Boiler Water Level Indicators PDF

Objective 3 Describe direct and indirect type boiler water level indicators. Boiler water level can be determined through direct or indirect methods. Direct methods include gauge glasses of various designs. Indirect methods include various remote water level indicators that continuously measure, tr...

Objective 3 Describe direct and indirect type boiler water level indicators. Boiler water level can be determined through direct or indirect methods. Direct methods include gauge glasses of various designs. Indirect methods include various remote water level indicators that continuously measure, transmit, and display water level. Many power boilers use both methods. CODE REQUIREMENTS FOR BOILER WATER LEVEL INDICATORS Power Boilers ASME BPVC I Part PG-60.1 states that “All boilers having a fixed water level (steam and water interface) shall have at least one gage glass.” It further states that “boilers having a maximum allowable working pressure exceeding 400 psi (3 MPa) shall have two gage glasses.” This is because of how critical it is to know the boiler water level at all times, especially with boilers operating at higher pressures. PG-60.1 also states that “The lowest visible water level in a gage glass shall be at least 2 in. (50 mm) above the lowest permissible water level, as determined by the boiler Manufacturer.” ASME BPVC I Part PG-60.1.1.1 also provides for the use of remote water level indication: “Instead of one of the two required gage glasses, two independent remote water level indicators… may be provided.” Page 21 of 45 Heating Boilers ASME BPVC IV Part HG-603 requires each steam heating boiler to have “one or more water gage glasses attached to the water column or boiler by means of valved fittings not less than NPS ½ (DN 15).” The lower fitting must have a drain valve “having an unrestricted drain opening not less than ¼ in. (6 mm) in diameter to facilitate cleaning.” The lowest visible part of the gage glass must be “at least 1 in. (25 mm) above the lowest permissible water level recommended by the boiler Manufacturer.” To facilitate this, each boiler must be provided at the time of the manufacture with a permanent indicator of the lowest permissible water level. DIRECT METHODS A gauge glass is a transparent device that permits continuous visual determination of a liquid level. On a boiler, the liquid is the water level in the drum or shell. Various types of gauge glasses include: • Tubular • Armored-type (flat) • Bicolour Tubular Gauge Glass The simplest and least costly gauge glass is the tubular design. These are simply transparent vertical tubes, with their lowest visible point connected to the boiler at a certain distance above the lowest permissible water level (LPWL). Isolating valves are placed above and below the gauge glass connections. Page 22 of 45 Two slightly different valve types are shown in Figures 11 and 12. Figure 11 shows a gauge glass with slow closing valves. In Figure 12, the valves are the quick closing type. With these, a one-quarter turn of the valve handle fully opens or fully closes the valve. The valve spindles are fitted with levers to which chains may be attached to operate the valves from ground level if the boiler drum is located too high to reach. Figure 12 – Water Gauge with Quick-Closing Valves and Chains Figure 11 – Gauge Glass For boilers, drain valves or cocks must be installed on the lower gauge glass fitting to remove any solid material that may collect. Side Track CSA B51 Part 6.3.1.1 addresses gauge glass installations that are difficult to reach: When the top connection of a water gauge is more than 2 m and less than 6 m from the floor or working platform of a boiler room, it shall be fitted with rods or chains so that it can be operated from the floor or working platform. When the top connection of a water gauge is 6 m or more from the operating floor level, it shall be of the inclined type or other accepted type. Page 23 of 45 Since many boilers are not under continuous supervision, if a gauge glass breaks, a large amount of fluid can escape. To prevent this, the lower valve on the gauge glass is often equipped with a safety shutoff device consisting of a stainless steel check ball. This ball closes off the fluid passage when the glass breaks. Figure 13 shows this type of valve. Figure 13 – Safety Shut-Off Gauge Valve Under normal conditions, the steel ball remains in the recess in front of the valve seat. However, when the gauge glass breaks, the sudden rush of fluid through the valve forces the ball against the valve opening, thereby stopping the flow. The gauge glass is usually surrounded by a number of metal rods, or a transparent shield, to protect it from breakage. The shield also protects operators from flying glass in case the gauge shatters. Gauge glass tubing comes in several styles and pressure ratings. Some tubes have red lines to make it easier to view the water level. Pressure ratings are determined by the manufacturer, and vary according to the: • Glass material • Thickness of the glass • Diameter of the gauge glass • Temperature at which the gauge glass operates Page 24 of 45 Because of this, the gauge glass manufacturer’s literature must be consulted and adhered to when selecting replacement gauge glass material. The use of tubular gauge glasses is limited to relatively lower pressures and temperatures. Excessively long gauge glasses should be avoided. For applications where a broad range of levels must be observed (as with some types of electrode boilers), two or more gauge glasses should be installed. The visible portions of the gauge glasses must overlap, as shown in Figure 14. Asme bpvc I pg-60.1. ASME BPVC I Part PG-60.1 addresses this situation as follows: Figure 14 – Multiple Gauge Mounting Gage glass assemblies having multiple sections, whether of tubular or other construction, shall be designed in such manner that will ensure a minimum of 1 in. (25 mm) overlap of all adjoining sections in which the water level may be visible. The gauge glass tube is held tightly in place at each end by a washer or packing ring and a nut. If the gauge glass leaks, the isolating valves should be closed, and the drain opened, to prevent injuries before any maintenance is done. Page 25 of 45 Boiler Gauge Glass Installations ASME BPVC I Part PG-60.3.1 and BPVC IV Part HG-603 both permit gauge glasses to be connected either directly to the shell or drum of the boiler, or to an intervening water column. When two gauge glasses are required by ASME I, both may be connected to a single water column. According to ASME BPVC I Part PG-60.3.4, the size of piping connecting a water column to a steam boiler shall be at least NPS 1 (DN 25). Part PG-60.2.3 states that each water column must have a connection of at least NPS ¾ (DN 20) to install a valved drain for blowing down the water column. Part PG-60.3.6 stipulates that the steam and water connections to a water column or a gauge glass shall be readily accessible for internal inspection and cleaning. ASME BPVC IV Part HG-604 agrees with ASME I Parts PG-60.3.4, PG-60.2.3, and PG-60.3.6. The installation shown in Figure 15 uses a gauge glass similar to the one shown in Figure 11. Note the drain valve which permits all the connections to be blown through. This daily activity ensures the gauge glass connections are not plugged with sludge or sediment. The valves can be slow or quick opening. This method of attachment is used only on some Figure 15 – Direct Connected Gauge Glasses on a Firebox or Locomotive Boiler firebox, locomotive, vertical firetube, and cast iron sectional boilers. Page 26 of 45 More often, gauge glasses are connected to a water column which, in turn, is connected to the boiler as shown in Figure 16. The water column acts as a reservoir to dampen agitation in the water. Figure 16 – Water Column and Gauge Glass In addition, the column traps sludge or sediment, and prevents it from collecting in the glass connections. The column also provides a place for installation of high and low level alarms and controls. Try cocks were once commonly installed on the column to provide a means of point level detection when the gauge glass is being replaced. Try cocks are no longer an ASME BPVC requirement. The ASME codes have very specific rules for the location of the gauge glass in relation to the lowest permissible water level in a boiler. The rules are not the same for all boilers. Page 27 of 45 ASME BPVC I Part PG-60.1 states that “The lowest visible water level in a gage glass shall be at least 2 in. (50 mm) above the lowest permissible water level, as determined by the boiler Manufacturer.” However, for horizontal firetube boilers and locomotive boilers, ASME BPVC I Part PFT-47.1 and 47.2 provide different measurements, depending on boiler shell diameter. PFT-47.1 states that for a horizontal firetube boiler with diameter greater than 400 mm, the “lowest visible water level in the gauge glass must be at least 75 mm above the lowest permissible water level.” Steam heating boilers have different requirements. ASME BPVC IV Part HG603 states “The lowest visible part of the water gage glass shall be at least 1 in. (25 mm) above the lowest permissible water level recommended by the boiler manufacturer.” Obviously, the ASME BPVC requires a greater safety margin for power boiler water level indicators as opposed to those for heating boilers. Tubular Gauge Glass Replacement Gauge glasses are susceptible to corrosion caused by alkalinity and silica depletion at higher temperatures. Alkalinity causes thinning of tubular glasses below the water line. This action becomes more aggressive as the pH of the water rises. Condensate formed due to cooling of steam in the gauge glass dissolves some of the silica in the glass and weakens it. Both cause eventual failure of the glass. Misalignment of fittings also causes premature gauge glass failure. Page 28 of 45 The following steps should be taken when a gauge glass fails: 1. Shut off the steam and water valves on the gauge. These valves are usually equipped with chains and levers so they can be closed from the operating floor. 2. Open the drain valve on the gauge. 3. Unscrew the nuts at each end of the glass, and remove the washer and broken glass. 4. Crack open the gauge glass valves to blow out any fragments of glass, and then close the valves again. A suitable face shield should be worn to avoid injury. 5. If there is no gauge glass of correct length, cut a new glass to the correct length using a glass cutter. Ensure the new glass is made of the correct material, and meets the pressure and temperature specifications of the boiler. 6. Place the nuts and new washers on the glass. Install the gauge in the gauge fittings. Putting graphite on the washers to act as a lubricant between the washers and nuts will prevent the glass from turning when tightening the nuts. The nuts should be only hand tightened. Tighten them alternately by holding one while tightening the other. If “O” rings are used instead of washers, tightening with a wrench will be required. Note that the top fitting of the gauge glass is deeper than the bottom so that the gauge glass must be inserted into the steam fitting first, and then lowered into the bottom of the water fitting before tightening the nuts. 7. Heat the glass slowly by cracking open the steam valve and leaving the drain valve open. Then, when the gauge glass is up to temperature, close the drain valve, and partially open the water valve. Open the gauge steam and water valves fully when the water level in the glass stabilizes. video The operator should wear a face shield to prevent injury when opening the valves, especially if the gauge valves are not equipped with chains for remote operation. Page 29 of 45 If the gauge glass leaks when put into service, do not tighten the nuts while the glass is under pressure. Always close the steam and water valves on the gauge, and open the drain before tightening the nuts. During the time that the gauge glass is out of service, the boiler drum level may be checked by means of try cocks, a second gauge glass, or a drum level recorder if so equipped. Armored-Type (Flat) Gauge Glass Tubular gauge glasses are not available for pressures greater than about 3000 kPa. For higher pressures, a flat type gauge glass is used, consisting of glass plates bolted in a steel forged housing. A flat gauge glass is shown Figure 17 – Water Column and Flat Glass attached to a water column in Figure 17. Transparent and reflex gauge glasses use armoured glass. They are suitable for temperatures exceeding 250°C, and pressures up to 70 000 kPa. Transparent Gauge Glass The transparent gauge glass, shown in Figure 18, has a one piece central chamber with cover plates on each side that hold the two glass windows. Figure 18 – Transparent Gauge Glass (Cross-Sectional Views) Page 30 of 45 The chamber and cover plates have machined recesses that keep all the parts aligned, and prevent the gaskets and cushions from shifting. The inside surfaces of both glasses are lined with a protective coating of transparent mica. After prolonged exposure to high temperature chemically treated water, the mica becomes opaque. This discolouration indicates failure of the mica, and that water is now directly contacting the glass. When this condition is observed, the glass should be changed before it fails. The glass is tempered for resistance, to both mechanical and thermal shock. Care must be taken when assembling the unit and tightening the bolts to prevent glass failure. It is safest to use the crossover method of tightening by starting at the centre and working outwards. Besides being suitable for high pressure steam applications, flat gauge glasses are also effective for caustic and acidic liquids, dirty materials, and other service where it is necessary to illuminate the glass from the rear. Page 31 of 45 Reflex Gauge Glass A reflex gauge is shown in Figure 19. This gauge has special optical properties that create a sharp line of demarcation at the liquid level. A dark area represents the liquid in the glass gauge, contrasted by a light area above the liquid. Armored-Type (Flat) Gauge Glass Replacement The following procedure is recommended when changing the glass: video 1. Close the steam and water valves on the gauge glass. Open the gauge glass drain. 2. Remove the bolted covers, glass, gaskets, and the mica. At this time, the threads on the studs should be coated with graphite, and the nuts run down to clean the threads. 3. Remove any remaining gasket material, being careful not to create low spots on the surfaces of the joints. Scraping the gasket off the metal surfaces may form burrs. 4. Clean both ends of the gauge so gasket material will not plug the valves on the gauge. 5. Polish the gauge surfaces perfectly smooth. Check the surfaces to be sure they are perfectly level with no high or low spots. Checking includes the surfaces of the gauge body and the bolted covers. 6. Apply molybdenum disulfide on the contact surfaces of the new glass. This permits the glass to slip into place easily. Never reuse old gauge glasses. Be sure that the Figure 19 – Reflex Gauge Glass glass is suitable for high temperature high-pressure service. 7. Install a new gasket, new Page 32 of 45 mica, and new glass on one side, and install the cover. Replace the nuts on the cover. 8. Tighten the nuts on the cover evenly. It is best to start at the centre of the glass, and tighten evenly on both sides of the glass. 9. Repeat steps 6, 7, and 8 on the other side of the glass. 10. If the boiler is in service, allow the new glass to warm up gradually, through heat conduction. Never open the gauge valves until the new glass is heated up. 11. With the drain valve still open, crack open the steam valve, and permit steam to slowly blow through to heat the glass further. 12. When the glass is at operating temperature, close the drain valve, and crack open the water valve to allow water into the glass. If everything appears normal and a water level is visible in the glass, open the steam and water valves fully. Caution On high-pressure boilers, many gauge glass failures occur because the new glass is not heated gradually Bicolour Gauge Glass Some gauge glasses show the steam and water as different colours. This permits easy identification of the water level at a distance. These are known as bicolour gauge glasses. Page 33 of 45 Figure 20 shows a type of bicolour multiport gauge glass using the point level method of indication. Instead of a water column, this gauge is attached to a circulating tie bar that has top and bottom connector blocks with gauge valves, plus a bottom connection for a drain line. The gauge glass consists of a number of sealed circular glasses (or double bullseye assemblies) with floodlights connected at the back. The steam space is indicated in red, while the water space is green. Figure 20 – Bicolour Multiport Gauge Glass Page 34 of 45 Figure 21 shows this type of gauge glass in service. Figure 21 – Column with Low Water Fuel Cut-Off Figure 22 shows the method used for indicating the drum level. Green and red filters are placed between the lamp and each circular glass. These glasses are placed at a slight angle, to permit the light to be diffracted. Figure 22(a) shows the passage of light through the water in the gauge glass. Only the green light can pass Figure 22 – Bicolour Gauge Glass Operation through the water and both glasses without excessive diffraction. The red light is diffracted so that it does not pass through. Therefore, only the green light is visible in the section of the gauge glass occupied by water. Figure 22(b) shows the passage of light through the steam in the gauge glass. Only the red light can pass through the steam and both glasses without excessive diffraction. The green light is diffracted, so that it does not pass through. Therefore, only the red light is visible in the section of the gauge glass occupied by steam. Page 35 of 45 When boilers are several stories tall, the gauge glass is located high above the operating floor. Several means can be used to inform the operator of the drum level. ASME BPVC I Part PG-60.1.1.1 states: When the water level in at least one gage glass is not readily visible to the operator in the area where control actions are initiated, either a fiber optic cable (with no electrical modification of the optical signal) or mirrors shall be provided to transfer the optical image of the water level to the control area. Figure 23 shows how a bicolour gauge glass can indicate the drum level to a person on the operating floor below. A hooded mirror is connected directly to the front of the gauge glass and adjusted to a proper angle, so it will reflect the red and green light to another mirror on the operating floor. The photo shows an application for an eight-storey tall boiler. The red ports of the gauge glass can be seen in the lower part of the mirror. Figure 23 – Mirror Arrangement Page 36 of 45 Gauge Glass Error During normal boiler operation, the gauge glass generally indicates a water level lower than the actual level in the boiler drum. This is because the water in the gauge glass, water column, and interconnecting piping is cooler and denser than the water within the boiler drum. The amount of the error depends on the temperature difference between the water in the gauge and its connection, and the water in the drum. Error is affected by such factors as the: • Ambient temperature • Length of the gauge glass • Level of the liquid in the gauge glass Testing of Water Column and Gauge Glass During Operation Water and steam connections to both a water column and a gauge glass must always be unobstructed. Obstructions may be caused by closed valves on water level control piping, or due to sediment deposits within the control piping. If the steam connection piping is blocked (say, with a closed valve), the steam in the gauge glass, water column, or level control float cage will condense, and form a vacuum. This will draw water in through the water connection, filling the void left by the condensed steam. The result will be a high water level that can fool both operators and controllers into believing the boiler is full of water, when it may, in fact, be empty. If the water connection piping is blocked (say, with a closed valve or heavy deposits), the steam in the gauge glass, water column, or level control float cage will condense. The condensate accumulates in the float cage, column, or gauge glass, but will be unable to return to the boiler through the obstructed water connection. Again, the result will be a high water level that can fool both operators and controllers into believing the boiler is full of water, when it may, in fact, be empty. Page 37 of 45 For these reasons, it is imperative that all piping connections to water level controls, water columns, and gauge glasses be kept clear, so that the boiler water level will be properly communicated to the level instruments. This is done by regular water column, gauge glass, and level control blow down. On Track Some high-pressure steam boilers have isolation valves in the steam and water piping, between the water column and the boiler. ASME BPVC I Part PG-60.3.7 permits this only under specific conditions. If valves are provided, they must indicate whether they are open or shut, and must be locked in the open position. For steam heating boilers, ASME BPVC IV Part HG-604 entirely prohibits such isolation valves. The water column and gauge glass should be blown down every shift to remove any sediment that may collect. This procedure is highly recommended on smaller highpressure boilers. On large boilers, where the gauge glass contains mica, blowing down of the gauge glass would be less frequent. Frequent blowing down will shorten the life of the mica, and increase maintenance costs. Gauge glasses should be renewed if they become obscured by internal corrosion or deposits. Every plant should carry a substantial reserve of gauge glasses, and washers or packing rings. Gauge glasses should be stored in a safe place where they will not be damaged. Side Track Gauge glasses and water column blow down procedures are covered in Part B, Unit 4, Chapter 4 Operational Checks. Page 38 of 45 INDIRECT (REMOTE) LEVEL INDICATORS Remote water level indicators are used to show the water level at a remote location, such as in a control room, adjacent to boiler blowoff valves, or at particular operating location. ASME BPVC I Part PG-60.1.1.1 states: When the water level in at least one gage glass is not readily visible to the operator in the area where control actions are initiated, either a fiber optic cable (with no electrical modification of the optical signal) or mirrors shall be provided to transfer the optical image of the water level to the control area. Alternatively, any combination of two of the following shall be provided: (a) an independent remote water level indicator (b) an independent continuous transmission and display of an image of the water level in a gage glass. Figure 24 shows a type of remote water level Figure 24 – Remote Water Level Indicator indicator of entirely mechanical operation. The operating element consists of a large sensitive diaphragm with the top side connected to the steam space of the boiler, and the bottom to the water space. A condenser at the boiler drum maintains a constant head of water pressure on the steam side. The waterside is connected at the minimum permissible water level. This connection varies in head with variations in drum level. The difference in head pressure on the two sides of the diaphragm is balanced by a spring when the level is at minimum, so the diaphragm moves up and down in accordance with the water level. Page 39 of 45 As the water level in the drum rises, the Figure 24 – Remote Water Level Indicator pressure due to the varying head of water increases. This causes the diaphragm to rise and move the indicator upwards. As full boiler pressure is exerted equally on both sides of the diaphragm, boiler pressure has no effect upon its movement. Coloured screens illuminate the inside of the indicator with blue in the lower portion, representing the water, and red in the upper part to represent steam. These two colours are separated by a shutter. The motion of the diaphragm is transmitted to the shutter through a lever mechanism. If the drum level rises, the increased force under the diaphragm causes it to rise, and move the shutter upwards so more blue shows. The opposite occurs when the drum level drops. The shutter may also be used to energize high and low drum level alarms. Page 40 of 45 Another type of remote level indicator is shown in Figure 25. The indicator is basically a manometer with one transparent leg. The manometer is filled with a blue or green insoluble liquid that is denser than water. The steam condenser maintains a constant head of water acting on one leg of the manometer. The other leg of the manometer is connected to the waterside of the boiler. The head Figure 25 – Remote Water Level Indicator pressure in this leg varies with boiler water level. When the level in the boiler steam drum is at a minimum, the pressure differential between the two heads will be the greatest. The indicating liquid will be forced to the bottom of the indicator glass by the difference in pressure. As the drum level rises, the indicating liquid rises in the glass, and shows the level to engineers on the operating floor. Dirt traps are installed to prevent contamination of the indicating liquid. Care must be taken when adding indicating liquid. Overfilling will make the indicated level too high. An advantage of this remote indicator is that there are no moving mechanical parts. Some remote water level indicators are electronic devices. They use various methods to sense and transmit level. The level indication can be graphical representations of the water level, digital representations, or displays on computer screens. Page 41 of 45 Figure 26 shows a common type of remote water level indicator. The system is comprised of three major components: • A water column with up to 24 water detection probes. • An electronic detection and verification unit. • One or more remote LED displays, customized according to the number of water level probes. Figure 26 – Aquarian Electronic Water Level Gauge The probes detect the presence of water or steam. The detection and verification unit determines the validity of the reading, and transmits the level to LED displays. The displays can be located up to three different locations. Alarm set points and shutdowns can be programmed in the control system. Often, one LED display is located near the gauge glass. This is so the drum level shown on the electronic display can be verified against the actual gauge glass indication. Page 42 of 45 Figure 27 shows a digital water level indicator beside a set of bottom blowoff valves. This indicator receives a standard 4 to 20 mA signal from a drum level transmitter, and displays the level on an LCD display, in user-configurable engineering units, such as millimetres or inches. This indicator shows drum level in millimetres. When the reading is “0 mm,” the drum level is at set point (usually ½ way up the gauge glass). Levels above or below set point are indicated as positive or negative readings, also in millimetres. The display shown in Figure 27 is located conveniently for operators to monitor boiler water level when blowing off the boiler. Other useful locations for such a digital readout include near boiler feed pumps, and feedwater bypass valves. Figure 27 – Loop-Powered Digital Indicator for Drum Level Page 43 of 45 Figure 28 shows four reflex-type gauge glasses for four different boilers, displayed on a single computer display, in a central control room. Cameras monitor the water levels and transmit the information to the control room in real time. The control room operator will continuously rely on this computer screen. Figure 28 – Digital Remote Water Level Indicator In order for any remote water level indicator to be used to meet the requirements of ASME BPVC I Part PG-60.1.1.1, it must be designed and installed to meet the requirements of ASME BPVC I Part PG-60.1.1.2, which states: The display of a remote water level indicator shall have a clearly marked minimum water level reference at least 2 in. (50 mm) above the lowest permissible water level, as determined by the Manufacturer. This does not exclude other remote indication devices (like the cameras and loop powered indicators discussed) from being used as operator aids. Page 44 of 45