In today's laboratory, water environment, as the basic environment of most laboratories, plays a very important role in the experiment. In the field of laboratory scientific research, the starting point of purification is tap water. If there is distilled water, deionized water or reverse osmosis water in the laboratory, it only pays attention to the process of super purification. Water quality often determines the authenticity and repeatability of many experimental results. For most experimental experts, they usually require the concentration of impurities and compounds in pure water to be at PPB level or lower. So the front section of the pure water system design link is particularly important, need to consider the overall layout, material selection, installation and maintenance and other factors will cause any adverse impact and give the overall solution.

The laboratory pure water supply is changing from the traditional "single machine" mode to the "multi-machine" mode

Generally speaking, the whole pure water system of the laboratory, its application range is between 200 liters and 8000 liters per day. The total consumption of pure water in most laboratories or experimental buildings is between 500 and 2000 liters, and only a few laboratories or experimental buildings consume more than 2000 liters of water per day. In these very few laboratories or experimental buildings, more than 2000 liters or more of pure water is used due to the availability of more water-intensive automatic testing equipment, or flushing equipment, and possible special applications (such as electronics and materials industries). For these cases, careful definition of water consumption and special design are required. Generally speaking, excessive redundancy will lead to insufficient system startup, and the purification module will not play a full role. Also, due to insufficient liquidity, the system may be potentially contaminated.

Because of the central water treatment chamber and the experimental apparatus ask a certain distance, to make pure water to the water points, usually use water pump to ensure pressure, the size of the pressure of water consumption and the experimental apparatus size and its associated with towel epicenter distance of pure water treatment chamber, water consumption, the greater the distance, the required pressure, the greater the vice versa. In the process of use, the water pressure should be adjusted properly, the water pressure is too low, can not ensure the smooth water use of the instrument, causing the instrument water shortage alarm; If the water pressure is too high, the pure water will contain gas. When the water is added to the reaction cup of the water instrument, a large number of bubbles will be released due to the reduced pressure, which will affect the quality of inspection, especially for some items using concentrated reagents.

17 Tertiary pure water The physical purity of tertiary pure water is generally less than 50 μs /cm. Single distilled water, double distilled water, ordinary deionized water and reverse osmosis water all belong to this grade. It is usually purified from tap water. Three levels of pure water is mainly used for cleaning bottles and dishes, high pressure disinfection device water, artificial environment room water and ultra-pure water meter water intake.

26 Secondary pure water Secondary pure water is a vague range, usually expressed as 5~5M-cm. However, the secondary pure water is by no means strictly limited to this range. It can be said that the range of 1~ 17m-cm is considered as the secondary pure water. Secondary pure water is usually made of tertiary pure water by ion exchange or electrodialysis. It is mainly used for the preparation of general reagents, ordinary chemical experiment water and water supply to ultrapure water meter.

Grade 1 ultrapure water refers to the water with physical purity greater than 18m-cm. It is customary to say that the resistivity of 18.2m-cm is the index of grade 1 ultrapure water. Primary ultrapure water must be purified from tertiary or secondary pure water by nucleon-grade ion exchange resin. It is mainly used in high-precision analytical experiments and life science experiments with high purity requirements.

What are the supply patterns of pure water in the laboratory?

The laboratory pure water supply mode can be divided into central pure water supply mode and decentralized pure water supply mode.

What is the central pure water supply model? The central pure water supply mode refers to the setting of pure water production device, laboratory water is transported to each laboratory water point through the water supply pipeline, whether it is a single laboratory or a laboratory building, to realize the direct access to laboratory pure water or ultra-pure water from the pure water tap at the laboratory water point.

Advantages :(1) low operating cost and centralized management. (2) Collective use, there is no idle machine. (3) Large output, the use of water pipe network, the same laboratory more water.

Disadvantages: The system must ensure long-term safe operation, otherwise there is a risk of water interruption.

What is the decentralized pure water supply model? The distributed pure water supply mode refers to the setting of pure water machine or finished water at each water point in the laboratory.

Advantages: The instrument has a separate use right, high utilization rate.

Disadvantages :(1) high operation cost, decentralized management, relatively high consumption cost. (2) Fixed-point desktop installation, fixed-point water intake, model output is small, small flow, low efficiency. (3) If each experimental group buys separately, the total investment of owners in such 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 pipeline network and centralization of laboratory water supply has become the development direction of pure water supply in large laboratory buildings. Not only "central pure water system" and "stand-alone pure water system"

Simple central pure water system or single water point pure water system are only two extreme cases in the overall pure water system design and selection. The overall pure water system can have many different designs and choices for different situations. Clearly defining the project's requirements early in the design process can help design and select results that are most appropriate for practical use.

How to design the overall pure water system for a new or renovated laboratory?

When the new laboratory or the old laboratory with the need of transformation is placed on the overall pure water system, it will often face the comprehensive challenges of system design, product selection, management and maintenance and many other practical problems. Throughout the whole water system from the distribution of water production module, storage module, module and monitoring equipment, pipes, purification, purification module of matching at the end of the terminal, involves the understanding of the experimental application, laboratory management of cost control, thinking, system running and maintenance requirements, installation, operation and engineering implementation of various factors, such as users tend to involve a lot of energy, It didn't work very well.

The CHOICE of pure water system pipe is very important, SEE the reason of European and American high pure water is polluted, one is the introduction of impurities from outside, the other is the dissolution of pollutants contained in all kinds of materials in the system. The decrease of pure water quality caused by pipeline material mainly has the following two points :(1) the impure material in pipeline material dissolved in high purity water causes the increase of Yang, anion, resistivity and TOC in water. (2) Due to the inner wall of the pipeline is not smooth, joints, valves and other reasons, bacteria retention and reproduction and other particles accumulation, resulting in the increase of particles in the water. In order to reduce the influence of the above adverse factors, the pipe with low availability and smooth inner wall should be selected and the uneven joints and fittings should be reduced as much as possible. Of course, we should also select materials according to the level of pure water quality, and pay attention to the price of materials, etc., to take into account. According to the different materials, the high pure water pipe can be divided into organic pipe and stainless steel pipe two categories. The common varieties of organic piping include polyvinyl chloride (PVC), polypropylene (PP), acrylonitrile-butaethylene styrene (ABS), polyvinylidene difluoride (PVDF) and other 4 kinds. At present, there is no complete comparative data and reliable test data report on the effect of organic pipe quality on high purity water quality. Each country has its own selection habits.

For example, the United States more PVC pipe, the United Kingdom more ABS pipe, and some European countries are commonly used PP pipe. PVDF pipe is mostly used in water quality requirements especially high pure water processing system.

What are the important components of the whole pure water system? Pure water transmission and distribution pipeline system is an important part of the whole pure water system, which delivers high-quality pure water produced by the central pure water station to each use site with as little as possible water quality reduction. However, due to the high purity of the water, it is easy to be polluted, and the water points are scattered and irregular and the range of change is large, which brings considerable difficulty to the transportation of pure water as far as possible.

At present, there are two major difficulties in piping design:

(1) How to maintain a high design flow rate of the pipeline, that is, the flow rate in the pipeline should be as high as possible under the condition that the pressure loss of the pipeline system allows, so as to prevent bacterial propagation and particle deposition in the pipeline.

(2) How to prevent stagnant water, short circuit and reverse flow phenomena in the pure water pipeline system. After continuous exploration and practice, several alternative approaches are proposed from the design point of view: ① the design uses pure water circulation system, and must be 24h continuous operation, in order to eliminate the dead water in the pipe. ② In order to ensure that no matter how much pure water is used in operation, pure water is in the flow state, the additional circulation flow rate should be 50% ~ 100% of the designed flow rate. ③ The flow rate of pure water pipeline should be guaranteed. The minimum flow rate of the main pipe of pure water circulation should be greater than 1.5m/s, and the flow rate of the branch pipe should be greater than 1.0m/s. (4) The branch pipe of the butt point should be shortened as far as possible to reduce the stagnant pipe section. It has been reported that the length of branch pipe should not exceed 30 times the length of pipe diameter. ⑤ The circulation pipe should be arranged by double pipe, that is, the pure water circulation pipe system with independent feed pipe and return pipe. Take a pure water conveyor system (PVC pipe) as an example. The water quality resistivity of the conveyor system is required to be ≥ 15mω. Cm (25℃), the pipe is SCH80PVC pipe, the circulation pipe diameter is 32mm, the total length is 250m. Through the test run, the water quality meets the process requirements. Before the installation of clean: because of the use of PVC pipe is introduced, the pipeline have been encapsulated after cleaning before they go out, therefore, generally do not consider cleaning before installation, to individual contaminated pipe fittings, valves can take the following steps to cleaning: 1% ~ 2% with alkaline solution (NaOH) scrub, to remove oil dirt, and water washing to neutral; Soak in 1% ~ 2% hydrochloric acid (HCl) solution for 2h ~ 4h, remove rust, metal ions and oxides, and then rinse with water to neutral; Dry with purified compressed air blown dry, and then wrapped in clean plastic sheeting.

Cleaning before commissioning: hydraulic test must be carried out on the installed pure water conveying system before commissioning, and it can be carried out after passing; The pipe was cleaned with 3% ~ 5% hydrogen peroxide for 40min in order to achieve the purpose of disinfection. Use the pure water or reverse osmosis water supplied by the pure water station to rinse the residual liquid in the pipeline until the water quality of the use point reaches the specified water quality requirements.