Best Practices for installing steam system heat transfer components
There are 12 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 greatly enhance likelihood of proper performance and longevity of the heat transfer units and the associated components.
Overview of Heat Transfer
Failure to employ basic fundamentals and establishing appropriate specifications for selecting the correct steam heat transfer components are just some of the factors that lead to premature failure or underperforming heat transfer results.
We reviewed numerous industrial heat transfer applications in various locations and industries, and compiled the following summary of the most common problems caused by incorrect component selection or improper installation practices:
- Unacceptable product quality
- Premature failure of components
- Poor temperature control
- Water hammer
- Low process temperatures
- Fouling of the heat transfer equipment
- Code violations
Best Practice Guidelines
Following some simple rules and proven field techniques can help you avoid the problems listed above. The “Best Practices” items listed below should be reviewed and implemented into the steam system design, maintenance, and specification program for each facility.
1. Steam control valves and the condensate drip leg
A steam control valve is a modulating component. Low or no flow will result in a build-up of condensate prior to the inlet of the control valve. Condensate accumulation before the valve can cause water hammer, or condensate passing through the steam control valve will cause premature failure of the control valve.
You can eliminate the build-up of condensate by installing a drip leg prior to the valve. This allows the condensate to fall into the drip pocket and then evacuate with the assistance of a steam trap.
2. Lockout ball valves
Ball valves provide a tight shut off (Class 4 or higher) in steam service. All ball valves 2 in. or smaller should be purchased or outfitted with locking handles. This provides the best safety procedure for lockout/tagout. Be sure to check with your safety officer to ensure compliance with any company, local, state, or federal regulations concerning lockout/tagout procedures.
3. Install a strainer ahead of the control valve
a. Foreign particles may become prevalent in a steam line. A common cause of the foreign material is corrosion and its byproducts. The foreign material 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.
b. When installing the strainer, always install a blow-off valve with locking kit on the strainer and pipe the discharge from the valve to a safe location.
c. Always install the strainer with the strainer section in the horizontal position as detailed in the drawing. Never install the strainer with the strainer section vertical.
4. Turndown and control valves
Heat transfer components require properly sized control valves for proper process temperature control (heat sink principle). A primary factor in selecting control valves is the turndown capability or working range of the valve. Following are some guidelines for control valves.
a. Cage control = 40:1 turndown ratio provides the highest degree of controllability
b. Globe control valve = 30:1 turndown ratio
c. Regulating valve = 20:1 turndown ratio
5. Install pressure gauges before and after the control valve
Pressure 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 passing through the valves. Additionally, install all pressure gauges with a siphon pipe (pigtail) and isolation valve.
6. Air vents
a. The existence of air in a steam system has several detrimental effects on heat transfer. Air in the system can form thin films on heat transfer surfaces. Air is a very efficient insulator (thermal conductivity 0.2). A film of air of only 1/1000 of an inch has the same effect as a thickness of 13 in. of copper or 3 in. of cast iron.
b. Air not only insulates but also reduces the heat transfer rate by lowering the temperature of the steam. The saturation temperature of steam is reduced when mixed with air in accordance with the law of partial pressures. Air contributes to the pressure of the mixture but does not contribute to the available heat content.
7. Saturated steam versus superheated steam
Typical 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 entrapped in the vapor. The addition of any superheated steam to a heat transfer process can cause performance problems if the original design did not anticipate any superheat. Furthermore, superheated steam may require material changes in order to handle difference in pressure and temperature of the steam.
8. Condensate removal
a. When 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 crucial in any application that has a modulating steam control valve.
b. Other applications do not permit gravity drainage and, therefore, care should be taken to ensure that 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 water hammer and inadequate temperature control. Poor condensate drainage can also result in corrosion problems for the heat transfer unit.
c. 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 will then 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.
d. The horizontal distance from the vertical drop leg (condensate outlet of heat exchanger) to the steam trap should never be more than 8 in. Any length of greater than 8 in. can lead to steam locking.
e. Install a test valve or a visual sight glass after the steam trap for visual indication of performance.
Insulate all exposed surface areas in a heat transfer application. Please refer to the DOE Best Practices Steam Tip Sheet on insulation for further details on pay back and material selection.
10. Control valve piping
a. The 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.
b. The control valve should be located at least 10 pipe diameters away from the heat transfer unit with the expanded pipe.
11. Vacuum Breakers
All heat transfer components, whether shell-and-tube, plate-and-frame, or any other device, requires vacuum breakers. Vacuum breakers protect heat-exchanging equipment when a system is shut down by preventing a vacuum to occur. Additionally, the vacuum breaker maintains the condensate in the heat transfer equipment. It is generally 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.
12. Codes and Standards
As mentioned earlier, know and follow all relative jurisdictional codes.
For more information, visit www.swagelokenergy.com.
The above material is part of Swagelok Energy Advisors’ series of Best Practice papers, authored by Kelly Paffel. Kelly is a recognized authority in steam and condensate systems. He is a frequent lecturer and instructor on the technical aspects of steam systems. In addition, Kelly has published many papers on the topics of steam system design and operation. Over the past 30 years, he has conducted thousands of steam system audits and training sessions in the United States and overseas, which has made Kelly an expert in trouble-shooting actual and potential problems in the utilities of steam. Kelly is a member of the U.S. Department of Energy’s (DOE) Steam Best Practices and Steam Training Committees.
Images courtesy of Swagelok Company