Jun-2021

Designing steam heat exchangers and tracing systems

Fundamentals must be considered when designing an industrial steam system to avoid premature failure or under-performance.

ALEX CHU
Swagelok

Article Summary

When designing a steam heat exchanger or steam heat tracing system, a full understanding of the overall process and how the system will operate is essential. Inadequate performance can often be attributed to a design engineer’s failure to consider all characteristics of a steam system. Before engaging in the design process, a thorough review of the steam system’s operating parameters and documentation must be conducted to understand the context of the application (see Figure 1). Failure to do so will result in inappropriate control or selection of system components.

Engineers must understand the basic fundamentals of industrial steam systems to prevent premature failure or under-performance. After a comprehensive review of industrial heat transfer applications across different locations and industries, the most common issues can be categorised into incorrect component selection or poor installation practices. The most common issues that are observed include:
- Unacceptable quality of end products
- Premature failure of components
- Poor temperature control
- Inadequate heat transfer
- Water hammer
- Fouling of the heat transfer equipment
- Code and standard violations

Best practices for steam system design
Designing a proper industrial steam system does not have to be complicated if engineers adhere to some simple guidelines and proven field techniques. The following recommendations should be reviewed and implemented into your facility’s steam system design, maintenance, and specification program before determining the equipment for your next steam heat exchanger and tracing system.

Eliminate steam supply condensate build-up and carry-over
Control valves are required to control process steam flow between 0% and 100%. When there is either low or no steam flow in the system, condensate can build up before the inlet of the control valve and cause water hammer. In addition, if condensate flows through a steam control valve at high velocities, it can cause premature wear to the valve internals and subsequent failure to control or seal.

There are several methods to keep condensate from accumulating. For example, installing a drip leg prior to the valve, adding insulation improvements, correctly grading the steam line, or installing a steam separator prior to the control valve. Each method on its own, or a combination of different methods, will prevent condensation from forming or ensure that if it does form it is diverted away from the control valve to a steam trap where it can be evacuated from the system.

Follow turndown ratio guidelines for control valves
To control process temperatures effectively, correctly sized control valves must be specified. The most important consideration when choosing a control valve is the turndown capability, rangeability, or working range of the valve.

Due to physical valve design constraints, all valves will exhibit some uncontrollable flow because of sealing tolerances and the linearity of flow, especially at the extreme ranges of the valve stroke.

Here are some guidelines for the turndown ratios of control valves:
- 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

For steam applications, in addition to the turndown ratio, it is recommended to avoid using the lower and upper 20% of the valve stroke — the lower 20% to avoid excessive velocities across the valve seat and the upper 20% for controllability.

If control valves are not sized properly, the results can be poor process temperature control, premature wear of the valve seats, and excessive noise.

Install pressure gauges before and after the control valve
Pressure gauges (see Figure 2) will provide the basic information necessary to understand the conditions at each part of the system. This will aid performance verification and troubleshooting. It is good practice to install a pressure gauge before and after a control valve and on the condensate return leg after the steam trap. This provides accurate data to assist in understanding the pressure characteristics of the steam passing through the steam heat exchanger. In addition, all pressure gauges should be installed with a coil siphon (pigtail) to prevent high temperature damage and double block isolation valves to allow maintenance.

Install vacuum breakers
Vacuum breakers are essential equipment when steam systems are isolated from the steam supply and are not open to atmosphere. Equipment that requires vacuum breakers includes steam lines, kettles, plate or shell and tube heat exchangers, and any other equipment not rated for vacuum use.
The reason they are necessary is that when steam cools down inside a closed volume, it will condense. Since condensate can occupy only up to 1/1700th of the total volume of its mass compared to when it is in gas phase, a vacuum will be formed.

When steam systems are isolated, vacuum breakers prevent system collapse from external pressures. As a result, most steam equipment requiring air vents and vacuum breakers will have designated installation points specified by the equipment manufacturer. These should not be ignored.

Install automatic air vents
When a steam system is shut down or being maintained, air will leak into the system. This air must be purged before the system can be returned back to service. If air is not purged properly from the system, it will result in slow start-ups and could form thin boundary layers on heat transfer surfaces, creating an insulating effect that will inhibit efficient heat transfer. A boundary layer of air only one thousandth of an inch thick has the same efficiency of heat transfer as 13in of copper or 3in of cast iron.

Steam traps should not be relied on to vent air as they are usually located at the lowest part of the system and therefore cannot be relied on to do it properly since hot air tends to rise and be trapped at the highest point. An air vent fitted on the end of a steam main, or at the highest point on a piece of equipment in conjunction with a vacuum breaker, will open when air is present. This occurs due to the differential temperature between the air and the steam.