Active, Most Current. Prices subject to change without notice. About Us. Contact Us.
|Published (Last):||22 June 2010|
|PDF File Size:||14.17 Mb|
|ePub File Size:||19.47 Mb|
|Price:||Free* [*Free Regsitration Required]|
In a power plant, the primary use of vacuum systems is to remove air and other noncondensable gases from the shell side of the condenser in order to maintain design heat transfer and thus design vacuum. If holding condenser vacuum is a persistent problem, one often-overlooked cause is an inadequately sized vacuum system. The primary application of vacuum systems in the power generation industry is for the evacuation of air and other noncondensable gases from the shell side and the waterbox side of a steam surface condenser.
The removal of air and other noncondensable gases from the condenser shell side is required for proper heat transfer from steam to cooling water in the condenser and, thus, to maintain high vacuum in the condenser. Without a vacuum system, air and other noncondensable gases would severely reduce the heat transfer in the condenser, and the plant would require a condenser with significantly more surface area for the same thermal load. This article provides plant designers with an understanding of published design standards for air-venting equipment used in condenser shell side applications and the vacuum system sizing methodology used by equipment suppliers.
The capacity in the holding mode is more critical, as it has a direct effect on megawatt generation. T sat is the saturation temperature of the liquid in the condenser at the given vacuum, and T cw,inlet is the temperature of the incoming cooling water. If the subcooling is less than 7. Note that the design conditions selected by the HEI are used to physically size the venting equipment; actual operating conditions are not necessarily the same.
In general, the standard is based on the number of openings exhausting steam to the condenser and the effective steam flow through each opening. Note that the German VGB code is significantly different regarding the requirement for air-venting equipment Figure 1. Your purchase specification should clearly indicate whether the equipment should meet the HEI standards or the VGB code.
The three most common vacuum-producing devices are steam-jet air ejectors SJAE, also called air jet ejectors , liquid ring vacuum pumps LRVP , and a hybrid of ejector and pump. The application of SJAEs requires motive steam, so in many plants the electric motor-driven vacuum pump is generally favored.
LRVPs are simple, reliable, low-speed devices with only one rotating part, and they can handle condensable vapors or even slugs of entrained liquid without damage to the pump. The hybrid helps in reducing the size of the vacuum pump. The LRVP is a rotary displacement pump using liquid as the principal element in gas compression performed in a single stage or two stages Figure 2.
Compression is performed by the liquid ring using the relative eccentricity between the casing and a rotating multi-bladed impeller. A portion of the liquid in the casing is continuously discharged with the gas, collected in a separator vessel, and cooled in a heat exchanger. The cooled liquid is introduced back to the pump casing to remove the heat of compression.
In power plant applications, the vacuum pump seal water is usually demineralized water from the condensate transfer system. The acceptance test is performed on dry air at various suction pressures and under ambient conditions present at the test location. Atmospheric air at normal room temperature is considered dry air because the small weight of water vapor present in ambient air is insignificant.
However, a problem arises because the acceptance test is based on dry air only, whereas the HEI gives values for air-water mixture, which represents the realistic conditions expected during plant operation, given the much higher moisture content inside the condenser than in atmospheric air used during factory tests. Consequently, the pump test results must be adjusted for the air-water mixture case, or the HEI values have to be adjusted for a proper comparison. Effect of Saturated Air-Water Mixture.
The saturated air-water mixture tends to increase the capacity of the pump, whereas dry air constrains the pump capacity. This effect is caused when the pump is handling an air-water mixture: Some of the incoming vapors are condensed due to the cooler seal water, thus allowing more room in the impeller bucket to handle more of the incoming mixture. Some pump vendors have published charts indicating the capacity increase with air-water mixtures. The amount of increased capacity is generally related to the pump inlet pressure, the temperature of the incoming mixture, and the temperature of seal water.
Effect of Seal Water Temperature. Pump capacity is decreased if the seal water temperature experienced during actual operation is higher than design or that used for pump testing. In this case, the test capacity has to be adjusted by the vendor-published factor for seal water temperature variation. Another option is to adjust the HEI correction factors. High seal water temperatures affect the capability of the pump, as it is no longer able to attain low suction pressures.
This is due to the increase in vapor pressure of the hot seal water, which begins to flash and cavitate the pump as suction pressure decreases. Cavitation in the pump casing can cause high vibration and could lead to damage to pump internals. As an example of the differences, consider the following test data taken during a vacuum pump factory test:. A quick review of the above data confirms that it is not a straightforward exercise to compare the pump test results with HEI requirements.
A proper comparison requires that either the HEI requirement for air-water mixture be decreased to account for condensation of part of the water vapor inside the pump casing and then compared with the pump test result, or that the pump test result be increased for handling an air-water mixture and then compared against the HEI requirements. In either case, specific correction factors from the pump vendors are required, and these factors must be correctly used as intended by the vendor.
The next section will continue this example by demonstrating how the pump test results should be compared with the HEI requirements. The methodology for comparing vacuum pump test results with the HEI requirements is reasonably straightforward. HEI requirements for vacuum pump capacity for 15 scfm see above can be found using the following calculations:. It appears that the HEI requirement for air removal is not being met, as the HEI standard requires a pump capacity of For example, if a condensation bonus factor of 0.
In other words, the pump capacity is governed by the value of the condensation bonus correction factor that is determined by the pump vendor. This factor is generally proprietary to each vendor and difficult for the purchaser to verify, especially if these factors are not published. Note that the condensation bonus factor for LRVPs is applied to the volumetric flow and not the mass flow.
In the hybrid arrangement, an air jet ejector is used as the first stage of the unit Figure 3. The ejector uses part of the pump discharge air as the motive air, and the ejector discharge is directed to the inlet of the vacuum pump.
The first-stage ejector also helps to minimize pump cavitation, as the pump is no longer operating at low suction pressures, which could cause vaporization of the seal fluid within the pump casing. Cavitation occurs at low suction pressures and high seal water temperatures. However, a different type of condensation bonus is used, taking advantage of a few degrees of adiabatic cooling within the ejector, which reduces the water vapor load.
This, in turn, reduces the loading on the downstream vacuum pump. Note that the amount of adiabatic cooling within the ejector is empirical and can vary from vendor to vendor.
The HEI standard requires removal of water vapor plus dry air while the ejector-pump system test is carried out with dry air at However, the effective water vapor loading inside the ejector is reduced due to cooling while the dry air loading remains the same. The HEI standards stipulate the capacity requirement for air-venting equipment for the condenser. However, equipment provided by the vendor is generally sized at a somewhat lower capacity, taking into consideration the condensation bonus.
However, vendors should be asked to justify, with data, their selection of the condensation bonus for your project. If the condensation bonus selected by the equipment vendor is too optimistic, proper venting of the condenser will not occur, and design heat transfer rates may not be achieved. Zaheer Akhtar, PE szakhtar bechtel. View more. Facebook Twitter LinkedIn. Defense Daily subscriber and registered users, please log in here to access the content. Get a Free Trial Here. Please contact clientservices accessintel.
ET , to start a free trial, get pricing information, order a reprint, or post an article link on your website. Hydro Feb 1, Start your free subscription. SHARE this article. Subscribe Today! Pick your standard. Source: Gardner Denver Nash. Pulling a vacuum. A typical liquid ring vacuum pump with separator vessel and seal water cooler.
Courtesy: Bechtel Power Corp. Best of both worlds. This schematic shows a hybrid arrangement using an air ejector and a vacuum pump. Source: Bechtel Power Corp.
Condenser Backpressure High? Check Vacuum System Sizing
In a power plant, the primary use of vacuum systems is to remove air and other noncondensable gases from the shell side of the condenser in order to maintain design heat transfer and thus design vacuum. If holding condenser vacuum is a persistent problem, one often-overlooked cause is an inadequately sized vacuum system. The primary application of vacuum systems in the power generation industry is for the evacuation of air and other noncondensable gases from the shell side and the waterbox side of a steam surface condenser. The removal of air and other noncondensable gases from the condenser shell side is required for proper heat transfer from steam to cooling water in the condenser and, thus, to maintain high vacuum in the condenser.
Cancellation of Spring Meeting
Thank you for interesting in our services. We are a non-profit group that run this website to share documents. We need your help to maintenance this website. Please help us to share our service with your friends. Share Embed Donate. Florence, New J ersey ,.. General Considerations
HEI Issues 12th Edition Of Standards For Steam Surface Condensers
HEI 2629-2012 (E11) Standards for Steam Surface Condensers