When it comes to heating systems, the word “efficiency” describes the focus of most plant managers like no other. Painstaking attention to detail is given to main
system areas like boiler construction, main steam-lines layout and piping insulation. Yet the efficiency of even the most advanced system can be compromised by its weak links — even when those links are small and unassuming.
Indeed, often it is the sum of those inconspicuous components that proves to be the greatest cause of preventable energy waste. Three commonly overlooked components that can contribute notably to energy costs are:
• Pressure-reducing valves.
• Steam traps.
This article provides a closer look at each of these energy wasters, and how users may be able to mitigate the associated energy costs by adding insulation.
A type of pressure regulator, a pressure-reducing valve (PRV) performs the function of lowering the pressure of an incoming high-pressure medium to safely meet the needs of a particular application. For instance, in steam systems, steam often is highly pressurized when it is made. Pressure-reducing valves reduce the incoming steam pressure for equipment and smaller lines (figure 1). Most pressure-reducing valves are constructed of a conductive metal, so those that carry steam or hot mediums can radiate a significant amount of heat.
A pressure-reducing valve that is not working properly can cause a good deal of damage. For this reason, pressure-reducing valves often need maintenance and testing. This need to access the pressure-reducing valve, coupled with the typically odd shape of pressure-reducing valves, make stay-in-place insulation impractical in some applications. And, as with many heating system components, when traditional insulation is impractical, insulation often is ignored completely.
However, there is another reason that many pressure-reducing valves are not insulated. If diaphragm-type pressure-reducing valves are insulated improperly, the valve will not work correctly and even can break down. The diaphragm is a flexible membrane that restricts flow as needed in response to changes in incoming pressure. The diaphragm chamber typically is found on the valve’s underside, but it can be located in other areas. When a diaphragm chamber is insulated, however, heat is trapped in the diaphragm area, which artificially increases pressure. This means the pressure-reducing valves may not respond exactly as desired to incoming pressure. In steam systems, an insulated pressure-reducing valve diaphragm may produce flash steam — where significant condensate builds up and bursts as a flash of steam. Flash steam can rupture the diaphragm, rendering the pressure-reducing valve inoperable and possibly causing a system shutdown.
While insulating the diaphragm chamber causes some scary consequences, the rest of the pressure-reducing valve — and the entire body of non-diaphragm pressure-reducing valves — can be insulated with removable insulation jackets (figure 2).
Expansion joints are an important part of many high temperature and high vibration piping systems. When there is extreme temperature volatility, materials expand and contract; in fact, drastic expansion and contraction can cause pipes to crack. Heavy vibration also increases pipe stress. Expansion joints allow media to pass through pipes but absorb movement and vibration to reduce stress in the piping systems. In some industries, pipe expansion joints sometimes are known as compensators — so named because they compensate for movement (figure 3).
Obviously, insulation that would restrict the expansion joints’ ability to move would limit functionality and not be viable. Likewise, insulation that cannot withstand the stress of expansion-joint movement would break down and be rendered useless.
It is ironic that expansion joints often are necessary because of the high temperatures in a heating system, yet they often go uninsulated. Fortunately, most pipe expansion joints can be insulated, though the solution is not always obvious.
Specialized insulation jackets can be used to prevent heat loss through expansion joints. Finding an insulation that suits the application can be complex, but it is worth the effort. Expansion joints have different movement requirements, temperature ranges and mechanics. In addition, expansion joints may be made of one of several materials such as metal, rubber or fabric. Each material has different conductive properties and tolerances to external pressure.
Qualified insulation specialists typically are able to assist plant engineering in determining which insulation jackets are appropriate for the job. In addition, standard insulation jackets are available for many common metal pipe expansion joints (figure 4). Because it is used on typically high temperature joints, insulation for pipe expansion joints often produces a surprising return on investment.
Steam traps are valves used to discharge condensate and noncondensable gases from steam systems (figure 5). This function is very important. Condensation
Steam traps are valves used to discharge condensate and noncondensable gases from steam systems (figure 5). This function is very important.
in a steam system can cause three problems:
• Condensation reduces efficiency. Liquid is a less efficient heat-transfer medium than steam.
• The sudden release of built-up condensation in piping is a cause of the water hammer effect. Water hammering puts the system through unwanted stress and increases the risk of pipe bursts and system malfunction.
• Condensation increases the rate of corrosion, which also increases system degradation.
When steam traps do not perform properly, they either fail to discharge adequately, or they fail to trap steam. According to studies, 15 to 25 percent of steam traps leak steam. This — combined with the fact that many steam traps remain uninsulated — make them prime offenders for steam system inefficiency.
A typical uninsulated 1" steam trap radiates approximately $100 to $200 worth of wasted heat annually. Because monitoring and maintenance are so highly recommended, steam traps should not be insulated with traditional stay-in-place insulation. In addition, only certain steam traps can be insulated at all, even with removable insulation covers.
Two types of steam traps should never be insulated: thermostatic steam traps and thermodynamic traps. The first, thermostatic steam traps, pass steam based on responding to detected variations in temperature between the steam and condensate. Basically, this class of trap opens to trap cooler gas and liquid but passes hot steam. If thermostatic traps are insulated, the condensate stays hot longer, and the valve will not open enough to eliminate condensate. Three common types of thermostatic traps are liquid-expansion traps, bimetallic traps and balanced-pressure traps.
The second type of steam trap that should not be insulated is the thermodynamic steam trap. These traps rely on flash steam to cause the trap to close. Insulating such traps may artificially raise the internal temperature and increase the rate of flash steam. An insulated thermodynamic steam trap — like an insulated thermostatic trap — may not open enough to adequately eliminate condensate. Common sub-types of thermodynamic traps include the traditional disc-type steam trap as well as impulse-type traps and labyrinth-type traps.
However, one main type of steam traps — mechanical steam traps — can be insulated without a problem. Sometimes called density steam traps, mechanical traps work by sensing the difference in density between steam and condensate. They open to trap more-dense fluids and close to pass less-dense fluids (steam). Two main types of mechanical traps are float traps and bucket traps. Common subtypes or names of mechanical steam traps that can be insulated include:
• Float and thermostatic (F&T).
• Inverted bucket.
These common density steam traps generally can be insulated with box-shape removable insulation jackets (figure 6). The payback on insulating them generally is less than a year.
Pressure-reducing valves, expansion joints and steam traps all are commonly uninsulated components in process heating applications. Yet with a little knowledge and attention to detail, you can identify opportunities to insulate these components and other weak links in your heating system — and save energy.
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Editor's note: This article was originally published in the May 2013 issue of Process Heating with the headline, "3 Commonly Overlooked Energy Wasters."