Top 10 best practices for designing steam heat exchangers and tracing systems
Mike EdwardsFeatures Swagelok
When developing a steam heat exchanger or steam heat tracing system, designers must consider every element of the system before finalizing its design to avoid premature failure or under performance.
By Alex Chu
That means thoroughly understanding the system’s complete operating parameters and documentation before selecting the components that will manage specific functions.
The most common problems associated with industrial heat transfer applications are caused by either incorrect component selection or improper installation practices. Some common problems include:
- Unacceptable end product quality
- Premature component failures
- Poor temperature control
- Inadequate heat transfer
- Water hammer
- Fouling of heat transfer equipment
- Code and standard violations
The following best practices will help industrial steam system designers avoid these and other problems to enhance the life and performance of their steam system components.
1. Eliminate Steam Condensate Build-Up and Carryover
At low- or no-flow scenarios, condensate can accumulate prior to the inlet of the steam control valve, which can lead to water hammer. In addition, when the control valve is open, any condensate passing through may degrade the valve and lead to premature failure. Therefore, it’s helpful to eliminate condensate build-up. The following methods, and others, can either prevent condensation or allow the condensate to be diverted away from the control valve where it can be removed via a steam trap:
- Installing a drip leg prior to the valve
- Improving insulation
- Grading the steam line properly
- Installing a steam separator prior to the valve
2. Follow Control Valve Turndown Ratio Guidelines
Incorrectly sized control valves in heat transfer systems can lead to poor process temperature control, premature valve seat wear, and excessive noise. To properly size a control valve for effective control and prolonged component life, look to the turndown capability, rangeability, or working range of the valve.
Here are some helpful turndown ratio guidelines:
- Cage control = 40:1 turndown ratio provides the highest degree of controllability
- Globe control valve = 30:1 turndown ratio
- Regulating valve = 20:1 turndown ratio
3. Install Pressure Gauges on Either Side of the Control Valve
Installing a pressure gauge before and after a control valve, as well as on the condensate return leg after a steam trap, will provide an accurate pressure profile for steam passing through the heat exchanger. Such gauges should be installed with a coil siphon (pigtail) to prevent high-temperature damage, as well as double block isolation valves to permit maintenance.
4. Install Vacuum Breakers
If a steam system not rated for vacuum service can be isolated from the steam supply and is not open to atmosphere, it must be fitted with a vacuum breaker to protect the system from implosion when it is shut down. Steam that cools down inside equipment will condense and form a vacuum because condensate typically only occupies around 1/150th to 1/500th of the total volume when compared to its gas phase. Installing a vacuum breaker and an air vent in all steam equipment will help to prevent a vacuum occurrence.
5. Install Automatic Air Vents
Any air that enters steam systems during shutdowns and maintenance must be purged out before returning the system to service to enhance heat transfer and startup times. Air trapped in a steam heat exchanger system can form thin boundary layers on heat transfer surfaces, creating an insulating effect and hindering heat transfer. One of the most efficient ways to remove air within a steam system is via automatic air vents. Such vents fitted on the end of a steam main, or at the highest point on equipment in conjunction with a vacuum breaker, will open when air is present.
6. Avoid Backpressure within Heat Transfer Equipment
Condensate drainage within heat transfer equipment is accomplished by either gravity or a pressure differential. If possible, the equipment should be installed to promote gravitational drainage with no vertical lift before or after steam traps.
For applications that do not permit gravitational drainage, ensure that no undue backpressure is placed on condensate devices like steam traps or control valves. Unanticipated backpressure can lead to premature failures and performance problems that cause condensate to accumulate in the equipment, resulting in water hammer, inadequate temperature control, reduced efficiency, and corrosion issues.
7. Prevent Superheated Steam
Superheated steam, or steam at a higher temperature than its saturation point, contains less energy per unit volume and presents poorer heat transfer compared to saturated steam, which can cause performance problems if not anticipated within the original design. The goal should be to prevent superheated steam from entering the heat transfer process and instead have a 100% quality saturated steam supply that contains no entrained droplets of condensate within the steam.
8. Lockout Ball Valves
Ball valves can provide a safe, reliable, and cost-effective solution for leak tight isolation in a steam system. Look for valves that allow for quick identification of their isolation state via their handle orientation to enhance safety. In addition, consider using valves with locking handles that allow system operators to prevent inadvertent valve actuations.
9. Correctly Select and Size Steam Traps
To enhance the reliability of steam heat exchanger applications, a mechanical type steam trap is best suited for modulating flow. To improve steam heat tracing reliability, a variety of steam trap types can be used depending on the application needs. When selecting any type of steam trap, consider sizing it based on the system’s maximum and minimum flow, its requirements for priming, air venting, and functional testing, as well as its mounting options.
10. Install a Strainer to Prevent Foreign Contamination
Contaminants inside a steam line can lodge within the control valve trims, isolation valve seats, and steam trap seats, causing premature failure or fouling of these components. Installing a strainer will help to prevent foreign materials from entering the steam system. When doing so, always install a blowdown valve with a lockout facility and vent the discharge from the valve to a safe location. In addition, install the strainer with the straining section in the horizontal position to prevent accumulation of condensate within the strainer body.
Reinforce Best Practices
Following the best practices noted above will help to avoid several common problems associated with designing industrial steam systems. In addition, it may be helpful to sign up system designers and operators for some additional training to refresh their understanding of steam system management.
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