Selecting the optimum steam pressure for process applications
What is the Correct Pressure Set Point for This System?
A common question that every person involved with steam systems must address at some point is what is the correct steam pressure for the application? Selecting higher than required steam pressure to a process application will result in a number of negative effects. High steam pressures have a higher steam temperature, but the negative is a lower usable (latent) energy. This is often an overlooked fact in the “more steam pressure is better” world. A review of the steam tables shows that as we increase the steam pressure, the latent energy decreases as the sensible energy increases.
Typical steam process applications utilize only the latent energy in the steam. The sensible energy is removed from the system with the condensate. By contrast, one should select the lowest effective steam pressure and temperature that permits operational process success.
Using the lowest effective steam pressure to achieve the process objective will result in the following:
- Lower steam consumption
- Lower thermal expansion of components
- Lower flash steam volume, particularly at the discharge of the trap
- Better temperature control, due to the closer proximity to the process heat sink
- Longer equipment life
Unfortunately, history has shown that the steam process plant accepts the recommendation of the steam equipment manufacturers to designate the required steam pressure. In true practice, the plant should be the designator of the steam pressure.
Another faulty approach is providing the process equipment manufacturer with the boiler operating steam pressure. At the process application location in the plant, the steam pressure will not be the same as the boiler operating pressure, due to steam line pressure drops and other restrictive devices such as valves in the steam line. A steam control will require a pressure drop to achieve flow. A low pressure drop steam control valve is very large and costly. An oversized steam control valve for will be difficult or impossible to control in a steam application. So selecting valves to overcome a poor operating point selection at the boiler is not a solution.
Choosing a steam pressure set point that is too high will result in the following negative conditions:
- Higher energy cost to produce the steam
- More severe duty operation for all steam components
- Greater thermal expansion
- High flash steam volume
- Temperature control problems
- Shorter operational life of equipment
- Higher surface thermal losses to atmosphere
Manufacturers usually can provide process equipment that can operate at any pressure with one exception, the lower or minimal steam pressure limit. The best “correct” steam pressure limit is the lowest reasonable steam pressure that can be used and meet the specification of the process. Understanding and selecting the correct steam pressure will enhance the steam process application.
Correct Steam Pressure for Low Temperature Applications
Applications that should always use low pressure steam (less than 15 psi) are the following heating applications:
- Heating water
- Unit heaters
- Space heating
- Steam coils for comfort heating
There is no advantage for using high pressure steam for these applications. Therefore, always supply general heating loops with steam pressures lower than 15 psi.
Example: Water is required to be heated to 180°F; therefore, steam pressures from 1 psi to 15 psi can used for this application. The steam pressure 1 psi is not commonly used for heating water, but 1 psi is commonly used in vent condensers. When discussing these applications, we are not including the direct injection of steam for heating water.
Correct Steam Pressure for High Process Temperature Applications
Process applications typically require high process temperatures; therefore, higher steam pressures are needed to meet the process requirements. How does a process person find the correct steam pressure?
An example in the steps to find the correct steam pressure:
- Assume a process requires 280°F temperatures
- Review the steam tables and find 280°F
- The equivalent “steam” pressure at 280°F is 35 psi
- Add 45 psi to the required pressure
- Add 45 psi to 35 psi = 80 psi steam pressure
Steam 80 psi pressure that is provided to the equipment manufacturer for design of the process heat transfer. 80 psi shall be the pressure after the steam control valve or the steam pressure that can be delivered to the process.
Example: Process temperature requirements are 310°F; using the steam tables, locate the corresponding steam pressure.
- 310°F = 63 psi
- Add 45 psi to 63 psi = 108 psi
- 108 psi is provided to the manufacturer for the delivered steam pressure
Selecting the correct steam pressure for each application will result in optimal performance of the process equipment while ensuring system reliability.
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.