Pressure gauges are essential safety accessories for boilers, measuring and indicating the pressure inside the boiler drum. If a pressure gauge is missing or malfunctioning, the pressure inside the boiler will not be displayed, directly threatening safety. A sensitive and accurate pressure gauge is installed on the boiler, allowing the boiler operator to operate the boiler correctly and ensure safe and economical operation. Let's learn about the function, uses, and installation of pressure gauges!
The structure and principle of pressure gauges: Pressure gauges commonly used in industrial boilers include liquid column type, spring type, bellows type, etc.
1. Liquid column pressure gauge
A liquid column manometer, commonly known as a U-tube manometer, utilizes the pressure generated by the height difference between the liquid columns on both sides of a U-shaped glass tube to balance the pressure of the measured medium. The greater the difference in liquid column height, the greater the pressure of the measured medium, and the pressure value can be read directly from the scale.
Liquid column pressure gauges have advantages such as simple structure, easy use, and low price, but they cannot measure high pressure and cannot automatically indicate and record, thus limiting their application range. They are usually used to measure pressure values in boiler furnaces and flues.
2. Bourdon tube pressure gauge
The structure of a tubular pressure gauge: Inside a circular casing is a spring-loaded tube with an elliptical cross-section (the major axis of the ellipse is parallel to the central axis of the needle). One end of the spring-loaded tube connects to a water storage pipe leading from the boiler steam space, becoming the fixed end. The other end is a closed free end, connected to a connecting rod. At the other end of the connecting rod is a sector gear, which connects to a small gear on the central shaft. The pressure gauge's pointer is fixed to the central shaft.
When the specially designed Bourdon tube is subjected to pressure from the medium, its free end moves outward, causing the sector gear and pinion to rotate via a connecting rod, thus rotating the pointer clockwise. The greater the pressure of the medium, the greater the rotation angle of the pointer. At this time, the scale value indicated by the pointer on the pressure gauge dial is the steam pressure value inside the boiler. When the pressure drops, the Bourdon tube contracts inward, and the pointer returns to the appropriate position. After the pressure disappears, the Bourdon tube returns to its original shape, and under the action of the spiral spring, the pointer returns to the starting point (zero point). The Bourdon tube pressure gauge has the advantages of compact structure, high accuracy, wide measurement range, and ease of use, and is widely used for measuring the pressure of media in boiler drums, liquid collection tanks, and piping.
3. Diaphragm type pressure gauge
Diaphragm pressure gauges, also known as wind pressure gauges or micro pressure gauges, are commonly used to replace glass tube pressure gauges to measure micro-pressures and micro-negative pressures in systems such as air supply, ventilation, smoke generation, and pulverization. The structure of a diaphragm pressure gauge mainly consists of two parts: a diaphragm and a transmission and amplification mechanism. The diaphragm is a cylindrical, box-shaped pressure-sensitive element made of alloy. When subjected to pressure changes, it expands or contracts, displacing itself. This displacement is transmitted through the transmission and amplification system, and the needle displays the measured micro-pressure or micro-negative pressure value on a grooved scale. Diaphragm pressure gauges can also achieve control and protection functions by energizing the contacts. (The text also mentions auxiliary parts of the pressure gauge.)
1. Collection tube
Please install a bend between the pressure gauge and the boiler. The condensed steam will then enter the bend. The bend's function is to create a water seal, preventing steam from directly entering the bend and causing thermal deformation under high temperatures, which would affect the accuracy of the pressure gauge reading. Common forms of storage pipes are described. The inner diameter of the bend should not exceed 6mm when using copper pipe and should not exceed 10mm when using steel pipe.
2. Three-way cock
Install a three-way stopcock between the pressure gauge and the bend, and clean the pipeline. Inspect, check, and disassemble the pressure gauge.
The safety technical requirements for pressure gauges: Currently, the most commonly used pressure gauges in boilers are ordinary Bourdon tube pressure gauges; therefore, the following explanations will use Bourdon tube pressure gauges as an example.
1) In addition to the pressure gauge directly connected to the steam space of the boiler tank, each steam boiler must also have a pressure gauge installed at the following inlets: before the regulating valve on the water supply pipe, at the outlet of the segmented economizer, between the outlet of the steam superheater and the main steam valve, at the reheater outlet, at the start-up separator of the once-through boiler, before the valve of the primary steam-water system of the once-through boiler, at the inlet of the boiler water circulation pump of the forced circulation boiler, and at the outlet and outlet valve of the hot water boiler. Pressure gauges should also be installed on the water supply and outlet pipes of the circulation pump.
2) The higher the working pressure of the boiler, the higher the accuracy requirement of the pressure gauge must be. For pressure gauge accuracy requirements: For steam boilers, when the rated steam pressure is less than 2.45 MPa, the accuracy must not be lower than 2.5; when the rated steam pressure is above 2.45 MPa, the accuracy must not be set above 1.5. For hot water boilers, the accuracy of the pressure gauge must be above 2.5. The accuracy class of a pressure gauge is expressed as a percentage of the allowable error of the dial scale limit value, generally divided into seven classes: 0.5, 1, 1.5, 2, 2.5, 3, and 4. For example, for a pressure gauge with a dial scale value of 0~2.45 MPa and an accuracy class of 2.5, the allowable error between the pressure value indicated by the pointer and the actual pressure value of the measured medium must not exceed 2.45 (±2.5%) = ±0.06 MPa; when the pressure gauge indicates a medium pressure of 0.98 MPa, the error value is 0.98 (2.5).
3) The limit value of the pressure gauge's dial scale should be 1.5 to 3 times the working pressure. It's best to choose twice that value. The larger the limit value of the pressure gauge's dial scale, the greater the allowable error and the greater the visual error, thus making the reading inaccurate. When the working pressure approaches the limit value of the pressure gauge's dial scale, the internal spring tube is often in a state of significant deformation, which is prone to extreme deformation. This not only increases the pressure gauge's error, but also, in the event of a pressure increase, the pointer may cross the dial scale limit value and point to zero, causing the operator to have a misperception and making the pressure appear even higher.
4) The size of the pressure gauge dial should be such that the boiler operator can clearly see the pressure reading. When the pressure gauge is installed less than 2 meters from the operating platform, the nominal diameter of the dial must be at least 10 mm; when the distance is 2-4 meters, the nominal diameter must be at least 150 mm; and when the distance exceeds 4 meters, the nominal diameter must be at least 200 mm.
5) The selected pressure gauge should meet the requirements of relevant technical standards. Its installation, calibration, and maintenance should comply with the regulations of the metrology department. Before installing the pressure gauge, check it and draw a red line on the dial (not the dial glass) to indicate the working pressure. After installation, check it at least once every six months and seal it after the check.
6) The pressure gauge should be installed in a location that allows for easy observation and cleaning, and the dial should be tilted forward at a 15-degree angle. It must be protected from the effects of high temperatures, freezing, vibration, etc.
The above describes the function, uses, and installation of pressure gauges.
7) Stop using the pressure gauge if any of the following conditions apply.
