Wednesday, July 4, 2012

Pump Cavitation & NPSH

Both cavitation and NPSH are terms very frequently encountered by chemical engineers during their entire career either as a design or operations engineer. Most fresh chemical engineers have some idea of these terms but are not fully conversant with the concept. Many a times there are also a lot of misconceptions about the actual meaning of these frequently encountered terms. My blog entry is a humble attempt to bring forth some explanation regarding these often misunderstood or partially understood concepts of cavitation and NPSH.
Let me begin with cavitation. A dictionary definition for cavitation is as follows:

“The sudden formation and collapse of low-pressure bubbles in liquids by means of mechanical forces, such as those resulting from rotation of a marine propeller”.
In context with centrifugal pumps it can be said to be a phenomena where vapor bubbles form and move across the vane of the pump impeller. As these vapor bubbles move along the impeller vane, the pressure around the bubbles begins to increase. When a point is reached where the pressure on the outside of the bubble is greater than the pressure inside the bubble, the bubble collapses. The bubble collapse is not by “explosion” but by “implosion”. This collapse occurs simultaneously for hundreds of bubbles moving across the impeller vane at practically the same point on the impeller vane. The figure below should illustrate this phenomenon.




The phenomenon of the formation and subsequent collapse of these vapor bubbles, known as cavitation, has several effects on a centrifugal pump. First, the collapsing bubbles make a distinctive noise which is akin to a rattling sound, or a sound like the pump is pumping gravel. This can be a nuisance in an extreme situation where a cavitating pump is operating where people are working. This physical symptom is usually the area of least concern with cavitation, however. Of far greater concern is the effect of cavitation on the hydraulic performance and the mechanical integrity of the pump. A cavitating pump causes its hydraulic performance to drop off from its expected performance as illustrated in the figure below:

Effect of Cavitation on the performance of a Centrifugal Pump

A much more serious effect is the mechanical damage that can be caused by excessive vibration in the pump. This vibration is due to the uneven loading of the impeller as the mixture of vapor and liquid passes through it, and to the local shock wave that occurs as each bubble collapses.

The shock waves can physically damage the impeller, causing the removal of material from the surface of the impeller. The amount of material removed varies, depending on the extent of the cavitation and the impeller material. Ferrous-based materials such as ductile iron are more susceptible to cavitation shock waves compared to stainless steels which are not only superior in corrosion resistance but work harden against the hammer like impact of the collapsing bubbles. If the impeller material is more corrosion resistant but softer, ordinary bronze, for example, the damage that cavitation causes is similar to a peening operation, in which a piece of relatively soft bronze is repeatedly struck with a small ball peen hammer.

As long as the cavitation persists, this removal of material can continue. Pits can be formed gradually on the impeller vanes and, in the extreme, the removal of material can actually cause a hole to be eaten clear through an impeller vane, as illustrated in the figure below:

Material loss from impeller vane due to cavitation.

This removal of material from the impeller has the obvious effect of upsetting the dynamic balance of the rotating component.
It is very important to remember that excessive vibration from cavitation can occur even without the material loss from the impeller described above. This is true because the vibration from cavitation is caused by the uneven loading of the impeller due to the shock waves produced by the collapsing vapor bubbles.
One of the most common and visible effects of cavitation is the failure of the pump’s seal and/or bearings. What causes the formation of the vapor bubbles in the first place, without which the cavitation would not have a chance to occur? To a layman, the most obvious way to create vapor bubbles that is, to make a liquid boil is by raising the temperature of the liquid. However, this is not what occurs in a cavitating pump because, in the higher flow range where cavitation is likely to occur, the temperature of a liquid as it moves through a centrifugal pump remains very nearly constant.
Another way to make a liquid boil, without increasing its temperature, is if the pressure of the liquid is allowed to decrease. This physical property of liquids is known as vapor pressure.
Every liquid has a characteristic vapor pressure that varies with temperature, as the table below shows for water. Many handbooks carry this data for various liquids.
For any liquid, as temperature goes up, vapor pressure increases. One way to interpret the vapor pressure data for a liquid is that it shows the temperature at which the liquid boils when it is at a certain pressure. For example, from Table 2.3, we see that at 14.7 psia (atmospheric pressure at sea level), water boils at 212°F (100°C).
If the water is subjected to a pressure of 90 psia, the liquid does not boil until it reaches a temperature of 320°F (160°C). This is the principle upon which a pressure cooker is based. With the pressure cooker operating at a pressure above atmospheric pressure, the liquid boils at a much higher temperature than it would in an open pot on the stove, so the food in the pressure cooker cooks faster.
If a liquid is at a certain temperature in a pressurized container and the pressure in the container is allowed to drop below the vapor pressure of the liquid at that particular temperature. the liquid boils. As an example (using Table below), if water at 300°F (148.9°C) is in a vessel which is maintained at a pressure of 100 psia, the water is in a liquid state, i.e., is not boiling. However, if the pressure in the vessel is allowed to drop, when it goes below 67 psia (the vapor pressure at 300°F), the liquid begins to boil.

In analyzing a pump operating in a system to determine if cavitation is likely, there are two aspects of NPSH to consider: NPSHa and NPSHr
NPSHa
Net positive suction head available (NPSHa) is the suction head present at the pump suction over and above the vapor pressure of the liquid. NPSHa is a function of the suction system and is independent of the type of pump in the system. It should be calculated by the engineer or pump user, and supplied to the pump manufacturer as part of the application criteria or pump specification. The general formula for calculating NPSHa is:
NPSHa = P ± H – Hf – Hvp         ……………………………..(1)
 where:
P =absolute pressure on the surface of the liquid in the suction vessel, expressed in feet (meter) of liquid
H = static distance from the surface of the liquid in the supply vessel to the centerline of the pump impeller, in feet (meter); the term is positive if the pump has a static suction head, and negative if the pump has a static suction lift. For the purpose of NPSHa calculations, both the static suction head and the static suction lift should be considered at the “minimum operating liquid level” of the suction vessel. In other words, the NPSHa should be calculated with the minimum static head or the maximum static lift as the case maybe. The term “minimum operating liquid level” although is quite debatable since many engineering professionals as well as engineering companies differ on it’s definition.  It would suffice to say that the minimum static head or maximum static lift may differ on a case-to-case and operating philosophy basis and the engineer performing the NPSHa calculations would require doing a careful analysis of this value before using it in his or her calculation. 
Hf = friction loss in the suction line, including all piping, valves, fittings, filters, etc., expressed in feet (meter) of liquid; this term varies with flow, so NPSHa must be calculated based on a particular flow rate
Hvp= vapor pressure of the liquid at the pumping temperature, expressed in feet (meter) of liquid
In a new pump application, NPSHa (and the static term H in the above formula) must be given to the manufacturer with reference to some known datum point such as the elevation of the pump mounting base. This is because the location of the pump impeller centerline elevation is generally not known when the NPSHa calculations are made. It is important that the datum point of reference be mentioned in the specification, as well as the calculated value of NPSHa.
New engineers often get confused when the suction vessel is not a vented vessel to atmosphere and there is absence of data about the maximum operating pressure in the vapor space of the vessel. A way out is that if the vessel is provided with a relief device in the vapor space of the vessel, the set pressure of the relief device may be used as the maximum vessel pressure for the purpose of NPSHa calculations.
NPSHr
Net positive suction head required (NPSHr) is the suction head required at the impeller centerline over and above the vapor pressure of the liquid. NPSHr is strictly a function of the pump inlet design, and is independent of the suction piping system. The pump requires a pressure at the suction flange greater than the vapor pressure of the liquid because merely getting the liquid to the pump suction flange in a liquid state is not sufficient. The liquid experiences pressure losses when it first enters the pump, before it gets to the point on the impeller vane where pressure begins to increase. These losses are caused by frictional effects as the liquid passes through the pump suction nozzle, moves across the impeller inlet, and changes direction to begin to flow along the impeller vanes.
NPSHr is established by the manufacturer using a special test, and the value of NPSHr is shown on the pump curve as a function of pump capacity.
It is important to note that the NPSHr increases with higher flow rate due to the increased amount of friction loss inside the pump inlet before the liquid reaches the pump impeller. In certain cases the NPSHr also increases with the flow remaining unchanged but the impeller diameter reduced.
For a pump to operate free of cavitation, NPSHa must be greater than NPSHr. In determining the acceptability of a particular pump operating in a particular system with regard to NPSH, the NPSHa for the system must be calculated by the engineer, and then the NPSHr for the pump to be used must be examined at the same flow rate by looking at this information on the pump curve. This comparison should be made at all possible operation points of the pump, with the worst case usually being at the maximum expected flow, also called the runout flow.
Safe Margin NPSHa vs. NPSHr
An often-asked question is: “What is a safe margin to maintain between NPSHa and NPSHr?” Unfortunately, like so many questions related to pumps, the answer must begin with “That depends.…” For the majority of pumping applications, it is good practice to have a reasonable margin between the available and required NPSH.
When considering the margin that should be maintained between NPSHa and NPSHr for a particular application, the questions to ask include:
a. How conservatively was NPSHa calculated for the system, and for what percentage of the pump’s duty cycle is this low value of NPSHa actually present?
b. What is the pump impeller material, and how resistant is it to cavitation damage?
c. Does the pump system make use of a gas blanket that may become dissolved in the liquid and subsequently liberated in the low pressure area of the impeller inlet?
Depending on the answers to these questions, the recommended minimum margin between calculated NPSHa and NPSHr can range from 0 to 35%. A rule of thumb often used in industry is that NPSHa should exceed NPSHr by a minimum of 3 feet (1 meter).
The safe margin between NPSHa and NPSHr needs to be critically evaluated and established when the liquid pumped is close to its boiling point (saturated liquid) or when vey high pumping flow rates are required. 
Remedies for Cavitation
From the discussion till now it is obvious that the higher the value of NPSHa or greater the difference between NPSHa and the NPSHr lesser the possibility of cavitation. For a given value of NPSHr at the pump rated flow the endeavor of any engineer would be to increase the value of NPSHa as given by the equation 1. Pump system modifications that can increase the NPSHa by manipulating the four terms in the right hand side of equation 1 can be described as follows:

1.  Increase the static suction ‘head’ or decrease the static suction ‘lift’ absolute value. This can be done by having the pump operate at a higher suction vessel level or by changing the pump location to a lower elevation.   
2.  Decrease the value of friction losses (Hf) in equation 1. This can be done in several ways:
     a. Make the suction pipe shorter by locating the pump close to the vessel.
     b. Increasing the size of the suction pipe to reduce friction losses.
     c. Reducing the number of fittings and valves in the suction pipe to reduce friction losses
     d. Using a suction filter / strainer which gives the minimum pressure drop (low differential 
         pressure) at clogged conditions while still protecting the pump from ingress of solids / particles.
3. Decrease the pumping temperature of the liquid. This option has to be evaluated with respect to the process requirements of the fluid being pumped.
4. Increase the pressure in the vapor space of the suction vessel by providing a pressure blanket using inert gas or any other compatible gas which will increase the value of ‘P’ in equation 1. This option requires a very careful evaluation since this may prove detrimental to the pump performance in terms of increased cavitation if the blanket gas has good solubility in the liquid and can be liberated at the lower pressure area at the inlet of the pump.
The topic of pump cavitation and NPSH can be discussed even more extensively. However, the endeavor of this brief refresher is to provide an insight to the younger generation of chemical engineers on the fundamentals of cavitation and NPSH.

Reference
 “Pump Characteristics and Applications” by Michael Volk, 2nd Edition.

Prepared by: Ankur Srivastava





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