Concentrate on condensate
Manage the many factors that contribute to steam system efficiency
The best method for improving steam system energy efficiency, reducing chemical costs, and reducing makeup water costs is to return the maximum quantity of condensate to the boiler plant. There are several factors that impact the reliability, performance, longevity, and maintenance requirements for the condensate piping system. Some of these factors are listed below:
- Condensate line sizing that factors condensate liquid, and flash steam quantities
- Location of the condensate line with respect to the process equipment
- Locations of the condensate branch line connection into the main condensate headers
- Insulation techniques.
An important factor to increase overall steam system efficiency is to maximize the temperature of the returning condensate. This permits high thermal cycle efficiency for the overall steam system.
Condensate contains a relatively large percentage (16% in some cases, depending on pressure) of the energy used to produce the steam. With today’s rising energy costs, facilities must return all possible condensate back to the boiler plant. The condensate should be maintained in a high-energy state or simply as hot as possible.
A typical reason for condensate loss in the system is due to condensate component failure. In this paper, we will address the major reasons for component failure and provide recommendations on achieving energy savings with a proper operating condensate system.
Condensate Piping Codes
The Power Piping Code B31.1 describes the minimum requirements for construction of power and auxiliary service piping. B31.1 applies to condensate piping when the pressure and temperatures are greater than 100 psi. However, it is a recommended practice to apply these standards to all condensate systems.
A reasonable specification for condensate system design is to provide a reliable and long operational life span of more than 20 years without a primary condensate system failure. Plant personnel must assume that the condensate system designs shall include reasonable maintenance and plant services. Forgoing a proactive maintenance plan will reduce the anticipated life span of the condensate system.
Because condensate pipelines are potentially subjected to carbonic acid, a damaging corrosive element, material selection is important. The recommended material to use for a condensate system is stainless steel. Stainless steel greatly enhances the pipes’ ability to withstand the corrosive attack for a longer, more reliable operational life.
However, understanding the cost limitations to an all-stainless-steel condensate system, other alternatives are available. If carbon steel piping is used for economical consideration, schedule 80 pipe is used because of the heavier wall thickness, which prolongs the life of the pipe in a corrosive environment.
Welding the condensate pipe or using tubing with tube connectors will minimize leaks. Condensate pipe will expand and contract during normal thermal cycling of a steam system operation. Unfortunately, steam component manufacturers still provide a large number of components with threaded (NPT) connections.
These threaded connections are inherently a weak point in the steam/condensate system and will be the first item attacked by the corrosive carbonic acid, particularly the threads near the bottom of the pipe. Also, the threaded connections do not have the ability to withstand the expansion and contraction of the steam/condensate system, and therefore leaks will occur. The most common condensate piping connections are listed below in order of preference:
- Welded joints
- Tube material with tube connectors
- Threaded pipe only when necessary.
Pipe vs. Tubing
Tubing is an acceptable method of piping, yet it is typically underutilized. Tubing provides an improved connection of steam components and other devices in the system. Welding smaller pipe sizes (below 1 in.) is time consuming and expensive. Using tubing material reduces the number of welds needed in an installation.
Most mechanical systems operate at peak performance levels following a new installation. However, system maintainability really determines the resiliency and reliability of the system. The system’s components, including piping, tubing, steam traps, condensate pumps, etc., must be designed and installed with maintenance functions in mind. Frankly, if the devices are not accessible by plant personnel, there will be little or no maintenance performed and the overall system integrity will deteriorate.
Condensate Pipe/Tubing Sizing
Sizing of condensate lines is calculated differently from sizing other fluids transferred in pipes. Although condensate is hot water, sizing a condensate line as if it were hot water would result in an undersized line. Undersized condensate lines will create excessive back pressure in the system, as well as maintenance and process problems throughout the system.
The key item to remember is that there are two major differences between condensate and hot water. Condensate lines will contain two phases, condensate (liquid) and flash steam (gas). Therefore, the correct size of a condensate line is somewhere between a hot water line and a steam line. With proper knowledge, a condensate line may be sized for the following:
- Condensate liquid load
- Flash steam load
- Neglect factor.
This is defined as steam loss resulting from faulty steam traps or open bypass valves. This is more common in systems than typically acknowledged. Blow-by steam will add steam flow to the return line and must be included in the calculations. Condensate that is free of flash steam may be pumped and sized as liquid only (single-phase flow).
Condensate pipe velocities (liquid and flash steam) must be lower than 4500 ft/min to prevent system waterhammer and other damaging effects. Condensate piping velocities (liquid only) must be lower than 7 ft/s.
Correct Identification of Condensate Type
The placement of condensate return lines is crucial to insure proper operation of the process equipment. The first step is to understand and identify the type of the condensate line.
This describes all process equipment with a modulating inlet steam valve and a very low steam pressure application where the condensate return line must be at or close to atmospheric conditions. Therefore, the condensate drains by gravity to a vented (atmospheric) condensate collection tank.
- Condensate return that is less than 15 psi
- Medium-pressure return
- Condensate return that is between 15 and 100 psi
- Condensate return piping system pressures of 100 psi or higher
Most of the condensate system problems result from the location of the condensate lines in relationship to the heat transfer equipment, steam trap, and other drainage-type devices.
Connecting into the Condensate Header
It is imperative that all condensate branch lines are connected into the top dead center of the main condensate header on a horizontal plane. This cannot be overstated, and there is no exception to this rule. Improper condensate connections are listed below:
- Connection to the bottom of a condensate header
- Connection to the side of a condensate header
- Connection to a vertical condensate header.
The condensate connections listed above will cause system problems, primarily waterhammer. Flash steam introduced to the main condensate header due to an improper connection location will interact with cooler condensate causing waterhammer. Waterhammer is the leading cause of premature component failures in a steam/condensate system.
Finally, a note regarding pressure gauges. These devices, when properly installed in the condensate return system, are a great advantage to assisting in identifying the process and steam system malfunctions. If pressure gauges are not installed, always put the necessary taps in the system for a pressure gauge. This will allow maintenance personnel to install a gauge during troubleshooting procedures. It is necessary to include a siphon pipe (pigtail) and isolation valve with each pressure gauge. The isolation valve must be rated for the pressure and temperature of the operating system. Additionally, a liquid-filled pressure gauge will be more resilient to system vibrations.
Condensate contains a high percentage of the energy (typically 16%) used to produce steam. Recovery and return of all possible condensate back to the boiler plant is the best method for cost containment and improved energy efficiency.
Remember these tips:
- Recover and return all possible condensate to the boiler plant as hot as possible
- Accept no component failure within three years of operation.
- Install components with maintenance in mind.
- Size condensate lines understanding the medium will be two-phase flow.
- Utilize connections that minimize leaks.
- Understand the various pressures of condensate returns available in order to design the piping system with proper flow.
- Remember to allow for pressure gauge installations throughout the system. These inexpensive devices are a key aid in troubleshooting the steam and condensate system.
- Following these rules will help to ensure a reliable and long-lasting condensate system.
- Kelly Paffel 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 troubleshooting actual and potential problems in the utilities of steam. Kelly is a member of the U.S. Department of Energy’s Steam Best Practices and Steam Training Committees.