There are several primary factors that impact the performance, longevity and ongoing maintenance requirements for steam heat transfer components. Generally, a steam component should be evaluated in terms of a 10-year operational life cycle.
Proper selection of a component first requires a full understanding of the operational characteristics where the steam component will function. A thorough review should be conducted of the steam system’s operating parameters and documentation. Failing to understand the context of the application commonly results in improperly sized and applied steam heat transfer components.
In addition to the understanding the application, all pertinent codes and design specifications must be understood. TEMA, ASME and B31.1 are some of the codes and standards that should be reviewed to ensure safety and proper documentation. Adhering to appropriate installation recommendations for steam components will eliminate premature failures and enhance the likelihood of proper performance and longevity. By contrast, failure to employ basic fundamentals and establish appropriate specifications for selecting the correct steam heat transfer components may lead to premature failure or underperforming heat transfer results.
To establish the best practices for installing steam system heat transfer components, many industrial heat transfer applications in various locations and industries were reviewed. The survey revealed that improper component selection and/or installation practices can lead to common problems. These include:
- Unacceptable product quality.
- Premature failure of components.
- Poor temperature control.
- Low process temperatures.
- Fouling of the heat transfer equipment.
- Code violations.
1. Steam Control Valves and the Condensate Drip LegA steam control valve is a modulating component. Low or no flow will result in the buildup of condensate prior to the inlet of the control valve. Condensate accumulation before the valve can cause waterhammer or condensate passing through the steam control valve; both will cause premature failure of the valve.
Condensate buildup can be eliminated by installing a drip leg prior to the valve. This allows the condensate to fall into the drip pocket and be evacuated with the assistance of a steam trap.
2. Lock Out Ball ValvesBall valves provide a tight shut-off (Class 4 or higher) in steam service. All ball valves should be purchased or outfitted with locking handles for 2" or smaller valves. This provides the best safety procedure for lock out/tag out (LOTO). Be sure to check with your safety officer to ensure compliance with any company, local, state or federal regulations concerning LOTO procedures.
3. Install a Strainer Ahead of the Control ValveForeign particles may become established in a steam line. A common cause of the foreign material is corrosion and its byproducts. The foreign materials can lodge in the control valve trim, causing premature failure of the steam control valve. A strainer will act as a filter and prevent any foreign material from entering the steam valve.
When installing the strainer, always install a blow-off valve with a locking kit on the strainer, and pipe the discharge from the valve to a safe location. Always install the strainer with the strainer section in the horizontal position. Never install the strainer with the strainer section vertical.
4. Turndown and Control ValvesSeveral factors are relevant when selecting control valves. One primary factor is the turndown capability or working range of the valve. Following are some guidelines for control valves.
- Cage control. A 40:1 turndown ratio provides the
highest degree of controllability.
- Globe control valve. Use a 30:1 turndown ratio.
- Regulating valve. Use a 20:1 turndown ratio.
5. Install Pressure Gauges Before and After the Control ValvePressure gauges provide the information needed to understand the conditions inside the pipe and components. It is always a good practice to install a pressure gauge before and after a control valve. This provides accurate data to assist in understanding the flow characteristics of the medium while it passes through the valves. Additionally, all pressure gauges should be installed with a siphon pipe (pig tail) and isolation valve.
6. Air VentsThe existence of air in a steam system has several detrimental effects on heat transfer. Air in the system can form thin films on the heat transfer surfaces. Air is a very efficient insulator. A film of air of only 1/1000th of an inch has the same effect as a thickness of 13" of copper or 3" of cast iron.
Air not only insulates but also reduces the heat transfer rate by lowering the temperature of the steam. In accordance with the law of partial pressures, the saturation temperature of steam is reduced when mixed with air. Air contributes to the pressure of the mixture but does not contribute to the available heat content. Therefore, always install and maintain air vents in your steam piping to eliminate this heat-robbing media from your steam system.
7. Saturated Steam vs. Superheated SteamTypical steam applications require a steam quality of 100 percent at saturated steam conditions. This level of quality refers to steam containing no minute droplets of condensate entrained in the vapor. Superheated steam is steam with its temperature raised above that of saturation.
If your system is designed for saturated steam (and the original design did not anticipate any superheat), remember that the addition of any superheated steam to a heat transfer process can cause performance problems. Furthermore, superheated steam may require material changes in order to handle the differences in pressure and temperature of the steam.
8. Condensate RemovalWhen designing heat transfer units, condensate drainage is accomplished by either gravity or pressure differential. Heat transfer equipment should be installed to promote gravity drainage, with no vertical lift before or after steam traps if possible. This is very crucial in any application that has a modulating steam control valve.
Other applications do not permit gravity drainage, and therefore, care should be taken to ensure no undue backpressure is placed on the drain devices (steam trap or control valve). Numerous premature failures and performance problems are due to unanticipated backpressure on the drain devices, which causes condensate to accumulate in the heat transfer unit. This will result in waterhammer and inadequate temperature control. Poor condensate drainage also can result in corrosion problems for the heat transfer unit.
If the heat transfer unit has a steam supply modulating control valve, all condensate drains must flow by gravity to a collection tank or pumping system, which then will pump the condensate back to the boiler area. To ensure proper control of any of the heat transfer, it is essential to have zero backpressure or vertical lifts in the condensate piping.
The horizontal distance from the vertical drop leg (condensate outlet of the heat exchanger) to the steam trap should never be more than 8". Any length of greater than 8" can lead to steam locking. Also, install a test valve or a visual sight glass after the steam trap for visual indication of performance.
9. InsulationAll exposed surface areas in a heat transfer application should be insulated. One good source for information on adequately insulating steam lines is the U.S. Department of Energy’s steam tip sheets. They describe payback and material selection in detail.
10. Control Valve PipingThe sizing and length of pipe from the control valve outlet to the inlet nozzle on the heat transfer unit is critical. Control valve outlet piping must be increased to be equal to or larger than the inlet connection to the heat transfer unit. Also, the control valve should be located at least 10 pipe diameters away from the heat transfer unit with the expanded pipe.
11. Vacuum BreakersAll heat transfer components, whether shell-and-tube, plate-and-frame or any other device, require vacuum breakers. Vacuum breakers protect heat exchanging equipment when a system is shut down by preventing a vacuum from occurring. Additionally, the vacuum breaker maintains the condensate in the heat transfer equipment. Generally, it is recommended that all heat transfer devices have an air vent and vacuum breaker installed at points designated by the heat transfer manufacturer. The normal locations are close to the steam inlet or on the top portion of the heat transfer unit.
In any steam application, first review and follow all relative jurisdictional codes. Then, eliminate premature failures and enhance the likelihood of proper performance and longevity of heat transfer units (and their associated components) by understanding selection, installation and maintenance requirements.