Steam Pressure Gauges PDF

Objective 2 Describe the code requirements for pressure gauges on steam boilers STEAM PRESSURE GAUGES Boilers are designed strong enough to withstand a certain amount of internal pressure. Excessive pressure may cause a pressure vessel explosion, especially if a boiler is nearing the end of its use...

Objective 2 Describe the code requirements for pressure gauges on steam boilers STEAM PRESSURE GAUGES Boilers are designed strong enough to withstand a certain amount of internal pressure. Excessive pressure may cause a pressure vessel explosion, especially if a boiler is nearing the end of its useful service life, or has been poorly maintained. Therefore, each steam boiler must be equipped with an accurate pressure gauge to indicate the boiler’s internal pressure. The bourdon tube gauge is commonly used for this purpose. BASME BPVC I: HIGH-PRESSURE STEAM BOILERS ASME BPVC I Part PG-60.6.1 ASME BPVC I Part PG-60.6.1 states: Each boiler shall have a pressure gauge so located that it is easily readable. The pressure gauge shall be installed so that it shall at all times indicate the pressure in the boiler. It also says: The dial of the pressure gauge shall be graduated to approximately double the pressure at which the safety valve is set, but in no case to less than 1 ½ times this pressure. This is important for three reasons: 1. When the pressure gauge is calibrated to double the safety valve setting, the operator can tell at a distance if the boiler is at its normal operating pressure. The gauge pointer will be nearly vertical. 2. When a pressure gauge is reading at mid-span, it is most accurate. 3. The pressure gauge will be suitable for use during a hydrostatic test, which is performed at 1 ½ times the maximum allowable working pressure of the boiler. Page 11 of 33 Part PG-60.6.1 also says the pressure gauge must be connected “to the steam space or to the water column or its steam connection.” This reduces the likelihood of blockage in the piping connection, due to the accumulation of sediment or corrosion products, which would render the gauge inoperative. As well, at this location, the pressure indication will not be affected by the boiler’s hydrostatic head. Part PG-60.6.1 requires a shutoff “valve or cock… placed in the gauge connection adjacent to the gauge,” and prohibits the installation of any other shutoff valve between the gauge and the boiler. The shutoff valve is important for two reasons: 1. It permits the replacement of the pressure gauge while the boiler is in operation. 2. It permits the installation of a master test pressure gauge, to verify the accuracy of the boiler pressure gauge. Because the pressure gauge must be attached to the steam space of the boiler, it must be protected from the action of high pressure and high temperature steam. Steam can reduce the elasticity of the Bourdon tube, and weaken its brazed joints. These conditions will affect the accuracy of the gauge and reduce its service life. For this reason, Part PG-60.6.1 states: For a steam boiler, the gauge or connection shall contain a syphon or equivalent device that will develop and maintain a water seal that will prevent steam from entering the gauge tube. Figure 1 – Pigtail Siphon Figure 1 shows a pigtail siphon. The “equivalent device” is usually a piping arrangement that traps condensate in its lower parts, keeping steam away from the pressure gauge. This piping arrangement must be equipped with tees, cross-tees, and plugs so that the pressure piping can be inspected and cleaned on a regular basis. Page 12 of 33 To ensure the siphon or the equivalent device does not plug off, the piping must be of adequate cross-section. Part PG-60.6.1 requires piping or siphons of the following minimum dimensions: The connections to the boiler, except the syphon, if used, shall not be less than NPS ¼ (DN 8) but where steel or wrought iron pipe or tubing is used, they shall not be less than ½ in. (13 mm) inside diameter. The minimum size of a syphon, if used, shall be ¼ in. (6 mm) inside diameter. Figure 2 – Finned Siphon on a Steam Pressure Gauge Figure 2 shows a fined siphon. This design builds condensate in a small internal reservoir to protect the pressure gauge. This siphon also acts as a snubber. ASME BPVC I Part PG-60.6.3 ASME BPVC I Part PG-60.6.3 requires that: Each boiler shall be provided with a valve connection at least NPS ¼ (DN 8) for the exclusive purpose of attaching a test gauge when the boiler is in service, so that the accuracy of the boiler pressure gauge can be ascertained. This is usually accomplished by installing a tee with an NPS ¼ (DN 8) plug between the siphon and boiler pressure gauge. This is shown in Figure 3. For the inspector to connect the test gauge, it is necessary to shut off the lever-handle cock, and remove the plug from the tee. The inspector’s test Figure 3 Pigtail with Inspector Test Connection and Isolation Valve gauge is a master gauge used to verify the pressure shown on the boiler gauge, while it is in operation. Page 13 of 33 Pigtail siphons are liquid seals, shaped in a spiral pattern or a “U”. Regardless of the shape, siphons all work the same way. Steam enters the body of the siphon, and gives off heat. As it loses heat, the steam condenses. The condensate becomes trapped in the lower portion of the pigtail or U. This trapped condensate keeps steam away from the pressure-sensing device, while still permitting the pressure to be transmitted to the gauge. Because of the damaging effects of steam on pressure sensors, siphons should be filled with distilled water prior Figure 4 – Operation of a Pigtail Siphon to installation. This ensures the pressure-sensing device is protected as soon as steam is admitted to the siphon. Siphons are also used to protect pressure limit controls, modulating pressure controllers, and electronic pressure sensors. Page 14 of 33 ASME BPVC IV: LOW-PRESSURE STEAM BOILERS ASME BPVC IV Part HG-602 states the requirements for pressure gauges used on lowpressure steam heating boilers. These include the following points: • Each steam boiler shall have a steam gauge or a compound steam gauge connected to its steam space or to its water column or to its steam connection. • The gauge or piping to the gauge shall contain a siphon or equivalent device that will develop and maintain a water seal that will prevent steam from entering the gauge tube. • The piping shall be so arranged that the gauge cannot be shut off from the boiler except by a cock placed in the pipe at the gauge and provided with a tee‐ or lever‐handle arranged to be parallel to the pipe in which it is located when the cock is open. • The gauge connection boiler tapping, external siphon, or piping to the boiler shall not be less than DN 8 (NPS ¼). Where steel or wrought iron pipe or tubing is used, the boiler connection and external siphon shall be not less than DN 15 (NPS ½). • The scale on the dial of a steam boiler gauge shall be graduated to not less than 200 kPa (30 psi) nor more than 414 kPa (60 psi). • The travel of the pointer from 0 kPa to 200 kPa (0 psi to 30 psi) pressure shall Figure 5 – Pressure Gauge Mounted on a Low Pressure Boiler be at least 75 mm (3 in.). Figure 5 shows an installation compliant with these rules. Though an inspector gauge connection is not mandatory for low-pressure boilers, it is a good idea to install a connection anyway, for the ability to verify the reading of the boiler gauge during operation. Page 15 of 33