Centrifugal Pumps- Part II- NPSH and Cavitation

Defined in the terminology section, NPSH stands for “Net Positive Suction Head”. During the operation of the pump, pressure will reduce in the suction nozzle of the pump. During academic course we learnt if pressure at the surface of the liquid reduces below the vapor pressure of the liquid, liquid will starts to boil. Boiling, in this way, is called cavitation. Small bubbles will form. These bubbles travel inside the pump through the path and will reach higher pressure areas. In higher pressure areas, bubbles will collapse inside them, a phenomenon called implosion. During this implosion, a wave of tiny but high pressure will be created. Upon collision of this wave with internals of the pump, small part of internal materials will be cut off. A wave caused by single bubble case really tiny portion of the pump material to erode. From this statement two important results can be driven:

1. Small duration or non-continuous cavitation is not important. For example if cavitation happens “only” during startup, it will not cause a problem. Pump material can withstand these kind of erosion.
2. Even with really small power of each bubble, when this phenomenon happens continuously, pump materials will start to erode. Major signs of pumps working in cavitation are:

• Sound: when pump works in cavitation it produce a sound like passing the sand in the system.
• Vibration: Pumps with cavitation have higher vibration

the results of cavitation are:

• Reduce head: Pumps can only work with liquid, upon cavitation, a bulk of gas phase will introduce to flow. This bulk results in lower discharge head.
• Erosion: Describe in the previous paragraphs, cavitation will lead to erosion. The higher bubbles formed, the more erosion will happens. Erosion can shorten life span of the pump from 20 years (as per clause 6.1.1API 610,11 edition) to few years or even less that few months depending on the degree of erosion.

Figure 1- Erosion due to cavitation

1- NPSHA

NPSHA stands for “Net Positive Suction Head Available”. The value of NPSHA is calculated by client or its consultant company and will be advised to pump manufacturer. NPSHA means how much total head we have on the suction side. In next parts calculation of NPSHA will be illustrated by examples. NPSHA is function of:

• Drum head before pump, from drum discharge to pump datum (for definition of the datum please refer to API 610, 11 edition, clause 6.1.8). This head is called “static head”
• Pipe losses, shall be expressed in meters.
• Drum pressure, shall be expressed in Absolute values During the course of a project these figures are usually, but not always, fixed, thus NPSHA is fixed.Common exception is NPSHA for the vertical pump. Sump height may vary during course of project in result the NPSHA will vary. This will be covered in detail in the next parts.

2- NPSHR

NPSHR stands for “Net Positive Suction Head Required”. This Figure is stated by pump manufacturer and it is function of inner geometry of the pumps like vane shape, impeller angles and etc. and can be calculated from below formula:

Figure 2- NPSHr calculation formula

For explanation of different terms of this formula please refer to: “Practical Centrifugal Pumps Design, Operation and Maintenance” by P.Girdhar and O.Moniz , 2005 Page.67.

There shall exists a margin between NPSHA and NPSHR. This margin, also called NPSH margin, will guarantee that pump will operate in a safe condition. The value from this margin will come from company practice or one's experience. As an example in the book “Practical Centrifugal Pumps Design, Operation and Maintenance”, page 72 below table is inserted for suggested NPSH margin that can be useful:

Figure 3- Suggested NPSH margin for different application

It is common for selected pumps in some cases do not meet this margin. For margins lower than 1m (again this value is company practice), NPSH test shall be carried out. NPSH test will be covered in next parts but for reference please look at page 74 of the standards of "The Hydraulic Institute" (Fourteenth edition).

3- NPSH3

As mentioned in the first part of this chapter, cavitation will reduce the discharge head of the pump because of flow blockage due to vapor formed from cavitation. As mentioned in the formula in the previous part, NPSH is a function of pump geometry. In order to make the job easier, Hydraulic Institute specify another term NPSH3. NPSH3 is equal to value of NPSH when the total head or head at first stage for multi stage pumps, is reduced by 3%. In the tests, vendor starts to reduce NPSHA until head drops to 97% (3% drop). The value for NPSHA at that point is reported as NPSH3.

With above mentioned definition there are other NPSH related terms like NPSH1 and NPSH5m but they are less likely to be used. For more information please refer to: Practical Centrifugal Pumps Design, Operation and Maintenance by P.Girdhar and O.Moniz , 2005 Page.66. During the bidding stage NPSHA will be estimated, either by hand calculation or by different commercial software. This estimation will be handed to pump manufacturers for selecting suitable pump model. However this value may change in the course of the project due to changes such as change in pipe line routes and etc. This is the reason behind considering a margin between NPSHA and NPSHR.

As mentioned in previous part, NPSHA is fixed because it is function of different parameter which is set at the beginning of the project. To go more in detail, please find these two examples for calculating NPSHA. These examples are taken from “Practical Centrifugal Pumps Design, Operation and Maintenance” by P.Girdhar and O.Moniz , 2005.

3.1- EXAMPLE 1

Figure 4- Example 1

3.2- EXAMPLE 2

Terms used in the figure 5 are same as terms used in figure 4.

Figure 5- Example 2

3-3 EXAMPLE 3. An example from a live project

In this example we will work on a real problem raised during course of bid evaluation for a project. A vertical pump (P-3007) is mounted on the top of sump (SU-3001). Vapor pressure for the liquid at NORMAL condition 0.8 barA, stated in datasheet. And the Specific gravity is 0.8 or density is 800kg/m3. sump dimension is as follow:

Figure 6- Example 3

To calculate NPSHA we have to consider worst case. For static head, the worst case is the moment when liquid is at low liquid level (LLL). The distance between LLL and pump suction is (500-100=400 mm or 0.4m) => H=0.4m

Tank is atmospheric meaning that above liquid surface the pressure is 1 barA. We have to convert this pressure to liquid head

1*100,000(P)=800(Ruh)*9.81 (g)*h=> h= 12.74.

We have to calculate vapor pressure to m of liquid then

0.8* 100,000=800*9.81*h=> h =10.19m

NPSHA= H-H+H=12.74-10.19+0.4= 2.95m.

Contrary to normal pumps, since vertical pump sucks the liquid from the sump, we can adjust H by making the sump deeper. For example if vendor submitted NPSHR as 2.8 we can make the sump boot 0.85m deeper in order to meet NPSH margin. For vertical pumps this issue is easy to address while for horizontal pumps H(st) is usually fixed and since other parameters are variable of temperature, thus NPSHA is fixed.

4- Conclusion of NPSHA calculation

As illustrated in the above examples, calculating NPSHA is a straight forward procedure. Parameters that affect NPSHA for horizontal pumps (OHs and BBs) are constant through the course of project. For vertical pumps (VS) since these pumps are in the sump (” example 3” in previous part) or in a well (VS6- Can type pumps) Static head above suction nozzle can be increased to cover the requirement for NPSH margin. The question that may trigger one curious mind is “Will having sufficient NPSH margin prevents cavitation?”

The answer is NO. Even having 10m of NPSH margin, will not prevent cavitation completely. There are areas of pump, for example the tip of the vanes, which have relatively high speed. As per Bernoulli’s law, with increasing the speed, pressure will drop and it is possible to have cavitation.

So does this kind of cavitation with for pumps with high NPSH margin will affect the pump performance or lead to erosion?

The answer is NO again. As described in the first part of this chapter only cavitation with rapid and continuous implosion of the bubbles are problematic. In this case the bubble formation is not continuous and the amount of produced bubbles is too low. Meaning that implosion of the bubbles will not cause a problem for pump. Pump manufacturer also consider an additional thickness for pump material to cover this kind of erosion. The reason for introducing NPSH3 is now clear. HIS (Hydraulic Institute Standard) defines a boundary limit so engineers can safely select the pump. When a pump is selected with sufficient margin between NPSHA and NPSH3, together with recommendation on materials, it means that the pump can operate safely for extended life or as per clause 6.1.1, API 610,11 edition for 20 years with normal maintenance.

5- NPSH Test

In clause 8.3.1.2 of API 610, 11th edition, it is stated that NPSH test shall be according to methods and uncertainties of ISO 9906 grade 1, ANSI/HI 1.6 (for centrifugal pumps) or ANSI/HI 2.6 (for vertical pumps).NPSH test procedure has its requirements. In addition to standards mentioned in API good practice is suggested by Hydraulic Institute. (HIS, Page 74). The suggested loops for testing horizontal and vertical pumps are as follow. Readers are requested to refer to HIS (Hydraulic Institute Standard) for more in depth article. For large pumps or pumps with special conditions (like low or high operating speed) the tests will be carried out on a model then correction factors are applied or NPSH is calculated using affinity laws. These topics are also covered in HIS.

Figure 7- NPSH test rig

In the table 16 of API 610, tolerances in testing are stated. It means that vendor can deviate from the data he stated in datasheet to certain degrees. But the only exception is NPSH at point which is 0%. It means in the test vendor shall develop the exact NPSHR he stated in the datasheet or else the pump can be rejected.

HINT: According to clause 6.1.8 API 610-11th edition ” The vendor shall specify on the data sheet the NPSH3 based on water [at a temperature of less than 55 °C (130 °F)] at the rated flow and rated speed. A reduction or correction factor for liquids other than water (such as hydrocarbons) shall NOT be applied.”

The NPSH required test shall determine the actual NPSH required at a 3 % head drop. Unless otherwise Specified or agreed, curves shall be developed at constant flow by reducing the NPSHA to a point where the head curves break away from that developed with sufficient NPSHA (API610- 11edition clause 8.3.4.3.2) by at least 3 %.

6- Other causes of Cavitations

Low NPSH margin is not the only source of cavitation. There is another phenomenon called recirculation that can lead to forming cavitation. Recirculation occurs when flow go in the reverse direction. Contrary to the NPSH cavitation that happens usually in the suction side, cavitation due to recirculation can happen both at the inlet and outlet. The suction and discharge recirculation may occur at different points as shown in below figure.

Figure 8-Formation of different phenomena

One theory suggests that recirculation cavitation (rotating stall or separation) is the formation of vapor-filled pockets. This type of cavitation is different from the classical cavitation described earlier due to low NPSH margin. Typical formation of recirculation zones are shown in the below picture. For more information please refer to “Practical Centrifugal Pumps Design, Operation and Maintenance”, section 3.12.2

Figure 9- Recirculations

For more clarification please see below picture (same reference), for the regions of formation of:

1. Cavitation due to NPSH margin
2. Suction recirculation cavitation
3. Discharge recirculation cavitation

Figure 10- Recirculation and cavitations formation regions

It is often believed that only high-energy pumps (as per API 610 – 11 edition clause 6.1.15 states: High-energy pumps are defined as pumping to a head greater than 650 ft (200 m) and more than 300 HP (225 kW) per stage) are affected by recirculation cavitation. However, an impeller constructed of cast iron or bronze can erode badly at much lower energy levels.

Related Articles:

1- Centrifugal Pumps- Part I- Mechanical seals

References:

1- American Petroleum Institute (API) Standard 610, 11th Edition

2- Paresh Girdhar, Octo Moniz and Steve Mackay, 1st Ed., "Practical Centrifugal Pumps Design, Operation and Maintenance": Elsevier (2005)

3-Hydraulic Institute "Hydraulic Institute Standards for Centrifugal, Rotary & Reciprocating Pumps", 14th Ed. (1983)

4-Pictures are taken from mentioned books and standards. In addition Cover photois courtesy of Grundfos Pumps Ltd., figure 1 taken from Engineerlive

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