Vaporization cavitation

Vaporization cavitation represents about 70% of all cavitation. Sometimes it's called 'classic cavitation'. At what temperature does water boil? Well, this depends on the pressure. Water will boil if the temperature is high enough. Water will boil if the pressure is low enough.

According to Bernoulli's Law, when velocity goes up, pressure goes down. This was explained in Chapter 1. A centrifugal pump works by acceleration and imparting velocity to the liquid in the eye of the impeller. Under the right conditions, the liquid can boil or vaporize in the eye of the impeller. When this happens we say that the pump is suffering from vaporization cavitation.

This type of cavitation is also called inadequate NPSHa cavitation. To prevent this type of cavitation, the NPSHa in the system (the available energy in the system), must be higher than the NPSHr of the pump (the pump's minimum energy requirement).

A good suggestion to prevent vaporization cavitation is:

NPSHa > NPSHr + 3 ft or more safety margin

Remember from Chapter 2, the NPSHa formula is: NPSHa = Ha + Hs - Hvp - Hf - Hi. If you want to raise the NPSHa, it will be necessary to increase the elements (Ha, Hs) that add energy to the fluid, or decrease the elements (Hvp, Hf, Hi) that rob energy from the fluid. Also remember that the NPSHr reading, printed on a pump curve, currently represents a point where the pump is already suffering a 3% loss in function due to cavitation. Some people in the industry are calling for a more precise definition of NPSHr, and higher safety margins on NPSHa.

With the pump disassembled in the shop, the damage from vaporization cavitation is seen behind the impeller blades toward the eye of the impeller as illustrated below (Figure 3-1).

To resolve and prevent this type of cavitation damage:

1. Lower the temperature. This reduces the Hvp

2. Raise the liquid level in the suction vessel. This elevates the Hs.

3. Change the pump.

■ Reduce the speed. This reduces the Hf.

■ Increase the diameter of the eye of the impeller. This reduces Hf and Hi.

■ Use an impeller inducer. This reduces the Hi, and increases Ha.

■ Use two lower capacity pumps in parallel. This reduces Hf and Hi.

■ Use a booster pump to feed the principal pump. This increases the Ha.

A typical situation often resulting in vaporization cavitation is a boiler

feed water pump where the pump drains the deaerator (d-a) tank. Because this pump generates high discharge pressure, it must also generate a strong vacuum in the eye of the impeller. Typically, this pump can generate 200 psi (460 ft of head) and require 30 ft of NPSHr at its duty point.

The d-a tank is normally one of two varieties:

■ Vented and exposed to atmosphere.

■ Closed and pressurized.

Once again, the formula and elements of NPSHa are: NPSHa = Ha + Hs - Hvp - Hf- Hi, where:

Ha Atmospheric Head. It is 33.9 ft at sea level.

Hs Static Head. It is the level in the d-a tank above the pump centerlinc. This is normally about 12 to 15 ft.

Hvp Vapor Head. It is based on the feed water temperature. Sec Chapter 2, Properties of Water I and II.

Hf Friction Head, or the friction losses in the suction piping. We could assign this a value of 1 ft.

Hi Inlet Head. The losses in the pump suction throat to the impeller eye. These losses could be insignificant up to 2 ft, depending on design.

The feed water in the d-a tank normally runs about 190°F in an open tank. Then we have:

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Responses

  • Kevin
    What is vaporization in civil engineering?
    2 years ago

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