leads to the increase of cation and anion in water, the decrease of resistivity and the increase of TOC. (2) Because of the unsmooth inner wall of the pipeline, joints, valves and other reasons, bacteria stay and multiply and other particles accumulate, resulting in the increase of particles in water. In order to reduce the influence of the above unfavorable factors, the pipes with low extractability and smooth inner wall should be selected and the unevenness of joints and pipe fittings should be reduced as much as possible. Of course, we should also select materials according to the level of pure water quality, pay attention to the price of materials, and make overall plans. According to different materials, high-purity water piping can be divided into two categories: organic piping and stainless steel piping. The common varieties of organic piping include polyvinyl chloride (PVC), polypropylene (PP), acrylonitrile-vinyl butadiene-styrene (ABS) and polyvinylidene fluoride (PVDF). At present, there is no complete comparative data and reliable test data report on the influence of organic piping materials on high purity water quality. Each country has its own choice habits.
 
 
 
For example, the United States more use PVC pipe, the United Kingdom more use ABS pipe, and some European countries are commonly used PP pipe. PVDF pipes are mostly used in pure water polishing systems with high water quality requirements.
 
 
 
What are the important components of the whole pure water system? Pure water transportation and distribution pipeline system is an important part of the whole pure water system, which transports the high-quality pure water produced by the central pure water station to every use place with the smallest water quality drop. However, due to the high purity of water, it is easy to be polluted, and the water points are scattered, irregular and vary greatly, which brings considerable difficulty to the transportation of pure water with good quality as much as possible.
 
 
 
At present, two major problems in piping design are:
 
 
 
(1) How to maintain a high design flow rate of the pipeline, that is, when the pressure loss of the pipeline system allows, the flow rate in the pipeline should be as large as possible to prevent bacteria reproduction and particle deposition in the pipeline.
 
 
 
(2) How to prevent stagnant water, short circuit and reverse flow in pure water pipeline system. After continuous exploration and practice, several alternative approaches are put forward from the design point of view: ① Pure water circulation system is adopted in the design, and it must be operated continuously for 24 hours to prevent stagnant water from being produced in the pipe. 2. In order to ensure that pure water is flowing no matter how much pure water is used in operation, the additional circulating flow rate should be 50% ~ 100% of the designed flow rate. ③ The flow rate of pure water pipeline should be ensured. The minimum flow velocity of pure water circulation main pipe should be greater than 1.5 m/s, and the flow velocity of branch pipe should be greater than 1.0 m/s. (4) The branch pipes used in butt joint should be shortened as far as possible to reduce the dead water pipe section. It is reported in literature that the length of branch pipe should not exceed 30 times of pipe diameter. ⑤ Circulating pipeline should adopt double-pipe layout, that is, pure water circulating pipeline system with independent water supply pipe and return pipe. Taking the pure water conveying system (PVC pipe) of a research institute as an example, the water quality of the conveying system of the research institute requires resistivity ≥ 15 M Ω. Cm (25 ℃), the pipe is made of SCH80PVC pipe, the diameter of circulating main pipe is 32mm, and the total length is 250m. Through trial operation, the water quality meets the process requirements. Cleaning before installation: Because the PVC pipes used are imported, and the pipes have been cleaned and packaged before leaving the factory, cleaning is generally not considered before installation, and the following steps can be taken to clean individual contaminated pipe fittings and valves: scrub with 1% ~ 2% alkali (NaOH) solution to remove grease dirt, and rinse with tap water to neutrality; Soak in 1% ~ 2% hydrochloric acid (HCl) solution for 2h ~ 4h to remove rust, metal ions and oxides, and then flush with water to neutrality; Dry with purified compressed air, and then encapsulate with clean plastic sheets.
 
 
 
Cleaning before operation and debugging: Before operation and debugging, the installed pure water conveying system must be tested by hydrostatic test, which can be carried out only after passing the test; Cleaning the pipeline circularly with 3% ~ 5% hydrogen peroxide for 40min to achieve the purpose of disinfection and sterilization; Use pure water or reverse osmosis water supplied by pure water station to flush the residual liquid medicine in the pipeline until the effluent quality at the use point reaches the specified water quality requirements.