Appendix C BOILERS AND FIRED HEATERS
C.3 Steam systems
C.3.1 Insulate hot pipe lines
Heat loss from steam pipe work and vessels is a significant cause of additional cost. To counter this, pipe work, boilers and vessels containing hot fluids are insulated using a range of materials such as mineral fibre, thermal textiles or polyurethane foam. Older installations may have been insulated using asbestos which is now considered a highly hazardous material, so it is important to be aware of the possibility of asbestos being present under older pipe work cladding.
Insulation should be applied according to a relevant national or international standard. In this guide we refer to BS5422:2009: “Method for specifying thermal insulating materials for pipes, tanks, vessels, ductwork and equipment operating within the temperature range –40 °C to +700 °C”. This provides a means of calculating the minimum economic thickness of insulation based on the thermal conductivity of the insulation material, the diameter of the pipe and the
temperature of the fluid the pipe is carrying. These are represented in the standard by the use of generic tables.
Insulation is equally important for refrigeration systems; the heat gain in a poorly insulated chilled water system can cost more in energy terms than heat loss from a steam line.
Observing the condition of insulation should be part of the audit process.
Cost of
Appendix C – BOILERS AND FIRED HEATERS
Exposed material can become wet, which reduces the effectiveness of the insulation. Worse still, wet insulation can accelerate corrosion of steam pipe work, and because the corrosion is hidden under the insulation it can go undetected until a leak occurs.
C.3.2 Maintain steam traps
Leaking steam traps can be serious sources of energy waste, they act as a short circuit from the high-pressure portion of the system to the condensate system, and the loss continues as long as the steam system is operating. Where the trap drains to a condensate line the steam loss is invisible and may persist for years. The worst energy loss occurs when the steam trap fails in a fully open position; here the rate of steam loss is proportional to the size of the orifice in the trap and to the pressure difference across the trap. These same characteristics determine the rated load of the trap, so steam loss from a stuck-open trap is proportional to its rated capacity. This is an important reason to avoid over-sizing traps.
The range of steam loss in typical applications varies enormously. A leaky steam trap on a radiator in a low-pressure steam system may lose 2.5 kg of steam (about 1.5 kWh) per hour. At the other extreme, a large steam trap with a valve seat diameter of ½ inch, serving a process application at 20 bars, may waste over 1 tonne of steam per hour (over 500 kWh) if the trap sticks fully open.
Failure of a trap in the closed position may have serious consequences, for example a closed trap will prevent steam using equipment from working, or allow condensate to accumulate in the pipe forming slugs that are propelled at high speed and which can destroy pipe, valves, and equipment.
There are different methods of testing steam traps, none are completely reliable, and most require special skill. The list below briefly describes the most common methods.
Appendix C – BOILERS AND FIRED HEATERS
C.3.3 Optimise condensate return systems
It is important to keep condensate as warm as possible until it is returned back to the feed water tank. However, if condensate returns in a condensate system that works at atmospheric pressure, some of the heat will be lost in the form of flash steam. Only liquid water returns to the boiler; any flash steam that remains by the time the condensate gets back to the receiver is lost through the vent on the receiver. The amount of flash steam depends on the pressure, and hence the temperature, at the discharge of the steam using equipment.
If steam equipment operates at high temperature, much of the energy of the condensate may be wasted. For example if steam is used to heat a vat to 160oC, the condensate temperature is also 160oC inside the heating equipment. When the condensate drains to a condensate system operating at atmospheric pressure, approximately half of the condensate will flash into steam. Part of this flash steam condenses in the return pipe, and keeps the condensate warm. The rest is lost through the vent at the condensate receiver. A variety of techniques to recover the heat of high-temperature condensate can be used. The list below describes some approaches:
1) Checking condensate system vents: In condensate systems operating at atmospheric pressure, leakage of steam traps can be detected from steam blowing at the condensate system vents.
2) Test valves: With test valve the output of the trap from the condensate system can be temporarily diverted to the atmosphere, so the trap output can be seen. The test valve setup helps to briefly shut off the trap discharge and vent the condensate system, so it can be seen if any blowing steam is coming from the condensate system rather than from the trap itself.
3) Audible sounds of trap operation: Most traps snap audibly as they close. Rapid rattling of an inverted bucket trap indicates that it is stuck open. Cycling of a disc trap at too high a rate indicates that it is leaking excessively.
4) Ultrasonic devices: Steam leaking through a narrow restriction emits a large amount of sound in the ultrasonic range, but little in the audible range. Therefore, listening for ultrasound is a valuable method of diagnosing leakage.
5) Sight glasses: Steam traps may be fitted with sight glasses or devices to verify operation. These allow you to directly observe the water level inside the traps.
6) Infrared imaging scanners: Infrared cameras are a versatile tool that allows a user to actually see patterns of temperature variation. With the right equipment and enough experience, heat patterns can be observed on the surface of the trap and adjacent pipe that indicate whether the trap is operating properly.
Appendix C – BOILERS AND FIRED HEATERS
C.3.4 Heat recovery from unrecoverable condensate
The condensate from some equipment may be contaminated, making it undesirable to return it to the boiler and is sent to drain. Even so, the heat from this condensate can still be recovered with a heat exchanger. A flash tank can be also used, if the condensate is delivered at high pressure. Ensure the heat exchanger is especially easy to clean.
C.4 Heat recovery
The following sections provide a short description of a few heat recovery options that help minimise fuel consumption.
C.4.1 Heat recovery from blow down
Heat recovery from unrecoverable condensate can be combined with blow down heat recovery. A blow down heat recovery system is a method of extracting heat from blow down water that is dumped to the sewer. If the condensate is being dumped near the boiler, it might be able to use a common heat exchanger to recover heat from condensate and blow down.
C.4.2 Heat recovery from flue gases
For fuel-fired industrial heating processes, one of the best ways to improve efficiency is to preheat the combustion air going to the burners. The source of this heat energy can be the exhaust gas stream, which leaves the process at elevated temperatures. A heat exchanger, placed in the exhaust tack or ductwork, can extract a large portion of the thermal energy in the flue gases and transfer it to the incoming combustion air.
Local heat recovery: If there is a low-temperature heating application near the point where the condensate is discharged, the condensate can be discharged through a heat exchanger that serves the lower-temperature process. This cools the condensate, so it will return through the atmospheric condensate system without flashing. If the recovered heat is needed in the form of low pressure steam, a flash tank can be used. The main feasibility question is whether the low temperature application can be synchronized in time with the operation of the equipment that discharges the high-temperature condensate.
Low pressure steam line: If a low-pressure application exists elsewhere, a low-pressure steam line can be used to allow the condensate to flash into it. If no low-temperature heat recovery application is available, the flash steam can be returned in a separate line. The high pressure condensate can be drained to a flash tank, which produces low-pressure steam. The steam is returned to the boiler plant or elsewhere for use in low-pressure applications, such as feed-water heating. This method is feasible only with large volumes of high-pressure condensate. A small quantity of low-pressure steam would condense before it is able to travel any great distance.
Closed, high-pressure condensate system: There is no inherent reason why a condensate system must operate at atmospheric pressure. The condensate system could operate at the discharge pressure of the steam-using equipment, returning the condensate as high-temperature liquid. A closed system eliminates water loss from vents, and has reduced water treatment requirements. A high-pressure condensate is restricted to applications that discharge at the same high pressure. It cannot accept condensate from any lower-temperature applications. Also, the higher pressure requirements on pipe, receivers, and other hardware may add significant cost.
Appendix C – BOILERS AND FIRED HEATERS
C.4.3 Heat (and water) recovery from steam vents
A blowing steam vent is clear evidence that energy is being wasted and should be seen as an opportunity to recover heat and save energy. Another reason to recover vented steam is to recover the pure water that comes from condensing the steam. Recovering the water saves the cost of replacing it, and also reduces water treatment costs.
Recovering the heat and the water is usually a simple matter of heat exchange. Any liquid, gas, or solid medium can be used to condense the steam if its temperature is below the boiling point of water. Most of the energy of steam is latent heat, so the temperature of the condensing medium does not matter, as long as it is cooler than the boiling temperature.
For example, warm condensate can be used to condense venting steam. This warms the condensate, reducing fuel cost, and allows the condensed steam to be used as makeup water. If a large amount of steam is being vented, it can be used for heating domestic water, heating fuel tanks, process heating, etc.
There is no need for a heat exchanger if the vented steam can be mixed with the condensing medium. One method is to pipe the steam into a tank of liquid; or, spray the liquid into a pipe that contains the vent steam. A vent condenser is a specialized heat exchanger designed specifically to recover heat and water at steam vents. A vent condenser is cooled by a liquid, and the condensed steam falls back into the vessel. Vent condensers are common on deaerator feed-water heating tanks. If there is no application for heat recovery, steam can be passed through an air coil to condense it.
C.4.4 How to find leaking steam vents
There are many industrial processes that involve steam vessels with vents. Plumes of steam are generally visible, and a major plume of steam may persist for a long time because everyone assumes that it is intentional or necessary. Within the boiler plant itself, the deaerator feed-water tank is the equipment most likely to be venting steam. Deaerators use steam to separate air from the feed-water. At the vent where the air is ejected from the tank, a considerable amount of steam may be lost along with the air. The tank may already have a vent condenser installed. If so, the vent condenser should operate properly.