AM-EX is an HEI member company, and design our feedwater heaters to HEI standards. However, if you are an engineer, you will tend to question everything just like me, and just because the standards (which were written long ago) tell us something, myself and AM-EX engineers in general feel it is important to verify. Yes, we are nerds, as myself and another engineer performed this research on our own free time, and sometimes at very early morning over weekends, etc. In general, if you tell me that a car has X horse power, I am going to ask for the torque and rpm and verify that the information you are providing is accurate. I did not expect the standards to be inaccurate, however simply wanted to verify that it was in fact correct, as there was no evidence that it had ever been verified by an outside source.
Anyhow, feedwater heater designs incorporate many design parameters which are provided by standards and codes, such as HEI, TEMA, and ASME code. The standards are there to provide a uniform set of guidelines to provide the heat exchanger industry a common set of guidelines to follow for safety, long term operation, and heat exchanger performance. The objective of this research was to simply verify, the existing approach, and that the standards used have an adequate safety factor. HEI and TEMA have published standards governing the feedwater heater design temperature. It is the focus of this blog to examine this standard within the context of a larger thermal evaluation of the top extraction feedwater heater in a supercritical power plant.
Common practice selects a design shell side temperature for the skirt section of a three zone FWH by the HEI Standards for Feedwater Heaters suggested method of utilizing the Mollier diagram. The Mollier diagram is a graphic approach to depict the relationships between the surrounding air temperature, pressure, enthalpy, entropy, and quality. HEI Standards for Closed Feedwater Heaters Seventh Edition section 3.3 for Design Temperatures states that shell side skirt design temperature is to be found by entering the Mollier diagram at normal operating steam temperature and pressure and follow the isentropic line to the maximum operating pressure. These pressures are provided to the heat exchanger designer by the purchaser. The normal operating and maximum steam pressures are obtained by collaboration between the turbine manufacturer and the architect engineer derived from various heat balance iterations to obtain the optimum cost and capitol cost for the end purchaser. Once this optimal situation is agreed upon, the turbine manufacturer provides the referenced pressures. The temperature at that pressure rounded to the nearest 10 degrees F is the design temperature. This method is completely dependent upon the maximum allowable pressure, which is provided by the customer. For verification, these numbers, various data samples were gathered near the shell side extraction steam inlet on an existing FWH to offer some indication on actual operation temperatures at the skirt, which is compared to the HEI standard method. These temperature data samples are skewed due to the use of a steam box DHZ shroud inside the skirt where the temperatures were measured with type K thermocouples. To double check the data which could be gathered by the thermal couples on the shell, actual DCS data was provided by the power plant to acquire actual steam temperature.
Design barrel temperatures of the shell are selected at the saturation point corresponding to the design pressure provided by the purchaser. This value is a more reliable number versus the skirt design temperature simply due to its direct correlation to analytical steam tables and data which have been established to be effective sources of information over hundreds of years. Shell data was collected with thermocouples on the DHZ exhaust where shell temperature would be large. It is speculated that the design shell side temperature is insufficient at the DHZ exhaust location.
|Feedwater Heater with Thermal Couples Rigged Up|
Contact me for the data and the methods used to acquire the shell side temperatures and for the hard fast data. Unless you contact me, you are going to have to take my word for the below conclusion.
After taking, and analyzing the data - which upon request I would be happy to forward on - Design temperatures provided by HEI standards appear to be adequate for the skirt side design temperatures. For the shell, design temperature appears to be too low in the area of the DHZ exhaust. These design temperatures are based upon pressure which shall be verified before heat exchanging equipment is purchased. With 220 ̊F of temperature difference between the measured skirt temperature and that of design temperature, it is clear to say that the design temperature is adequate with a sufficient safety factor. With only 20 ̊F of difference between the shell temperature near the DHZ and the design temperature, it appears the design temperature may at times be insufficient. Design temperature and operating temperature are below the temperature where creep and stress failure mechanism generally occur for the provided materials. When upgrading a turbine, it is important to note the affect of possible increase in temperature and pressure and how this may adversely affect the feedwater heaters.
If you are interested more in the methods used for this blog post, please do not hesitate to contact me at 262-670-6600 and ask for Kellen Muldoon or shoot me an email at KellenMuldoon@amexservices.com.