On line monitoring of oil content in compressed air of power plant with PID photo ionization detector

Compressed air is an important public system in power plants , The quality decline of the compressed air may cause the failure of the pneumatic actuator and affect the safe operation of the unit . Oil content is an important monitoring index of compressed air , But the existing absorption - The operation of infrared detection method is tedious , Susceptible to pollution , Large deviation of test results , The test cannot be completed on site . In order to realize fast and accurate detection of oil content in compressed air on site , The absorption of oil content in compressed air is compared and analyzed - Infrared detection and PID（ Photoionization detector , abbreviation “PID”） Detection method , Through the analysis of the test results , Think PID The detection method can realize the rapid and accurate detection of oil content in compressed air , And it can be used for on-line monitoring of oil content in compressed air .

Compressed air system of power plant is an indispensable public system , Compared with the emphasis on power supply reliability , Compressed air quality monitoring is often ignored , Therefore, the compressed air quality is reduced 、 The pneumatic actuator refuses to operate 、 Impurity deposition , Reduce the heat exchange capacity, etc , Accidents endangering the safe operation of the unit occur from time to time . When the oil vapor mixed in compressed air accumulates to a certain extent, it will form a flammable and explosive source . The vaporization of lubricating oil will form an organic acid , Easy to corrode the inner surface of compressed air pipeline , Make rubber 、 plastic 、 The sealing material is deteriorated , Plug the small hole , Cause valve action failure . therefore , The quality index of compressed air must be controlled , To ensure the safety and stability of the system .DL/T 774-2004《 Code for maintenance and operation of thermal automation system for thermal power generation 》 and DL/T 261-2012《 Technical guide for reliability evaluation of thermal automation system in thermal power plants 》 It also clearly stipulates that the oil content index of compressed air shall be tested .

At present, semi quantitative and quantitative methods are commonly used to detect the oil content of compressed air , The representative of semi quantitative detection are detection tube method and detection box method , It can only be tested by grade , It is difficult to issue specific test values . At present, most of the existing quantitative tests are absorption - Instrument detection method , All need gas pretreatment . At present, the common pretreatment methods of oil in compressed air mainly include absorption and solvent absorption 、 The absorbent material absorbs 、 Condensation enrichment absorption, etc , But the pretreatment and absorption process of compressed air is complicated , And it is easy to affect the test results , For example, the common solvent absorption method is to use carbon tetrachloride solvent for absorption , Carbon tetrachloride for on-site detection （ New distillation on site is required ）、 Long absorption time , After absorption, it should be tested in the laboratory , The standard solution with the same carbon tetrachloride background shall also be prepared for laboratory testing , And the purity of the absorption solution 、 Absorption containers have a great impact on the test results , The prepared standard curve is easy to be polluted . The main methods for testing the on-site absorption solution in the laboratory are gravimetric method 、 Ultraviolet spectrophotometry 、 Infrared spectrophotometry 、 Fluorescence spectrometry 、 Turbidimetry 、 Gas detection tube method 、 Gas chromatography, etc , The common ones are ultraviolet spectrophotometry and infrared spectrophotometry , Therefore, it is difficult to detect the oil content of compressed air on site . according to the understanding of , Many power plants have less detection of oil content in compressed air , The quality of compressed air cannot be fully monitored . In order to meet the testing requirements for oil content of compressed air on site , This paper compares PID Law （ Photoionization detector , abbreviation “PID”） And absorption - The detection results of oil in compressed air by infrared spectroscopy , A method for determining the oil content of compressed air is proposed PID Field test method .

1 Experimental part

1.1 Instruments and materials

Fourier infrared spectrometer ; Carbon tetrachloride distillation unit ; Volumetric flask ; Gas oil separator ; Wet gas flowmeter ,0.5 m3/h, Accuracy is 1%; Box barometer , Graduation 1 h Pa;PID Oil detector .

carbon tetrachloride , Analytical purity , New distillation ; Compressed air ;30 Compressor oil No .

1.2 Experimental scheme

1.2.1 absorption - Infrared spectroscopy

(1） New distillation 4 L carbon tetrachloride , As absorption solution and standard solution preparation solution .

(2） Use the absorption device , Add... Into two dry and clean gas washing bottles respectively 25 m L carbon tetrachloride , in 0℃ Connect the air circuit in the ice water bath , Pictured 1 Shown , Record the start of the wet gas flowmeter 、 End volume , The gas to be measured is lower than 10 L/h The flow rate of flow steadily flows through the washing cylinder , Approximate flow 20 L Gas time , Record the gas flow of the wet gas flowmeter , And the beginning 、 Termination gas temperature 、 pressure , Absorption is over .

(3） Preparation of standard working fluid . Prepare separately 0.2 mg/L、0.5mg/L、0.8 mg/L、1.0 mg/L、1.5 mg/L、2.0 mg/L、4.0 mg/L Carbon tetrachloride standard solution containing oil .

(4） Detection of absorption solution . Determination of carbon tetrachloride standard solution containing oil by Fourier infrared spectrometer 2 930 cm-1 Absorbance of the absorption peak , And draw the working curve ; The absorption solution was detected by Fourier infrared spectrometer 2 930cm-1 Absorbance of the absorption peak , Then find out the absorbance in the absorption solution from the working curve .

(5） Calculation of test results . The oil content of compressed air shall be calculated according to the following formula ：

In style ：Oc— Oil content in compressed air ,mg/m3;a— The concentration of oil in the absorption solution ,mg/L;Vc—20℃ and 101 325 Pa Correction volume at ;100— Volume of absorption solution ,mL.

Press the formula （1） Calculated at 20℃ and 101 325 Pa Correction volume at Vs(L）, namely ：

In style ：p1 and p2— The atmospheric pressure at the beginning and end ,Pa;t1 and t2— Ambient temperature at start and end ,℃;V1 and V2— The volume at the beginning and end of the wet gas flowmeter ,L.

1.2.2 PID Detection method

Connect PID The oil detector detects the pipeline , Regulate the gas flow of compressed air , Test according to instrument requirements . After testing , Record the test results 、 Gas pressure 、 Gas temperature .

PID The detection principle is to use ultraviolet lamp （UV） The light source ionizes organic molecules into positive and negative ions that can be detected by the detector （ ionization ）, The detector captures the positive and negative charges of the ionized gas and converts them into current signals , Realize the measurement of gas concentration .

2 Results and analysis

2.1 Experimental study on Preparation of standard solution

In order to ensure the reliability of oil-containing working fluid , In this paper, a group of oil-bearing standard solutions were prepared by distillation of carbon tetrachloride , Then, non distilled carbon tetrachloride is used respectively 、 Store carbon tetrachloride in different containers 、 Carbon tetrachloride is stored in the washing cylinder to prepare oil-containing working fluid , The absorbance of the standard solution was detected by Fourier infrared spectrometer , See table for test results 1.

From the table 1 The test results show that , Use non distilled carbon tetrachloride 、 Carbon tetrachloride stored in different glass containers or carbon tetrachloride stored in washing cylinders may affect the concentration of oily standard solution , And the detected oil content has certain dispersibility , There are no rules , therefore , May cause inaccuracy of the standard curve , Poor linearity , Test results that affect the oil content of compressed air . When preparing the oil containing standard solution , Carbon tetrachloride freshly distilled and stored in the same glass container shall be used for preparation , Avoid affecting the test results of oil content of compressed air due to impure or polluted carbon tetrachloride .

2.2 Experimental study on two different detection methods

In this paper, absorption - The oil content of compressed air with different concentrations is detected by infrared spectroscopy and infrared spectroscopy , See table for test results 2 Shown .

In order to judge and use more clearly PID Detection method and absorption - Whether the infrared detection results are consistent , In this paper, the test results of the two methods are plotted , See the picture 2 Shown .

From the figure 2 See , utilize PID Method and absorption - The detection results of oil content in compressed air by infrared detection method are basically the same , The change trend of the curve is consistent , The test results are close to , Explain the use of PID The detection method can also detect the oil content in compressed air . From the picture 2 We can also see that , Adopt absorption - Infrared detection , The repeatability of the test results is poor , Large offset , Especially when the oil content of compressed air is low , The deviation of test results is obvious , The test results are highly dispersive , It may have a great impact on the test results , After analysis, it is believed that , Mainly due to the use of absorption - Infrared detection , Sample treatment requires the use of carbon tetrafluoride to absorb the oil in the gas sample , The absorption process has a great influence on the test results , If the bottle washer is not cleaned 、 The test results of oil content in compressed air may be too high due to the absorption liquid contacting the connecting pipe .

use PID Detection method found in the detection process ,PID Detection method is a non-destructive detection method , Will not change the gas molecules to be tested , after PID The detected gas can still be collected for further use , therefore ,PID The detection method can be considered for on-line monitoring of oil content in compressed air . The industrial mining network provides... Of various ranges PID Photoionization detector , As follows ：

NO.	Sensor model	range	The resolution of the	Technical principle
1	PID-AH5	3ppb-40ppm	1ppb	PID
2	PID-AR5	10ppb-200ppm	1ppb	PID
3	PID-AY5	1.5ppb-20ppm	1ppb	PID
4	PID-A15	100ppb-4000ppm	1ppb	PID
5	PID-AH	1ppb-50ppm	1ppb	PID
6	PID-A1	50ppb-6000ppm	1ppb	PID

3 Conclusion

(1） use PID Detection technology can effectively replace absorption - Infrared method to carry out on-site detection of compressed air quality , It can realize the fast detection of compressed air in power plant , The test results are accurate and reliable , It solves the problem of traditional absorption - In the process of infrared detection , Sample pretreatment is susceptible to contamination 、 Cumbersome operation 、 The test cannot be completed on site 、 The test results are subject to manual operation 、 Many factors, such as containers, etc., have a great impact .

(2)PID It is a non-destructive detector , It can be used for on-line monitoring of oil content in compressed air , It is suitable for on-line monitoring of compressed air in power plants .

(3） Adopt absorption - When the oil content of compressed air is detected by infrared method , The test results are highly dispersive , It may be caused by contamination of the absorption process .

(4） When preparing carbon tetrachloride standard solution containing oil , Glass containers shall be used , And the glass measuring tools used 、 Utensils must be cleaned , Avoid contact with latex tubes 、 Pollution caused by washing gas cylinders and other containers .