Water level controls and devices include:
- water column
- level indicator, including:
- gage glass
- indirect sensing method (ASME BPV Code Section IV)
- independent remote (ASME BPV Code Section I)
- low-water fuel cutoff
- feedwater regulator
- pump controller
A water column is used on a steam boiler to reduce the turbulence and fluctuation of the water level so the gage glass can provide a steady, accurate water level reading. The use of a water column is not mandatory.
A level indicator, whether it be a gage glass, indirect sensing method, or independent remote, is used on a steam boiler to provide a water level reading. A gage glass is the most common form of level indicator found on steam boilers. Depending upon ASME BPV Code requirements, the gage glass can be a round glass "window" known as a bull's-eye, a glass tube with a circular cross-section, or a flat glass assembly held in a special frame. An indirect sensing method, as allowed in ASME BPV Code Section IV, could be in the form of a floating magnet which causes colored "flags" external to the mechanism to flip over, indicating the water level. If an indirect sensing method is used, an operable gage glass must also be installed. An independent remote level indicator, as allowed in ASME BPV Code Section I, could include fiber-optic cables, video camera, electronic representation on a computer monitor, and magnetic devices. Section I has specific requirements which must be followed when using these types of indicators.
A low-water fuel cutoff is used on both steam and hot water boilers to shut off the fuel or source of heat when the water drops below a predetermined, safe operating level. The most common type of low water fuel cutoff is a float inside a chamber. The float mechanism actuates an electrical switch when the float reaches a preset level. Another popular form of low-water fuel cutoff is called a probe type. This variation uses rod-like electrical probes inserted into the boiler or in an external chamber. When the probes are in contact with the water, an electrical circuit is completed using the water as a conductor. If the probes lose contact with the water, the electrical circuit is opened. Some low-water fuel cutoffs have a manual reset device built in. The purpose of this is to alert the operator that the low-water fuel cutoff has activated. The operator should then look for the problem that caused the low-water condition before resetting the device and restarting the boiler.
A feedwater regulator can be found in different forms. This guide will address only those most commonly used on small- to medium-sized boilers. The feedwater regulator is essentially a valve installed in conjunction with the boiler to ensure the water is maintained at the desired level. The valve can be opened either mechanically by direct actuation of a float, electrically, or pneumatically. Many times the feedwater regulator is installed as a mechanically integral component of the low-water fuel cutoff or it may be controlled by a signal from a low-water fuel cutoff.
A pump controller is an electrical switch used for controlling a feed pump on a steam boiler. While this may seem to be similar to one of the variations of a feedwater regulator, it does serve a different purpose.
All the devices listed above must be constructed and rated for the pressure and temperature applicable to the installation.
All water level controls and devices must operate as designed in order to perform their required tasks. Different types of problems can prevent normal operation:
- A water column, if used, must be connected to the boiler using a cross or equivalent pipe fitting at each right angle connection in order to allow visual inspection and cleaning of the connecting pipes. Sludge or sediment of any kind in the water column or connecting pipes can cause false water level indications.
- External gage glasses must be installed as required by ASME BPV Code Sections I and IV. Typical gage glasses have a valve in the top fitting, a valve in the bottom fitting, and a drain valve directly below the glass. The ASME BPV Code requires external gage glasses to be installed in such a manner so the glass can be replaced while the boiler is under pressure. Sludge or sediment of any kind in the gage glass or its connections can cause false water level indications.
- Boilers equipped with an indirect sensing method or independent remote level indicator must have these devices installed in accordance with all ASME BPV Code and jurisdictional requirements. Due to the technology involved with some of these devices, it is difficult to summarize the different problems which may inhibit their proper operation.
- A low-water fuel cutoff must be installed as required by the ASME BPV Code and the device manufacturer's instructions. ASME BPV Code Section I does not require a low water fuel cutoff; however, it accepts the use of such a device, under specific conditions outlined in PG-61, as an alternative to a second source of feedwater supply.
- A float-type low-water fuel cutoff can be rendered inoperative if sludge or sediment is allowed to accumulate in the float chamber. A punctured or collapsed float can also prevent the device from operating as required.
- A probe-type low-water fuel cutoff can be rendered inoperative if sludge or sediment is allowed to cover the probes or make contact with the chamber walls. This can occur if the device is installed in a water column or other external chamber that is not flushed out periodically. In a boiler with extremely poor water chemistry, scale can build up on the probes or bridge across them, rendering the device inoperative.
- Either type of low-water fuel cutoff can be bypassed electrically with jumper wires. Jumper wires can be used legitimately by qualified service personnel during maintenance and testing, but must be removed before returning the boiler to normal operation. Jumper wires could be used inappropriately in an attempt to permanently bypass a device that has malfunctioned and will not allow the boiler to operate.
- Some jurisdictional authorities require two functioning low-water fuel cutoffs on each boiler. In addition, the second low water fuel cutoff must be equipped with a manual reset in many instances.
- A feedwater regulator must be installed as required by the manufacturer's instructions and any applicable code and jurisdictional requirements. Feedwater must not be introduced directly into a low-water fuel cutoff or the piping connected to it.
- A feedwater regulator is usually installed to actuate when the water drops below the normal operating water level but before the level at which the low-water fuel cutoff would actuate. The difference between the normal operating water level and the point at which the feedwater regulator actuates can vary depending upon the design of the boiler and the type of service for which the boiler is used. Some service conditions can cause the water level to fluctuate a great deal but would still be considered normal.
- If the feedwater regulator incorporates a float chamber and float, it can suffer the same problems associated with a low-water fuel cutoff.
- A pump controller is usually a float and float chamber type of device whether it is included within a low-water fuel cutoff or as a separate unit. As such, it can be affected by the same problems experienced by a low-water fuel cutoff or a feedwater regulator.
- A water column must be flushed out while the boiler is under pressure. The owner or owner's representative can do this by opening the drain valve connected to the bottom of the water column. A brief daily flush will help ensure the water column and its connecting piping remain clear of sludge.
- An external gage glass must be blown down while the boiler is under pressure. This should be performed on a daily basis. During an inspection, the owner or owner's representative can do this by opening the drain valve connected to the bottom of the gage glass. If the water level does not return quickly to a stable reading, the steam and water connections to the gage glass should be blown down separately. This is accomplished by performing the following steps:
- Close the steam connection valve at the top of the gage glass and open the drain valve. This will flush the water connection to the gage glass.
- Close the drain valve and open the steam connection valve.
- Close the water connection valve on the lower portion of the gage glass and open the drain valve. This will flush the steam connection to the gage glass.
- Open the water connection valve and close the drain valve. The water level should return and stabilize quickly. If it does not, the boiler should be taken out of service so that all piping and connections associated with the gage glass can be mechanically cleaned.
- A flat glass-type gage should only be blown down in accordance with the glass manufacturer's instructions. Some of these are lined with mica to protect the interior surface of the glass. The mica may be damaged when steam is blown across the surface.
- If at any time a water level cannot be observed in a gage glass, the boiler should be taken out of service and allowed to cool before adding make-up water.
- If a boiler is equipped with more than one level indicator, the inspector should compare readings between them to determine if they are all indicating the same level.
- The operation of a float-type low-water fuel cutoff should be tested while the burner or other source of heat is operating. The owner or owner's representative can do this by opening the drain valve on the bottom of the float chamber. When the float drops in the chamber, the fuel or other source of heat should immediately shut down. If it does not, the boiler should be taken out of service until the problem can be corrected.
- The quick drain test described above does not work well with a hot water boiler since the low-water fuel cutoff is constantly flooded with water and the drain valve will not typically flush enough water at one time from the float chamber to allow the float to drop. ASME BPV Code Section IV, paragraph HG-614 requires a means to test the device without draining the entire system. Please review HG-614 for specific requirements.
- Testing of a probe-type low-water fuel cutoff is usually accomplished by depressing a test switch on the device. The device manufacturer's instructions should be reviewed closely to determine the correct test procedure.
- Another type of low-water fuel cutoff test involves slowly draining a steam boiler which would more closely mimic an actual low-water condition. It usually requires two people to perform this test safely. Both people should be representatives of the boiler owner. The feed valve should be closed first, and then one person operates the boiler drain valve while maintaining communication with the person watching the water level in the gage glass. If the water level reaches the bottom visible portion of the gage glass and the low-water fuel cutoff has not shut off the fuel or other source of heat, the test should be halted immediately and the boiler drain valve closed. The boiler should then be taken out of service until the problem can be corrected. If the test is successful, all valves must be returned to their normal operating positions before leaving the boiler.
- While performing an internal inspection of a boiler, the float-type low-water fuel cutoff should be disassembled so the float, float chamber, and mechanical linkage can be inspected. A probe type low water fuel cutoff should be removed so the probe surfaces can be inspected.
- A feedwater regulator or pump controller employing a float mechanism can be tested in much the same way as a float-type low-water fuel cutoff. When the float reaches its predetermined setting, feedwater should be admitted to the boiler or the pump should operate as applicable to the type of device.
- An internal inspection of a float-type feedwater regulator or pump controller would require disassembly of the float and float chamber.
While inspecting low-pressure or high-pressure boilers, the inspector will be observing water level controls and devices. The inspector must take the time necessary to completely evaluate the condition and operational effectiveness of these controls and devices. The inspector should consider:
- National Board Inspection Code
- ASME BPV Code Section I
- ASME BPV Code Section IV
- ASME BPV Code Section VI
- ASME BPV Code Section VII
- ASME Standard CSD-1
- Manufacturer's Installation, Operation, and Maintenance Documentation
- Jurisdictional Laws, Rules, and Directives