Can Your Boiler Feed Pump Handle a Deaerator Pressure Transient?

 

“This is part of an article talking about the importance of DA and BFP positions at power plant “

In a typical steam power plant, the boiler feedwater (BFW) pump takes suction from the deaerator (DA) and discharges high-pressure water to the boiler through the feedwater heaters. During normal operation, the DA is supplied with steam turbine extraction steam to mix with and heat the feedwater. Other purposes for the DA are to provide the required net positive suction head (NPSH) for the BFW pump and to serve as a storage tank to ensure a continuous supply of feedwater during rapid changes in BFW demand.

The available net positive suction head provided to a boiler feedwater pump can drop enough during a pressure excursion to cause cavitation and damage to the pump’s internal parts. A careful analysis of various operating profiles can ensure that the pump operates safely during the pressure fluctuations that occur after a steam turbine trip or large load change.

How does the plant designer or operator determine the adequacy of the BFW pump selection or the DA and feedwater system design? It’s not uncommon to find that the BFW pump was originally specified based on steady-state conditions and did not consider the DA pressure transients that occur during a steam turbine trip (with the boiler remaining in service) or a sudden steam turbine load reduction. If the NPSH available to the BFW pump during the pressure transient drops below that required by the pump for only a short period of time, cavitation and damage to the pump internals often result.

An NPSH deficit in an existing system or a new system under development can be avoided by using some very simple analytic tools.

Find the NPSH margin

The deaerator is installed at some elevation above the BFW pump to provide the NPSH required by the pump. By definition, the NPSHr is the total suction head over and above the vapor pressure of the liquid pumped.

The DA elevation minus the dynamic losses in the BFW suction piping between the DA and the BFW pump equals the NPSH available (NPSHa) to the pump. The difference between the value of the NPSHa and that required (NPSHr) by the pump gives the NPSH margin.

The NPSH margin or the NPSH margin ratio (NPSHa/NPSHr) is an important factor in ensuring adequate service life of the pump and minimizing noise, vibration, cavitation, and seal damage. The NPSH margin requirement increases as the suction energy level (for example, high suction specific speed, high peripheral velocity of impeller, and the like) of the pump increases. In the case of the BFW pump, this ratio could be in the range of 1.8 to 2.5. These margins are typically based on steady-state operation.

In addition, the NPSH margin improves the ability of the BFW pump to handle a DA pressure transient. Once a design is determined to have an adequate NPSH margin, the next step is to determine if the NPSH margin is adequate during a pressure transient.

Expect Deaerator Pressure Decay

Immediately after a steam turbine generator trip, turbine extraction steam is no longer available to the deaerator, resulting in decay of the DA pressure. Also during a sudden steam turbine generator load reduction, the extraction steam pressure decreases until the extraction stage supplying the DA can no longer maintain DA pressure. This also results in DA pressure decay as the lower-temperature condensate continues to enter the DA, cooling the stored feedwater

The decrease in DA pressure causes some of the water in the DA storage tank to flash to steam until saturation pressure is reached at the new DA pressure. The water in the BFW pump suction line has a static head exerted on it by the level in the DA storage tank, preventing it from flashing immediately. Therefore, the water in the suction line can be considered as a slug of hot fluid that must be moved through the pump in some finite amount of time. In other words, the pump will not perceive a decrease in vapor pressure (or a decrease in water temperature) until the entire slug of hot water has passed through the pump.

During the passage of the hot-water slug, the combination of high vapor pressure at the pump suction along with a decrease in pump suction pressure (due to DA pressure decay), results in a "critical point" at which the suction pressure may drop below the minimum required pressure (that is, the vapor pressure of the hot-water slug plus the pressure equivalent of the NPSHr). This low suction pressure could result in cavitation damage to the pump internals due to insufficient net positive suction head

Short Residence Time

The time required for passage of the hot-water slug through the pump suction line is the "residence time." Residence time can be expressed as the suction line volume divided by the volumetric flow rate (or, alternatively, as the mass of liquid in the suction line divided by the mass flow rate)

Note that because the vapor pressure at pump suction is modeled to decay only after the residence time has elapsed, the critical point occurs at the end of the residence time interval. The challenge is to determine the DA pressure at this critical point and thereby the system NPSH margin.

Options for Adding NPSH to the System

The main BFW pumps are generally large, high-energy pumps needing large amounts of NPSHr. One solution would be to raise the DA to a higher elevation to increase the NPSHa. This solution is normally not practical or cost-effective. Another approach is to install a low-speed, low-NPSH booster pump upstream of the BFW pump. The booster pump discharge pressure then provides the added NPSH required by the BFW pump. In addition, the same NPSH analysis must be made on the booster pump. The only difference is that in the case of the booster pump arrangement, the critical point and the critical point margin need to be evaluated at the booster pump suction as well as the BFW pump suction.

Additional Transient Condition

An additional transient condition that the system designer must consider occurs during a "hot start." In this situation, steam flash (water-steam mixture) can occur at the pump suction and cause cavitation damage to the pump internals. However, the mechanism causing steam flash is slightly different than what was discussed earlier.

On a plant trip, the DA pressure drops and the water temperature inside the DA drops. However, the pump and suction piping near the pump remain at a higher temperature due to the mass of the metal. As a result, when the pump is operated on a hot restart of the plant, steam flash and cavitation are likely to occur at the pump suction

“This article for, Magdy Mahmoud is manager of engineering for PGESCo., Egypt.”